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NONLINEAR AXISYMMETRIC AND THREE-DIMENSIONAL VORTICITY DYNAMICS IN A SWIRLING JET MODEL?

J.E. MARTIN
Department of Mathematics
Christopher Newport University
Newport News, VA 23606-2998

E. MEIBURG
Department of Aerospace Engineering
University of Southern California
Los Angeles, CA 90089-1191

Abstract

The mechanisms of vorticity concentration, reorientation, and stretching 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 vortex dynamics simulations demonstrate the nonlinear interaction and competition between a centrifugal instability and Kelvin-Helmholtz instabilities feeding on both components of the base flow vorticity. Under axisymmetric flow conditions, it is found that the swirl leads to the emergence of counterrotating vortex rings, whose circulation, in the absence of viscosity, can grow without bounds. Scaling laws are provided for the growth of these rings, which trigger a pinch-off mechanism resulting in a strong decrease of the local jet diameter. In the presence of an azimuthal disturbance, the nonlinear evolution of the flow depends strongly on the initial ratio of the azimuthal and axisymmetric perturbation amplitudes. The long term dynamics of the jet can be dominated by counterrotating vortex rings connected by braid vortices, by like-signed rings and streamwise braid vortices, or by wavy streamwise vortices alone.

?Support by the National Science Foundation under grant CTS-9196004 to EM, and through matching funds provided by the Electric Power Research Institute, is gratefully acknowledged. JEM was supported by the National Aeronautics and Space Administration under NASA Contract No. NAS1-19480 while in residence at the Institute for Computer Applications in Science and Engineering (ICASE), NASA Langley Research Center, Hampton, VA 23681-0001. Computing resources were provided by the NSF-supported San Diego Supercomputer Center.

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