Propulsive efficiency of a biomorphic pulsed-jet underwater vehicle

Abstract

The effect of the velocity program and duty cycle (St(L)) on the propulsive efficiency of pulsed-jet propulsion was studied experimentally on a self-propelled, pulsed-jet underwater vehicle, dubbed Robosquid due to the similarity of essential elements of its propulsion system with squid jet propulsion. Robosquid was tested for jet slug length-to-diameter ratios (L/D) in the range 2-6 and St(L) in the range 0.2-0.6 with jet velocity programs commanded to be triangular or trapezoidal. Digital particle image velocimetry was used for measuring the impulse and energy of jet pulses to calculate the pulsed-jet propulsive efficiency and compare it with an equivalent steady jet system. Robosquid's Reynolds number (Re) based on average vehicle velocity and vehicle diameter ranged between 1300 and 2700 for the conditions tested. The results indicated better propulsive efficiency of the trapezoidal velocity program (up to 20% higher) compared to the triangular velocity program. Also, an increase in the ratio of the pulsed-jet propulsive efficiency to the equivalent steady jet propulsive efficiency (eta(P)/eta(P, ss)) was observed as St(L) increased and L/D decreased. For cases of short L/D and high St(L), eta(P)/eta(P, ss) was found to be as high as 1.2, indicating better performance of pulsed jets. This result demonstrates a case where propulsion using essential elements of a biological locomotion system can outperform the traditional mechanical system equivalent in terms of efficiency. It was also found that changes in St(L) had a proportionately larger effect on propulsive efficiency compared to changes in L/D. A simple model is presented to explain the results in terms of the contribution of over-pressure at the nozzle exit plane associated with the formation of vortex rings with each jet pulse.

Optimal vortex formation in a self-propelled vehicle

“…Although we are unable to make a similar quantitative comparison of the steady and unsteady jets in the current work, the results of Ruiz et al (2011) suggest a similar advantage of pulsed jet propulsion neart/F = 1 over steady jet propulsion in the present study. Moslemi & Krueger (2010 and Nichols & Krueger (2012) studied selfpropulsion using a vehicle called 'robosquid' which used a pulsation mechanism different from either Ruiz et al (2011) or the current work. The results obtained using vortex cores in the images for the major axis and the streamwise extent of the darkest dye region for the minor axis, as the darkest dye was from the most recent ejection.…”

“…The 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.…”

“…Ruiz et al (2011) achieved significant increases in propulsive efficiency using vortex-enhanced propulsion, but did not explore the dependence of propulsive efficiency on vortex formation time. Moslemi & Krueger (2010 and Nichols & Krueger (2012) explored a range of formation time for their self-propelled vehicle, however only for relatively low Reynolds numbers (2 × 10 2 -2 × 10 4 , based on the vehicle length). The present work explores a range of vortex ring formation times at higher Reynolds number, relevant to larger autonomous and manned underwater vehicle propulsion, to better understand the dependence of propulsive efficiency on vortex ring formation.…”

Numerical investigation of the influence of nozzle geometrical parameters on thrust of synthetic jet underwater

“…Studies about pulsed jet found that compared with the steady jet, pulsed jet possess higher propulsion efficiency [1][2][3][4][5][6]. Weihs [7], Seikman [8] and Krueger and Gharib [6] have shown that pulsed jet can give rise to a greater average thrust than steady jet of equivalent mass flow rate.…”

An elastic pulsed-jet thruster for Soft Unmanned Underwater Vehicles

“…serchi@sssup.it recognition (i.e. [4], [5], [6], [7], [8]) and never in the context of soft robotics. This is somewhat surprising given that this mode of swimming has two major merits: first, pulsed jetting is known to provide more thrust than the continuous outflow case, due to vortex ring formation at the nozzle exit; in addition, this mode of locomotion has an efficiency comparable to that of fish but does not require an underlying rigid structure.…”