A Priori Analysis of Acoustic Source Terms from Large-Eddy Simulation in Turbulent Pipe Flow

The absence of combustion engine noise pushes increasingly attention to the sound generation from other, even much weaker, sources in the acoustic design of electric vehicles. The present work focusses on the numerical computation of flow induced noise, typically emerging in components of flow guiding devices in electro-mobile applications. The method of Large-Eddy Simulation (LES) represents a powerful technique for capturing most part of the turbulent fluctuating motion, which qualifies this approach as a highly reliable candidate for providing a sufficiently accurate level of description of the flow induced generation of sound. Considering the generic test configuration of turbulent pipe flow, the present study investigates in particular the scope and the limits of incompressible Large-Eddy Simulation in predicting the evolution of turbulent sound sources to be supplied as source terms into acoustic analogies, which have been proposed for the computation of the acoustic pressure field. To this end, a comprehensive a priori analysis of fully resolved flow fields obtained from Direct Numerical Simulations (DNS) was carried out for different Reynolds numbers. The analysis of the highly resolved DNS data gives a detailed insight into the instantaneous spatial distribution of the sound sources associated with different acoustic analogies. The subsequent spatial filtering of the DNS data on a typical coarse LES-grid quantifies the contribution from the turbulent structures larger than the grid-scale, which are directly resolved by LES, the contribution from the smaller subgrid-scale (SGS) structures, which affect the larger scales and need to be modelled, and the contribution from unresolved small scale structures, which are principally not captured by LES, to the source terms. The effect of the numerical error inherent in the discretized representation of the source terms is investigated as well.