49 results found
Deskos G, Laizet S, Piggott M, 2019, Turbulence-resolving simulations of wind turbine wakes, Renewable Energy, Vol: 134, Pages: 989-1002, ISSN: 1879-0682
Margnat F, Ioannou V, Laizet S, 2018, A diagnostic tool for jet noise using a line-source approach and implicit large-eddy simulation data, COMPTES RENDUS MECANIQUE, Vol: 346, Pages: 903-918, ISSN: 1631-0721
Schuch FN, Pinto LC, Silvestrini JH, et 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
Chandramouli P, Heitz D, Laizet S, et al., 2018, Coarse large-eddy simulations in a transitional wake flow with flow models under location uncertainty, COMPUTERS & FLUIDS, Vol: 168, Pages: 170-189, ISSN: 0045-7930
Ioannou V, Laizet S, 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
© 2018 The Authors 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.
Wu JL, Sun R, Laizet S, et al., 2018, Representation of stress tensor perturbations with application in machine-learning-assisted turbulence modeling, Computer Methods in Applied Mechanics and Engineering, ISSN: 0045-7825
© 2018 Elsevier B.V. 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.
Francisco EP, Espath LFR, Laizet S, et al., 2018, Reynolds number and settling velocity influence for finite-release particle-laden gravity currents in a basin, COMPUTERS & GEOSCIENCES, Vol: 110, Pages: 1-9, ISSN: 0098-3004
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.
Diaz-Daniel C, Laizet S, Vassilicos JC, 2017, Direct numerical simulations of a wall-attached cube immersed in laminar and turbulent boundary layers, INTERNATIONAL JOURNAL OF HEAT AND FLUID FLOW, Vol: 68, Pages: 269-280, ISSN: 0142-727X
Mahfoze O, Laizet S, 2017, Skin-friction drag reduction in a channel flow with streamwise-aligned plasma actuators, INTERNATIONAL JOURNAL OF HEAT AND FLUID FLOW, Vol: 66, Pages: 83-94, ISSN: 0142-727X
Dairay T, Lamballais E, Laizet S, et al., 2017, Numerical dissipation vs. subgrid-scale modelling for large eddy simulation, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 337, Pages: 252-274, ISSN: 0021-9991
Diaz-Daniel C, Laizet S, Vassilicos JC, 2017, Wall shear stress fluctuations: Mixed scaling and their effects on velocity fluctuations in a turbulent boundary layer, PHYSICS OF FLUIDS, Vol: 29, ISSN: 1070-6631
Benard N, Laizet S, Moreau E, 2017, PIV-based dynamic model of EHD volume force produced by a surface dielectric barrier discharge
© 2017 by N Benard. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. In this paper, an experimental measurement of the flow produced by a surface DBD plasma actuator has been conducted. One original aspect of these measurements by particle image velocimetry is the high acquisition rate for a PIV system (20 kHz). By using these highlyresolved flow measurements, the fluid flow velocity is used to estimate the spatial and temporal evolution of the EHD volume force. A reduced order model of this force has been constructed by proper orthogonal decomposition. Based on the analysis of the time-resolved expansion coefficients and their associated spatial modes, it is shown that the volume force can be reconstructed by using a limited number of POD modes (6 modes). This spatial and temporal filtering of the force fields remains faithful to the original data and it will help in view of an implementation of such a source term in a numerical solver. The resulting dynamic model shows an alternation of positive and negative volume forces. The strong positive EHD force developing during the glow regime of the DBD plasma discharge is visualized in a time-resolved manner. This positive force is immediately followed by a strong negative volume force probably caused by the local flow deceleration.
ahlfeld, laizet, Geraci G, et al., 2016, Multi-Fidelity Uncertainty Quanti cation Using RANS and DNS, CTR Stanford Summer Program
Boschung J, Peters N, Laizet S, et al., 2016, Streamlines in stationary homogeneous isotropic turbulence and fractal-generated turbulence, FLUID DYNAMICS RESEARCH, Vol: 48, ISSN: 0169-5983
Espath L, Francisco E, Moser C, et al., 2016, Particle-laden gravity currents in non-axisymmetric lock-exchange configurations, Second Conference on Forward Modelling of Sedimentary Systems, Pages: 110-114
ï¿½ 2016, European Association of Geoscientists and Engineers, EAGE. All rights reserved. High-fidelity simulations of particle-laden gravity currents in non-axisymmetric lock-exchange configurations are presented and compared with more conventional channelized and axysymmetric lock-exchange configurations. We limit our investigations to gravity currents over a flat bed in which density differences are small enough for the Boussinesq approximation to be valid. The concentration of particles is described in an Eulerian fashion by using a transport equation combined with the incompressible Navier-Stokes equations. Our non-axisymmetric results highlight similarities and differences with axisymmetric and channelized lock-exchange configurations and show that the dynamics of the current and final deposition maps are significantly influenced by the geometry of the initial reservoir.
