Some effects of compressibility on small horizontal-axis wind turbines

Many small wind turbines operate at high tip spt:ed ratio and this gives rise to situations where compressibility may influence performance. The most important example is runaway, the operating point for an unloaded turbine where the rotational speed of the blades is maximised. Because compressibility significantly increases drag through the action of ''shock-stall'', it may provide an inherent mechanism for overspeed protection. This possibility is tested computationally for a turbine with a high optimum tip speed ratio, having an aerofoil section (NACA 0012) whose behaviour in compressible flow is well known. Calculations of turbine performance for differing windspeeds, and hence Mach numbers, Show that compressibility effects occur too slowly to prevent overspeeding. However, it is suggested that aerofoils could be designed to maximise the onset of shock stall. Copyright (C) 1996 Elsevier Science Ltd.