verify_interface_flux

For two coupled solvers: checks that what leaves one side arrives at the other.

Group: Check the answer.

Parameters

Parameter	Type	Required	Default
interface_files	string	yes
solution_files	string	no	''
interface_axis	integer	no	0

What the model reads

The text below is the tool's own description, exactly as the AI model receives it.

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On a COUPLED task: is your interface flux the right sign, and do
your two sides actually agree there? Runs on YOUR OWN files, with no
reference solution.

THIS EXISTED ONLY INSIDE AN INDEPENDENT CHECK UNTIL NOW, which is why
it is here. The check below is the one that decided one coupled
development run: a result set whose two codes both genuinely ran,
whose coupling genuinely iterated over three mesh levels, and whose
interface FIELD matched to 0.000e+00 across the seam, was still
complete but unphysical — because one side reported its flux with the
INWARD normal. The relative flux jump came out 8.139e-01, 9.066e-01,
9.530e-01 over the three levels: not shrinking, and growing. The
agent had no way to see that before handing in. Now it does.

WHAT IT CHECKS, all of it key-free:

1. SIGN AND SELF-CONSISTENCY, per side. For a flux you really computed
   from your own solution,

       q_n(x) / (-du/dn)(x)  ==  k   at every interface point,

   so the ratio is CONSTANT along the interface whatever k is — and
   POSITIVE. A constant NEGATIVE ratio means your normal points the
   wrong way: the task defines q_n = -(K grad u) . n_out with n_out
   pointing OUT of the subdomain. A ratio that is not constant means
   the profile did not come from the field you delivered.

   The trap this catches most often: on the NEUMANN side the flux you
   IMPORT and the flux you REPORT have OPPOSITE signs. Kratos's
   FACE_HEAT_FLUX is the INWARD normal flux (measured against a closed
   form: u came out +0.875 where the inward reading predicts +0.875),
   so the number you write into each per-level interface file is the
   NEGATIVE of the one you applied.

2. THE TWO-SIDED JUMP, and its refinement trend. The field trace must
   be continuous and the two fluxes must cancel, because the normals
   are anti-parallel. A jump that stays O(1) as h halves means the
   iteration converged to a fixed point of the WRONG transmission
   condition — and your observed order cannot see that, because
   convergence to a wrong answer is still convergence.

Args:
    interface_files: comma-separated per-level interface files, one
        per side, in the shape `x, y, u, qn`; each name must carry
        `level<k>_<side>`, which is how level and side are read. Give
        BOTH sides and all levels; the trend is the informative part.
    solution_files: comma-separated per-level field files, one per
        side, `x, y, u`, named the same way. Needed for check 1 —
        without them the sign cannot be tested, only the jump.
    interface_axis: 0 if the interface is a line of constant x, 1 if
        constant y.

Returns: per-side sign verdicts, per-level jumps, the refinement trend,
    and NOT_ASSESSED wherever a check could not look at anything — a
    check that could not run never reports success.