Acoustic Control of Vortex-Induced Vibrations of a Tethered Sphere

An active flow-control technique is used to reduce or eliminate vortex-induced vibrations (VIV) on a tethered body that is subjected to acoustic perturbations. Experiments were performed in a small blowdown wind tunnel having a 9.5:1 inlet contraction and square test section with transparent acrylic windows for optical access. The symmetric nature of the vortices was expected to reduce the amplitude of the unsteady forcing. Results obtained by initially setting the excitation frequency and varying the freestream velocity are found to be similar to those obtained by setting a freestream velocity and subsequently initiating the acoustic excitation. It is also found that terminating the excitation results in the sphere's return to its baseline response. A different boundary-layer structure is found where instabilities are not damped and cause the breakup of large structures into smaller ones.