Jet Noise – Acoustic Analogy informed by Large Eddy Simulation

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. Far-field propagation is modelled by solution of a system of adjoint Linear Euler Equations, capturing convective and refraction effects using 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 fourth-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 also taken from the RANS CFD prediction. The constants of proportionality are, however, not determined empirically, but extracted by comparison with turbulence length- and time-scales obtained from a LES prediction. The LES results are shown to be in good agreement with experimental data for the fourth-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. This model is applied to the prediction of sound to the experimental configuration of the EU JEAN project, and gives encouraging agreement with experimental data across a wide spectral range and for both sideline and peak noise angles. This paper also examines the accuracy of various commonly made simplifications, for example: a locally parallel mean flow approximation rather than consideration of the spatially-evolving mean jet flow and scattering from the nozzle; the assumption of small radial variation in Green function over the turbulence correlation length; the application of the far-field approximation in the Green function; and the impact of isotropic assumptions made in previous acoustic source models.