[CoSim] Incorrect residuals in `ConvergenceCriteria`

[juancamarotti]

[CoSim] Incorrect residuals in `ConvergenceCriteria`

[philbucher]

nice catch!

No wonder it never really worked for us, we basically always used the other criteria

[juancamarotti]

After further analysis of the problem, I have concluded that this is not a CoSim issue. The bug is related to the strategy used. Let's consider the ResidualBasedNewtonRaphsonStrategy as an example:

1. In ResidualBasedNewtonRaphsonStrategy::SolveSolutionStep, the method ConvergenceCriteria::InilitializeNonLinearIteration is called with the LHS and RHS even before invoking the BuilderAndSolver. This implies that the method is called with zero LHS and RHS vectors.

2. The second issue is that inside ResidualCriteria, the ConvergenceCriteria::InitializeNonLinearIteration method is not overriden. The residual is calculated just once per time step inside ResidualCriteria::InitializeSolutionStep and not once per coupling iteration.

@KratosMultiphysics/technical-committee Why the ConvergenceCriteria::InilitializeNonLinearIteration is called before the BuilderAndSolver?

FYI @matekelemen @philbucher

[juancamarotti]

Yes! This issue is causing slow convergence with the non-linear shell elements we are using in our coupled simulations.

[matekelemen]

The question is how to fix the bug. As far as I can tell, calling the solver's InitializeSolutionStep and FinalizeSolutionStep during coupling iterations is a no-go because it would flush the solution buffer and thus break time stepping.

A hackish solution I can think of is storing the initial residual in ProcessInfo or ModelPart's DataValueContainer and manually overriding it during coupling iterations, but I don't particularly like this fix.

Any other ideas?