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  • 201.
    Schlatter, Philipp
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Li, Qiang
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne V.
    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.
    Simulations of spatially evolving turbulent boundary layers up to Re-theta=43002010In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 31, no 3, p. 251-261Article in journal (Refereed)
    Abstract [en]

    A well-resolved large-eddy simulation (LES) of a spatially developing turbulent boundary layer under zero-pressure-gradient up to comparably high Reynolds numbers (Re-theta = 4300) is performed. The laminar inflow is located at Re-delta = 450 (Re-theta approximate to 1180), a position where natural transition to turbulence can be expected. The simulation is validated and compared extensively to both numerical data sets, i.e. a recent spatial direct numerical simulation (DNS) up to Re-theta = 2500 (Schlatter et al., 2009) and available experimental measurements, e.g. the ones obtained by Osterlund (1999). The goal is to provide the research community with reliable numerical data for high Reynolds-number wall-bounded turbulence, which can in turn be employed for further model development and validation, but also to contribute to the characterisation and understanding of various aspects of wall turbulence. The results obtained via LES show that good agreement with DNS data at lower Reynolds numbers and experimental data can be obtained for both mean and fluctuating quantities. In addition, turbulence spectra characterising large-scale organisation in the flow have been computed and compared to literature results with good agreement. In particular, the near-wall streaks scaling in inner units and the outer layer large-scale structures can clearly be identified in both spanwise and temporal spectra. (C) 2010 Elsevier Inc. All rights reserved.

  • 202.
    Schlatter, Philipp
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Li, Qiang
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne V.
    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.
    Structure of a turbulent boundary layer studied by DNS2011In: Direct and Large-Eddy Simulation VIII, 2011, p. 9-14Conference paper (Refereed)
    Abstract [en]

    Turbulent boundary layers constitute one of the basic building blocks for understanding turbulence, particularly relevant for industrial applications. Although the geometries in technical but also geophysical applications are complicated and usually feature curved surfaces, the flow case of a canonical boundary layer developing on a flat surface has emerged as an important setup for studying wall turbulence, both via experimental and numerical studies. However, only recently spatially developing turbulent boundary layers have become accessible via direct numerical simulations (DNS). The difficulties of such setups are mainly related to the specification of proper inflow conditions, the triggering of turbulence and a careful control of the free-stream pressure gradient. In addition, the numerical cost of such spatial simulations is high due to the long, wide and high domains necessary for the full development of all relevant turbulent scales. We consider a canonical turbulent boundary layer under zero-pressure-gradient via large-scale DNS. The boundary layer is allowed to develop and grow in space. The inflow is a laminar Blasius boundary layer, in which laminar-turbulent transition is triggered by a random volume force shortly downstream of the inflow. This trip force, similar to a disturbance strip in an experiment (Schlatter and Örlü, 2010; Schlatter et al., 2009), is located at a low Reynolds number to allow the flow to develop over a long distance. The simulation covers thus a long, wide and high domain starting at Re θ =180 extending up to the (numerically high) value of Re θ =4300, based on momentum thickness θ and free-stream velocity U ∞. Fully turbulent flow is obtained from Re θ ≈500. The numerical resolution for the fully spectral numerical method (Chevalier et al., 2007) is in the wall-parallel directions Δx +=9 and Δz +=4, resolving the relevant scales of motion. The simulation domain requires a total of 8⋅109 grid points in physical space, and was thus run massively parallel with 4096 processors.

  • 203.
    Schlatter, Philipp
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Li, Qiang
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Towards large-eddy simulations of high-Reynolds number turbulent boundary layers2009In: Proc. 6th Intl Symp. on Turbulence and Shear Flow Phenomena, 2009, p. 271-276Conference paper (Refereed)
  • 204.
    Schlatter, Philipp
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Li, Qiang
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hussain, Fazle
    Henningson, Dan S.
    On the vortical structures of a turbulent boundary layer at high Reynolds number2011Report (Other academic)
    Abstract [en]

    A recent data base from direct numerical simulation of a turbulent boundary layer up to Reθ = 4300 [Schlatter & Örlü, J. Fluid Mech. 659, 2010] has been analysed in an effort to educe the dominant flow structures populating the near-wall region. In particular, the question of whether hairpin vortices are indeed observable as a dominant building block of near-wall turbulence is addressed. It is shown that during the initial phase, dominanted by the specific laminar-turbulent transition induced via the tripping mechanism, hairpin vortices are very numerous, and can certainly be considered as the dominant structure. This is in agreement with previous experiments and low Reynolds number simulations such as Wu & Moin [J. Fluid Mech. 630, 2009]. At sufficient distance from transition, the flow is dominated by a staggered array of quasi-streamwise vortices which is the same situation as in previous channel flows. It turns out that even quantitatively, no major differences between boundary layers and channels can be detected; structures are about 200 viscous units in length, and inclined by about 9 degrees [Jeong et al., J. Fluid Mech. 332, 1997]. The present results clearly show that the regeneration process of turbulence does not involve the generation of (symmetric) hairpin vortices, and that their dominant appearance as instantaneous flow structures in the outer boundary-layer region is at least very unlikely.

