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  • 351.
    Rosti, Marco E.
    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.
    Niazi Ardekani, Mehdi
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    The effect of elastic walls on suspension flow2018Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114Artikel i tidskrift (Övrigt vetenskapligt)
  • 352.
    Rosti, Marco E.
    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.
    Olivieri, S.
    DICCA, University of Genova, Via Montallegro 1, Genova, 16145, Italy ; INFN, Genova Section, Via Dodecaneso 33, Genova, 16146, Italy.
    Banaei, Arash Alizad
    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.
    Mazzino, A.
    DICCA, University of Genova, Via Montallegro 1, Genova, 16145, Italy ; INFN, Genova Section, Via Dodecaneso 33, Genova, 16146, Italy.
    Flowing fibers as a proxy of turbulence statistics2020Ingår i: Meccanica (Milano. Print), ISSN 0025-6455, E-ISSN 1572-9648, Vol. 55, s. 357-370Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The flapping states of a flexible fiber fully coupled to a three-dimensional turbulent flow are investigated via state-of-the-art numerical methods. Two distinct flapping regimes are predicted by the phenomenological theory recently proposed by Rosti et al. (Phys. Rev. Lett. 121:044501, 2018) the under-damped regime, where the elasticity strongly affects the fiber dynamics, and the over-damped regime, where the elastic effects are strongly inhibited. In both cases we can identify a critical value of the bending rigidity of the fiber by a resonance condition, which further provides a distinction between different flapping behaviors, especially in the under-damped case. We validate the theory by means of direct numerical simulations and find that, both for the over-damped regime and for the under-damped one, fibers are effectively slaved to the turbulent fluctuations and can therefore be used as a proxy to measure various two-point statistics of turbulence. Finally, we show that this holds true also in the case of a passive fiber, without any feedback force on the fluid.

  • 353.
    Rosti, Marco E.
    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.
    Omidyeganeh, Mohammad
    City Univ London, Sch Math Comp Sci & Engn, London EC1V 0HB, England..
    Pinelli, Alfredo
    City Univ London, Sch Math Comp Sci & Engn, London EC1V 0HB, England..
    Numerical Simulation of a Passive Control of the Flow Around an Aerofoil Using a Flexible, Self Adaptive Flaplet2018Ingår i: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 100, nr 4, s. 1111-1143Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Self-activated feathers are used by almost all birds to adapt their wing characteristics to delay stall or to moderate its adverse effects (e.g., during landing or sudden increase in angle of attack due to gusts). Some of the feathers are believed to pop up as a consequence of flow separation and to interact with the flow and produce beneficial modifications of the unsteady vorticity field. The use of self adaptive flaplets in aircrafts, inspired by birds feathers, requires the understanding of the physical mechanisms leading to the mentioned aerodynamic benefits and the determination of the characteristics of optimal flaps including their size, positioning and ideal fabrication material. In this framework, this numerical study is divided in two parts. Firstly, in a simplified scenario, we determine the main characteristics that render a flap mounted on an aerofoil at high angle of attack able to deliver increased lift and improved aerodynamic efficiency, by varying its length, position and its natural frequency. Later on, a detailed direct numerical simulation analysis is used to understand the origin of the aerodynamic benefits introduced by the flaplet movement induced by the interaction with the flow field. The parametric study that has been carried out, reveals that an optimal flap can deliver a mean lift increase of about 20% on a NACA0020 aerofoil at an incidence of 20 (o) degrees. The results obtained from the direct numerical simulation of the flow field around the aerofoil equipped with the optimal flap at a chord Reynolds number of 2 x 10(4) shows that the flaplet movement is mainly induced by a cyclic passage of a large recirculation bubble on the aerofoil suction side. In turns, when the flap is pushed downward, the induced plane jet displaces the trailing edge vortices further downstream, away from the wing, moderating the downforce generated by those vortices and regularising the shedding cycle that appears to be much more organised when the optimal flaplet configuration is selected.

  • 354.
    Rosti, Marco Edoardo
    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.
    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.
    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.
    Effect of elastic walls on suspension flow2019Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, nr 6, artikel-id 062301Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study suspensions of rigid particles in a plane Couette flow with deformable elastic walls. We find that, in the limit of vanishing inertia, the elastic walls induce shear thinning of the suspension flow such that the effective viscosity decreases as the wall deformability increases. This shear-thinning behavior originates from the interactions between rigid particles, soft walls, and carrier fluids; an asymmetric wall deformation induces a net lift force acting on the particles which therefore migrate towards the bulk of the channel. Based on our observations, we provide a closure for the suspension viscosity which can be used to model the rheology of suspensions with arbitrary volume fraction in elastic channels.

