Effect of Strouhal number on propulsion of tandem flapping foils

Abstract

Motivated by the energy efficient locomotion by flock of birds and school of fish to develop biomimetic systems, the current study studies the propulsive performance of the tandem configuration of two-foil system. The effects of the kinematic motion parameters such as heave and pitch amplitudes and flapping frequency on the propulsive performance of the tandem two-foil system is yet to be explored comprehensively due to the complex nature of the wake-foil interaction. Here, we numerically investigate these effects by utilizing a finite element based moving mesh framework to model the flapping foil motion subjected to fluid flow. The performance of the upstream foil is found to be similar to that of an isolated single foil. However, for the downstream foil, a beating phenomenon is observed in the thrust response when the flapping frequency is different for both the foils. This feature can help tune propulsive performance of the foil system. Furthermore, through a control volume analysis, we show that sole reliance on the time-averaged streamwise velocity may not yield correct trends for the mean thrust forces, which has been typically followed in the literature. Variation in the time-averaged pressure in the wake is significant in predicting correct thrust forces.