embedded3 S2-3d: face-momentum unsteady term for the box (Mask::box_momentum_rates, Solver::previous_apertures); falsifier prints the residual against it

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

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

@@ -485,3 +485,86 @@ impl Mask {
         force
     }
 }
+
+impl Mask {
+    /// S2-3d: the rate of change of the momentum the SCHEME carries in a
+    /// box — on the face control volumes, `Σ_f ρ h A (α_f u_f − α_fⁿ u_fⁿ)/dt`
+    /// (faces on the box's own surfaces at half weight) — against the
+    /// cell-volume form of the box route's unsteady term, returned second.
+    /// Near an accelerating wall the two differ at O(h) in the cut cells.
+    #[must_use]
+    pub fn box_momentum_rates(
+        &self,
+        f: &Field,
+        dt: f64,
+        rho: f64,
+        (i0, i1, j0, j1, k0, k1): (usize, usize, usize, usize, usize, usize),
+        apertures_old: Option<&[Vec<f64>; 3]>,
+        vol_old: Option<&[f64]>,
+    ) -> ([f64; 3], [f64; 3]) {
+        let g = self.grid();
+        let dv = g.dx * g.dy * g.dz;
+        let mut faces = [0.0; 3];
+        let mut cells = [0.0; 3];
+        let old = |c: usize, idx: usize, new: f64| apertures_old.map_or(new, |a| a[c][idx]);
+        for k in k0..k1 {
+            for j in j0..j1 {
+                for i in i0..=i1 {
+                    let idx = g.uface(k, j, i);
+                    let w = if i == i0 || i == i1 { 0.5 } else { 1.0 };
+                    let a = self.a_u(idx);
+                    faces[0] += w * rho * dv * (a * f.u[idx] - old(0, idx, a) * f.u_old[idx]) / dt;
+                }
+            }
+            for j in j0..=j1 {
+                for i in i0..i1 {
+                    let idx = g.vface(k, j, i);
+                    let w = if j == j0 || j == j1 { 0.5 } else { 1.0 };
+                    let a = self.a_v(idx);
+                    faces[1] += w * rho * dv * (a * f.v[idx] - old(1, idx, a) * f.v_old[idx]) / dt;
+                }
+            }
+        }
+        for k in k0..=k1 {
+            for j in j0..j1 {
+                for i in i0..i1 {
+                    let kk = if k == g.nz && self.periodic_z() { 0 } else { k };
+                    let idx = g.wface(kk, j, i);
+                    let w = if (k == k0 || k == k1) && !self.periodic_z() {
+                        0.5
+                    } else if k == k1 {
+                        0.0
+                    } else {
+                        1.0
+                    };
+                    let a = self.a_w(idx);
+                    faces[2] += w * rho * dv * (a * f.w[idx] - old(2, idx, a) * f.w_old[idx]) / dt;
+                }
+            }
+        }
+        for k in k0..k1 {
+            for j in j0..j1 {
+                for i in i0..i1 {
+                    let idx = g.cell(k, j, i);
+                    let v_new = if self.is_fluid_cell(idx) {
+                        self.vol(idx)
+                    } else {
+                        0.0
+                    };
+                    let v_old = vol_old.map_or(v_new, |vo| vo[idx]);
+                    let c = |a: &[f64], f0: usize, f1: usize| 0.5 * (a[f0] + a[f1]);
+                    let (fu0, fu1) = (g.uface(k, j, i), g.uface(k, j, i + 1));
+                    let (fv0, fv1) = (g.vface(k, j, i), g.vface(k, j + 1, i));
+                    let (fw0, fw1) = (g.wface(k, j, i), g.wface(k + 1, j, i));
+                    cells[0] +=
+                        rho * dv * (v_new * c(&f.u, fu0, fu1) - v_old * c(&f.u_old, fu0, fu1)) / dt;
+                    cells[1] +=
+                        rho * dv * (v_new * c(&f.v, fv0, fv1) - v_old * c(&f.v_old, fv0, fv1)) / dt;
+                    cells[2] +=
+                        rho * dv * (v_new * c(&f.w, fw0, fw1) - v_old * c(&f.w_old, fw0, fw1)) / dt;
+                }
+            }
+        }
+        (faces, cells)
+    }
+}

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

@@ -183,6 +183,8 @@ pub struct Solver {
     /// The previous mask's cell volumes (moving bodies; the box route's
     /// unsteady term).
     pub(super) vol_old: Vec<f64>,
+    /// The previous mask's face apertures (moving bodies; S2-3d).
+    pub(super) apertures_old: Option<[Vec<f64>; 3]>,
     boundary_velocity: Option<Vec3Fn>,
     body: Option<Body>,
     /// The body moves: the mask is rebuilt at every step's new time.
@@ -216,6 +218,7 @@ impl Solver {
             floor_source: std::cell::Cell::new([0.0; 3]),
             pressure_lag: std::cell::Cell::new([0.0; 3]),
             vol_old: Vec::new(),
+            apertures_old: None,
             boundary_velocity: None,
             body: None,
             moving: false,
@@ -310,6 +313,13 @@ impl Solver {
         self.pressure_lag.get()
     }
 
+    /// The previous step's cell volumes (fraction of the cell; 0 for a
+    /// solid cell), once a moving body's mask has been rebuilt.
+    #[must_use]
+    pub fn previous_apertures(&self) -> Option<&[Vec<f64>; 3]> {
+        self.apertures_old.as_ref()
+    }
+
     /// The previous step's cell volumes (fraction of the cell; 0 for a
     /// solid cell), once a moving body's mask has been rebuilt.
     #[must_use]

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

@@ -70,6 +70,9 @@ impl Solver {
             self.last_ghost_correction = correction;
         }
         if let Some(old) = &self.mask {
+            self.apertures_old = old
+                .cut()
+                .map(|c| [c.a_u.clone(), c.a_v.clone(), c.a_w.clone()]);
             self.vol_old = (0..field.grid.cells())
                 .map(|i| {
                     if old.is_fluid_cell(i) {