  • 355.
    Rosti, Marco Edoardo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Pramanik, Satyajit
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Mitra, Dhrubaditya
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, SE-10691 Stockholm, Sweden.
    The breakdown of Darcy's law in a soft porous material2020Ingår i: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 16, nr 4, s. 939-944Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We perform direct numerical simulations of the flow through a model of deformable porous medium. Our model is a two-dimensional hexagonal lattice, with defects, of soft elastic cylindrical pillars, with elastic shear modulus G, immersed in a liquid. We use a two-phase approach: the liquid phase is a viscous fluid and the solid phase is modeled as an incompressible viscoelastic material, whose complete nonlinear structural response is considered. We observe that the Darcy flux (q) is a nonlinear function - steeper than linear - of the pressure-difference (Delta P) across the medium. Furthermore, the flux is larger for a softer medium (smaller G). We construct a theory of this super-linear behavior by modelling the channels between the solid cylinders as elastic channels whose walls are made of material with a linear constitutive relation but can undergo large deformation. Our theory further predicts that the flow permeability is an universal function of Delta P/G, which is confirmed by the present simulations.

  • 356.
    Rubio-Magnieto, Jenifer
    et al.
    Univ Mons UMONS, Ctr Innovat Mat & Polymers, Lab Chem Novel Mat, 20 Pl Parc, B-7000 Mons, Belgium.;Univ Jaume 1, Dept Quim Inorgan & Organ, Bioinspired Supramol Chem & Mat Grp, Avda Sos Baynat S-N, E-12071 Castellon de La Plana, Spain..
    Kajouj, Sofia
    Univ Libre Bruxelles, Chim Organ & Photochim CP160 08, 50 Ave FD Roosevelt, B-1050 Brussels, Belgium..
    Di Meo, Florent
    Univ Limoges, Sch Pharm, INSERM, U1248,IPPRITT, 2 Rue Dr Marcland, F-87025 Limoges, France..
    Fossepre, Mathieu
    Univ Mons UMONS, Ctr Innovat Mat & Polymers, Lab Chem Novel Mat, 20 Pl Parc, B-7000 Mons, Belgium..
    Trouillas, Patrick
    Univ Limoges, Sch Pharm, INSERM, U1248,IPPRITT, 2 Rue Dr Marcland, F-87025 Limoges, France.;Palacky Univ, Fac Sci, RCPTM, Slechtitelu 27, Olomouc 78371, Czech Republic..
    Norman, Patrick
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi.
    Linares, Mathieu
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Teoretisk kemi och biologi. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Moucheron, Cecile
    Univ Libre Bruxelles, Chim Organ & Photochim CP160 08, 50 Ave FD Roosevelt, B-1050 Brussels, Belgium..
    Surin, Mathieu
    Univ Mons UMONS, Ctr Innovat Mat & Polymers, Lab Chem Novel Mat, 20 Pl Parc, B-7000 Mons, Belgium..
    Binding Modes and Selectivity of Ruthenium Complexes to Human Telomeric DNA G-Quadruplexes2018Ingår i: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 24, nr 58, s. 15577-15588Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Metal complexes constitute an important class of DNA binders. In particular, a few ruthenium polyazaaromatic complexes are attractive as light switches because of their strong luminescence enhancement upon DNA binding. In this paper, a comprehensive study on the binding modes of several mononuclear and binuclear ruthenium complexes to human telomeric sequences, made of repeats of the d(TTAGGG) fragment is reported. These DNA sequences form G-quadruplexes (G4s) at the ends of chromosomes and constitute a relevant biomolecular target in cancer research. By combining spectroscopy experiments and molecular modelling simulations, several key properties are deciphered: the binding modes, the stabilization of G4 upon binding, and the selectivity of these complexes towards G4 versus double-stranded DNA. These results are rationalized by assessing the possible deformation of G4 and the binding free energies of several binding modes via modelling approaches. Altogether, this comparative study provides fundamental insights into the molecular recognition properties and selectivity of Ru complexes towards this important class of DNA G4s.

