发动机喷流噪声的声源识别与声源机理研究

Unsteady turbulent eddies inside the jet mixing layer of the aircraft engine exhaust perturb the surrounding low-speed air and radiate energy as sound waves. This is the largest contribution to the far-field noise radiation when the aircraft takes off, and poses extreme health threats to the ground crew. Base on the cause-effect reasoning, various acoustic equations, such as the acoustic analogy theories and the pressure wave equation, were proposed to describe the (generation and) propagation of sound waves inside the background turbulent flow that produces the noise radiation, and led to some consensuses on the noise source characteristics and the noise generation mechanisms. However, controversies also exist due to the lack of consensus in separating the background flow and the acoustic perturbations from each other, which in turn results in the difficulty in designing accurate governing equations of the former and the invalidity of the latter, especially in the noise source region. The proposed study aims first to resolve these controversies by clearly identifying the different characteristics of the acoustic perturbations and the background flows. After establishing physically reasonable models and governing equations, this study will extract both terms from the entire solutions and clearly formulate the acoustic perturbations as functions of the background flow. Therefore, this procedure unambiguously identifies the radiating sources and reveals the noise generation mechanisms of various modes of the noise sources. All these will serve the ultimate goals of designing more accurate source models for noise prediction and more sophisticated noise reduction devices.

在发动机高速尾喷流剪切层中，非定常湍流涡扰动邻近气体而产生的辐射声波，是航空器起飞时最主要的噪声来源，可对地勤人员造成严重的健康威胁。过去的研究提出了声比拟理论、压力波动方程等声扰动方程，从因果律角度描述声扰动在导致此声扰动的背景湍流中的（产生和）传播，获取了对湍流声源分布与声源物理机理既有初步共识、也有诸多争议的认知。这些争议的根源是由于对声源区中背景流动和声扰动缺乏公认合理清晰的定义，以致缺乏准确的背景流动方程，而声扰动方程也在声源区失效。本课题拟从厘清二者的物理特征和区别出发，澄清现有理论的局限和争议，对二者均构建合理的物理模型和控制方程，以高精度数值模拟为手段实现背景流动和噪声的分离，将辐射声源和声扰动明确表达为背景流动的函数，从动力学角度回答如何准确识别实时湍流声源、以及不同模态的辐射声源如何产生声扰动这两个关键科学问题，为设计更精确的声源模型和更高效的降噪装置奠定理论基础。

缺乏对发动机喷流剪切层中湍流涡如何产生噪声辐射这一物理过程的深刻理解，是我们难以设计出高效降噪装置的根本原因。在已有的声源机理研究中，对声源区中背景流动和声扰动缺乏公认合理清晰的定义，以致缺乏准确的背景流动方程，而声扰动方程也在声源区失效。在此背景下，本项目发展了耦合PSE/CDE的非辐射流场脉动和噪声脉动的分离方法。该方法一方面通过迭代算法进一步抑制传统PSE解中向下游传播的声模态，利用PSE解模拟不同流动模态沿流向的演化；另一方面利用CDE的完整形式为NS方程的特征，将定常层流与PSE解的全部或不同模态作为参考解，从而求解不同流动模态的噪声解。对准二维剪切流和三维喷流的模拟表明，PSE解几乎不包含噪声脉动分量，并能准确模拟近场流动涡结构的演化；而CDE解可准确还原不同流动模态的远场噪声解。同时我们以声比拟理论为基础，以PSE流动模态重构声源模型，准确预测了远场噪声，并分析了不同流向、周向模态的声源机理。研究结果表明，该方法证实了以不可压流动方程模拟流动模态的局限性，以及以相速度区分流动和噪声模态的合理性，澄清了现有理论的局限和争议，为进一步发展先进的噪声预测模型、设计先进的降噪方法提供了理论基础。