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  • 1.
    Bobke, Alexandra
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    History effects and near equilibrium in adverse-pressure-gradient turbulent boundary layers2017In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 820, p. 667-692Article in journal (Refereed)
    Abstract [en]

    Turbulent boundary layers under adverse pressure gradients are studied using well-resolved large-eddy simulations (LES) with the goal of assessing the influence of the streamwise pressure-gradient development. Near-equilibrium boundary layers were characterized through the Clauser pressure-gradient parameter β. In order to fulfil the near-equilibrium conditions, the free stream velocity was prescribed such that it followed a power-law distribution. The turbulence statistics pertaining to cases with a constant value of β (extending up to approximately 40 boundary-layer thicknesses) were compared with cases with non-constant β distributions at matched values of β and friction Reynolds number Reδ∗. An additional case at matched Reynolds number based on displacement thickness Reδ∗ was also considered. It was noticed that non-constant β cases appear to approach the conditions of equivalent constant β cases after long streamwise distances (approximately 7 boundary-layer thicknesses). The relevance of the constant β cases lies in the fact that they define a 'canonical' state of the boundary layer, uniquely characterized by β and Re. The investigations on the flat plate were extended to the flow around a wing section overlapping in terms of β and Re. Comparisons with the flat-plate cases at matched values of β and Re revealed that the different development history of the turbulent boundary layer on the wing section leads to a less pronounced wake in the mean velocity as well as a weaker second peak in the Reynolds stresses. This is due to the weaker accumulated effect of the β history. Furthermore, a scaling law suggested by Kitsios et al. (Intl J. Heat Fluid Flow, vol. 61, 2016, pp. 129-136), proposing the edge velocity and the displacement thickness as scaling parameters, was tested on two constant-pressure-gradient parameter cases. The mean velocity and Reynolds-stress profiles were found to be dependent on the downstream development. The present work is the first step towards assessing history effects in adverse-pressure-gradient turbulent boundary layers and highlights the fact that the values of the Clauser pressure-gradient parameter and the Reynolds number are not sufficient to characterize the state of the boundary layer.

  • 2.
    Bobke, Alexandra
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Large-eddy simulations of adverse pressure gradient turbulent boundary layers2016In: 2nd Multiflow Summer School on Turbulence, Institute of Physics (IOP), 2016, article id 012012Conference paper (Refereed)
    Abstract [en]

    Adverse pressure-gradient (APG) turbulent boundary layers (TBL) are studied by performing well-resolved large-eddy simulations. The pressure gradient is imposed by defining the free-stream velocity distribution with the description of a power law. Different inflow conditions, box sizes and upper boundary conditions are tested in order to determine the final set-up. The statistics of turbulent boundary layers with two different power-law coefficients and thus magnitudes of adverse pressure gradients are then compared to zero pressure-gradient (ZPG) data. The effect of the APG on TBLs is manifested in the mean flow through a much more prominent wake region and in the Reynolds stresses through the existence of an outer peak. The pre-multiplied energy budgets show, that more energy is transported from the near-wall region to farther away from the wall.

  • 3.
    Hosseini, Seyed Mohammad
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. Swedish Defence Research Agency, Sweden.
    Heninngson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Direct numerical simulation of the flow around a wing section at moderate Reynolds number2016In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 61, p. 117-128Article in journal (Refereed)
    Abstract [en]

    Abstract A three-dimensional direct numerical simulation has been performed to study the turbulent flow around the asymmetric NACA4412 wing section at a moderate chord Reynolds number of R e c = 400 , 000 , with an angle of attack of A o A = 5 ∘ . The mesh was optimized to properly resolve all relevant scales in the flow, and comprises around 3.2 billion grid points. The incompressible spectral-element Navier–Stokes solver Nek5000 was used to carry out the simulation. An unsteady volume force is used to trip the flow to turbulence on both sides of the wing at 10% of the chord. Full turbulence statistics are computed in addition to collection of time history data in selected regions. The Reynolds numbers on the suction side reach Reτ ≃ 373 and R e Ξ = 2 , 800 with the pressure-gradient parameter ranging from β ≈ 0.0 to β ≈ 85. Similarly, on the pressure side, the Reynolds numbers reach Reτ ≈ 346 and R e Ξ = 818 while β changes from β ≈ 0.0 to β ≈ − 0.25 . The effect of adverse pressure gradients on the mean flow is consistent with previous observations, namely a steeper incipient log law, a more prominent wake region and a lower friction. The turbulence kinetic energy profiles show a progressively larger inner peak for increasing pressure gradient, as well as the emergence and development of an outer peak with stronger APGs. The present simulation shows the potential of high-order (spectral) methods in simulating complex external flows at moderately high Reynolds numbers.

