Analysis of Gas Expansion in a Rarefied Atmosphere

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Abstract
Similarity solutions for spherically and cylindrically symmetric gas expansions in rarefied atmospheres are refined and then compared with the corresponding numerical hydrodynamic calculations. All of the hydrodynamic solutions are then shown to be invalid except for the initial radius-time dependence in a very limited region, after which time diffusion becomes the dominant expansion method. Apparent agreement with observed gas expansions at later times is due only to the similar radial growth for hydrodynamics (∼t0.4) and diffusion (∼t0.5).