Abstract
This paper uses the actuator line method (ALM) to investigate the hydrodynamic performance and wake evolution of a floating tidal stream turbine under single and coupled yaw and heave motion with varying frequencies. As demonstrated, the fluctuations of power and thrust coefficients in coupled scenarios are dominated by the double yaw frequency (2fy), the primary spectral component influencing the energy output. The normal and tangential force coefficients on individual blades are 0.2 and 0.051 respectively during coupled yaw-heave motion, inheriting the dual-peak fluctuation patterns characteristic of the pure yaw motion. The coupled motion induces oblique stretching effects in the wake, hence generating vortex structures, somewhat akin to the single yaw and heave motions. The effect of yaw period is to reduce the performance alteration and rate of wake restoration. The enstrophy evolution analysis is used to correlate the spatial distributions of the vortex stretching term (R1) with the bundle formation mechanisms. In general, the motion period (more with yaw than heave) enhances the dominance of the wake patterns, while vortex evolution remains fundamentally governed by alternating fluid stretching mechanisms along the motion trajectories. The research, whilst limited to two degrees of freedom of motion, makes several noteworthy contributions to comprehend the unsteady wake formation, a base for future studies in floating farm optimization.