Ducted fan acoustic radiation including the effects of nonuniform mean flow and acoustic treatment

"" Forward and aft acoustic propagation and radiation from a ducted fan is modelled using a finite element discretization of the acoustic field equations. The fan noise source is introduced as equivalent body forces representing distributed blade loading. The flow in and around the nacelle is assumed to be nonuniform, reflecting the effects of forward flight and flow into the inlet. Refraction due to the fan exit jet shear layer is not represented. Acoustic treatment on the inlet and exhaust duct surfaces provides a mechanism for attenuation. In a region enclosing the fan a pressure formulation is used with the assumption of locally uniform flow. Away from the fan a velocity potential formulation is used and the flow is assumed nonuniform but irrotational. A procedure is developed for matching the two regions by making use of local duct modal amplitudes as transition state variables and determining the amplitudes by enforcing natural boundary conditions at the interface between adjacent regions in which pressure and velocity potential are used. Simple models of rotor alone and rotor/exit guide vane generated noise are used to demonstrate the calculation of the radiated acoustic field and to show the effect of acoustic treatment. The model has been used to asses the success of four techniques for acoustic lining optimization in reducing far field noise. INTRODUCTION Figure 1 shows in idealized form a rotor/exit guide vane configuration imbedded in a nacelle with a centerbody or a core engine. The rotor represents the fan in a high bypass turbofan engine or a ducted propeller. The exit guide vanes provide a source of interaction noise. In the numerical examples considered in this investigation the number of blades and exit guide vanes is characteristic of a ducted propeller. The rotor/exit guide vane source generates noise which is propagated through the inlet and exhaust ducts and is radiated to the far field. Nonuniform steady flow exists in and around the nacelle due to inflow, outflow, and forward flight effects. It is required to predict the far field radiated noise at harmonics of the blade passage frequency. In a previous investigation [1] ducted fan noise was studied with the assumption that the mean flow in and around the inlet could be assumed to be uniform. This allowed the