Identification of the Linear Sound Sources in Turbulent free Shear Flows:Non-modal Analysis and Direct Numerical Simulation Study

基于20世纪90年代剪切流流体动力稳定性理论的突破，研究了嵌入在混合层和射流中的纯涡旋扰动产生声音的线性过程。这一突破与对非均匀流动系统非正态性引起的现象的理解有关，并使我们能够研究线性声音产生过程。我们的初步结果表明，涡旋扰动产生波浪的唯一线性机制是由这些流动系统的非正态性引起的模态耦合。这种机制对所有马赫数都是有效的，所产生的声音的近场与水动力场相当：剪切流中线性产生的声波的能量与湍流漩涡的能量相当。非模态方法（开尔文模态方法）是我们研究的基本理论方法，它使我们能够分析被研究流动中嵌入的涡流产生线性声音的基本过程。我们还知道，声场在频谱空间中表现出各向异性的性质，Lighthill声学类比中线性声源表示的拓扑结构与所提出的各向异性线性波产生机制的拓扑结构具有内在的不同，基本上是互不相容的。因此，我们考虑自由剪切流动，以了解这些流动中声音产生的潜在现象的基本物理。因此，该项目的主要目的是对声音产生中线性现象的效率进行全面和定量的研究。在自由剪切流中，线性和非线性过程在声波产生和传播中的相对作用的分析也将是本提案的重点。

The linear process of sound generation by pure vortex mode perturbations embedded in mixing layer and jet flows is the main objective of the proposed research which is based on a theoretical breakthrough in the hydrodynamic stability theory of shear flows in the 1990s. This breakthrough is related to the understanding of phenomena induced by the non-normality of non-uniform flow systems and it enables us to study the linear sound generation processes. Our preliminary results show that the only linear mechanism of wave generation by vortex disturbances is the mode coupling induced by the non-normality of these flow systems. This mechanism is efficient for all Mach numbers and the near field of the generated sound is comparable to the hydrodynamic one: the energy of the linearly generated acoustic waves in shear flows is comparable to the energy of the turbulent eddies. The non-modal approach (Kelvin-mode approach) is the basic theoretical approach in our study and allows us to analyse the basic processes of linear sound generation by vortices embedded in the flows under investigation. It is also known that the sound field reveals an anisotropic nature in the spectral space and the topology of the linear source representation in Lighthill's acoustic analogy and of the presented mechanism of anisotropic linear wave generation are inherently different and principally incompatible with each other. Accordingly, we consider free shear flows to understand the basic physics of the underlying phenomena of sound generation in these flows. Therefore, the main aim of the project is the comprehensive and quantitative investigation of the efficiency of linear phenomena in sound generation. The analysis of the relative roles of linear and nonlinear processes in the generation and propagation of acoustic waves in the free shear flow will also be the focus of this proposal.