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  • 301. Sarlak, H.
    et al.
    Mikkelsen, R.
    Sarmast, Sasan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Sørensen, J. N.
    Aerodynamic behaviour of NREL S826 airfoil at Re=100,0002014Ingår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 524, nr 1, artikel-id 012027Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper presents wind tunnel measurements of the NREL S826 airfoil at Reynolds number Re 100,000 for angles of attack in a range of -10° to 25° the corresponding Large Eddy Simulation (LES) for selected angles of attack. The measurements have been performed at the low speed wind tunnel located at Fluid Mechanics laboratory of the Technical University of Denmark (DTU). Lift coefficient is obtained from the forge gauge measurements while the drag is measured according to the integration of the wake profiles downstream of the airfoil. The pressure distribution is measured by a set of pressure taps on the airfoil surface. The lift and drag polars are obtained from the LES computations using DTU's inhouse CFD solver, EllipSys3D, and good agreement is found between the measurement and the simulations. At high angles of attack, the numerical computations tend to over-predict the lift coefficients, however, there is a better agreement between the drag measurements and computations. It is concluded that LES computations are able to capture the lift and drag polars as well as the pressure distribution around the airfoil with an acceptable accuracy.

  • 302.
    Sarmast, Sasan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. DTU Wind Energy, Denmark Technical University, Denmark .
    Chivaee, Hamid Sarlak
    Ivanell, Stefan
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Wind Energy Campus Gotland, Department of Earth Sciences, Uppsala University, Sweden.
    Mikkelsen, Robert F.
    Comparison of the near-wake between a simplied vortex model and actuator line simulations of a horizontal-axis wind turbineManuskript (preprint) (Övrigt vetenskapligt)
  • 303.
    Sarmast, Sasan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. DTU Wind Energy, Denmark Technical University, Denmark .
    Chivaee, Hamid Sarlak
    Ivanell, Stefan
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Wind Energy Campus Gotland, Department of Earth Sciences, Uppsala University, Sweden.
    Mikkelsen, Robert F.
    Numerical investigation of the wake interaction between two model wind turbines with span-wise offset2014Ingår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 524, nr 1Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Wake interaction between two model scale wind turbines with span-wise offset is investigated numerically using Large Eddy Simulation (LES) and the results are validated against the experimental data. An actuator line technique is used for modeling the rotor. The investigated setup refers to a series of experimental measurements of two model scale turbines conducted by NTNU in low speed wind tunnel in which the two wind turbines are aligned with a span-wise offset resulting in half wake interaction. Two levels of free-stream turbulence are tested, the minimum undisturbed level of about Ti 0.23% and a high level of about Ti = 10% using a passive upstream grid. The results show that the rotor characteristics for both rotors are well captured numerically even if the downstream rotor operates into stall regimes. There are however some difficulties in correct prediction of the thrust level. The interacting wake development is captured in great details in terms of wake deficit and streamwise turbulence kinetic energy. The present work is done in connection with Blind test 3 workshops organized jointly by NOWITECH and NORCOWE.

  • 304.
    Sarmast, Sasan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Tech Univ Denmark.
    Dadfar, Reza
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Mikkelsen, R. F.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Ivanell, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. Uppsala Univ, Sweden.
    Sorensen, Jens N.
    Henningson, Dans S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Mutual inductance instability of the tip vortices behind a wind turbine2014Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, s. 705-731Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Two modal decomposition techniques are employed to analyse the stability of wind turbine wakes. A numerical study on a single wind turbine wake is carried out focusing on the instability onset of the trailing tip vortices shed from the turbine blades. The numerical model is based on large-eddy simulations (LES) of the Navier-Stokes equations using the actuator line (ACL) method to simulate the wake behind the Tj ae reborg wind turbine. The wake is perturbed by low-amplitude excitation sources located in the neighbourhood of the tip spirals. The amplification of the waves travelling along the spiral triggers instabilities, leading to breakdown of the wake. Based on the grid configurations and the type of excitations, two basic flow cases, symmetric and asymmetric, are identified. In the symmetric setup, we impose a 120 degrees symmetry condition in the dynamics of the flow and in the asymmetric setup we calculate the full 360 degrees wake. Different cases are subsequently analysed using dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD). The results reveal that the main instability mechanism is dispersive and that the modal growth in the symmetric setup arises only for some specific frequencies and spatial structures, e.g. two dominant groups of modes with positive growth (spatial structures) are identified, while breaking the symmetry reveals that almost all the modes have positive growth rate. In both setups, the most unstable modes have a non-dimensional spatial growth rate close to pi/2 and they are characterized by an out-of-phase displacement of successive helix turns leading to local vortex pairing. The present results indicate that the asymmetric case is crucial to study, as the stability characteristics of the flow change significantly compared to the symmetric configurations. Based on the constant non-dimensional growth rate of disturbances, we derive a new analytical relationship between the length of the wake up to the turbulent breakdown and the operating conditions of a wind turbine.

  • 305.
    Sarmast, Sasan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. Technical University of Denmark, Denmark.
    Segalini, Antonio
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Mikkelsen, Robert F.
    Ivanell, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. Uppsala University, Sweden.
    Comparison of the near-wake between actuator-line simulations and a simplified vortex model of a horizontal-axis wind turbine2016Ingår i: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 19, nr 3, s. 471-481Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The flow around an isolated horizontal-axis wind turbine is estimated by means of a new vortex code based on the Biot-Savart law with constant circulation along the blades. The results have been compared with numerical simulations where the wind turbine blades are replaced with actuator lines. Two different wind turbines have been simulated: one with constant circulation along the blades, to replicate the vortex method approximations, and the other with a realistic circulation distribution, to compare the outcomes of the vortex model with real operative wind-turbine conditions (Tjaereborg wind turbine). The vortex model matched the numerical simulation of the turbine with constant blade circulation in terms of the near-wake structure and local forces along the blade. The results from the Tjaereborg turbine case showed some discrepancies between the two approaches, but overall, the agreement is qualitatively good, validating the analytical method for more general conditions. The present results show that a simple vortex code is able to provide an estimation of the flow around the wind turbine similar to the actuator-line approach but with a negligible computational effort.

  • 306.
    Schlatter, Philipp
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Deusebio, Enrico
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    de Lange, Rick
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Numerical study of the stabilisation of boundary-layer disturbances by finite amplitude streaks2010Ingår i: International Journal of Flow Control, ISSN 1756-8250, Vol. 2, nr 4, s. 259-288Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Well-resolved large-eddy simulations of passive control of the laminar-turbulent transition process in flat-plate boundary-layer flows are presented. A specific passive control mechanism is studied, namely the modulation of the laminar boundary-layer profile by a periodic array of steady boundary-layer streaks. This has been shown experimentally to stabilise the exponential growth of Tollmien-Schlichting (TS) waves and delay transition to turbulence. Here we examine the effect of the steady modulations on the amplification of different types of disturbances such as TS-waves, stochastic noise and free-stream turbulence. In our numerical simulations, the streaks are forced at the inflow as optimal solutions to the linear parabolic stability equations (PSE), whereas the additional disturbances are excited via volume forcing active within the computational domain. The simulation results show, in agreement with experimental and theoretical studies, significant damping of unstable two-dimensional TS-waves of various frequencies when introduced into a modulated base flow: The damping characteristics are mainly dependent on the streak amplitude. A new phenomenon is also identified which is characterised by the strong amplification via nonlinear interactions of the second spanwise harmonic of the streak when the streak amplitude is comparable to the TS amplitude. Furthermore, we demonstrate that control by streaks can be effective also in case of stochastic two-dimensional noise. However, as soon as a significant three-dimensionality is dominant, as in e.g. oblique or bypass transition, control by streaks leads often to premature transition. Visualisations of the flow fields are used to highlight the different vortical structures and their interactions that are relevant to the various transition scenarios and the corresponding control by streamwise streaks.

  • 307.
    Schlatter, Philipp
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Li, Q.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Hussain, F.
    Henningson, Dan Stefan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    On the near-wall vortical structures at moderate Reynolds numbers2014Ingår i: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 48, s. 75-93Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A recent database from direct numerical simulation (DNS) of a turbulent boundary layer up to Re-theta = 4300 (Schlatter and Orlu, 2010) is analysed to extract the dominant flow structures in the near-wall region. In particular, the question of whether hairpin vortices are significant features of near-wall turbulence is addressed. A number of different methods based on the lambda(2) criterion (Jeong and Hussain, 1995) is used to extract turbulent coherent structures: three-dimensional flow visualisation with quantitative estimates of hairpin population, conditional averaging and planar hairpin vortex signatures (HVS). First, visualisations show that during the initial phase of laminar turbulent transition induced via tripping, hairpin vortices evolving from transitional A vortices are numerous and can be considered as the dominant structure of the immediate post-transition stage of the boundary layer. This is in agreement with previous experiments and low-Reynolds-number simulations such as Wu & Moin (2009). When the Reynolds number is increased, the fraction of hairpin vortices decreases to less than 2% for Re-theta > 4000. Second, conditional ensemble averages (Jeong et al., 1997) find hairpins close to the wall at low Reynolds number, while at a sufficient distance downstream from transition, the flow close to the wall is dominated by single quasi-streamwise vortices; even quantitatively, no major differences between boundary layer and channel can be detected. Moreover, three-dimensional visualisations of the neighbourhood of regions of strong swirling motion in planar cuts through the layer (the HVS) do not reveal hairpin vortices, thereby impairing statistical evidences based on HVS. The present results thus clearly confirm that transitional hairpin vortices do not persist in fully developed turbulent boundary layers, and that their dominant appearance as instantaneous flow structures in the outer boundary-layer region is very unlikely .

