Application of a ray theory model to the prediction of noise emissions from isolated wind turbines and wind parks

Abstract

Various propagation models have been developed to estimate the level of noise near residential areas. Predictions and measurements have proven that proper modelling of the propagation medium is of particular importance. In the present work, calculations are performed using a ray theory methodology. The ray trajectory and transport equations are derived from the linear acoustics equations for a moving medium in three dimensions. Ground and atmospheric absorption, wave refraction and diffraction and atmospheric turbulence are taken into account by introducing appropriate coefficients in the equations. In the case of a wind turbine (W/T) it is assumed that noise is produced by a point source located at the rotor centre. Given the sound power spectrum, the noise spectrum at the receiver is obtained by solving the axisymmetric propagation problem. The procedure consists of (a) finding the eigenrays, (b) calculating the energy losses along the eigenrays and (c) synthesizing the sound pressure level (SPL) by superposing the contributions of the eigenrays. In the case of a wind park the total SPL is calculated by superposing the contributions of all W/Ts. Application is made to five cases of isolated W/Ts in terrains of varying complexity. In flat or even smooth terrain the predictions agree well with the measurements. In complex terrain the predictions can be considered satisfactory, taking into account the assumption of constant wind velocity profile. Application to a wind park shows clearly the influence of the terrain on the wind velocity and consequently on the SPL. Copyright © 2006 John Wiley &Sons, Ltd.

Coupling of an aeroacoustic model and a parabolic equation code for long range wind turbine noise propagation

“…This approach has been used for instance in the ray-tracing calculations of Ref. [8], and in the parabolic equation calculations 15 of Refs. [9,10].…”

Determination of equivalent sound speed profiles for ray tracing in near-ground sound propagation

“…Although the model provided satisfactory results using the mean logarithmic wind velocity profiles estimated by similarity theory, in cases of wind turbine sound propagation 22 Springs, Radlett and Hatfield, where the source height is less than 2 m. Because of that, a spatial variation of the effective sound speed profiles was sought to better simulate the high wind velocity gradients caused by turbulence at such low heights. The lack of measurements of near-ground wind speed and temperature profiles does not allow to judge how far a realistic effective sound speed profile deviates during measurement period from a constant, linear or logarithmic profile, and how such deviation affects the predicted sound pressure levels and their consistency to the measured levels.…”

“…For a source height between 30 and 100 m, as in the case of a wind turbine, the mean wind velocity and temperature profiles estimated by the similarity theory produce satisfactory results. 22 However, in near-ground propagation ͑source height up to 5 m͒, the use of mean velocity profiles may not well simulate the high temporal fluctuations of the steep gradients which significantly affect the number of eigenrays. L'Esperance et al 18 used an equivalent linear sound speed profile in their simple geometrical model, the gradient of which is derived from the mean logarithmic profiles according to the theory of the first Fresnel zone.…”

Wind Energy: Status and Trends

“…However, care has to be taken with siting, particularly in complex terrain where noise can propagate differentially in different stability conditions. This is the subject of ongoing research (Prospathopoulos and Voutsinas 2006).…”