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Negi, P. S., Mishra, M., Schlatter, P. & Skote, M. (2019). Bypass transition delay using oscillations of spanwise wall velocity. Physical Review Fluids, 4(6), Article ID 063904.
Open this publication in new window or tab >>Bypass transition delay using oscillations of spanwise wall velocity
2019 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 6, article id 063904Article in journal (Refereed) Published
Abstract [en]

Large eddy simulations are performed to investigate the possibility of bypass transition delay in spatially developing boundary layers. An open loop wall control mechanism is employed which consists of either spatial or temporal oscillations of the spanwise wall velocity. Both spatial and temporal oscillations show a delay in the sharp rise in skin friction coefficient which is characteristic of laminar-turbulent transition. An insight into the mechanism is offered based on a secondary filtering of the continuous Orr-Sommerfeld-Squire (OSQ) modes provided by the Stokes layer, and it is shown that the control mechanism selectively affects the low-frequency penetrating modes of the OSQ spectrum. This perspective clarifies the limitations of the mechanism's capability to create transition delay. Furthermore, we extend the two-mode model of bypass transition proposed by T. Zaki and P. Durbin [j Fluid Mech. 531, 85 (2005)] to cases with wall control and illustrate the selective action of the wall oscillations on the penetrating mode in this simplified case.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-255189 (URN)10.1103/PhysRevFluids.4.063904 (DOI)000472000300002 ()2-s2.0-85069727063 (Scopus ID)
Note

QC 20190904

Available from: 2019-09-04 Created: 2019-09-04 Last updated: 2019-09-04Bibliographically approved
Guemes, A., Vila, C. S., Örlü, R., Vinuesa, R., Schlatter, P., Ianiro, A. & Discetti, S. (2019). Flow organization in the wake of a rib in a turbulent boundary layer with pressure gradient. Experimental Thermal and Fluid Science, 108, 115-124
Open this publication in new window or tab >>Flow organization in the wake of a rib in a turbulent boundary layer with pressure gradient
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2019 (English)In: Experimental Thermal and Fluid Science, ISSN 0894-1777, E-ISSN 1879-2286, Vol. 108, p. 115-124Article in journal (Refereed) Published
Abstract [en]

The effect of a streamwise pressure gradient on the wake developed by wall-attached square ribs in a turbulent boundary layer is investigated experimentally. Favourable-, adverse- and zero-pressure-gradient conditions (FPG, APG and ZPG, respectively) are reproduced at matched friction Reynolds number and non-dimensional rib height. Flow-field measurements are carried out by means of Particle Image Velocimetry (PIV). Turbulence statistics are extracted at high resolution using an Ensemble Particle Tracking Velocimetry approach. Modal analysis is performed with Proper Orthogonal Decomposition (POD). We demonstrate that a non-dimensional expression of the pressure gradient and shear stress is needed to quantify the pressure-gradient effects in the wake developing past wall-attached ribs. We suggest the Clauser pressure-gradient parameter beta, commonly used in the literature for the characterization of turbulent boundary layers under the effect of a pressure gradient, as a suitable parameter. The results show that, in presence of an adverse pressure gradient, the recirculation region downstream of the rib is increased in size, thus delaying the reattachment, and that the peak of turbulence intensity and the shed eddies are shifted towards larger wall-normal distances than in the ZPG case. The observed changes with respect to the ZPG configuration appear more intense for larger magnitude of beta, which are more likely to be obtained in APG than in FPG due to the reduced skin friction and increased displacement thickness.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE INC, 2019
Keywords
Turbulent boundary layer, Pressure gradient flows, Ribs
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-260990 (URN)10.1016/j.expthermflusci.2019.05.022 (DOI)000484651400012 ()2-s2.0-85068442439 (Scopus ID)
Note

QC 20191003

Available from: 2019-10-03 Created: 2019-10-03 Last updated: 2019-10-16Bibliographically approved
Otero, E., Gong, J., Min, M., Fischer, P., Schlatter, P. & Laure, E. (2019). OpenACC acceleration for the PN-PN-2 algorithm in Nek5000. Journal of Parallel and Distributed Computing, 132, 69-78
Open this publication in new window or tab >>OpenACC acceleration for the PN-PN-2 algorithm in Nek5000
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2019 (English)In: Journal of Parallel and Distributed Computing, ISSN 0743-7315, E-ISSN 1096-0848, Vol. 132, p. 69-78Article in journal (Refereed) Published
Abstract [en]

Due to its high performance and throughput capabilities, GPU-accelerated computing is becoming a popular technology in scientific computing, in particular using programming models such as CUDA and OpenACC. The main advantage with OpenACC is that it enables to simply port codes in their "original" form to GPU systems through compiler directives, thus allowing an incremental approach. An OpenACC implementation is applied to the CFD code Nek5000 for simulation of incompressible flows, based on the spectral-element method. The work follows up previous implementations and focuses now on the P-N-PN-2 method for the spatial discretization of the Navier-Stokes equations. Performance results of the ported code show a speed-up of up to 3.1 on multi-GPU for a polynomial order N > 11.

