Analysis of noise emission from the Bonus Kombi wind turbine

Part of the problem in comparing noise emission levels from wind turbines with theory is that it is difficult to know exactly which mechanism is responsible for noise in each part of the spectrum. The majority of noise measurements are made at ground level, some distance away from the machine, and generally the results are expressed as average values over many cycles of the turbine. The aim of these experiments was to address this problem, and to develop a technique for investigating each noise mechanism separately. A microphone was placed behind a rotating blade, and the output voltage sampled rapidly. We would expect to detect leading edge noise before detecting trailing edge noise, and therefore it should be possible to separate these two noise sources. More importantly, the very low frequency microphone trace can provide information about the aerodynamic conditions on the blade. A calculation to see if it is possible to deduce the bound circulation on the blade directly from the low frequency pressure fluctuation detected by the microphone is described. An accurate description of the decrease in bound circulation near the tip could be a useful input to a tip noise theory, especially if comparisons were made for tips of different shapes. The analysis technique also allows the separation of mechanical discrete frequency noise from aerodynamic broad band noise, and in the final section, the question of whether mechanical noise is radiated by the blades is addressed. (author)