Direct Kinetic Modeling of the Plasma Generated in a Rarefied Hypersonic Shock Layer

Under hypersonic entry conditions into an atmosphere, gas may be ionized via shock waves to create a weakly ionized plasma. Accurate kinetic modeling of these flows is essential for deriving macroscopic properties relevant to vehicular design, such as surface heat flux. Of particular interest are regions with a high degree of charge separation. One region is the plasma sheath at the vehicle surface, whose self-induced electric fields lead to ion acceleration and thus increased heat flux to the vehicle. The second region is the interface between the shock front and the ambient freestream gas, whose field structure is not well understood. Due to statistical scatter and the computational cost associated with simulating high density flows with electric field effects, these regions have yet to be fully characterized by prior investigations using traditional particle-based kinetic modeling. In this study, we explore both regions by developing a one-dimensional numerical model of a rarefied, hypersonic shock layer for a helium atmosphere. The model employs the grid-based direct kinetic method, which directly solves the Boltzmann-Poisson-BGK equations for a discretized velocity distribution function. The charge density, ion and electron currents, and convective heat flux are computed and compared with results from the standard direct simulation Monte Carlo model, which invokes the ambipolar diffusion approximation to simulate plasma dynamics. Additionally, the electric field and magnitude of charge separation are evaluated throughout the flowfield to assess the validity of the quasineutral assumption along the stagnation streamline.