An experimental evaluation of blockage effects on the wake of a cross-flow current turbine

Abstract

While flow confinement, or blockage, is known to affect current turbine performance and wake evolution, there have been limited experimental investigations of the wake evolution of cross-flow turbines under variable confinement. In this study, velocity data were collected in the near wake of a laboratory-scale (0.51 m diameter) cross-flow turbine under two different blockage conditions. To isolate the effects of blockage, other parameters that affect turbine performance, such as the Reynolds number, Froude number, turbine submergence depth, and free-stream turbulence intensity, were held constant. The turbine was operated at the tip-speed ratio corresponding to peak power for each blockage case. Increasing the blockage caused faster streamwise flow speeds through and around the turbine, a decreased overall wake size, higher levels of turbulent kinetic energy in the wake, and an increased viscous dissipation rate. This suggests that higher blockage could increase the power output and reduce the physical footprint of current turbine arrays. However, these benefits must be weighed against the potential for high blockage arrays to reduce a turbine’s “basin efficiency”, which is influenced by how thrust changes with blockage. Furthermore, we observed that decreasing the width of the experimental channel while holding the depth constant decreased the extent of the wake in the lateral direction only. The wake was unaffected in the vertical direction, which suggests that lateral and vertical blockages have independent effects on turbine wakes. Consistent with prior studies, we also observed significant wake mixing in the vertical (i.e., spanwise) direction and negligible wake mixing in the lateral direction for both blockage conditions.