Numerical analysis and reduced-order modelling of a variable area mixer

Abstract Variable cycle engines (VCEs) are an emerging class of propulsion systems designed to adapt their thermodynamic cycle to changing flight conditions, enabling both high efficiency at subsonic speeds and high specific thrust at supersonic speeds. A key enabler of this flexibility is the variable area bypass injector (VABI), which controls the bypass-to-core mass flow ratio and influences overall engine performance. Despite its importance, the aerodynamic behaviour of VABIs, particularly in terms of bypass flow entrainment and associated pressure losses, has received limited attention in the literature. This paper focuses on the rear VABI, a component analogous to the mixer in conventional turbofans but equipped with an area-regulating mechanism to vary the bypass cross-sectional area. The study consists of two parts: first, high-fidelity three-dimensional computational fluid dynamics (CFD) simulations are performed to characterise the rear VABI. A range of operating conditions and area regulator positions have been assessed, emphasising the total pressure losses and bypass flow entrainment. Secondly, the CFD results have been processed into a non-dimensional performance database used to develop a reduced-order VABI model. This VABI model has been integrated into a notional three-stream VCE configuration to demonstrate its applicability in system-level analyses. The proposed methodology bridges detailed CFD simulations and a computationally efficient representation, enabling accurate modelling of VABI behaviour within broader engine design frameworks.