Effect of the asymmetric geometry on the wake structures of a pitching foil

The two-dimensional wake produced by a time-periodic pitching foil with the asymmetric geometry is investigated in the present work. Through numerically solving nonlinear Navier-Stokes equations, we discuss the relationship among the kinematics of the prescribed motion, the asymmetric parameter K ranged as -1 <= K <= 1, and the types of the wakes including the mP+nS wake, the Benard-von Karman wake, the reverse Benard-von Karman wake, and the deviated wake. Compared with previous studies, we reveal that the asymmetric geometry of a pitching foil directly affects the foil's wake structures. The numerical results show that the reverse Benard-von Karman wake is easily deviated at K < 0, while the symmetry-breaking of the reverse Benard-von Karman wake is delayed at K > 0. Through the vortex dynamic method, we understand that the initial velocity of the vortex affected by the foil's asymmetry plays a key role in the deviation of the reverse Benard-von Karman wake. Moreover, we provide a theoretical model to predict the wake deviation of the asymmetric foil.