Jet Switching Precedes Chaos in the Wake of a Simultaneously Pitching-Plunging Airfoil

This study investigates the transitional flow dynamics in the wake of a pitching-plunging airfoil in the low Reynolds number regime. In the periodic regime, the transition from Karman to reverse Karman vortex-streets and the subsequent wake deflection with a gradual increase in the amplitude based Strouhal number (St) are well investigated in the literature. However, a further increase in St may result in chaotic transition of the wake which has largely remained unexplored. The present study aims to understand the role of the main flow-field structures in triggering chaos in the wake and resolve the underlying flow physics. It is observed that the deflected trailing jet undergoes switching of direction in the far-field prior to the chaotic regime. The switching location in the downstream approaches the trailing-edge and the switching frequency increases as St is increased. Eventually, the wake becomes chaotic through a series of rapid aperiodic mode-switching of the jet. Leading-edge vortex is also seen to play an important part in triggering the jet switching phenomenon. The behavior of the unsteady flow-field during these dynamical changes is studied in terms of underlying vortex interactions in the near and far-fields.