Measurements on an oscillating aerofoil with applications to wind energy conversion

Abstract

An open-jet wind tunnel and an experimental test rig were built to perform oscillation tests under two-dimensional conditions on an NACA 0012 aerofoil of 0.2 m chord and 0.8 t span. The Reynolds number ranged between 0.5 x IO5 and 3.3 x IO5 and the reduced frequency, ωc∕2V, from 0.03 to 0.45. The aerofoil was driven mechanically at amplitudes up to 34° in pitch (about the half-chord axis), 0.2 chord in heave, and in a number of combined motion modes. Forces and moments were measured by a strain gauge balance interposed between the driving mechanism and the aerofoil, and results were corrected for tunnel interference. The results were compared with the potential flow theory for flat plates by von Karman and Sears. The amplitude of motion had definite effects on the measured Iift and moment co efficients in all three types of motion (pure heave, pure pitch and combined heave and pitch) The agreement between the theory, which is for small amplitude oscillations, and experiment became better for increasing reduced frequencies, with close convergence from 0.4 onwards. Forces in the air stream direction were also measured. For pure heave and pure pitch their time-averaged values agreed well with potential flow predictions by Garrick. The dynamic stall delays in pure pitch at the present test Reynolds numbers (Re - 105) were compared with those observed by other researchers at other Reynolds numbers. The stall delay at Re ^∙ IO5 were found to be significantly longer than those at Re ~ 104s but about the same as those at Re ~ 106. The performance of an aerofoil oscillating in the combined mode as a Wind Energy Converter was compared with the Darrieus-type Vertical Axis Wind Turbine. It was found to be comparable or better in aerodynamic efficiency, although probably Iess productive for the same amount of blade material. Data for pure pitch oscillations were used in calculating the performance of Darrieus-type turbines. The calculating procedure was based on Templin1S method adapted for use with unsteady aerofoil data, and turned out to be much Iess complicated than might have been expected. The results of these calculations revealed significant power increases relative to quasi-steady predictions at Iow rotor tip speed ratios.