Expansion Characteristics of Jets into Low-ambient Pressures using Kinetic-Continuum Approaches

A hybrid kinetic–continuum solver, HAVAHybridFoam, is developed to simulate supersonic and hypersonic jet expansion into very low ambient pressures. The domain is dynamically partitioned using the gradient‐length local Knudsen criterion, with a density-based continuum approach applied in continuum regions and a kinetic-based direct simulation Monte Carlo (DSMC) approach used in rarefied regions. A flux-based coupling strategy enables consistent mass, momentum, and energy exchange between the solvers, and both one-way and two-way approaches are implemented and assessed. Validation against stationary-shock experiments demonstrates that the hybrid method captures rarefied shock structure with accuracy comparable to full DSMC with a significantly reduced cost. Plume-impingement results for a 1~kPa argon jet into vacuum show close agreement with a fully kinetic hybrid BGK-DSMC approach. Finally, a strongly underexpanded argon jet at a nozzle pressure ratio of a thousand (NPR=1000) confirms that the continuum solver predicts the Mach-disk location further downstream, while the hybrid framework resolves rarefaction effects inside the expansion and the shock region. Our ongoing studies show that significant differences in the flow field are observed. It is also found that the Mach disk location becomes noticeably influenced by the nozzle temperature ratio as rarefaction increases. The proposed method provides DSMC-level fidelity with considerably improved efficiency for plume modeling in space-propulsion applications.