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

Abstract

Cross-flow turbine (known as vertical-axis wind turbines or ``VAWTs'' in wind) 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 single-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 downstream sweep consumes power beyond the optimal tip-speed ratio due to unfavorable lift and drag directions relative to rotation and a potentially detrimental pitching moment arising from rotation-induced virtual camber. Downstream power degradation increases faster than upstream power generation, such that downstream sweep performance determines the optimal tip-speed ratio. In addition to performance measurements, 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 limiting 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 significance of the downstream sweep.