A parametric evaluation of the interplay between geometry and scale on cross-flow turbines

The effects of geometry on cross-flow (vertical-axis) turbine performance can be challenging to predict due to the unsteady interactions the rotor experiences during each rotation. We experimentally characterize the interplay between scale and geometry by varying the ratio of the chord length to turbine radius, the preset pitch angle between the chord line and rotational tangent, and the blade count. In total, 223 unique experiments were conducted, evaluating geometric trends in cases spanning diameter-based Reynolds numbers (Re) from 8x10⁴ - 8x10⁵. We observe that maximum performance generally increases with Re and decreases with blade count. In addition, for a given Re and blade count, the optimal preset pitch angle increases with chord-to-radius ratio. Because these experiments parametrically vary the chord-to-radius ratio and blade count, we can evaluate the performance of multiple turbines with the same solidity (the ratio of total blade chord to rotor circumference). We find that while solidity can be a poor predictor of maximum performance, across all scales and geometries investigated, it is an excellent predictor of the tip-speed ratio corresponding to maximum performance.