Effect of Aspect Ratio on Cross-Flow Turbine Performance

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 efficiency is found to be invariant for the range of aspect ratios tested (0.95 – 1.63), which we ascribe to minimal blade-support interactions characteristic of the particular turbine investigated. Finally, a subset of experiments is repeated without controlling for the Froude number and the efficiency 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.