Spring-supported cylinder wake control

A novel technique is proposed for the vortex control in the wake of a freely vibrating bluff body. The essence of the technique is to create a local perturbation on the surface of the body using piezoelectric actuators, thus modifying interactions between the flow and the structure. A square cylinder, flexibly supported on springs at both ends, was placed in a uniform flow and allowed to vibrate laterally. 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. Measurements were conducted at the synchronization condition when the vortex shedding frequency fscoincided with the natural frequency of the fluid-structure system. As the perturbation frequency fpof the actuators falls in the synchronization range, both particle image velocimetry and laser-induced fluorescence flow visualization captured dramatically enhanced vortices shed from the cylinder. The circulation of these enhanced vortices doubled. On the other hand, when fpwas shifted away from the synchronization the vortex circulation dropped by about 50%. The spectral analysis of the structural displacement signal Y and hot-wire signal u points to the fact that the perturbation has altered fluid-structure interactions, the spectral phase φYuat fsbetween fluid excitation and structural vibration changing from 0 to -π, namely, from reinforcing each other to dissipating each other. The perturbation effect on the drag coefficient and the crossflow distribution of Reynolds stresses is also investigated.