Fluid–structure interaction of an inverted flexible panel in the wake of a cylinder

We experimentally study the effect of a stationary upstream cylinder on the flapping dynamics of an inverted flexible panel, which has a free leading edge and a fixed trailing edge. In particular, we explore the influence of the gap between the panel and the cylinder, the diameter of the cylinder, and the reduced velocity. Panel deflection is measured using a laser displacement sensor, and the panel's oscillation dynamics are recorded using a high-speed camera. Flow dynamics are captured using hot-wire anemometry and fog visualization. The panel's tip amplitude, flapping frequency, flow frequency response, and flow patterns are characterized. Based on amplitude and frequency responses, various flapping regimes are identified, namely, small-amplitude flapping (SAF), periodic large-amplitude flapping (PLAF), aperiodic large-amplitude flapping (ALAF), bistable flapping, and deflected flapping. The present measurements show a lock-in between the panel flapping frequency and the vortex shedding frequency behind the panel in the PLAF regime. A threshold reduced velocity and gap are required to transition from SAF to PLAF or ALAF, depending on the cylinder diameter. Additionally, beyond a critical cylinder diameter, PLAF transitions to ALAF, which exists over a wider range of reduced velocities as compared to PLAF. These findings are relevant for the design of small-scale flow-induced vibration-based energy harvesters and other low-power fluid–structure interaction applications.