Towards Numerical Simulation of Fan Broadband Noise Propagation and Radiation from Aero-Engines

Abstract

A first attempt to numerically simulate the broadband noise transmitted through and radiated from a turbofan bypass duct is presented in this paper. Turbulent source assessment on the fan rows is left out here, and a generic power spectral density of in-duct acoustic pressure is imposed as an input to a computational aeroacoustics simulation performed using a time-domain Euler solver. The acoustic field is expanded over usual Fourier-Bessel modes, and the prescribed spectrum is re-generated by assuming an equi-modal energy distribution. The broadband noise behavior is modeled by introducing a random phase when imposing each acoustic mode and by performing a quadratic averaging of the numerical solution over a few independent runs. This broadband-like source generation approach is firstly validated for a semi-infinite annular duct with uniform flow by comparison with analytical solutions. Decrease of the statistical errors depending on the number of run is checked showing that at least 10 runs have to be considered. The exhaust radiation problem from an idealized nozzle in uniform and shear mean flow is then investigated and validated (for single mode case) by comparison to a Wiener-Hopf based solution. Far-field predictions are obtained by coupling the Euler code with a Kirchhoff integral. Computed results appear to be stable and accurate even in the presence of vorticity waves created in the shear layer. Expected shear effect, leading to a deviation of the directivity lobe from the duct axis, is confirmed for broadband noise. Then, the computation method is applied to a realistic nozzle, including a RANS solution providing the mean flow. The only problem arising is the computational cost resulting in a limitation of high frequency range limited here to 2 3 kHz (with around 600 propagating modes). Anyway, computed far-field directivities relative to idealized and realistic nozzle cases with similar input conditions are found to be rather close up to 90 o of radiation angle.