  Cross-flow turbines harness kinetic energy in wind or moving water. Due to
their unsteady fluid dynamics, it can be difficult to predict the interplay
between aspects of rotor geometry and turbine performance. This study considers
the effects of three geometric parameters: the number of blades, the preset
pitch angle, and the chord-to-radius ratio. The relevant fluid dynamics of
cross-flow turbines are reviewed, as are prior experimental studies that have
investigated these parameters in a more limited manner. Here, 223 unique
experiments are conducted across an order of magnitude of diameter-based
Reynolds numbers ($\approx 8\!\times\!10^4 - 8\!\times\!10^5$) in which the
performance implications of these three geometric parameters are evaluated. In
agreement with prior work, maximum performance is generally observed to
increase with Reynolds number and decrease with blade count. The broader
experimental space clarifies parametric interdependencies; for example, the
optimal preset pitch angle is increasingly negative as the chord-to-radius
ratio increases. As these experiments vary both the chord-to-radius ratio and
blade count, the performance of different rotor geometries with the same
solidity (the ratio of total blade chord to rotor circumference) can also be
evaluated. Results demonstrate that while solidity can be a poor predictor of
maximum performance, across all scales and tested geometries it is an excellent
predictor of the tip-speed ratio corresponding to maximum performance. Overall,
these results present a uniquely holistic view of relevant geometric
considerations for cross-flow turbine rotor design and provide a rich dataset
for validation of numerical simulations and reduced-order models.