  • 205.
    Schlatter, Philipp
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Malm, Johan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne V.
    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.
    Large-scale Simulations of Turbulence: HPC and Numerical Experiments2011In: 2011 Seventh IEEE International Conference on eScience, 2011, p. 319-324Conference paper (Refereed)
    Abstract [en]

    In this paper, we describe some of the methods used at KTH Mechanics in the field of turbulence simulations. High performance computing (HPC) resources, including the dedicated system "Ekman" with 10000 cores at KTH, are employed to perform some of the largest turbulence simulations with up to 10 billion grid points. The results are used to assess the fidelity of such types of simulations by comparing in detail to wind tunnel experiments, excellent agreement is obtained in general. Simulations can thus be considered numerical experiments, and are subject to the same scrutiny as "real" experimental data. Validated simulation data is extremely valuable as it may provide unprecedented insight into the turbulence dynamics. However, given the large size of the computations, the massively parallel simulation codes, post processing tools and storage solutions have to be specifically adapted, making turbulence simulations an interdisplinary area of e-Science.

  • 206. Schlatter, Philipp
    et al.
    Stolz, S.
    Kleiser, L.
    Evaluation of high-pass filtered eddy-viscosity models for large-eddy simulation of turbulent flows2005In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 6, no 5Article in journal (Refereed)
    Abstract [en]

    Large-eddy simulations (LES) of incompressible homogeneous isotropic turbulence and turbulent channel flow are performed at moderate Reynolds numbers using high-pass filtered (HPF) eddy-viscosity models. This family of models computes the subgrid-scale (SGS) terms from a high-pass filtered velocity field using classical closure relations, e. g. the Smagorinsky or the structure-function model closure. Unlike the classical fixed-coefficient eddy-viscosity models, the HPF models are able to accurately describe the viscous sublayer of near-wall turbulence. Moreover, it has been shown recently that the HPF models are also capable of predicting transitional flows. Detailed results of energy and dissipation spectra are given for forced isotropic turbulence at microscale Reynolds number up to Re-gimel approximate to 5500. For turbulent channel flow at friction Reynolds number Re-tau approximate to 590, results are presented for first- and second-order statistics as well as for the energy budget including the SGS terms. The overall performance of the HPF eddy-viscosity models is very good for both flow cases using a constant model coefficient. An empirical adaptation of the model coefficient to the cutoff wavenumber of the chosen high-pass filter is given. In contrast to classical eddy-viscosity models, the HPF models allow the prediction of backscatter effects, which are important for wall-bounded flows close to the walls.

  • 207. Schlatter, Philipp
    et al.
    Stolz, S.
    Kleiser, L.
    Large-eddy simulation of spatial transition in plane channel flow2006In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 7, no 33, p. 1-24Article in journal (Refereed)
    Abstract [en]

    Spatial large-eddy simulations (LES) of forced transition in plane incompressible channel flow are presented and compared to temporal simulations. Using the fringe method, spectral Fourier discretization is employed also in the streamwise, spatially evolving flow direction. Various subgrid-scale (SGS) models are examined including the dynamic Smagorinsky model, high-pass filtered (HPF) eddy-viscosity models and the relaxation-term model (ADM-RT). The applicability of the fringe method in conjunction with SGS models is demonstrated. Good results are obtained even at rather low LES resolution at which a coarse-grid no-model calculation is inaccurate. The most accurate prediction of transitional flow structures is obtained using the ADM-RT model. For this model, a detailed comparison between spatial and temporal simulation results is given. A clear representation of the transitional flow structures by LES up to the multi-spike stage could be established. Our results also show that the SGS models behave similarly in temporal and spatial simulations, thus allowing us to perform SGS model testing with the more straightforward and inexpensive temporal approach. The same SGS models work well without any change also in the fully developed turbulent flow.

  • 208. Schlatter, Philipp
    et al.
    Stolz, S.
    Kleiser, L.
    LES of transitional flows using the approximate deconvolution model2004In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 25, no 3, p. 549-558Article in journal (Refereed)
    Abstract [en]

    Large-eddy simulations of transitional incompressible channel flow on rather coarse grids are performed. The standard approximate deconvolution model (ADM) as well as two modifications are compared to fully resolved direct numerical simulation (DNS) calculations. The results demonstrate that it is well possible to simulate transitional flows on the basis of ADM. During the initial phase of transition, the models remain inactive and do not disturb the flow development as long as it is still sufficiently resolved on the coarse large-eddy simulation (LES) grid. During the later stages of transition the model contributions provide necessary additional dissipation. Due to the dynamic determination of the model coefficient also employed for the standard ADM, no ad hoc constants or adjustments are needed. The results of the modified ADM show excellent agreement with DNS already on coarser meshes than the standard ADM, e.g. in the skin friction throughout the transitional phase, while preserving the accuracy for the fully developed turbulent channel flow. A grid-resolution study demonstrates convergence of LES to the DNS results. Results of the dynamic Smagorinsky model are included for comparison.

