overset: the stamp-energy instrument (P5-3's energy-source measurement) — OversetResult gains stamp_energy (½ρ(u_stamped² − u_before²)·dx·dy summed over the fringe faces after the flux balance on a reclassification step) and reclass_energy (the faces touching a cell whose class changed); the replay CSV logs both plus the reclassified-cell count per step — the comparator showed the plate loads per unit motion agree with ours to ~10 %, so the discrepancy is our limit-cycle amplitude, an h-growing energy source, and the hole boundary's reclassification is the candidate

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_0116sg1Qz1gMv9hdcKP1XUam

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

@@ -134,6 +134,14 @@ pub struct OversetResult {
     pub reclassified_cells: usize,
     /// Background cells that jumped hole → active (patch moved > 1 cell).
     pub fresh_cells: usize,
+    /// Kinetic energy the exchange wrote into the background's fringe faces
+    /// on this step's reclassification (`½ ρ (u_stamped² − u_before²)` per
+    /// face times `dx·dy`, summed over the stamped `u` and `v` faces after
+    /// the fringe flux balance) [J/m]; 0 when the patch did not move.
+    pub stamp_energy: f64,
+    /// The part of `stamp_energy` on faces that touch a cell whose class
+    /// changed in this rebuild — the reclassification's own share.
+    pub reclass_energy: f64,
 }
 
 /// Restorable state of the composite (the coupling re-runs a step).
@@ -510,6 +518,7 @@ impl OversetPisoSolver {
         //    path to the bit.
         let mut reclassified = 0usize;
         let mut fresh = 0usize;
+        let (mut stamp_energy, mut reclass_energy) = (0.0_f64, 0.0_f64);
         // P3b locating trace (print-only, `RTX_OVERSET_TRACE_SP`): the class
         // of every background cell before this step, kept only when a
         // reclassification happens.
@@ -581,11 +590,10 @@ impl OversetPisoSolver {
             // instead of taking the patch's interpolation when they turn
             // prescribed.
             let h1 = std::env::var("RTX_OVERSET_H1").is_ok();
+            let u_before = field.background.u.clone();
+            let v_before = field.background.v.clone();
             let (u_keep, v_keep) = if h1 {
-                (
+                (Some(u_before.clone()), Some(v_before.clone()))
-                    Some(field.background.u.clone()),
-                    Some(field.background.v.clone()),
-                )
             } else {
                 (None, None)
             };
@@ -606,6 +614,31 @@ impl OversetPisoSolver {
                 }
             }
             self.balance_fringe(&mut field.background);
+            // The energy the stamp wrote (P5-3's energy-source instrument):
+            // every stamped face, and the faces touching a reclassified cell.
+            {
+                let rho = self.background.config().density;
+                let vol = dx * dy;
+                let changed = |jj: usize, ii: usize| {
+                    jj < ny && ii < nx && old_map.class(jj, ii) != self.overlap.class(jj, ii)
+                };
+                for e in &self.overlap.fringe_u {
+                    let (a, b) = (u_before[(e.j, e.i)], field.background.u[(e.j, e.i)]);
+                    let de = 0.5 * rho * (b * b - a * a) * vol;
+                    stamp_energy += de;
+                    if changed(e.j, e.i) || (e.i > 0 && changed(e.j, e.i - 1)) {
+                        reclass_energy += de;
+                    }
+                }
+                for e in &self.overlap.fringe_v {
+                    let (a, b) = (v_before[(e.j, e.i)], field.background.v[(e.j, e.i)]);
+                    let de = 0.5 * rho * (b * b - a * a) * vol;
+                    stamp_energy += de;
+                    if changed(e.j, e.i) || (e.j > 0 && changed(e.j - 1, e.i)) {
+                        reclass_energy += de;
+                    }
+                }
+            }
             // Knock-out H2 (`RTX_OVERSET_H2`): cells that turn active → fringe
             // keep the background's own pressure this step instead of the
             // patch's stamped value (their p' is still Dirichlet from the
@@ -805,6 +838,8 @@ impl OversetPisoSolver {
             overlap_flux_scale: scale,
             reclassified_cells: reclassified,
             fresh_cells: fresh,
+            stamp_energy,
+            reclass_energy,
         })
     }
 }

crates/specialized/rtx-fsi/tests/turek_hron_fsi2_overset.rs

@@ -411,7 +411,7 @@ fn fsi2_overset_prescribed_motion() {
         let mut f = std::fs::File::create(p).expect("csv");
         writeln!(
             f,
-            "t,uy_tip,drag,lift,power,wall_flux,area_rate,p_faces,p_normal,p_tangential,p_root,p_mid,p_tip,rounds_max,schwarz_ok,bg_residual,patch_div,defect_patch,defect_bg"
+            "t,uy_tip,drag,lift,power,wall_flux,area_rate,p_faces,p_normal,p_tangential,p_root,p_mid,p_tip,rounds_max,schwarz_ok,bg_residual,patch_div,defect_patch,defect_bg,reclassified,e_stamp,e_reclass"
         )
         .unwrap();
         f
@@ -436,12 +436,15 @@ fn fsi2_overset_prescribed_motion() {
         if let Err(e) = fluid.set_geometry(&d_r, &v_r) {
             panic!("set_geometry died at step {step}, t = {t_new:.4}: {e:?}");
         }
-        let (rounds_max, schwarz_ok, bg_res, patch_div) = match fluid.step() {
+        let (rounds_max, schwarz_ok, bg_res, patch_div, reclassified, e_stamp, e_reclass) = match fluid.step() {
             Ok(r) => (
                 r.rounds.iter().copied().max().unwrap_or(0),
                 r.schwarz_converged,
                 r.background_residual,
                 r.patch_max_divergence,
+                r.reclassified_cells,
+                r.stamp_energy,
+                r.reclass_energy,
             ),
             Err(e) => panic!("fluid step died at step {step}, t = {t_new:.4}: {e:?}"),
         };
@@ -522,7 +525,7 @@ fn fsi2_overset_prescribed_motion() {
             use std::io::Write as _;
             writeln!(
                 f,
-                "{t_new:.6},{uy_tip:.6e},{drag:.4},{lift:.4},{power:.6e},{wall_flux:.6e},{area_rate:.6e},{p_faces:.6e},{p_normal:.6e},{p_tangential:.6e},{:.6e},{:.6e},{:.6e},{rounds_max},{},{bg_res:.3e},{patch_div:.3e},{defect_patch:.3e},{defect_bg:.3e}",
+                "{t_new:.6},{uy_tip:.6e},{drag:.4},{lift:.4},{power:.6e},{wall_flux:.6e},{area_rate:.6e},{p_faces:.6e},{p_normal:.6e},{p_tangential:.6e},{:.6e},{:.6e},{:.6e},{rounds_max},{},{bg_res:.3e},{patch_div:.3e},{defect_patch:.3e},{defect_bg:.3e},{reclassified},{e_stamp:.4e},{e_reclass:.4e}",
                 p_thirds[0], p_thirds[1], p_thirds[2], schwarz_ok as u8
             )
             .unwrap();