Toward Numerical Simulation of Fan Broadband Noise Aft Radiation from Aeroengines

A first attempt to numerically simulate 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 aeroacoustic simulation performed using a time-domain Euler solver. The acoustic field is expanded over Fourier―Bessel modes, and the broadband noise behavior is modeled by introducing a random phase and by performing a quadratic averaging over independent runs. This broadband source generation approach is first validated in a semi-infinite annular duct by comparison with analytical solutions. The exhaust radiation problem from an idealized nozzle is then validated (for a single mode) by comparison to a Wiener―Hopf solution. In a second step, refraction effect through the shear layer is investigated for broadband noise. Computed results appear to be stable even in the presence of vorticity waves created in the shear layer. Then, the computation method is applied to a realistic nozzle. 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). Computed far-field directivities relative to idealized or realistic nozzle with similar input conditions are found to be rather close up to 90° of radiation angle.