  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.
