Low Frequency Noise from Wind Turbines Mechanisms of Generation and its Modelling

Abstract

SUMMARYThe objective of the present paper is to present an overview of LFN characteristics of modern MW turbines based on numerical simulations. Typical sizes of modern turbines are from 1-3 MW nominal generator power and a rotor diameter ranging from 80-100 m but larger prototypes up to 5 MW and with a rotor diameter of 126 m have now been installed. The numerical investigations comprise the common upwind rotor concept but also the turbines with a downwind rotor are considered. The reason to include the downwind rotor concept is that this turbine design has some advantages which could lead to future competitive designs compared with the upwind threebladed rotor. The simulation package comprises an aeroelastic time simulation code HAWC2 and an acoustic low frequency noise (LFN) prediction model. Computed time traces of rotor thrust and rotor torque from the aeroelastic model are input to the acoustic model which computes the sound pressure level (SPL) at a specified distance from the turbine. The influences on LFN on a number of turbine design parameters are investigated and the position of the rotor relative to the tower (upwind or downwind rotor) is found to be the most important design parameter. For an upwind rotor the LFN levels are so low that it should not cause annoyance of neighbouring people. Important turbine design parameters with strong influence on LFN are the blade tip speed and the distance between rotor and tower.

Long term estimations of low frequency noise levels over water from an off-shore wind farm

“…51,52 Another possible source of inaccuracy in the source modeling is omitting the increased turbulence levels on downwind turbines from the wakes of the upwind turbines. As earlier shown the driving mechanism behind low frequency noise from turbines is the incoming turbulence, 7 and therefore, increased levels of turbulence might generate increased levels of low frequency noise. The used meteorological model is optimized for higher altitudes than those important for sound sources at low heights as wind turbines and could thus be uncertain at the heights most important for the sound transmission.…”

Infrasound and low frequency noise from wind turbines: exposure and health effects

“…The second source originates from the in-flow turbulence which is the main sound source in frequencies from around 10 Hz up to a few hundreds of hertz (van den Berg 2005). A model for this source by Madsen (2008) has been experimentally verified and shows satisfying results from 10 to 50 Hz. The third source is the trailing edge noise, which has its peak frequency between 500 and 1000 Hz, that is, above the region of LFN.…”

Experimental and numerical investigation of blade–tower interaction noise

“…A similar method was employed by Madsen [14], who used blade element momentum theory and also assumed potential flow around the tower to approximate the forces generated on the turbine blades. The resulting noise was obtained by using the NASA-LeRC code [15].…”