  • 209.
    Schlatter, Philipp
    et al.
    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, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Assessment of direct numerical simulation data of turbulent boundary layers2010In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 659, p. 116-126Article in journal (Refereed)
    Abstract [en]

    Statistics obtained from seven different direct numerical simulations (DNSs) pertaining to a canonical turbulent boundary layer (TBL) under zero pressure gradient are compiled and compared. The considered data sets include a recent DNS of a TBL with the extended range of Reynolds numbers Re-theta = 500-4300. Although all the simulations relate to the same physical flow case, the approaches differ in the applied numerical method, grid resolution and distribution, inflow generation method, boundary conditions and box dimensions. The resulting comparison shows surprisingly large differences not only in both basic integral quantities such as the friction coefficient c(f) or the shape factor II12, but also in their predictions of mean and fluctuation profiles far into the sublayer. It is thus shown that the numerical simulation of TBLs is, mainly due to the spatial development of the flow, very sensitive to, e. g. proper inflow condition, sufficient settling length and appropriate box dimensions. Thus, a DNS has to be considered as a numerical experiment and should be the subject of the same scrutiny as experimental data. However, if a DNS is set up with the necessary care, it can provide a faithful tool to predict even such notoriously difficult flow cases with great accuracy.

  • 210.
    Schlatter, Philipp
    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.
    Quantifying the interaction between large and small scales in wall-bounded turbulent flows: A note of caution2010In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, no 5, p. 051704-Article in journal (Refereed)
    Abstract [en]

    Turbulent flow close to solid walls is dominated by an ensemble of fluctuations of large and small spatial scales. Recent work by Mathis [J. Fluid Mech. 628, 311 (2009); Phys. Fluids 21, 111703 (2009)] introduced and used a decoupling procedure based on the Hilbert transformation applied to the filtered small-scale component of the fluctuating streamwise velocity. This method is employed as a robust tool to quantify a dominant amplitude modulation of the small scales by the large scales found in the outer part of the boundary layer. In the present study, however, we demonstrate by means of experimental and synthetic signals that the correlation coefficient used to quantify the amplitude modulation is related to the skewness of the original signal, and hence, for the Reynolds numbers considered here, may not be an independent tool to unambiguously detect or quantify the effect of large-scale amplitude modulation of the small scales.

  • 211.
    Schlatter, Philipp
    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.
    Turbulent asymptotic suction boundary layers studied by simulation2011In: 13th European Turbulence Conference (ETC13): Wall-Bounded Flows And Control Of Turbulence, Institute of Physics (IOP), 2011Conference paper (Refereed)
    Abstract [en]

    The turbulent asymptotic suction boundary layer (ASBL) is studied using numerical simulations. Uniform suction is applied on the wall in order to compensate for the momentum loss inflicted by the wall friction. Four Reynolds numbers, defined as the ratio of free-stream velocity and suction rate, Re = 333, 400 and 500, are considered, whereas Re = 280 relaminarised. In agreement with previous studies, suction causes the fluctuation intensities to decrease, and the near-wall anisotropy to increase. The shape of the mean velocity profile is considerably changed yielding a decreased slope in the overlap region. It is shown that even for moderate suction rates large values for the friction Reynolds number Re-tau = delta(+)(99) are obtained; at Re = 333 a value of Re-tau = 1900 is reached and Re = 400 yields Re-tau = 5700. Artificially using smaller computational domains, limiting the size of the largest turbulent structures, gives unexpected results: The mean velocity profile starts to show a distinct wake region which only disappears for large enough domains. Moreover, the boundary layer thickness delta(99) strongly depends on the chosen domain size. Spectral maps of the flow are analysed, showing an outer peak appearing at a spanwise size of about 0.6 delta(99), albeit with considerably lower amplitude compared to cases without suction. Visualisations of the flow are also discussed.

  • 212.
    Schlatter, Philipp
    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.
    Turbulent boundary layers at moderate Reynolds numbers: inflow length and tripping effects2012In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 710, p. 5-34Article in journal (Refereed)
    Abstract [en]

    A recent assessment of available direct numerical simulation (DNS) data from turbulent boundary layer flows (Schlatter & Orlu, J. Fluid Mech., vol. 659, 2010, pp. 116-126) showed surprisingly large differences not only in the skin friction coefficient or shape factor, but also in their predictions of mean and fluctuation profiles far into the sublayer. While such differences are expected at very low Reynolds numbers and/or the immediate vicinity of the inflow or tripping region, it remains unclear whether inflow and tripping effects explain the differences observed even at moderate Reynolds numbers. This question is systematically addressed by re-simulating the DNS of a zero-pressure-gradient turbulent boundary layer flow by Schlatter et a l. (Phys. Fluids, vol. 21, 2009, art. 051702). The previous DNS serves as the baseline simulation, and the new DNS with a range of physically different inflow conditions and tripping effects are carefully compared. The downstream evolution of integral quantities as well as mean and fluctuation profiles is analysed, and the results show that different inflow conditions and tripping effects do indeed explain most of the differences observed when comparing available DNS at low Reynolds number. It is further found that, if transition is initiated inside the boundary layer at a low enough Reynolds number (based on the momentum-loss thickness) Re-theta < 300, all quantities agree well for both inner and outer layer for Re-theta > 2000. This result gives a lower limit for meaningful comparisons between numerical and/or wind tunnel experiments, assuming that the flow was not severely over-or understimulated. It is further shown that even profiles of the wall-normal velocity fluctuations and Reynolds shear stress collapse for higher Re-theta irrespective of the upstream conditions. In addition, the overshoot in the total shear stress within the sublayer observed in the DNS of Wu & Moin (Phys. Fluids, vol. 22, 2010, art. 085105) has been identified as a feature of transitional boundary layers.