, 2016, Modelling of dielectric barrier discharge plasma actuators for direct numerical simulations
© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. In recent years the development of devices known as plasma actuators has advanced the promise of controlling flows in new ways that increase lift, reduce drag and improve aerodynamic efficiencies; advances that may lead to safer, more efficient and quieter aircraft. The large number of parameters (location of the actuator, orientation, size, relative placement of the embedded and exposed electrodes, materials, applied voltage, frequency) affecting the performance of plasma actuators makes their development, testing and optimisation a very complicated task. Several approaches have been proposed for developing numerical models for plasma actuators. The discharge can be modelled by physics-based kinetic methods based on first principles, by semi-empirical phenomenological approaches and by PIV-based methods where the discharge is replaced by a steady-state body force. The latter approach receives a recent interest for its easy implementation in RANS and U-RANS solvers. Here, a forcing term extracted from experiments is implemented into our high-order Navier-Stokes solver (DNS) in order to evaluate its robustness and ability to mimic the effects of a surface dielectric barrier discharge. This experimental forcing term is compared to the numerical forcing term developed by Suzen & Huang (1, 2) with an emphasis on the importance of the wall-normal component of each model.
Laizet S, Nedic J, Vassilicos JC, 2015, The spatial origin of-5/3 spectra in grid-generated turbulence, Physics of Fluids, Vol: 27, ISSN: 1089-7666
A combined wind tunnel and computational study of grid-generated turbulencealong the centreline shows that the close to −5/3 power law signature of energyspectra in the frequency domain originates relatively close to the grid not only wherethe velocity derivative statistics become quite suddenly isotropic but also wherethe turbulent fluctuating velocities are very intermittent and non-Gaussian. As theinlet flow velocity increases, these power laws are increasingly well defined andincreasingly close to −5/3 over an increasing range of frequencies. However, thisrange continuously decreases with streamwise distance from the grid even though thelocal Reynolds number first increases and then decreases along the same streamwiseextent. The intermittency at the point of origin of the close to −5/3 power spectraconsists of alternations between intense vortex tube clusters with shallow broad-bandspectra and quiescent regions where the velocity fluctuations are smooth with steepenergy spectra.
Espath LFR, Pinto LC, Laizet S, et al., 2015, High-fidelity simulations of the lobe-and-cleft structures and the deposition map in particle-driven gravity currents, Physics of Fluids, Vol: 27, Pages: 056604-056604, ISSN: 1089-7666
The evolution of a mono-disperse gravity current in the lock-exchange configurationis investigated by means of direct numerical simulations for various Reynoldsnumbers and settling velocities for the deposition. We limit our investigations togravity currents over a flat bed in which density differences are small enough for theBoussinesq approximation to be valid. The concentration of particles is described inan Eulerian fashion by using a transport equation combined with the incompressibleNavier-Stokes equations. The most interesting results can be summarized asfollows: (i) the settling velocity is affecting the streamwise vortices at the head ofthe current with a substantial reduction of their size when the settling velocity isincreased; (ii) when the Reynolds number is increased the lobe-and-cleft structuresare merging more frequently and earlier in time, suggesting a strong Reynoldsnumber dependence for the spatio-temporal evolution of the head of the current;(iii) the temporal imprint of the lobe-and-cleft structures can be recovered fromthe deposition map, suggesting that the deposition pattern is defined purely andexclusively by the structures at the front of the current.
