embedded3 S2-1: the traction sampler on the cut field (pressure fit excludes merged cells; sampler route in the sphere MMS and DFG drivers)

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

crates/specialized/rtx-cfd/tests/embedded3_sphere/mod.rs

@@ -107,9 +107,14 @@ pub struct Measurement {
     pub l2_velocity: f64,
     pub max_div: f64,
     pub ghost_correction: f64,
+    /// The scheme's wall route (the traction sampler on the binary wall,
+    /// the operator route on the cut wall).
     pub force_surface: [f64; 3],
     pub skipped: usize,
     pub force_cv: [f64; 3],
+    /// The traction sampler on the cut wall (S2-1); the wall route again
+    /// on the binary wall.
+    pub force_sampler: [f64; 3],
 }
 
 /// March the manufactured solution with the sphere at `c` to steady state on grid `n`.
@@ -250,6 +255,10 @@ pub fn measure(n: usize, scheme: WallScheme, c: (f64, f64, f64)) -> Measurement
     let (i0, i1) = (n / 8, n - n / 8);
     let src = |x: f64, y: f64, z: f64| source3(x, y, z);
     let force_cv = mask.control_volume_force(&f, dt, RHO, MU, Some(&src), (i0, i1, i0, i1, i0, i1));
+    let force_sampler = match scheme {
+        WallScheme::GhostBinary => surface.f,
+        WallScheme::CutCell => mask.surface_force(body, &f, MU, t, 0.5 * h).f,
+    };
     Measurement {
         l2_velocity: (sq / vol).sqrt(),
         max_div,
@@ -257,5 +266,6 @@ pub fn measure(n: usize, scheme: WallScheme, c: (f64, f64, f64)) -> Measurement
         force_surface: surface.f,
         skipped: surface.skipped,
         force_cv,
+        force_sampler,
     }
 }