  • 213.
    Schlatter, Philipp
    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.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Chin, C.
    Hutchins, N.
    Monty, J.
    Large-scale friction control in turbulent wall flow2015In: 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015, TSFP-9 , 2015Conference paper (Refereed)
    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.

  • 214.
    Schlatter, Philipp
    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.
    Eitel-Amor, G
    Li, Q
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    On the near-wall vortical structures at moderate Reynolds numbers2013Conference paper (Refereed)
  • 215.
    Schlatter, Philipp
    et al.
    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), Centres, Linné Flow Center, FLOW.
    Li, Qiang
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. 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.
    Turbulent boundary layers up to Re-theta=2500 studied through simulation and experiment2009In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Physics of Fluids, Vol. 21, no 5, p. 051702-Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations (DNSs) and experiments of a spatially developing zero-pressure-gradient turbulent boundary layer are presented up to Reynolds number Re-theta=2500, based on momentum thickness theta and free-stream velocity. For the first time direct comparisons of DNS and experiments of turbulent boundary layers at the same (computationally high and experimentally low) Re-theta are given, showing excellent agreement in skin friction, mean velocity, and turbulent fluctuations. These results allow for a substantial reduction of the uncertainty of boundary-layer data, and cross validate the numerical setup and experimental technique. The additional insight into the flow provided by DNS clearly shows large-scale turbulent structures, which scale in outer units growing with Re-theta, spanning the whole boundary-layer height.

  • 216.
    Schlatter, Philipp
    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.
    Li, Qiang
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brethouwer, Gert
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Johansson, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Progress in simulations of turbulent boundary layers2011In: Proc. 7th International Symposium on Turbulence and Shear Flow Phenomena, 2011Conference paper (Refereed)
    Abstract [en]

    Recent efforts in the simulation of turbulent boundary layers using direct and large-eddy simulations are described. The focus is naturally on a series of simulations performed at KTH Stockholm. These results have been used to examine various aspects of the boundary layer; starting from estimates of the extent of the transitional region, the detailed comparison to wind-tunnel experiments, the effect of ambient freestream turbulence on the boundary layer and to quantifications of the spectral composition of the turbulent signal. Furthermore, selected aspects of boundary layers with coupled scalar (e.g. heat) transport are summarised, including profiles of the turbulent Prandtl number.

  • 217.
    Segalini, Antonio
    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.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rüedi, Jean-Daniel
    Talamelli, Alessandro
    A method to estimate turbulence intensity and transverse Taylor microscale in turbulent flows from spatially averaged hot-wire data2011In: Experiments in Fluids, ISSN 0723-4864, Vol. 51, no 3, p. 693-700Article in journal (Refereed)
    Abstract [en]

    A new approach to evaluate turbulence intensity and transverse Taylor microscale in turbulent flows is presented. The method is based on a correction scheme that compensates for probe resolution effects and is applied by combining the response of two single hot-wire sensors with different wire lengths. Even though the technique, when compared to other correction schemes, requires two independent measurements, it provides, for the same data, an estimate of the spanwise Taylor microscale. The method is here applied to streamwise turbulence intensity distributions of turbulent boundary layer flows but it is applicable generally in any turbulent flow. The technique has been firstly validated against spatially averaged DNS data of a zero pressure-gradient turbulent boundary layer showing a good capacity to reconstruct the actual profiles and to predict a qualitatively correct and quantitatively agreeing transverse Taylor microscale over the entire height of the boundary layer. Finally, the proposed method has been applied to available higher Reynolds number data from recent boundary layer experiments where an estimation of the turbulence intensity and of the Taylor microscale has been performed.

  • 218. Skote, Martin
    et al.
    Mishra, M.
    Negi, P. S.
    Wu, Y.
    Lee, H. M.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wall oscillation induced drag reduction of turbulent boundary layers2016In: Springer Proceedings in Physics, Springer, 2016, p. 161-165Conference paper (Refereed)
    Abstract [en]

    Spanwise oscillation applied on the wall under a turbulent boundary layer flow is investigated using direct numerical simulation. The temporal wall-forcing produces considerable drag reduction over the region where oscillation occurs. The turbulence fluctuations downstream of the oscillations are presented for the first time. Simulations with identical oscillation parameters have been performed at different Reynolds numbers to investigate the effect on the drag reduction. One of the simulations replicates an earlier experiment to test the fidelity of the current simulations. In addition, we present the future work in this area with an integrated experimental and computational investigation to explore the possibility of applying travelling waves (oscillations in both time and space) as the mode of wall motion for active control of near-wall turbulence. © Springer International Publishing Switzerland 2016.