  • 308.
    Schlatter, Philipp
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Chin, C.
    Hutchins, N.
    Monty, J.
    Large-scale friction control in turbulent wall flow2015Ingår i: 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015, TSFP-9 , 2015Konferensbidrag (Refereegranskat)
    Abstract [en]

    The present study reconsiders the control scheme proposed by Schoppa & Hussain [Phys Fluids 10:1049-1051 (1998)], using new sets of numerical simulations in a turbulent channel at a friction Reynolds number of 180. In particular, it is aimed at better characterising the physics of the control as well as investigate the optimal parameters. Results indicate that a clear maximum efficiency in drag reduction is reached for the case with a viscous-scaled spanwise wavelength of the vortices of 1200, which yields a drag reduction of 18%, contrary to the smaller wavelength of 400 suggested as the most efficient vortex in Schoppa & Hussain.

  • 309.
    Schliephake, Michael
    et al.
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Laure, Erwin
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Performance Analysis of Irregular Collective Communication with the Crystal Router Algorithm2015Ingår i: Solving software challenges for exascale, 2015, s. 130-140Konferensbidrag (Refereegranskat)
    Abstract [en]

    In order to achieve exascale performance it is important to detect potential bottlenecks and identify strategies to overcome them. For this, both applications and system software must be analysed and potentially improved. The EU FP7 project Collaborative Research into Exascale Systemware, Tools & Applications (CRESTA) chose the approach to co-design advanced simulation applications and system software as well as development tools. In this paper, we present the results of a co-design activity focused on the simulation code NEK5000 that aims at performance improvements of collective communication operations. We have analysed the algorithms that form the core of NEK5000's communication module in order to assess its viability on recent computer architectures before starting to improve its performance. Our results show that the crystal router algorithm performs well in sparse, irregular collective operations for medium and large processor number but improvements for even larger system sizes of the future will be needed. We sketch the needed improvements, which will make the communication algorithms also beneficial for other applications that need to implement latency-dominated communication schemes with short messages. The latency-optimised communication operations will also become used in a runtime-system providing dynamic load balancing, under development within CRESTA.

  • 310.
    Schliephake, Michael
    et al.
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Laure, Erwin
    KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Towards improving the communication performance of CRESTA's co-design application NEK50002012Ingår i: Proceedings - 2012 SC Companion: High Performance Computing, Networking Storage and Analysis, SCC 2012, IEEE , 2012, s. 669-674Konferensbidrag (Refereegranskat)
    Abstract [en]

    In order to achieve exascale performance, all aspects of applications and system software need to be analysed and potentially improved. The EU FP7 project 'Collaborative Research into Exascale Systemware, Tools & Applications' (CRESTA) uses co-design of advanced simulation applications and system software as well as related development tools as a key element in its approach towards exascale. In this paper we present first results of a co-design activity using the highly scalable application NEK5000. We have analysed the communication structure of NEK5000 and propose new, optimised collective communication operations that will allow to improve the performance of NEK5000 and to prepare it for the use on several millions of cores available in future HPC systems. The latency-optimised communication operations can also be beneficial in other contexts, for instance we expect them to become an important building block for a runtime-system providing dynamic load balancing, also under development within CRESTA.

  • 311. Schmidt, Oliver T.
    et al.
    Hosseini, Seyed M.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Rist, Ulrich
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. Swedish Defence Research Agency, FOI, Sweden.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Optimal wavepackets in streamwise corner flow2015Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 766Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The global non-modal stability of the flow in a right-angled streamwise corner is investigated. Spatially confined linear optimal initial conditions and responses are obtained by use of direct-adjoint looping. Two base states are considered, the classical self-similar solution for a zero streamwise pressure gradient, and a modified solution that mimics leading-edge effects commonly observed in experimental studies. The latter solution is obtained in a reverse engineering fashion from published measurement data. Prior to the global analysis, a classical local linear stability and sensitivity analysis of both base states is conducted. It is found that the base-flow modification drastically reduces the critical Reynolds number through an inviscid mechanism, the so-called corner mode. A survey of the geometry of the two base states confirms that the modification greatly aggravates the inflectional nature of the flow. Global optimals are calculated for subcritical and supercritical Reynolds numbers, and for two finite optimization times. The optimal initial conditions are found to be self-confined in the spanwise directions, and symmetric with respect to the corner bisector. They evolve into streaks or streamwise modulated wavepackets, depending on the base state. Substantial transient growth caused by the Orr mechanism and the lift-up effect is observed.

  • 312.
    Semeraro, Onofrio
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Bagheri, Shervin
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Transition delay in a boundary layer flow using active control2013Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 731, s. 288-311Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Active linear control is applied to delay the onset of laminar-turbulent transition in the boundary layer over a flat plate. The analysis is carried out by numerical simulations of the nonlinear, transitional regime. A three-dimensional, localized initial condition triggering Tollmien-Schlichting waves of finite amplitude is used to numerically simulate the transition to turbulence. Linear quadratic Gaussian controllers based on reduced-order models of the linearized Navier-Stokes equations are designed, where the wall sensors and the actuators are localized in space. A parametric analysis is carried out in the nonlinear regime, for different disturbance amplitudes, by investigating the effects of the actuation on the flow due to different distributions of the localized actuators along the spanwise direction, different sizes of the actuators and the effort of the controllers. We identify the range of parameters where the controllers are effective and highlight the limits of the device for high amplitudes and strong control action. Despite the fully linear control approach, it is shown that the device is effective in delaying the onset of laminar-turbulent transition in the presence of packets characterized by amplitudes a approximate to 1% of the free stream velocity at the actuator location. Up to these amplitudes, it is found that a proper choice of the actuators positively affects the performance of the controller. For a transitional case, a approximate to 0.20 %, we show a transition delay of Delta Re-x = 3 .0 x 10(5).

  • 313.
    Shahmardi, Armin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Zade, Sagar
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Niazi Ardekani, Mehdi
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Poole, Rob J.
    Lundell, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Rosti, Marco E.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Turbulent duct flow with polymers2019Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 859, s. 1057-1083Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have performed direct numerical simulation of the turbulent flow of a polymer solution in a square duct, with the FENE-P model used to simulate the presence of polymers. First, a simulation at a fixed moderate Reynolds number is performed and its results compared with those of a Newtonian fluid to understand the mechanism of drag reduction and how the secondary motion, typical of the turbulent flow in non-axisymmetric ducts, is affected by polymer additives. Our study shows that the Prandtl's secondary flow is modified by the polymers: the circulation of the streamwise main vortices increases and the location of the maximum vorticity moves towards the centre of the duct. In-plane fluctuations are reduced while the streamwise ones are enhanced in the centre of the duct and dumped in the corners due to a substantial modification of the quasi-streamwise vortices and the associated near-wall low- and high-speed streaks; these grow in size and depart from the walls, their streamwise coherence increasing. Finally, we investigated the effect of the parameters defining the viscoelastic behaviour of the flow and found that the Weissenberg number strongly influences the flow, with the cross-stream vortical structures growing in size and the in-plane velocity fluctuations reducing for increasing flow elasticity.We have performed direct numerical simulation of the turbulent flow of a polymer solution in a square duct, with the FENE-P model used to simulate the presence of polymers. First, a simulation at a fixed moderate Reynolds number is performed and its results compared with those of a Newtonian fluid to understand the mechanism of drag reduction and how the secondary motion, typical of the turbulent flow in non-axisymmetric ducts, is affected by polymer additives. Our study shows that the Prandtl's secondary flow is modified by the polymers: the circulation of the streamwise main vortices increases and the location of the maximum vorticity moves towards the centre of the duct. In-plane fluctuations are reduced while the streamwise ones are enhanced in the centre of the duct and dumped in the corners due to a substantial modification of the quasi-streamwise vortices and the associated near-wall low- and high-speed streaks; these grow in size and depart from the walls, their streamwise coherence increasing. Finally, we investigated the effect of the parameters defining the viscoelastic behaviour of the flow and found that the Weissenberg number strongly influences the flow, with the cross-stream vortical structures growing in size and the in-plane velocity fluctuations reducing for increasing flow elasticity.

  • 314.
    Shahriari, Nima
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    On stability and receptivity of boundary-layer flows2016Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    This work is concerned with stability and receptivity analysis as well as studies on control of the laminar-turbulent transition in boundary-layer flows through direct numerical simulations. Various flow configurations are considered to address flow around straight and swept wings. The aim of this study is to contribute to a better understanding of stability characteristics and different means of transition control of such flows which are of great interest in aeronautical applications.

    Acoustic receptivity of flow over a finite-thickness flat plate with elliptic leading edge is considered. The objective is to compute receptivity coefficient defined as the relative amplitude of acoustic disturbances and TS wave. The existing results in the literature for this flow case plot a scattered image and are inconclusive. We have approached this problem in both compressible and incompressible frameworks and used high-order numerical methods. Our results have shown that the generally-accepted level of acoustic receptivity coefficient for this flow case is one order of magnitude too high.

