Influence of the downstream blade sweep on cross-flow turbine performance

Cross-flow turbine blades encounter a relatively homogenous inflow for half their rotation (the upstream sweep) and then pass through their own wake for the other half (the downstream sweep). Most research on cross-flow turbine optimization has focused on the power-generating phases of the upstream sweep, but the downstream sweep significantly affects net turbine performance. Here, a single-bladed cross-flow turbine is used to experimentally isolate power contributions from the upstream and downstream sweeps. Two-component, phase-locked, PIV data is obtained inside the turbine swept area to investigate the hydrodynamic mechanisms underlying the observed degradation in downstream performance at high tip-speed ratios. We find that power generation from the upstream sweep continues to increase beyond the optimal tip-speed ratio. In contrast, power generation from the downstream sweep is net neutral until just before the optimal tip-speed ratio, after which it decreases faster than the upstream power generation increases. This indicates that the optimal tip-speed ratio is driven by the point at which the downstream sweep consumes appreciable power. At high tip-speed ratios, lift production in the downstream sweep becomes increasingly normal to the rotation direction while drag increases with rotation rate. These results highlight the need to consider the role of the downstream sweep on overall turbine performance and the projection of lift and drag into the direction of blade rotation.