A Coupled LES-Synthetic Turbulence Method for Jet Noise Prediction

Large-eddy simulation (LES)-based jet noise predictions do not resolve the entire broadband noise spectra, often under-predicting high frequencies that correspond to un-resolved small-scale turbulence. The coupled LES-synthetic turbulence (CLST) model is presented which aims to model the missing high frequencies. The CLST method resolves large-scale turbulent fluctuations from coarse-grid large-eddy simulations (CLES) and models small-scale fluctuations generated by a synthetic eddy method (SEM). Noise is predicted using a formulation of the linearized Euler equations (LEE), where the acoustic waves are generated by source terms from the combined fluctuations of the CLES and the stochastic fields. Sweeping and straining of the synthetic eddies are accounted for by convecting eddies with the large turbulent scales from the CLES flow field. The near-field noise of a Mach 0.9 jet at a Reynolds number of 100,000 is predicted with LES. A high-order numerical algorithm, involving a dispersion relation preserving scheme for spatial discretization and an Adams-Bashforth scheme for time marching, is used for both LES and LEE solvers. Near-field noise spectra from the LES solver are compared to published results. Filtering is applied to the LES flow field to produce an under-resolved CLES flow field, and a comparison to the un-filtered LES spectra reveals the missing noise for this case. The CLST method recovers the filtered high-frequency content, agreeing well with the spectra from LES and showing promise at modeling the high-frequency range in the acoustic noise spectrum at a reasonable expense.

Automated summary

This study introduces the coupled LES-synthetic turbulence (CLST) model, which resolves large-scale turbulent fluctuations from LES and models small-scale fluctuations using the synthetic eddy method to predict jet noise. The noise is calculated using the linearized Euler equations, with the rolling and stretching of synthetic eddies taken into account by convection with the large-scale turbulent scales. The results show that the CLST method is able to recover the filtered high-frequency content, agreeing well with the LES spectra, and has the potential to simulate the high-frequency range in the acoustic noise spectrum at a reasonable cost.