The dynamics of dual vortex ring flows is studied experimentally and numerically in a model system that consists of a piston-cylinder apparatus. The flows are generated by double identical strokes which have the velocity profile characterized by the sinusoidal function of half the period. By calculating the total wake impulse in two strokes in the experiments, it is found that the average propulsive force increases by 50% in the second stroke for the sufficiently small stroke length, compared with the first stroke. In the numerical simulations, two types of transient force augmentation are revealed, there being the transient force augmentation for the small stroke lengths and the absolute transient force augmentation for the large stroke lengths. The relative transient force augmentation increases to 78% for L/D = 1, while the absolute transient force augmentation for L/D = 4 is twice as much as that for L/D = 1. Further investigation demonstrates that the force augmentation is attributed to the interaction between vortex rings, which induces transport of vortex impulse and more evident fluid entrainment. The critical situation of vortex ring separation is defined and indicated, with vortex spacing falling in a narrow gap when the stroke lengths vary. A new model is proposed concerning the limiting process of impulse, further suggesting that apart from vortex formation timescale, vortex spacing should be interpreted as an independent timescale to reflect the dynamics of vortex interaction.
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January 2014
Research Article|
January 02 2014
Mechanism of transient force augmentation varying with two distinct timescales for interacting vortex rings
Zhidong Fu;
Zhidong Fu
a)
J. C. Wu Center for Aerodynamics, School of Aeronautics and Astronautics,
Shanghai Jiao Tong University
, Shanghai 200240, China
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Suyang Qin;
Suyang Qin
J. C. Wu Center for Aerodynamics, School of Aeronautics and Astronautics,
Shanghai Jiao Tong University
, Shanghai 200240, China
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a)
Also at Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, United Kingdom.
b)
Author to whom correspondence should be addressed. Electronic mail: hongliu@sjtu.edu.cn
Physics of Fluids 26, 011901 (2014)
Article history
Received:
May 22 2013
Accepted:
December 14 2013
Citation
Zhidong Fu, Suyang Qin, Hong Liu; Mechanism of transient force augmentation varying with two distinct timescales for interacting vortex rings. Physics of Fluids 1 January 2014; 26 (1): 011901. https://doi.org/10.1063/1.4859395
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