FSI: Subcycling incorrect displacement

[vidulejs]

While working on issue #58, I discovered the real reason for instability in FSI simulations with subcycling. Initially it ticked me off, that the mesh was changing during subcycling steps. Maybe this was intended but I was not sure.

However, I realized that at each subcycling step the displacement is read again. This is the total displacement and not multiplied by subcycling dt. This means that for N subcycling steps, the total displacement will be amplified N times.

[davidscn]

However, I realized that at each subcycling step the displacement is read again

For FSI and subcycling, this would be the expected behavior.

This is the total displacement and not multiplied by subcycling dt.

That's the expected behavior either. preCICE takes care of the time interpolation between the beginning and the end of the time window, so each intermediate state will just be in between those two displacement states. The read displacement is total, yes, but the state of the fluid grid within a simulation is already deformed, so there is only an incremental portion left to reach this absolute state. I think this might go wrong when using relative displacements. But from your description, I understand you are dealing with absolute displacements, right?

This is the total displacement and not multiplied by subcycling dt

Why should the displacement be multiplied by dt? What would be the unit of this quantity?

This means that for N subcycling steps, the total displacement will be amplified N times.

I dont understand this and disagree, could explain this more in detail?