Unexpected strong and high frequency sounds have been noted during test operation of wind turbine rotor blades which were manufactured to have a sharp trailing edge and smooth surface for acoustic tests. Previous tests to investigate the sound had revealed that it was composed of many discrete tones believed to be generated by T-S instability waves in the laminar boundary layer developed on the pressure side of the airfoil. To obtain basic data for clarifying how the sound is generated, the frequencies of the tones were measured through two experiments: firstly, the tonal component distribution along the full span of the rotor blade was measured using the microphone array technique, and secondly, the tonal frequency dependence on the section speed was measured for a wide range of rotor speed. The results of these experiments revealed that the sources of the tone are located within the span of 2 m from the blade tip and that the tonal frequency changes with the 0.8-th power of the section speed with a possible ladder-like structure.
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