Experimental Study of Vortex Ring Interaction with a Free Surface
MICHIGAN UNIV ANN ARBOR DEPT OF AEROSPACE ENGINEERING
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The results of an experimental investigation on the interaction of vortex rings with a free surface are presented. Laminar vortex rings with Reynolds numbers in the range 2 x 103 to 10 x 103 were formed underwater at several incidence angles to the free surface. When the vortex core closest to the surface interacts with it vortex lines are found to disconnect and reconnect with the free surface resulting in open vortex lines beginning and terminating at the surface. Under certain conditions a second reconnection process is observed resulting in a vortex line topology consisting of two semicircles propagating away from each other on divergent paths. This double reconnection of vortex rings at a free surface is documented in detail by flow visualization and surface velocity field measurements. A new technique was developed to measure the velocity field at the free surface using Laser Speckle Photography and automated digital image analysis. Test results show the accuracy of the velocity measurement to better than 1. It is further shown that the interaction geometry when the vortex ring moves initially parallel to the surface is independent of the ring Reynolds number. The effect of Reynolds number on reconnection time is also investigated and showed to be independent of Reynolds number. It is however quite sensitive to the incidence angle. The effect of free surface contamination is investigated at an incidence angle of 90 deg. The results show the formation of secondary and tertiary rings with a contaminated surface. With a cleaner surface the formation of a secondary ring is delayed.
- Fluid Mechanics