Experimental study of vortex ring interaction with a free surface.

Abstract

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 $\imes$ 10$\\sp3$ to 10$\\sp4$ 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 resulting in open vortex lines beginning and terminating at the surface. This process occurred at all vortex ring Reynolds numbers investigated and incidence angles up to 45$\\sp\\circ$. 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. The range of vortex ring Reynolds numbers and incidence angles resulting in double reconnection was determined. It is shown that the nondimensional reconnection time based on core parameters is independent of vortex ring Reynolds number and increases as the incidence angle is reduced. The double reconnection process was documented in detail by flow visualization and surface velocity field measurements for a vortex ring Reynolds number of 5 $\imes$ 10$\\sp3$ and incidence angle of 20$\\sp\\circ$. The surface velocity field measurements were obtained using Laser Speckle Photography combined with automated digital image processing techniques specifically developed for this investigation. The results show that the reconnection process is driven by the development during the interaction of strong three-dimensional straining fields which cause reduction of the vorticity component parallel to the surface and enhancement of the vorticity component normal to the surface. At zero incidence the vortex ring path turns toward the free surface as a result of the velocity field of the image vortex above the surface. It is shown that the distance from the vortex generator to the first reconnection location and the extent of the interaction normalized by the ring diameter depend only on the normalized ring depth. They are independent of the Reynolds number. It was further found that the second interaction process was strongly influenced by surface active agents on the surface. The effect of free surface contamination was further investigated at an incidence angle of 90$\\sp\\circ$ i.e. normal incidence. The results show the formation of secondary and tertiary rings when the surface is contaminated by surface active agents. With a cleaner surface a secondary vortex is still formed although the formation is delayed.