Imperial College London

DrSylvainLaizet

Faculty of EngineeringDepartment of Aeronautics

Reader in Computational Fluid Mechanics
 
 
 
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Contact

 

+44 (0)20 7594 5045s.laizet Website

 
 
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Location

 

339City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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78 results found

Giannenas A, Laizet S, 2021, A simple and scalable Immersed Boundary Method for high-fidelity simulations of fixed and moving objects on a Cartesian mesh, Applied Mathematical Modelling: simulation and computation for engineering and environmental systems, Vol: 99, Pages: 606-627, ISSN: 0307-904X

A simple and scalable Immersed Boundary Method based on cubic spline reconstructionsis presented for high-fidelity simulations of immersed objects in a turbulent flow on aCartesian mesh. The novelty of the proposed IBM lies in its simplicity, accuracy, scalabilityand its ability to simulate both fixed and moving immersed objects. The new IBM is thoroughly validated against a 1D benchmark, with the 2D flow around a cylinder at Re = 40and 300 and the 3D flow around a sphere at Re = 300 and Re = 3700. Convergence studies and detailed error maps showing the spatial distribution of the velocity L2-Norm errorcompared to a spectral reference solution for the cylinders at Re = 40 show the robustnessof the proposed method. The cost and performance of the method are also presented formulti-billion mesh node simulations with up to 65,536 computational cores. The potentialof the method in handling multiple moving objects for practical applications is demonstrated with the control of a square bluff body wake by two rear pitching flaps.

Journal article

Mahfoze O, Laizet S, 2021, Non-explicit Large Eddy Simulations of turbulent channel flows from Reτ= 180 up to Reτ= 5,200, Computers and Fluids, Vol: 228, Pages: 1-19, ISSN: 0045-7930

This numerical study based on high-order finite-difference schemes presents LES-NWR (Large Eddy Simulation with near-wall resolution) of turbulent channel flows up to Reτ = 5200 using non-explicit approaches for which numerical dissipation is introduced via the discretisation of viscous terms of the Navier-Stokes equations. These models are cheaper than explicit LES models as no extra terms are needed in the equations toaccount for the contribution of the unresolved scales. The Approximate Deconvolution Method with Relaxation Term (ADM-RT) approach is also assessed and the present LES data are compared with reference Direct Numerical Simulations (DNS) data for first and second order moments as well as for the turbulent kinetic energy budget. Even if the viscous sublayer is not resolved, the proposed non-explicit LES approaches are in excellent agreement with the reference DNS data and to a certain extent with the ADM-RT model, for a fraction of the cost of the DNS. The proposed non-explicit models, for which the possibility to locally adjust the added numerical dissipation is investigated, are straightforward to implement and come with a negligible additional computational cost while the ADM-RT model is 30% more expensive than the non-explicit models.The parameters of the models are defined before the simulations and no modifications of the parameters are needed when the Reynolds number and the mesh resolution are changed. It is shown that a modulation of the magnitude of the numerical dissipation in time and in space is not necessarily needed, at least for the mesh resolutions and Reynolds numbers considered in the present study. The main conclusion is that non-explicit models can replace advantageously explicit models when high-order finite-difference methods are used. They can generate accurate LES-NWR of turbulent channel flows over a wide range of Reynolds numbers at a fraction of a cost of DNS.

Journal article

Hamzehloo A, Lusher D, Laizet S, Sandham Net al., 2021, Direct numerical simulation of compressible turbulence in acounter-flow channel configuration, Physical Review Fluids, ISSN: 2469-990X

Counter-flow configurations, whereby two streams of fluid are brought together from oppositedirections, are highly efficient mixers due to the high turbulence intensities that can be maintained.In this paper, a simplified version of the problem is introduced that is amenable to direct numericalsimulation. The resulting turbulent flow problem is confined between two walls, with one non-zeromean velocity component varying in the space direction normal to the wall, corresponding to asimple shear flow. Compared to conventional channel flows, the mean flow is inflectional and themaximum turbulence intensity relative to the maximum mean velocity is nearly an order of magnitude higher. The numerical requirements and turbulence properties of this configuration are firstdetermined. The Reynolds shear stress is required to vary linearly by the imposed forcing, witha peak at the channel centreline. A similar behaviour is observed for the streamwise Reynoldsstress, the budget of which shows an approximately uniform distribution of dissipation, with largecontributions from production, pressure-strain and turbulent diffusion. A viscous sublayer is obtained near the walls and with increasing Reynolds number small-scale streaks in the streamwisemomentum are observed, superimposed on the large-scale structures that buffet this region. Whenthe peak local mean Mach number reaches 0.55, turbulent Mach numbers of 0.6 are obtained,indicating that this flow configuration can be useful to study compressibility effects on turbulence.

