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"The aim of this project was to undertake a thorough investigation of blade design for variable speed wind turbines with specific emphasis being on techniques to reduce aerodynamic noise emission from the rotor. The main tasks of the project were:

i. to perform comprehensive noise measurements on a variable speed wind turbine;
ii. to optimise blade geometry and specify the required aerofoil characteristics for a low noise blade;
iii. to develop aerofoil and blade designs to achieve the required aerodynamic noise and performance characteristics.

The measurements were correlated with tip speed, angle of attack, and 50m wind speed, using the method of partial residuals. The results showed that the dependence of noise on tip speed varies with the frequency of the noise, lower frequency noise (< 400 Hz) generally being less sensitive to tip speed. Correlations of residuals with angle of attack showed no clear pattern despite the wide range of angle of attack residuals (-5 to +2 degrees). Similarly, correlations with wind speed residuals showed no clear pattern.

The theoretical study was based on the noise models of Brooks, Pope and Marcolini, and Lowson. Aerofoil boundary layer parameters were calculated using the program XFOIL. The study demonstrated that the method of calculation of these parameters has a very strong influence on the noise predictions; the most important issues being;

i. whether free or forced boundary layer transition should be assumed
ii. if free transition is assumed, the choice of the parameter N in the Tollmein-Schlichting equation
iii. the chordwise position at which the aerofoil boundary layer parameters are specified.

Each of these issues can be more important than the choice of aerofoil. To clarify these questions, detailed wind tunnel measurements would be required.

Having decided on a standardised method of calculating boundary layer parameters, the effects on predicted noise were investigated. It was demonstrated tha

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