granularity of multiapp transfer and update

[mcacace]

Check these boxes if you have followed the posting rules.

• Q&A General is the most appropriate section for my question
• I have consulted the posting Guidelines on the Discussions front page
• I have searched the Discussions forum and my question has not been asked before
• I have searched the MOOSE website and the documentation does not answer my question
• I have formatted my post following the posting guidelines (screenshots as a last resort, triple back quotes around pasted text)

Question

Dear all,
I might have a naive question, but ... I'm working on linking an external library to a moose application, doing that via the multiapp system. My moose app runs as master and the external library as sub app at timestep_end. Coupling is done via transfer from the master to the sub (all is fine) and then back from the sub to the master (and here my question). The sub app passes a source/sink contribution as a auxvariable (monomial) which I then use in the master app to compute a kernel contribution). So my question: given that the transfer is done at the timestep_end, what would be the value of the auxvariable in the master app at the new time step? Is the value I passed from the previous solve? Or, should I "lag in time" the auxvariable (taking the old value)? Or, how can I control the syncing between the two?
Thanks for any feedback,
mauro

[GiudGiud]

Hello

Use Problem/verbose_multiapps to see when the transfers and multiapps are being executed and that will let you know which field is at the end of the time step and which field is at the beginning.

If you use fixed point iterations between the two applications, by setting fixed_point_max_its in the executioner. That can help you do an implicit coupling between the two solves

Lagging is not always a bad idea, but it s usually first order in time. As in the time integration scheme error will be first order of the time step

Guillaume

[mcacace]

Thanks @GiudGiud. Will try to follow your suggestion. Sure I'll be back as my coding (slowly) develops further.