Journal article

Khojasteh AR, Yang Y, Heitz D, Laizet Set al., 2021, Lagrangian coherent track initialisation, Physics of Fluids, ISSN: 1070-6631

Advances in time-resolved three-dimensional Particle Tracking Velocimetry (4D-PTV)techniques have been consistently revealed more accurate Lagrangian particle motions.A novel track initialisation technique as a complementary part of 4D-PTV, based on localtemporal and spatial coherency of neighbour trajectories, is proposed. The proposed Lagrangian Coherent Track Initialisation (LCTI) applies physics-based Finite Time LyapunovExponent (FTLE) to build four frame coherent tracks. We locally determine LagrangianCoherent Structures (LCS) among neighbour trajectories by using the FTLE boundaries(i.e., ridges) to distinguish clusters of coherent motions. To evaluate the proposed technique, we created an open-access synthetic Lagrangian and Eulerian dataset of the wakedownstream of a smooth cylinder at a Reynolds number equal to 3900 obtained fromthree-dimensional (3D) Direct Numerical Simulation (DNS). Performance of the proposedmethod based on three characteristic parameters, temporal scale, particle concentration(i.e., density), and noise ratio, showed robust behaviour in finding true tracks compared tothe recent initialisation algorithms. Sensitivity of LCTI to the number of untracked andwrong tracks are also discussed. We address the capability of using the proposed methodas a function of a 4D-PTV scheme in the Lagrangian Particle Tracking (LPT) challenge.We showed that LCTI prevents 4D-PTV divergence in flows with high particle concentrations. Finally, the LCTI behaviour was demonstrated in a jet impingement experiment.LCTI was found to be a reliable tracking tool in complex flow motions, with a strengthrevealed for flows with high velocity and acceleration gradients.

Journal article

Özbay AG, Hamzehloo A, Laizet S, Tzirakis P, Rizos G, Schuller Bet al., 2021, Poisson CNN: Convolutional neural networks for the solution of the Poisson equation on a Cartesian mesh, Data-Centric Engineering, Vol: 2, Pages: 1-31, ISSN: 2632-6736

<jats:title>Abstract</jats:title> <jats:p>The Poisson equation is commonly encountered in engineering, for instance, in computational fluid dynamics (CFD) where it is needed to compute corrections to the pressure field to ensure the incompressibility of the velocity field. In the present work, we propose a novel fully convolutional neural network (CNN) architecture to infer the solution of the Poisson equation on a 2D Cartesian grid with different resolutions given the right-hand side term, arbitrary boundary conditions, and grid parameters. It provides unprecedented versatility for a CNN approach dealing with partial differential equations. The boundary conditions are handled using a novel approach by decomposing the original Poisson problem into a homogeneous Poisson problem plus four inhomogeneous Laplace subproblems. The model is trained using a novel loss function approximating the continuous <jats:inline-formula> <jats:alternatives> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S2632673621000071_inline1.png" /> <jats:tex-math>$ {L}^p $</jats:tex-math> </jats:alternatives> </jats:inline-formula> norm between the prediction and the target. Even when predicting on grids denser than previously encountered, our model demonstrates encouraging capacity to reproduce the correct solution profile. The proposed model, which outperforms well-known neural network models, can be included in a CFD solver to help with solving the Poisson equation. Analytical test cases indicate that our CNN architecture is capable of predicting the correct solution of a Poisson problem with mean percentage errors below 10%, an improvement by comparison to the first step of conventional iterative methods. Predictions from our model, used as the initial guess to iterative algorithms like Multigrid, can reduce the root mean square error af

Journal article

Hamzehloo A, Bartholomew P, Laizet S, 2021, Direct numerical simulations of incompressible Rayleigh–Taylor instabilities at low and medium Atwood numbers, Physics of Fluids, Vol: 33, Pages: 1-23, ISSN: 1070-6631

Direct numerical simulations of two-dimensional (2D) and three-dimensional (3D), single-mode and multi-mode, incompressible immiscible Rayleigh–Taylor (RT) instabilities are performed using a phase-field approach and high-order finite-difference schemes. Various combinations of Atwood number, Reynolds number, surface tension, and initial perturbation amplitude are investigated. It is found that at high Reynolds numbers, the surface tension, if significant, could prevent the formation of Kelvin–Helmholtz type instabilities within the bubble region. A relationship is proposed for the vertical distance of the bubble and spike vs the Atwood number. The spike and bubble reaccelerate after reaching a temporary plateau due to the reduction of the friction drag as a result of the formation of the spike vortices and also the formation of a momentum jet traveling upward within the bubble region. The interface for a 3D single-mode instability grows exponentially; however, a higher Reynolds number and/or a lower Atwood number could result in a noticeably larger surface area after the initial growth. It is also shown that a 3D multi-mode RT instability initially displays an exponential interface growth rate similar to single-mode RT instabilities. Due to the collapse and merging of individual single-mode instabilities, the interface area for a multi-mode RT instability is strongly dependent to the mesh resolution after the exponential growth rate. However, the ratio of kinetic energy over released potential energy exhibits an almost steady state after the initial exponential growth, with values around 0.4, independently of the mesh resolution.