Laizet S, Nedic J, Vassilicos C, 2015, Influence of the spatial resolution on fine-scale features in DNS of turbulence generated by a single square grid, INTERNATIONAL JOURNAL OF COMPUTATIONAL FLUID DYNAMICS, Vol: 29, Pages: 286-302, ISSN: 1061-8562
Laizet S, Vassilicos JC, 2015, Stirring and scalar transfer by grid-generated turbulence in the presence of a mean scalar gradient, JOURNAL OF FLUID MECHANICS, Vol: 764, Pages: 52-75, ISSN: 0022-1120
Gautier R, Laizet S, Lamballais E, 2014, A DNS study of jet control with microjets using an immersed boundary method, INTERNATIONAL JOURNAL OF COMPUTATIONAL FLUID DYNAMICS, Vol: 28, Pages: 393-410, ISSN: 1061-8562
Espath LFR, Pinto LC, Laizet S, et al., 2014, Two- and three-dimensional Direct Numerical Simulation of particle-laden gravity currents, COMPUTERS & GEOSCIENCES, Vol: 63, Pages: 9-16, ISSN: 0098-3004
Laizet S, Vassilicos JC, Cambon C, 2013, Interscale energy transfer in decaying turbulence and vorticity-strain-rate dynamics in grid-generated turbulence, FLUID DYNAMICS RESEARCH, Vol: 45, ISSN: 0169-5983
Laizet S, Sakai Y, Vassilicos JC, 2013, Special issue of selected papers from the second UK-Japan bilateral Workshop and First ERCOFTAC Workshop on Turbulent Flows Generated/Designed in Multiscale/Fractal Ways, London, March 2012, FLUID DYNAMICS RESEARCH, Vol: 45, ISSN: 0169-5983
Baggaley AW, Laizet S, 2013, Vortex line density in counterflowing He II with laminar and turbulent normal fluid velocity profiles, PHYSICS OF FLUIDS, Vol: 25, ISSN: 1070-6631
, 2013, Coexistence of regions of equilibrium and non-equilibrium two-point turbulence dynamics in grid-generated turbulence both with -5/3 spectra but different underlying physics
In this paper we present results concerning the spatial development of energy spectra E11( f ) and their associated integral and Taylor scales in conjunction with the spatial developments of vorticity, strain and production rates of vorticity and strain obtained from Direct Numerical Simulations of spatially developing grid-generated turbulence. We use a fractal square grid and a single mesh grid where the mesh is similar to the largest square on the fractal square grid. We find two adjacent but physically different regions in these flows relatively close to the grid: one where the Q-R diagram has not yet formed its well-known, presumed universal, tear-drop shape (Tsinober (2009)) but E11( f ) ∼ f-5/3 over more than a decade of a frequency range which is set by inlet conditions rather than Kolmogorov scalings: and one where the Q-R diagram immediately adopts the well-known tear-drop shape and E11( f ) ∼ f-5/3 over a Kolmogorov range of frequencies which increases as the local Reynolds number increases. In the one case with the higher local Reynolds numbers, the first region gives rise, as one moves downstream, to the non-equilibrium behaviour Cϵ ∼ 1/Reλ whilst the second region leads to Cϵ =Const.
, 2013, Stirring and mixing by grid-generated turbulence in the presence of a mean scalar gradient
The stirring and mixing of a passive scalar by gridgenerated turbulence in the presence of a mean scalar gradient is studied in three dimensions by DNS (Direct Numerical Simulation). Using top-end high fidelity computer simulations, we calculate and compare the effects of various fractal and regular grids on scalar transfer and turbulent diffusion efficiencies. We demonstrate the existence of a new mechanism present in turbulent flows generated by multiscale/fractal objects which has its origin in the multiscale/ fractal space-scale structure of such turbulent flow generators. As a result of this space-scale unfolding (SSU) mechanism, fractal grids can enhance scalar transfer and turbulent diffusion by one order of magnitude while at the same time reduce pressure drop by half. The presence of this SSU mechanism when turbulence is generated by fractal grids means that the spatial distribution of length-scales unfolds onto the streamwise extent of the flow and gives rise to a variety of wake-meeting distances downstream. This SSU mechanism must be playing a decisive role in environmental, atmospheric, ocean and river transport processes wherever turbulence originates from multiscale/fractal objects such as trees, forests, mountains, rocky river beds and coral reefs. It also ushers in the new concept of fractal design of turbulence which may hold the power of setting entirely new mixing and cooling industrial standards.
, 2013, Direct numerical simulation of dilute suspension Particle-laden gravity currents
For this numerical work, we are interested in the prediction of a mono-disperse dilute suspension particle-laden flow in the typical lock-exchange configuration. The main objective is to investigate the influence of the Reynolds number in such flows. 2D and 3D Direct Numerical Simulations (DNS) with three different Reynolds numbers are presented with comparisons with previous experimental and numerical works.
Laizet S, Vassilicos JC, 2012, Fractal space-scale unfolding mechanism for energy-efficient turbulent mixing, PHYSICAL REVIEW E, Vol: 86, ISSN: 2470-0045
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