Axisymmetric vortex breakdown Part 2. Physical mechanisms

Abstract : Numerical solutions of the axisymmetric Navier Stokes equations are presented and compared with results from experiments for a confined cylindrical flow. The details of the vortex breakdown phenomenon are calculated with a high degree of accuracy. From solutions over a range of parameters the essential features of the flow are obtained. These solutions also provide flow quantities such as the vorticity and the pressure throughout the volume which would be difficult to obtain from experiments. The solutions are explored and the essential physical mechanisms of vortex breakdown in this particular geometry are identified. These mechanisms, which rely on the production of a negative azimuthal component of vorticity as a result of the stretching and tilting of the predominantly axially directed vorticity vector, are elucidated with the aid of a simple, steady, inviscid, axisymmetric equation of motion. This equation has been a starting point for most studies of vortex breakdown but a departure in the present study is that it is explored directly and not through perturbations of an initial stream function. The findings are then generalised to the case of vortex breakdown in swirling pipe flows. Australia.