Tidal stream turbines operate in combined wave–current conditions and the downstream wakes that develop due to momentum extraction can modify wave conditions through Doppler shift and refraction. We study this using a wave-ray tracing approach with an effective depth-integrated current representing idealised wakes at rated operation. Four lateral layouts are examined for full-scale turbines (radius R = 9 m, D = 18 m) in 4 R water depth with a typical freestream velocity U 0 = 2 . 5 ms − 1 . We simulate approximately 2500 regular wave cases, spanning periods 4–16 s and a range of relative headings to assess wave amplification fields (averaged over R × R windows), before creating weighted averages representing irregular and directionally spread wave fields. Peak amplification for regular waves reaches > 3 A 0 (with A 0 the incident wave amplitude) for near-opposing incidence and around 2.5 A 0 for near-following conditions. However, spatial integration reveals that while extreme amplifications ( > 1 . 5 A 0 ) are highly localised ( < 100 D 2 ), moderate amplifications ( 1 . 1 A 0 ) persist over expansive regions (up to 1000 D 2 ). Furthermore, sensitivity analysis demonstrates that this focusing mechanism is highly robust; despite ± 40 % variations in freestream velocity and wake deficit, peak amplifications remain remarkably stable. Unidirectional irregular seas smooth focusing (2.2 A 0 maximum) and directional spreading reduces amplification further to between 1.75 A 0 and 1.35 A 0 for moderate and large spreading repectively. Theoretical transmission errors are calculated to be small ( < 10 % ) for most wave conditions, and comparison against a phase-resolved Boussinesq model confirms that the spatial focusing regions are consistent. While the omission of diffraction leads to local amplitude over-predictions at focal points, the ray-tracing approach is shown to be a highly efficient and conservative screening tool for tidal array design. These results indicate that the wave-induced loading of turbines within an array could differ considerably from an isolated turbine, with amplification magnitude and location dependent on array geometry, turbine operation and site characteristics meriting further study. • Tidal turbine wakes act as refractive lenses for surface waves. • Ray tracing shows wave amplitudes can exceed three times incident for regular waves. • Irregular seas smooth hotspots but peaks still reach 2.2 times incident amplitude. • Large directional spreading limits amplification to 1.35 times incident amplitude. • Array layout shifts amplification locations.
Draycott et al. (Sat,) studied this question.