Control of Rayleigh-Taylor instability onset time and convective velocity by differential diffusion effects.

Fingering instabilities of a miscible interface between two fluids in a gravitational field can develop due to adverse density gradients as in the well-known Rayleigh-Taylor (RT) and double-diffusive (DD) instabilities. In the absence of differential diffusion, the mixing rate and the onset time of the RT instability developing when a denser solution of a given solute A overlies a less dense solution of a solute B are respectively proportional and inversely proportional to the initial density difference Δρ_{0} between the two superposed layers. We show here both experimentally and theoretically for porous media flows that when the mechanisms of RT and DD instabilities are combined, the properties of the convective growth of the fingers are controlled by the dynamic density jump Δρ_{m} of the nonmonotonic density profile induced by the differential diffusion effects.

Enhanced steady-state dissolution flux in reactive convective dissolution.

Chemical reactions can accelerate, slow down or even be at the very origin of the development of dissolution-driven convection in partially miscible stratifications when they impact the density profile in the host fluid phase. We numerically analyze the dynamics of this reactive convective dissolution in the fully developed non-linear regime for a phase A dissolving into a host layer containing a dissolved reactant B. We show for a general A + B → C reaction in solution, that the dynamics vary with the Rayleigh numbers of the chemical species, i.

Instabilities and transition in magnetohydrodynamic flows in ducts with electrically conducting walls.

This Letter presents a numerical study of a magnetohydrodynamic flow in a square duct with electrically conducting walls subject to a uniform, transverse magnetic field. Two regimes of instability and transition of Hunt's jets at the walls parallel to the magnetic field have been identified. The first one occurs for relatively low values of the Reynolds number Re and is associated with weak, periodic, counterrotating vortices discovered previously in linear stability studies.

Energy transfer in anisotropic magnetohydrodynamic turbulence.

A spectral analysis of anisotropic magnetohydrodynamic turbulence, in presence of a constant magnetic field, is presented using high-resolution direct numerical simulations. A method of decomposing the spectral space into ring structures is presented and the energy transfers between such rings are studied. This decomposition method takes into account the angular dependency of energy transfers in anisotropic systems, while it allows one to recover easily the known shell-to-shell energy transfers in the limit of isotropic turbulence.