Evaluating Reynolds number effects in small-scale wind turbine experiments

Abstract

A series of experiments was conducted to evaluate the effect of the Reynolds number on the initial wake expansion and on the thrust coefficient in scaled wind turbine tests. Two scaled horizontal axis wind turbines were tested in a 0.6m×0.8m closed-loop water channel at Reynolds numbers that ranged from 3620 to 31,400, based on the blade tip speed and tip chord. The first rotor was a three-bladed rotor custom-designed to operate in these low Reynolds number conditions. The second rotor was a 1/4.8 scaled model of an existing two-bladed rotor. The initial wake expansion was assessed by using quantitative dye visualisation to identify the propagation of tip vortices downstream of the rotor. Thrust coefficients were determined from data obtained with a six degree-of-freedom balance. Dye visualisation indicated that the initial wake expansion downstream of the custom three-bladed rotor narrowed when the Reynolds number was lower than 20,000. Additional experiments compared the wake expansion downstream of the scaled two-bladed rotor to existing data recorded for a medium-scale rotor. The wake in the scaled two-bladed rotor was 30–60% narrower than the medium-scale rotor′s wake depending on the Reynolds number. Similarly, the thrust coefficient was reduced by 25–60% in the scaled two-bladed rotor experiments as a result of lower Reynolds numbers than in the medium-scale experiment. The results have identified that the thrust coefficient of a geometrically-scaled rotor may be degraded in a low Reynolds number environment whereas a rotor designed for the particular Reynolds number regime can generate thrust coefficients that are representative of commercial-scale wind turbines.