    The continuous increase of computational power has enabled us to perform global stability analysis of three-dimensional boundary layers. A swept flat plate of FSC type boundary layer with surface roughness is considered. The aim is to determine the critical roughness height for which the flow becomes turbulent. Global stability characteristics of this flow have been addressed and sensitivity of such analysis to domain size and numerical parameters have been discussed.

    The last flow configuration studied here is infinite swept-wing flow. Two numerical set ups are considered which conform to wind-tunnel experiments where passive control of crossflow instabilities is investigated. Robustness of distributed roughness elements in the presence of acoustic waves have been studied. Moreover, ring-type plasma actuators are employed as virtual roughness elements to delay laminar-turbulent transition.

  • 315.
    Shahriari, Nima
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Bodony, Daniel J.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. Swedish Defence Research Agency, Sweden.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Acoustic receptivity simulations of flow past a flat plate with elliptic leading edge2016Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 800, artikel-id R2Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present results of numerical simulations of leading-edge acoustic receptivity for acoustic waves impinging on the leading edge of a finite-thickness flat plate. We use both compressible and incompressible flow solvers fitted with high-order high-accuracy numerical methods and independent methods of estimating the receptivity coefficient. The results show that the level of acoustic receptivity in the existing literature appears to be one order of magnitude too high. Our review of previous numerical simulations and experiments clearly identifies some contradictory trends. In the limit of an infinitely thin flat plate, our results are consistent with asymptotic theory and numerical simulations.

  • 316.
    Shahriari, Nima
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Interaction of acoustic waves and micron-sized surface roughness elements in a swept-wing boundary layer2016Rapport (Övrigt vetenskapligt)
    Abstract [en]

    E↵ect of acoustic waves on the control performance of distributed micron-sized roughness elements in a swept-wing boundary layer is investigated through direct numerical simulations. The flow configuration conforms to experiments by Kachanov et al. (2015) who observed either no significant influence of acoustic waves on the transition location or small stabilisation e↵ect. In this work, a base set up for natural transition scenario is first established by introducing unsteady background noise in the boundary layer. The natural transition is then delayed using control roughness elements. Introduction of acoustic waves to the controlled flow promotes the transition location. In all these flow cases, stationary primary crossflow vortices dominate the disturbance environment and unsteady disturbances experience an explosive growth prior to transition. The spatial distribution of the energy production associated with z-type modes shows an increase in the local transfer of energy from the modified mean flow to perturbations. Simulation of flow with control roughness elements and acoustic waves as the only source of unsteady disturbances shows no influence of acoustic wave in transition to turbulence. 

  • 317.
    Shahriari, Nima
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. henning@mech.kth.se.
    Application of biorthogonal eigenfunction system for extraction of Tollmien-Schlichting waves in acoustic receptivity simulations2016Rapport (Övrigt vetenskapligt)
    Abstract [en]

    Acoustic receptivity of a two-dimensional boundary layer on a flat plate with elliptic leading edge is studied through direct numerical simulation (DNS). Sound waves are modelled by a uniform oscillation of freestream boundaries in time which results to an infinite-wavelength acoustic wave. Acoustic disturbances interact with strong streamwise gradients at the leading edge or surface non- homogeneities and create Tollmien-Schlichting (TS) waves inside the boundary layer. Measuring amplitude of TS waves created by sound waves is challenging due to presence of Stokes wave (acoustic boundary layer) with the same temporal frequency of TS waves. In this study biorthogonal eigenfunction system of local linear stability equations has been utilised to extract TS wave amplitudes. This method is based on the concept of using adjoint mode as a projector where the TS amplitude is obtained by projecting the DNS solution onto adjoint TS modes. However, the computed TS wave amplitude employing this method found to be modulated. It is shown that the modulation is due to existence of a small amplitude wave in the DNS data that is not expandable onto the basis of local linear stability equations. 

  • 318.
    Shahriari, Nima
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Kollert, Matthias R.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Control of a swept-wing boundary layer using ring-type plasma actuators2018Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 844, s. 36-60Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Application of ring-type plasma actuators for control of laminar-turbulent transition in a swept-wing boundary layer is investigated thorough direct numerical simulations. These actuators induce a wall-normal jet in the boundary layer and can act as virtual roughness elements. The flow configuration resembles experiments by Kim et al. (2016 Technical Report. BUTERFLI Project TR D3.19, http://eprints.nottingham.ac.uk/id/eprint/46529). The actuators are modelled by the volume forces computed from the experimentally measured induced velocity field at the quiescent air condition. Stationary and travelling cross-flow vortices are triggered in the simulations by means of surface roughness and random unsteady perturbations. Interaction of vortices generated by actuators with these perturbations is investigated in detail. It is found that, for successful transition control, the power of the actuators should be increased to generate jet velocities that are one order of magnitude higher than those used in the experiments by Kim et al. (2016) mentioned above.

  • 319.
    Shahriari, Nima
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Kollert, Matthias R.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Passive control of a swept-wing boundary layer using ring-type plasma actuatorsIngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Application of the ring-type plasma actuators for passive control of laminar- turbulent transition in a swept-wing boundary layer is investigated thorough direct numerical simulations. These actuators induce a wall-normal jet in the boundary layer and can act as virtual roughness elements. The flow configuration resembles experiments of Kim et al. (2016). The actuators are modelled by the volume forces computed from the experimentally measured induced velocity filed at the quiescent air condition. The natural surface roughness and unsteady perturbations are also included in the simulations. The interaction of generated vortices by the actuators with these perturbations is investigated in details. It is found that for a successful transition control the power of the actuator should be increased to generate a jet velocity one order of magnitude higher than that in the considered experiments. 

  • 320.
    Shukla, Isha
    et al.
    Ecole Polytech Fed Lausanne, Lab Fluid Mech & Instabil, CH-1015 Lausanne, Switzerland..
    Kofman, Nicolas
    Ecole Polytech Fed Lausanne, Lab Fluid Mech & Instabil, CH-1015 Lausanne, Switzerland..
    Balestra, Gioele
    Ecole Polytech Fed Lausanne, Lab Fluid Mech & Instabil, CH-1015 Lausanne, Switzerland..
    Zhu, Lailai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. Ecole Polytech Fed Lausanne, Lab Fluid Mech & Instabil, CH-1015 Lausanne, Switzerland.;Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA.
    Gallaire, Francois
    Ecole Polytech Fed Lausanne, Lab Fluid Mech & Instabil, CH-1015 Lausanne, Switzerland..
    Film thickness distribution in gravity-driven pancake-shaped droplets rising in a Hele-Shaw cell2019Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 874, s. 1021-1040, artikel-id PII S0022112019004531Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study here experimentally, numerically and using a lubrication approach, the shape, velocity and lubrication film thickness distribution of a droplet rising in a vertical Hele-Shaw cell. The droplet is surrounded by a stationary immiscible fluid and moves purely due to buoyancy. A low density difference between the two media helps to operate in a regime with capillary number $Ca$ lying between $0.03$ and $0.35$ , where $Ca=\unicode[STIX]{x1D707}_{o}U_{d}/\unicode[STIX]{x1D6FE}$ is built with the surrounding oil viscosity $\unicode[STIX]{x1D707}_{o}$ , the droplet velocity $U_{d}$ and surface tension $\unicode[STIX]{x1D6FE}$ . The experimental data show that in this regime the droplet velocity is not influenced by the thickness of the thin lubricating film and the dynamic meniscus. For iso-viscous cases, experimental and three-dimensional numerical results of the film thickness distribution agree well with each other. The mean film thickness is well captured by the Aussillous & Quere (Phys. Fluids, vol. 12 (10), 2000, pp. 2367-2371) model with fitting parameters. The droplet also exhibits the 'catamaran' shape that has been identified experimentally for a pressure-driven counterpart (Huerre et al., Phys. Rev. Lett., vol. 115 (6), 2015, 064501). This pattern has been rationalized using a two-dimensional lubrication equation. In particular, we show that this peculiar film thickness distribution is intrinsically related to the anisotropy of the fluxes induced by the droplet's motion.

  • 321. Siegel, M.
    et al.
    Tornberg, Anna-Karin
    KTH, Skolan för teknikvetenskap (SCI), Matematik (Inst.). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    A local target specific quadrature by expansion method for evaluation of layer potentials in 3D2018Ingår i: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 364, s. 365-392Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Accurate evaluation of layer potentials is crucial when boundary integral equation methods are used to solve partial differential equations. Quadrature by expansion (QBX) is a recently introduced method that can offer high accuracy for singular and nearly singular integrals, using truncated expansions to locally represent the potential. The QBX method is typically based on a spherical harmonics expansion which when truncated at order p has O(p2) terms. This expansion can equivalently be written with p terms, however paying the price that the expansion coefficients will depend on the evaluation/target point. Based on this observation, we develop a target specific QBX method, and apply it to Laplace's equation on multiply-connected domains. The method is local in that the QBX expansions only involve information from a neighborhood of the target point. An analysis of the truncation error in the QBX expansions is presented, practical parameter choices are discussed and the method is validated and tested on various problems.

