On the impact of waves and turbulence on the power fluctuations and wake structure of a tidal-stream turbine

Abstract

Anticipating the power output of tidal-stream turbines (TSTs) is relevant for its technological advance and success. Therefore, it is important to clarify the behavior of TSTs under tidal-stream environments, which include both waves and currents. In this study, a scale-model horizontal-axis TST was examined in turbulent conditions and in the presence of waves. The impact of waves on the wake structure and output power fluctuations were analyzed in terms of integral length scales, the turbine's power statistics, and spectral density. Due to wave–current interactions, the distribution of integral length scales is asymmetric at downstream positions behind the rotor's lateral tips. The fluctuations of the wave speed in the wake lead to a homogenized but reduced length scale. In the spectral domain, waves mix with the turbulent cascade, effectively reducing the energy decay from f 0 to f−1 in the low-frequency region and from f−5/3 to f−11/3 in the intermediate-decay region. The bandwidth of the mechanical noise in the high-frequency range, along with the periodic components, intensifies with increasing wave height and length. Although they are preliminary, these findings will prove useful for understanding the disruption of turbine flow and power output in wave–current flows. This is a first step toward assessing the damage caused to ecology and the structural parts of TSTs in real-world conditions.