  • 219.
    Srinivasan, P. A.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Electrical Engineering and Computer Science (EECS). KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Guastoni, L.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH Mech, Linne FLOW Ctr, SE-10044 Stockholm, Sweden.;Swedish E Sci Res Ctr SeRC, SE-10044 Stockholm, Sweden..
    Azizpour, Hossein
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Electrical Engineering and Computer Science (EECS).
    Schlatter, Philipp
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vinuesa, Ricardo
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Predictions of turbulent shear flows using deep neural networks2019In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 5, article id 054603Article in journal (Refereed)
    Abstract [en]

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

  • 220. Stolz, S.
    et al.
    Schlatter, Philipp
    Kleiser, L.
    High-pass filtered eddy-viscosity models for large-eddy simulations of transitional and turbulent flow2005In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 17, no 6Article in journal (Refereed)
    Abstract [en]

    Classical fixed-coefficient eddy-viscosity models for large-eddy simulations (LES), e. g., the Smagorinsky [Mon. Weather Rev. 93, 99 (1963)] and the structure-function model [Metais and Lesieur, J. Fluid Mech. 239, 157 (1992)], have deficiencies with accurately describing laminar flow regions and the viscous sublayer of near-wall turbulence. This is mainly due to unphysically large model contributions in such flow regions and is one main reason for the difficulty with correctly predicting laminar-turbulent transition using such models. For transitional flows, subgrid-scale (SGS) models must be able to properly deal with at least the two limits of the laminar and the turbulent fluid state. The dynamic Smagorinsky model [Germano et al., Phys. Fluids A 3, 1760 (1991)] alleviates this problem by dynamically computing a small, but not necessarily vanishing, value for the model coefficient in laminar regions and in the vicinity of the wall. In this contribution we analyze high-pass filtered (HPF) eddy-viscosity models which were proposed recently and independently by Vreman [Phys. Fluids 15, L61 (2003)] and Stolz et al. [Direct and Large-Eddy Simulation V (Kluwer, Dordrecht, 2004), pp. 81-88]. We investigate high-pass filtering suitable for such HPF eddy-viscosity models, e.g., the Smagorinsky or the (filtered) structure-function model. Furthermore, we suggest suitable high-pass filters and examine the influence of different high-pass filters on the results of LES of transitional and turbulent incompressible channel flow. Except for the filter shape, the cutoff wavenumber of the high-pass filter is the only parameter besides the eddy-viscosity model coefficient, and its influence can be minimized by a proposed adaptation of the model coefficient. We find that high-pass filtering employed to the computational quantities prior to the calculation of the eddy-viscosity and strain rate in the SGS model significantly improves the quality of the prediction of transitional and turbulent flows without using any ad-hoc adaptation or dynamic procedure. Of particular importance is that the sensitivity of the results to the model coefficient is considerably reduced by the high-pass filtering. Furthermore, the proposed high-pass filters enable the computation of the structure function in the filtered or HPF structure-function models in all spatial directions also for inhomogeneous flows and on non-equidistant grids, removing the arbitrariness of a special treatment of selected (e.g., wall-normal) directions. Simulation results are presented for incompressible turbulent channel flow at Reynolds numbers Re-tau (based on the channel half-width and the friction velocity) of 180 and 590 and for forced laminar-turbulent transition in a plane channel, demonstrating the effectiveness of the proposed approach. The results are compared to data of direct numerical simulations and to data obtained using the dynamic Smagorinsky model with different test filters.

  • 221. Stolz, Steffen
    et al.
    Schlatter, Philipp
    Kleiser, Leonhard
    Large-eddy simulations of subharmonic transition in a supersonic boundary layer2007In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 45, no 5, p. 1019-1027Article in journal (Refereed)
    Abstract [en]

    We investigate the performance of two recently developed subgrid-scale models, the approximate deconvolution model and the high-pass filtered Smagorinsky model, in large-eddy simulations of laminar-turbulent transition in a supersonic boundary layer. Subharmonic transition in a boundary layer at a freestream Mach number of 4.5 and a Reynolds number (based on initial displacement thickness) of 10,000 is considered, which has been studied previously in detail by direct numerical simulations. For computational efficiency, the temporal simulation approach has been adopted. The discretization is based on Fourier collocation and various high-order finite difference schemes in the wall-parallel and wall-normal directions, respectively. Large-eddy simulations results are assessed by comparing statistical and instantaneous quantities during transition with data obtained from a sufficiently resolved simulations accurately reproduce the direct direct numerical simulation. The results show that the large-eddy numerical simulations data from the slightly disturbed laminar flow through transition into the turbulent stage, with a computational effort of two orders of magnitude less than the direct numerical simulations. Both subgrid-scale models are formulated locally in space and in a fully three-dimensional manner and do not need an ad hoc adaptation to nonturbulent or near-wall regions.

  • 222.
    Straub, S.
    et al.
    Institute of Fluid Mechanics, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany.
    Forooghi, P.
    Institute of Fluid Mechanics, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany.
    Marocco, L.
    Dipartimento di Energia, Politecnico di Milano, Milano, 20156, Italy.
    Wetzel, T.
    Institute of Thermal Process Engineering, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany.
    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.
    Frohnapfel, B.
    Institute of Fluid Mechanics, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany.
    The influence of thermal boundary conditions on turbulent forced convection pipe flow at two Prandtl numbers2019In: International Journal of Heat and Mass Transfer, ISSN 0017-9310, E-ISSN 1879-2189, Vol. 144, article id 118601Article in journal (Refereed)
    Abstract [en]

