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

嵌入在混合层和射流的纯涡模式扰动的声音产生的线性过程是拟议的研究，这是基于在20世纪90年代的剪切流的流体动力学稳定性理论的理论突破的主要目标。这一突破与对非均匀流动系统的非常态性引起的现象的理解有关，它使我们能够研究线性声音产生过程。我们的初步结果表明，涡扰动波产生的唯一线性机制是由这些流动系统的非正常引起的模式耦合。这种机制对于所有马赫数都是有效的，并且所产生的声音的近场与流体动力学的近场相当：剪切流中线性产生的声波的能量与湍流涡流的能量相当。非模态方法（开尔文模式方法）是我们研究的基本理论方法，使我们能够分析嵌入在所调查的流动中的涡产生线性声音的基本过程。还已知的是，声场揭示了频谱空间中的各向异性性质，并且Lighthill的声学类比中的线性源表示的拓扑结构和所提出的各向异性线性波生成机制的拓扑结构本质上是不同的，并且主要彼此不兼容。因此，我们考虑自由剪切流，以了解这些流动中的声音产生的基本现象的基本物理。因此，该项目的主要目的是全面和定量调查线性现象在声音产生中的效率。在自由剪切流中声波的产生和传播中线性和非线性过程的相对作用的分析也将是本建议的重点。

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.