Large amplitude cross-stream sphere vibration generated by applied rotational oscillation.

A new class of vibrations has been discovered where the vibrations are induced by forced sinusoidal rotary oscillation of a sphere. They have been termed Rotary-Induced Vibrations (RIV). RIV is observed only for selected forcing parameters, where the vibrations are locked to the forcing frequency. For these cases, the vibration amplitude increases monotonically with an increase in U ∗ , similar to a galloping response. However, unlike the galloping response which is typically observed for high flow velocities and is generated by lift caused by body asymmetry, RIV is observed for the entire fundamental synchronisation regime of a sphere. RIV is found to be driven by vortex shedding as well as the imposed rotary oscillation, which causes an oscillatory Magnus force. As is typical of forced oscillations, the band of forcing frequencies that leads to RIV becomes wider with forcing amplitude, and is centred on the resonant condition for which the forcing frequency matches the natural system frequency. At a reduced velocity of 20, which is approximately 3 times the resonant reduced velocity, the vibration amplitude was more than 50% greater than the unforced case. [ABSTRACT FROM AUTHOR]