Place, publisher, year, edition, pages
Academic Press, 2019
Keywords
Nek5000; OpenACC; GPU programming; Spectral element method; High performance computing
National Category
Computer and Information Sciences
Identifiers
urn:nbn:se:kth:diva-253811 (URN)10.1016/j.jpdc.2019.05.010 (DOI)000476580400006 ()2-s2.0-85066835225 (Scopus ID)
Funder
EU, Horizon 2020Swedish e‐Science Research CenterSwedish Foundation for Strategic Research
Note

QC 20190625

Available from: 2019-06-18 Created: 2019-06-18 Last updated: 2019-08-16Bibliographically approved
Offermans, N., Peplinski, A., Marin, O., Merzari, E. & Schlatter, P. (2019). Performance of preconditioners for large-scale simulations using Nek5000. In: : . Paper presented at ICOSAHOM18 conference, July 9 - 13, 2018, London, United Kingdom.
Open this publication in new window or tab >>Performance of preconditioners for large-scale simulations using Nek5000
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2019 (English)Conference paper, Published paper (Refereed)
Abstract [en]

BoomerAMG, the algebraic multigrid solver from the hypre library, is used to solve a coarse grid problem which is part of the preconditioning strategy for thepressure equation arising from the numerical resolution of the Navier–Stokes equations. A set of optimal parameters for the setup phase is determined and used for selected strong scaling tests on two different supercomputers, namely Mira and Hazel Hen, on up to 131, 072 compute cores. The results are compared to an existing algebraic multigrid solver, designed specifically for the coarse gridproblem at hand. It is shown that the BoomerAMG solver is fast and scalable, and that performance depends on the computer architecture. The test cases considered are the turbulent flow past a NACA4412 airfoil and the turbulent flow inside wire-tapped pin bundles.

National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-251632 (URN)
Conference
ICOSAHOM18 conference, July 9 - 13, 2018, London, United Kingdom
Note

QC 20190520

Available from: 2019-05-16 Created: 2019-05-16 Last updated: 2019-05-20Bibliographically approved
Srinivasan, P. A., Guastoni, L., Azizpour, H., Schlatter, P. & Vinuesa, R. (2019). Predictions of turbulent shear flows using deep neural networks. Physical Review Fluids, 4(5), Article ID 054603.
Open this publication in new window or tab >>Predictions of turbulent shear flows using deep neural networks
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2019 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 5, article id 054603Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-252606 (URN)10.1103/PhysRevFluids.4.054603 (DOI)000467744500004 ()
Note

QC 20190610

Available from: 2019-06-10 Created: 2019-06-10 Last updated: 2019-06-10Bibliographically approved
Dogan, E., Örlü, R., Gatti, D., Vinuesa, R. & Schlatter, P. (2019). Quantification of amplitude modulation in wall-bounded turbulence. Paper presented at Inernational Camp-Style Seminar on Dynamics of Wall-Bounded Shear Flows, AUG 31-SEP 02, 2016, Kyoto, JAPAN. Fluid Dynamics Research, 51(1), Article ID 011408.
Open this publication in new window or tab >>Quantification of amplitude modulation in wall-bounded turbulence
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2019 (English)In: Fluid Dynamics Research, ISSN 0169-5983, E-ISSN 1873-7005, Vol. 51, no 1, article id 011408Article in journal (Refereed) Published
Abstract [en]

Many recent investigations on the scale interactions in wall-bounded turbulent flows focus on describing so-called amplitude modulation, the phenomenon that deals with the influence of large scales in the outer region on the amplitude of the small-scale fluctuations in the near-wall region. The present study revisits this phenomenon regarding two aspects, namely the method for decomposing the scales and the quantification of the modulation. First, the paper presents a summary of the literature that has dealt with either or both aspects. Second, for decomposing the scales, different spectral filters (temporal, spatial or both) and empirical mode decomposition (EMD) are evaluated and compared. The common data set is a well-resolved large-eddy simulation that offers a wide range of Reynolds numbers spanning Re-theta = 880-8200. The quantification of the amplitude modulation is discussed for the resulting scale components. Particular focus is given to evaluate the efficacy of the various filters to separate scales for the range of Reynolds numbers of interest. Different to previous studies, the different methods have been evaluated using the same data set, thereby allowing a fair comparison between the various approaches. It is observed that using a spectral filter in the spanwise direction is an effective approach to separate the small and large scales in the flow, even at comparably low Reynolds numbers, whereas filtering in time should be approached with caution in the low-to-moderate Re range. Additionally, using filters in both spanwise and time directions, which would separate both wide and long-living structures from the small and fast scales, gives a cleaner image for the small-scales although the contribution to the scales interaction from that filter implementation has been found negligible. Applying EMD to decompose the scales gives similar results to Fourier filters for the energy content of the scales and thereby for the quantification of the amplitude modulation using the decomposed scales. No direct advantage of EMD over classical Fourier filters could be seen. Potential issues regarding different decomposition methods and different definitions of the amplitude modulation are also discussed.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2019
Keywords
amplitude modulation, turbulent boundary layer, scale interaction
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-243936 (URN)10.1088/1873-7005/aaca81 (DOI)000456203700009 ()2-s2.0-85061429605 (Scopus ID)
Conference
Inernational Camp-Style Seminar on Dynamics of Wall-Bounded Shear Flows, AUG 31-SEP 02, 2016, Kyoto, JAPAN
Note

