Experimental study of flow-induced vibration in a two degrees-of-freedom flexibly mounted triangular prism in crossflow

This study experimentally investigates the flow-induced vibration (FIV) of a flexibly mounted triangular prism with two degrees of freedom (DoF), capable of oscillating in its first two vibrational modes in the crossflow direction. The experiments were conducted by varying the angle of attack (⁠ $α$⁠) in the range of 0°–60° and eigenfrequency ratio (R), which is the ratio between the second and first eigenfrequency, from R = 1.5 to 3. For α = 0° and 15°, no significant oscillations were observed. At $α=30°$⁠, both vortex-induced vibration (VIV) and galloping-type response were detected. For higher angles of attack, α = 45° and 60°, a pure galloping type response was identified. Hydrogen bubble flow visualization was employed to analyze the vortex-dominated wake and to discern the nature of the FIV response—whether it was VIV or galloping. Although the prism was free to oscillate in both its first and second modes, only the first mode contributed to the FIV response at α = 45° and 60°, where galloping was dominant. The FIV response remained primarily influenced by the first mode, except when mode two was externally excited, resulting in pronounced hard mode-two galloping behavior. Because mode one predominantly drives the system's overall FIV response, variations in the eigenfrequency ratio had minimal impact on the system's overall FIV behavior.