Using Large Eddy Simulation within an Acoustic Analogy Approach for Jet Noise Modelling

A novel approach to the development of a hybrid prediction methodology for jet noise is described. Modelling details and numerical techniques are optimised for each of the three components of the model individually. Far field propagation is modelled via solution of a system of adjoint Linear Euler Equations, capturing convective and refraction effects via use of a spatially developing jet mean flow provided by a RANS CFD solution. Sound generation is modelled following Goldstein's acoustic analogy, including a Gaussian function model for the two-point cross-correlation of the 4th order velocity fluctuations in the acoustic source. Parameters in this model describing turbulent length- and time-scales are assumed to be proportional to turbulence information taken also from the RANS CFD prediction. The constants of proportionality are, however, not determined empirically, but extracted via comparison with turbulence length- and time-scales obtained from an LES prediction. The LES results are shown to be in good agreement with experimental data for the 4th order two-point cross-correlation functions. The LES solution is then used to determine the amplitude parameter and also to examine which components of the cross-correlation are largest, enabling inclusion of all identified dominant terms in the Gaussian source model. The acoustic source description in the present approach is therefore determined with no direct input from experimental data. The paper also examines the accuracy of various commonly made simplifications, for example: the inclusion of an evolving jet flow and scattering from the nozzle, the assumption of small variation in Green's function over the correlation length, the application of the far-field approximation in the Green's function, and the impact of isotropic assumptions made in previous acoustic source models. The final optimised model is applied to prediction of experimental data from the JEAN project, and gives excellent agreement across a wide spectral range and for both sideline and peak noise angles.