Wake-mode transitions of a triangular prism undergoing vortex-induced vibration and galloping

An experimental investigation was conducted to examine the wake dynamics and mode transitions of an elastically mounted triangular prism undergoing vortex-induced vibration (VIV) and galloping. Particle image velocimetry measurements were performed in a circulating water channel over the Reynolds number of 4.44 × 104 ≤ Re ≤ 1.14 × 105 in TrSL3 (transition in shear layer, 20 000 ≤ Re ≤ 200 000). The wake structures were analyzed together with the measured displacement histories, allowing the wake evolution to be related to the response branches of the prism. For the stationary prism, alternating vortices shed symmetrically, with increased coherence and a reduced formation length as the reduced velocity increases. For the freely oscillating prism, distinct wake modes emerge within different response branches. The wake evolved from a 2S mode in the VIV initial branch to a 2P mode in the VIV upper branch, and further developed into a 2P + S mode in both the VIV–galloping transition branch and the galloping branch. The wake-mode transitions indicate that the wake of the triangular prism undergoes a progressive reorganization with increasing reduced velocity, involving changes in vortex formation, pairing, and shedding. Compared with the wake behavior commonly studied for circular cylinders, the persistence of the 2P + S mode at high reduced velocities reveals a fundamental influence of cross section on the nonlinear fluid–structure interaction. The present results provide experimental evidence for the wake-mediated mechanisms underlying VIV and galloping of non-circular bluff bodies.