Numerical investigation into the time-asymmetry effect on a plunging foil

In the present study, the effect of time-asymmetry motion trajectory on force generation, propulsion performance and wake structure of a plunging foil is evaluated. The proposed motion pattern is inspired by kinematics of natural flyers and manta rays. For this simulation, a wide range of time-asymmetry parameters S adjust the proportion of plunge-down in the whole cycle. The results show that a noticeable increase is achieved in thrust, lift and kinematic velocity when the more asymmetric (low S) motion is experienced. High propulsion efficiency mainly focuses within the domain of 30% ≤ S ≤ 50% (sinusoidal motion). The wake evolution gets complex under asymmetry motion. When S<50%, a faster plunge-down produces a single positive vortex and one or more negative vortices generated during slower plunge-up, the number depends on the reduced frequency, asymmetry and plunge amplitude. The vortex trajectories and streamwise velocities downstream the trailing edge exhibit asymmetric behavior. The present work provides helpful findings for better understanding the kinematic behavior of actual animals and designing bio-inspired robotics.