Impacts of hard marine growth on 2DoF VIV-based piezoelectric energy harvesting

Two-degree-of-freedom (2DoF) oscillators are highly effective for renewable Vortex-Induced Vibration (VIV) energy harvesting due to their capacity to exhibit larger fluctuations in comparison to one-degree-of-freedom (1DoF) oscillators. However, when submerged in the ocean, these oscillators are colonized by marine growth, altering the flow field around them and thus affecting oscillation amplitudes and energy harvesting performance. This paper assesses the effects of hard marine growth on 2DoF-VIV energy harvesting from circular cylinders. Towing tank experiments are conducted within a Reynolds number range of 6400 to 52,000 to determine the output power and efficiency of both clean and biofouled oscillators under optimal piezoelectric coupling. The study utilizes an additive manufacturing technique for 3D printing artificial communities of aggregate barnacles at different marine growth coverage ratios: 33 %-100 %. The results reveal that early-stage hard biofouling, on average, reduces the energy harvesting performance of 2DoF circular oscillators by 81 %. Although performance shows a slight improvement over time, it remains approximately 75 % lower than that of the clean model. The adverse effects of hard marine growth are significant enough to eliminate the advantage of the second degree of freedom, resulting in an average performance 49 % lower than that of the 1DoF clean oscillator.