Influence of vortical structure impingement on the oscillation and rotation of flat plates

Wind tunnel experiments were performed to study the impact of coherent vortical structures, shed from a cylinder, on the oscillations and rotations of vertical flat plates with thickness ratio b/c=1/16, 3/16 and 5/16. Various combinations of cylinder diameter to plate chord ratio of d/c=1.1,0.8 and 0.5, and cylinder-plate spacing L_d/c∈[1,14] were studied to determine the role of the strength and scale of the vortices on the motion of the plates. The plate motion was characterized with high-temporal resolution accelerometer, gyroscope sensor and a laser tachometer. High temporal- and spatial-resolution flow measurements were performed using hotwire and planar particle image velocimetry (PIV). Plates were found to oscillate around two distinctive equilibrium positions depending on the strength of the vortices shed from the cylinder. For comparatively low cylinder-plate spacing L_d/c, the frequency of the shed vortices dominates the plate oscillation and the plate aligns with the mean direction of the flow. However, the plate motion transitions to an oscillation around an axis perpendicular to the mean flow with larger L_d/c and, eventually, the oscillation become dominated by the vortices shed by the plate. Given an initial impulse, the plates are also able to sustain multi-frequency rotations for large L_d/c ratios. In addition to governing the plate oscillations and equilibrium position, the spacing L_d/c is found to significantly modify the characteristics of the recirculation bubble and turbulence levels in the wake of the plate.