Experimental exploration of unpowered propellers for vortex-induced vibration suppression of flexible riser

Passive control of the vortex-induced vibrations (VIVs) of flexible risers has been typically studied to alleviate their VIV responses by means of breaking the spanwise correlation, modifying the structural characteristic, or disturbing the flow field. However, this paper proposed using innovative suppression devices-unpowered propellers-to mitigate VIVs and output energy through passive rotation under flowing fluids. The effectiveness of three-, four-, and six-blade propellers for VIV suppression was experimentally examined in the Reynolds number range of 2600-10,400. Seven identical propellers were installed uniformly along a riser facing the incoming flow for VIV suppression. The response parameters of the riser models, including the displacement, frequency, trajectory, and fatigue damage, were investigated. Moreover, a comprehensive evaluation of the unpowered propellers, determined by the suppression efficiency and the cost, was also conducted. The experi-mental results showed that all three types of propellers significantly decreased the VIV amplitudes and inhibited high-order frequency components. Increasing the number of blades effectively enhanced the amplitude sup-pression of the propellers, whereas it had almost no effect on the vibration frequencies of the controlled risers. It is worth noting that the three-blade propellers with the best comprehensive evaluation may not be a reasonable choice in high-velocity water.