Volume 18, Issue 12, December 2006
Index of content:
18(2006); http://dx.doi.org/10.1063/1.2404948View Description Hide Description
We use numerical simulations to investigate the dynamics in suspensions of ideally polarizable rods sedimenting under gravity in a vertical electric field. While such suspensions are unstable to concentration fluctuations when no field is applied, we show that the induced-charge electrophoresis that results from the application of the field provides control over the concentration instability by causing particle alignment in the field direction. A phase diagram is obtained for the occurrence of the instability in terms of field strength and volume fraction. In stable suspensions velocity hindrance is shown to occur, and results for the hindered settling function are presented.
18(2006); http://dx.doi.org/10.1063/1.2409729View Description Hide Description
We describe the first DNS-based measurement of the complete mean response of a turbulent channel flow to small external disturbances. Space-time impulsive perturbations are applied at one channel wall, and the linear response describes their mean effect on the flow field as a function of spatial and temporal separations. The turbulent response is shown to differ from the response of a laminar flow with the turbulent mean velocity profile as the base flow.
18(2006); http://dx.doi.org/10.1063/1.2424496View Description Hide Description
The large-scale dynamics in classical two-dimensional Euler and surface quasigeostrophic flows are studied by examining the evolution of the mean-square stream function and of the Fourier mode for small wave number . Upper bounds for and are derived. The growth of is at most quadratic in time and nearly quadratic in time for surface quasigeostrophic and Euler flows, respectively. At the modal level, it is found that and , where and are constant, for the surface quasigeostrophic and Euler cases, respectively. These bounds imply a steep energy spectrum at small respectively, and . The latter is consistent with previous statistical predictions and numerical results.