  • 357.
    Runborg, Olof
    KTH, Skolan för teknikvetenskap (SCI), Matematik (Inst.), Numerisk analys, NA. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Analysis of high order fast interface tracking methods2014Ingår i: Numerische Mathematik, ISSN 0029-599X, E-ISSN 0945-3245, Vol. 128, nr 2, s. 339-375Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Fast high order methods for the propagation of an interface in a velocity field are constructed and analyzed. The methods are generalizations of the fast interface tracking method proposed in Runborg (Commun Math Sci 7:365-398, 2009). They are based on high order subdivision to make a multiresolution decomposition of the interface. Instead of tracking marker points on the interface the related wavelet vectors are tracked. Like the markers they satisfy ordinary differential equations (ODEs), but fine scale wavelets can be tracked with longer timesteps than coarse scale wavelets. This leads to methods with a computational cost of rather than for markers and reference timestep . These methods are proved to still have the same order of accuracy as the underlying direct ODE solver under a stability condition in terms of the order of the subdivision, the order of the ODE solver and the time step ratio between wavelet levels. In particular it is shown that with a suitable high order subdivision scheme any explicit Runge-Kutta method can be used. Numerical examples supporting the theory are also presented.

  • 358. Ryzhenkov, V.
    et al.
    Ivashchenko, V.
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Mullyadzhanov, R.
    Simulation of heat and mass transfer in turbulent channel flow using the spectral-element method: Effect of spatial resolution2016Ingår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 754, nr 6, artikel-id 062009Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We use the open-source code nek5000 to assess the accuracy of high-order spectral element large-eddy simulations (LES) of a turbulent channel flow depending on the spatial resolution compared to the direct numerical simulation (DNS). The Reynolds number Re = 6800 is considered based on the bulk velocity and half-width of the channel. The filtered governing equations are closed with the dynamic Smagorinsky model for subgrid stresses and heat flux. The results show very good agreement between LES and DNS for time-averaged velocity and temperature profiles and their fluctuations. Even the coarse LES grid which contains around 30 times less points than the DNS one provided predictions of the friction velocity within 2.0% accuracy interval. © Published under licence by IOP Publishing Ltd.

  • 359. Ryzhenkov, V.
    et al.
    Ivashchenko, V.
    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.
    Mullyadzhanov, R.
    Spectral-element simulations of variable-density turbulent flow in a plane channel2017Ingår i: EPJ Web of Conferences, EDP Sciences, 2017, Vol. 159, artikel-id 0041Konferensbidrag (Refereegranskat)
    Abstract [en]

    We perform Large-eddy simulations (LES) of the turbulent flow in a channel with isothermal heated walls with the temperature ratio equal to 2. The variable properties of the fluid are accounted for by using the low Mach number approximation. The Reynolds number based on the bulk velocity, half-width of the channel, density and dynamic viscosity near the cold wall is 6800. We study the effect of spatial resolution on the accuracy of Large-eddy simulations with dynamic Smagorinsky model. The very good agreement of LES results is shown even for the coarse meshes which is attributed to the high accuracy of the spectral method.

  • 360.
    Råsander, Mikael
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Bergqvist, Lars
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Delin, Anna
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Centra, SeRC - Swedish e-Science Research Centre. Uppsala University, Sweden.
    Electronic structure and lattice dynamics in the FeSb3 skutterudite from density functional theory2015Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, nr 1, artikel-id 014303Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have performed density functional calculations of the electronic structure and lattice dynamics of the binary skutterudite FeSb3. We find that the ground state of FeSb3 is a near half-metallic ferromagnet with T-c = 175 K. Furthermore, we find that FeSb3 is softer than CoSb3 based on an analysis of the relation of the elastic constants and the shape of the phonon density of states in the two systems, which is in agreement with experimental observation. Based on these observations we find it plausible that FeSb3 will have a lower thermal conductivity than CoSb3. Additionally, our calculations indicate that FeSb3 may be stable towards decomposition into FeSb2 and Sb. Furthermore, for ferromagnetic FeSb3 we obtain real-valued phonon frequencies and also a c44 greater than zero, indicating that the system is mechanically as well as dynamically stable.

