Efficiency of pulsed-jet propulsion via thrust-drag decomposition
Xiaobo Bi, Qiang Zhu
Abstract
By using an axisymmetric immersed-boundary model, fluid dynamics of a cephalopod-inspired propeller undergoing periodic inflation–deflation deformation in background flow is numerically studied in a low Reynolds number regime. A thrust-drag decoupling method based on physical analysis is proposed, in which the jet-related thrust is obtained as the summation of three parts: the jet momentum flux, the normal stress at the exit plane, and the flow acceleration inside the body. This method enables the calculation of the propulsive efficiency, especially the efficiency at the steady-swimming state. Systematic simulations are then conducted to study the effects of the Reynolds number and stroke ratio on force generation and efficiency. Two Reynolds numbers, the incoming-flow Reynolds number Re∞ and the jet-flow Reynolds number Rej, are involved. When Re∞ is fixed, the thrust generation is found to depend mostly on jet-flow velocity at high Rej, while the effect of incoming-flow velocity is pronounced at relatively low Rej, mostly through its influence on the excessive pressure at the nozzle. Within the range of incoming-flow Reynolds number considered in this study (40–150), our results show that the whole-cycle propulsive efficiency of the propeller lies in the range of 11%–30%.