Parallel 3D Hybrid Continuum/DSMC Method for Unsteady Expansions Into a Vacuum

Abstract

We present the application of a unidirectional unsteady coupling between a continuum solver and a three dimensional parallel Direct Simulation Monte Carlo (DSMC) code. Two different problems have been considered: the spherically symmetric expansion of a water vapor cloud into a vacuum and the late stages of a comet impact on the Moon. In both cases, unsteady data pre-computed from a continuum solution are used as input to the DSMC code at a fixed interface. The DSMC results were then compared to the continuum results downstream of the interface in a region of mutual validity in order to validate our approach. The DSMC results for the expansion flow showed good agreement with the analytic solution for the density, velocity and temperature downstream of the interface. Similarly, for the comet impact simulations, the DSMC density agrees well with the solution from the continuum solver, the SOVA hydrocode. A slightly hotter solution is however obtained downstream of the interface using the DSMC code compared to the hydrocode solution.

Transport of water in a transient impact-generated lunar atmosphere

“…Fig. 1 is a schematic depiction of the interfacing between the two codes (described and validated in Stewart et al, 2009) and the problem set-up. Both DSMC and SOVA simulations are unsteady and three-dimensional.…”

“…Translational-rotational energy exchange between molecules during collisions is modeled using the standard Larsen-Borgnakke model, while translational-vibrational energy exchange is modeled using the discrete Larsen-Borgnakke model. We also use the collision limiting scheme developed by Stewart et al (2009), which reduces the computational cost of simulations by accounting for the fact that in regions at local thermal equilibrium, only a limited number of collisions is required to bring the gas to equilibrium; subsequent collisions simply redistribute energy between molecules without affecting macroscopic properties.…”

“…The inset diagram depicts the overlap between SOVA and DSMC cells at the interface, where DSMC molecules are initialized using continuum SOVA data for water vapor (gray). SeeStewart et al (2009Stewart et al ( , 2011 for details. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.…”

Simulating Irregular Source Geometries for Ionian Plumes

“…The same method has previously been used to simulate plumes evolving from round holes [3]. This planet-scale DSMC code has been developed over a number of years in our group [4,5,6].…”

“…Although, in reality, complex continuum processes underneath and near the surface determine the specific features of the flow as it expands, we at first simplify matters for this study by supposing that those processes can be modeled as flow out of an input reservoir, using a method adapted from work simulating comet impacts on Earth's moon, which includes collision limiting in the nearly continuous near-field [4]. We specify a vent geometry on the surface of the planet, and then project that geometry downward to obtain the particle reservoir in which molecules are created in each time step.…”