Journal article

Frantz R, Deskos G, Laizet S, Silvestrini Jet al., 2021, High-fidelity simulations of gravity currents using a high-order finite-difference spectral vanishing viscosity approach, Computers and Fluids, Vol: 221, Pages: 1-18, ISSN: 0045-7930

This numerical work investigates the potential of a high-order finite-difference spectralvanishing viscosity approach to simulate gravity currents at high Reynolds numbers.The method introduces targeted numerical dissipation at small scales through alteringthe discretisation of the second derivatives of the viscous terms in the incompressibleNavier-Stokes equations to mimic the spectral vanishing viscosity (SVV) operator, originally designed for the regularisation of spectral element method (SEM) solutions of pureadvection problems. Using a sixth-order accurate finite-difference scheme, the adoption ofthe SVV method is straightforward and comes with a negligible additional computationalcost. In order to assess the ability of this high-order finite-difference spectral vanishingviscosity approach, we performed large-eddy simulations (LES) of a gravity current ina channelised lock-exchange set-up with our SVV model and with the well-known explicit static and dynamic Smagorinsky sub-grid scale (SGS) models. The obtained dataare compared with a direct numerical simulation (DNS) based on more than 800 millionmesh nodes, and with experimental measurements. A framework for the energy budgetis introduced to investigate the behaviour of the gravity current. First, it is found thatthe DNS is in good agreement with the experimental data for the evolution of the frontlocation and velocity field as well as for the stirring and mixing inside the gravity current. Secondly, the LES performed with less than 0.4% of the total number of mesh nodescompared to the DNS, can reproduce the main features of the gravity currents, with theSVV model yielding slightly more accurate results. It is also found that the dynamicSmagorinsky model performs better than its static version. For the present study, thestatic and dynamic Smagorinsky models are 1.8 and 2.5 times more expensive than theSVV model, because the latter does not require the calculation of explicit SGS terms inthe Navier-Stokes equa

Journal article

Voet L, Ahlfeld R, Gaymann A, Laizet S, Montomoli Fet al., 2021, A hybrid approach combining DNS and RANS simulations to quantify uncertainties in turbulence modelling, Applied Mathematical Modelling: simulation and computation for engineering and environmental systems, Vol: 89, Pages: 885-906, ISSN: 0307-904X

Uncertainty quantification (UQ) has recently become an important part of the design process of countless engineering applications. However, up to now in computational fluid dynamics (CFD) the errors introduced by the turbulent viscosity models in Reynolds-Averaged Navier Stokes (RANS) models have often been neglected in UQ studies. Although Direct Numerical Simulations (DNS) are physically correct, obtaining a large enough set of DNS data for UQ studies is currently computationally intractable. UQ based only on RANS simulations or on DNS thus leads to physical and statistical inaccuracies in the output probability distribution functions (PDF). Therefore, three hybrid methods combining both RANS simulations and DNS to perform non-intrusive UQ are suggested in this work. Low-fidelity RANS simulations and high-fidelity DNS are combined to give an approximation of an output PDF using the advantages of both data sets: the physical accuracy via the DNS and the statistical accuracy via the RANS simulations. The hybrid methods are applied to the flow over 2D periodically arranged hills. It is shown that the Gaussian CoKriging (GCK) method is the best hybrid method and that a non-intrusive hybrid UQ approach combining both DNS and RANS simulations is possible, with both physically more accurate and statistically better PDF.

Journal article

Hamzehloo A, Lusher D, Laizet S, Sandham Net al., 2021, On the performance of WENO/TENO schemes to resolve turbulence in DNS/LES of high-speed compressible flows, International Journal for Numerical Methods in Fluids, Vol: 93, Pages: 176-196, ISSN: 0271-2091

High‐speed compressible turbulent flows typically contain discontinuities and have been widely modelled using Weighted Essentially Non‐Oscillatory (WENO) schemes due to their high‐order accuracy and sharp shock capturing capability. However, such schemes may damp the small scales of turbulence, and result in inaccurate solutions in the context of turbulence‐resolving simulations. In this connection, the recently‐developed Targeted Essentially Non‐Oscillatory (TENO) schemes, including adaptive variants, may offer significant improvements. The present study aims to quantify the potential of these new schemes for a fully‐turbulent supersonic flow. Specifically, DNS of a compressible turbulent channel flow with M = 1: 5 and Re τ = 222 is conducted using OpenSBLI, a high‐order finite difference CFD framework. This flow configuration is chosen to decouple the effect of flow discontinuities and turbulence and focus on the capability of the aforementioned high‐order schemes to resolve turbulent structures. The effect of the spatial resolution in different directions and coarse grid implicit LES are also evaluated against theWALE LES model. The TENO schemes are found to exhibit significant performance improvements over the WENO schemes in terms of the accuracy of the statistics and the resolution of the three‐dimensional vortical structures. The 6th order adaptive TENO scheme is found to produce comparable results to those obtained with non‐dissipative 4th and 6th order central schemes and reference data obtained with spectral methods. Although the most computationally expensive scheme, it is shown that this adaptive scheme can produce satisfactory results if used as an implicit LES model.

