Je. Martin et E. Meiburg, NONLINEAR AXISYMMETRICAL AND 3-DIMENSIONAL VORTICITY DYNAMICS IN A SWIRLING JET MODEL, Physics of fluids, 8(7), 1996, pp. 1917-1928
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.