The installed flow field for a variable pitch fan (VPF) operating in reverse thrust for the complete aircraft landing run is described in this paper. To do this, a VPF design to generate reverse thrust by reversing airflow direction is developed for a representative 40,000 lbf modern high bypass ratio engine. Thereafter, to represent the actual flow conditions that the VPF would face, an engine model that includes the nacelle, core inlet splitter, outlet guide vanes, bypass nozzle, core exhaust duct, aft-body plug, and core nozzle is designed. The engine model with the VPF is attached to a representative airframe in landing configuration to include the effects of installation. A rolling ground plane that mimics the runway during the landing run is also included to complete the model definition. Three-dimensional (3D) Reynolds-averaged Navier–Stokes (RANS) solutions are carried out for two different VPF stagger angle settings and rotational speeds to obtain the fan flow field. The dynamic installed VPF flow field is characterized by the interaction of the freestream and the reverse stream flows. The two streams meet in a shear layer in the fan passages and get deflected radially outward before turning back onto themselves. The flow field changes with stagger setting, fan rotational speed, and the aircraft landing speed because of the consequent changes in the momentum of the two streams. The description of the installed VPF flow field as generated in this study is necessary to (a) qualify VPF designs that are typically designed by considering only the uninstalled static flow field and (b) choose the VPF operating setting for different stages of the aircraft landing run.
A rationale for the level of model fidelity required to provide the most representative flow field information to ascertain the feasibility of using a Variable Pitch Fan (VPF) in a modern high bypass ratio aero engine to generate reverse thrust is described in this paper. This is done by comparing the 3D RANS flow field solution for a newly developed reverse flow VPF design from two research models: i) isolated engine model in which the bypass duct, guide vanes, splitter and VPF are wrapped in an axisymmetric nacelle and placed in a generic far-field domain and b) integrated model in which the engine is installed to an airframe in landing configuration through a pylon and placed in a far-field domain bound by a rolling runway. The flow field solution obtained at an aircraft landing speed of 80 knots indicates that even though both models can predict the general flow patterns, there are substantial differences in parameters such as the amount of reverse stream, circumferential distribution of flow properties and flow development downstream of the engine. These differences impact the levels of reverse thrust generated, flow distortion entering the core engine and resultant airframe forces. This study makes the case that it is necessary to use an integrated model that includes a full engine nacelle installed on an airframe, to answer design questions for engineering the VPF system to generate reverse thrust.
The flow distortion at core engine entry for a Variable Pitch Fan (VPF) in reverse thrust mode is described from a realistic flow field obtained using an integrated airframe-engine model. The model includes the VPF, core entry splitter, complete bypass nozzle flow path wrapped in a nacelle and installed to an airframe in landing configuration through a pylon. A moving ground plane to mimic the rolling runway is included. 3D RANS solutions are generated at two combinations of VPF stagger angle and rotational speed settings for the entire aircraft landing run from 140 to 20 knots. The internal reverse thrust flow field is characterized by bypass nozzle lip separation, pylon wake and recirculation of flow turned back from the VPF. A portion of the reverse stream flow turns 180° with separation at the splitter leading edge to feed the core engine. The core engine feed flow exhibits circumferential and radial non-uniformities that depend on the reverse flow development at different landing speeds. The temporal dependence of the distorted flow features is also explored by an URANS analysis. Total pressure and swirl angle distortion descriptors, as defined by the Society of Automotive Engineers (SAE) S-16 committee, and, total pressure loss into the core engine are described for the core feed flow at different operating conditions and landing speeds. It is observed that the radial intensity of total pressure distortion is critical to core engine operation, while the circumferential intensity is within acceptable limits. Therefore, the baseline sharp splitter edge is replaced by two larger rounded splitter edges of radii, ∼0.1x and ∼0.2x times the core duct height. This was found to reduce the radial intensity of total pressure distortion to acceptable levels. The description of the installed core feed flow distortion, as described in this study, is necessary to ascertain stable core engine operation, which powers the VPF in reverse thrust mode.
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