Unsteady Simulations of a Fan/Outlet-Guide-Vane System: Broadband-Noise Computation

The fan broadband noise of a rotor–stator system representing a turbofan aeroengine is investigated using the improved delayed detached-eddy-simulation approach. The baseline configuration of the NASA source diagnostic test with the entire hard-wall nacelle is simulated at approach and takeoff conditions. In preceding studies, the simulation methodology for the aerodynamics and the treatment of turbulence was established; subsequently, the blade-passing-frequency tones were analyzed. In this study, the forward- and aft-radiating broadband noise associated with the rotor–stator interaction is investigated. The radiated sound power levels are calculated using two methods, and the results are compared with the experimental data: 1) the Ffowcs Williams and Hawkings method with a permeable surface enclosing the nacelle, which is determined through careful studies of the integration-surface design; and 2) the mode-extraction method applied in the fan duct, in which the cost function is tailored for the broadband noise. The power-level spectra from these two numerical methods generally agree, except for the forward-radiating noise in the takeoff condition. Compared with the experiment, both methods predict the spectra typically within 3 dB for both the inlet and the exhaust in the approach condition, but they underpredict the broadband levels in the takeoff condition. The azimuthal-mode decompositions in the inlet and exhaust ducts are also generally consistent with the experiment.