IBPSA.Fluid.Examples.FlowSystem.Simplified1 (Dymola)

[DagBruck]

The simulation stops due to a failure to solve a nonlinear system. This happens during crossing function evaluation after an output time. When failing such an evaluation it is not possible for the integrator to recover. (Fortunately, this is a rare case.)

CVode does not have some features that our other solvers have. To solve this issue there would be two ways forward:

• Make a local modification in SUNDIALS to add this functionality. This is a path we'd rather not go.
• Implement a solution in our code that then calls SUNDIALS. As of yet we have not been able to study in detail how that should be done.

Work-around: the problem goes away by tightening the tolerance. The error message will be improved in the next release of Dymola.

[ErikHenningsson]

We have solved this issue for the upcoming Dymola 2023x Refresh 1. CVode in Dymola now has improved start guesses for nonlinear systems and is therefore more robust.

[mwetter]

@ErikHenningsson : Thanks for bringing this up. The model from this version of the library now works as tested with Dymola 2024x, and I closed this case.

However, in the meantime, the library has been updated and -- among other things -- the pump model is now different. With the new version, Dymola 2024x now fails. I opened a new case for this, https://ibpsa.github.io/modelica-challenge-problems/cases/FlowSystemDymola-2.html
I now this is a bit of transient issue, but that model really seems to have hard dynamics that we pushed with some model changes inadvertently to a point where the improved solver now has issues. I hope the updated model helps further improving the tool, or getting to the root cause of the model issue.

[ErikHenningsson]

I see, thank you for the feedback. Then it is back to the drawing board for us to try to figure out what goes wrong with the new version and if the solver can be improved.

[ErikHenningsson]

Actually, I just tested with some other improvements that we've made to CVode for the next Dymola release and the simulation of the new model version works fine with those improvements.

[mwetter]

Great to hear about the improved solver. I will update when the new Dymola version is released.

[DagBruck]

April 19.