IFOSMONDI Co-simulation Algorithm with Jacobian-Free Methods in PETSc

Co-simulation is a widely used solution to enable global simulation of a modular system via the composition of black-boxed simulators. Among co-simulation methods, the IFOSMONDI implicit iterative algorithm, previously introduced by the authors, enables us to solve the non-linear coupling function while keeping the smoothness of interfaces without introducing a delay. Moreover, it automatically adapts the size of the steps between data exchanges among the subsystems according to the difficulty of solving the coupling constraint. The latter was solved by a fixed-point algorithm, whereas this paper introduces the Jacobian-Free Methods version. Most implementations of Newton-like methods require a jacobian matrix which, except in the Zero-Order-Hold case, can be difficult to compute in the co-simulation context. As IFOSMONDI coupling algorithm uses Hermite interpolation for smoothness enhancement, we propose hereafter a new formulation of the non-linear coupling function including both the values and the time-derivatives of the coupling variables. This formulation is well designed for solving the coupling through jacobian-free Newton-type methods. Consequently, successive function evaluations consist in multiple simulations of the systems on a co-simulation time-step using rollback. The orchestrator-workers structure of the algorithm enables us to combine the PETSc framework on the orchestrator side for the non-linear Newton-type solvers with the parallel integrations of the systems on the workers' side thanks to MPI processes. Different non-linear methods will be compared to one another and to the original fixed-point implementation on a newly proposed 2-system academic test case with direct feedthrough on both sides. An industrial model will also be considered to investigate the performance of the method.