NONLINEAR AXISYMMETRICAL AND 3-DIMENSIONAL VORTICITY DYNAMICS IN A SWIRLING JET MODEL

The mechanisms of vorticity concentration, reorientation, and stretchi ng are investigated in a simplified swirling jet model, consisting of a line vortex along the jet axis surrounded by a jet shear layer with both azimuthal and streamwise vorticity. Inviscid three-dimensional vo rtex dynamics simulations demonstrate the nonlinear interaction and co mpetition between a centrifugal instability and Kelvin-Helmholtz insta bilities feeding on both components of the base flow vorticity. Under axisymmetric flow conditions, it is found that the swirl leads to the emergence of counter-rotating vortex rings, whose circulation, in the absence of viscosity, can grow without bounds. Scaling laws are provid ed for the growth of these rings, which trigger a pinch-off mechanism resulting in a strong decrease of the local jet diameter. In the prese nce of an azimuthal disturbance, the nonlinear evolution of the flow d epends strongly on the initial ratio of the azimuthal and axisymmetric perturbation amplitudes. The long term dynamics of the jet can be dom inated by counter-rotating vortex rings connected by braid vortices, b y like-signed rings and streamwise braid vortices, or by wavy streamwi se vortices alone. (C) 1996 American Institute of Physics.