    Different types of thermal boundary conditions are conceivable in numerical simulations of convective heat transfer problems. Isoflux, isothermal and a mixed-type boundary condition are compared by means of direct numerical simulations (for the lowest Reynolds number) and well-resolved large-eddy simulations of a turbulent forced convection pipe flow over a range of bulk Reynolds numbers from Reb=5300 to Reb=37700, at two Prandtl numbers, i.e. Pr=0.71 and Pr=0.025. It is found that, while for Pr=0.71 the Nusselt number is hardly affected by the type of thermal boundary condition, for Pr=0.025 the isothermal boundary condition yields ≈20% lower Nusselt numbers compared to isoflux and mixed-type over the whole range of Reynolds numbers. A decomposition of the Nusselt number is derived. In particular, we decompose it into four contributions: laminar, radial and streamwise turbulent heat flux as well as a contribution due to the turbulent velocity field. For Pr=0.71 the contribution due to the radial turbulent heat flux is dominant, whereas for Pr=0.025 the contribution due to the turbulent velocity field is dominant. Only at a moderately high Reynolds number, such as Reb=37700, both turbulent contributions are of similar magnitude. A comparison of first- and second-order thermal statistics between the different types of thermal boundary conditions shows that the statistics are not only influenced in the near-wall region but also in the core region of the flow. Power spectral densities illustrate large thermal structures in low-Prandtl-number fluids as well as thermal structures located right at the wall, only present for the isoflux boundary condition. A database including the first- and second-order statistics together with individual contributions to the budget equations of the temperature variance and turbulent heat fluxes is hosted in the open access repository KITopen (DOI:https://doi.org/10.5445/IR/1000096346).

  • 223. 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.

  • 224. Stroh, A.
    et al.
    Frohnapfel, B.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hasegawa, Y.
    A comparison of opposition control in turbulent boundary layer and turbulent channel flow2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 7, article id 075101Article in journal (Refereed)
    Abstract [en]

    A comparison between classical opposition control applied in the configuration of a fully developed turbulent channel flow and applied locally in a spatially developing turbulent boundary layer is presented. It is found that the control scheme yields similar drag reduction rates if compared at the same friction Reynolds numbers. However, a detailed analysis of the dynamical contributions to the skin friction coefficient reveals significant differences in the mechanism behind the drag reduction. While drag reduction in turbulent channel flow is entirely based on the attenuation of the Reynolds shear stress, the modification of the spatial flow development is essential for the turbulent boundary layer in terms of achievable drag reduction. It is shown that drag reduction due to this spatial development contribution becomes more pronounced with increasing Reynolds number (up to Re-tau = 660, based on friction velocity and boundary layer thickness) and even exceeds drag reduction due to attenuation of the Reynolds shear stress. In terms of an overall energy balance, it is found that opposition control is less efficient in the turbulent boundary layer due to the inherently larger fluctuation intensities in the near wall region.

  • 225. Stroh, A.
    et al.
    Hasegawa, Y.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Frohnapfel, B.
    Global effect of local skin friction drag reduction in spatially developing turbulent boundary layer2016In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 805, p. 303-321Article in journal (Refereed)
    Abstract [en]

    A numerical investigation of two locally applied drag-reducing control schemes is carried out in the configuration of a spatially developing turbulent boundary layer (TBL). One control is designed to damp near-wall turbulence and the other induces constant mass flux in the wall-normal direction. Both control schemes yield similar local drag reduction rates within the control region. However, the flow development downstream of the control significantly differs: persistent drag reduction is found for the uniform blowing case, whereas drag increase is found for the turbulence damping case. In order to account for this difference, the formulation of a global drag reduction rate is suggested. It represents the reduction of the streamwise force exerted by the fluid on a plate of finite length. Furthermore, it is shown that the far-downstream development of the TBL after the control region can be described by a single quantity, namely a streamwise shift of the uncontrolled boundary layer, i.e. a changed virtual origin. Based on this result, a simple model is developed that allows the local drag reduction rate to be related to the global one without the need to conduct expensive simulations or measurements far downstream of the control region.

  • 226.
    Talamelli, A.
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Malizia, F.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Cimarelli, A.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Temperature effects in hot-wire measurements on higher-order moments in wall turbulence2016In: Springer Proceedings in Physics, Springer, 2016, p. 185-189Conference paper (Refereed)
    Abstract [en]

    The effect of temperature fluctuations—as they are e.g. encountered in non-isothermal flows—on the mean, variance and spectra in wall-bounded turbulent flows when utilising hot-wire anemometry has recently been documented. The present work extends these efforts on the effect of temperature fluctuations on the skewness and flatness factors as well as assesses the performance of commonly employed temperature compensations.

  • 227. Talamelli, Alessandro
    et al.
    Segalini, Antonio
    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.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    A method to correct third and fourth order moments in turbulent flows2011Conference paper (Refereed)
    Abstract [en]

    It is well known that spatial averaging, resulting from the finite size of the probe, significantly affects the accuracy of hot-wire measurements close to the wall. Here, an analytical model, which describes the effect of the spatial filtering of hot-wire probes on the third and fourth order moments of the streamwise velocity in turbulent flows, is presented. The model, which is based on the three (four) points velocity correlation function for the third (fourth) order moment correction, shows that the filtering can be related to a characteristic length scale which is the equivalent of the Taylor transverse micro-scale for the second order moment spatial filtering correction approach. The capacity of the model to accurately describe the attenuation is validated against DNS data of a zero pressure-gradient turbulent boundary layer. The DNS data allow the filtering effect to be appraised for different wire lengths and for the different moments. A procedure, based on the developed model, to correct the measured moments in turbulent flows is finally presented. The method is applied by combining the response of two single hot-wire sensors with different wire lengths. The technique has also been validated against spatially averaged DNS data showing a good capacity to reconstruct the actual profiles over the entire height of the boundary layer except, for the third order moment, in the region where the latter is close to zero.