  • 4.
    Hosseini, Seyed Mohammad
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Swedish Defense Research Agency, FOI, Sweden.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Direct numerical simulation of the flow around a wing section at moderate Reynolds numberManuscript (preprint) (Other academic)
  • 5. Marin, O.
    et al.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Obabko, A. V.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Characterization of the secondary flow in hexagonal ducts2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 12, article id 125101Article in journal (Refereed)
    Abstract [en]

    In this work we report the results of DNSs and LESs of the turbulent flow through hexagonal ducts at friction Reynolds numbers based on centerplane wall shear and duct half-height Re-tau,Re- c similar or equal to 180, 360, and 550. The evolution of the Fanning friction factor f with Re is in very good agreement with experimental measurements. A significant disagreement between the DNS and previous RANS simulations was found in the prediction of the in-plane velocity, and is explained through the inability of the RANS model to properly reproduce the secondary flow present in the hexagon. The kinetic energy of the secondary flow integrated over the cross-sectional area < K >(yz) decreases with Re in the hexagon, whereas it remains constant with Re in square ducts at comparable Reynolds numbers. Close connection between the values of Reynolds stress (uw) over bar on the horizontal wall close to the corner and the interaction of bursting events between the horizontal and inclined walls is found. This interaction leads to the formation of the secondary flow, and is less frequent in the hexagon as Re increases due to the 120 degrees aperture of its vertex, whereas in the square duct the 90 degrees corner leads to the same level of interaction with increasing Re. Analysis of turbulence statistics at the centerplane and the azimuthal variance of the mean flow and the fluctuations shows a close connection between hexagonal ducts and pipe flows, since the hexagon exhibits near-axisymmetric conditions up to a distance of around 0.15D(H) measured from its center. Spanwise distributions of wall-shear stress show that in square ducts the 90 degrees corner sets the location of a high-speed streak at a distance z(nu)(+) similar or equal to 50 from it, whereas in hexagons the 120 degrees aperture leads to a shorter distance of z(nu)(+) similar or equal to 38. At these locations the root mean square of the wall-shear stresses exhibits an inflection point, which further shows the connections between the near-wall structures and the large-scale motions in the outer flow. Published by AIP Publishing.

  • 6.
    Negi, Prabal Singh
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Unsteady aerodynamic effects in small-amplitude pitch oscillations of an airfoil2018In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 71, p. 378-391Article in journal (Refereed)
    Abstract [en]

    High-fidelity wall-resolved large-eddy simulations (LES) are utilized to investigate the flow-physics of small-amplitude pitch oscillations of an airfoil at Rec=100,000. The investigation of the unsteady phenomenon is done in the context of natural laminar flow airfoils, which can display sensitive dependence of the aerodynamic forces on the angle of attack in certain “off-design” conditions. The dynamic range of the pitch oscillations is chosen to be in this sensitive region. Large variations of the transition point on the suction-side of the airfoil are observed throughout the pitch cycle resulting in a dynamically rich flow response. Changes in the stability characteristics of a leading-edge laminar separation bubble has a dominating influence on the boundary layer dynamics and causes an abrupt change in the transition location over the airfoil. The LES procedure is based on a relaxation-term which models the dissipation of the smallest unresolved scales. The validation of the procedure is provided for channel flows and for a stationary wing at Rec=400,000.

  • 7.
    Noorani, Azad
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Aspect ratio effect on particle transport in turbulent duct flows2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 11, article id 115103Article in journal (Refereed)
    Abstract [en]

    The dynamics of dilute micron-sized spherical inertial particles in turbulent duct flows is studied by means of direct numerical simulations of the carrier phase turbulence with one-way coupled Lagrangian particles. The geometries are a square and a rectangular duct with width-to-height aspect ratio AR of 3 operating at Re-tau,Re-c = 360 (based on the centerplane friction velocity and duct half-height). The present study is designed to determine the effect of turbulence-driven secondary motion on the particle dynamics. Our results show that a weak cross-flow secondary motion significantly changes the cross-sectional map of the particle concentration, mean velocity, and fluctuations. As the geometry of the duct is widened from AR = 1 to 3, the secondary vortex on the horizontal wall significantly expands in the spanwise direction, and although the kinetic energy of the secondary flow increases close to the corner, it decays towards the duct centreplane in the AR = 3 case so as the turbulent carrier phase approaches the behavior in spanwise-periodic channel flows, a fact that significantly affects the particle statistics. In the square duct the particle concentration in the viscous sublayer is maximum at the duct centreplane, whereas the maximum is found closer to the corner, at a distance of |z/h| approximate to 1.25 from the centreplane, in the AR = 3 case. Interestingly the centreplane concentration in the rectangular duct is around 3 times lower than that in the square duct. Moreover, a second peak in the accumulation distribution is found right at the corners for both ducts. At this location the concentration increases with particle inertia. The secondary motion changes also the cross-stream map of the particle velocities significantly in comparison to the fluid flow statistics. These directly affect the particle velocity fluctuations such that multiple peaks appear near the duct walls for the particle streamwise and wall-normal velocity fluctuations.

  • 8.
    Noorani, Azad
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Aspect-ratio effect on particle transport in turbulent duct flowsManuscript (preprint) (Other academic)
  • 9. Prus, C.
    et al.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tembras, E.
    Mestres, E.
    Ramirez, J. P. Berro
    Impact simulation and optimisation of elastic fuel tanks reinforced with exoskeleton for aerospace applications2017In: International Journal of Crashworthiness, ISSN 1358-8265, E-ISSN 1754-2111, Vol. 22, no 3, p. 271-293Article in journal (Refereed)
    Abstract [en]

    The main subject of the study is the impact simulation of an elastic fuel tank reinforced with a polymer exoskeleton. Thanks to its lightweight and failure resistance, this type of design shows potential to be used in aerospace applications. The simulation emulates a drop test from the height of 20 m on a rigid surface, in accordance with Military Handbook testing guidelines for fuel tanks. The focus is on providing an example of modelling and solving this type of problems. The computational methods are tested on a generic model of a rectangular prismatic tank with rounded edges. The walls of the tank are made of orthotropic fabric reinforced polymer. The simulation is performed for a 70% and a 100% water-filled tank. All calculations are performed using the Altair HyperWorks 13.0 software suite, in particular, the nonlinear RADIOSS solver and OptiStruct Solver and Optimiser. The fluid inside the tank is modelled using the SPH (Smoothed Particle Hydrodynamics) approach. The model serves as a basis for establishing a design optimisation procedure, aiming at reduction of mass of the tank components while ensuring structural integrity. The main insights of the current study are the successful modelling of the liquid and the air inside the tank by means of smoothed-particle hydrodynamics elements, and the structural optimisation methodology of a composite fuel tank.

