Formation and dynamics of a semi vortex ring connected to a free surface

TL;DR

This study experimentally investigates the formation and behavior of semi vortex rings connected to a free surface, revealing complex formation processes, shedding phenomena, and effects of disk characteristics across various Reynolds numbers.

Contribution

It provides new insights into semi vortex ring formation, including shedding mechanisms and the influence of disk parameters, extending understanding beyond classical vortex ring models.

Findings

Semi vortex rings form through complex processes including shedding.

Shedding is linked to free-shear instability in the boundary layer.

Disk thickness influences vortex properties and shedding frequency.

Abstract

Playing a role in the locomotion of some animals such as the water strider, the formation and dynamics of a semi vortex ring connected to a free surface are experimentally investigated. This semi vortex ring is generated by the circular motion of a flat circular disk in water. Digital Particle Image Velocimetry provides velocity fields and vortex properties. We show how in a broad range of Reynolds numbers, the properties of the semi vortex rings are related to the disk characteristics. In particular, our results highlight a formation process more complex than for a complete vortex ring produced by a piston stroke. In addition to the classical rolling up at the rear of the disk, a shedding phenomenon occurs on the leading edge, producing secondary vortices. The Strouhal number related to this shedding process reveals that it comes from the free-shear instability of the boundary layer identified in the near wake of a cylinder by Bloor. Finally we show that the disk thickness affects the final properties of the semi vortex rings through emission frequency of secondary vortices.