Wind Turbine Field Measurements With Compact Microphone Array Using Advanced Beamforming Methods

Abstract

The knowledge of noise source locations on a wind turbine is crucial for designing low noise wind turbines. It is also important to monitor the noise from the wind turbine, be it mechanical or aerodynamic noise, to monitor the health of the wind turbine. Currently compliance standards for wind turbine noise are based on single microphone measurements at various designated locations around a wind turbine. This only provides amplitude and frequency data of the overall wind turbine noise. It is however impossible to locate noise sources with a single microphone. A sophisticated method, namely the use of a microphone phased array with advanced beamforming is necessary. Typical microphone arrays that have been used in the past for monitoring aircraft noise and wind turbine noise are very large ranging up to 300 m 2 in area with about 150 microphones. The setup is xed for a particular wind turbine and takes considerable amount of time. Our idea is to use a compact microphone array to locate and separate wind turbine noise sources successfully with the use of advanced beamforming methods such as DAMAS2, CLSC and TIDY. Such a compact tool could be used to measure multiple wind turbines as the apparatus is not xed and will be much faster and easier to make measurements. A GE 1.5 MW full scale production wind turbine was used in our study with an Optinav 24 microphone array (of area 1.5 m 2 ). A wind screen was used to cover the microphone array to eliminate ambient air interaction with the microphones. On site calibration was performed to account for the loss in amplitude due to the wind screen. The frequency spectrum of the wind turbine was studied to identify the contribution of various noise sources on the wind turbine such as mechanical noise from the nacelle and tower and aerodynamic noise from the blades. The low frequency noise component was found to be due to amplication of noise from the nacelle through the tower of the wind turbine. Mid and high frequency noise components were attributed to trailing edge noise and blade tip vortex noise from the blade. The cooling fan in the nacelle was also observed to radiate high frequency noise. It was also observed that during the yaw motor operation the structural noise was amplied considerably. Detailed discussions and the results of the experiments are presented in this paper.