  • 10. Rezaeiravesh, S.
    et al.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Liefvendahl, M.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Assessment of uncertainties in hot-wire anemometry and oil-film interferometry measurements for wall-bounded turbulent flows2018In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 72, p. 57-73Article in journal (Refereed)
    Abstract [en]

    In this study, the sources of uncertainty of hot-wire anemometry (HWA) and oil-film interferometry (OFI) measurements are assessed. Both statistical and classical methods are used for the forward and inverse problems, so that the contributions to the overall uncertainty of the measured quantities can be evaluated. The correlations between the parameters are taken into account through the Bayesian inference with error-in-variable (EiV) model. In the forward problem, very small differences were found when using Monte Carlo (MC), Polynomial Chaos Expansion (PCE) and linear perturbation methods. In flow velocity measurements with HWA, the results indicate that the estimated uncertainty is lower when the correlations among parameters are considered, than when they are not taken into account. Moreover, global sensitivity analyses with Sobol indices showed that the HWA measurements are most sensitive to the wire voltage, and in the case of OFI the most sensitive factor is the calculation of fringe velocity. The relative errors in wall-shear stress, friction velocity and viscous length are 0.44%, 0.23% and0.22%, respectively. Note that these values are lower than the ones reported in other wall-bounded turbulence studies. Note that in most studies of wall-bounded turbulence the correlations among parameters are not considered, and the uncertainties from the various parameters are directly added when determining the overall uncertainty of the measured quantity. In the present analysis we account for these correlations, which may lead to a lower overall uncertainty estimate due to error cancellation Furthermore, our results also indicate that the crucial aspect when obtaining accurate inner-scaled velocity measurements is the wind-tunnel flow quality, which is more critical than the accuracy in wall-shear stress measurements.

  • 11. Ryzhenkov, V.
    et al.
    Ivashchenko, V.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, 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 resolution2016In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 754, no 6, article id 062009Article in journal (Refereed)
    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.

  • 12. Ryzhenkov, V.
    et al.
    Ivashchenko, V.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Mullyadzhanov, R.
    Spectral-element simulations of variable-density turbulent flow in a plane channel2017In: EPJ Web of Conferences, EDP Sciences, 2017, Vol. 159, article id 0041Conference paper (Refereed)
    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.

  • 13.
    Samanta, Arghya
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Lashgari, Iman
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Enhanced secondary motion of the turbulent flow through a porous square duct2015In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 784, p. 681-693Article in journal (Refereed)
    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.

  • 14. Sanmiguel Vila, Carlos
    et al.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Ianiro, Andrea
    Discetti, Stefano
    Adverse-Pressure-Gradient Effects on Turbulent Boundary Layers: Statistics and Flow-Field Organization2017In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 99, no 3-4, p. 589-612Article in journal (Refereed)
    Abstract [en]

    This manuscripts presents a study on adverse-pressure-gradient turbulent boundary layers under different Reynolds-number and pressure-gradient conditions. In this work we performed Particle Image Velocimetry (PIV) measurements supplemented with Large-Eddy Simulations in order to have a dataset covering a range of displacement-thickness-based Reynolds-number 2300 34000 and values of the Clauser pressure-gradient parameter beta up to 2.4. The spatial resolution limits of PIV for the estimation of turbulence statistics have been overcome via ensemble-based approaches. A comparison between ensemble-correlation and ensemble Particle Tracking Velocimetry was carried out to assess the uncertainty of the two methods. The effects of beta, R e and of the pressure-gradient history on turbulence statistics were assessed. A modal analysis via Proper Orthogonal Decomposition was carried out on the flow fields and showed that about 20% of the energy contribution corresponds to the first mode, while 40% of the turbulent kinetic energy corresponds to the first four modes with no appreciable dependence on beta and R e within the investigated range. The topology of the spatial modes shows a dependence on the Reynolds number and on the pressure-gradient strength, in line with the results obtained from the analysis of the turbulence statistics. The contribution of the modes to the Reynolds stresses and the turbulence production was assessed using a truncated low-order reconstruction with progressively larger number of modes. It is shown that the outer peaks in the Reynolds-stress profiles are mostly due to large-scale structures in the outer part of the boundary layer.