  • 322.
    Simmendinger, Christian
    et al.
    T Syst Solut Res, Stuttgart, Germany..
    Iakymchuk, Roman
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST).
    Cebamanos, Luis
    Univ Edinburgh, EPCC, Edinburgh, Midlothian, Scotland..
    Akhmetova, Dana
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST).
    Bartsch, Valeria
    Fraunhofer ITWM, HPC Dept, Kaiserslautern, Germany..
    Rotaru, Tiberiu
    Fraunhofer ITWM, Kaiserslautern, Germany..
    Rahn, Mirko
    Fraunhofer ITWM, HPC Dept, Kaiserslautern, Germany..
    Laure, Erwin
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC. KTH Royal Inst Technol, High Performance Comp, Stockholm, Sweden.;KTH Royal Inst Technol, PDC Ctr, High Performance Comp Ctr, Stockholm, Sweden..
    Markidis, Stefano
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST). KTH Royal Inst Technol, High Performance Comp, Stockholm, Sweden..
    Interoperability strategies for GASPI and MPI in large-scale scientific applications2019Ingår i: The international journal of high performance computing applications, ISSN 1094-3420, E-ISSN 1741-2846, Vol. 33, nr 3, s. 554-568Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    One of the main hurdles of partitioned global address space (PGAS) approaches is the dominance of message passing interface (MPI), which as a de facto standard appears in the code basis of many applications. To take advantage of the PGAS APIs like global address space programming interface (GASPI) without a major change in the code basis, interoperability between MPI and PGAS approaches needs to be ensured. In this article, we consider an interoperable GASPI/MPI implementation for the communication/performance crucial parts of the Ludwig and iPIC3D applications. To address the discovered performance limitations, we develop a novel strategy for significantly improved performance and interoperability between both APIs by leveraging GASPI shared windows and shared notifications. First results with a corresponding implementation in the MiniGhost proxy application and the Allreduce collective operation demonstrate the viability of this approach.

  • 323.
    Smith, Anderson D.
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Elektronik, Integrerade komponenter och kretsar.
    Elgammal, Karim
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Fan, Xuge
    KTH, Skolan för elektro- och systemteknik (EES), Mikro- och nanosystemteknik.
    Lemme, Max C.
    RWTH Aachen, Otto-Blumenthal-Str., 52074 Aachen, Germany .
    Delin, Anna
    KTH, Skolan för elektro- och systemteknik (EES), Mikro- och nanosystemteknik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Råsander, Mikael
    KTH, Skolan för elektro- och systemteknik (EES), Mikro- och nanosystemteknik.
    Bergqvist, Lars
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Schröder, Stephan
    KTH, Skolan för elektro- och systemteknik (EES), Mikro- och nanosystemteknik. SenseAir AB, Sweden..
    Fischer, Andreas C.
    KTH, Skolan för elektro- och systemteknik (EES), Mikro- och nanosystemteknik. Karlsruhe Institute of Technology (KIT), Germany..
    Niklaus, Frank
    KTH, Skolan för elektro- och systemteknik (EES), Mikro- och nanosystemteknik.
    Östling, Mikael
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Elektronik, Integrerade komponenter och kretsar.
    Graphene-based CO2 sensing and its cross-sensitivity with humidity2017Ingår i: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 7, nr 36, s. 22329-22339Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present graphene-based CO2 sensing and analyze its cross-sensitivity with humidity. In order to assess the selectivity of graphene-based gas sensing to various gases, measurements are performed in argon (Ar), nitrogen (N2), oxygen (O2), carbon dioxide (CO2), and air by selectively venting the desired gas from compressed gas bottles into an evacuated vacuum chamber. The sensors provide a direct electrical readout in response to changes in high concentrations, from these bottles, of CO2, O2, nitrogen and argon, as well as changes in humidity from venting atmospheric air. From the signal response to each gas species, the relative graphene sensitivity to each gas is extracted as a relationship between the percentage-change in graphene's resistance response to changes in vacuum chamber pressure. Although there is virtually no response from O2, N2 and Ar, there is a sizeable cross-sensitivity between CO2 and humidity occurring at high CO2 concentrations. However, under atmospheric concentrations of CO2, this cross-sensitivity effect is negligible – allowing for the use of graphene-based humidity sensing in atmospheric environments. Finally, charge density difference calculations, computed using density functional theory (DFT) are presented in order to illustrate the bonding of CO2 and water molecules on graphene and the alterations of the graphene electronic structure due to the interactions with the substrate and the molecules.

  • 324.
    Smith, Anderson D.
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Elgammal, Karim
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Fan, Xuge
    KTH, Skolan för elektro- och systemteknik (EES), Mikro- och nanosystemteknik.
    Lemme, Max
    Delin, Anna
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, SeRC - Swedish e-Science Research Centre. Uppsala Univ, Sweden.
    Niklaus, Frank
    KTH, Skolan för elektro- och systemteknik (EES), Mikro- och nanosystemteknik.
    Östling, Mikael
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Toward Effective Passivation of Graphene to Humidity Sensing Effects2016Ingår i: 2016 46TH EUROPEAN SOLID-STATE DEVICE RESEARCH CONFERENCE (ESSDERC), IEEE, 2016, s. 299-302Konferensbidrag (Refereegranskat)
    Abstract [en]

    Graphene has a number of remarkable properties which make it well suited for both transistor devices as well as for sensor devices such as humidity sensors. Previously, the humidity sensing properties of monolayer graphene on SiO2 substrates were examined - showing rapid response and recovery over a large humidity range. Further, the devices were fabricated in a CMOS compatible process which can be incorporated back end of the line (BEOL). We now present a way to selectively passivate graphene to suppress this humidity sensing effect. In this work, we experimentally and theoretically demonstrate effective passivation of graphene to humidity sensing - allowing for future integration with other passivated graphene devices on the same chip.

  • 325.
    Smith, Anderson D.
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Elgammal, Karim
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Niklaus, Frank
    KTH, Skolan för elektro- och systemteknik (EES), Mikro- och nanosystemteknik.
    Delin, Anna
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Skolan för elektro- och systemteknik (EES), Mikro- och nanosystemteknik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Fischer, Andreas C.
    KTH, Skolan för elektro- och systemteknik (EES), Mikro- och nanosystemteknik.
    Vaziri, Sam
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Forsberg, Fredrik
    KTH, Skolan för elektro- och systemteknik (EES), Mikro- och nanosystemteknik.
    Råsander, Mikael
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. Univ London Imperial Coll Sci Technol & Med, Dept Mat, England.
    Hugosson, Håkan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Bergqvist, Lars
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schröder, Stephan
    KTH, Skolan för elektro- och systemteknik (EES), Mikro- och nanosystemteknik.
    Kataria, Satender
    Östling, Mikael
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar.
    Lemme, Max C.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Integrerade komponenter och kretsar. Univ Siegen, D-57076 Siegen, Germany.
    Resistive graphene humidity sensors with rapid and direct electrical readout2015Ingår i: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 7, nr 45, s. 19099-19109Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We demonstrate humidity sensing using a change of the electrical resistance of single-layer chemical vapor deposited (CVD) graphene that is placed on top of a SiO2 layer on a Si wafer. To investigate the selectivity of the sensor towards the most common constituents in air, its signal response was characterized individually for water vapor (H2O), nitrogen (N-2), oxygen (O-2), and argon (Ar). In order to assess the humidity sensing effect for a range from 1% relative humidity (RH) to 96% RH, the devices were characterized both in a vacuum chamber and in a humidity chamber at atmospheric pressure. The measured response and recovery times of the graphene humidity sensors are on the order of several hundred milliseconds. Density functional theory simulations are employed to further investigate the sensitivity of the graphene devices towards water vapor. The interaction between the electrostatic dipole moment of the water and the impurity bands in the SiO(2)d substrate leads to electrostatic doping of the graphene layer. The proposed graphene sensor provides rapid response direct electrical readout and is compatible with back end of the line (BEOL) integration on top of CMOS-based integrated circuits.

  • 326.
    Sorensen, Jens N.
    et al.
    Uppsala University.
    Mikkelsen, Robert F.
    DTU Wind Energy.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Ivanell, Stehan
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. Uppsala University.
    Sarmast, Sasan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Andersen, Soren J.
    DTU Wind Energy.
    Simulation of wind turbine wakes using the actuator line technique2015Ingår i: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, ISSN 1364-503X, Vol. 373, s. 2035-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The actuator line technique was introduced as a numerical tool to be employed in combination with large eddy simulations to enable the study of wakes and wake interaction in wind farms. The technique is today largely used for studying basic features of wakes as well as for making performance predictions of wind farms. In this paper, we give a short introduction to the wake problem and the actuator line methodology and present a study in which the technique is employed to determine the near-wake properties of wind turbines. The presented results include a comparison of experimental results of the wake characteristics of the flow around a three-bladed model wind turbine, the development of a simple analytical formula for determining the near-wake length behind a wind turbine and a detailed investigation of wake structures based on proper orthogonal decomposition analysis of numerically generated snapshots of the wake.