Journal article

Wang C, Muñóz-Simon A, Deskos G, Laizet S, Palacios R, Campagnolo F, Bottasso CLet al., 2020, Code-to-code-to-experiment validation of LES-ALM wind farm simulators, Journal of Physics: Conference Series, Vol: 1618, Pages: 1-8, ISSN: 1742-6588

The aim of this work is to present a detailed code-to-code comparison of two Large-Eddy Simulation (LES) solvers. Corresponding experimental measurements are used as a reference to validate the quality of the CFD simulations. The comparison highlights the effects of solver order on the solutions, and it tries to answer the question of whether a high order solver is necessary to capture the main characteristics of a wind farm. Both solvers were used on different grids to study their convergence behavior. While both solvers show a good match with experimental measurements, it appears that the low order solver is more accurate and substantially cheaper in terms of computational cost.

Journal article

Bartholomew P, Deskos G, Frantz R, Schuch F, Lamballais E, Laizet Set al., 2020, Xcompact3D: An open-source framework for solving turbulence problems on a Cartesian mesh, SoftwareX, Vol: 12, ISSN: 2352-7110

Xcompact3D is a Fortran 90–95 open-source framework designed for fast and accurate simulations of turbulent flows, targeting CPU-based supercomputers. It is an evolution of the flow solver Incompact3D which was initially designed in France in the mid-90’s for serial processors to solve the incompressible Navier–Stokes equations. Incompact3D was then ported to parallel High Performance Computing (HPC) systems in the early 2010’s. Very recently the capabilities of Incompact3D have been extended so that it can now tackle more flow regimes (from incompressible flows to compressible flows at low Mach numbers), resulting in the design of a new user-friendly framework called Xcompact3D. The present manuscript presents an overview of Xcompact3D with a particular focus on its functionalities, its ready-to-run simulations and a few case studies to demonstrate its impact.

Journal article

Xiao H, Wu J-L, Laizet S, Duan Let al., 2020, Flows over periodic hills of parameterized geometries: a dataset for data-driven turbulence modeling from direct simulations, Computers and Fluids, Vol: 200, ISSN: 0045-7930

Computational fluid dynamics models based on Reynolds-averaged Navier–Stokes equations with turbulence closures still play important roles in engineering design and analysis. However, the development of turbulence models has been stagnant for decades. With recent advances in machine learning, data-driven turbulence models have become attractive alternatives worth further explorations. However, a major obstacle in the development of data-driven turbulence models is the lack of training data. In this work, we survey currently available public turbulent flow databases and conclude that they are inadequate for developing and validating data-driven models. Rather, we need more benchmark data from systematically and continuously varied flow conditions (e.g., Reynolds number and geometry) with maximum coverage in the parameter space for this purpose. To this end, we perform direct numerical simulations of flows over periodic hills with varying slopes, resulting in a family of flows over periodic hills which ranges from incipient to mild and massive separations. We further demonstrate the use of such a dataset by training a machine learning model that predicts Reynolds stress anisotropy based on a set of mean flow features. We expect the generated dataset, along with its design methodology and the example application presented herein, will facilitate development and comparison of future data-driven turbulence models.

Journal article

Deskos G, Laizet S, Palacios R, 2020, WInc3D: a novel framework for turbulence-resolving simulations of wind farm wake interactions, Wind Energy, Vol: 23, Pages: 779-794, ISSN: 1095-4244

A fast and efficient turbulence‐resolving computational framework, dubbed as WInc3D (Wind Incompressible 3‐Dimensional solver), is presented and validated in this paper. WInc3D offers a unified, highly scalable, high‐fidelity framework for the study of the flow structures and turbulence of wind farm wakes and their impact on the individual turbines' power and loads. Its unique properties lie on the use of higher‐order numerical schemes with “spectral‐like” accuracy, a highly efficient parallelisation strategy which allows the code to scale up to O(104) computing processors and software compactness (use of only native solvers/models) with virtually no dependence to external libraries. The work presents an overview of the current modelling capabilities along with model validation. The presented applications demonstrate the ability of WInc3D to be used for testing farm‐level optimal control strategies using turbine wakes under yawed conditions. Examples are provided for two turbines operating in‐line as well as a small array of 16 turbines operating under “Greedy” and “Co‐operative” yaw angle settings. These large‐scale simulations were performed with up to 8192 computational cores for under 24 hours, for a computational domain discretised with O(109) mesh nodes.

