Effects of reynolds number on the energy conversion and near-wake dynamics of a high solidity vertical-axis cross-flow turbine

Abstract

Experiments were performed with a large laboratory-scale high solidity cross-flow turbine to investigate Reynolds number effects on performance and wake characteristics and to establish scale thresholds for physical and numerical modeling of individual devices and arrays. It was demonstrated that the performance of the cross-flow turbine becomes essentially Re-independent at a Reynolds number based on the rotor diameter ReD ≈ 106 or an approximate average Reynolds number based on the blade chord length Rec ≈ 2 × 105. A simple model that calculates the peak torque coefficient from static foil data and cross-flow turbine kinematics was shown to be a reasonable predictor for Reynolds number dependence of an actual cross-flow turbine operating under dynamic conditions. Mean velocity and turbulence measurements in the near-wake showed subtle differences over the range of Re investigated. However, when transport terms for the streamwise momentum and mean kinetic energy were calculated, a similar Re threshold was revealed. These results imply that physical model studies of cross-flow turbines should achieve ReD ~ 106 to properly approximate both the performance and wake dynamics of full-scale devices and arrays.