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

为了对空调设备内部复杂的湍流流场进行数值模拟，提出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通道中的湍流直接模拟，我们证实了我们的格式具有足够的精度来达到我们的目的。在此基础上，建立了一个广义的正应力模型(GNS模型)来预测波纹壁上的流动分离。为了研究气动噪声的产生机理，寻找一种降低湍流涡流产生的宽带噪声的算法，我们制定了合适的声源指数。本文用Hardin和Pope提出的直接方法模拟了二维空腔流动和旋涡对的声场。通过对压力脉动的二次导数来识别远场传播的声源。基于这一指标，我们研究了壁面湍流的声场。我们揭示了湍流噪声是由流向涡和发夹状涡的相干结构引起的。在远离壁面的地方观察到声压的共格气泡状结构，而声场的频谱表现为宽频和无序。此外，还进行了通过控制近壁区涡旋结构来降低噪声强度和噪声源的数值实验。结果表明，近壁结构的衰减导致了远场声压级的降低。从而证实了湍流控制技术在降噪方面也是有效的。

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.