Die Natur der Turbulenz in kompressibler, homentroper konstanter Scherströmung: Wirbel- und Wellenanteile sowie Selbsterhaltung.
Zusammenfassung der Projektergebnisse
The aim of the project was to substantiate the importance of non-normality of shear flow linear operators and its consequence – the non-modal dynamics of the perturbations – in the formation of acoustic wave output of shear layers. The non-modal approach allows to capture the only linear mechanism of the acoustic wave generation – the linear vortex-wave mode coupling induced by the shear flow non-normality. To be able to quantify this approach, a simple yet realistic enough canonical flow was chosen, a compressible temporal mixing layer, the central core of which can be approximated by a constant shear flow, allowing the non-modal analysis to be fully applied to the investigated flow. In an extension to previous work and projects, new simulations were performed to get a better data base, increasing resolution and adding various solution domain aspect ratios. Therefore, direct numerical simulations of compressible turbulent mixing layers for convective Mach numbers of Mc = 0.3, 0.7, 1.4 and various simulation boxes (Lx, Ly, Lz) with different streamwise-spanwise aspect ratios (Lx /Lz = 0.5, 1, 2) were performed. The simulations identify the origin of the acoustic wave output now more clearly: the dominance of a linear generation process of acoustic waves in the shear layer core region, induced by the flow non-normality, observable in the near-far field of acoustic waves emitted by the flow, supporting former theoretical work. DNS with subsequent analysis made it possible to consider a shear flow where not only the largest streamwise vortex harmonics are acoustically active, but also the subsequent shorter ones. In this case, the spatial coherence of the wave-packet would be added to the temporal coherence of the acoustic wave generation mechanism described in the paper attached. This would further amplify the generated acoustic waves while maintaining their regular/smooth pattern and eventually result in the formation of a regular/smooth near to far acoustic field subsequently transforming into Mach waves. At a first glance, the mechanism seems to be that of wave-packet generated acoustic waves. However, the waves are generated by the vortex harmonics due to the mechanism induced by the shear flow non-normality instead - the wave-packet only collects the generated acoustic wave harmonics by constructive interference of the spatially and temporally coherent vortex harmonics of the wave-packet.
