Effects of soft marine growth on vortex-induced vibration: A comparative analysis with hard marine growth

The experimental modeling of vortex-induced vibration (VIV) poses a significant challenge when dealing with marine growth due to its diversity in species, size, and texture. Soft marine growth, in particular, presents a more complex behavior compared to hard marine growth, and its influence on the VIV phenomenon has received less attention. Therefore, the present study experimentally investigates the VIV of a vertical cylinder covered with soft marine growth. As a representative model for soft marine growth, hydroids are selected and tested at three different densities. The oscillator has 1 Degree of Freedom (1DoF) in the cross-flow direction, and the mass ratio ranges between 0.99 and 1.65. The Reynolds number varies from 5.5 x 103 to 5.5 x 104. The results indicate that even at the early stages of soft marine growth, there is a significant reduction in displacement amplitude, lift force, and drag force coefficients, amounting to approximately 30-40 % decrease. Under complete biofouling conditions, the VIV responses are further diminished by 8-17 %. Soft biofouling demonstrates a greater mitigating effect on hydrodynamic forces compared to hard marine growth. At the highest stage of biofouling, the soft marine growth results in VIV responses that are at least 10 % lower than those observed with hard fouling.