54 results found
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
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
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
Laizet S, Vassilicos JC, 2011, DNS of Fractal-Generated Turbulence, FLOW TURBULENCE AND COMBUSTION, Vol: 87, Pages: 673-705, ISSN: 1386-6184
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
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
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
Laizet S, Fortuneé V, Lamballais E, et al., 2011, Fractal grid turbulence and acoustic predictions
© 2011 International Symposium on Turbulence and Shear Flow Phenomena TSFP. All rights reserved. In this numerical work, we compare the acoustic properties of a fractal square grid with those of a regular grid by means of a hybrid approach based on Lighthill's analogy and Direct Numerical Simulation (DNS). It has been shown that the turbulence generated by fractal objects can have different properties than those generated by a regular object: whereas a regular object effectively introduces a single length-scale into the flow, a fractal object can introduce a wide range of length-scales into the flow. These distinct turbulent states depending on the grid are found to lead to different acoustic radiations both in terms of sound pressure levels and frequencies.
Laizet S, Vassilicos JC, 2011, Passive scalar stirring by multiscale-generated turbulence
© 2018 International Symposium on Turbulence and Shear Flow Phenomena, TSFP07. All rights reserved. The stirring and mixing of a passive scalar by fractal gen- erated turbulence in the presence of a mean scalar gradient is studied in three dimensions by DNS (Direct Numerical Simu- lation). Passive scalar behaviour is of preliminary importance in turbulent mixing and can also provide an opportunity to understand turbulence itself. Recent experiments in wind tun- nels [1, 2] have shown that it is possible to tune fractal objects (see figure 1) as very efficient turbulence generators of po- tential use for static inline mixers. In this numerical work, turbulent mixing in regular and fractal grid turbulence is in- vestigated with a Prandtl number of 0.1. The results show that it is possible to improve turbulent stirring and heat transfer mixing by using a fractal square grid instead of a regular grid, even with the same blockage ratio and the same input velocity.
Lamballais E, Silvestrini J, Laizet S, 2010, Direct numerical simulation of flow separation behind a rounded leading edge: Study of curvature effects, INTERNATIONAL JOURNAL OF HEAT AND FLUID FLOW, Vol: 31, Pages: 295-306, ISSN: 0142-727X
Laizet S, Lamballais E, Vassilicos JC, 2010, A numerical strategy to combine high-order schemes, complex geometry and parallel computing for high resolution DNS of fractal generated turbulence, COMPUTERS & FLUIDS, Vol: 39, Pages: 471-484, ISSN: 0045-7930
Laizet S, Lardeau S, Lamballais E, 2010, Direct numerical simulation of a mixing layer downstream a thick splitter plate, Physics of Fluids, Vol: 22, Pages: 1-15, ISSN: 1070-6631
In this numerical study, the flow obtained behind a trailing edge separating two streams of different velocities is studied by means of direct numerical simulation. The main originality of this work is that the splitter plate itself is included in the computational domain using an immersed boundary method. The influence of the trailing-edge shape is considered through the analysis of the destabilizing mechanisms and their resulting effect on the spatial development of the flow. The streamwise evolution of the different flows is found to be very different for each of the configurations considered, both in terms of mean quantities and flow dynamics. Present results suggest that the wake component, which dominates the flow close to the trailing edge, is still influential further downstream, as already observed in pure wake flows but only conjectured in mixing layer. A detailed analysis of the vortex dynamics is proposed using instantaneous visualizations, statistical/stability analysis considerations, and proper orthogonal decomposition in order to better understand how the transition regime from the wake to the mixing layer occurs and why it can influence the self-similarity, in a region where no wake influence can be locally detected. © 2010 American Institute of Physics.
Laizet S, Lardeau S, Lamballais E, 2010, Direct numerical simulation of a mixing layer downstream a thick splitter plate, PHYSICS OF FLUIDS, Vol: 22, ISSN: 1070-6631
Laizet S, Vassilicos JC, 2010, Direct Numerical Simulation of Fractal-Generated Turbulence, 7th International ERCOFTAC Workshop on Direct and Large-Eddy Simulation, Publisher: SPRINGER, Pages: 19-+, ISSN: 1382-4309
Laizet S, Lamballais E, 2009, High-order compact schemes for incompressible flows: A simple and efficient method with quasi-spectral accuracy, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 228, Pages: 5989-6015, ISSN: 0021-9991
Laizet S, Vassilicos JC, 2009, Multiscale generation of turbulence, Journal of Multiscale Modeling, Vol: 1, Pages: 177-196, ISSN: 1756-9737
This paper presents a brief but general introduction to the physics and engineering of fractals, followed by a brief introduction to fluid turbulence generated by multiscale flow actuation. Numerical computations of such turbulent flows are now beginning to be possible because of the immersed boundary method (IBM) and terascale parallel high performance computing capabilities. The first-ever direct numerical simulation (DNS) results of turbulence generated by fractal grids are detailed and compared with recent wind tunnel measurements. © 2009 Imperial College Press.
Laizet S, Vassilicos JC, 2009, Direct numerical simulations of turbulent flows generated by regular and fractal grids using an immersed boundary method, Pages: 717-722
© 2009 TSFP4 Symposium. All Rights Reserved. Two turbulent flows, one generated by a regular grid and the other by a fractal square grid are studied by means of Direct Numerical Simulations (DNS). An innovative approach which combines high order compact schemes, Immersed Boundary Method and an efficient domain decomposition method is used in this study. Turbulent statistics such as Reynolds stresses are investigated with the objective to analyse the two different regions (production and decay regions) downstream from the grid, as already observed in the experimental results of Hurst & Vassilicos (2007). The main goal of this numerical study is to identify the physical mechanisms implicated in the generation of turbulent flows, especially when generated at different scales, but also to compare the different levels of turbulence intensity generated by each grid.
Lamballais E, Silvestrini J, Laizet S, 2008, Direct numerical simulation of a separation bubble on a rounded finite-width leading edge, INTERNATIONAL JOURNAL OF HEAT AND FLUID FLOW, Vol: 29, Pages: 612-625, ISSN: 0142-727X
Laizet S, Lamballais E, 2006, Direct-numerical simulation of the splitting-plate downstream-shape influence upon a mixing layer., COMPTES RENDUS MECANIQUE, Vol: 334, Pages: 454-460, ISSN: 1631-0721
Laizet S, Lamballais E, 2006, Direct numerical simulation of a spatially evolving flow from an asymmetric wake to a mixing layer, 6th International ERCOFTAC Workshop on Direct and Large-Eddy Simulation, Publisher: SPRINGER, Pages: 467-+, ISSN: 1382-4309
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