The Experts below are selected from a list of 22773 Experts worldwide ranked by ideXlab platform
Marcel Lesieur - One of the best experts on this subject based on the ideXlab platform.
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a numerical investigation of the coherent vortices in turbulence behind a backward facing step
Journal of Fluid Mechanics, 1993Co-Authors: Aristeu Silveira Neto, Dominique Grand, Olivier Metais, Marcel LesieurAbstract:This paper presents a statistical and topological study of a complex turbulent flow over a backward-facing step by means of direct and large-Eddy simulations. Direct simulations are first performed for an isothermal two-dimensional case. In this case, shedding of coherent vortices in the mixing layer is demonstrated. Both direct and large-Eddy simulations are then carried out in three dimensions. The subgrid-scale model used is the Structure-function model proposed by Metais & Lesieur (1992). Lowstep computations corresponding to the geometry of Eaton & Johnston's (1980) laboratory experiment give turbulence statistics in better agreement with the experimental data than both Smagorinsky's method and K-e modelling. Furthermore, calculations for a high step show that the Eddy Structure of the flow presents striking analogies with forced plane mixing layers: large billows are shed behind the step with intense longitudinal vortices strained between them.
Tie Wei - One of the best experts on this subject based on the ideXlab platform.
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designing large Eddy simulation of the turbulent boundary layer to capture law of the wall scalinga
Physics of Fluids, 2010Co-Authors: James G Brasseur, Tie WeiAbstract:Law-of-the-wall (LOTW) scaling implies that at sufficiently high Reynolds numbers the mean velocity gradient ∂U/∂z in the turbulent boundary layer should scale on u∗/z in the inertia-dominated surface layer, where u∗ is the friction velocity and z is the distance from the surface. In 1992, Mason and Thomson pointed out that large-Eddy simulation (LES) of the atmospheric boundary layer (ABL) creates a systematic peak in ϕ(z)≡(∂U/∂z)/(u∗/z) in the surface layer. This “overshoot” is particularly evident when the first grid level is within the inertial surface layer and in hybrid LES/Reynolds-averaged Navier–Stokes methods such as “detached-Eddy simulation,” where the overshoot is identified as a “logarithmic layer mismatch.” Negative consequences of the overshoot—spurious streamwise coherence, large-Eddy Structure, and vertical transport—are enhanced by buoyancy. Several studies have shown that adjustments to the modeling of the subfilter scale (SFS) stress tensor can alter the degree of the overshoot. A com...
Yimin Huang - One of the best experts on this subject based on the ideXlab platform.
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evolution of unmagnetized and magnetized shear layers
The Astrophysical Journal, 2008Co-Authors: M L Palotti, Fabian Heitsch, Ellen G Zweibel, Yimin HuangAbstract:We present numerical simulations of the growth and saturation of the Kelvin-Helmholtz instability in a compressible fluid layer with and without a weak magnetic field. In the absence of a magnetic field, the instability generates a single Eddy that flattens the velocity profile, stabilizing it against further perturbations. Adding a weak magnetic field?weak in the sense that it has almost no effect on the linear instability?leads to a complex flow morphology driven by MHD forces and to enhanced broadening of the layer due to Maxwell stresses. We corroborate earlier studies, which showed that magnetic fields destroy the large-scale Eddy Structure through periodic cycles of windup and resistive decay, but we show that the rate of decay decreases with decreasing plasma resistivity ?, at least within the range of ? accessible to our simulations. Magnetization increases the efficiency of momentum transport, and the transport increases with decreasing ?.
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evolution of unmagnetized and magnetized shear layers
arXiv: Astrophysics, 2008Co-Authors: M L Palotti, Fabian Heitsch, Ellen G Zweibel, Yimin HuangAbstract:We present numerical simulations of the growth and saturation of the Kelvin-Helmholtz instability in a compressible fluid layer with and without a weak magnetic field. In the absence of a magnetic field, the instability generates a single Eddy which flattens the velocity profile, stabilizing it against further perturbations. Adding a weak magnetic field - weak in the sense that it has almost no effect on the linear instability - leads to a complex flow morphology driven by MHD forces and to enhanced broadening of the layer, due to Maxwell stresses. We corroborate earlier studies which showed that magnetic fields destroy the large scale Eddy Structure through periodic cycles of windup and resistive decay, but we show that the rate of decay decreases with decreasing plasma resistivity, at least within the range of resistivity accessible to our simulations. Magnetization increases the efficiency of momentum transport, and the transport increases with decreasing resistivity.
Aristeu Silveira Neto - One of the best experts on this subject based on the ideXlab platform.
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a numerical investigation of the coherent vortices in turbulence behind a backward facing step
Journal of Fluid Mechanics, 1993Co-Authors: Aristeu Silveira Neto, Dominique Grand, Olivier Metais, Marcel LesieurAbstract:This paper presents a statistical and topological study of a complex turbulent flow over a backward-facing step by means of direct and large-Eddy simulations. Direct simulations are first performed for an isothermal two-dimensional case. In this case, shedding of coherent vortices in the mixing layer is demonstrated. Both direct and large-Eddy simulations are then carried out in three dimensions. The subgrid-scale model used is the Structure-function model proposed by Metais & Lesieur (1992). Lowstep computations corresponding to the geometry of Eaton & Johnston's (1980) laboratory experiment give turbulence statistics in better agreement with the experimental data than both Smagorinsky's method and K-e modelling. Furthermore, calculations for a high step show that the Eddy Structure of the flow presents striking analogies with forced plane mixing layers: large billows are shed behind the step with intense longitudinal vortices strained between them.
Luca Ridolfi - One of the best experts on this subject based on the ideXlab platform.
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turbulent boundary layers over permeable walls scaling and near wall Structure
Journal of Fluid Mechanics, 2011Co-Authors: Costantino Manes, Davide Poggi, Luca RidolfiAbstract:This paper presents an experimental study devoted to investigating the effects of permeability on wall turbulence. Velocity measurements were performed by means of laser Doppler anemometry in open channel flows over walls characterized by a wide range of permeability. Previous studies proposed that the von Karman coefficient associated with mean velocity profiles over permeable walls is significantly lower than the standard values reported for flows over smooth and rough walls. Furthermore, it was observed that turbulent flows over permeable walls do not fully respect the widely accepted paradigm of outer-layer similarity. Our data suggest that both anomalies can be explained as an effect of poor inner–outer scale separation if the depth of shear penetration within the permeable wall is considered as the representative length scale of the inner layer. We observed that with increasing permeability, the near-wall Structure progressively evolves towards a more organized state until it reaches the condition of a perturbed mixing layer where the shear instability of the inflectional mean velocity profile dictates the scale of the dominant eddies. In our experiments such shear instability eddies were detected only over the wall with the highest permeability. In contrast attached eddies were present over all the other wall conditions. On the basis of these findings, we argue that the near-wall Structure of turbulent flows over permeable walls is regulated by a competing mechanism between attached and shear instability eddies. We also argue that the ratio between the shear penetration depth and the boundary layer thickness quantifies the ratio between such Eddy scales and, therefore, can be used as a diagnostic parameter to assess which Eddy Structure dominates the near-wall region for different wall permeability and flow conditions