  • 361.
    Råsander, Mikael
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Imperial College London, United Kingdom.
    Hugosson, Håkan W.
    Delin, Anna
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. KTH, Centra, SeRC - Swedish e-Science Research Centre. Uppsala University, Sweden.
    Density functional study of carbon vacancies in titanium carbide2018Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 30, nr 1, artikel-id 015702Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    It is well established that TiC contains carbon vacancies not only in carbon-deficient environments but also in carbon-rich environments. We have performed density functional calculations of the vacancy formation energy in TiC for C-as well as Ti-rich conditions using several different approximations to the exchange-correlation functional, and also carefully considering the nature and thermodynamics of the carbon reference state, as well as the effect of varying growth conditions. We find that the formation of carbon vacancies is clearly favorable under Ti-rich conditions, whereas it is slightly energetically unfavorable under C-rich conditions. Furthermore, we find that the relaxations of the atoms close to the vacancy site are rather long-ranged, and that these relaxations contribute significantly to the stabilization of the vacancy. Since carbon vacancies in TiC are also experimentally observed in carbon-rich environments, we conclude that kinetics may play an important role. This conclusion is consistent with the experimentally observed high activation energies and sluggish diffusion of vacancies in TiC, effectively causing a freezing in of the vacancies.

  • 362.
    Sadeghian, Parastoo
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Hållbara byggnader.
    Wang, Cong
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Hållbara byggnader.
    Duwig, Christophe
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Sadrizadeh, Sasan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Hållbara byggnader.
    Impact of surgical lamp design on the risk of surgical site infections inoperating rooms with mixing and unidirectional airflow ventilationManuskript (preprint) (Övrigt vetenskapligt)
  • 363.
    Saglietti, Clio
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    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.
    Wadbro, Eddie
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Berggren, Martin
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Henningson, Dan S.
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Topology optimization of heat sinks in a square differentially heated cavity2018Ingår i: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 74, s. 36-52Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Innovative designs of heat sinks are generated in the present paper through numerical optimization, by applying a material distribution topology optimization approach. The potential of the method is demonstrated in a two-dimensional differentially heated cavity, in which the heat transfer is increased by means of introducing a solid structure that acts as a heat sink. We simulate the heat transfer in the whole system by performing direct numerical simulations of the conjugated problem, i.e. temperature diffusion and convection in the entire domain and momentum conservation in the fluid surrounding the solid. The flow is driven by the buoyancy force, under the Boussinesq approximation, and we describe the presence of solid material as the action of a Brinkman friction force in the Navier–Stokes equations. To obtain a design with a given length scale, we apply regularization techniques by filtering the material distribution. Two different types of filters are applied and compared for obtaining the most realistic solution. Given the large scale of the problem, the optimization is solved with a gradient based method that relies on adjoint sensitivity analysis. The results show the applicability of the method by presenting innovative geometries that are increasing the heat flux. Moreover, the effect of various factors is studied: We investigate the impact of boundary conditions, initial designs, and Rayleigh number. Complex tree-like structures are favored when a horizontal temperature gradient is imposed on the boundary and when we limit the amount of solid volume in the cavity. The choice of the initial design affects the final topology of the generated solid structures, but not their performance for the studied cases. Additionally, when the Rayleigh number increases, the topology of the heat exchanger is able to substantially enhance the convection contribution to the heat transfer. 

  • 364.
    Sahlin, Kristoffer
    et al.
    KTH, Centra, Science for Life Laboratory, SciLifeLab.
    Frånberg, M.
    Arvestad, Lars
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsvetenskap och beräkningsteknik (CST). KTH, Centra, SeRC - Swedish e-Science Research Centre. Stockholm University, Sweden.
    Structural Variation Detection with Read Pair Information: An Improved Null Hypothesis Reduces Bias2017Ingår i: Journal of Computational Biology, ISSN 1066-5277, E-ISSN 1557-8666, Vol. 24, nr 6, s. 581-589Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Reads from paired-end and mate-pair libraries are often utilized to find structural variation in genomes, and one common approach is to use their fragment length for detection. After aligning read pairs to the reference, read pair distances are analyzed for statistically significant deviations. However, previously proposed methods are based on a simplified model of observed fragment lengths that does not agree with data. We show how this model limits statistical analysis of identifying variants and propose a new model by adapting a model we have previously introduced for contig scaffolding, which agrees with data. From this model, we derive an improved null hypothesis that when applied in the variant caller CLEVER, reduces the number of false positives and corrects a bias that contributes to more deletion calls than insertion calls. We advise developers of variant callers with statistical fragment length-based methods to adapt the concepts in our proposed model and null hypothesis.