  • 327.
    Sorgentone, Chiara
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Matematik (Inst.). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Tornberg, Anna-Karin
    KTH, Skolan för teknikvetenskap (SCI), Matematik (Inst.). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    A highly accurate boundary integral equation method for surfactant-laden drops in 3D2018Ingår i: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 360, s. 167-191Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The presence of surfactants alters the dynamics of viscous drops immersed in an ambient viscous fluid. This is specifically true at small scales, such as in applications of droplet based microfluidics, where the interface dynamics become of increased importance. At such small scales, viscous forces dominate and inertial effects are often negligible. Considering Stokes flow, a numerical method based on a boundary integral formulation is presented for simulating 3D drops covered by an insoluble surfactant. The method is able to simulate drops with different viscosities and close interactions, automatically controlling the time step size and maintaining high accuracy also when substantial drop deformation appears. To achieve this, the drop surfaces as well as the surfactant concentration on each surface are represented by spherical harmonics expansions. A novel reparameterization method is introduced to ensure a high-quality representation of the drops also under deformation, specialized quadrature methods for singular and nearly singular integrals that appear in the formulation are evoked and the adaptive time stepping scheme for the coupled drop and surfactant evolution is designed with a preconditioned implicit treatment of the surfactant diffusion.

  • 328.
    Srinivasan, P. A.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för elektroteknik och datavetenskap (EECS). KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Guastoni, L.
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH Mech, Linne FLOW Ctr, SE-10044 Stockholm, Sweden.;Swedish E Sci Res Ctr SeRC, SE-10044 Stockholm, Sweden..
    Azizpour, Hossein
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för elektroteknik och datavetenskap (EECS).
    Schlatter, Philipp
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Vinuesa, Ricardo
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Predictions of turbulent shear flows using deep neural networks2019Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, nr 5, artikel-id 054603Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the present work, we assess the capabilities of neural networks to predict temporally evolving turbulent flows. In particular, we use the nine-equation shear flow model by Moehlis et al. [New J. Phys. 6, 56 (2004)] to generate training data for two types of neural networks: the multilayer perceptron (MLP) and the long short-term memory (LSTM) networks. We tested a number of neural network architectures by varying the number of layers, number of units per layer, dimension of the input, and weight initialization and activation functions in order to obtain the best configurations for flow prediction. Because of its ability to exploit the sequential nature of the data, the LSTM network outperformed the MLP. The LSTM led to excellent predictions of turbulence statistics (with relative errors of 0.45% and 2.49% in mean and fluctuating quantities, respectively) and of the dynamical behavior of the system (characterized by Poincare maps and Lyapunov exponents). This is an exploratory study where we consider a low-order representation of near-wall turbulence. Based on the present results, the proposed machine-learning framework may underpin future applications aimed at developing accurate and efficient data-driven subgrid-scale models for large-eddy simulations of more complex wall-bounded turbulent flows, including channels and developing boundary layers.

  • 329. Straub, Steffen
    et al.
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Frohnapfel, Bettina
    Gatti, Davide
    Turbulent Duct Flow Controlled with Spanwise Wall Oscillations2017Ingår i: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 99, nr 3-4, s. 787-806Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The spanwise oscillation of channel walls is known to substantially reduce the skin-friction drag in turbulent channel flows. In order to understand the limitations of this flow control approach when applied in ducts, direct numerical simulations of controlled turbulent duct flows with an aspect ratio of A R = 3 are performed. In contrast to channel flows, the spanwise extension of the duct is limited. Therefore, the spanwise wall oscillation either directly interacts with the duct side walls or its spatial extent is limited to a certain region of the duct. The present results show that this spanwise limitation of the oscillating region strongly diminishes the drag reduction potential of the control technique. We propose a simple model that allows estimating the achievable drag reduction rates in duct flows as a function of the width of the duct and the spanwise extent of the controlled region.

  • 330.
    Tabaei Kazerooni, Hamid
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Fornari, Walter
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hussong, Jeanette
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Inertial migration in dilute and semidilute suspensions of rigid particles in laminar square duct2017Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 2, nr 8, artikel-id 084301Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the inertial migration of finite-size neutrally buoyant spherical particles in dilute and semidilute suspensions in laminar square duct flow. We perform several direct numerical simulations using an immersed boundary method to investigate the effects of the bulk Reynolds number Re-b, particle Reynolds number Re-p, and duct to particle size ratio h/a at different solid volume fractions phi, from very dilute conditions to 20%. We show that the bulk Reynolds number Re-b is the key parameter in inertial migration of particles in dilute suspensions. At low solid volume fraction (phi = 0.4%), low bulk Reynolds number (Re-b = 144), and h/a = 9 particles accumulate at the center of the duct walls. As Re-b is increased, the focusing position moves progressively toward the corners of the duct. At higher volume fractions, phi = 5%, 10%, and 20%, and in wider ducts (h/a = 18) with Re-b = 550, particles are found to migrate away from the duct core toward the walls. In particular, for phi = 5% and 10%, particles accumulate preferentially at the corners. At the highest volume fraction considered, phi = 20%, particles sample all the volume of the duct, with a lower concentration at the duct core. For all cases, we find that particles reside longer times at the corners than at the wall centers. In a duct with lower duct to particle size ratio h/a = 9 (i.e., with larger particles), phi = 5%, and high bulk Reynolds number Re-b = 550, we find a particle concentration pattern similar to that in the ducts with h/a = 9 regardless of the solid volume fraction phi. Instead, for lower Bulk Reynolds number Re-b = 144, h/a = 9, and phi = 5%, a different particle distribution is observed in comparison to a dilute suspension phi = 0.4%. Hence, the volume fraction plays a key role in defining the final distribution of particles in semidilute suspensions at low bulk Reynolds number. The presence of particles induces secondary cross-stream motions in the duct cross section, for all phi. The intensity of these secondary flows depends strongly on particle rotation rate, on the maximum concentration of particles in focusing positions, and on the solid volume fraction. We find that the secondary flow intensity increases with the volume fraction up to phi = 5%. However, beyond phi = 5% excluded-volume effects lead to a strong reduction of cross-stream velocities for Re-b = 550 and h/a = 18. Inhibiting particles from rotating also results in a substantial reduction of the secondary flow intensity and in variations of the exact location of the focusing positions.

  • 331.
    Tammisola, Outi
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Loiseau, Jean-Christophe
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Effect of viscosity ratio on the self-sustained instabilities in planar immiscible jets2017Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 2, nr 3, artikel-id 033903Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Previous studies have shown that intermediate magnitude of surface tension has a counterintuitive destabilizing effect on two-phase planar jets. In the present study, the transition process in confined two-dimensional jets of two fluids with varying viscosity ratio is investigated using direct numerical simulations (DNSs). The outer fluid coflow velocity is 17% of that of the central jet. Neutral curves for the appearance of persistent oscillations are found by recording the norm of the velocity residuals in DNS for over 1000 nondimensional time units or until the signal has reached a constant level in a logarithmic scale, either a converged steady state or a "statistically steady" oscillatory state. Oscillatory final states are found for all viscosity ratios ranging from 10-1 to 10. For uniform viscosity (m = 1), the first bifurcation is through a surface-tension-driven global instability. On the other hand, for low viscosity of the outer fluid, there is a mode competition between a steady asymmetric Coanda-type attachment mode and the surface-tension-induced mode. At moderate surface tension, the first bifurcation is through the Coanda-type attachment, which eventually triggers time-dependent convective bursts. At high surface tension, the first bifurcation is through the surface-tension-dominated mode. For high viscosity of the outer fluid, persistent oscillations appear due to a strong convective instability, although it is shown that absolute instability may be possible at even higher viscosity ratios. Finally, we show that the jet is still convectively and absolutely unstable far from the inlet when the shear profile is nearly constant. Comparing this situation to a parallel Couette flow (without inflection points), we show that in both flows, a hidden interfacial mode brought out by surface tension becomes temporally and absolutely unstable in an intermediate Weber and Reynolds regime. By an energy analysis of the Couette flow case, we show that surface tension, although dissipative, can induce a velocity field near the interface that extracts energy from the flow through a viscous mechanism. This study highlights the rich dynamics of immiscible planar uniform-density jets, where different self-sustained and convective mechanisms compete and the nature of the instability depends on the exact parameter values.

  • 332.
    Tempelmann, David
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schrader, L. -U
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Modelling roughness and receptivity in three-dimensional boundary layers2011Ingår i: 7th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2011, International Symposium on Turbulence and Shear Flow Phenomena, TSFP , 2011, s. 1-6Konferensbidrag (Refereegranskat)
    Abstract [en]

    The receptivity of a swept-wing boundary layer to localised surface roughness is studied by means of direct numerical simulations (DNS). The flow case considered is meant to model wind tunnel experiments performed at the Arizona State University by Saric & coworkers. The receptivity amplitude of the crossflow disturbances predicted by the DNS is 40% of that measured in the experiments. The DNS results are then used to evaluate the performance of different receptivity models based on either the parabolised stability equations or the finite Reynolds number theory (FRNT). In general it is found that receptivity amplitudes are well predicted for micron sized roughness elements if non-parallel effects are accounted for. 

