Neumann BC Input in MMS Testing

Dr. Aagaard, @baagaard

As discussed the other day, I am using pylith::bc:NeumannTimeDependent to specify the neumann BC in an MMS testing version of the Terzaghi problem. I am still a little unclear how the auxiliary field data is passed to the BC variable. Going on the example put forth for the DirichletUserFn objects, the assignment setup I have at the moment is as follows:

    // Y Positive Traction

I know something needs to be added to take the place of the setUserFn call for DirichletUserFn, in order to specify values, however I am not sure the syntax required to replicate what would be in a .cfg file.

I would love some insight into what I hope is a minor bump in the road.

Thank you,


The bc::NeumannTimeDependent object inherits from bc::BoundaryCondition which inherits from problem::Physics. To find these yourself, just follow the inheritance as given in the header (.hh) files. The “setters” you will want to call are:

Implemented in NeumannTimeDependent:

  • setTimeHistoryDB() if applicable
  • useInitial()
  • useRate()
  • useTimeHistory()
  • setScaleName()

Implemented in BoundaryCondition:

  • setMarkerLabel()
  • setSubfieldName()
  • setRefDir1()
  • setRefDir2()

Implemented in Physics:

  • setNormalizer()
  • setAuxiliaryFieldDB()
  • setAuxiliaryFieldSubfieldDiscretization() for each subfield in the auxiliary field

You are already calling most of these, so it is just adding calls to the ones you are missing.

You should be able to use the default reference directions (used to determine the tangential directions) and not have to call setRefDir1() and setRefDir2() (I included those for completeness).

Apologies in advance if some of these are stupid questions, but I am still unclear about a bit.

I am unsure about setting the AuxiliaryFieldDB for the neumann BC. I believe the only example of an AuxiliaryFieldDB in the MMS tests available is that where the material parameters are setup. Using your example from the IncompressibleElasticity MMS tests, as well as the [assumed] precedent from the .cfg files, I am still not exactly sure how the equivalent of setting something like “spatialdata.spatialdb.UniformDB” is located/accomplished.

Continuing from the IncompressibleElasticity example, I am also going to guess that a separate (from the material properties) and new auxDB will be required; will this also be of the form spatialdata::spatialdb::UserFunctionDB? If so, will this also require its own variables for auxSubfields, auxDiscretizations, and the same setup process in the base (, for example) as for the auxDB for the material properties? Similarly, will the pylith::topology::Field::Discretization values need to be updated for each element class, as in the specific examples?

Apologies again, this setup process is a bit confusing, for me, at least.


Yes, the Neumann BC setup for an MMS test closely follows that of the bulk material. It has its own spatial database for the auxiliary field. In the MMS tests we use the UserFunctionDB, which is an analytical function (again, see the material for examples). For the auxiliary field you will need to specify the discretization of each subfield (initial value, rate value, etc) as is done for the material.