HEAD-ON COLLISION OF 2 COAXIAL VORTEX RINGS - EXPERIMENT AND COMPUTATION

Head-on collision of two coaxial vortex rings has been studied by join t experimental and numerical investigation. The Reynolds number, Re-Ga mma, based on the initial circulation of the vortex rings, ranged from 400 to 2700. Besides numerical data, the vorticity field was also res olved by a non-intrusive visualization technique, LIPA, which enabled simultaneous measurement of velocities at multiple locations on a plan e area. It was found that the enstrophy, rather than circulation, reve aled three stages of evolution of the vortex rings prior to their brea kdown. These include the free-travelling stage, stage of vortex stretc hing and the stage of viscous dissipation dominance. The results indic ate that it would be incorrect to neglect the viscous effect, in parti cular, for the latter two stages of flow development. In fact, the reb ound behaviour of the vortex rings for lower Re-Gamma is essentially a viscous phenomenon and is found to be closely related to the dissipat ion of enstrophy when the vortex rings are brought to interact activel y with each other and is also related to the increase of the vorticity core diameter in the stage of dominance of viscous dissipation. Furth ermore, an instant dimensionless group, N-t/Re-Gamma, based on the loc al vorticity distribution and the radius of a vortex ring, is found to be appropriate to characterize the onset of instability. Our investig ation indicates that, in the range of observation, bulging instability will be observed during collision when N-t/Re-Gamma, exceeds a critic al value, (N-t/Re-Gamma)(er), which is a function of the initial core- size of the vortex ring. Comparisons showed that the numerical, measur ed, and visualization results were in consistent agreement; this not o nly enables us to assess the range of validity of the axisymmetry assu med for the numerical simulation, but also provides us with a rational basis for further analysis of azimuthal instability.