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

Cross-flow turbine blades encounter a relatively undisturbed inflow for the first half of each rotational cycle ("upstream sweep") and then pass through their own wake for the latter half ("downstream sweep"). While most research on cross-flow turbine optimization focuses on the power-generating upstream sweep, we use singled-bladed turbine experiments to show that the downstream sweep strongly affects time-averaged performance. We find that power generation from the upstream sweep continues to increase beyond the optimal tip-speed ratio. In contrast, the power consumption from the downstream sweep begins to increase approximately linearly beyond the optimal tip-speed ratio due in part to an increasingly unfavorable orientation of lift and drag relative to the rotation direction as well as high tangential blade velocities. Downstream power degradation increases faster than upstream power generation, indicating the downstream sweep strongly influences the optimal tip-speed ratio. In addition, particle image velocimetry data is obtained inside the turbine swept area at three tip-speed ratios. This illuminates the mechanisms underpinning the observed performance degradation in the downstream sweep and motivates an analytical model for a limited case with high induction. Performance results are shown to be consistent across 55 unique combinations of chord-to-radius ratio, preset pitch angle, and Reynolds number, underscoring the general relevance of the downstream sweep.