A Perturbation Technique for Flow-Induced Vibration Control

This paper presents a novel technique to perturb the vortex shedding from a bluff body and subsequently suppress the flow-induced structural vibration. The essence of the technique is to create a local perturbation on the surface of a bluff body using piezoelectric actuators. Experiments were carried out in a wind tunnel. A square cylinder of a height h, flexibly supported on springs at both ends, was allowed to vibrate only in the lift direction. Three actuators were embedded underneath one side, parallel to the flow, of the cylinder. They were simultaneously activated by a sinusoidal wave, thus causing the cylinder surface to oscillate. The structural displacement Y and flow velocity u were simultaneously measured using a laser vibrometer and a single hot-wire, respectively. When the vortex shedding frequency ƒ_s synchronized with the natural frequency of the dynamic system, Y was estimated to be about 0.08h. This displacement collapsed to 25% once the actuators were excited at a normalized frequency ƒh/U_∞ = 0.1 (U_∞ is the free-stream velocity) and amplitude of 0.028h. The laser-illuminated flow visualization captured drastically weakened vortices shed from the cylinder. Spectral analysis of the Y and u signals points to the fact that the imposed perturbation has altered the spectral phase at ƒ_s between fluid excitation and structural vibration from 0 to π, and meanwhile decreased the spectral coherence at ƒ_s from 0.65 to 0.15. It is expected that the perturbation technique presently investigated will have an important role to play in the flow-induced vibration control, especially with the active control element assimilated into the system.