Journal article

Mahfoze O, Moody A, Wynn A, Whalley R, Laizet Set al., 2019, Reducing the skin-friction drag of a turbulent boundary-layer flow with low-amplitude wall-normal blowing within a Bayesian optimisation framework., Physical Review Fluids, Vol: 4, Pages: 094601-1-094601-23, ISSN: 2469-990X

A Bayesian optimisation framework is developed to optimise low-amplitude wall-normal blowing control of a turbulent boundary-layer flow. The Bayesian optimisation framework determines the optimum blowing amplitude and blowing coverage to achieve up to a 5% net-power saving solution within 20 optimisation iterations, requiring 20 Direct Numerical Simulations (DNS). The power input required to generate the low-amplitude wall-normal blowing is measured experimentally for two different types of blowing device, and is used in the simulations to assess control performance. Wall-normal blowing with amplitudes of less than 1% of the free-stream velocity generate a skin-friction drag reduction of up to 76% over the control region, with a drag reduction which persists for up to 650δ0 downstream of actuation (where δ0 is the boundary-layer thickness at the start of the simulation domain). It is shown that it is the slow spatial recovery of the turbulent boundary-layer flow downstream of control which generates the net-power savings in this study. The downstream recovery of the skin-friction drag force is decomposed using the Fukagata-Iwamoto-Kasagi (FIK) identity, which shows that the generation of the net-power savings is due to changes in contributions to both the convection and streamwise development terms of the turbulent boundary-layer flow.

Journal article

Bartholomew P, Laizet S, 2019, A new highly scalable, high-order accurate framework for variable-density flows: application to non-Boussinesq gravity currents, Computer Physics Communications, Vol: 242, Pages: 83-94, ISSN: 0010-4655

This paper introduces a new code “QuasIncompact3D” for solving the variabledensity Navier-Stokes equations in the low-Mach number limit. It is derived from the Incompact3D framework which is designed for incompressible flows [1]. QuasIncompact3D is based on high-order accurate compact finite-differences [2], an efficient 2D domain decomposition [3] and a spectral Poisson solver. The first half of the paper focuses on introducing the low-Mach number governing equations, the numerical methods and the algorithm employed by QuasIncompact3D to solve them. Two approaches to forming the pressure-Poisson equation are presented: one based on an extrapolation that is efficient but limited to low density ratios and another one using an iterative approach suitable for higher density ratios. The scalability of QuasIncompact3D is demonstrated on several TIER-1/0 supercomputers using both approaches, showing good scaling up to 65k cores. Validations for incompressible and variable-density low-Mach number flows us-ing the Taylor-Green vortex and a non-isothermal mixing layer, respectively, as test cases are then presented, followed by simulations of non-Boussinesq gravity currents in two- and three-dimensions. To the authors’ knowledge this is the first investigation of 3D non-Boussinesq gravity currents by means of Direct Numerical Simulation over a relatively long time evolution. It is found that 2D and 3D simulations of gravity currents show differences in the locations of the fronts, specifically that the fronts travel faster in three dimensions, but that it only becomes apparent after the initial stages. Our results also show that the difference in terms of front location decreases the further the flow is from Boussinesq conditions.

Journal article

Pinier B, Mémin E, Laizet S, Lewandowski Ret al., 2019, Stochastic flow approach to model the mean velocity profile of wall-bounded flows, Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol: 99, ISSN: 1539-3755

There is no satisfactory model to explain the mean velocity profile of the whole turbulent layer in canonical wall-bounded flows. In this paper, a mean velocity profile expression is proposed for wall-bounded turbulent flows based on a recently proposed stochastic representation of fluid flows dynamics. This original approach, called modeling under location uncertainty, introduces in a rigorous way a subgrid term generalizing the eddy-viscosity assumption and an eddy-induced advection term resulting from turbulence inhomogeneity. This latter term gives rise to a theoretically well-grounded model for the transitional zone between the viscous sublayer and the turbulent sublayer. An expression of the small-scale velocity component is also provided in the viscous zone. Numerical assessments of the results are provided for turbulent boundary layer flows, pipe flows and channel flows at various Reynolds numbers.

