FaceDivFree interpolation for a staggered field

[farhaddaei]

Hi everyone!

In our code, we have a staggered magnetic field (each component is face-centred in its respective direction), so each component has its own index type. Therefore, in the variableSetUp method of the class inheriting from the AmrLevel class, we need to add a separate element to desc_lst (correct?). However, to use the face_divfree_interp interpolator (of type FaceDivFree, to preserve the divergence of the magnetic field) we need to inform AMReX that the components provided in separate elements of desc_lst should participate in the interpolation. Unfortunately, I did not find any example of such interpolation in the tutorials, code examples, some other codes which use AMReX, or any explicit instruction about this subject in the AMReX documentation.

I appreciated any hint about using the mentioned interpolation method.

Regards,
Farhad

[WeiqunZhang]

I assume you wanted to use the divergence free interpolater in FillPath. You should be able to call

amrex/Src/AmrCore/AMReX_FillPatchUtil.H

Line 277 in e792933

FillPatchTwoLevels (Array<MF*, AMREX_SPACEDIM> const& mf, IntVect const& nghost, Real time,

and

amrex/Src/AmrCore/AMReX_FillPatchUtil.H

Line 685 in e792933

InterpFromCoarseLevel (Array<MF*, AMREX_SPACEDIM> const& mf, IntVect const& nghost, Real time,

directly without using AmrLevel::FillPatch. For convenience, you might want to create a function say FillPatchBfield that does what AmrLevel::FillPatch does for other variables.

Aslo, AmrLevel has two virtual functions virtual void init (AmrLevel &old) and virtual void init () that are used during regrid. Here is an example. https://github.com/AMReX-Fluids/IAMR/blob/37327b3ed63d3ac29e2c6f2937aa071a2b06e02c/Source/NavierStokesBase.cpp#L1713. In these two functions, you should be able to call FillPatchTwoLevels and InterpFromCoarseLevel directly or your FillPatchBfield instead of FillPatch for the B field.

[farhaddaei]

@WeiqunZhang thanks for the hints and the link to IAMR repo. As you mentioned we needed face_divfree_interp to fill patches in both directions (coarse <=> fine) and the methods you suggested are perfect for the task.
We will pack magnetic field components as implemented in "amrex/Tests/DivFreePatch/"

Just out of curiosity, how could one use the "FaceDivFree" class? The name and the description of the class (the comments in the source code) are very tempting for scenarios including "AMR + staggered magnetic field"

[WeiqunZhang]

One uses FaceDivFree when calling fillpatch functions by passing &amrex::face_divfree_interp as the Interp* argument. For MHD, you probably also need amrex::EdgeFluxRegister for E (or v x B).