Multivariable analysis of obstacle noise

Noise emitted by cylinders in a subsonic wind tunnel was studied, and the effectiveness of multivariable analysis involving partial and multiple coherent functions for the treatment of numerical data was demonstrated. Since the significance of relative coherence scales and the insignificance of coherent angles was shown, it was maintained that the coherence surface, which emits low frequency noise, is very weak. Fluctuation of the separation line on the cylinder, whose lateral extension is limited, was proposed as a possible mechanism. Results supported the hypothesis of fluctuation very close to separation lines. It was also found that the partial coherence module between acoustic pressure and wall pressure was high, and that the partial coherence module between acoustic pressure and unsteady velocity in the near wake was low in all operation modes. Applications include the management of aircraft, helicopter blades, and especially fan noise.