embedded3 S2-5: the exchange route mirrors the predictor's centroid spacing toward solid neighbours (operator = box back to the box's own error)

Co-Authored-By: Claude Fable 5.1 <[email protected]>

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/exchange.rs

@@ -72,6 +72,7 @@ impl Mask {
                         }
                         let cv = self.cv_geometry(c, p);
                         let u0 = vals[c][idx];
+                        let shift0 = self.face_shift(c, p);
                         let cell_minus = add(p, ec, -1);
                         let cell_plus = p;
                         for d in 0..3 {
@@ -99,6 +100,17 @@ impl Mask {
                                     0.5 * (flux(cell_minus) + flux(cell_plus)) * a_d,
                                 )
                             };
+                            // The spacing the predictor uses toward a solid
+                            // neighbour (its shift is zero): the centroid
+                            // spacing in a cross direction (S2-5), else the
+                            // exchange distance.
+                            let solid_spacing = |sign: f64| -> f64 {
+                                if self.diffusion_centroid && d != c && !self.wall_exchange_axis {
+                                    (h[d] - sign * shift0[d]).clamp(0.25 * h[d], 2.0 * h[d])
+                                } else {
+                                    self.exchange_delta(&cv, d)
+                                }
+                            };
                             // Plus side: a prescribed neighbour face.
                             if let Some(fp) = lat.face(c, add(p, ed, 1)) {
                                 if kind(c, fp) != FaceKind::Fluid {
@@ -112,8 +124,7 @@ impl Mask {
                                     };
                                     let u_face = upwind(m_plus, u0, un) + delta;
                                     let on_fluid = -rho * m_plus * (u_face - u0)
-                                        + mu * cv.ap[d][1] * a_d * (un - u0)
-                                            / self.exchange_delta(&cv, d);
+                                        + mu * cv.ap[d][1] * a_d * (un - u0) / solid_spacing(1.0);
                                     force[c] -= on_fluid;
                                 }
                             }
@@ -130,8 +141,7 @@ impl Mask {
                                     };
                                     let u_face = upwind(m_minus, ud, u0) + delta;
                                     let on_fluid = rho * m_minus * (u_face - u0)
-                                        + mu * cv.ap[d][0] * a_d * (ud - u0)
-                                            / self.exchange_delta(&cv, d);
+                                        + mu * cv.ap[d][0] * a_d * (ud - u0) / solid_spacing(-1.0);
                                     force[c] -= on_fluid;
                                 }
                             }