Journal article

Wu J, Sun R, Laizet S, Xiao Het al., 2019, Representation of stress tensor perturbations with application in machine-learning-assisted turbulence modeling, Computer Methods in Applied Mechanics and Engineering, Vol: 346, Pages: 707-726, ISSN: 0045-7825

Numerical simulations based on Reynolds-Averaged Navier–Stokes (RANS) equations are widely used in engineering design and analysis involving turbulent flows. However, RANS simulations are known to be unreliable in many flows of engineering relevance, which is largely caused by model-form uncertainties associated with the Reynolds stresses. Recently, a machine-learning approach has been proposed to quantify the discrepancies between RANS modeled Reynolds stress and the true Reynolds stress. However, it remains a challenge to represent discrepancies in the Reynolds stress eigenvectors in machine learning due to the requirements of spatial smoothness, frame-independence, and realizability. This challenge also exists in the data-driven computational mechanics in general where quantifying the perturbation of stress tensors is needed. In this work, we propose three schemes for representing perturbations to the eigenvectors of RANS modeled Reynolds stresses: (1) discrepancy-based Euler angles, (2) direct-rotation-based Euler angles, and (3) unit quaternions. We compare these metrics by performing a priori and a posteriori tests on two canonical flows: fully developed turbulent flows in a square duct and massively separated flows over periodic hills. The results demonstrate that the direct-rotation-based Euler angles representation lacks spatial smoothness while the discrepancy-based Euler angles representation lacks frame-independence, making them unsuitable for being used in machine-learning-assisted turbulence modeling. In contrast, the representation based on unit quaternion satisfies all the requirements stated above, and thus it is an ideal choice in representing the perturbations associated with the eigenvectors of Reynolds stress tensors. This finding has clear importance for uncertainty quantification and machine learning in turbulence modeling and for data-driven computational mechanics in general.

Journal article

Deskos G, Laizet S, Piggott M, 2019, Turbulence-resolving simulations of wind turbine wakes, Renewable Energy, Vol: 134, Pages: 989-1002, ISSN: 1879-0682

Turbulence-resolving simulations of wind turbine wakes are presented using a high-order flow solver combined with both a standard and a novel dynamic implicit spectral vanishing viscosity (iSVV and dynamic iSVV) model to account for subgrid-scale (SGS) stresses. The numerical solutions are compared against wind tunnel measurements, which include mean velocity and turbulent intensity profiles, as well as integral rotor quantities such as power and thrust coefficients. For the standard (also termed static) case the magnitude of the spectral vanishing viscosity is selected via a heuristic analysis of the wake statistics, while in the case of the dynamic model the magnitude is adjusted both in space and time at each time step. The study focuses on examining the ability of the two approaches, standard (static) and dynamic, to accurately capture the wake features, both qualitatively and quantitatively. The results suggest that the static method can become over-dissipative when the magnitude of the spectral viscosity is increased, while the dynamic approach which adjusts the magnitude of dissipation locally is shown to be more appropriate for a non-homogeneous flow such that of a wind turbine wake.

Journal article

Mahfoze OA, Wynn A, Whalley RD, Laizet Set al., 2019, Bayesian optimisation of intermittent wall blowing in a turbulent boundary layer for net power saving

© 2019 International Symposium on Turbulence and Shear Flow Phenomena, TSFP. All rights reserved. A Bayesian optimisation framework is used to optimise low-amplitude wall-normal blowing control of a turbulent boundary-layer (TBL) flow in order to achieve skin-friction drag reduction and net-power saving. The study is carried out using Direct Numerical Simulations (DNS) and Implicit Large Eddy Simulations (ILES). Control performance is assessed by using the power consumption from two different sets of experimental data from two different types of blowing device. The simulations demonstrate that wall-normal blowing control can generate a local skin-friction drag reduction of up to 75%, which persists far downstream of the control. This slow spatial recovery of the skin-friction coefficient back to its canonical counterpart can generate net-power savings up to 5% in the present study. When combined with DNS or ILES, Bayesian optimisation, with its fast convergence (within a dozen iterations with three parameters to optimise) is an ideal tool to find the optimal set of parameters to maximise net-power saving. The evolution of the skin-friction coefficient is decomposed using the Fukagata-Iwamoto-Kasagi (FIK) identity, which shows that the generation of the net-power savings is due to changes in contributions to both the convection and streamwise development terms of the turbulent boundary-layer flow.

Conference paper

Deskos G, Piggott MD, Laizet S, 2019, Development and validation of the higher-order finite-difference wind farm simulator, winc3d, Pages: 721-728

High-fidelity wind farm models typically employ Large–Eddy Simulation (LES) formulations and turbine parametrisations (e.g. actuator disc models) to resolve the turbine wakes at spatial and temporal scales so that all flow features of engineering importance are well–captured. Such features include the low frequency dynamic wake meandering, which plays a key role in the fatigue loading expe-rienced by downstream turbines clustered in arrays. By the term ‘Wind Farm Simulator’ (WFS) we refer to an integrated framework which offers these capabilities and can be used as a research tool to study wake–to–wake and turbine–to–wake interactions. In this work, we present a validation study for WInc3D, a WFS based on the powerful, sixth-order finite-difference flow solver, incompact3d. For our validation study, we use operational scenarios from the Horns Rev offshore wind farm. The comparison of the present model with existing Supervisory Control and Data Acquisition (SCADA) measurements and previous LES studies shows an overall good agreement.

