Broadband noise radiation models for aircraft engines

Abstract

Far-field noise radiation models are presented for broadband random noise radiation from an aircraft engine. Models include radiation from the fan inlet and aft duct, and the engine core duct. Turbomachinery broadband noise generation models which define the acoustic power generation in the duct as distributed among the duct modes or as a mode cut-off ratio distribution can be used as input. In this paper simple acoustic power distributions have been assumed to demonstrate the far-field radiation field. The inlet radiation model includes the influence of the bellmouth used in static tests or the inlet lip used for wind-tunnel or flight tests. The aft duct models include the effect of the nozzle area change and the jet slip layer on the propagation and radiation of the internally generated broadband noise. Acoustic power has been conserved and the far-field pressure properly evaluated even for the difftcult aft radiation cases where the radiation angle is drastically altered by the shear layer. A new termination transmission loss model is also presented which is valid for the inlet and the aft ducts. The noise radiation models are intended to be approximate but adequate representations where very rapid calculations are needed for many modes. The broadband random noise radiation models developed here are intended to be reasonably accurate and provide extremely fast computational capability. This speed is required since extremely large numbers of duct modes can propagate in modem turbofan engines at the high frequencies usually encountered with turbomachinery broadband noise. The noise radiation models are intended to be used in conjunction with noise source models which predict the broadband noise generation in the engine ducts in terms of the duct modes or other convenient modal representation such as the modal cut-off ratio power