Effect of farfield assumption on calculation of Green's function for predicting jet noise

To simplify the solution procedure of Green's function, most popular Reynolds-averaged Navier-Stokes (RANS) based jet noise prediction methods suggest to make the assumption that the jet flow is parallel and the observers are located at the infinity farfield. With the development of the solution method of Green's function, the effect of parallel flow assumption on calculation of the Green's function has been studied recently. However, the effect of farfield assumption on calculation of the Green's function has not yet been studied. To study the effect of farfield assumption, the adjoint method is used to calculate the Green's function in this paper. For actual observer 90°-150°and assumed farfield observer, the adjoint Green's functions are solved separately by a computational aeroacoustics (CAA) method. Comparison of calculation results of Green's function for actual observer and for assumed farfield observer are given in this paper. It is found that for different observation angle, the calculated deviation caused by farfield assumption is different. It is also found that there is a greater derivation of calculation results of Green's function to the point farther away from the nozzle exit. For the observer at 150°, the deviations of calculation results of Green's function caused by farfield assumption at some point are as large as-15 dB. Consequently, for observers close to the jet axis, calculation of adjoint Green's function should avoid farfield assumption to reduce the prediction error. © 2016, Press of Chinese Journal of Aeronautics. All right reserved.