On Techniques for Barely Coupled Multiphysics

TL;DR

The paper proposes a barely coupled timestepping framework to efficiently solve multiphysics problems with widely separated timescales by advancing each field to a quasi-steady state before exchanging data. This loose-coupling strategy enables flexible inter-code data exchange and tolerates diverse discretizations and codes, while providing practical stopping criteria for each field. The evaporative cooling of electric motors serves as a representative case, illustrating rapid quasi-steady flow behavior alongside slower thermal evolution and particle-based heat transfer, with detailed models and numerics provided in the appendices. Open questions identified include optimal ramping of time increments, automatic detection of stop conditions, integration with optimization and adjoint methods, and broader adoption across engineering workflows.

Abstract

A technique to combine codes to solve barely coupled multiphysics problems has been developed. Each field is advanced separately until a stop is triggered. This could be due to a preset time increment, a preset number of timesteps, a preset decrease of residuals, a preset change in unknowns, a preset change in geometry, or any other physically meaningful quantity. The technique allows for a simple implementation in coupled codes using the loose coupling approach. Examples from evaporative cooling of electric motors, a problem that has come to the forefront with the rise of electric propulsion in the aerospace sector (drones and air taxis in particular) shows the viability and accuracy of the proposed procedure.