Effect of aspect ratio on the propulsive performance of tandem flapping foils

Abstract In this work, we describe the impact of aspect ratio ($AR$) on the performance of optimally phased, identical flapping flippers in a tandem configuration. Three-dimensional simulations are performed for seven sets of single and tandem finite foils at a moderate Reynolds number, with thrust producing, heave-to-pitch coupled kinematics. Increasing slenderness (or $AR$) is found to improve thrust coefficients and thrust augmentation but the benefits level off towards higher values of $AR$. However, the propulsive efficiency shows no significant change with increasing $AR$, while the hind foil outperforms the single by a small margin. Further analysis of the spanwise development and propagation of vortical structures allows us to gain some insights into the mechanisms of these wake interactions and provide valuable information for the design of novel biomimetic propulsion systems.