Mitigation of the aerodynamic noise of small axial wind turbines - methods and experimental validation

Although the characteristic Reynolds number of small horizontal axis wind turbines is only in the order of 100,000, their noise emissions can be annoying. Three different modifications of the blades aiming at attenuating the aerodynamic noise mechanisms are investigated: (i) Optimization of the blade profile, (ii) application of boundary layer tripping and (iii) addition of serrations to the trailing edges of the blades. All modifications were designed using either a novel optimization method or state-of-the-art design rules. They were tested on a three-bladed research wind turbine with 3 m diameter, which was placed on a large flat roof of an isolated tall building. Shaft and sound power were measured during substantial time periods, statistically analyzed and compared. Depending on the type and thickness of the blade profile, both, tripping and trailing edge serrations, were found to reduce noise but always degraded the turbine shaft power. The model-based optimization of the 15 % thick baseline profile S834 resulted in a new 10 % thick profile with a novel shape of the pressure and suction sides. Blades made from this profile, without trips but with trailing edge serrations, provided the best compromise between noise reduction and degradation of the aerodynamic turbine performance.