Coupling CFD and MD

The prohibitive computational cost of MD simulations at the macroscale has motivated the development of hybrid techniques. In these methods, the MD simulation is limited to a small spatial region, for example interfaces. The rest of the flow domain is computed using a continuum approach. An overlap between the MD and continuum regions allows the exchange of the two states, and thus couples the two descriptions.

Independently, the MD and continuum descriptions are well established. The difficulty arises in the overlap region where they must be coupled. The coupling must satisfy conservation laws and ensure continuity of density, momentum, and energy and their fluxes. The upscaling problem (MD to continuum) is straightforward: microscopic properties are integrated locally over space and in time to provide boundary conditions for the continuum region. The inverse problem, however, is non-trivial and in fact non-unique. The continuum-molecular coupling adopted in this work is based on flux exchange, and was pioneered by Flekkøy and coworkers. It naturally ensures that fluxes from one configuration ``flow" to the other, but special care must be taken in order to ensure continuity of the state variables.

The above coupling methodology has been tested in canonical problems, for example laminar Couette and Poiseuille flow were simulated over a molecular representation of flat surfaces and two-dimensional, organised roughness. A significant leap in modelling and computational ability is required in order to translate these canonical studies to simulations of engineering flows, where the outer flow is multiscale and surface textures and coatings are modelled more accurately using MD. Coupling of $\mathcal {T}rans$ $\mathcal {F}low$and $\mathcal {S}tream$$MD$for massively parallel simulations on HECToR is intended to serve this purpose. It should be noted that the coupler module, however, is designed to be a general purpose library, or a multi-purpose computational coupling utility, which is available to the wider research and HECToR community.