Numerical Investigation of Freestream Composition Perturbations in a Scramjet Combustor

This study employs three-dimensional unsteady numerical simulations to analyze fluid–combustion interactions in a scramjet combustor under freestream oxygen perturbations at Mach 6 flight conditions. The combustor utilizes ethylene fuel injection, with systematic examinations of dynamic responses to abrupt reductions in freestream oxygen mole fractions from baseline 0.21 to 0, 0.05, and 0.10. A characteristic mode shift occurs from jet-wake stabilization to cavity shear-layer stabilization during oxygen depletion, followed by reversion to the original mode upon freestream recovery. This transition correlates strongly with perturbations in combustion intensity and heat release redistribution. Rapid alterations in shock train positioning and flow separation characteristics were observed, directly influencing static pressure distributions along the flowpath. The resultant pressure fluctuations exhibited temporal coherence with thrust variations. Thrust modulation magnitudes demonstrated linear dependence on oxygen depletion severity, while combustion sustainability remained intact during 0.2 ms perturbations. Prolonged perturbations can induce cumulative effects and are capable of altering hypersonic vehicle trajectories through thrust vectoring changes. These results highlight the critical time-dependent coupling between freestream composition transients and scramjet operability, providing quantitative benchmarks for robust combustor design in atmospheric disturbance scenarios.