Modeling of Inlet Distortion Using a Combined Turbofan and Nacelle Inlet Model During Crosswind and Low Speed Forward Operation

Abstract

Inlet distortion on turbofan nacelles during crosswind and low speed forward operation is an area of concern in the gas turbine engine community. Specifically for aft fuselage mounted nacelles, flow into the inlet is affected by the turning of the airflow at the inlet for crosswind operation, and by fuselage interference effects, such as fuselage based vortices, during low-speed forward operation. A common process of modeling airflow through a turbofan inlet in crosswind and low-speed forward operation is to model the flow boundary, at the fan leading edge location, as a static pressure boundary. For this process, the fan face is represented by a two-dimensional static pressure profile as a function of radius. This process assumes the static pressure at the fan face is uniform circumferentially. In conditions where non-axisymmetric flow effects are present at the fan boundary, such as flow separation in the inlet or local fuselage based ground vortices, an axisymmetric pressure boundary at the fan face is inappropriate. The ingestion of a fuselage based ground vortex will also impact the prediction of inlet distortion into a turbofan inlet. An improved methodology is to model the complete fan and fan stator system using a frozen rotor in a rotating reference frame. This allows the three dimensional flow effects of the fan and stator system to be better modeled within the CFD analysis by allowing the physical geometry of the modeled fan to set the flow characteristics circumferentially in the inlet. The CFD analyses were performed using two methods: (1) with airflow through the nacelle driven by a static pressure boundary at the fan face, and (2) with the fan system modeled as a frozen rotor in a rotating reference frame. The CFD results were evaluated using ARP 1419 circumferential and radial distortion descriptors (Reference 1) at the nacelle’s aerodynamic interface plane location. Results from the fan system CFD analyses are compared to typical values from distortion testing and to CFD results using a static pressure profile boundary condition at the fan face. The goal of this evaluation is to combine the aircraft nacelle and the fan rotor and stator in order to model the impact of a fuselage vortex on inlet distortion where flow through the inlet is set by the fan geometry and fan speed. Initial studies on the isolated nacelle have predicted the effects of ground based vortices on the fan flow at various crosswind velocities. In the current study the effects of fuselage and ground based vortices are studied at various crosswind and head wind velocities at ground idle and takeoff operating conditions.