Add support for function-based velocity advection on constant and higher order discontinuous variables

[smpark7]

Description

PostprocessorVelocityFunctionInflowBC and VelocityFunctionConservativeAdvection are similar to PostprocessorInflowBC and ConservativeAdvection, but with function-based velocity profiles.

PostprocessorPenaltyDirichletBC is similar to PenaltyDirichletBC, but with a postprocessor to set the boundary value.

PostprocessorVelocityFunctionInflowBC is required to fix arfc/moltres#183.

VelocityFunctionConservativeAdvection and PostprocessorPenaltyDirichletBC are required to fix arfc/moltres#265.

Changes

• Add PostprocessorVelocityFunctionInflowBC
• Add VelocityFunctionConservativeAdvection
• Add PostprocessorPenaltyDirichletBC

[LukeSeifert]

Looks good! I only had some questions on the documentation and the variable names. I have noticed when looking at MOOSE source code it uses really short variable names like _t, why is that?

[LukeSeifert]

Is documentation going to be added in a future PR?

[LukeSeifert]

Same here

[LukeSeifert]

These variable names don't seem great to me, how come something longer isn't used?

[smpark7]

@LukeSeifert Thanks for the quick review! I added documentation to the markdown files and I renamed the variables to _penalty and _postprocessor. I think the MOOSE team used to be more lax about variables names. To be fair the names usually quite obvious (e.g. _t usually means time). As the framework grew over the years, naming conflicts have cropped up. However, it can be difficult to "rectify" some of the names without impacting a large number of users and MOOSE-based apps downstream.

Feel free to merge if you approve!

[LukeSeifert]

Looks good!

[cticenhour]

I think the MOOSE team used to be more lax about variables names.

@LukeSeifert @smpark7 I have no doubt that we might have been a bit more lax in the past. 😄 The decision on _t was a decent bit before my time though. I would like to think that short-named spatial and temporal variables that get inherited (like _t) come from how one naturally writes a simple function down on paper (i.e., f(x,y,z,t) = 5xt). I should ask one day. For my code, I try to make my object variable names longer so as to be more descriptive. There are almost certainly many places we could fix up in MOOSE and many apps though 😆