The dynamics of vortex breakdown

Abstract

Swirling flows with jet-like axial velocities are studied by means of direct numerical simulations for various control parameters. The numerical code solves the low-Mach number approximation of the Navier-Stokes equations in cylindrical coordinates which allows the incorporation of compressible and variable-density effects without the adverse effect of acoustic waves on the numerical time-step. The governing parameters, such as swirl and coflow, are varied, and their effect on the breakdown type and breakdown location is studied. The nonlinear direct numerical simulation traces the steady states of the swirling flow, and the structure of the bifurcation is described as a function of the swirl parameter, the Reynolds number and the coflow parameter. In particular, unstable branches are computed and their physical relevance is discussed. The analysis gives new insight into the prevalence of coherent states and their controllability.