亚、超音速射流中不稳定波和相干结构的噪音产生机理

Facing the continuing expansion of air travel/transportation and tightening noise regulation, there is an imperative need for the aviation industry to cut substantially the level of aircraft noise, of which jet noise constitutes a significant part. Innovative design of quiet aircrafts depends crucially on improving our understanding of noise generation mechanisms. One of the important mechanisms is thought to be associated with instability waves (in transitional jets) and large-scale coherent structures (in turbulent jets). Significant theoretical progress has been made by using asymptotic approach for aeroacoustics, to elucidate how instability waves in the fully developed region of supersonic and subsonic jets emit noise. Recent experiments further confirm that coherent structures in a turbulent jet and instability waves of the mean-flow profile are highly similar, and the data supports the notation that coherent structures can be represented by instability modes of the mean flow. Prompted by this exciting finding and the recent theoretical progress, the present project aims to treat noise generation by coherent structures and instability waves in a unified mathematical framework of asymptotic approach that the applicant has been advancing. Specifically, we plan (1) to investigate the radiation of sound by multiple interacting instability modes in the near-field shear layer of sub- and supersonic jets, (2) to propose and validate a mathematical model describing the nonlinear dynamics of coherent structures, (3) to apply the model to predict sound generation in turbulent jets, and (4) to develop, by combining the nonlinear PSE with asymptotic techniques, a method for predicting noise emission of instability modes with a broadband spectrum.

面临不断增长的航空旅客、货运流量和更严的噪音限制规定，民用航空工业迫切需要大幅度地降低飞机噪音。设计低噪音飞机关键依赖于对噪音产生机理有更好的理解。射流噪音的产生被认为与不稳定波(层流情形)和相干结构（湍流情形）紧密相关。气动声学的渐近理论已经能够比较成功地揭示亚、超音速层流射流完全发展区中的不稳定波是如何产生噪音的。最近的实验研究进一步证实湍流射流中相干结构的特性与平均流剖面的不稳定模态高度相似，后者在很大程度上代表相干结构。受这一重要实验发现和最近理论进展驱动，本项目将把相干结构和不稳定波产生噪音的机理和过程，在气动声学渐近理论这一数学框架内，统一地加以处理。具体内容包括：研究亚、超音速射流近场剪切层中多波相互作用的发声机理和特性；提出并验证描述相干结构非线性动力学的数学模型；将相干结构动力学模型用于湍流射流的发声机理研究；发展基于非线性PSE与渐近理论相结合的射流噪音预测方法。

不断增长的航空旅客、货运流量和更严的噪音限制规定，迫切要求民用航空工业大幅度地降低飞机噪音。设计低噪音飞机依赖于对噪音产生机理有更好的理解。射流噪音是飞机噪音的主要部分，它的产生被认为与不稳定波(层流情形)和相干结构（湍流情形）紧密相关。气动声学的渐近理论已能比较成功地揭示亚、超音速层流射流完全发展区中的不稳定波是如何产生噪音的。最近的实验研究进一步证实湍流射流中相干结构的特性与平均流剖面的不稳定模态高度相似，后者在很大程度上代表相干结构。受这一实验发现和最近理论进展驱动，本项目把相干结构和不稳定波产生噪音的机理和过程，在气动声学渐近理论这一数学框架内，统一地加以处理。主要研究内容，重要结果和意义如下。（1）基于对层流稳定性的非线性临界层理论推广，提出描述湍流自由剪切流中相干结构非线性动力学的数学模型, 该理论成功地描述了相干结构的最重要的特征：‘涡卷绕 (vortex roll-up)’ 和非线性饱和，理论结果与有关实验数据定性、甚至定量地相符。（2）在相干结构动力学模型的基础上，分析了亚音速湍流混合层和轴对称射流的发声物理过程，发现相干结构（波包）非线性作用导致的非定常平均流直接辐射低频噪音，由此了提供了噪音产生机理的第一原理描述。（3）提出描述亚、超音速射流近场剪切层中多波（螺旋波）相互作用和非线性演化的数学理论，并分析了其 发声机理和特性。（4）正在发展基于非线性PSE与渐近理论相结合的射流噪音预测方法：推广被广泛采用的非线性PSE来预测宽频谱相干结构演化，并根据渐进理论结果计算声场。以上结果为控制相干结构及其噪音通过了理论指导，为有效预测噪音提供了新途径。