Vortex-Induced Vibrations of a square prism free to oscillate in the inline direction

Abstract

We study the response of a prism placed in flow and free to oscillate in the direction of flow (the inline direction) for different possible angles of attack. The tests were conducted in a water tunnel for a range of reduced velocities of 2.4 <U* < 5.8 and Reynolds numbers of 757 <Re < 1900, and over angles of attack of 0° ≤α≤ 45°, where α= 0° corresponded to the case where one of the square’s sides was perpendicular to the incoming flow. We observed Vortex-Induced Vibrations in the inline direction for angles of attack of α= 0°, 2.5°, 5°, 7.5°, and we showed that lock-in has occurred in the cases where oscillations are observed, by showing that the shedding frequency and the oscillation frequency were equal in this range. Throughout the lock-in range, the fluctuating force and the displacement remained in phase as well. The width of the lock-in range and the maximum amplitude of oscillations decreased with increasing angles of attack, and no oscillations were observed for α > 7.5°. The wake structures were found to be predominantly symmetric for α= 0°, and transitioned toward asymmetry with increasing α. By calculating the ratio of the fluctuating lift force to the fluctuating drag force, we quantified how the asymmetry in fluid forces increased with increasing both the angle of attack and the reduced velocity. We also showed how for larger angles of attack where oscillations were not observed, giving an initial disturbance to the prism resulted in formation of vortices for only few cycles and the prism’s oscillations decayed and were damped by the fluid forces.