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## Development of a Hybrid CFD-DSMC Solver

As spacecraft enter the Earth’s atmosphere, they move through different flow regimes. The rarefied flow at high altitudes is characterized by low density and thermal non-equilibrium and is usually described by the Direct Simulation Monte Carlo (DSMC) method which is a probabilistic particle technique. After a transition zone, the spacecraft will encounter continuum flow where conventional Computational Fluid Dynamics (CFD) can be utilized. As these flow regimes do not have distinct boundaries, the continuum assumption may hold for the incoming flow, while regions of rarefied, non-equilibrium flow can appear locally. In order to properly describe the flow around spacecraft, a hybrid CFD-DSMC solver is proposed to take advantage of both methods in their respective regimes. The accuracy of DSMC for rarefied flow and the transition regime is coupled with the fast calculation times of CFD in the continuum regime. Moreover, a hybrid solver is applicable to microscale flows such as in micro-thrusters.

A hybrid solver requires the definition of a continuum breakdown parameter which determines the preferred method for the respective domain. A possible parameter is the rarefaction of the gas that can be quantified by the local Knudsen number as a ratio of the mean free path and the local gradient of a selected property. As this parameter is not known a priori, it has to be approximated, e.g. with a full CFD calculation. As soon as the domains have been identified, treatment of the interface between CFD and DSMC poses the next challenge. While the CFD method solves equations for the macroscopic flow properties, the DSMC method models the gas with discrete particles on a microscopic level. Flow properties such as temperature and pressure are found through averaging over time and particles in a cell. The interface can introduce significant computational overhead due to the completely different approaches of both methods.

The proposed research project is concerned with the development of such a hybrid CFD-DSMC solver. The aim is to develop a solver which enables the calculation of a complete re-entry trajectory while efficiently taking advantage of the CFD and DSMC methods. CFD and DSMC solvers, as part of the OpenFOAM software package, are already being developed at the University of Strathclyde. Possible breakdown parameters and methods for the treatment of the interface will be identified. After investigating and comparing the different parameters and methods, the hybrid solver will be implemented into the OpenFOAM framework. Finally, the solver is to be verified and validated with several test cases.

#### Summary of the proposed work

- Definition of a continuum breakdown parameter
- Development of an interface between CFD and DSMC
- Implementation into the OpenFOAM framework
- Verification and validation of the code

For more information about the project contact Dr Tom Scanlon (tom.scanlon [at] strath [dot] ac [dot] uk), Senior Lecturer at the Department of Mechanical and Aerospace Engineering at the University of Strathclyde.

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