  • 15. Sanmiguel Vila, Carlos
    et al.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Ianiro, Andrea
    Discetti, Stefano
    Adverse-Pressure-Gradient Effects on Turbulent Boundary Layers: Statistics and Flow-Field Organization2017In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 99, no 3-4, p. 589-612Article in journal (Refereed)
    Abstract [en]

    This manuscripts presents a study on adverse-pressure-gradient turbulent boundary layers under different Reynolds-number and pressure-gradient conditions. In this work we performed Particle Image Velocimetry (PIV) measurements supplemented with Large-Eddy Simulations in order to have a dataset covering a range of displacement-thickness-based Reynolds-number 2300 34000 and values of the Clauser pressure-gradient parameter beta up to 2.4. The spatial resolution limits of PIV for the estimation of turbulence statistics have been overcome via ensemble-based approaches. A comparison between ensemble-correlation and ensemble Particle Tracking Velocimetry was carried out to assess the uncertainty of the two methods. The effects of beta, R e and of the pressure-gradient history on turbulence statistics were assessed. A modal analysis via Proper Orthogonal Decomposition was carried out on the flow fields and showed that about 20% of the energy contribution corresponds to the first mode, while 40% of the turbulent kinetic energy corresponds to the first four modes with no appreciable dependence on beta and R e within the investigated range. The topology of the spatial modes shows a dependence on the Reynolds number and on the pressure-gradient strength, in line with the results obtained from the analysis of the turbulence statistics. The contribution of the modes to the Reynolds stresses and the turbulence production was assessed using a truncated low-order reconstruction with progressively larger number of modes. It is shown that the outer peaks in the Reynolds-stress profiles are mostly due to large-scale structures in the outer part of the boundary layer.

  • 16. Straub, Steffen
    et al.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Frohnapfel, Bettina
    Gatti, Davide
    Turbulent Duct Flow Controlled with Spanwise Wall Oscillations2017In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 99, no 3-4, p. 787-806Article in journal (Refereed)
    Abstract [en]

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

  • 17.
    Vidal, A.
    et al.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Nagib, H. M.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Secondary flow in spanwise-periodic in-phase sinusoidal channels2018In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 851, p. 288-316Article in journal (Refereed)
    Abstract [en]

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

  • 18.
    Vidal, A.
    et al.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Nagib, H. M.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Turbulent rectangular ducts with minimum secondary flow2018In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 72, p. 317-328Article in journal (Refereed)
    Abstract [en]

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

  • 19.
    Vinuesa, R.
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, P.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, D. S.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Characterization of the massively separated wake behind a square cylinder by means of direct numerical simulation2016In: Springer Proceedings in Physics, Springer Science+Business Media B.V., 2016, p. 259-266Conference paper (Refereed)
    Abstract [en]

    The massively separated wake behind a wall-mounted square cylinder is investigated by means of direct numerical simulation (DNS). The effect of inflow conditions is assessed by considering two different cases with matching momentum thickness Reynolds numbers Reθ ≃ 1000 at the location of the cylinder: one with a fully-turbulent boundary layer as inflow condition, and another one with a laminar boundary layer. The main simulation is performed by using the spectral element code Nek5000. While in the laminar-inflow simulation the horseshoe vortex forming around the cylinder can be observed in the instantaneous flow fields, this is not the case in the turbulent-inflow simulation. Besides, the streaks in the turbulent case become greatly attenuated on both sides of the obstacle. By analyzing the Reynolds shear stress uv, we show that this is due to the modulation of the horseshoe vortex by the turbulence from the incoming boundary layer. © Springer International Publishing Switzerland 2016.

  • 20.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bartrons, E.
    Chiu, D.
    Rüedi, J. -D
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Obabko, A.
    Nagib, H. M.
    On minimum aspect ratio for experimental duct flow facilities2016In: 2nd WALLTURB Workshop on Understanding and modelling of wall turbulence, 2014, Springer, 2016, p. 201-211Conference paper (Refereed)
    Abstract [en]

    To the surprise of some of our colleagues, we recently recommended aspect ratios of at least 24 (instead of accepted values over last few decades ranging from 5 to 12) to minimize effects of sidewalls in turbulent duct flow experiments, in order to approximate the two-dimensional channel flow. Here we compile avail- able results from hydraulics and civil engineering literature, where this was already documented in the 1980s. This is of great importance due to the large amount of computational studies (mainly Direct Numerical Simulations) for spanwise-periodic turbulent channel flows, and the extreme complexity of constructing a fully developed duct flow facility with aspect ratio of 24 for high Reynolds number with adequate probe resolution. Results from this nontraditional literature for the turbulence com- munity are compared to our recent database of DNS of turbulent duct flows with aspect ratios ranging from 1 to 18 and Reτ,c ≃ 180 and 330, leading to very good agreement between their experimental and our computational results. © Springer International Publishing Switzerland 2016.

  • 21.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Bobke, Alexandra
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    On determining characteristic length scales in pressure-gradient turbulent boundary layers2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 5, article id 055101Article in journal (Refereed)
    Abstract [en]

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

  • 22.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI).
    Duncan, Richard D.
    Nagib, Hassan M.
    Alternative interpretation of the Superpipe data and motivation for CICLoPE: The effect of a decreasing viscous length scale2016In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 58, p. 109-116Article in journal (Refereed)
    Abstract [en]

    Pressurization and cryogenic conditions have been used in some experiments to change the kinematic viscosity v of the flowing gas by many orders of magnitude in order to achieve high Reynolds number conditions in facilities of limited size. This leads to a substantial reduction of the viscous length scale l* = v/u(tau), as in the so-called Princeton "Superpipe" experiments, We demonstrate that the limited dimensions of the facilities and probes can lead to inaccuracies in the near-wall measurements for increasing Reynolds number. Specifically, a lack of accurate wall-normal probe positioning is simulated using three different datasets of wall-bounded turbulent flows. Relatively large errors in the overlap region parameters are observed for position errors of small physical magnitude that become greatly amplified in wall units as l* is reduced. This offers an alternative interpretation to some of the key findings reported by the Superpipe team, such as the increasing lower limit of the logarithmic region y(log,min)(+), the existence of a power law region between the wall and the logarithmic layer, and the "mixing transition" phenomenon in wall bounded turbulence.