  • 333.
    The, Matthew
    et al.
    KTH, Centra, Science for Life Laboratory, SciLifeLab.
    Edfors, Fredrik
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Proteinvetenskap, Systembiologi.
    Perez-Riverol, Yasset
    EBI, EMBL, Wellcome Trust Genome Campus, Cambridge CB10 1SD, England..
    Payne, Samuel H.
    Pacific Northwest Natl Lab, Biol Sci Div, Richland, WA 99352 USA..
    Hoopmann, Michael R.
    Inst Syst Biol, Seattle, WA 98109 USA..
    Palmblad, Magnus
    Leiden Univ, Med Ctr, Ctr Prote & Metabol, NL-2300 RC Leiden, Netherlands..
    Forsström, Björn
    KTH, Skolan för bioteknologi (BIO), Centra, Albanova VinnExcellence Center for Protein Technology, ProNova. KTH, Centra, Science for Life Laboratory, SciLifeLab.
    Käll, Lukas
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Centra, Science for Life Laboratory, SciLifeLab.
    A Protein Standard That Emulates Homology for the Characterization of Protein Inference Algorithms2018Ingår i: Journal of Proteome Research, ISSN 1535-3893, E-ISSN 1535-3907, Vol. 17, nr 5, s. 1879-1886Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A natural way to benchmark the performance of an analytical experimental setup is to use samples of known measured analytes are peptides and not the actual proteins one of the inherent problems of interpreting data is that the composition and see to what degree one can correctly infer the content of such a sample from the data. For shotgun proteomics, themselves. As some proteins share proteolytic peptides, there might be more than one possible causative set of proteins resulting in a given set of peptides and there is a need for mechanisms that infer proteins from lists of detected peptides. A weakness of commercially available samples of known content is that they consist of proteins that are deliberately selected for producing tryptic peptides that are unique to a single protein. Unfortunately, such samples do not expose any complications in protein inference. Hence, for a realistic benchmark of protein inference procedures, there is a need for samples of known content where the present proteins share peptides with known absent proteins. Here, we present such a standard, that is based on E. coli expressed human protein fragments. To illustrate the application of this standard, we benchmark a set of different protein inference procedures on the data. We observe that inference procedures excluding shared peptides provide more accurate estimates of errors compared to methods that include information from shared peptides, while still giving a reasonable performance in terms of the number of identified proteins. We also demonstrate that using a sample of known protein content without proteins with shared tryptic peptides can give a false sense of accuracy for many protein inference methods.

  • 334.
    Thoman, Peter
    et al.
    Univ Innsbruck, A-6020 Innsbruck, Austria..
    Dichev, Kiril
    Queens Univ Belfast, Belfast BT7 1NN, Antrim, North Ireland..
    Heller, Thomas
    Univ Erlangen Nurnberg, D-91058 Erlangen, Germany..
    Iakymchuk, Roman
    KTH.
    Aguilar, Xavier
    KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hasanov, Khalid
    IBM Ireland, Dublin 15, Ireland..
    Gschwandtner, Philipp
    Univ Innsbruck, A-6020 Innsbruck, Austria..
    Lemarinier, Pierre
    IBM Ireland, Dublin 15, Ireland..
    Markidis, Stefano
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH Royal Inst Technol, S-10044 Stockholm, Sweden..
    Jordan, Herbert
    Univ Innsbruck, A-6020 Innsbruck, Austria..
    Fahringer, Thomas
    Univ Innsbruck, A-6020 Innsbruck, Austria..
    Katrinis, Kostas
    IBM Ireland, Dublin 15, Ireland..
    Laure, Erwin
    KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Nikolopoulos, Dimitrios S.
    Queens Univ Belfast, Belfast BT7 1NN, Antrim, North Ireland..
    A taxonomy of task-based parallel programming technologies for high-performance computing2018Ingår i: Journal of Supercomputing, ISSN 0920-8542, E-ISSN 1573-0484, Vol. 74, nr 4, s. 1422-1434Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Task-based programming models for shared memory-such as Cilk Plus and OpenMP 3-are well established and documented. However, with the increase in parallel, many-core, and heterogeneous systems, a number of research-driven projects have developed more diversified task-based support, employing various programming and runtime features. Unfortunately, despite the fact that dozens of different task-based systems exist today and are actively used for parallel and high-performance computing (HPC), no comprehensive overview or classification of task-based technologies for HPC exists. In this paper, we provide an initial task-focused taxonomy for HPC technologies, which covers both programming interfaces and runtime mechanisms. We demonstrate the usefulness of our taxonomy by classifying state-of-the-art task-based environments in use today.

  • 335. Tofigh, A.
    et al.
    Sjölund, E.
    Höglund, M.
    Lagergren, Jens
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsbiologi, CB. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Centra, Science for Life Laboratory, SciLifeLab.
    A global structural em algorithm for a model of cancer progression2011Ingår i: Adv. Neural Inf. Process. Syst.: Annu. Conf. Neural Inf. Process. Syst., NIPS, 2011Konferensbidrag (Refereegranskat)
    Abstract [en]

    Cancer has complex patterns of progression that include converging as well as diverging progressional pathways. Vogelstein's path model of colon cancer was a pioneering contribution to cancer research. Since then, several attempts have been made at obtaining mathematical models of cancer progression, devising learning algorithms, and applying these to cross-sectional data. Beerenwinkel et al. provided, what they coined, EM-like algorithms for Oncogenetic Trees (OTs) and mixtures of such. Given the small size of current and future data sets, it is important to minimize the number of parameters of a model. For this reason, we too focus on tree-based models and introduce Hidden-variable Oncogenetic Trees (HOTs). In contrast to OTs, HOTs allow for errors in the data and thereby provide more realistic modeling. We also design global structural EM algorithms for learning HOTs and mixtures of HOTs (HOT-mixtures). The algorithms are global in the sense that, during the M-step, they find a structure that yields a global maximum of the expected complete log-likelihood rather than merely one that improves it. The algorithm for single HOTs performs very well on reasonable-sized data sets, while that for HOT-mixtures requires data sets of sizes obtainable only with tomorrow's more cost-efficient technologies.

  • 336. Toth, Gabor
    et al.
    Chen, Yuxi
    Gombosi, Tamas I.
    Cassak, Paul
    Markidis, Stefano
    KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Peng, Ivy Bo
    KTH.
    Scaling the Ion Inertial Length and Its Implications for Modeling Reconnection in Global Simulations2017Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 10, s. 10336-10355Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate the use of artificially increased ion and electron kinetic scales in global plasma simulations. We argue that as long as the global and ion inertial scales remain well separated, (1) the overall global solution is not strongly sensitive to the value of the ion inertial scale, while (2) the ion inertial scale dynamics will also be similar to the original system, but it occurs at a larger spatial scale, and (3) structures at intermediate scales, such as magnetic islands, grow in a self-similar manner. To investigate the validity and limitations of our scaling hypotheses, we carry out many simulations of a two-dimensional magnetosphere with the magnetohydrodynamics with embedded particle-in-cell (MHD-EPIC) model. The PIC model covers the dayside reconnection site. The simulation results confirm that the hypotheses are true as long as the increased ion inertial length remains less than about 5% of the magnetopause standoff distance. Since the theoretical arguments are general, we expect these results to carry over to three dimensions. The computational cost is reduced by the third and fourth powers of the scaling factor in two-and three-dimensional simulations, respectively, which can be many orders of magnitude. The present results suggest that global simulations that resolve kinetic scales for reconnection are feasible. This is a crucial step for applications to the magnetospheres of Earth, Saturn, and Jupiter and to the solar corona.

  • 337.
    Ullah, Ikram
    et al.
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsbiologi, CB. KTH, Centra, Science for Life Laboratory, SciLifeLab.
    Parviainen, Pekka
    Lagergren, Jens
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsbiologi, CB. KTH, Centra, Science for Life Laboratory, SciLifeLab. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Species tree inference using a mixture model2015Ingår i: Molecular biology and evolution, ISSN 0737-4038, E-ISSN 1537-1719Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Species tree reconstruction has been a subject of substantial research due to its central role across biology and medicine. A species tree is often reconstructed using a set of gene trees or by directly using sequence data. In either of these cases, one of the main confounding phenomena is the discordance between a species tree and a gene tree due to evolutionary events such as duplications and losses. Probabilistic methods can resolve the discordance by co-estimating gene trees and the species tree but this approach poses a scalability problem for larger data sets.

    We present MixTreEM-DLRS: a two-phase approach for reconstructing a species tree in the presence of gene duplications and losses. In the first phase, MixTreEM, a novel structural EM algorithm based on a mixture model is used to reconstruct a set of candidate species trees, given sequence data for monocopy gene families from the genomes under study. In the second phase, PrIME-DLRS, a method based on the DLRS model ( ̊Akerborg et al., 2009), is used for selecting the best species tree. PrIME-DLRS can handle multicopy gene families since DLRS, apart from modeling sequence evolution, models gene duplication and loss using a gene evolution model (Arvestad et al., 2009).

    We evaluate MixTreEM-DLRS using synthetic and biological data, and compare its performance to a recent genome-scale species tree reconstruction method PHYLDOG (Boussau et al., 2013) as well as to a fast parsimony-based algorithm Duptree (Wehe et al., 2008). Our method is competitive with PHYLDOG in terms of accuracy and runs significantly faster and our method outperforms Duptree in accuracy. The analysis constituted by MixTreEM without DLRS may also be used for selecting the target species tree, yielding a fast and yet accurate algorithm for larger data sets. MixTreEM is freely available at http://prime.scilifelab.se/mixtreem.

