  Cross-flow turbines convert kinetic power in wind or water currents to
mechanical power. Unlike axial-flow turbines, the influence of geometric
parameters on turbine performance is not well-understood, in part because there
are neither generalized analytical formulations nor inexpensive, accurate
numerical models that describe their fluid dynamics. Here, we experimentally
investigate the effect of aspect ratio - the ratio of the blade span to rotor
diameter - on the performance of a straight-bladed cross-flow turbine in a
water channel. To isolate the effect of aspect ratio, all other non-dimensional
parameters are held constant, including the relative confinement, Froude
number, and Reynolds number. The coefficient of performance is found to be
invariant for the range of aspect ratios tested (0.95 - 1.63), which we ascribe
to minimal blade-support interactions for this turbine design. Finally, a
subset of experiments is repeated without controlling for the Froude number and
the coefficient of performance is found to increase, a consequence of Froude
number variation that could mistakenly be ascribed to aspect ratio. This
highlights the importance of rigorous experimental design when exploring the
effect of geometric parameters on cross-flow turbine performance.
