Unified Methodology for Simulation of Continuum and Rarefied Flows

We propose to develop stand-alone computational modules for seamlessly extending the validity of continuum CFD codes into transitional and rarefied flow regimes. The modules will be designed for implementation in to existing legacy codes for use in the characterization of high altitude plume flows. The approach is based on a novel, regularized set of Navier-Stokes equations (RNS) that is extended to account for kinetic effects (intermediate Knudsen number, fluctuations) in the continuum approximation. RNS has several important features not found in classical NS equations that are of direct relevance in high altitude plume flows: (a) Natural accounting of both continuum and rarefied gas flow regimes; and (b) Kolmogorov-scale field fluctuations resulting from a mathematical model that accounts for turbulent diffusion in a natural manner that allows direct simulation of phenomena such as laminar-turbulent transition and wall slip effects. Phase I has completed the development of stand-alone, computational module prototypes incorporating a simplified version of the RNS approach. The modules were connected to a characteristics-based high order compressible flow code with particle transport capabilities, and an unstructured finite-element code, and were successfully exercised and evaluated for a model problem that contained features of both continuum and rarefied flows. Phase II will complete the development of the modules, provide a detailed comparison to Navier-Stokes results for low and high altitude flow regimes, identify the parameter region where the approach is advised, and provide connections to plume signatures sponsored and managed by the MDA/DES modeling and simulation effort.