  • 23.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fdez. De Arévalo, L.
    Luna, M.
    Cachafeiro, H.
    Simulations and experiments of heat loss from a parabolic trough absorber tube over a range of pressures and gas compositions in the vacuum chamber2016In: Journal of Renewable and Sustainable Energy, ISSN 1941-7012, E-ISSN 1941-7012, Vol. 8, no 2, article id 023701Article in journal (Refereed)
    Abstract [en]

    A new simulation tool aimed at characterizing the thermal behavior of parabolic trough collectors over a wide range of pressures and gas mixtures in the vacuum chamber is presented in this study. The model is able to accurately predict overall efficiency and heat losses obtained in a number of experimental sources based on both on-sun and off-sun testing, with vacuum in the annulus and no hydrogen. Excellent agreement of the simulations with experiments including different gas compositions, especially with high hydrogen concentrations, is achieved through the use of molecular dynamics results for the determination of the accommodation coefficient of the mixture. Additional experiments were carried out to validate the accuracy of the model over a range of pressures between 10-4 and 130 mbar. The accurate modelling of rarefied gas dynamics presented here also leads to an excellent agreement between simulations and experiments over the whole pressure range. An accurate determination of the performance in such extreme conditions is critical for an adequate Operation &amp; Maintenance strategy, as well as the development of effective predictive and preventives plans.

  • 24.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. Department of Mechanical, Materials and Aerospace Engineering (MMAE), Illinois Institute of Technology (IIT), Chicago, IL, United States.
    Hites, Michael
    Wark, Candace
    Nagib, Hassan
    Documentation of the role of large-scale structures in the bursting process in turbulent boundary layers2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 10Article in journal (Refereed)
    Abstract [en]

    The scaling of the bursting frequency with Reynolds number is investigated experimentally by means of hot-wire measurements in turbulent boundary layers over the range 1580 < ReΘ < 23 700. Bursting events are detected by means of the u-level technique, and the effect of filtering and the choice of the threshold are also assessed. We find that the inner scaling produces excellent collapse of the high-pass filtered data in the buffer and log regions, regardless of the cutofffrequency and the threshold. This is due to the fact that the high-pass filter attenuates the low frequency component of the streamwise velocity signal, which is associated with large-scale structures in the flow. On the other hand, the bursting frequency scales with outer variables when the time-series is low-pass filtered with very low cutofffrequencies, highlighting the connection between bursting phenomena and the outer flow.

  • 25.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hosseini, Seyed M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Pressure-gradient turbulent boundary layers developing around a wing section2017In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 99, no 3-4, p. 613-641Article in journal (Refereed)
    Abstract [en]

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

  • 26.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Nagib, Hassan M.
    Enhancing the accuracy of measurement techniques in high Reynolds number turbulent boundary layers for more representative comparison to their canonical representations2016In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 55, p. 300-312Article in journal (Refereed)
    Abstract [en]

    Existing differences between experimental, computational and theoretical representations of a particular flow do not allow one-to-one comparisons, prevent us from identifying the absolute contributions of the various sources of uncertainty in each approach, and highlight the importance of developing suitable corrections for experimental techniques. In this study we utilize the latest Pitot tube correction schemes to develop a technique which improves on the outcome of hot-wire measurements of mean velocity profiles in ZPG turbulent boundary layers over the range 11 500 < Re-theta < 21 500. Measurements by Bailey et al. (2013), carried out with probes of diameters ranging from 0.2 to 1.89 mm, supplemented by other data with larger diameters up to 12.82 mm, are used first to develop a somewhat improved Pitot tube correction which is based on viscous, shear and near-wall schemes (which contribute with around 85% of the effect), together with a turbulence scheme which accounts for 15% of the whole correction. The correction proposed here leads to similar agreement with available high-quality datasets in the same Reynolds number range as the one proposed by Bailey et al. (2013), but this is the first time that the contribution of the turbulence scheme is quantified. In addition, four available algorithms to correct wall position in hot-wire measurements are tested, using as benchmark the corrected Pitot tube profiles with artificially simulated probe shifts and blockage effects. We find that the kappa B-Musker correction developed in this study produces the lowest deviations with respect to the introduced shifts. Unlike other schemes, which are based on a prescribed near-wall region profile description, the kappa B-Musker is focused on minimizing the deviation with respect to the (kappa) over tilde(B) over tilde relation, characteristic of wall-bounded turbulent flows. This general approach is able to locate the wall position in probe measurements of the wall-layer profiles with around one half the error of the other available methods. The difficulties encountered during the development of adequate corrections for high-Re boundary layer measurements highlight the existing gap between the conditions that can be reproduced and measured in the laboratory and the so-called canonical flows.

