Experimental investigation of wake dynamics and power fluctuations of a horizontal-axis tidal stream turbine under co- and counter-directional wave-current environment

The study investigates the wake characteristics and power fluctuation behavior of a horizontal-axis tidal turbine subjected to co- and counter-directional wave–current interactions. A series of physical model experiments were conducted in a wave-current flume using Acoustic Doppler Velocimetry and Particle Image Velocimetry to characterize the wake development and power response under varying wave periods. Results show that the velocity deficit in the wake exhibits a Gaussian distribution with limited sensitivity to wave period or flow direction. High turbulence intensity regions were primarily located downstream of blade tips, driven by coherent tip vortices. The turbulence integral scale displayed a non-monotonic response to wave period, with larger values observed under countercurrent conditions, thus enhancing the wake recovery. Proper Orthogonal Decomposition analysis revealed that nonlinear interactions between tip vortices and Kármán vortex streets dominated the large-scale wake deformation. Power spectral analysis identified dominant frequencies at wave, rotor, and their modulated harmonics components. A power fluctuation model based on Fourier series decomposition was proposed, accurately quantifying the contribution of major frequency components. The findings offer theoretical insights and practical guidance for turbine layout optimization, load mitigation, and performance forecasting in marine renewable energy systems.