QC 20190306

Available from: 2019-03-06 Created: 2019-03-06 Last updated: 2019-03-18Bibliographically approved
Rinaldi, E., Canton, J. & Schlatter, P. (2019). The vanishing of strong turbulent fronts in bent pipes. Journal of Fluid Mechanics, 866, 487-502
Open this publication in new window or tab >>The vanishing of strong turbulent fronts in bent pipes
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 866, p. 487-502Article in journal (Refereed) Published
Abstract [en]

Isolated patches of turbulence in transitional straight pipes are sustained by a strong instability at their upstream front, where the production of turbulent kinetic energy (TKE) is up to five times higher than in the core. Direct numerical simulations presented in this paper show no evidence of such strong fronts if the pipe is bent. We examine the temporal and spatial evolution of puffs and slugs in a toroidal pipe with pipe-to-torus diameter ratio delta = D/d = 0.01 at several subcritical Reynolds numbers. Results show that the upstream overshoot of TKE production is at most one-and-a-half times the value in the core and that the average cross-flow fluctuations at the front are up to three times lower if compared to a straight pipe, while attaining similar values in the core. Localised turbulence can be sustained at smaller energies through a redistribution of turbulent fluctuations and vortical structures by the in-plane Dean motion of the mean flow. This asymmetry determines a strong localisation of TKE production near the outer bend, where linear and nonlinear mechanisms optimally amplify perturbations. We further observe a substantial reduction of the range of Reynolds numbers for long-lived intermittent turbulence, in agreement with experimental data from the literature. Moreover, no occurrence of nucleation of spots through splitting could be detected in the range of parameters considered. Based on the present results, we argue that this mechanism gradually becomes marginal as the curvature of the pipe increases and the transition scenario approaches a dynamical switch from subcritical to supercritical.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
nonlinear instability, pipe flow boundary layer, transition to turbulence
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-248058 (URN)10.1017/jfm.2019.120 (DOI)000461032400001 ()2-s2.0-8506290521 (Scopus ID)
Note

QC 20190426

Available from: 2019-04-26 Created: 2019-04-26 Last updated: 2019-04-26Bibliographically approved
Sasaki, K., Vinuesa, R., Cavalieri, A. V. G., Schlatter, P. & Henningson, D. S. (2019). Transfer functions for flow predictions in wall-bounded turbulence. Journal of Fluid Mechanics, 864, 708-745
Open this publication in new window or tab >>Transfer functions for flow predictions in wall-bounded turbulence
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2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 864, p. 708-745Article in journal (Refereed) Published
Abstract [en]

Three methods are evaluated to estimate the streamwise velocity fluctuations of a zero-pressure-gradient turbulent boundary layer of momentum-thickness-based Reynolds number up to using as input velocity fluctuations at different wall-normal positions. A system identification approach is considered where large-eddy simulation data are used to build single and multiple-input linear and nonlinear transfer functions. Such transfer functions are then treated as convolution kernels and may be used as models for the prediction of the fluctuations. Good agreement between predicted and reference data is observed when the streamwise velocity in the near-wall region is estimated from fluctuations in the outer region. Both the unsteady behaviour of the fluctuations and the spectral content of the data are properly predicted. It is shown that approximately 45 % of the energy in the near-wall peak is linearly correlated with the outer-layer structures, for the reference case. These identified transfer functions allow insight into the causality between the different wall-normal locations in a turbulent boundary layer along with an estimation of the tilting angle of the large-scale structures. Differences in accuracy of the methods (single- and multiple-input linear and nonlinear) are assessed by evaluating the coherence of the structures between wall-normally separated positions. It is shown that the large-scale fluctuations are coherent between the outer and inner layers, by means of an interactions which strengthens with increasing Reynolds number, whereas the finer-scale fluctuations are only coherent within the near-wall region. This enables the possibility of considering the wall-shear stress as an input measurement, which would more easily allow the implementation of these methods in experimental applications. A parametric study was also performed by evaluating the effect of the Reynolds number, wall-normal positions and input quantities considered in the model. Since the methods vary in terms of their complexity for implementation, computational expense and accuracy, the technique of choice will depend on the application under consideration. We also assessed the possibility of designing and testing the models at different Reynolds numbers, where it is shown that the prediction of the near-wall peak from wall-shear-stress measurements is practically unaffected even for a one order of magnitude change in the corresponding Reynolds number of the design and test, indicating that the interaction between the near-wall peak fluctuations and the wall is approximately Reynolds-number independent. Furthermore, given the performance of such methods in the prediction of flow features in turbulent boundary layers, they have a good potential for implementation in experiments and realistic flow control applications, where the prediction of the near-wall peak led to correlations above 0.80 when wall-shear stress was used in a multiple-input or nonlinear scheme. Errors of the order of 20 % were also observed in the determination of the near-wall spectral peak, depending on the employed method.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
turbulence modelling, turbulent boundary layers
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-245119 (URN)10.1017/jfm.2019.27 (DOI)000458488900001 ()2-s2.0-85061456245 (Scopus ID)
Note