  • 338.
    Ullah, Ikram
    et al.
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsbiologi, CB. KTH, Centra, Science for Life Laboratory, SciLifeLab.
    Sjöstrand, Joel
    Andersson, Peter
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsbiologi, CB.
    Sennblad, Bengt
    Lagergren, Jens
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsbiologi, CB. KTH, Centra, Science for Life Laboratory, SciLifeLab. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Integrating Sequence Evolution into Probabilistic Orthology Analysis2015Ingår i: Systematic Biology, ISSN 1063-5157, E-ISSN 1076-836X, Vol. 64, nr 6, s. 969-982Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Orthology analysis, that is, finding out whether a pair of homologous genes are orthologs - stemming from a speciation - or paralogs - stemming from a gene duplication - is of central importance in computational biology, genome annotation, and phylogenetic inference. In particular, an orthologous relationship makes functional equivalence of the two genes highly likely. A major approach to orthology analysis is to reconcile a gene tree to the corresponding species tree, (most commonly performed using the most parsimonious reconciliation, MPR). However, most such phylogenetic orthology methods infer the gene tree without considering the constraints implied by the species tree and, perhaps even more importantly, only allow the gene sequences to influence the orthology analysis through the a priori reconstructed gene tree. We propose a sound, comprehensive Bayesian MCMC-based method, DLRSOrthology, to compute orthology probabilities. It efficiently sums over the possible gene trees and jointly takes into account the current gene tree, all possible reconciliations to the species tree, and the, typically strong, signal conveyed by the sequences. We compare our method with PrIME-GEM, a probabilistic orthology approach built on a probabilistic duplication-loss model, and MrBayesMPR, a probabilistic orthology approach that is based on conventional Bayesian inference coupled with MPR. We find that DLRSOrthology outperforms these competing approaches on synthetic data as well as on biological data sets and is robust to incomplete taxon sampling artifacts.

  • 339. Ulman, Kanchan
    et al.
    Narasimhan, Shobhana
    Delin, Anna
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Tuning spin transport properties and molecular magnetoresistance through contact geometry2014Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 140, nr 4, s. 044716-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Molecular spintronics seeks to unite the advantages of using organic molecules as nanoelectronic components, with the benefits of using spin as an additional degree of freedom. For technological applications, an important quantity is the molecular magnetoresistance. In this work, we show that this parameter is very sensitive to the contact geometry. To demonstrate this, we perform ab initio calculations, combining the non-equilibrium Green's function method with density functional theory, on a dithienylethene molecule placed between spin-polarized nickel leads of varying geometries. We find that, in general, the magnetoresistance is significantly higher when the contact is made to sharp tips than to flat surfaces. Interestingly, this holds true for both resonant and tunneling conduction regimes, i.e., when the molecule is in its "closed" and "open" conformations, respectively. We find that changing the lead geometry can increase the magnetoresistance by up to a factor of similar to 5. We also introduce a simple model that, despite requiring minimal computational time, can recapture our ab initio results for the behavior of magnetoresistance as a function of bias voltage. This model requires as its input only the density of states on the anchoring atoms, at zero bias voltage. We also find that the non-resonant conductance in the open conformation of the molecule is significantly impacted by the lead geometry. As a result, the ratio of the current in the closed and open conformations can also be tuned by varying the geometry of the leads, and increased by similar to 400%.

  • 340.
    Ulman, Kanchan
    et al.
    Jawaharlal Nehru Ctr Adv Sci Res, Theoret Sci Unit, Bangalore 560064, Karnataka, India..
    Narasimhan, Shobhana
    Jawaharlal Nehru Ctr Adv Sci Res, Theoret Sci Unit, Bangalore 560064, Karnataka, India.;Jawaharlal Nehru Ctr Adv Sci Res, Sheikh Saqr Lab, ICMS, Bangalore 560064, Karnataka, India..
    Delin, Anna
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Tuning spin transport properties and molecular magnetoresistance through contact geometry (vol 140, 044716, 2014)2014Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 140, nr 22, artikel-id 229903Artikel i tidskrift (Refereegranskat)
  • 341.
    Urbanaviciute, Indre
    et al.
    Linkoping Univ, Complex Mat & Devices, Dept Phys Chem & Biol IFM, S-58183 Linkoping, Sweden..
    Bhattacharjee, Subham
    Eindhoven Univ Technol, Lab Macromol & Organ Chem, POB 513, NL-5600 MB Eindhoven, Netherlands..
    Biler, Michal
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Lugger, Jody A. M.
    Eindhoven Univ Technol, Lab Macromol & Organ Chem, POB 513, NL-5600 MB Eindhoven, Netherlands..
    Cornelissen, Tim D.
    Linkoping Univ, Complex Mat & Devices, Dept Phys Chem & Biol IFM, S-58183 Linkoping, Sweden..
    Norman, Patrick
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Linares, Mathieu
    KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Sijbesma, Rint P.
    Eindhoven Univ Technol, Lab Macromol & Organ Chem, POB 513, NL-5600 MB Eindhoven, Netherlands..
    Kemerink, Martijn
    Linkoping Univ, Complex Mat & Devices, Dept Phys Chem & Biol IFM, S-58183 Linkoping, Sweden..
    Suppressing depolarization by tail substitution in an organic supramolecular ferroelectric2019Ingår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 21, nr 4, s. 2069-2079Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Despite being very well established in the field of electro-optics, ferroelectric liquid crystals so far lacked interest from a ferroelectric device perspective due to a typically high operating temperature, a modest remnant polarization and/or poor polarization retention. Here, we experimentally demonstrate how simple structural modification of a prototypical ferroelectric liquid-crystal benzene-1,3,5-trisamide (BTA) - introduction of branched-tail substituents - results in materials with a wide operating temperature range and a data retention time of more than 10 years in thin-film solution-processed capacitor devices at room temperature. The observed differences between linear- and branched-tail compounds are analyzed using density functional theory (DFT) and molecular dynamics (MD) simulations. We conclude that morphological factors like improved packing quality and reduced disorder, rather than electrostatic interactions or intra/inter-columnar steric hindrance, underlay the superior properties of the branched-tailed BTAs. Synergistic effects upon blending of compounds with branched and linear side-chains can be used to further improve the materials' characteristics.

  • 342.
    Vidal, A.
    et al.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Nagib, H. M.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Secondary flow in spanwise-periodic in-phase sinusoidal channels2018Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 851, s. 288-316Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Direct numerical simulations (DNSs) are performed to analyse the secondary flow of Prandtl's second kind in fully developed spanwise-periodic channels with in-plane sinusoidal walls. The secondary flow is characterized for different combinations of wave parameters defining the wall geometry at Re-h = 2500 and 5000, where h is the half-height of the channel. The total cross-flow rate in the channel Q(yz) is defined along with a theoretical model to predict its behaviour. Interaction between the secondary flows from opposite walls is observed if lambda similar or equal to h similar or equal to A, where A and lambda are the amplitude and wavelength of the sinusoidal function defining the wall geometry. As the outer-scaled wavelength (lambda/h) is reduced, the secondary vortices become smaller and faster, increasing the total cross-flow rate per wall. However, if the inner-scaled wavelength (lambda(+)) is below 130 viscous units, the cross-flow decays for smaller wavelengths. By analysing cases in which the wavelength of the wall is much smaller than the half-height of the channel lambda << h, we show that the cross-flow distribution depends almost entirely on the separation between the scales of the instantaneous vortices, where the upper and lower bounds are determined by lambda/h and lambda(+), respectively. Therefore, the distribution of the secondary flow relative to the size of the wave at a given Re-h can be replicated at higher Re-h by decreasing lambda/h and keeping lambda(+) constant. The mechanisms that contribute to the mean cross-flow are analysed in terms of the Reynolds stresses and using quadrant analysis to evaluate the probability density function of the bursting events. These events are further classified with respect to the sign of their instantaneous spanwise velocities. Sweeping events and ejections are preferentially located in the valleys and peaks of the wall, respectively. The sweeps direct the instantaneous cross-flow from the core of the channel towards the wall, turning in the wall-tangent direction towards the peaks. The ejections drive the instantaneous cross-flow from the near-wall region towards the core. This preferential behaviour is identified as one of the main contributors to the secondary flow.

  • 343.
    Vidal, A.
    et al.
    IIT, MMAE Dept, Chicago, IL 60616 USA..
    Nagib, H. M.
    IIT, MMAE Dept, Chicago, IL 60616 USA..
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Vorticity fluxes and secondary flow: Relevance for turbulence modeling2018Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 3, nr 7, artikel-id 072602Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Vorticity fluxes are analyzed in fully developed turbulent flow through rectangular ducts with a width-to-height ratio of 3, and both straight and semicylindrical side walls, at a centerplane friction Reynolds number Re-tau,(c) similar or equal to 180. The transport of secondary Reynolds stresses by the secondary flow of Prandtl's second kind is analyzed from a vorticity-flux perspective. This analysis reveals that the in-plane transport of viscous stresses locally counteracts the inhomogeneous distribution of the turbulent shear-stress gradient in the spanwise direction. A relationship is established between the mean and fluctuating transport terms that can be useful to improve turbulence models and their ability to accurately predict the secondary flow. Finally, quadrant analysis is used to evaluate the contribution from the different types of bursting events to the fluctuating transport terms.

