DR. S.R. RAJESH

Hydrodynamic perturbations on shear flows of different angular momentum distributions are of great importance in astrophysical fluid mechanics. In explaining the formation and sustainment of astrophysical accretion discs the existence of instability and hence the origin of turbulence is crucial. Since any linear instability is ruled out by the Rayleigh criterion, for pure hydrodynamic instability in accretion discs one has to look for a nonlinear mechanism for instability and turbulence. Here, the speaker will present a method originally devised for non-rotating flows, in the case of flows with angular momentum and try to understand how much energy has to be imposed on the flow to get nonlinear instability and how the angular momentum distribution affects the threshold. Apart from the instability and turbulent viscosity, hydrodynamic perturbations are invoked to explain different kind of variabilities seen in the spectrum of accretion discs (e.g. QPO’s). The speaker will present a model to systematically analyse the nonlinear modes which are supposed to be responsible for the above phenomenon. For that a “dynamical system” is reduced from the nonlinear perturbation. Such a model can be used to improve the understanding about the interconnectivity of flows of different scales and their consequences seen in the spectrum.