Fluctuations and continuity in particle-continuum hybrid simulations of unsteady flows based on flux-exchange

– This letter describes the treatment of unsteady liquid flow by a hybrid particlecontinuum scheme. The scheme couples a particle region described by molecular dynamics with a coarse-grained domain solved by continuum fluid dynamics. The particle and continuum domains overlap in the coupling region, where two-way transfer of momentum flux is established. We demonstrate that this flux-coupling scheme is able to describe high-frequency oscillatory flows and to ensure the continuity of velocity across the particle-continuum interface. The effect of fluctuations within the particle system is also analysed and establishes the range in frequency and flow wave number for which hydrodynamic fluctuations need to be taken into account within the continuum description. Many systems are governed by a dynamic interplay between rapid molecular processes occurring within a small localised region and slower long-ranged hydrodynamic processes mediated by hydrodynamic interactions with the bulk fluid. This sort of scenario is broadly encountered in complex flows near interfaces, wetting, drop formation, melting, crystal growth from a fluid phase or moving interfaces of immiscible fluids or membranes. Such problems are usually too computationally expensive for any standard molecular dynamics (MD) simulation, while they cannot be solved by continuum fluid dynamics (CFD). An alternative is to retain the atomistic description only where it is needed and solve the bulk flow by much faster CFD methods. This multiscale approach is the essential idea of the present hybrid particlecontinuum model. A quite general procedure used in hybrid particle-continuum models, based on domain decomposition, is to connect the particle (P) and the continuum (C) domains at an overlapping or handshaking region consisting in two buffers, C → P and P → C, where the two-way exchange of information is performed. At C → P, the particle dynamics are modified to adhere to the local prescriptions of the C-flow, while within the P → C domain the microscopic (∗) E-mail: [email protected] (∗∗) E-mail: [email protected] (∗∗∗) E-mail: [email protected]