The propulsion of tandem flapping foil following fishtailed flapping trajectory

It has always been a challenge to implement the natural flyer and swimmer kinematics into human-made aero/hydro vehicles for the enhancement of their performance. The propulsive performance of underwater vehicles can be enhanced by following the fishtailed kinematics. In the present study, a two-dimensional simulation has been performed on a tandem flapping foil by altering the simple flapping trajectory motion to a fishtailed trajectory by varying the Strouhal number ( St) in the range of 0.1–0.5. The effect of the inter-foil spacing and phasing between the foils on wake interaction is also investigated. The results show that fishtailed trajectory motion and inter-foil spacing of 2 cm–3 cm (where cm is the mean chord length) between the foils would enhance the propulsive efficiency of the downstream foil by up to 41%. The unfavorable spacing between the foils results in adverse wake interaction, which reduces the propulsive efficiency compared to solo flapping foil.