Aeroacoustic Noise Measurements of a Wind Turbine with BSDS Blades Using an Acoustic Array

Environmental noise restrictions play a role in the performance of wind turbines and the dominant source of noise on modern turbines is aeroacoustic noise produced by the blades. A 32 sensor acoustic array has been developed in cooperation between the National Renewable Energy Laboratory (NREL) and the University of Colorado to conduct measurements of aeroacoustic noise from a 100 kW wind turbine. The array is capable of broadband measurements from 125 Hz to 8 kHz and can locate noise sources with a spatial resolution of 2 meters. Successful noise measurements have been made on a 36.4 meter hub height wind turbine at the National Wind Technology Center (NWTC) in Colorado. Currently the array is deployed at the USDA Agricultural Research Service in Bushland, Texas. Sandia National Laboratories is field testing the BSDS blades on a 23 meter hub height turbine and the array is being used to study their noise signature. Trailing edge noise near the blade tip has traditionally been the focus of acoustic array field experiments, but a goal of this test campaign is to observe the inboard sections of the blades as well as the tip region. The many challenges in designing an array for broadband analysis of small wind turbines using a limited number of sensors are presented here along with an overview of the general design process. Initial results from the Bushland, TX field measurements are provided including analysis of low frequency inboard blade noise and higher frequency noise near the blade tip. Noise near the blade tip is strongest on the downward half cycle of rotation as expected, while the inboard noise is present at all rotor azimuth angles.