  • 365.
    Sakr, Mahmoud A.
    et al.
    AUC, Grad Program Nanotechnol, New Cairo 11835, Egypt.;AUC, Dept Mech Engn, New Cairo 11835, Egypt..
    Elgammal, Karim
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Delin, Anna
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. KTH, Centra, SeRC - Swedish e-Science Research Centre. Uppsala University, Box 516, Uppsala, SE-75120, Sweden.
    Serry, Mohamed
    AUC, Dept Mech Engn, New Cairo 11835, Egypt..
    Performance-Enhanced Non-Enzymatic Glucose Sensor Based on Graphene-Heterostructure2020Ingår i: Sensors, ISSN 1424-8220, E-ISSN 1424-8220, Vol. 20, nr 1, artikel-id 145Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Non-enzymatic glucose sensing is a crucial field of study because of the current market demand. This study proposes a novel design of glucose sensor with enhanced selectivity and sensitivity by using graphene Schottky diodes, which is composed of graphene (G)/platinum oxide (PtO)/n-silicon (Si) heterostructure. The sensor was tested with different glucose concentrations and interfering solutions to investigate its sensitivity and selectivity. Different structures of the device were studied by adjusting the platinum oxide film thickness to investigate its catalytic activity. It was found that the film thickness plays a significant role in the efficiency of glucose oxidation and hence in overall device sensitivity. 0.8-2 mu A output current was obtained in the case of 4-10 mM with a sensitivity of 0.2 mu A/mM.cm(2). Besides, results have shown that 0.8 mu A and 15 mu A were obtained by testing 4 mM glucose on two different PtO thicknesses, 30 nm and 50 nm, respectively. The sensitivity of the device was enhanced by 150% (i.e., up to 30 mu A/mM.cm(2)) by increasing the PtO layer thickness. This was attributed to both the increase of the number of active sites for glucose oxidation as well as the increase in the graphene layer thickness, which leads to enhanced charge carriers concentration and mobility. Moreover, theoretical investigations were conducted using the density function theory (DFT) to understand the detection method and the origins of selectivity better. The working principle of the sensors puts it in a competitive position with other non-enzymatic glucose sensors. DFT calculations provided a qualitative explanation of the charge distribution across the graphene sheet within a system of a platinum substrate with D-glucose molecules above. The proposed G/PtO/n-Si heterostructure has proven to satisfy these factors, which opens the door for further developments of more reliable non-enzymatic glucometers for continuous glucose monitoring systems.

  • 366.
    Samanta, Arghya
    et al.
    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), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Lashgari, Iman
    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, Centra, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Enhanced secondary motion of the turbulent flow through a porous square duct2015Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 784, s. 681-693Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Direct numerical simulations of the fully developed turbulent flow through a porous square duct are performed to study the effect of the permeable wall on the secondary cross-stream flow. The volume-averaged Navier-Stokes equations are used to describe the flow in the porous phase, a packed bed with porosity epsilon(c) = 0.95. The porous square duct is computed at Re-b similar or equal to 5000 and compared with the numerical simulations of a turbulent duct with four solid walls. The two boundary layers on the top wall and porous interface merge close to the centre of the duct, as opposed to the channel, because the sidewall boundary layers inhibit the growth of the shear layer over the porous interface. The most relevant feature in the porous duct is the enhanced magnitude of the secondary flow, which exceeds that of a regular duct by a factor of four. This is related to the increased vertical velocity, and the different interaction between the ejections from the sidewalls and the porous medium. We also report a significant decrease in the streamwise turbulence intensity over the porous wall of the duct (which is also observed in a porous channel), and the appearance of short spanwise rollers in the buffer layer, replacing the streaky structures of wall-bounded turbulence. These spanwise rollers most probably result from a Kelvin-Helmholtz type of instability, and their width is limited by the presence of the sidewalls.

