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Publications (10 of 245) Show all publications
Amor, C., Perez, J. M., Schlatter, P., Vinuesa, R. & Le Clainche, S. (2020). Soft Computing Techniques to Analyze the Turbulent Wake of a Wall-Mounted Square Cylinder. In: Alvarez, FM Lora, AT Munoz, JAS Quintian, H Corchado, E (Ed.), 14th International Conference on Soft Computing Models in Industrial and Environmental Applications, SOCO 2019: . Paper presented at 14th International Conference on Soft Computing Models in Industrial and Environmental Applications (SOCO), MAY 13-15, 2019, Seville, Spain (pp. 577-586). Springer, 950
Open this publication in new window or tab >>Soft Computing Techniques to Analyze the Turbulent Wake of a Wall-Mounted Square Cylinder
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2020 (English)In: 14th International Conference on Soft Computing Models in Industrial and Environmental Applications, SOCO 2019 / [ed] Alvarez, FM Lora, AT Munoz, JAS Quintian, H Corchado, E, Springer, 2020, Vol. 950, p. 577-586Conference paper, Published paper (Refereed)
Abstract [en]

This paper introduces several methods, generally used in fluid dynamics, to provide low-rank approximations. The algorithm describing these methods are mainly based on singular value decomposition (SVD) and dynamic mode decomposition (DMD) techniques, and are suitable to analyze turbulent flows. The application of these methods will be illustrated in the analysis of the turbulent wake of a wall-mounted cylinder, a geometry modeling a skyscraper. A brief discussion about the large and small size structures of the flow will provide the key ideas to represent the general dynamics of the flow using low-rank approximations. If the flow physics is understood, then it is possible to adapt these techniques, or some other strategies, to solve general complex problems with reduced computational cost. The main goal is to introduce these methods as machine learning strategies that could be potentially used in the field of fluid dynamics, and that can be extended to any other research field.

Place, publisher, year, edition, pages
Springer, 2020
Series
Advances in Intelligent Systems and Computing, ISSN 2194-5357 ; 950
Keywords
Soft computing, Fluid dynamics, Turbulence flow, CFD, Data science, POD, DMD
National Category
Computer Systems
Identifiers
urn:nbn:se:kth:diva-263684 (URN)10.1007/978-3-030-20055-8_55 (DOI)000490706700055 ()2-s2.0-85065927402 (Scopus ID)978-3-030-20055-8 (ISBN)978-3-030-20054-1 (ISBN)
Conference
14th International Conference on Soft Computing Models in Industrial and Environmental Applications (SOCO), MAY 13-15, 2019, Seville, Spain
Note

QC 20191108

Available from: 2019-11-08 Created: 2019-11-08 Last updated: 2019-11-08Bibliographically approved
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
Beneitez Galan, M., Duguet, Y., Schlatter, P. & Henningson, D. S. (2019). Edge tracking in spatially developing boundary layer flows. Journal of Fluid Mechanics, 881, 164-181
Open this publication in new window or tab >>Edge tracking in spatially developing boundary layer flows
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 881, p. 164-181Article in journal (Refereed) Published
Abstract [en]

Recent progress in understanding subcritical transition to turbulence is based on the concept of the edge, the manifold separating the basins of attraction of the laminar and the turbulent state. Originally developed in numerical studies of parallel shear flows with a linearly stable base flow, this concept is adapted here to the case of a spatially developing Blasius boundary layer. Longer time horizons fundamentally change the nature of the problem due to the loss of stability of the base flow due to Tollmien-Schlichting (TS) waves. We demonstrate, using a moving box technique, that efficient long-time tracking of edge trajectories is possible for the parameter range relevant to bypass transition, even if the asymptotic state itself remains out of reach. The flow along the edge trajectory features streak switching observed for the first time in the Blasius boundary layer. At long enough times, TS waves co-exist with the coherent structure characteristic of edge trajectories. In this situation we suggest a reinterpretation of the edge as a manifold dividing the state space between the two main types of boundary layer transition, i.e. bypass transition and classical transition.

Place, publisher, year, edition, pages
Cambridge University Press, 2019
Keywords
boundary layer stability, nonlinear dynamical systems, transition to turbulence, Aerodynamics, Boundary layer flow, Boundary layers, Dynamical systems, Parallel flow, Shear flow, Trajectories, Turbulence, Basins of attraction, Blasius boundary layer, Boundary layer stabilities, Boundary layer transitions, Classical transition, Subcritical transition, Tollmien-Schlichting waves, Atmospheric thermodynamics, boundary layer, fluid dynamics, fluid flow, nonlinearity
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-263766 (URN)10.1017/jfm.2019.763 (DOI)2-s2.0-85074285559 (Scopus ID)
Note

QC 20191112

Available from: 2019-11-12 Created: 2019-11-12 Last updated: 2019-11-12Bibliographically 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
Research subject
Applied and Computational Mathematics, Numerical Analysis
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: 2019-11-26Bibliographically 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
Straub, S., Forooghi, P., Marocco, L., Wetzel, T., Vinuesa, R., Schlatter, P. & Frohnapfel, B. (2019). The influence of thermal boundary conditions on turbulent forced convection pipe flow at two Prandtl numbers. International Journal of Heat and Mass Transfer, 144, Article ID 118601.
Open this publication in new window or tab >>The influence of thermal boundary conditions on turbulent forced convection pipe flow at two Prandtl numbers
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2019 (English)In: International Journal of Heat and Mass Transfer, ISSN 0017-9310, E-ISSN 1879-2189, Vol. 144, article id 118601Article in journal (Refereed) Published
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).

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Forced convection, Low Prandtl number, Pipe, Thermal boundary conditions, Turbulence, Boundary conditions, Budget control, Isotherms, Large eddy simulation, Numerical models, Nusselt number, Pipe flow, Prandtl number, Reynolds number, Spectral density, Velocity, Convective heat transfer, Isoflux boundary conditions, Isothermal boundary conditions, Mixed type boundary conditions, Turbulent forced convection, Turbulent velocity fields, Heat flux
National Category
Fluid Mechanics and Acoustics
Research subject
Applied and Computational Mathematics, Numerical Analysis; Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-263479 (URN)10.1016/j.ijheatmasstransfer.2019.118601 (DOI)000494883300040 ()2-s2.0-85071569272 (Scopus ID)
Note

QC 20191205

Available from: 2019-12-05 Created: 2019-12-05 Last updated: 2019-12-05Bibliographically 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
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ORCID iD: ORCID iD iconorcid.org/0000-0001-9627-5903

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