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

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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.