An application of the parabolic equation method to long-range propagation of wind turbine noise

This paper describes the initial development of the parabolic equation (PE) method for the long-range propagation of wind turbine noise. The PE method has been used extensively for underwater propagation, but to a lesser extent for atmospheric noise propagation. The PE method is attractive because it is efficient computationally. In this paper the Crank-Nicholson Parabolic Equation (CNPE) is used to solve the PE equation numerically although the Green's Function Parabolic Equation (GFPE) method is discussed. The CNPE method is demonstrated for axisymmetric noise propagation in both homogeneous and inhomogeneous atmospheres. In order to reduce the total computational time further, an adjoint or reciprocal version of the CNPE method has been developed. The effects of irregular and absorbing terrain are modeled through the use of the Impedance Mismatch Method (IMM), which has been applied to the PE method for the first time. This technique replaces the actual irregular surface with a region of different acoustic properties. This greatly simplifies the modeling of complex terrain because the computation grid does not have to be "fitted" to the ground. Example computations are presented.