Drag–thrust transition and wake structures of a pitching foil undergoing asymmetric oscillation

Abstract

The drag–thrusttransition and wake structures of a pitching foil undergoing asymmetric sinusoidal oscillation are numerically investigated for foil thickness-based Strouhal number St = 0.1–0.3 and amplitude ratio A = 0.5–2.0. The asymmetry in the oscillation is introduced by making one half-stroke (e.g. from the lower extreme to the upper extreme) faster than the other. The results reveal that the drag–thrust​ transition advances with increasing the pitching asymmetry because of enhanced thrust in the faster stroke. Similar to the reverse Kármán wake generated from the symmetric oscillation, the P+S (paired and single vortices) wake identified in the case of the asymmetric oscillation can produce both drag and thrust, hence appearing around the drag–thrust transition boundary. The formation and evolution of wake structures produced by the asymmetrically oscillating foil are discussed, showing how the asymmetric oscillation affects fluid dynamics, drag–thrust transition, vortex strength, and wake jet. This work provides some new perspectives to understand the swimming and flying performance and some useful findings to design bio-inspired robots.