Research on Aeroacoustic Control using Direct Numerical Simulation.

为了建立一种降低气动噪声的技术，我们研究了流场和声场的协同分析，以预测流动控制对声场的影响。这是因为这两个场的尺度有很大的不同，而且声压比流场的要小一个数量级。在这个项目中，开发了以下方法。首先，采用直接数值模拟（DNS）或大涡模拟（LES）技术求解非定常不可压缩的Navier-Stokes方程，得到声源；然后，利用界面场的Hardin和Pope方法将声分量从流场中分离出来。然后根据声学近似方程计算远场声压。所考虑的流场是湍流圆形射流和叶片周围的非定常流场。通过引入高阶一致有限差分法，提高了DNS和LES再现声源的精度。对于声场，提出了Hardin-Pose方法在高阶精度曲线坐标系上的应用。在此基础上，对平板附近准流向涡的形成机理进行了研究，并进行了控制准流向涡的数值实验。

To establish a technique for the reduction of aerodynamic noise, we investigated a cooperative analysis of flow and sound fields to predict the effect of flow control on the acoustic field. The difficulty is caused by the facts that these two fields have significantly different scales and that the acoustic pressure is some order of magnitude less than that of flow field. In this project, the following approach has been developed. First, the unsteady incompressible Navier-Stokes equations are solved by the Direct Numerical Simulation (DNS) or Large-eddy Simulation (LES) technique to obtain the sound source. Next, the acoustic component is separated from the flow field using the method of Hardin and Pope in the interfacial field. The far-filed sound-pressure is then calculated based on the acoustically approximated equation. The flow fields considered are turbulent round jet and the unsteady flow around a blade. The accuracy for reproducing the sound source by DNS and LES has been improved by introducing the higher-order and consistent finite-difference method. For the acoustic field, the application of the Hardin-Pose's method on the curvilinear coordinate system with higher-order accuracy has been developed. In addition, the mechanism of quasi-streamwise vortices near a flat plate and a numerical experiment of their control were carried out by the DNS.