That’s a good question.

Think of the coupling dt as a window size. Currently this window size is fixed (but we might make it adaptive in the future). There is probably no perfect recipe on how to choose the window size. The trade-off you explain is valid.

Inside a window, both solvers can use their own timestep sizes until they reach the end of the window. In an implicit coupling loop, they can also use different timestep sizes in each iteration.

An example:

- Window size 1.0.
- The fluid solver does 2 timesteps of 0.5, the structure solver 1 timestep of 1.0.
- We don’t converge and the adaptive timestep criterion of the fluid solvers suggests smaller steps.
- In the next iteration we do 4 timestep in the fluid solver of 0.25 and again 1 timestep in the structure solver.

All this is already supported by preCICE right now. This wiki page gives more information on the configuration. The current limitation is that for such a subcyling within one timestep, we currently use a constant extrapolation, which deteriorates the order of both solvers typically to one and which might also introduces stability problems. We are working on a general interpolation scheme, see e.g. this paper. Rough estimate would be that we have this working in preCICE at the end of 2020.

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