## Publications

52 results found

Pinier B, Mémin E, Laizet S,
et al., A stochastic flow approach to model the mean velocity profile of wall-bounded flows, *Physical Review E (Statistical, Nonlinear, and Soft Matter Physics)*, ISSN: 1539-3755

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.

Bartholomew P, Laizet S, A new highly scalable, high-order accurate framework for variable-density flows: application to non-Boussinesq gravity currents, *Computer Physics Communications*, 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.

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

© Springer Nature Switzerland AG 2019. 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.

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°.

Wu J, 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

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.

Schuch FN, Pinto LC, Silvestrini JH,
et al., Three‐dimensional turbulence‐resolving simulations of the plunge phenomenon in a tilted channel, *Journal of Geophysical Research: Oceans*, 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.

Laizet S, Chandramouli P, Heitz D,
et 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.

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.

Dairay T, Lamballais E, Laizet S,
et 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.

Laizet S, Diaz Daniel C, Vassilicos C, 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

A wall-attached cube immersed in a zero pressure gradient boundary layer is studied by means of Direct Numerical Simulations (DNS) at various Reynolds numbers ReH (based on the cube height and the free-stream velocity) ranging from 500 to 3000. The cube is either immersed in a laminar boundary layer (LBL) or in a turbulent boundary layer (TBL), with the aim to understand the mechanisms of the unsteady flow structures generated downstream of the wall-attached cube. The mean locations of the stagnation and recirculation points around the cube immersed in a TBL are in good agreement with reference experimental and numerical data, even if in those studies the cube was immersed in a turbulent channel. In the TBL simulation, a vortex shedding can be identified in the energy spectra downstream of the cube, with Strouhal number of St=0.14. However, the frequency of the vortex shedding is different in the LBL simulations, showing a significant dependence on the Reynolds number. Furthermore, in the TBL simulation, a low frequency peak with St=0.05 can be observed far away from the boundary layer, at long streamwise distances from the cube. This peak cannot be identified in the LBL simulations nor in the baseline TBL simulation without the wall-attached cube.

Francisco EP, Espath LFR, Laizet S,
et al., 2017, Reynolds number and settling velocity influence for finite-release particle-laden gravity currents in a basin, *Computers and Geosciences*, Vol: 110, Pages: 1-9, ISSN: 0098-3004

Three-dimensional highly resolved Direct Numerical Simulations (DNS) of particle-laden gravity currents are presented for the lock-exchange problem in an original basin configuration, similar to delta formation in lakes. For this numerical study, we focus on gravity currents over a flat bed for 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, with the possibility of particles deposition but no erosion nor re-suspension. The focus of this study is on the influence of the Reynolds number and settling velocity on the development of the current which can freely evolve in the streamwise and spanwise direction. It is shown that the settling velocity has a strong influence on the spatial extent of the current, the sedimentation rate, the suspended mass and the shape of the lobe-and-cleft structures while the Reynolds number is mainly affecting the size and number of vortical structures at the front of the current, and the energy budget.

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

Direct Numerical Simulations in a turbulent channel flow at a moderate Reynolds number are performed in order to investigate the potential of Dielectric Barrier Discharge (DBD) plasma actuators for the reduction of the skin-friction drag. The idea is to use a sparse array of streamwise-aligned plasma actuators to produce near-wall spanwise-orientated jets in order to destroy the events which transport high-speed fluid towards the wall. It is shown that it is possible to reduce the drag by about 33.5% when the streamwise-aligned actuators are configured to generate appropriate spanwise-orientated jets very close to the wall so that the sweeps which are mainly responsible for the skin-friction are destroyed. We demonstrate that it is possible to achieve significant drag reduction with a sparse array of streamwise-aligned plasma actuators, with one order of magnitude less actuators than previous experiments in a similar set-up.

Diaz Daniel, 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: 0031-9171

The present work investigates numerically the statistics of the wall shear stress fluctuations in a turbulent boundary layer (TBL) and their relation to the velocity fluctuations outside of the near-wall region. The flow data are obtained from a Direct Numerical Simulation (DNS) of a zero pressure-gradient TBL using the high-order flow solver Incompact3D [S. Laizet and E. Lamballais, “High-order compact schemes for incompressible flows: A simple and efficient method with quasi-spectral accuracy,” J. Comput. Phys. 228(16), 5989 (2009)]. The maximum Reynolds number of the simulation is Re ≈2000, based on the free-stream velocity and the momentum thickness of the boundary layer. The simulation data suggest that the root-mean-squared fluctuations of the streamwise and spanwise wall shear-stress components τx and τz follow a logarithmic dependence on the Reynolds number, consistent with the empirical correlation of Örlü and Schlatter [R. Örlü and P. Schlatter, “On the fluctuating wall-shear stress in zero pressure-gradient turbulent boundary layer flows,” Phys. Fluids 23, 021704 (2011)]. These functional dependencies can be used to estimate the Reynolds number dependence of the wall turbulence dissipation rate in good agreement with reference DNS data. Our results suggest that the rare negative events of τx can be associated with the extreme values of τzand are related to the presence of coherent structures in the buffer layer, mainly quasi-streamwise vortices. We also develop a theoretical model, based on a generalisation of the Townsend-Perry hypothesis of wall-attached eddies, to link the statistical moments of the filtered wall shear stress fluctuations and the second order structure function of fluctuating velocities at a distance y from the wall. This model suggests that the wall shear stress fluctuations may induce a higher slope in the turbulence energy spectra of streamwise velocities than the on

