Vibration reduction of a sphere through shear-layer control

To date, it has been shown that the vibration response of an elastically mounted sphere undergoing vortex-induced vibration (VIV) can be controlled by imposing rotary oscillations at frequencies close to the vibration frequency. Here, we demonstrate that rotary oscillations imposed at significantly higher frequencies can be used to directly influence shear-layer vortex shedding and consequently reduce vibration. This approach contrasts with aiming to directly target the large-scale wake structures, using lower frequency perturbations. The oscillation frequencies imposed were between 5 and 35 times the natural frequency of the system and the amplitude of the rotational velocities were only 10% of the free-stream velocity. The effects of the rotary oscillations were found to vary significantly across sphere vibration modes. In the mode III transition regime significant attenuation of the vibration response was observed for a narrow band of rotary oscillation frequencies. Time-resolved particle image velocimetry revealed that the shear-layer vortex structures locked to the forcing frequency, where suppression of the vibration response occurred. Optimal tuning of the oscillation frequency reduced the vibration amplitude in the mode III transition regime by 84%, with a rotational velocity amplitude of only 10% of freestream. These results show low-amplitude shear-layer forcing is a promising method of more efficiently suppressing VIV of three-dimensional geometries.