The influence of waves propagating with the current on the wake of a tidal stream turbine

The presence of the wind-waves enhances the flow turbulent complexity and modifies the dynamic behaviour of the tidal-stream turbine and causes extra challenges for its performance and survivability. A sign of these effects is the modification of the turbine wake under the action of wind-waves, thus assessing the resulting wake characteristics is fundamental for technological development of such devices in single or array configurations. Laboratory experiments, involving regular waves propagating with the current, were conducted to describe the wake structure of a fixed scale model tidal-stream turbine. In order to highlight wave effects, the laboratory measurements are analysed in term of: (i) time-mean velocity profiles, (ii) swirl numbers, (iii) turbulence intensities and (iv) turbulent anisotropy maps. In each case, distinctive features are discussed in the downstream rotor near and far fields. The velocity profile is slightly affected by the waves in both near and far fields. The swirl number in the near field is drastically influenced by waves presence where a significant correlation with the wave-steepness is highlighted. Wave presence also affects the turbulence intensities both in the near and far field. This phenomenon is more evident at the top-tip and rotor–centreline water depth instead of in the bottom-tip level. The analysis of turbulent anisotropy maps shows that waves act on the wake flow leading to axisymmetric turbulent conditions mainly dominated by the axial turbulence component. These results constitute an initial step toward the understanding of unsteady wake interactions, in arrays of tidal stream turbines.