Perturbed interaction between vortex shedding and induced vibration

This paper presents a novel technique to perturb interactions between vortex shedding from a bluff body and vortex-induced vibration of the body, with a view to provide a possible control of both flow and structural vibration. The essence of the technique is to create a local perturbation on the surface of a bluff body using piezoelectric ceramic actuators. Experiments were carried out in a wind tunnel. A square cylinder of 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 normalized vortex shedding frequency f*ssynchronized with the natural frequency, f1n, 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 perturbation frequency of f*p= 0.1 and amplitude of 0.028h. Flow visualization captured drastically impaired vortices shed from the cylinder. Spectral analysis of the Y and u signals points to the fact that the perturbation has altered the spectral phase φYuat fsbetween fluid excitation and structural vibration from 0 to π, and meanwhile decreased the spectral coherence CohYuat fsfrom 0.65 to 0.15. However, as f*pfalls within the possible synchronization range (f*p= 0.11-0.26 or 0.8f1n∼2f1n) where fn= fs, φYuat fsremains near 0, the maximum CohYueven reaching 0.9. As a result, both vortex shedding and the structural vibration are enhanced. 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.