The effects of piston stroke length and exit geometry on the kinematics of vortex rings

This investigation looked at the relationships between the piston stroke length and the exit geometry of a vortex generator and the dependent characteristics of a vortex ring. The vortex generator was operated with two exit geometries, one a conventional straight tube and the other a sharp edged orifice with the same exit diameter as the straight tube. Rings were generated in the University of Tennessee Space Institute water tunnel in order to facilitate the study of ring characteristics in both a quiescent fluid and in the presence of a uniform cross flow. Quantitative measurements were taken using hot film anemometry and qualitative observations were made using flow visualization techniques. Experimental results indicated that within the range of parameters used for these experiments, ring circulation and diameter increased with increasing stroke length. Additionally, the diameter of highly turbulent rings generated with the sharp edged exit increased slowly during the initial stage of ring motion. This was then followed by a rapid increase in ring growth. In addition, the sharp edged rings had larger cores and smaller overall diameters, and shed more of their initial vorticity into their wakes than the straight tube rings. Finally, tests yielded conclusive evidence of the tilting of vortex rings in the presence of a cross flow.