Numerical Investigation of Fluid Injection to Reduce Trailing-Edge Noise

欧盟2020年愿景预计每架飞机运行时的噪音减少10 dB。也就是说，在着陆时，除了发动机噪声之外，还必须降低由增升装置和起落架产生的机身噪声，因为它是同样重要的噪声源。由于代表细长体的增升装置和由钝头体组成的起落架的流动特性的差异，在本建议中只考虑细长体问题的降噪。作为一种通用的配置，有限厚度的平板的后缘被用来数值分析的潜力，以减少噪音时，“声学软”后缘的设计。更准确地说，涡流在边缘上对流时所经历的不连续性将被降低，以降低边缘散射的强度。这种不连续性的减少是通过在边缘上游注入额外的流体以产生近壁流动结构来实现的，该近壁流动结构类似于接近边缘时的较软表面，并且通过在钝后缘处注入流体以减少自由剪切层中的剪切应力。应用成熟的混合LES-APE方法分析了其机理，从而实现了与频率和方向性无关的噪声降低的总体目标。

The EU Vision 2020 expects a 10dB noise reduction per aircraft operation. That is, at landing besides engine noise especially airframe noise generated by high-lift devices and landing gears has to be reduced since it is an equally significant noise source. Due to the disparity of the flow characteristics of high-lift devices, which represent a slender body, and landing gears, which consist of blunt bodies, only the noise reduction of the slender-body problem will be considered in this proposal. As a generic configuration the trailing edge of the flat plate of finite thickness is used to numerically analyze the potential to reduce the noise when an “acoustically soft” trailing edge is designed. To be more precise, the discontinuity experienced by an eddy in convecting over the edge is to be lowered to decrease the intensity of edge scattering. This reduction of the discontinuity is to be realized by injecting an additional fluid upstream of the edge to generate a near-wall flow structure that resembles a softer surface as the edge is approached and by injecting fluid right at the blunt trailing edge to decrease the shear stress in the free-shear layer. The well-established hybrid LES-APE method is applied to analyze the mechanisms such that the overall objective of a frequency and directivity independent noise reduction is achieved.