苏彩虹. 发动机外罩波纹形尾缘降噪机理初探[J]. 空气动力学学报, 2018, 36(3): 410-416. DOI: 10.7638/kqdlxxb-2018.0066
引用本文: 苏彩虹. 发动机外罩波纹形尾缘降噪机理初探[J]. 空气动力学学报, 2018, 36(3): 410-416. DOI: 10.7638/kqdlxxb-2018.0066
SU Caihong. Tentative explanation of the mechanism of noise reduction for jet engine with chevron at the trailing edge of the nacelle[J]. ACTA AERODYNAMICA SINICA, 2018, 36(3): 410-416. DOI: 10.7638/kqdlxxb-2018.0066
Citation: SU Caihong. Tentative explanation of the mechanism of noise reduction for jet engine with chevron at the trailing edge of the nacelle[J]. ACTA AERODYNAMICA SINICA, 2018, 36(3): 410-416. DOI: 10.7638/kqdlxxb-2018.0066

发动机外罩波纹形尾缘降噪机理初探

Tentative explanation of the mechanism of noise reduction for jet engine with chevron at the trailing edge of the nacelle

  • 摘要: 喷气发动机的射流是起飞过程中主要的噪声源之一。发动机外罩外的流动和外涵道内的环形射流在尾缘下游形成一个强剪切层。由剪切层的不稳定性产生的大尺度结构是一个重要的噪声源。近年发现,一些现代航空发动机外罩采用了波纹形尾缘的设计,被认为是一种降噪的措施。本文采用简化模型,即一个分割两层流体的平板后缘形成的剪切层,从流动稳定性的角度探讨其降噪的机理。研究发现,对于平直尾缘的情况,尾缘后会产生二维的非定常涡,对应的是剪切层中最不稳定的模态。而对于波纹形尾缘的情况,则不存在二维模态。对三维平均流的全局稳定性分析显示,所得三维最不稳定模态的增长率显著小于平滑尾缘的情况。数值模拟结果也证实了这一结论。因此,波纹形尾缘降噪的机理可以归结为,波纹形设计降低了平均流的不稳定性,从而降低了大尺度结构的增长率和幅值,使得Lighthill声源项中雷诺应力的二阶导数项也相应大幅减小,从而降低了噪声。

     

    Abstract: Jet engine exhaust is one of the main noise sources during airplane take-off. There is a strong shear layer formed between the flow outside the nacelle and the circular jet flow in its bypass duct downstream of the trailing edge. The large vortex structure due to the instability of the shear layer is an important noise source. Recently, some modern jet engines have chevrons at their trailing edge, which is believed to be a measure for noise reduction. In this paper, from the perspective of flow instability, a simplified model, i.e. the shear layer downstream of the trailing edge of a flat plate splitting two uniform streams, is proposed to explore the possible cause of the noise reduction. It was found that for the case without chevron, unsteady vortex roll-up appears downstream of the trailing edge, whose frequency is actually chosen to match the most unstable 2-D shear mode by the stability characteristics of the mean flow. For the case with chevron, there is no 2-D layer mode. Biglobal stability analysis of the 3-D mean flow shows that the most unstable mode is significantly smaller compared with the case without chevron, which is also verified by the numerical simulation. Therefore, the mechanism of noise reduction for using chevron can be concluded as the following. The existence of chevron changes the stability characteristics of the mean flow, so that the amplification rate and the amplitude of large scale vortices are reduced. So are the second derivatives of Reynolds stress, which is one of source terms in noise generation and the noise is thus reduced.

     

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