The drag–thrusttransition 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 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.

基于箔厚度的斯特劳哈尔数St，对不对称正弦振动的节距箔的阻力-推力转变和尾流结构进行了数值研究${}_d$= 0.1–0.3和振幅比A${}_d$= 0.5–2.0。通过使一个半冲程（例如，从较低极限到较高极限）比另一半更快来引入振荡中的不对称性。结果表明，由于更快的冲程中的推力增加，阻力-推力过渡随着俯仰不对称的增加而前进。与对称振荡产生的反向Kármán尾流相似，在非对称振荡情况下识别出的P + S（成对和单个涡旋）尾流既可以产生阻力也可以产生推力，因此出现在阻力-推力过渡边界附近。讨论了由非对称振荡箔产生的尾流结构的形成和演化，表明了不对称振荡如何影响流体动力学，阻力-推力过渡，涡旋强度和尾流射流。