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 the acoustic analogy of Lighthill. 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 spatial filtering of the DNS data onto a typical coarse LES-grid allowed us to quantify the contribution from the turbulent structures, 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. Additionally, a posteriori LES were performed to investigate the contribution of a specially selected SGS-model to the Lighthill source term as well as the effect of the time-dependent dynamics of the turbulent vortical motion basically not captured by a priori LES.