Transport phenomena in rotating turbulence

\u3cp\u3eThe role of rotation on turbulence and some of its transport properties will be reviewed with emphasis on two specific cases: statistically steady or decaying rotating turbulence and rotating thermally driven turbulence. For this purpose we briefly address a few basic concepts relevant for understanding processes in rotating (turbulent) flows such as the emergence of coherent structures, the Taylor-Proudman theorem, quasi-two-dimensional turbulence, inertial waves and Ekman boundary layers. The effect of rotation on turbulence will subsequently be illustrated with two sets of laboratory experiments: one with steadily forced rotating turbulence and another with rotating turbulent convection.\u3c/p\u3

Benchmark computations of the normal and oblique collision of a dipole with a no-slip boundary

Benchmark results are reported of two separate sets of numerical experiments on the collision of a dipole with a no-slip boundary at several Reynolds numbers. One set of numerical simulations is performed with a finite differences code while the other set concerns simulations conducted with a Chebyshev pseudospectral code. Well-defined initial and boundary conditions are used and the accuracy and convergence of the numerical solutions have been investigated by inspection of several global quantities like the total kinetic energy, the enstrophy and the total angular momentum of the flow, and the vorticity distribution at the no-slip boundaries. It is found that the collision of the dipole with the no-slip wall and the subsequent flow evolution is dramatically influenced by small-scale vorticity produced during and after the collision process. The trajectories of several coherent vortices are tracked during the simulation and show that in particular underresolved high-amplitude vorticity patches near the no-slip walls are potentially responsible for deteriorating accuracy of the computations. Our numerical simulations clearly indicate that it is extremely difficult to obtain mode or grid convergence for this seemingly rather simple two-dimensional vortex-wall interaction problem

Turbulent oscillating channel flow subjected to wind stress

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Dispersion of inertial particles in stratified turbulence

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Dispersion of inertial particles in stably stratified turbulence

The dispersion of heavy particles in statistically stationary stably stratified turbulence is studied by means of direct numerical simulations. The distribution of the particles over the domain clearly shows the effect of preferential concentration. This particle distribution reflects the anisotropy of the flow. Large-scale horizontal structures can be seen, whereas in vertical direction thin, sheared layers are observed. It is found that with increasing stratification the effect of preferential concentration decreases. Also single-particle dispersion displays different behavior in horizontal and vertical directions. In horizontal direction its behavior for inertial particles is very similar to that for fluid particles. An increased long-time behavior (O($t^{2}$)) is found compared to the classical diffusive regime ($\propto t$) in isotropic turbulence. In vertical direction, however, with increasing inertia the long-time dispersion is clearly enhanced compared to that of fluid particles. The typical plateau found for vertical fluid particle dispersion becomes less pronounced and the transition to a final linear diffusion limit sets in at earlier times. To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2007.DFD.GS.

Simulation of finite-size particles in turbulent flows using the lattice Boltzmann method

Particle laden turbulent flows occur in a variety of industrial applications. While the numerical simulation of such flows has seen significant advances in recent years, it still remains a challenging problem. Many studies investigated the rheology of dense suspensions in laminar flows as well as the dynamics of point-particles in turbulence. Here we will present results on the development of numerical algorithms, based on the lattice Boltzmann method, suitable for the study of suspensions of finite-size particles under turbulent flow conditions. The turbulent flow is modeled by the lattice Boltzmann method, and the interaction between particles and carrier fluid is modeled using the bounce-back rule. Direct contact and lubrication force models for particle-particle interactions and particle-wall interaction are taken into account to allow for a full four-way coupled interaction. The accuracy and robustness of the method is discussed by validating the velocity profile in turbulent pipe flow, the sedimentation velocity of spheres in duct flow and the resistance functions of approaching particles. Preliminary results from the turbulent pipe flow simulations with particles show that the angular and axial velocities of the particles are scattered around values of mean axial velocity and shear rate obtained from the Eulerian velocity field

Effect of particle shape on fluid statistics and particle dynamics in turbulent pipe flow

\u3cp\u3eAnisotropic particles are present in many natural and industrial flows. Here we perform direct numerical simulation (DNS) of turbulent pipe flows with dispersed finite-size prolate spheroids simulated by means of the lattice Boltzmann method (LBM). We consider three different particle shapes: spheroidal (aspect ratio 2 and 3) and spherical. These three simulations are complemented with a reference simulation of a single-phase flow. For the sake of comparison, all simulations, laden or unladen have the same energy input. The flow geometry used is a straight pipe with length eight times its radius where the fluid is randomly seeded with 256 finite-size particles. The volume fraction of particles in the flow has been kept fixed at 0.48% by varying the major and minor axis of each particle such that their volume remains the same. We studied the effect of different particle shapes on particle dynamics and orientation, as well as on the flow modulation. We show that the local accumulation of spheres close to the wall decreases for spheroids with increasing aspect ratio. These spheroidal particles rotate slower than spheres near to the wall and tend to stay with their major axes aligned to the flow streamwise direction. Despite the lower rotation rates, a higher intermittency in the rotational rates was observed for spheroids and this increase at increasing the aspect ratio. The drag reduction observed for particles with higher aspect ratio have also been investigated using the one-dimensional energy and dissipation spectra. These results point to the relevance of particle shapes on their dynamics and their influence on the turbulent flow.\u3c/p\u3

Studies on quasi-2D turbulence-the effect of boundaries

This paper addresses the effects of domain boundaries on the behaviour of quasi-two-dimensional flows, thereby distinguishing between lateral boundaries of horizontal flow domains and the horizontal boundaries confining shallow fluid layers. As already discussed in some recent papers, the lateral walls may play an essential role in acting as sources of filamentary high-amplitude vorticity, which usually affects the flow evolution in the interior of the domain. Besides, walls exert forces that may promote the self-organization of the flow, and hence contribute to a change in the net angular momentum of the flow. Both aspects will be reviewed briefly. In contrast to what is commonly assumed, shallow-layer flows may develop an essentially three-dimensional structure, with vertical gradients in the principal horizontal flow field and with significant vertical velocity components. These features have been found recently in experiments on electromagnetically generated flows in a shallow fluid layer. In this paper, we will discuss some of these experimental observations as well as some numerical simulation results that are helpful in explaining the observed flow dynamics. © 2009 The Japan Society of Fluid Mechanics and IOP Publishing Ltd