Optimization of variable geometry scheme during modal transition for variable cycle engines based on high fidelity cavity models

Variable cycle engines (VCEs) offer significant advantages for wide speed range and multi-mission aircraft, and accurate prediction of transient behavior during modal transition is essential for their design and control. However, most existing transient simulation approaches rely on lumped-parameter inter-component volume (ICV) method, which neglects the spatial distribution of flow inside engine cavities and consequently limit prediction accuracy. To address this limitation, this paper develops a high-fidelity and computationally efficient transient modeling framework for modal transition analysis and variable geometry optimization of VCEs, with particular emphasis on improving cavity dynamic representation. A one-dimensional volume (ODV) method is first developed to provide a higher accuracy description of cavity dynamics. Considering the typical length scales of aero-engine cavities, the ODV method is further simplified to derive a simplified one-dimensional volume (SODV) method, which significantly improves computational efficiency while maintaining high accuracy. Meanwhile, transient models and solution strategies for key variable geometry components are established and improved to ensure numerical robustness during modal transition. Based on the developed transient modeling framework, collaborative optimization of multiple variable geometry components during the modal transition process is carried out. The results show that instability and supersonic flow are prone to occur during the modal transition process, which can only be improved through the collaborative regulation of multiple components. Based on the developed transient modeling framework, the optimized variable geometry scheme achieves a final specific thrust of 736 m/s. When evaluated against a reference adjustment strategy, this corresponds to a relative improvement of approximately 17.4%, while satisfying the imposed operational constraints.