  • 27.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Negi, Prabal Singh
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Atzori, M.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Turbulent boundary layers around wing sections up to Re-c=1, 000, 0002018In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 72, p. 86-99Article in journal (Refereed)
    Abstract [en]

    Reynolds-number effects in the adverse-pressure-gradient (APG) turbulent boundary layer (TBL) developing on the suction side of a NACA4412 wing section are assessed in the present work. To this end, we analyze four cases at Reynolds numbers based on freestream velocity and chord length ranging from Re-c = 100, 000 to 1,000,000, all of them with 5 degrees angle of attack. The results of four well-resolved large-eddy simulations (LESs) are used to characterize the effect of Reynolds number on APG TBLs subjected to approximately the same pressure-gradient distribution (defined by the Clauser pressure-gradient parameter beta). Comparisons of the wing profiles with zero pressure-gradient (ZPG) data at matched friction Reynolds numbers reveal that, for approximately the same beta distribution, the lower-Reynolds-number boundary layers are more sensitive to pressure-gradient effects. This is reflected in the values of the inner-scaled edge velocity U-e(+), the shape factor H, the components of the Reynolds-stress tensor in the outer region and the outer-region production of turbulent kinetic energy. This conclusion is supported by the larger wall-normal velocities and outer-scaled fluctuations observed in the lower-Re-c cases. Thus, our results suggest that two complementing mechanisms contribute to the development of the outer region in TBLs and the formation of large-scale energetic structures: one mechanism associated with the increase in Reynolds number, and another one connected to the APG. Future extensions of the present work will be aimed at studying the differences in the outer-region energizing mechanisms due to APGs and increasing Reynolds number.

  • 28.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Negi, Prabal Singh
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    High-fidelity simulations of the flow around wings at high reynolds numbers2017In: 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, 2017, Vol. 2Conference paper (Refereed)
    Abstract [en]

    Reynolds-number effects in the adverse-pressure-gradient (APG) turbulent boundary layer (TBL) developing on the suction side of a NACA4412 wing section are assessed in the present work. To this end, we conducted a well-resolved large-eddy simulation of the turbulent flow around the NACA4412 airfoil at a Reynolds number based on freestream velocity and chord length of Rec = 1;000;000, with 5° angle of attack. The results of this simulation are used, together with the direct numerical simulation by Hosseini et al. (Int. J. Heat Fluid Flow 61, 2016) of the same wing section at Rec = 400;000, to characterize the effect of Reynolds number on APG TBLs subjected to the same pressure-gradient distribution (defined by the Caluser pressure-gradient parameter β). Our results indicate that the increase in inner-scaled edge velocity U+e, and the decrease in shape factor H, is lower in the APG on the wing than in zero-pressure-gradient (ZPG) TBLs over the same Reynolds-number range. This indicates that the lower-Re boundary layer is more sensitive to the effect of the APG, a conclusion that is supported by the larger values in the outer region of the tangential velocity fluctuation profile in the Rec = 400;000 wing. Future extensions of the present work will be aimed at studying the differences in the outer-region energizing mechanisms due to APGs and increasing Reynolds number.

  • 29.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prus, Cezary
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Nagib, Hassan M.
    Convergence of numerical simulations of turbulent wall-bounded flows and mean cross-flow structure of rectangular ducts2016In: Meccanica (Milano. Print), ISSN 0025-6455, E-ISSN 1572-9648, Vol. 51, no 12, p. 3025-3042Article in journal (Refereed)
    Abstract [en]

    Convergence criteria for direct numerical simulations of turbulent channel and duct flows are proposed. The convergence indicator for channels is defined as the deviation of the nondimensional total shear-stress profile with respect to a linear profile, whereas the one for the duct is based on a nondimensional streamwise momentum balance at the duct centerplane. We identify the starting () and averaging times () necessary to obtain sufficiently converged statistics, and also find that optimum convergence rates are achieved when the spacing in time between individual realizations is below . The in-plane structure of the flow in turbulent ducts is also assessed by analyzing square ducts at and 360 and rectangular ducts with aspect ratios 3 and 10 at . Identification of coherent vortices shows that near-wall streaks are located in all the duct cases at a wall-normal distance of as in Pinelli et al. (J Fluid Mech 644:107-122, 2010). We also find that large-scale motions play a crucial role in the streamline pattern of the secondary flow, whereas near-wall structures highly influence the streamwise vorticity pattern. These conclusions extend the findings by Pinelli et al. to other kinds of large-scale motions in the flow through the consideration of wider ducts. They also highlight the complex and multiscale nature of the secondary flow of second kind in turbulent duct flows.

  • 30.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Malm, Johan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mavriplis, Catherine
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Direct numerical simulation of the flow around a wall-mounted square cylinder under various inflow conditions2015In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 16, no 6, p. 555-587Article in journal (Refereed)
    Abstract [en]

    The flow around a wall-mounted square cylinder of side d is investigated by means of direct numerical simulation (DNS). The effect of inflow conditions is assessed by considering two different cases with matching momentum-thickness Reynolds numbers Re-theta similar or equal to 1000 at the obstacle: the first case is a fullyturbulent zero pressure gradient boundary layer, and the second one is a laminar boundary layer with prescribed Blasius inflow profile further upstream. An auxiliary simulation carried out with the pseudo-spectral Fourier-Chebyshev code SIMSON is used to obtain the turbulent time-dependent inflow conditions which are then fed into the main simulation where the actual flow around the cylinder is computed. This main simulation is performed, for both laminar and turbulent-inflows, with the spectral-element method code Nek5000. In both cases the wake is completely turbulent, and we find the same Strouhal number St similar or equal to 0.1, although the two wakes exhibit structural differences for x > 3d downstream of the cylinder. Transition to turbulence is observed in the laminar-inflow case, induced by the recirculation bubble produced upstream of the obstacle, and in the turbulent-inflow simulation the streamwise fluctuations modulate the horseshoe vortex. The wake obtained in our laminar-inflow case is in closer agreement with reference particle image velocimetry measurements of the same geometry, revealing that the experimental boundary layer was not fully turbulent in that dataset, and highlighting the usefulness of DNS to assess the quality of experimental inflow conditions.

