rtx-cfd: overset_cfd23 — Turek–Hron CFD2/CFD3 on the overset (the harness's ramped inflow from rest, TVD background and patch, 3-round cap), loads by the patch wall stress with the solver-flux-form box and the CV formula beside it as window statistics, CFD3 frequency from lift crossings, the solver-metric momentum chain at the final state (solved-face residual is the pin), save/load of the fields by case; momentum_residual checks the far-upwind neighbours explicitly under a limited background (the van Albada limiter swallows a NaN silently)

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_0116sg1Qz1gMv9hdcKP1XUam

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

@@ -89,7 +89,9 @@ pub struct FaceResidual {
     /// Between two fringe cells (else fringe–hole).
     pub fringe_fringe: bool,
     /// The pieces of `r` (N/m): unsteady, convective, diffusive, pressure
-    /// (`r = time + conv − diff + pres`; upwind, interior faces).
+    /// (`r = time + conv − diff + pres` under upwind on interior faces;
+    /// under a limited scheme the convective piece is the upwind part
+    /// only and the four do not reconstruct `r`).
     pub pieces: [f64; 4],
 }
 
@@ -201,6 +203,27 @@ impl OversetPisoSolver {
         };
         let vol = dx * dy;
         let mut prescribed = Vec::new();
+        // Under a limited scheme the far-upwind value enters through
+        // `r > 0`, which a NaN fails silently (a silent upwind fallback),
+        // so the two-away neighbours are checked explicitly.
+        let limited = self.background.parameters().convection_scheme
+            != crate::solvers::incompressible::ConvectionScheme::Upwind;
+        let far_ok_u =
+            |m: &crate::solvers::incompressible::flow_field::FlowField, j: usize, i: usize| {
+                !limited
+                    || ((i < 2 || m.u_old[(j, i - 2)].is_finite())
+                        && (i + 2 > nx || m.u_old[(j, i + 2)].is_finite())
+                        && (j < 2 || m.u_old[(j - 2, i)].is_finite())
+                        && (j + 2 >= ny || m.u_old[(j + 2, i)].is_finite()))
+            };
+        let far_ok_v =
+            |m: &crate::solvers::incompressible::flow_field::FlowField, j: usize, i: usize| {
+                !limited
+                    || ((j < 2 || m.v_old[(j - 2, i)].is_finite())
+                        && (j + 2 > ny || m.v_old[(j + 2, i)].is_finite())
+                        && (i < 2 || m.v_old[(j, i - 2)].is_finite())
+                        && (i + 2 >= nx || m.v_old[(j, i + 2)].is_finite()))
+            };
         let mu = self.background.config().viscosity;
         let upw = |f: f64, a: f64, b: f64| if f >= 0.0 { a } else { b };
         // The upwind predictor's pieces on an interior u face, × ρ·vol.
@@ -286,7 +309,10 @@ impl OversetPisoSolver {
                     }
                 };
                 let rhs = self.background.u_rhs(&m, j, i, t_old);
-                let r = rho * ((m.u[(j, i)] - m.u_old[(j, i)]) / dt - rhs) * vol;
+                let mut r = rho * ((m.u[(j, i)] - m.u_old[(j, i)]) / dt - rhs) * vol;
+                if !far_ok_u(&m, j, i) {
+                    r = f64::NAN;
+                }
                 bucket.add(r, true);
                 if ghost_u(j, i) {
                     prescribed.push(FaceResidual {
@@ -334,7 +360,10 @@ impl OversetPisoSolver {
                     }
                 };
                 let rhs = self.background.v_rhs(&m, j, i, t_old);
-                let r = rho * ((m.v[(j, i)] - m.v_old[(j, i)]) / dt - rhs) * vol;
+                let mut r = rho * ((m.v[(j, i)] - m.v_old[(j, i)]) / dt - rhs) * vol;
+                if !far_ok_v(&m, j, i) {
+                    r = f64::NAN;
+                }
                 bucket.add(r, false);
                 if ghost_v(j, i) {
                     prescribed.push(FaceResidual {