  • 367. Sanmiguel Vila, C.
    et al.
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Discetti, S.
    Ianiro, A.
    Schlatter, P.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Large-scale energy in turbulent boundary layers: Reynolds-number and pressure-gradient effects2019Ingår i: Springer Proceedings in Physics, Springer Science and Business Media, LLC , 2019, s. 69-74Konferensbidrag (Refereegranskat)
    Abstract [en]

    Adverse-pressure-gradient (APG) turbulent boundary layers (TBLs) are studied using hot-wire measurements which cover a Clauser pressure-gradient-parameter range up to β ≈ 2.4. Constant and non-constant β distributions with the same upstream history are studied. The pre-multiplied power-spectral density is employed to study the differences in the large-scale energy content throughout the boundary layer. Two different large-scale phenomena are identified, the first one due to the pressure gradient and the second one due to the Reynolds number; the latter is also present in high-Re ZPG TBLs. A decomposition of the streamwise velocity fluctuations using a temporal filter shows that the small-scale velocity fluctuations do not scale in APG TBL flows since the effect of the large-scale features extends up to the near-wall region.

  • 368. Sardina, G.
    et al.
    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.
    Transition Delay and its Implications for Drag Reduction in Particle-laden Channel Flow2015Ingår i: International Conference of Computational Methods in Sciences and Engineering (ICCMSE),, American Institute of Physics (AIP), 2015, artikel-id 180009Konferensbidrag (Refereegranskat)
    Abstract [en]

    Direct numerical simulations of a two-way coupled particle-laden channel flow are performed in the transitional and turbulent regime. Our aim is to investigate whether particles, assumed solid, spherical and heavy delay transition and diminish drag in wall bounded flows. We consider that the only interaction between the carrier and dispersed phase is due to the Stokes drag. The simulations have been performed with a pseudo-spectral solver for the incompressible Navier-Stokes equations while the particle equations are solved in a Lagrangian formulation. In the transitional regime, we study the evolution of disturbances added to a plane Poiseuille base flow with Reynolds number 2000, based on the channel half-height and centerline velocity. By fixing the Reynolds number, the only two free parameters are the particle Stokes number St and the particle mass fraction f. Two different scenarios are considered depending on the initial condition of the velocity field. The first scenario consists of counter-rotating streamwise vortices and one three-dimensional mode while the second is a nonlinear interaction of two symmetric oblique waves. The effects of the particles on the flow transition depend on the particle mass fraction. In particular, at lower concentrations, the transition is facilitated by the presence of the particles, conversely at higher mass fractions the particles tend to stabilize the flow. The results indicate that the particle can play an important role by influencing secondary instabilities and streaks breakdown. These effects can be linked to the reduced drag observed in turbulent channel flow laden with heavy particles and specific simulations in the turbulent regime have been performed.

  • 369.
    Sardina, Gaetano
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    An Efficient-high Performance Code for Particle Transport in Homogeneous Turbulence2016Ingår i: Proceedings of the International Conference of Computational Methods in Sciences and Engineering 2016 (ICCMSE-2016), American Institute of Physics (AIP), 2016, artikel-id UNSP 090004Konferensbidrag (Refereegranskat)
    Abstract [en]

    We have developed a fully parallel fortran/MPI code for tracking particles in homogeneous turbulent flows. The fluid is discretized in a uniform Eulerian grid while the particles are evolved via a Lagrangian tracking framework. The code is pseudo-spectral and employs the libraries FFTw, time integration has a third or fourth-order accuracy. The carrier phase can transport several equations for active/passive scalars that can act like a source of mass/energy transfer to the particles. We were able to simulate a fully-turbulent flow in an Eulerian grid of about 10(10) points and to track in a Lagrangian framework at least 10(9) point particles. The code is fully modular, can be easily extended or modified and available upon request.

  • 370. Sardina, Gaetano
    et al.
    Picano, Francesco
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Processteknisk strömningsmekanik. 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, Processteknisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Caballero, Rodrigo
    Continuous Growth of Droplet Size Variance due to Condensation in Turbulent Clouds2015Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, nr 18, artikel-id 184501Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We use a stochastic model and direct numerical simulation to study the impact of turbulence on cloud droplet growth by condensation. We show that the variance of the droplet size distribution increases in time as t(1/2), with growth rate proportional to the large-to-small turbulent scale separation and to the turbulence integral scales but independent of the mean turbulent dissipation. Direct numerical simulations confirm this result and produce realistically broad droplet size spectra over time intervals of 20 min, comparable with the time of rain formation.

