embedded3 S2-7b A1: the moving wall's mass flux from the body's true normal at the wall foot (RTX_E3_WALL_FLUX=true, host prototype, device refuses it) — the wall flux of a rotating cylinder 5e-3 → 1e-9 of ΩR h², the rotating wall's cut-cell pressure error 0.33 → 0.11 (n 16) / 0.30 → 0.09 (n 32) of ρ(ΩR)², full cells second order; the rotating wall's offset unchanged (−0.02 h)

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

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

@@ -223,6 +223,7 @@ impl Mask {
             wall_order2_centroid: false,
             wall_exchange_foot: false,
             conv_sides_exact: false,
+            wall_flux_true_normal: false,
             grad_weights: None,
             diffusion_centroid: false,
             face_shifts: None,
@@ -699,6 +700,18 @@ impl Mask {
             (j as f64 + 0.5) * g.dy,
             (k as f64 + 0.5) * g.dz,
         ];
+        // S2-7b A1: the flux through the TRUE surface at the wall foot —
+        // `A_w (u_b · n)` with n the body's own normal into the body.
+        if self.wall_flux_true_normal {
+            let (s, n) = self.interpolant_distance_and_normal(cut, x);
+            let foot = [x[0] - s * n[0], x[1] - s * n[1], x[2] - s * n[2]];
+            let v = body.surface_velocity(foot[0], foot[1], foot[2], t);
+            let eps = 1e-6 * g.dx.min(g.dy).min(g.dz);
+            let (n1, n2, n3) = body.normal(foot[0], foot[1], foot[2], t, eps);
+            let a_w = (w[0] * w[0] + w[1] * w[1] + w[2] * w[2]).sqrt();
+            // `normal` points out of the solid: into the body is its negative.
+            return -a_w * (v.0 * n1 + v.1 * n2 + v.2 * n3);
+        }
         let mut flux = 0.0;
         for (c, wc) in w.iter().enumerate() {
             flux += self.surface_velocity_at(body, x, c, t) * wc;