Experimental investigation of the effect of high damping on the VIV energy converter near the free surface

The vortex induced vibration (VIV) energy converter is a novel device to extract hydrokinetic energy from ocean currents. The flow velocity near the sea level is relatively higher, which makes ocean surface currents more exploitable. However, due to the influence from free surface (such as wave impact, etc.), the total system damping of the converter increases rapidly. Therefore, this study focuses on the effect of high damping on the converter near the free surface. The experiments are carried out in a towing tank to measure the VIV responses and energy conversion performance. The VIV energy converter is operated at high damping by changing the load resistor from infinity to 30Ω. Correspondingly, the damping ratio increases from 0.38 to 0.50. The main conclusions are: (1) High damping suppresses the VIV amplitudes. (2) Three typical VIV branches can't be distinguished when at high damping. (3) The VIV frequency increases with damping and approaches the vortex-shedding frequency of a stationary cylinder (Strouhal frequency). (4) When the damping ratio increases from 0.38 to 0.50, the maximum converted power decreases from 1.37W to 0.66W. (5) The optimal efficiency η_VIV = 29% is achieved at the beginning of the VIV region.