远场假设对喷流噪声预测中格林函数求解的影响

目前基于雷诺平均Navier-Stokes（RANS）的喷流噪声预测方法在格林函数求解时， 为简化求解过程，通常对喷流流动做平行流假设， 对观测点做远场假设。随着格林函数求解方法发展，近年来的研究表明平行流假设对下游观测点格林函数的计算会引起较大偏差，而目前远场假设对格林函数求解的影响仍不清楚。为研究远场假设对喷流格林函数求解的影响，以二维喷流为例， 采用计算气动声学方法（CAA）分别数值求解了观测点远场假设条件与实际条件下90°~150°方向喷流内伴随格林函数，进而分析远场假设对格林函数求解的影响。研究结果表明，对于不同方向的观测点，由观测远场假设导致的伴随格林函数求解偏差不尽相同，且对于越靠近喷流中心线方向的观测点，远场假设导致的偏差越大，其中150°方向观测点，采用远场假设后，格林函数计算结果最大偏差达到-15 dB以上。因此，对于靠近喷流中心线方向的噪声观测点而言，为避免预测偏差，应采用实际观测条件求解喷流格林函数。

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.