Research Article
Peng J
Abstract
Vortex ring formation is a limited growth process. Upon reaching a non-dimensional formation number, the vortex ring pinches off and secondary vortices form in the trailing jet. However, since a larger vortex ring is associated with higher propulsive efficiency, it is desired to delay vortex ring pinch-off. In this study, a numerical model based on vortex sheet method was used to study vortex ring formation from a piston-cylinder apparatus, with the focus on the vortex ring pinch-off at formation number F. Studies on the various piston velocity programs showed that piston acceleration is able to delay the vortex ring pinch-off and the formation of secondary vortices. The delay is more significant when the vortex ring pinch-off occurs while the piston is accelerating. Acceleration of piston before a constant velocity period would still delay the vortex ring pinch-off, but to a much smaller extent.