Generating periodic vortex pairs using flexible structures.

When a planar flow starts through a narrow slit, it creates a unique mass of fluid characterized by counter-rotating vortex pairs. These vortex pairs typically remain attached to the fluid layer at the tip without separating in a phenomenon known as "pinch-off," usually observed in 3D vortex ring formation process. Our research aims to trigger this pinch-off in vortex pairs using flexible slit plates, enhancing momentum transport and self-propulsion. By employing a flow evolution model, we demonstrate that the growth rate of these ejected vortex pairs is proportional to the square root of time. We discovered a critical case of plate flexibility which induces pinch-off in the vortex pairs — a phenomenon not observed with rigid plates (slit). This setup generates a continuous sequence of vortex pairs, with their generation period closely matching the oscillation period of the plates. The streamwise velocity of the leading vortex pair varies non-monotonically with Ca, increasing up to Ca ≈ 0.04 before decreasing due to the upstream movement of smaller vortices. These findings provide new insights into the control and induction of vortex pair behavior, opening up innovative possibilities for fluid transport and advanced flow manipulation techniques.