On the basic phenomena of acoustic wave generation and dynamics in compressible shear flows (English shop)

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Especially in the design of aircraft engines, noise regulations and applicable airworthiness requirements make it impossible to neglect aeroacoustic properties. This is the starting point of the present work, which investigates aerodynamic noise generation and propagation in modelled shear flows of engineering relevance. To date, neither of the two phenomena is fully understood, although regions of (constant) shear rate are ubiquitous. The analysis of the temporal behaviour of small-amplitude perturbations enables a fundamental understanding of aeroacoustic phenomena. In this way, the only linear mechanism of acoustic wave generation in shear flows in two and three dimensions is identified. The spectral anisotropy of this linear mechanism leads to highly directional noise radiation. The classification of all linear and nonlinear processes involved in acoustic wave generation and propagation, and the comparison of their effectiveness on the basis of direct numerical simulations, demonstrate the dominance of the linear mechanisms and hence their relevance. Nevertheless, these remain unaccounted for in the source term of the acoustic analogy established in Lighthill’s seminal publications and in its extensions. The theory developed in this work makes it possible to explain highly directional noise radiation and its further propagation in shear flows of engineering relevance.