Experimental investigation into effects of boundary proximity and blockage on horizontal-axis tidal turbine wake

Abstract This study examined how the wake of a scaled turbine was changed by a bypass using three different water depths with the hub at mid-depth, and one water depth with the hub at 40%, 50%, and 60% of the channel depth. The velocity of the flow field was sampled to compare the distributions of the turbulence intensity, velocity deficit, shear layer structure, and wake rotation among the different cases. In the experiments, the shear layer had an expansion rate of 0.14–0.22 and was transversally elliptical, as also shown in other works, as a result of the compression of the wake. The downstream turbulence intensities changed from a trimodal into a decreasing bimodal distribution until the recovery of flow was detected at 12 times the rotor diameter. The rotational velocity of the wake followed a sigmoid function and was larger in the transversal than in the lateral plane by virtue of the mass flow differences within the bypass and pile interference. Overall, an increase in the wake deficit was associated with the blockage ratio and proximity of the turbine to the water surface. It could be argued that the benefits of the blockage and greater current speeds in power extraction were offset by the wake prolongation, which has implications for multi-unit designs.