  • 344. Vidal, A.
    et al.
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Nagib, H. M.
    Influence of corner geometry on the secondary flow in turbulent square ducts2017Ingår i: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 67, s. 69-78Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Direct numerical simulations of fully-developed turbulent flow through a straight square duct with increasing corner rounding radius r were performed to study the influence of corner geometry on the secondary flow. Unexpectedly, the increased rounding of the corners from r=0 to 0.75 does not lead to a monotonic trend towards the pipe case of r=1. Instead, the secondary vortices relocate close to the region of wall-curvature change. This behavior is connected to the inhomogeneous interaction between near-wall bursting events, which are further characterized in this work with the definition of their local preferential direction. We compare our results with those obtained for the flow through a square duct (which corresponds to r=0) and through a round pipe (r=1), focusing on the influence of r on the wall-shear stress distribution and the turbulence statistics along the centerplane and the corner bisector. The former shows that high-speed streaks are preferentially located near the transition between straight and curved surfaces. The Reynolds numbers based on the centerplane friction velocity and duct half-height are Reτ, c ≃ 180 and 350 for the cases under study.

  • 345.
    Vidal, A.
    et al.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Processteknisk strömningsmekanik.
    Nagib, H. M.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Turbulent rectangular ducts with minimum secondary flow2018Ingår i: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 72, s. 317-328Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the present study we perform direct numerical simulations (DNSs) of fully-developed turbulent rectangular ducts with semi-cylindrical side-walls at Re-t,Re- c similar or equal to 180 with width-to-height ratios of 3 and 5. The friction Reynolds number Re-tau,Re- (c) is based on the centerplane friction velocity and the half-height of the duct. The results are compared with the corresponding duct cases with straight side-walls (Vinuesa et al., 2014), and also with spanwise-periodic channel and pipe flows. We focus on the influence of the semi-cylindrical side-walls on the mean cross-stream secondary flow and on further characterizing the mechanisms that produce it. The role of the secondary and primary Reynolds-shear stresses in the production of the secondary flow is analyzed by means of quadrant analysis and conditional averaging. Unexpectedly, the ducts with semi-cylindrical side-walls exhibit higher cross-flow rates and their secondary vortices relocate near the transition point between the straight and curved walls. This behavior is associated to the statistically preferential arrangement of sweeping events entering through the curved wall and ejections arising from the adjacent straight wall. Therefore, the configuration with minimum secondary flow corresponds to the duct with straight side-walls and sharp corners. Consequences on experimental facilities and comparisons between experiments and various numerical and theoretical models are discussed revealing the uniqueness of pipe flow.

  • 346.
    Vinuesa, Ricardo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Bobke, Alexandra
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    On determining characteristic length scales in pressure-gradient turbulent boundary layers2016Ingår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, nr 5, artikel-id 055101Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the present work, we analyze three commonly used methods to determine the edge of pressure gradient turbulent boundary layers: two based on composite profiles, the one by Chauhan et al. ["Criteria for assessing experiments in zero pressure gradient boundary layers," Fluid Dyn. Res. 41, 021404 (2009)] and the one by Nickels ["Inner scaling for wall-bounded flows subject to large pressure gradients," J. Fluid Mech. 521, 217-239 (2004)], and the other one based on the condition of vanishing mean velocity gradient. Additionally, a new method is introduced based on the diagnostic plot concept by Alfredsson et al. ["A new scaling for the streamwise turbulence intensity in wall-bounded turbulent flows and what it tells us about the 'outer' peak," Phys. Fluids 23, 041702 (2011)]. The boundary layers developing over the suction and pressure sides of a NACA4412 wing section, extracted from a direct numerical simulation at chord Reynolds number Re-c = 400 000, are used as the test case, besides other numerical and experimental data from favorable, zero, and adverse pressure-gradient flat-plate turbulent boundary layers. We find that all the methods produce robust results with mild or moderate pressure gradients, although the composite-profile techniques require data preparation, including initial estimations of fitting parameters and data truncation. Stronger pressure gradients (with a Rotta-Clauser pressure-gradient parameter beta larger than around 7) lead to inconsistent results in all the techniques except the diagnostic plot. This method also has the advantage of providing an objective way of defining the point where the mean streamwise velocity is 99% of the edge velocity and shows consistent results in a wide range of pressure gradient conditions, as well as flow histories. Collapse of intermittency factors obtained from a wide range of pressure-gradient and Re conditions on the wing further highlights the robustness of the diagnostic plot method to determine the boundary layer thickness (equivalent to delta(99)) and the edge velocity in pressure gradient turbulent boundary layers.

  • 347. Vinuesa, Ricardo
    et al.
    Bobke, Alexandra
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    On determining characteristic length scales in pressure-gradient turbulent boundary layers2016Ingår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the present work we analyze three commonly used methods to determine the edge of pressure gradient turbulent boundary layers: two based on composite profiles, the one by Chauhan et al. (Fluid Dyn. Res. 41:021401, 2009) and the one by Nickels (J. Fluid Mech. 521:217–239, 2004), and the other onebased on the condition of vanishing mean velocity gradient. Additionally, a new method is introduced based on the diagnostic plot concept by Alfredsson et al. (Phys. Fluids 23:041702, 2011). The boundary layers developing over the suction and pressure sides of a NACA4412 wing section, extracted from a directnumerical simulation at chord Reynolds number Rec = 400, 000, is used as the test case, besides other numerical and experimental data from favorable, zero and adverse pressure-gradient flat-plate turbulent boundary layers. We find that all the methods produce robust results with mild or moderate pressure gradients, although the composite-profile techniques require data preparation, including initial estimations of fitting parameters and data truncation. Stronger pressure gradients (with a Rotta–Clauser pressure-gradient parameter β larger than around 7) lead to inconsistent results in all the techniques except the diagnosticplot. This method also has the advantage of providing an objective way of defining the point where the mean streamwise velocity is 99% of the edge velocity, and shows consistent results in a wide range of pressure gradient conditions, as well as flow histories. Collapse of intermittency factors obtained from a wide range of pressure-gradient and Re conditions on the wing further highlightsthe robustness of the diagnostic plot method to determine the boundary layert hickness (equivalent to δ99 ) and the edge velocity in pressure gradient turbulent boundary layers.

  • 348.
    Vinuesa, Ricardo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Bobke, Alexandra
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Scaling of adverse-pressure-gradient turbulent boundary layers in near-equilibrium conditions2017Ingår i: Progress in Turbulence VII, Springer, 2017, Vol. 196, s. 73-78Konferensbidrag (Refereegranskat)
    Abstract [en]

    Well-resolved large-eddy simulations are used to study adverse-pressure-gradient (APG) turbulent boundary layers (TBLs) under near-equilibrium conditions. In particular, we focus on two near-equilibrium cases where the power-law freestream velocity distribution is adjusted in order to produce long regions with a constant value of the Clauser pressure-gradient parameter β. In the first case we obtain an APG TBL with a constant value of β ≃ 1 over 37 average boundary-layer thicknesses, and in the second one a constant value of β ≃ 2 for around 28 average boundary-layer thicknesses. The scaling law suggested by Kitsios et al. (Int J Heat Fluid Flow 61:117–128, 2016, [10]), proposing the edge velocity and the displacement thickness as scaling parameters, was tested on the two constant-pressure-gradient parameter cases. The mean velocity and Reynolds-stress profiles were found to be dependent on the downstream development, a conclusion in agreement with classical theory.

  • 349.
    Vinuesa, Ricardo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hosseini, Seyed M.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Pressure-gradient turbulent boundary layers developing around a wing section2017Ingår i: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 99, nr 3-4, s. 613-641Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A direct numerical simulation database of the flow around a NACA4412 wing section at R e (c) = 400,000 and 5(ay) angle of attack (Hosseini et al. Int. J. Heat Fluid Flow 61, 117-128, 2016), obtained with the spectral-element code Nek5000, is analyzed. The Clauser pressure-gradient parameter beta ranges from ae integral 0 and 85 on the suction side, and from 0 to - 0.25 on the pressure side of the wing. The maximum R e (oee integral) and R e (tau) values are around 2,800 and 373 on the suction side, respectively, whereas on the pressure side these values are 818 and 346. Comparisons between the suction side with zero-pressure-gradient turbulent boundary layer data show larger values of the shape factor and a lower skin friction, both connected with the fact that the adverse pressure gradient present on the suction side of the wing increases the wall-normal convection. The adverse-pressure-gradient boundary layer also exhibits a more prominent wake region, the development of an outer peak in the Reynolds-stress tensor components, and increased production and dissipation across the boundary layer. All these effects are connected with the fact that the large-scale motions of the flow become relatively more intense due to the adverse pressure gradient, as apparent from spanwise premultiplied power-spectral density maps. The emergence of an outer spectral peak is observed at beta values of around 4 for lambda (z) ae integral 0.65 delta (99), closer to the wall than the spectral outer peak observed in zero-pressure-gradient turbulent boundary layers at higher R e (oee integral) . The effect of the slight favorable pressure gradient present on the pressure side of the wing is opposite the one of the adverse pressure gradient, leading to less energetic outer-layer structures.

  • 350.
    Vinuesa, Ricardo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Negi, Prabal
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    High-fidelity simulations of the flow around wings at high Reynolds numbers2017Manuskript (preprint) (Övrigt vetenskapligt)
45678 301 - 350 av 381
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