Motion of a Flexible Plate in the Wind: Modes, Transitions, and Drag Implications

Inspired by the fluid-structure interaction between wind and vegetation—a critical process in ecology and engineering design—we study the dynamics of flexible wall-mounted plates. High-speed imaging from side and top views captures the plate’s motions in a wind tunnel, while the wake flow is simultaneously characterized by particle image velocimetry and the instantaneous drag via a force balance. Two distinct motion modes are identified: simple oscillation and inflexional flapping. The transition between them is quantitatively governed by the Cauchy number, i.e., the ratio between fluid inertia and the plate bending rigidity. Spectral analysis of drag force and plate motion reveals that the dominant frequencies correspond to the plate’s natural frequencies. We attribute the complex motions in the flapping regime to higher-order vibrational modes and to the coupling with vortex shedding from the tip and the sides of the plate. At high Cauchy numbers, the plate bends to align with the wind and exhibits flag-like lateral flapping, though this motion has negligible influence on the drag.