Self-induced flapping dynamics of a horizontal flexible sheet in a uniform flow

The flapping of conventional flexible sheets in a uniform flow has long been studied to understand the passive coupled dynamics between a structure and the surrounding flow. While gravity is known to influence the flapping instability and post-critical dynamics, previous investigations have often neglected gravity or considered it along the length or span of the sheet. This study experimentally examines the self-induced, sustained flapping of a thin, flexible sheet positioned horizontally, with gravity acting along its bending direction. Wind-tunnel experiments are conducted across a range of sheet parameters, including length (L) and aspect ratio (), and for different wind speeds, to explore the coupled effects of gravitational, aerodynamic, and structural forces on the onset of instability and post-critical dynamics. For comparison, the flapping behavior of a vertically mounted sheet with gravity along its span is also investigated. It is observed that the gravity along the bending direction does not affect the onset of flapping instability, while horizontally mounted sheets exhibit higher transverse tip displacements and frequencies than their vertical counterparts. These findings have direct relevance for smart propulsion and energy-harvesting devices.