Torque measurement on a simple vertical-axis windmill using a finite flat plate

We conduct subsonic wind-tunnel experiments, in order to investigate the basic aerodynamic characteristics of an autorotating finite flat plate in uniform mainstream, which has a potential for new windmills and water-wheels with very simple structures in addition to new mixers/diffusers. The present study focuses on both the effects of an aspect ratio AR and a tip-speed ratio Q* upon the tumbling of the flat plate with a low thickness-to-breadth ratio λ = 2.0 × 10^-2 - 5.0 × 10^-2. We carry out the measurements using a torquemeter together with the synchronised measurements of the plate´s attack angle α for the analyses with phase-averaging technique, in addition to measurements using a hot-wire anemometer and flow visualisations. As a result, the torque coefficient C_T linearly decreases with increasing 0*, and C_T tends to become large with increasing AR. Then, neutral tip-speed ratio ΩP*_CT0 where C_T crosses zero tends to become large with increasing AR. Power coefficient C_P shows similar tendencies with C_T. In addition, we confirm that there exist two torque modes T-mode I and T-mode II, and the transition between them. The T-mode I tends to appear at low AR and low Ω*. And, the T-mode II tends to appear at high AR and high Q*.