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Approaches to predict flowfields that display rarefaction effects incur a cost in computational time and memory that is considerably higher than methods commonly employed for continuum flows. For this reason, to simulate flowfields where continuum and rarefied regimes coexist, hybrid techniques have been introduced. In the present work, analytically defined gas-kinetic schemes based on the Shakhov and Rykov models, for monatomic and diatomic gas flows, respectively, are proposed and evaluated, with the aim of being used in the context of hybrid simulations. This should reduce the region where more expensive methods are needed by extending the validity of the continuum formulation. Moreover, because, for rarefied gas flows at high velocities, it is necessary to take into account the nonequilibrium among the internal degrees of freedom, the extension of the approach to employ diatomic gas models with rotational relaxation is a mandatory first step toward realistic simulations. Compared to the previous works the presented scheme is defined directly on the basis of kinetic models that involve a Prandtl number correction. Moreover, the methods are defined fully analytically instead of making use of Taylor expansion for the evaluation of the required derivatives. The scheme has been tested for various test cases and Mach numbers proving to produce reliable predictions in agreement with other approaches for near-continuum flows. Finally, the performance of the scheme, in terms of memory and computational time, compared to discrete velocity methods makes it a compelling alternative in place of more complex methods for hybrid simulations of weakly rarefied flows. [ABSTRACT FROM AUTHOR]

Numerical meshes of both cork and carbon fiber ablative materials in their virgin and pyrolized states, with realistic porosity and tortuosity, have been created from microcomputed tomography scans. The porosity of each material has been calculated from the microcomputed scans and used to extract smaller representative sample volumes to perform numerical simulations on. Direct simulation Monte Carlo simulations of rarefied gas flow through these materials have been performed to find the permeability of each material to argon gas and to a gas mixture. The method has been validated by comparing the measured permeability for a Berea sandstone material with previously published experimental values. For the specific pressure conditions investigated here, the cork-phenolic material becomes around 10 times more permeable after being pyrolized, whereas the carbon-phenolic material only becomes five times more permeable than its virgin form. The permeability to the gas mixture is found to be greater than to argon for most of the samples, showing the importance of choosing the correct gas for rarefied permeability studies. The form of the pressure and Mach number profiles through the materials is independent of the applied pressure gradient. [ABSTRACT FROM AUTHOR]