  • 371.
    Sardina, Gaetano
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Picano, Francesco
    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.
    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.
    Casciola, C. M.
    Statistics of Particle Accumulation in Spatially Developing Turbulent Boundary Layers2014Ingår i: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 92, nr 1-2, s. 27-40Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present the results of a Direct Numerical Simulation of a particle-laden spatially developing turbulent boundary layer up to Re (theta) = 2500. Two main features differentiate the behavior of inertial particles in a zero-pressure-gradient turbulent boundary layer from the more commonly studied case of a parallel channel flow. The first is the variation along the streamwise direction of the local dimensionless parameters defining the fluid-particle interactions. The second is the coexistence of an irrotational free-stream and a near-wall rotational turbulent flow. As concerns the first issue, an inner and an outer Stokes number can be defined using inner and outer flow units. The inner Stokes number governs the near-wall behavior similarly to the case of channel flow. To understand the effect of a laminar-turbulent interface, we examine the behavior of particles initially released in the free stream and show that they present a distinct behavior with respect to those directly injected inside the boundary layer. A region of minimum concentration occurs inside the turbulent boundary layer at about one displacement thickness from the wall. Its formation is due to the competition between two transport mechanisms: a relatively slow turbulent diffusion towards the buffer layer and a fast turbophoretic drift towards the wall.

  • 372. Sardina, Gaetano
    et al.
    Poulain, Stephane
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    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.
    Caballero, Rodrigo
    Broadening of Cloud Droplet Size Spectra by Stochastic Condensation: Effects of Mean Updraft Velocity and CCN Activation2018Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 75, nr 2, s. 451-467Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The authors study the condensational growth of cloud droplets in homogeneous isotropic turbulence by means of a large-eddy simulation (LES) approach. The authors investigate the role of a mean updraft velocity and of the chemical composition of the cloud condensation nuclei (CCN) on droplet growth. The results show that a mean constant updraft velocity superimposed onto a turbulent field reduces the broadening of the droplet size spectra induced by the turbulent fluctuations alone. Extending the authors' previous results regarding stochastic condensation, the authors introduce a new theoretical estimation of the droplet size spectrum broadening that accounts for this updraft velocity effect. A similar reduction of the spectra broadening is observed when the droplets reach their critical size, which depends on the chemical composition of CCN. The analysis of the square of the droplet radius distribution, proportional to the droplet surface, shows that for large particles the distribution is purely Gaussian, while it becomes strongly non-Gaussian for smaller particles, with the left tail characterized by a peak around the haze activation radius. This kind of distribution can significantly affect the later stages of the droplet growth involving turbulent collisions, since the collision probability kernel depends on the droplet size, implying the need for new specific closure models to capture this effect.

  • 373.
    Sardina, Gaetano
    et al.
    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, Centra, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Picano, F.
    Department of Mechanics and Aeronautics, "Sapienza" University of Rome, Via Eudossiana 18, 00184 Rome, Italy.
    Casciola, C. M.
    Department of Mechanics and Aeronautics, 'Sapienza' University of Rome, Via Eudossiana 18, 00184 Rome, Italy.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Tidigare Institutioner (före 2005), Mekanik.
    Statistics of particle accumulation in spatially developing turbulent boundary layers2012Ingår i: Proceedings of the International Symposium on Turbulence, Heat and Mass Transfer, 2012, s. 1715-1723Konferensbidrag (Refereegranskat)
    Abstract [en]

    A Direct Numerical Simulation of a particle-laden spatially developing turbulent boundary layer up to Reθ = 2500 has been performed. The peculiar feature of a boundary layer flow seeded with heavy particles is the variation of the local dimensionless parameters defining the fluid-particle interactions along the streamwise direction. An inner and an outer Stokes number can be defined using inner and outer flow units. These two parameters show different decay rates in the streamwise direction so that it is possible to find a decoupled particle dynamics between the inner and the outer region of the boundary layer. Preferential near-wall particle accumulation is similar to that observed in turbulent channel flow, while a self-similar behavior characterizes the outer region.

  • 374. 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.

  • 375.
    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)
  • 376.
    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.

  • 377.
    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.

