rtx-cfd: overset P4-0 — the O-grid around the Turek–Hron rigid body (cylinder + flag), gated

patch_gen::{cylinder_flag_outline, cylinder_flag_patch, winslow_smooth, respace_rays}
(+ convex_hull, offset_convex_polygon, nearest_on_polyline): the outline CCW then
reversed to clockwise (tip semicircle 16 cells, junction fillets of a FIXED radius
with 3 cells, straights graded 0.3 h → h, cylinder arc at h); the outer ring the
6 h normal offset of the body's convex hull; initial pairing by the inner point's
normal offset projected onto the hull offset (an arclength-proportional pairing
folded the transfinite grid at the tip: rays crossed where the curvatures differ);
Winslow (TTM) smoothing of the interior with the outer nodes SLIDING along the
hull offset (each re-placed at the nearest point to the extrapolated ray), then
re-spacing along the smoothed rays to the across stretch. Gates
(tests/patch_cylinder_flag.rs): ny = 41/62/82 → 143×12 / 183×12 / 225×12 cells,
positive, wall row 0.23 h (fillet max 0.37 / 0.43 / 0.50 h), worst
non-orthogonality 76.6 / 69.1 / 63.3° at the concave fillets (structural: a
concave arc's normals converge at its centre), classification of the benchmark
background with both donor invariants. P0 MMS on these meshes
(tests/cylinder_flag_mms.rs, exact acceptors, line-implicit): Stokes orders 2.17 /
2.13, upwind 2.00 / 1.86 (cell Péclet ≈ 0.1), divergence ≤ 9e-14 — the fillet skew
costs nothing measurable. Rule: a refinement ladder's geometry must be fixed in
physical units — with fillet = h/2 the Stokes orders read 1.74 → 1.31, the O(h)
boundary perturbation masquerading as a scheme defect; the fillet is a parameter
(5 mm across the ladder). Also: the P3b knock-outs H1/H2 on the balanced default
(no effect), the P3 §5.10 record in the falsifier's header.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01X2GmJXeQ2njUecEKiJZ1G2

crates/specialized/rtx-cfd/src/solvers/incompressible/overset/mod.rs

@@ -564,6 +564,7 @@ impl OversetPisoSolver {
                         .collect(),
                 );
             }
+            let p_before_stamp = field.background.p.clone();
             let old_map = std::mem::replace(&mut self.overlap, new);
             self.background
                 .set_overlap(self.overlap.background_mask(), self.overlap.fringe_flags());
@@ -597,6 +598,17 @@ impl OversetPisoSolver {
                 }
             }
             self.balance_fringe(&mut field.background);
+            // Knock-out H2 (`RTX_OVERSET_H2`): cells that turn active → fringe
+            // keep the background's own pressure this step instead of the
+            // patch's stamped value (their p' is still Dirichlet from the
+            // patch).
+            if std::env::var("RTX_OVERSET_H2").is_ok() {
+                for e in &self.overlap.fringe_cells {
+                    if old_map.class(e.j, e.i) == CellClass::Active {
+                        field.background.p[(e.j, e.i)] = p_before_stamp[(e.j, e.i)];
+                    }
+                }
+            }
             // Knock-out H4 (`RTX_OVERSET_H4`): no temporal warm start on a
             // reclassification step.
             if std::env::var("RTX_OVERSET_H4").is_ok() {