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

The drag-thrust transition and wake structures of a pitching foil undergoing asymmetric sinusoidal oscillation are numerically investigated for foil thickness-based Strouhal number St(D) = 0.1-0.3 and amplitude ratio A(D) = 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 Karman 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. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study numerically investigates the drag-thrust transition and wake structures of a pitching foil undergoing asymmetric sinusoidal oscillation. The results show that pitching asymmetry advances the drag-thrust transition and produces P+S wake structures, affecting fluid dynamics and the formation of wake jets.