  • 378.
    Sarmast, Sasan
    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.
    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, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Mikkelsen, Robert F.
    DTU Wind Energy, Lyngby, Denmark..
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Instability of the Helical Tip Vortices behind a Single Wind Turbine2014Ingår i: Wind Energy - Impact Of Turbulence / [ed] Holling, M Peinke, J Ivanell, S, Springer, 2014, s. 165-174Konferensbidrag (Refereegranskat)
    Abstract [en]

    A numerical study on a single wind turbine wake has been carried out focusing on the instability properties 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 together with the actuator line method to simulate the wake behind the Tjaereborg wind turbine. The wake is perturbed by low amplitude stochastic excitations located in the neighborhood of the tip spiral, giving rise to spatially developing instabilities. Dynamic mode decomposition (DMD) is then utilized for identification of the coherent flow structures. The DMD results indicate that the amplification of specific waves along the spiral is responsible for triggering the instability leading to wake breakdown. Two types of dynamic structures dominates the flow; low and high frequency groups. Examination of these structures reveals that the dominant modes have the largest spatial growth.

  • 379.
    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.

  • 380.
    Scapin, Nicolo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Costa, Pedro
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. Univ Iceland, Fac Ind Engn Mech Engn & Comp Sci, Hjardarhagi 2-6, IS-107 Reykjavik, Iceland..
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KNorwegian Univ Sci & Technol NTNU, Dept Energy & Proc Engn, Trondheim, Norway..
    A volume-of-fluid method for interface-resolved simulations of phase-changing two-fluid flows2020Ingår i: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 407, artikel-id 109251Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a numerical method for interface-resolved simulations of evaporating two-fluid flows based on the volume-of-fluid (VoF) method. The method has been implemented in an efficient EFT-based two-fluid Navier-Stokes solver, using an algebraic VoF method for the interface representation, and extended with the transport equations of thermal energy and vaporized liquid mass for the single-component evaporating liquid in an inert gas. The conservation of vaporizing liquid and computation of the interfacial mass flux are performed with the aid of a reconstructed signed-distance field, which enables the use of well-established methods for phase change solvers based on level-set methods. The interface velocity is computed with a novel approach that ensures accurate mass conservation, by constructing a divergence-free extension of the liquid velocity field onto the entire domain. The resulting approach does not depend on the type of interface reconstruction (i.e. can be employed in both algebraic and geometrical VoF methods). We extensively verified and validated the overall method against several benchmark cases, and demonstrated its excellent mass conservation and good overall performance for simulating evaporating two-fluid flows in two and three dimensions.

  • 381.
    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.

  • 382.
    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 .

  • 383.
    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.

  • 384.
    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.

  • 385.
    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.

  • 386. 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.

  • 387.
    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).

  • 388.
    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.

  • 389.
    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.

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  • 390.
    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.

  • 391.
    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. 

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  • 392.
    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. 

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  • 393.
    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.

  • 394.
    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. 

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  • 395.
    Shamshirgar, D. S.
    et al.
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Matematik (Inst.).
    Yokota, R.
    Tokyo Inst Technol, Global Sci Informat & Comp Ctr, Tokyo, Japan..
    Tornberg, Anna-Karin
    KTH, Skolan för teknikvetenskap (SCI), Matematik (Inst.), Numerisk analys, NA. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hess, Berk
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Biofysik.
    Regularizing the fast multipole method for use in molecular simulation2019Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 151, nr 23, artikel-id 234113Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The parallel scaling of classical molecular dynamics simulations is limited by the communication of the 3D fast Fourier transform of the particle-mesh electrostatics methods, which are used by most molecular simulation packages. The Fast Multipole Method (FMM) has much lower communication requirements and would, therefore, be a promising alternative to mesh based approaches. However, the abrupt switch from direct particle-particle interactions to approximate multipole interactions causes a violation of energy conservation, which is required in molecular dynamics. To counteract this effect, higher accuracy must be requested from the FMM, leading to a substantially increased computational cost. Here, we present a regularization of the FMM that provides analytical energy conservation. This allows the use of a precision comparable to that used with particle-mesh methods, which significantly increases the efficiency. With an application to a 2D system of dipolar molecules representative of water, we show that the regularization not only provides energy conservation but also significantly improves the accuracy. The latter is possible due to the local charge neutrality in molecular systems. Additionally, we show that the regularization reduces the multipole coefficients for a 3D water model even more than in our 2D example.

  • 396.
    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.

  • 397. 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.

  • 398.
    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.

  • 399.
    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.

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  • 400.
    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.

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