Computation of exhaust mixing noise using large-eddy simulation turbulence modeling and Lighthill's acoustic analogy

This research involves computational study of free, heated jet flow and resultant far-field sound performed using large-eddy simulation (LES) and Lighthill's acoustic analogy. A subgrid-scale model for small-scale compressible turbulence was developed using a combination of the popular Smagorinsky model and a deductive model. An existing software package for compressible flow field computation was substantially modified to perform temporal LES and aerosound simulations. Cases studied extend to large Reynolds number (Re), high subsonic (compressible) flow with realistic geometries more representative of aircraft engine exhausts than typically considered using direct numerical simulation (DNS). Flow field fluctuations are stored over a period of time and used to calculate RMS turbulence within the computational domain. The far field sound and directivity is computed using the time-derivative form of Lighthill's source-integral result formulated in terms of quadrupole sources from the simulated flow field, which is integrated in time and contains the fluctuations set up by the time-varying stress tensor. A simulation for a WR19-4 turbofan engine exhaust (Re > 106 based on exit velocity and diameter) is presented, and propagated jet noise results are compared with experimental acoustics data. Methods to account for effects of source convection and thermal refraction to obtain realistic frequency spectra and directivity will be considered.; The acoustic radiation calculation is performed in three steps; (1) an approximate result for the mean flow field using a compressible flow code employing a k-ε turbulence model, (2) unsteady turbulent fluid field simulation using the CFD code appended with a LES turbulence model (the k-ε prediction serving as an initial guess) for the mean flow and (3) far field acoustics obtained using Lighthill's analogy. Extensive far field noise data gathered from ground static measurements of a WR19-4 mini-turbofan engine were used for comparisons between computed results and measurements.