Optimal vortex formation in a self-propelled vehicle

Abstract

Previous studies have shown that the formation of coherent vortex rings in the nearwake of a self-propelled vehicle can increase propulsive efficiency compared with a steady jet wake. The present study utilizes a self-propelled vehicle to explore the dependence of propulsive efficiency on the vortex ring characteristics. The maximum propulsive efficiency was observed to occur when vortex rings were formed of the largest physical size, just before the leading vortex ring would pinch off from its trailing jet. These experiments demonstrate the importance of vortex ring pinch off in self-propelled vehicles, where coflow modifies the vortex dynamics.Key words: biological fluid dynamics, propulsion, vortex dynamics IntroductionThe presence of coherent vortical structures in the near-wake of a jet, hereafter referred to as 'vortex-enhanced propulsion', has been studied extensively and shown to increase propulsive performance in self-propelled vehicles (Siekmann 1962;Weihs 1977;Müller et al. 2000a;Krueger 2001;Finley & Mohseni 2004;Choutapalli 2007;Bartol et al. 2008;Krieg & Mohseni 2008, 2013Moslemi & Krueger 2010Ruiz, Whittlesey & Dabiri 2011). Weihs (1977, using many assumptions, analytically predicted an increase of 50 % in the average thrust through the use of vortex-enhanced propulsion. Later experimental work by Krueger & Gharib (2005) measured increases of up to 90 % of the propulsive thrust by optimization of the vortex ring characteristics. Ruiz et al. (2011) extended the results from stationary nozzles to a self-propelled vehicle, showing that vortex-enhanced propulsion can yield up to a 50 % increase in the propulsive efficiency over a steady jet.The vortices formed during vortex-enhanced propulsion can be characterized using the vortex formation time,t GRS , defined aŝ

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