Experimental investigation of torsional effects on cross-flow oscillations in flow-induced vibrations of a triangular prism

This study experimentally investigates the flow-induced vibration (FIV) response of a rigid equilateral triangular prism with one and two degrees of freedom (DoF). The primary focus is on assessing how introducing rotational oscillations (a second DoF) influences cross-flow (CF) oscillations (the first DoF) through a series of water tunnel experiments. The system’s dynamic response is characterized at five initial angles of attack ($\alpha =0°,15°,30°,45°,60°$), within a Reynolds number range of 525 to 3,817. For the one-DoF configuration in the CF direction, a galloping-type instability is observed at $\alpha =45°$ and $60°$. However, when torsional motion is introduced as a second DoF, the onset of oscillations is delayed, and the amplitude of CF oscillations is significantly reduced. This suppression is attributed to changes in the mean angle of attack and the influence of periodic rotational oscillations. The periodic prism rotation modifies the flow-afterbody interaction, leading to weakened flow forces in the CF direction and further reducing the vibration amplitude. Particle Image Velocimetry (PIV) measurements reveal notable differences in the wake structures between the one- and two-DoF cases. In the two-DoF configuration, the prism’s rotation shifts the separation points, leading to asymmetric vortex shedding between the upper and lower sides. This asymmetry periodically modulates the wake dynamics and sustains the rotational oscillations of the prism.