Onset of Turbulence in Magnetohydrodynamic, Compressible, non Homogeneous, Mixing Layers: Instability and Large Eddie Simulations

GRATTON, F. T.; BILBAO, L.; GNAVI, G.; FARRUGIA, C. J.

São Pedro, SP, Brasil

Congreso; IX Brazilian Meeting on Plama Physics; 2007

Sociedade Brasileira de Fisica

Mixing layers between flows of different velocity, and generally of different density, are ubiquitous features of space plasmas. We report a study of the magnetohydrodinamic (MHD) turbulence that develops in compressible, non homogeneous, mixing layers caused by the instability of the velocity gradient. The onset of the process is analyzed with stability theory, while the non linear evolution is computed with large eddie simulations (LES) of temporal mixing layers. The turbulent layer is characterized by the presence of large scale vortices, and it becomes a site of mass mixing and enhanced dffusion. In MHD, the Lorentz force and its associated baroclinic term, together with the ordinary baroclinic term and the stratified entropy across the mixing layer, they all hinder the conservation of the vorticity flux. The non conservation of vorticity becomes manifest in the LES, where in addition to vortices with positive rotation (with the same sign of the original vorticity sheet) other coherent structures with strong negative vorticity also arise, a noteworthy effect we point out. We find that vorticity concentrates in cores of both signs, with absolute value several times larger than the maximum vorticity of the initial shear layer.