  • 228.
    Talamelli, Alessandro
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. University of Bologna, Italy.
    Segalini, Antonio
    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.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. University of Bologna, Italy.
    Correcting hot-wire spatial resolution effects in third- and fourth-order velocity moments in wall-bounded turbulence2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 4, p. 1496-Article in journal (Refereed)
    Abstract [en]

    Spatial averaging, resulting from the finite size of a hot-wire probe, significantly affects the accuracy of velocity measurements in turbulent flows close to walls. Here, we extend the theoretical model, introduced in Segalini et al. (Meas Sci Technol 22: 104508, 2011) quantifying the effect of a linear spatial filter of hot-wire probes on the mean and the variance of the streamwise velocity in turbulent wall-bounded flows, to describe the effect of the spatial filtering on the third-and fourth-order moments of the same velocity component. The model, based on the three-(four) point velocity-correlation function for the third-(fourth-) order moment, shows that the filtering can be related to a characteristic length scale which is an equivalent of the Taylor transverse microscale for the second-order moment. The capacity of the model to accurately describe the attenuation is validated against direct numerical simulation (DNS) data of a zero pressure-gradient turbulent boundary layer. The DNS data allow the filtering effect to be appraised for different wire lengths and for the different moments. The model shows good accuracy except for the third-order moment in the region where a zero-crossing of the third-order function is observed and where the equations become ill-conditioned. An "a posteriori" correction procedure, based on the developed model, to correct the measured third-and fourth-order velocity moments is also presented. This procedure, based on combining the measured data by two single hot-wire sensors with different wire lengths, is a natural extension of the one introduced by Segalini et al. (Exp Fluids 51:693-700, 2011) to evaluate both the turbulence intensity and the transverse Taylor microscale in turbulent flows. The technique is validated against spatially averaged simulation data showing a good capacity to correct the actual profiles over the entire height of the boundary layer except, as expected, for the third-order moment in the region where the latter exhibits a zero-crossing. Moreover, the proposed method has been tested on experimental data from turbulent pipe flow experiments.

  • 229.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    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.
    Wehrfritz, Armin
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Global linear and nonlinear stability of viscous confined plane wakes with co-flow2011In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 675, p. 397-434Article in journal (Refereed)
    Abstract [en]

    The global stability of confined wakes is studied numerically, using two-dimensionallinear global modes and nonlinear direct numerical simulations (DNS).The wake inflow velocity is varied between different amounts of co-flow (basebleed), while the density and viscosity are assumed to be constant everywherein the flow domain. In accordance with previous studies, we find that thefrequencies of both the most unstable linear and the saturated nonlinear globalmode increase with confinement. Here, we also find that for wake Reynoldsnumber Re = 100, the confinement is stabilising. It decreases both the growthrate of the linear and the saturation amplitude of the nonlinear modes. Weconclude that the dampening effect is connected to the streamwise developmentof the base flow, and for higher Reynolds numbers this effect decreases, sincethe flow becomes more parallel. The linear analysis reveals that the criticalwake velocities below which the flow becomes unstable are almost identicalfor unconfined and confined wakes at Re ≈ 400. Also, the present resultsare compared with literature data for an inviscid parallel wake due to thesimilarity of inflow profile. The confined wake is found to be more stable thanits inviscid counterpart, while the unconfined wake is more unstable than theinviscid wake. The main reason to both can be explained by the base flowdevelopment. A detailed comparison of the linear and nonlinear results revealsthat the most unstable linear global mode gives an excellent prediction of theinitial nonlinear behaviour and therefore the stability boundary, in all cases.However, the nonlinear saturated state is quite different in particular for higherReynolds numbers. For Re = 100, the saturated frequency also differs less than5% from the linear frequency, and trends regarding confinement observed in thelinear analysis are confirmed.141

  • 230. Tanarro, Álvaro
    et al.
    Mallor, Fermı́n
    Offermans, Nicolas
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Peplinski, Adam
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Using adaptive mesh refinement to simulate turbulent wings at high Reynolds numbers2019Conference paper (Refereed)
    Abstract [en]

    The implementation of adaptive mesh refinement (AMR) in Nek5000 is used for the first time on the simulation of the flow over wings. This is done by simulating the flow over a NACA4412 profile with 5 degrees angle of attack at chord-based Reynolds number 200,000. The mesh is progressively refined by means of AMR which allows for high resolution near the wall whereas significantly larger elements are used in the far-field. The resultant mesh shows higher resolution than previous conformal meshes, and it allows for larger computational domains,which avoid the use of RANS to determine the boundary condition, all of this with, approximately, 3 times lower total number of grid points. The results ofthe turbulence statistics show a good agreement with the ones obtained with the conformal mesh. Finally, using AMR on wings leads to simulations at higher Reynolds numbers (i.e. Rec = 850, 000) in order to analyse the effect of adverse pressure gradients at high Reynolds numbers.

