Non-axisymmetric vertical shear and convective instabilities as a mechanism of angular momentum transport

Abstract

Discs with a rotation profile depending on radius and height are subject to an axisymmetric linear instability, the vertical shear instability. Here we show that non-axisymmetric perturbations, while eventually stabilized, can sustain huge exponential amplifications with growth rate close to the axisymmetric one. Transient growths are therefore to all effects genuine instabilities. The ensuing angular momentum transport is positive. These growths occur when the product of the radial times the vertical wavenumbers (both evolving with time) is positive for a positive local vertical shear, or negative for a negative local vertical shear. We studied, as well, the interaction of these vertical shear induced growths with a convective instability. The asymptotic behaviour depends on the relative strength of the axisymmetric vertical shear (s_v) and convective (s_c) growth rates. For s_v > s_c we observed the same type of behaviour described above - large growths occur with asymptotic stabilization. When s_c > s_v the system is asymptotically unstable, with a growth rate which can be slightly enhanced with respect to s_c. The most interesting feature is the sign of the angular momentum transport. This is always positive in the phase in which the vertical shear driven transients growths occur, even in the case s_c > s_v . Thermal diffusion has a stabilizing influence on the convective instability, specially for short wavelengths.