Simulation of Low Frequency Noise from a Downwind Wind Turbine Rotor

Abstract

One of the major drawbacks of a wind turbine with a downwind rotor is the generation of considerable low frequency noise (so-called thumping noise) which can cause annoyance of people at a considerable distance. This was experienced on a number of full-scale turbines in e.g. US and Sweden in the period from around 1980 to 1990. One of the common characteristics of this low frequency noise, emerging from analysis of the phenomenon, was that the sound pressure level is strongly varying in time. We have investigated this phenomenon using a model package by which the low frequency noise of a downwind rotor can be simulated. In order to investigate the importance of wake unsteadiness, time true CFD computations of the flow past a 4 m diameter cylinder were performed at 8 m/s, and the wake characteristics were subsequently read into the aeroelastic code HAWC, which finally gives output to the aero acoustic model. The results for a 5 MW two-bladed turbine with a downwind rotor showed an increase in the sound pressure level of 5-20 dB due to the unsteadiness in the wake caused mainly by vortex shedding. However, in some periods the sound pressure level can increase additionally 0-10 dB when the blades directly pass through the discrete shed vortices behind the tower. The present numerical results strongly confirm the experiences with full scale turbines showing big variations of sound pressure level in time due to the wake unsteadiness, as well as a considerable increase in sound pressure level if the blade passing frequency is close to the Strouhal number controlling the vortex shedding from the tower.