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: 1090-2716

This study presents an alternative way to perform large eddy simulation based on a targeted numerical dissipation introduced by the discretization of the viscous term. It is shown that this regularisation technique is equivalent to the use of spectral vanishing viscosity. The flexibility of the method ensures high-order accuracy while controlling the level and spectral features of this purely numerical viscosity. A Pao-like spectral closure based on physical arguments is used to scale this numerical viscosity a priori. It is shown that this way of approaching large eddy simulation is more efficient and accurate than the use of the very popular Smagorinsky model in standard as well as in dynamic version. The main strength of being able to correctly calibrate numerical dissipation is the possibility to regularise the solution at the mesh scale. Thanks to this property, it is shown that the solution can be seen as numerically converged. Conversely, the two versions of the Smagorinsky model are found unable to ensure regularisation while showing a strong sensitivity to numerical errors. The originality of the present approach is that it can be viewed as implicit large eddy simulation, in the sense that the numerical error is the source of artificial dissipation, but also as explicit subgrid-scale modelling, because of the equivalence with spectral viscosity prescribed on a physical basis.

Benard N, Laizet S, Moreau E, 2017, PIV-based dynamic model of EHD volume force produced by a surface dielectric barrier discharge, 55th AIAA Aerospace Sciences Meeting, Publisher: American Institute of Aeronautics and Astronautics

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 highly-resolved 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 in 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

Brauner T, Laizet S, Benard N, et al., 2016, Modelling of dielectric barrier discharge plasma actuators for direct numerical simulations, 8th AIAA Flow Control Conference 2016, Publisher: AAAI

In recent years the development of devices known as plasma actuators has advancedthe promise of controlling flows in new ways that increase lift, reduce drag and improveaerodynamic efficiencies; advances that may lead to safer, more efficient and quieter aircraft.The large number of parameters (location of the actuator, orientation, size, relativeplacement of the embedded and exposed electrodes, materials, applied voltage, frequency)affecting the performance of plasma actuators makes their development, testing and optimisationa very complicated task. Several approaches have been proposed for developingnumerical models for plasma actuators. The discharge can be modelled by physics-basedkinetic methods based on first principles, by semi-empirical phenomenological approachesand 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 andU-RANS solvers. Here, a forcing term extracted from experiments is implemented intoour high-order Navier-Stokes solver (DNS) in order to evaluate its robustness and abilityto mimic the effects of a surface dielectric barrier discharge. This experimental forcingterm is compared to the numerical forcing term developed by Suzen & Huang (1, 2) withan emphasis on the importance of the wall-normal component of each model.

Vassilicos C, 2016, Streamlines in stationary homogeneous isotropic turbulenceand fractal-generated turbulence, *Fluid Dynamics Research*, Vol: 48, ISSN: 1873-7005

We compare streamline statistics in stationary homogeneous isotropic turbulence and in turbulence generated by a fractal square grid. We examine streamline segments characterised by the velocity difference ${\rm{\Delta }}u$ and the distance l between extremum points. We find close agreement between the stationary homogeneous isotropic turbulence and the decay region of the fractal-generated turbulence as well as the production region of the fractal flow for small segments. The statistics of larger segments are very similar for the isotropic turbulence and the decay region, but differ for the production region. Specifically, we examine the first, second and third conditional mean $\langle {[{\rm{\Delta }}u]}^{n}| l\rangle $. Noticeably, non-vanishing $\langle {[{\rm{\Delta }}u]}^{n}| l\rangle $ for $n=1,3$ are due to an asymmetry of positive and negative segments, i.e. those for which ${\rm{\Delta }}u\gt 0$ and ${\rm{\Delta }}u\lt 0$, respectively. This asymmetry is not only kinematic, but is also due to dissipative effects and therefore $\langle {[{\rm{\Delta }}u]}^{n}| l\rangle $ contains cascade information.

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.

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

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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

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- Citations: 18

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

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- Citations: 10

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

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- Citations: 14

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

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

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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

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Laizet S, Christos Vassilicos J, 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.

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