Propulsive efficiency of a biomorphic pulsed-jet underwater vehicle

Propulsive efficiency of a self-propelled pulsed-jet underwater vehicle, dubbed Robosquid due to similarity of its propulsion system with squid, was experimentally studied using Digital Particle Image Velocimetry (DPIV). The effect of jet velocity program (the variation of jet velocity over time) and duty cycle (StL) as well as Reynolds number ( Re) on propulsive efficiency was investigated. In the first part of this study, Robosquid was tested for jet slug length-to-diameter ratios (L/D) in the range 2--6 and StL in the range 0.2--0.6 with triangular and trapezoidal jet velocity programs. DPIV 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 tests performed in water. 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 pulsed-jet propulsive efficiency to equivalent steady jet propulsive efficiency (eta P/etaP,ss) was observed as StL increased and L/D decreased. For cases of short L/D and high StL (dimensionless frequency) etaP/etaP,ss was found to be as high as 1.2, indicating better performance of pulsed jets over steady jet. This result clearly demonstrates a case where a biologically inspired propulsion system can outperform its mechanical equivalent in terms of efficiency. It was also found that changes in StL 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.;In the second part of this study, the average Re was lowered from 2000 to 50 using a glycerin-water mixture to investigate the effect of scaling on pulsed-jet propulsive efficiency. Robosquid was tested with nominal test parameters identical to those used in water, except that its Re based on average vehicle velocity and vehicle diameter ranged between 37 and 60. The results for propulsive efficiency at the mentioned Re were compared to the results in water where Robosquid's Re ranged between 1300--2700. The results showed that the average propulsive efficiency decreased by 26% as the average Re decreased from 2000 to 50 while the ratio of pulsed-jet to steady jet efficiency (etaP/etaP,ss) increased up to 0.15 (26%) as the Re decreased over the same range for similar pulsing conditions. The improved etaP/etaP,ss at lower Re suggests that pulsed jet propulsion can be used as an efficient propulsion system for small scale propulsion applications. The Re = 37--60 conditions in the present investigation, showed a reduced dependence of etaP and etaP/etaP,ss on L/D compared to higher Re results. This may be due to the lack of clearly observed vortex ring pinch off as L/D increased for this Re regime.