The interaction between the rotor and the tower wake is an important source of noise for wind turbines with downwind rotors. These noise levels may significantly impact the immediate environment. During rotation the rotor blades encounter periodic changes in flow conditions as a result of the tower presence. Typically turbine towers have a circular or modified circular cross section which significantly modifies the flow in the vicinity of the tower. Upstream, the tower causes the flow to decelerate and, hence, causes a rise in pressure. Because of its bluff shape, the flow separates prematurely from the tower and this tends to create a wide, unsteady, vortical wake. The wake characteristics are dependent on the cross-sectional shape of the tower, its surface properties, the Reynolds number (based on tower diameter and wind velocity) of the flow, and the turbulence level of the incoming flow. The 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 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, 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.

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