  This paper presents a unified framework, called multiRegionFoam, for solving
multiphysics problems of the multi-region coupling type within OpenFOAM
(FOAM-extend). It is intended to supersede the existing solver with the same
name. The design of the new framework is modular, allowing users to assemble a
multiphysics problem region-by-region and coupling conditions
interface-by-interface. The present approach allows users to choose between
deploying either monolithic or partitioned interface coupling for each
individual transport equation. The formulation of boundary conditions is
generalised in the sense that their implementation is based on the mathematical
jump/transmission conditions in the most general form for tensors of any rank.
  The present contribution focuses on the underlying mathematical model for
these types of multiphysics problems, as well as on the software design and
resulting code structure that enable a flexible and modular approach. Finally,
deployment for different multi-region coupling cases is demonstrated, including
conjugate heat, multiphase flows and fuel-cells.
