VORTEX BREAKDOWN INCIPIENCE - THEORETICAL CONSIDERATIONS

The sensitivity of the onset and the location of vortex breakdowns in concentrated vortex cores, and the pronounced tendency of the breakdowns to move upstream have been characteristic observations of experimental investigations; they have also been features of numerical simulations and led to questions about the validity of these simulations. This movement upstream may be a migration in time for fixed values of the relevant parameters, or the movement of the breakdown location closer to the entrance to the flow or computational domain with small changes in these parameters. This behavior seems to be inconsistent with the strong time-like axial evolution of the flow, as expressed explicitly, for example, by the quasicylindrical approximate equations for this flow. An order-of-magnitude analysis of the equations of motion near breakdown leads to a modified set of governing equations, analysis of which demonstrates that the interplay between radial inertial, pressure, and viscous forces gives an elliptic character to these concentrated swirling flows. Analytical, asymptotic, and numerical solutions of a simplified nonlinear equation are presented; these qualitatively exhibit the features of vortex onset and location noted above. © 1995 American Institute of Physics.