Symmetry breaking in flow-induced vibration of rotating prisms with non-circular cross-sections

Flow-induced vibration of a flexibly-mounted triangular prism, free to oscillate in the crossflow direction with imposed rotation about its axis, was studied experimentally. Depending on the angle of attack, non-rotating prisms with triangular cross-sections could experience both Vortex-Induced Vibration (VIV) and galloping. In particular, the objective of this study was to investigate how the imposed rotation could affect the galloping instability for such prisms. The rotation rate, $\alpha $, defined as the ratio of the surface velocity and the free stream velocity, was varied in the range of $\alpha =$0-2.6. The amplitudes and frequencies of oscillations were measured in a Reynolds number range of Re$=$420-2100 and reduced velocity of Vr$=$2.8-14. The oscillation was found to be limited to a range of reduced velocities, and the lock-in range became narrower at higher rotation rates where the oscillation ceased beyond the rotation rate around $\alpha =$2.4. This rotation rate at which the oscillation ceased and the decrease in the width of the lock-in range was very similar to those observed previously for VIV of a rotating circular cylinder. The tests were repeated for a square-cross-section prism and similarly, the oscillation was observed to cease at the same range of rotation rate around $\alpha =$2.4.