Dynamics of a Flexible Square Prism in Crossflow Undergoing a VIV to Galloping Transition

This study presents an experimental investigation into dynamics of a flexible square prism in crossflow to understand the transition from vortex-induced vibrations (VIV) to galloping. A rotating water channel is used to impose a range of reduced velocities 3.6<U^*<18, at a range of Reynolds numbers 115<Re<1500. We observed three different regimes in the entire experimental range. For low flow velocities, U^*<12, VIV is observed and the prism vibrates with a low amplitude and at the same frequency as the vortex shedding. This synchronization is the characteristic of VIV. For high flow velocities, U^*>18, the amplitude of oscillations greatly increases, the oscillation frequency drops and decouples from the shedding frequency and approaches the prism's natural frequency in a classic galloping response. In both regimes, only first mode deformation of the prism is observed. The key finding of this study is the transition between the VIV and galloping regimes that occurs in the range of 12<U^*<18. In the transition regime, both VIV and galloping dynamics are observed and the dynamics are observed to intermittently switch between each other. We discuss this transition mechanism by analyzing the prism's cross-flow and in-line motions and the corresponding wake structures measured through PIV-generated vorticity maps.