Conference paper

Mahfoze OA, Wynn A, Whalley RD, Laizet Set al., 2019, Bayesian optimisation of intermittent wall blowing in a turbulent boundary layer for net power saving

A Bayesian optimisation framework is used to optimise low-amplitude wall-normal blowing control of a turbulent boundary-layer (TBL) flow in order to achieve skin-friction drag reduction and net-power saving. The study is carried out using Direct Numerical Simulations (DNS) and Implicit Large Eddy Simulations (ILES). Control performance is assessed by using the power consumption from two different sets of experimental data from two different types of blowing device. The simulations demonstrate that wall-normal blowing control can generate a local skin-friction drag reduction of up to 75%, which persists far downstream of the control. This slow spatial recovery of the skin-friction coefficient back to its canonical counterpart can generate net-power savings up to 5% in the present study. When combined with DNS or ILES, Bayesian optimisation, with its fast convergence (within a dozen iterations with three parameters to optimise) is an ideal tool to find the optimal set of parameters to maximise net-power saving. The evolution of the skin-friction coefficient is decomposed using the Fukagata-Iwamoto-Kasagi (FIK) identity, which shows that the generation of the net-power savings is due to changes in contributions to both the convection and streamwise development terms of the turbulent boundary-layer flow.

Conference paper

Frantz RAS, Farenzena BA, Laizet S, Silvestrini JHet al., 2019, High-Reynolds number scale-resolving simulations of axisymmetric density current

A three-dimensional implicit Large-Eddy simulation is presented for a cylindrical density current at a Reynolds number equal to 136,000, under the Boussinesq approximation for small density difference. The aim is to assess the ability of an implicit Spectral Vanishing Viscosity approach to numerically reproduce the main features of an axisymmetric release of heavy fluid into a light ambient fluid at high Reynolds numbers. Comparisons are made with experimental data and with numerical data at lower Reynolds numbers. It is found that the proposed approach is able to provide a detailed description of the structure of the current, information about the large-scale coherent structures, and to predict the evolution of the front velocity over the different stages of the current propagation.

Conference paper

Frantz RAS, Farenzena BA, Laizet S, Silvestrini JHet al., 2019, High-Reynolds number scale-resolving simulations of axisymmetric density current

© 2019 International Symposium on Turbulence and Shear Flow Phenomena, TSFP. All rights reserved. A three-dimensional implicit Large-Eddy simulation is presented for a cylindrical density current at a Reynolds number equal to 136,000, under the Boussinesq approximation for small density difference. The aim is to assess the ability of an implicit Spectral Vanishing Viscosity approach to numerically reproduce the main features of an axisymmetric release of heavy fluid into a light ambient fluid at high Reynolds numbers. Comparisons are made with experimental data and with numerical data at lower Reynolds numbers. It is found that the proposed approach is able to provide a detailed description of the structure of the current, information about the large-scale coherent structures, and to predict the evolution of the front velocity over the different stages of the current propagation.

Conference paper

Lamballais E, Dairay T, Laizet S, Vassilicos JCet al., 2019, Implicit/explicit spectral viscosity and large-scale SGS effects, ERCOFTAC Series, Pages: 107-113

In order to investigate the scale-selective influence of SGS on the large scale dynamics, DNS and LES are performed for the Taylor-Green vortex problem. An a priori analysis confirms the interest of the hyperviscous feature at small scale as used in implicit LES, SVV and VMS. However, the assumption of zero SGS dissipation at very large scales is found unrealistic for the high Reynolds number and coarse LES mesh considered. A posteriori analysis shows that SGS modelling based on the assumption of an inviscid cascade leads to a bottleneck effect on the kinetic energy spectrum with a significant underprediction of the total SGS dissipation. The simple addition of a constant eddy viscosity, even targeted to be optimal in terms of SGS dissipation, is unable to give realistic results. To allow accurate predictions by LES, a specific closure that incorporates both the hyperviscous feature (i.e. regularisation) and the expected SGS dissipation at large scales has to be developed.

Book chapter

Laizet S, Ioannou V, Margnat F, 2018, A diagnostic tool for jet noise using a line-source approach and implicit large-Eddy simulation data, Comptes Rendus Mécanique, Vol: 346, Pages: 903-918, ISSN: 1631-0721

In this work, we propose a cost-effective approach allowing one to evaluate the acoustic field generated by a turbulent jet. A turbulence-resolving simulation of an incompressible turbulent round jet is performed for a Reynolds number equal tothanks to the massively parallel high-order flow solver Incompact3d. Then a formulation of Lighthill's solution is derived, using an azimuthal Fourier series expansion and a compactness assumption in the radial direction. The formulation then reduces to a line source theory, which is cost-effective to implement and evaluate. The accuracy of the radial compactness assumption, however, depends on the Strouhal number, the Mach number, the observation elevation angle, and the radial extent of the source. Preliminary results are showing that the proposed method approaches the experimental overall sound pressure level by less than 4 dB for aft emission angles below 50°.

