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.
Highlights
Vortex ring formation is a limited growth process
Because vortex ring pinch-off occurs during the constant velocity period, this implies that the piston acceleration rate at the vortex ring pinch-off is the dominant factor in delay in vortex ring pinch-off and formation of secondary vortices, even though the piston acceleration preceding the vortex pinch-off still has some similar effect, though to a lesser degree
A numerical model based on vortex sheet method was used to study vortex ring formation from a piston-cylinder system, with the focus on the limited vortex ring growth at ring pinch-off at formation number F
Summary
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. Though F ≈ 4 for a starting vortex ring from a constant-velocity jet in a classical piston/cylinder arrangement, it is desired to increase F to generate larger vortex ring.
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