The effects of flexibility on the formation and evolution of wake vortices behind a heaving plate

The kinematics of vortical structures behind heaving plates with different flexibility has been investigated using particle image velocimetry. The rectangular plates with different thicknesses undergo a trapezoidal velocity profile (acceleration-steady-deceleration). The Reynolds number Re_c based on the chord length and the steady velocity is 5330. The formation and evolution processes of the wake vortices are investigated, with the focus on the comparison between the rigid wing and flexible wings. Wake vortices for the rigid wing initiated near the plate edges, while for flexible wings vortices initiated near the quarter-chord from the edges and slid toward the edges as the plates deform and move forward. During the first half cycle (moving forward), both the vortex trajectories and circulation indicate a global two-stage growth of the starting vortices for all the plates, corresponding to the acceleration and steady motion stages, respectively. The flexible plates have larger wake width and attain higher peak circulation initially; however, the circulation for the rigid plate achieves higher peak value during the steady stage. During the second half cycle (moving backward), the vortex trajectories present a two-stage process; however, the circulation is a single growth and decay process. Besides, detailed study reveals that during the acceleration, the circulation grows rapidly and there is a small decay at the end of the acceleration, thus forming the local peak for the first stage. The vortex trajectories for the flexible plates show greater range transversely, as well as the motion reversal in the streamwise direction.