Research on Noise Reduction of Air-conditioner based on the Acoustic Analysis using Large Eddy Simulation

为了模拟空调设备内部复杂的湍流流场，本文发展了一种基于广义坐标系的高阶有限差分格式。通过将其应用于波浪形通道中湍流流动的直接模拟，我们证实了我们的方案显示出足够的精度。此外，利用DNS数据库，一个广义的正应力模型（GNS模型）被发现能够预测流动分离的波浪壁。为了研究气动噪声的产生机制，并找到一种算法，以减少宽带噪声的湍流涡发射，我们制定了合适的声源指数。本文用哈丁和波普提出的直接法再现了二维空腔流和旋转涡对的声场。其结果是，传播到远场的声源被识别的压力波动的二阶时间导数。在此基础上，我们研究了壁湍流辐射的声场。我们发现流向涡和发夹状涡的相干结构是造成湍流噪声的原因。在离壁面较远的地方还观察到了一些相干的气泡状声压结构，而声场的频谱具有宽带和无序的特点。此外，通过控制近壁区的涡结构，对降低噪声强度和噪声源进行了数值试验研究。结果表明，近壁结构的衰减导致了远场声压级的降低。因此，我们证实了湍流控制技术对降噪也是有效的。

In order to simulate the complicated turbulence flow field concerning to the air-conditioner equipment, we developed a high-order finite difference scheme using the generalized coordinate system. By applying it to a direct simulation of turbulent flow in a wavy channel, we confirmed our scheme showed enough accuracy for our purpose. Furthermore using this DNS databases, a generalized normal stress model (GNS model) was found to be able to predict the flow separation on the wavy wall.To investigate the mechanism of generation of aerodynamic noise and to find an algorithm to reduce the broad band noise emitted from turbulent eddies ; we formulated suitable indices of sound source. In this study, acoustic fields from 2D Cavity flow and rotating vortex pair are reproduced with the direct method proposed by Hardin and Pope. As a result, the sources of propagated sound to the far field is identified by the second time derivative of pressure fluctuation. Based on this index, we examined the acoustic field emitted from wall turbulence. We revealed that the coherent structure of streamwise vortices and hair-pin-like vortices are responsible for turbulent noise. Also some coherent bubble-like structures of acoustic pressure were observed further away from the wall, while the frequency spectra of acoustic field is characterized to be broadband and disorder. Moreover, a numerical experiment to reduce the intensity and source of noise was examined by the control of the vortex structures in near-wall region. As a result, the attenuation of near-wall structure caused the reduction of sound pressure level in far field. Thus we confirmed that the turbulence control technique was also effective for noise reduction.