Publications
104 results found
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: 0021-9991
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
� 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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- Citations: 33
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
The stirring of a passive scalar by grid-generated turbulence in the presence of a mean scalar gradient is studied by direct numerical simulations (DNS) for six different grids: one fractal square grid with three fractal iterations, one fractal square grid with four fractal iterations, one fractal I grid and three different regular grids. Our results can be summarised as follows. (i) For all these grids, the turbulence intensity averaged over time and over a plane parallel to the grid takes its peak value when the streamwise position of this plane is between 0.75Meff and 1.5Meff where Meff is the effective mesh size introduced by Hurst & Vassilicos (Phys. Fluids, vol. 19, 2007, 035103). (ii) Downstream of the location of this peak, the turbulence intensity averaged in this way is greatly enhanced by the fractal grids relative to the regular grids even though the fractal grids have comparable or even lower blockage ratios. The novelty of this result lies in the fact that it concerns turbulence intensities averaged over lateral planes (as well as time). (iii) The pressure drop is about the same across grids of the same blockage ratio whether fractal or not, but the pressure recovery is longer for the fractal grids. (iv) Even so, the fractal grids enhance turbulent scalar fluxes by up to an order of magnitude in the region downstream of the aforementioned peak and they also greatly enhance the streamwise growth of the fluctuating scalar variance in that region. (v) We demonstrate on a simple planar model problem that the cause of this phenomenon lies in the fractality of the grids. (vi) The turbulence scalar flux coefficient is constant far enough downstream of all the present grids and is significantly dependent on the nature and details of the turbulence-generating grid.
Espath LFR, Pinto LC, Laizet S, et al., 2015, Direct numerical simulations of particle-driven gravity currents in a basin configuration
© TU Delft. Three-dimensional highly resolved Direct Numerical Simulations (DNS) of a particle-driven gravity current are presented for the lock-exchange problem in a basin configuration. Two Reynolds numbers are investigated in order to identify differences in the flow structures and dynamics. For this numerical study, 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, with possibility of particles deposition but no erosion and re-suspension. For this original flow configuration, it is found that the Reynolds number has a strong influence on the flow, in particular on the deposition pattern over the flat bed. Furthermore, we found out that the well-known lobe-anfcleft patterns at the head of the current have a different shape than what is usually observed for the lock-exchange problem in a channel configuration. The curvated shape of the front has a significant twisting effect on the structures at the head of the current.
Laizet S, Nedić J, Vassilicos JC, 2015, The spatial origin of -5/3 spectra in grid-generated turbulence
A combined wind tunnel and computational study of grid-generated turbulence along the centreline shows that the close to −5/3 power law signature of energy spectra in the frequency domain originates in the production region close to the grid where the velocity derivative statistics become quite suddenly isotropic but also where the turbulent fluctuating velocities are very intermittent and non-Gaussian. As the inlet flow velocity increases, these power laws are increasingly well defined and increasingly close to −5/3 over an increasing range of frequencies. However, this range continuously decreases with streamwise distance from the grid even though the local Reynolds number first increases and then decreases along the same streamwise extent. The intermittency at the point of origin of the close to −5/3 power spectra consists of alternations between intense vortex tube clusters with shallow broad-band spectra and quiescent regions where the velocity fluctuations are smooth with steep energy spectra.
Espath LFR, Pinto LC, Laizet S, et al., 2015, Direct numerical simulations of particle-driven gravity currents in a basin configuration
Three-dimensional highly resolved Direct Numerical Simulations (DNS) of a particle-driven gravity current are presented for the lock-exchange problem in a basin configuration. Two Reynolds numbers are investigated in order to identify differences in the flow structures and dynamics. For this numerical study, 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, with possibility of particles deposition but no erosion and re-suspension. For this original flow configuration, it is found that the Reynolds number has a strong influence on the flow, in particular on the deposition pattern over the flat bed. Furthermore, we found out that the well-known lobe-anfcleft patterns at the head of the current have a different shape than what is usually observed for the lock-exchange problem in a channel configuration. The curvated shape of the front has a significant twisting effect on the structures at the head of the current.
Laizet S, Nedić J, Vassilicos JC, 2015, The spatial origin of -5/3 spectra in grid-generated turbulence
© TU Delft. A combined wind tunnel and computational study of grid-generated turbulence along the centreline shows that the close to −5/3 power law signature of energy spectra in the frequency domain originates in the production region close to the grid where the velocity derivative statistics become quite suddenly isotropic but also where the turbulent fluctuating velocities are very intermittent and non-Gaussian. As the inlet flow velocity increases, these power laws are increasingly well defined and increasingly close to −5/3 over an increasing range of frequencies. However, this range continuously decreases with streamwise distance from the grid even though the local Reynolds number first increases and then decreases along the same streamwise extent. The intermittency at the point of origin of the close to −5/3 power spectra consists of alternations between intense vortex tube clusters with shallow broad-band spectra and quiescent regions where the velocity fluctuations are smooth with steep energy spectra.
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: 31
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: 33
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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- Citations: 30
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
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- Citations: 33
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.
Christos Vassilicos J, Laizet S, 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.
Espath LFR, Pinto LC, Laizet S, et al., 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, Fortune V, Lamballais E, et al., 2012, Low Mach number prediction of the acoustic signature of fractal-generated turbulence, 7th Symposium on Turbulence and Shear Flow Phenomena (TSFP), Publisher: ELSEVIER SCIENCE INC, Pages: 25-32, ISSN: 0142-727X
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- Citations: 7
Laizet S, vassilicos JC, 2012, Fractal space-scale unfolding mechanism for energy-efficient turbulent mixing, Physical Review E, Vol: 86(4)
Laizet S, Li N, 2011, Incompact3d: A powerful tool to tackle turbulence problems with up to O(105) computational cores, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Vol: 67, Pages: 1735-1757, ISSN: 0271-2091
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- Citations: 137
Laizet S, Vassilicos JC, 2011, DNS of Fractal-Generated Turbulence, FLOW TURBULENCE AND COMBUSTION, Vol: 87, Pages: 673-705, ISSN: 1386-6184
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- Citations: 62
Buxton ORH, Laizet S, Ganapathisubramani B, 2011, The effects of resolution and noise on kinematic features of fine-scale turbulence, EXPERIMENTS IN FLUIDS, Vol: 51, Pages: 1417-1437, ISSN: 0723-4864
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- Citations: 32
Buxton ORH, Laizet S, Ganapathisubramani B, 2011, The interaction between strain-rate and rotation in shear flow turbulence from inertial range to dissipative length scales, PHYSICS OF FLUIDS, Vol: 23, ISSN: 1070-6631
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- Citations: 16
Lamballais E, Fortune V, Laizet S, 2011, Straightforward high-order numerical dissipation via the viscous term for direct and large eddy simulation, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 230, Pages: 3270-3275, ISSN: 0021-9991
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- Citations: 53
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