QC 20190315

Available from: 2019-03-15 Created: 2019-03-15 Last updated: 2019-03-15Bibliographically approved
Rezaeiravesh, S., Vinuesa, R., Liefvendahl, M. & Schlatter, P. (2018). Assessment of uncertainties in hot-wire anemometry and oil-film interferometry measurements for wall-bounded turbulent flows. European journal of mechanics. B, Fluids, 72, 57-73
Open this publication in new window or tab >>Assessment of uncertainties in hot-wire anemometry and oil-film interferometry measurements for wall-bounded turbulent flows
2018 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 72, p. 57-73Article in journal (Refereed) Published
Abstract [en]

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

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Hot-wire anemometry, Oil-film interferometry, Uncertainty quantification, Wall-bounded turbulence
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-228692 (URN)10.1016/j.euromechflu.2018.04.012 (DOI)000447570200005 ()2-s2.0-85047057608 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20180530

Available from: 2018-05-30 Created: 2018-05-30 Last updated: 2018-11-06Bibliographically approved
Wang, Z., Örlü, R., Schlatter, P. & Chung, Y. M. (2018). Direct numerical simulation of a turbulent 90° bend pipe flow. International Journal of Heat and Fluid Flow, 73, 199-208
Open this publication in new window or tab >>Direct numerical simulation of a turbulent 90° bend pipe flow
2018 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 73, p. 199-208Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulation (DNS) has been performed for a spatially developing 90° bend pipe flow to investigate the unsteady flow motions downstream of the bend. A recycling method is implemented to generate a fully-developed turbulent inflow condition. The Reynolds number of the pipe flow is ReD=5300 and the bend curvature is γ=0.4. A long straight pipe section (40D) is attached in the downstream of the bend to allow the flow to develop. Flow oscillations downstream of the bend are measured using several methods, and the corresponding oscillation frequencies are estimated. It is found that different characteristic frequencies are obtained from various flow measurements. The stagnation point movement and single-point velocity measurements may not be good measures to determine the swirl-switching frequency. The oscillations of the lateral pressure force on the pipe wall and half-sided mass flow rate are proposed to be a more unambiguous measure of the unsteady flow motions downstream of the bend. 

Place, publisher, year, edition, pages
Elsevier B.V., 2018
Keywords
90 degrees bend, Conditional averaging, Curved pipe, DNS, Swirl switching, Turbulent pipe flow, Direct numerical simulation, Flow rate, Numerical models, Pipe flow, Reynolds number, Turbulent flow, Characteristic frequencies, Curved pipes, Oscillation frequency, Stagnation points, Turbulent inflow conditions, Oscillating flow
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-236662 (URN)10.1016/j.ijheatfluidflow.2018.08.003 (DOI)000452345000017 ()2-s2.0-85052536769 (Scopus ID)
Note

Export Date: 22 October 2018; Article; CODEN: IJHFD; Correspondence Address: Chung, Y.M.; School of Engineering and Centre for Scientific Computing, University of WarwickUnited Kingdom; email: y.m.chung@warwick.ac.uk; Funding details: University of Warwick; Funding details: EP/L000261/1, EPSRC, Engineering and Physical Sciences Research Council; Funding text: This work has been supported by the Engineering and Physical Sciences Research Council grant no EP/L000261/1 . The authors would like to thank Professor Paul Fischer for the help in using Nek5000 . Simulations were performed on ARCHER, the UK National Supercomputing Service. This work also used the HPC facilities (Tinis) at the Centre for Scientific Computing, University of Warwick. QC 20181113

Available from: 2018-11-13 Created: 2018-11-13 Last updated: 2019-10-18Bibliographically approved
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