Reduced-Order Modeling of Unstart in Liquid Fuel Scramjets

The overall goal of this project is to develop a reduced-order modelling (ROM) approach which rapidly and accurately predicts unstart in both gas- and liquid-fueled scramjet engines over a broad range of flight conditions, and which can be directly inserted into relevant analysis workflows including six degrees-of-freedom (6DOF) trajectory simulations. Large Eddy Simulations (LES) of experimental scramjet configurations shall be performed to support ROM development by providing insight into unstart physics, data for training, and verifying ROM predictions. Consequently, a secondary objective is to ensure the simulations are of the highest accuracy feasible, which entails careful consideration of the available physical models and numerical methods employed. The Phase I effort initiated this process by investigating different shock-capturing schemes and the use of flamelet progress variable (FPV) methods as an alternative to finite-rate chemistry (FRC) in the Hyshot II configuration, and Phase II will build upon this with targeted improvements to the liquid fuel spray and flamelet modeling approaches. The physics-based ROMs developed in Phase I demonstrated the ability to capture the transient unstart process including detailed chemistry, axially varying cross-sectional area, shock-capturing, and variable boundary conditions. This proof-of-concept will be further developed in Phase II by incorporating sub-models for specific physics and extending the 1D computational domain to account for more complex geometries. In addition to the quick insight into trends and sensitivities afforded by ROMs, they can be directly incorporated into system-level analyses such as trajectory simulations for mission planning. A major contribution of the Phase II effort will be a demonstration wherein a complete aerodynamic model of a representative air-breathing hypersonic vehicle will be generated and tested within a 6DOF flight simulation framework. The vehicle model will be two-way coupled to the ROM to simulate the combustor, and the complete system will be used to develop and test a feedback control loop to examine the feasibility of in-situ unstart detection and recovery. To prepare the ROMs for insertion into DoD workflows, a software package will be developed to encapsulate the process with a clearly defined user interface and API.