Blade-Wake Interaction Noise for Turbines With Downwind Rotors

Abstract

The interaction between the rotor and the tower wake is an important source of noise for wind turbines with downwind rotors. The tower wake modifies the dynamic pressure and the local flow incidence angle as seen by the blades and, hence, modifies the aerodynamic loading of the blade during blade passage. The resulting n per revolution fluctuation in the blade loading (where n is the number of blades) is the source of low frequency but potentially high amplitude sound levels. The Wind Turbine Company (WTC) Proof of Concept 250 kW (POC) wind turbine has been observed by field personnel to produce low-frequency emissions at the National Wind Technology Center (NWTC) site during specific atmospheric conditions. Consequently, WTC is conducting a three-phase program to characterize the low frequency emissions of its two-bladed wind turbines and to develop noise mitigation techniques if needed. This paper summarizes the first phase of this program including recent low-frequency noise measurements conducted on the WTC POC250 kW wind turbine, a review of the wake characteristics of circular towers as they pertain to the blade-wake interaction problem, and techniques to attenuate the sound pressure levels caused by the blade-wake interaction.