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research-article

A unified energy feature of vortex rings for identifying the pinch-off mechanism

[+] Author and Article Information
Yang Xiang

J.C.Wu Center for Aerodynamics, School of Aeronautics and Astronautics, Shanghai Jiao Tong University, Shanghai, China, 200240
xiangyang@sjtu.edu.cn

Hong Liu

J.C.Wu Center for Aerodynamics, School of Aeronautics and Astronautics, Shanghai Jiao Tong University, Shanghai, China, 200240
hongliu@sjtu.edu.cn

Suyang Qin

J.C.Wu Center for Aerodynamics, School of Aeronautics and Astronautics, Shanghai Jiao Tong University, Shanghai, China, 200240
liam_young@sjtu.edu.cn

1Corresponding author.

ASME doi:10.1115/1.4037506 History: Received March 14, 2017; Revised July 20, 2017

Abstract

Owing to the limiting effect of energy, vortex rings cannot grow indefinitely and thus pinch off. In this paper, experiments on the vortex rings generated using a piston-cylinder apparatus are conducted so as to investigate the pinch-off mechanisms and identify the limiting effect of energy. Both theoretical and experimental results show that the generated vortex rings share a unified energy feature, regardless of whether they are pinched-off or not. Moreover, the unified energy feature is quantitatively described by a dimensionless energy number ?, defined as ? = E/(I^2G?_max) and exhibiting a critical value ?_ring =0.14 for the generated vortex rings. This unified energy feature reflects the limiting effect of energy and specifies the target of vortex ring formation. Furthermore, based on the tendency of ? during vortex ring formation, criteria for determining the two timescales, i.e pinch-off time and separation time, which correspond to the onset and end of pinch-off process respectively, are suggested.

Copyright (c) 2017 by ASME
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