  • 231. Tsuji, Y.
    et al.
    Imahama, S.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Pressure fluctuation in high-Reynolds number turbulent boundary layer: results from experimentsand numerical simulations2011In: Proc. 7th International Symposium on Turbulence and Shear Flow Phenomena, 2011Conference paper (Refereed)
    Abstract [en]

    Pressure fluctuations are measured in zero-pressuregradientboundary layers. Following the previous studies,we developed the small pressure probe and measure both thestatic pressure inside boundary layer and wall pressure simultaneouslyin turbulent boundary layers up to Reynolds numbersbased on the momentum thickness Rq ' 20000. Discussionsare made on the background pressure in the free streamregion. It contaminates the physical pressure in the boundarylayer. We report on the pressure intensity profile normalizedby outer and inner variables. Once the background pressureis subtracted, they are compared with the results of direct numericalsimulations.

  • 232. Tsuji, Y.
    et al.
    Imayama, Shintaro
    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.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Marusic, I.
    Hutchins, N.
    Monty, J.
    Pressure fluctuation in high-Reynolds-number turbulent boundary layer: Results from experiments and DNS2012In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 13, no 50, p. 1-19Article in journal (Refereed)
    Abstract [en]

    We have developed a small pressure probe and measured both static pressure and wall pressure simultaneously in turbulent boundary layers up to Reynolds numbers based on the momentum thickness Rθ ≃ 44,620. The measurements were performed at large experimental facilities in Sweden, Australia, and Japan.We find that the measured pressure data are contaminated by the artificial background noise induced by test section and are also affected by the flow boundary conditions. By analyzing data from different wind tunnels acquired at the same Reynolds number, we evaluate the effect of background noises and boundary conditions on the pressure statistics. We also compare the experimental results with results of direct numerical simulations and discuss differences in boundary conditions between real and simulated wind tunnels.

  • 233.
    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.

  • 234.
    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.

  • 235.
    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.

  • 236.
    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.

  • 237.
    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.

  • 238. Vinuesa, Ricardo
    et al.
    Bobke, Alexandra
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics.
    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. 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. 28Article 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. (Fluid Dyn. Res. 41:021401, 2009) and the one by Nickels (J. Fluid Mech. 521:217–239, 2004), and the other onebased on the condition of vanishing mean velocity gradient. Additionally, a new method is introduced based on the diagnostic plot concept by Alfredsson et al. (Phys. Fluids 23:041702, 2011). The boundary layers developing over the suction and pressure sides of a NACA4412 wing section, extracted from a directnumerical simulation at chord Reynolds number Rec = 400, 000, is used as the test case, besides other numerical and experimental data from favorable, zero and adverse pressure-gradient flat-plate turbulent boundary layers. We find that all the methods produce robust results with mild or moderate pressure gradients, although the composite-profile techniques require data preparation, including initial estimations of fitting parameters and data truncation. Stronger pressure gradients (with a Rotta–Clauser pressure-gradient parameter β larger than around 7) lead to inconsistent results in all the techniques except the diagnosticplot. This method also has the advantage of providing an objective way of defining the point where the mean streamwise velocity is 99% of the edge velocity, and shows consistent results in a wide range of pressure gradient conditions, as well as flow histories. Collapse of intermittency factors obtained from a wide range of pressure-gradient and Re conditions on the wing further highlightsthe robustness of the diagnostic plot method to determine the boundary layert hickness (equivalent to δ99 ) and the edge velocity in pressure gradient turbulent boundary layers.

  • 239.
    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.

  • 240.
    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.

  • 241.
    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.

  • 242.
    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.

  • 243. Vinuesa, Ricardo
    et al.
    Rozier, Paul H.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Nagib, Hassan M.
    Experiments and Computations of Localized Pressure Gradients with Different History Effects2014In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 52, no 2, p. 368-384Article in journal (Refereed)
    Abstract [en]

    The study of high-Reynolds-number wall-bounded turbulent flows has become a very active area of research in the past decade, where several recent results have challenged current understanding. In this study, four different localized pressure gradient configurations are characterized by computing them using four Reynolds-averaged Navier-Stokes turbulence models (Spalart-Allmaras, k-epsilon, shear stress transport, and the Reynolds stress model) and comparing their predictions with experimental measurements of mean flow quantities and wall shear stress. The pressure gradients were imposed on high-Reynolds-number, two-dimensional turbulent boundary layers developing on a flat plate by changing the ceiling geometry of the test section. The computations showed that the shear stress transport model produced the best agreement with the experiments. It was found that what is called "numerical transition" (a procedure by which the laminar boundary conditions are transformed into inflow conditions to characterize the initial turbulent profile) causes the major differences between the various models, thereby highlighting the need for models representative of true transition in computational codes. Also, both experiments and computations confirm the nonuniversality of the von Karman coefficient kappa. Finally, a procedure is demonstrated for simpler two-dimensional computations that can be representative of flows with some mild three-dimensional geometries.

  • 244.
    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.

  • 245.
    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.

  • 246.
    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, E-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.

  • 247.
    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.

  • 248.
    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.

  • 249.
    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.

  • 250.
    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. 

23456 201 - 250 of 264
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