Flow-induced oscillations of a flexibly mounted triangular prism allowed to oscillate in the cross-flow direction are studied experimentally, covering the entire range of possible angles of attack. For angles of attack smaller than $\unicode[STIX]{x1D6FC}=25^{\circ }$ (where $0^{\circ }$ corresponds to the case where one of the vertices is facing the incoming flow), no oscillation is observed in the entire reduced velocity range tested. At larger angles of attack of $\unicode[STIX]{x1D6FC}=30^{\circ }$ and $\unicode[STIX]{x1D6FC}=35^{\circ }$, there exists a limited range of reduced velocities where the prism experiences vortex-induced vibration (VIV). In this range, the frequency of oscillations locks into the natural frequency twice: once approaching from the Strouhal frequencies and once from half the Strouhal frequencies. Once the lock-in is lost, there is a range with almost-zero-amplitude oscillations, followed by another range of non-zero-amplitude response. The oscillations in this range are triggered when the Strouhal frequency reaches a value three times the natural frequency of the system. Large-amplitude low-frequency galloping-type oscillations are observed in this range. At angles of attack larger than $\unicode[STIX]{x1D6FC}=35^{\circ }$, once the oscillations start, their amplitude increases continuously with increasing reduced velocity. At these angles of attack, the initial VIV-type response gives way to a galloping-type response at higher reduced velocities. High-frequency vortex shedding is observed in the wake of the prism for the ranges with a galloping-type response, suggesting that the structure’s oscillations are at a lower frequency compared with the shedding frequency and its amplitude is larger than the typical VIV-type amplitudes, when galloping-type response is observed.

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Vortex-induced vibration of a flexibly-mounted circular cylinder free to oscillate in the crossflow direction with imposed rotation around its axis was studied experimentally. The rotation rate, α, defined as the ratio of the surface velocity and free stream velocity, was varied from 0 to 2.6 in small steps. The amplitudes and frequencies of oscillations as well as the flow forces were measured in a Reynolds number range of Re = 350 -1000. The maximum amplitude of oscillations was limited to values less than a diameter of the cylinder at high rotation rates. Also, the lock-in range became narrower at higher rotation rates and finally the oscillations ceased beyond α = 2.4. Vortex shedding pattern was found to be 2S (two single vortices shed per cycle of oscillations) for rotation rates up to α = 1.4 and transitioned toward an asymmetric P shedding (one pair of vortices shed in a cycle of oscillations) for rotation rates within the range of 1.4 ≤ α ≤ 1.8. Vortex shedding was found to persist up to higher rotation rates than those observed for a non-oscillating cylinder. The phase difference between the flow forces and displacement of the cylinder in the crossflow direction was influenced as the rotation rate was increased: At high reduced velocities, the phase difference decreased from 180° for a non-rotating cylinder to values close to 90° for a rotating cylinder at large rotation rates. Different shedding patterns resulted in flow forces with different frequencies. In the crossflow direction, the dominant frequency of flow forces was found to be close to the system’s natural frequency for all the rotation rates tested with either 2S or P vortex shedding pattern. In the inline direction, however, the change from 2S to P shedding at high rotation rates resulted in a shift of the ratio of the dominant frequency of the inline flow forces to the natural frequency of the system from 2:1 to 1:1.

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