Abstract
A domain-decomposed method to simultaneously couple the classical Molecular Dynamics (MD) and Direct Simulation Monte Carlo (DSMC) methods is proposed. This approach utilises the MPI-based general coupling library, the Multiscale Universal Interface. The method provides a direct coupling strategy and utilises two OpenFOAM based solvers, mdFoam+ and dsmcFoam+, enabling scenarios where both solvers assume one discrete particle is equal to one molecule or atom. The ultimate goal of this work is to enable complex multi-scale simulations involving micro, meso and macroscopic elements, as found with problems like evaporation.Results are presented to show the fundamental capabilities of the method in terms of mass and kinetic energy conservation between simulation regions handled by the different solvers. We demonstrate the capability of the method by deploying onto a large supercomputing resource, with attention paid to the scalability for a canonical NVT ensemble (a constant number of atoms N, constant volume V and constant temperature T) of Argon atoms. The results show that the method performs as expected in terms of mass conservation and the solution is also shown to scale reasonably on a supercomputing resource, within the known performance limits of the coupled codes. The wider future of this work is also considered, with focus placed on the next steps to expand the capabilities of the methodology to allow for indirect coupling (where the coarse-graining capability of the DSMC method is used), as well as how this will then fit into a larger coupled framework to allow a complete micro-meso-macro approach to be tackled.
Highlights
Many fundamental physical processes are intrinsically multiscale in nature when they are considered from a modelling and simulation perspective
A method is proposed, along with associated software, that provides a fully-coupled simulation environment for problems where classical Molecular Dynamics (MD) and Direct Simulation Monte Carlo (DSMC) can be used simultaneously to consider a problem. This is the first part of a larger overall piece of work which will look at how to allow for coarse-graining with the DSMC method and allow for simultaneous coupling between MD and Computational Fluid Dynamics (CFD) as well as DSMC and CFD in order to complete the full tri-scale modelling capability demanded by an evaporation problem
Background approaches ranging from direct coupling strategies [15,16] that allow DSMC to be used as a direct replacement for MD in order to facilitate the simulation of significantly larger problem sizes, through to strategies that allow DSMC to be used as a coarse-grained method alongside MD [17,18,19]
Summary
Many fundamental physical processes are intrinsically multiscale in nature when they are considered from a modelling and simulation perspective. Background approaches ranging from direct coupling strategies [15,16] that allow DSMC to be used as a direct replacement for MD (or similar methods) in order to facilitate the simulation of significantly larger problem sizes (due to the inherent lower computational demand of the method compared to MD), through to strategies that allow DSMC to be used as a coarse-grained method alongside MD [17,18,19] These strategies are distinct; the first treats MD molecules (or atoms) and DSMC parcels (or particles) as functionally equivalent; the second relies on collecting statistical data from each domain that enables an interface-based coupling where length and time scales can be different. Where domains are significantly different an indirect approach is needed that allows the DSMC simulated domain to be coarse-grained
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