Multiscale Computation of Plume Expansion in Continuum and Rarefied Regimes

The plume expansion in rarefied ambient conditions exhibits significantly different shock dynamics and flow statistics compared to the expansion in the earth's atmospheric conditions. In particular, the plume flow in transitional regimes (Knudsen number between continuum and free-molecular flows) remains poorly understood. Numerical prediction of such flows is complicated because of the challenge of selecting appropriate governing equations that balance accuracy and computational cost across regimes. To address the challenge, the present study adapts the Unified Gas Kinetic Wave-Particle (UGKWP) method for plume (or jet) flows ranging from continuum to weakly rarefied regimes. The solver is first validated in the continuum regime through comparisons with the well-characterized underexpanded jet flows, followed by a parametric study of planar underexpanded jets over a range of pressure ratios to examine the transition from shock-train to Mach-cell structures. To assess performance in the rarefied regime, the UGKWP method is further evaluated against the vacuum-plume configuration from the literature. The results demonstrate the capability of the developed framework to model multiscale jet flows and establish a foundation for future investigations into jet expansion under various levels of rarefaction.