  • 31.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Nagib, H. M.
    Flow features in three-dimensional turbulent duct flows with different aspect ratios2016In: Springer Proceedings in Physics, Springer, 2016, p. 123-126Conference paper (Refereed)
    Abstract [en]

    Direct numerical simulations of turbulent duct flows with width-to-height ratios 1, 3, 5, 7 and 10, at a friction Reynolds number Reτ,c ≃ 180, are carried out with the spectral element code Nek5000. The aim of these simulations is to gain insight into the kinematics and dynamics of Prandtl’s secondary flow of second kind, and its impact on the flow physics of wall-bounded turbulence. The secondary flow is characterized in terms of the cross-plane mean kinetic energy K = (V2 + W2)/2, and its variation in the spanwise direction of the flow. Our results show that averaging times of at least 3, 000 time units are required to reach a converged state of the secondary flow, which extends up to z* ≃ 5 h from the side walls. We also show that if the duct is not wide enough to accommodate the whole extent of the secondary flow, then its structure is modified by means of a different spanwise distribution of energy. Future proposed work includes coherent structure eduction, quadrant analysis at the corner, and comparisons with spanwise-periodic channels at comparable Reynolds numbers. © Springer International Publishing Switzerland 2016.

  • 32.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Nagib, H. M.
    IIT, MMAE Dept, Chicago, IL 60616 USA.
    Secondary flow in turbulent ducts with increasing aspect ratio2018In: PHYSICAL REVIEW FLUIDS, ISSN 2469-990X, Vol. 3, no 5, article id 054606Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations of turbulent duct flows with aspect ratios 1, 3, 5, 7, 10, and 14.4 at a center-plane friction Reynolds number Re-tau,Re- c similar or equal to 180, and aspect ratios 1 and 3 at Re-tau,Re- c similar or equal to 360, were carried out with the spectral-element code NEK5000. The aim of these simulations is to gain insight into the kinematics and dynamics of Prandtl's secondary flow of the second kind and its impact on the flow physics of wall-bounded turbulence. The secondary flow is characterized in terms of the cross-plane component of the mean kinetic energy, and its variation in the spanwise direction of the flow. Our results show that averaging times of around 3000 convective time units (based on duct half-height h) are required to reach a converged state of the secondary flow, which extends up to a spanwise distance of around similar or equal to 5h measured from the side walls. We also show that if the duct is not wide enough to accommodate the whole extent of the secondary flow, then its structure is modified as reflected through a different spanwise distribution of energy. Another confirmation of the extent of the secondary flow is the decay rate of kinetic energy of any remnant secondary motions for z(c)/h > 5 (where z(c) is the spanwise distance from the corner) in aspect ratios 7, 10, and 14.4, which exhibits a decreasing level of energy with increasing averaging time t(a), and in its rapid rate of decay given by similar to t(a)(-1). This is the same rate of decay observed in a spanwise-periodic channel simulation, which suggests that at the core, the kinetic energy of the secondary flow integrated over the cross-sectional area, < K >(yz), behaves as a random variable with zero mean, with rate of decay consistent with central limit theorem. Long-time averages of statistics in a region of rectangular ducts extending about the width of a well-designed channel simulation (i.e., extending about similar or equal to 3h on each side of the center plane) indicate that ducts or experimental facilities with aspect ratios larger than 10 may, if properly designed, exhibit good agreement with results obtained from spanwise-periodic channel computations.

  • 33.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. MMAE Department, Illinois Institute of Technology, Chicago, IL, USA.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Nagib, Hassan M.
    On minimum aspect ratio for duct flow facilities and the role of side walls in generating secondary flows2015In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 16, no 6, p. 588-606Article in journal (Refereed)
    Abstract [en]

    To the surprise of some of our colleagues, we recently recommended aspect ratios of at least 24 (instead of accepted values over last few decades ranging from 5 to 12) to minimise effects of sidewalls in turbulent duct flowexperiments, in order to approximate the two-dimensional channel flow. Here we compile available results from hydraulics and civil engineering literature, where this was already documented in the 1980s. This is of great importance due to the large amount of computational studies (mainly direct numerical simulations, DNSs) for spanwise-periodic turbulent channel flows, and the extreme complexity of constructing a fully developed duct flow facility with aspect ratio of 24 for high Reynolds numbers with adequate probe resolution. Results from this nontraditional literature for the turbulence community are compared to our recent database of DNS of turbulent duct flows with aspect ratios ranging from 1 to 18 at Re-tau,Re- c values of 180 and 330, leading to very good agreement between their experimental and our computational results at these low Reynolds numbers. The DNS results also reveal the complexity of a multitude of streamwise vortical structures in addition to the secondary corner flows (which extend up to z similar or equal to 5h). These time-dependent and meandering streamwise structures are located at the core of the duct and scale with its half-height. Comparisons of these structures with the vortical motions found in spanwise-periodic channels reveal similitudes in their time-averages and the same rate of decay of their mean kinetic energy similar to T-A(-1), with T-A being the averaging time. However, differences between the two flows are identified and ideas for their future analysis are proposed.