Journal article

Schuch FN, Pinto LC, Silvestrini JH, Laizet Set al., 2018, Three‐dimensional turbulence‐resolving simulations of the plunge phenomenon in a tilted channel, Journal of Geophysical Research: Oceans, Vol: 123, Pages: 4820-4832, ISSN: 2169-9275

Hyperpycnal flows are produced when the density of a fluid flowing in a relatively quiescent basin is greater than the density of the fluid in the basin. The density differences can be due to the difference in temperatures, salinity, turbidity, concentration, or a combination of them. When the inflow momentum diminishes, the inflowing fluid eventually plunges under the basin fluid and flows along the bottom floor as an underflow density current. In the present work, 3‐D turbulence‐resolving simulations are performed for an hyperpycnal flow evolving at the bottom floor of a tilted channel. Using advanced numerical techniques designed for supercomputers, the incompressible Navier‐Stokes and transport equations are solved to reproduce numerically the experiments of Lamb et al. (2010, https://doi.org/10.1130/B30125.1) obtained inside a flume with a long tilted ramp. This study focuses on presenting and validating a new numerical framework for the correct reproduction and analysis of the plunge phenomenon and its associated flow features. A very good agreement is found between the experimental data of Lamb et al. (2010), the analytical models of Parker and Toniolo (2007, https://doi.org/10.1061/(ASCE)0733-9429(2007)133:6(690)), and the present turbulence‐resolving simulations. The mixing process between the ambient fluid and the underflow density current is also analyzed thanks to visualizations of vortical structures at the interface.

Journal article

Laizet S, Chandramouli P, Heitz D, Mémin Eet al., 2018, Coarse large-eddy simulations in a transitional wake flow with flow models under location uncertainty, Computers and Fluids, Vol: 168, Pages: 170-189, ISSN: 0045-7930

The focus of this paper is to perform coarse-grid large-eddy simulation (LES) using recently developed sub-grid scale (SGS) models of cylinder wake flow at Reynolds number (Re) of 3900. As we approach coarser resolutions, a drop in accuracy is noted for all LES models but more importantly, the numerical stability of classical models is called into question. The objective is to identify a statistically accurate, stable sub-grid scale (SGS) model for this transitional flow at a coarse resolution. The proposed new models under location uncertainty (MULU) are applied in a deterministic coarse LES context and the statistical results are compared with variants of the Smagorinsky model and various reference data-sets (both experimental and Direct Numerical Simulation (DNS)). MULU are shown to better estimate statistics for coarse resolution (at 0.46% the cost of a DNS) while being numerically stable. The performance of the MULU is studied through statistical comparisons, energy spectra, and sub-grid scale (SGS) contributions. The physics behind the MULU are characterised and explored using divergence and curl functions. The additional terms present (velocity bias) in the MULU are shown to improve model performance. The spanwise periodicity observed at low Reynolds is achieved at this moderate Reynolds number through the curl function, in coherence with the birth of streamwise vortices.

Journal article

Laizet S, Ioannou V, 2018, Numerical investigation of plasma-controlled turbulent jets for mixing enhancement, International Journal of Heat and Fluid Flow, Vol: 70, Pages: 193-205, ISSN: 0142-727X

Plasma-controlled turbulent jets are investigated by means of Implicit Large–Eddy Simulations at a Reynolds number equal to 460,000 (based on the diameter of the jet and the centreline velocity at the nozzle exit). Eight Dielectric Barrier Discharge (DBD) plasma actuators located just before the nozzle exit are used as an active control device with the aim to enhance the mixing of the jet. Four control configurations are presented in this numerical study as well as a reference case with no control and a tripping case where a random forcing is used to destabilize the nozzle boundary layer. Visualisations of the different cases and time-averaged statistics for the different controlled cases are showing strong modifications of the vortex structures downstream of the nozzle exit, with a substantial reduction of the potential core, an increase of the jet radial expansion and an improvement of the mixing properties of the flow.

Journal article

Dairay T, Lamballais E, Laizet S, Vassilicos JCet al., 2018, Physical scaling of numerical dissipation for LES, ERCOFTAC Series, Vol: 24, Pages: 149-155, ISSN: 1382-4309

In this work, we are interested in an alternative way to perform LES using a numerical substitute of a subgrid-scale model with a calibration based on physical inputs.

Journal article

Moulinec C, Laizet S, Emerson DR, 2018, Developing 2-D stretching in a high order DNS code: Application to turbulent flow in a square duct

A Direct Numerical Simulation open-source software supporting high order finite difference schemes, with 1 direction of stretching in space is modified to support stretching in 2 directions in space. The path to change the code is explained for general turbulent flows and some preliminarly results are presented.

Conference paper

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