embedded3 S2-5: centroid diffusion default ON (RTX_E3_DIFFUSION_CENTROID=0 restores the records); flat_wall_position_is_second_order gate; closure.rs split (cutwall.rs under 700)

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

crates/specialized/rtx-cfd/tests/embedded3_dfg_2d1_host.rs

@@ -108,7 +108,7 @@ fn dfg_2d1_on_the_host() {
             let fcv = mask.control_volume_force_with_walls(&field, dt, RHO, mu, None, bx, false);
             let cd = |f: [f64; 3]| coef * f[0];
             println!(
-                "  t {t:7.3}: c_D operator {:.4} (p {:.4} + s {:.4} + exchange {:.4}) | reconstructed {:.4} (p {:.4} + s {:.4}) | box {:.4} c_L {:.5} Δp {:.5}; residual {:.1e} [{:.0} s]",
+                "  t {t:7.3}: c_D operator {:.4} (p {:.4} + s {:.4} + exchange {:.4}) | reconstructed {:.4} (p {:.4} + s {:.4}) | box {:.4} c_L {:.5} Δp {:.5} (quadratic probes {:.5}); residual {:.1e} [{:.0} s]",
                 cd(po) + cd(so) + cd(ex),
                 cd(po),
                 cd(so),
@@ -123,6 +123,18 @@ fn dfg_2d1_on_the_host() {
                     - mask
                         .pressure_at(&field.p, CX + 0.5 * D, CY, 0.5 * lz)
                         .unwrap_or(f64::NAN),
+                {
+                    // Δp by quadratic extrapolation along the normal from probes
+                    // at 1.5 h, 2.5 h, 3.5 h (whole-fluid stencils only).
+                    let wall_p = |sign: f64| {
+                        let at = |d: f64| {
+                            mask.pressure_at(&field.p, CX + sign * (0.5 * D + d * h), CY, 0.5 * lz)
+                                .unwrap_or(f64::NAN)
+                        };
+                        4.375 * at(1.5) - 5.25 * at(2.5) + 1.875 * at(3.5)
+                    };
+                    wall_p(-1.0) - wall_p(1.0)
+                },
                 r.final_residual,
                 start.elapsed().as_secs_f64()
             );

crates/specialized/rtx-cfd/tests/embedded3_wall_position.rs

@@ -53,10 +53,8 @@ fn offset(ny: usize, theta: f64) -> (f64, Vec<(f64, f64, f64)>) {
     for k in 0..=nz {
         for j in 0..ny {
             for i in 0..nx {
-                if k < nz || true {
-                    let idx = g.wface(k.min(nz), j, i);
-                    field.w[idx] = exact((j as f64 + 0.5) * h);
-                }
+                let idx = g.wface(k, j, i);
+                field.w[idx] = exact((j as f64 + 0.5) * h);
             }
         }
     }
@@ -100,3 +98,24 @@ fn flat_wall_effective_position() {
         }
     }
 }
+
+/// The gate: with the centroid diffusion (the default) the effective wall
+/// position is second order — within 0.08 h at ny 16, halving in units of
+/// h at ny 32, the same at every cut fraction (before S2-5: −½θ h at
+/// every resolution).
+#[test]
+fn flat_wall_position_is_second_order() {
+    if std::env::var("RTX_E3_DIFFUSION_CENTROID").is_ok_and(|v| v == "0") {
+        return;
+    }
+    for theta in [0.25, 0.75] {
+        let (coarse, _) = offset(16, theta);
+        let (fine, _) = offset(32, theta);
+        println!("  θ {theta}: offset {coarse:+.4} h at ny 16, {fine:+.4} h at ny 32");
+        assert!(coarse.abs() < 0.08, "ny 16 offset {coarse}");
+        assert!(
+            fine.abs() < 0.6 * coarse.abs(),
+            "the offset does not halve: {coarse} → {fine}"
+        );
+    }
+}