  • 34.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Nagib, Hassan M.
    Role of data uncertainties in identifying the logarithmic region of turbulent boundary layers2014In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 55, no 6, p. 1751-Article in journal (Refereed)
    Abstract [en]

    Composite expansions based on the log-law and the power-law were used to generate synthetic velocity profiles of zero pressure gradient turbulent boundary layers (TBLs) in the range of Reynolds number 800 <= Re-theta <= 860; 000; based on displacement thickness and free-stream velocity. Several artificial errors were added to the velocity profiles to simulate typical measurement uncertainties. The effects of the simulated errors were studied by extracting log-law and power-law parameters from all these pseudo-experimental profiles. Various techniques were used to establish a measure of the deviations in the overlap region. When parameters extracted for the log-law and the power-law are associated with similar levels of deviations with respect to their expected values, we consider that the profile leads to ambiguous conclusions. This ambiguity was observed up to Re-theta 16; 000 for a 4 % dispersion in the velocity measurements, up to Re-theta 8.6 x 10(5) for a 400 mu m uncertainty in probe position (in air flow at atmospheric pressure), and up to Re-theta 32; 000 for 3 % uncertainty in the determination of u(tau): In addition, a new method for the determination of the log-law limits is proposed. The results clearly serve as a further note for caution when identifying either a log or a power-law in TBLs. Together with a number of available studies in the literature, the present results can be seen as a additional reconfirmation of the log-law.

  • 35.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sanmiguel Vila, Carlos
    Ianiro, Andrea
    Discetti, Stefano
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Revisiting history effects in adverse-pressure-gradient turbulent boundary layers2017In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 99, no 3-4, p. 565-587Article in journal (Refereed)
    Abstract [en]

    The goal of this study is to present a first step towards establishing criteria aimed at assessing whether a particular adverse-pressure-gradient (APG) turbulent boundary layer (TBL) can be considered well-behaved, i.e., whether it is independent of the inflow conditions and is exempt of numerical or experimental artifacts. To this end, we analyzed several high-quality datasets, including in-house numerical databases of APG TBLs developing over flat-plates and the suction side of a wing section, and five studies available in the literature. Due to the impact of the flow history on the particular state of the boundary layer, we developed three criteria of convergence to well-behaved conditions, to be used depending on the particular case under study. (i) In the first criterion, we develop empirical correlations defining the R e (oee integral) -evolution of the skin-friction coefficient and the shape factor in APG TBLs with constant values of the Clauser pressure-gradient parameter beta = 1 and 2 (note that beta = delta (au)/tau (w) dP (e) /dx, where delta (au) is the displacement thickness, tau (w) the wall-shear stress and dP (e) /dx the streamwise pressure gradient). (ii) In the second one, we propose a predictive method to obtain the skin-friction curve corresponding to an APG TBL subjected to any streamwise evolution of beta, based only on data from zero-pressure-gradient TBLs. (iii) The third method relies on the diagnostic-plot concept modified with the shape factor, which scales APG TBLs subjected to a wide range of pressure-gradient conditions. These three criteria allow to ensure the correct flow development of a particular TBL, and thus to separate history and pressure-gradient effects in the analysis.

  • 36.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Characterisation of backflow events over a wing section2017In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 18, no 2, p. 170-185Article in journal (Refereed)
    Abstract [en]

    Rare backflow (negative wall-shear stress) events have recently been found and quantified in the near-wall region of canonical wall-bounded turbulent flows. Although their existence and correlation with large-scale events have been established beyond numerical and measurement technique uncertainties, their occurrence at numerically high Reynolds numbers is still rare (less than 1 per thousand and 1 per million at the wall and beyond the viscous sublayer, respectively). To better quantify these rare events, the turbulent boundary layer developing over the suction side of a wing section, experiencing an increasing adverse pressure gradient (APG) without separation along its chord c, is considered in the present work. We find that the backflow level of 0.06% documented in turbulent channels and zero-pressure-gradient (ZPG) turbulent boundary layers is already exceeded on the suction side for x/c &gt; 0.3, at friction Reynolds numbers three times lower, while close to the trailing edge the backflow level reaches 30%. Conditional analysis of extreme events indicates that for increasing Clauser pressure-gradient parameters (reaching β ≃ 35), the flow reaches a state in which the extreme events are more likely aligned with or against the freestream, and that the otherwise strong spanwise component of the wall-shear stress reduces towards the vicinity of the trailing edge. Backflow events subjected to moderate up to strong APG conditions (0.6 &lt; β &lt; 4.1) exhibit an average width of Δz+ ≃ 20, and an average lifetime of Δt+ ≃ 2. This directly connects with the findings by Lenaers et al., and implies that there is a connection between high-Re ZPG and strong APG conditions. 

  • 37.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vila, C. S.
    DIscetti, S.
    Ianiro, A.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Towards canonical adverse-pressure-gradient turbulent boundary layers2017In: 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, International Symposium on Turbulence and Shear Flow Phenomena, TSFP10 , 2017, Vol. 3Conference paper (Refereed)
    Abstract [en]

    The present investigation focuses on the concerted investigation of pressure gradient and streamwise curvature effects on turbulent boundary layers. In particular, a number of direct and large-eddy simulations covering a wide range of pressure gradient parameters and streamwise histories on flat and curved surfaces is performed and will be compared with wind-tunnel experiments utilising hot-wire anemometry and particle image velocimetry that overlap and extend the Reynolds number range. Results are aimed at isolating the effects of pressure gradients, streamwise curvature and streamwise (pressure gradient) histories as well as Reynolds number, which have traditionally inhibited to draw firm conclusions from the available data.

1 - 37 of 37
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