Prediction of Crosswind Separation Velocity for Fan and Nacelle Systems Using Body Force Models: Part 2: Comparison of Crosswind Separation Velocity with and without Detailed Fan Stage Geometry

Quentin J Minaker,Jeffrey J Defoe

doi:10.3390/ijtpp4040041

Quentin J Minaker, Jeffrey J Defoe

Open Access

https://doi.org/10.3390/ijtpp4040041

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Abstract

Modern aircraft engines must accommodate inflow distortions entering the engines as a consequence of modifying the size, shape, and placement of the engines and/or nacelle to increase propulsive efficiency and reduce aircraft weight and drag. It is important to be able to predict the interactions between the external flow and the fan early in the design process. This is challenging due to computational cost and limited access to detailed fan/engine geometry. In this, the second part of a two part paper, we apply the fan gas path and body force model design process from Part 1 to the problem of predicting flow separation over an engine nacelle lip caused by crosswind. The inputs to the design process are based on NASA Stage 67. A body force model using the detailed Stage 67 geometry is also used to enable assessment of the accuracy of the design process based approach. In uniform flow, the model produced by the design process recreates the spanwise loading distribution of Rotor 67 with a 7% RMS error. Both models are then employed to predict crosswind separation velocity. The two approaches are found to agree in their prediction of the crosswind separation velocity to within 5%.

Highlights

Modern aircraft engine design is moving towards using larger bypass ratios with lower fan stagnation pressure ratios
Since discrete crosswind velocities are required in Computational fluid dynamics (CFD), identifying the precise velocity required for separation was challenging
Flow separation was identified by a region where the flow was locally travelling upstream; this could be identified by a negative value of the wall shear stress in the axial direction on the inside of the nacelle

Summary

Introduction

Modern aircraft engine design is moving towards using larger bypass ratios with lower fan stagnation pressure ratios This improves propulsive efficiency; it comes with the negative trade off of larger and heavier nacelles. The airframers’ ability to determine the impact of these changes is important during design To model such situations, the airframer must be able to model the engine fan stage as its operation will greatly impact the intake performance [1]; two major concerns arise while modelling these flows. The first is that it requires full wheel simulations to capture the non-uniform flow caused by inlet separation; using traditional bladed full wheel simulations to model non-uniform flow is very computationally expensive. The second issue is that access to detailed fan stage geometry may not be possible and that most airframers lack the expertise or time required to reproduce this geometry

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