Flow-induced vibrations of a square prism free to oscillate in the cross-flow and inline directions

We study flow-induced vibrations of a square prism free to oscillate in two degrees of freedom (cross-flow (CF) and inline (IL)), and placed in flow at varying angles of attack, by measuring the prism's displacement and flow-induced forces in both degrees of freedom experimentally, and conducting hydrogen bubble visualizations, as well as bubble image velocimetry. At large angles of attack (where alpha = 45 degrees corresponds to the case where one of the edges of the prism sees the flow first), we observe a two degree of freedom vortex-induced vibration (VIV) response with a figure-eight trajectory, similar to what has been observed for a cylinder with two degrees of freedom. As the angle of attack is decreased, the figure-eight trajectory transitions to a teardrop trajectory, suggesting a 1 : 1 ratio between the oscillation frequencies in the CF and IL directions. The VIV response remains to be the dominant response down to an angle of attack of alpha = 20 degrees. At angles of attack of alpha = 10 degrees and alpha = 15 degrees, the VIV response becomes negligible and elliptical trajectories are observed at higher reduced velocities. These elliptical trajectories then become four-lobe trajectories with amplitudes mainly in the CF direction at the lowest angles of attack (where a side of the square sees the flow first) and galloping-type response is observed, where the amplitude of oscillations is increased with increasing reduced velocity. Deviations from a typical galloping response are observed due to synchronizations between the shedding frequency and oscillation frequency at ranges of higher reduced velocities.