On the Flow Physics During the Transition of a Variable Pitch Fan From Nominal Operation to Reverse Thrust Mode

Abstract

The flow field during the transition of a VPF from nominal operation to reverse thrust mode at typical ‘Approach Idle’ engine power setting is described in this work. An integrated airframe-engine-VPF research model is used to explore the flow field in a fully transient URANS simulation with imposed wall motion. A novel methodology that implements an adaptation of a mesh displacement equation to mimic the fan blade aerofoil rotation is developed. The implementation of this method with gradual, small step deformation along with an automated mesh update routine enables a high quality, near ‘real-time’ simulation of the complete transition. The flow field during transition is characterised by the evolution from typical forward flow to the development of massive recirculation regions at the feather pitch setting and finally to development of a reverse flow. The transient development of the flow features, ingested mass flow, airframe decelerating force and core engine distorted flow, apropos the fan aerofoil rotation to reverse thrust mode are discussed. A hitherto unresolved fan power peaking during the middle of the transition and higher power requirement at reverse thrust mode is captured. The effect of fan rotational speed and touch down velocity on the transition flow physics is explored. A comparison of the transient approach with discrete steady state runs for different stagger angle settings is presented. The new capability to study the transition in a fully transient simulation can be used as a design development aid for engineering the reverse thrust VPF.