P5 audit exact on the moving path: the composite rebuilt around a saved instant carries the march's dt (from the mesh the march started on — the warm base under RTX_FSI2O_WARM_SWEEPS — whatever patch the instant holds; 0.2 % of dt read as 0.3–5 N/m of solved-face residual), and instants save dt; over 131 instants of the FSI2 march to t = 16 s the solved-face residual is 1.7e-10 at every one

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

crates/specialized/rtx-fsi/tests/fsi2_harness/overset.rs

@@ -227,15 +227,13 @@ impl OversetFluid {
             sweeps,
         )?;
         // With the warm-started regeneration (`RTX_FSI2O_WARM_SWEEPS`), the
-        // initial patch is the converged fixed point of the warm build's
+        // march starts on the converged fixed point of the warm build's
         // sweep + re-spacing map, so the chain starts where it stays.
         let warm_sweeps: usize = std::env::var("RTX_FSI2O_WARM_SWEEPS")
             .ok()
             .and_then(|v| v.parse().ok())
             .unwrap_or(0);
-        let patch_mesh = if let Some(m) = initial {
-            m
-        } else if warm_sweeps > 0 {
+        let start_mesh = if warm_sweeps > 0 {
             let t0 = std::time::Instant::now();
             let (m, (it, last)) = rtx_cfd::mesh::patch_gen::cylinder_flag_patch_deformed_from(
                 Some(&cold_mesh),
@@ -259,13 +257,15 @@ impl OversetFluid {
         } else {
             cold_mesh
         };
-        // The patch's along-body explicit limit (its wall row is
-        // line-implicit) beside the background's combined criterion.
+        // The fluid step is the march's: from the mesh the march started
+        // on, whatever patch this composite is built around (a saved
+        // deformed instant) — 0.2 % of dt read as 0.3–5 N/m of solved-face
+        // residual in the audit before this.
         let mut hs = f64::INFINITY;
-        for c in 0..patch_mesh.cell_count() {
-            for (f, _) in patch_mesh.cell_faces(c) {
-                if patch_mesh.is_sface(f) {
-                    let d = patch_mesh.faces()[f].d;
+        for c in 0..start_mesh.cell_count() {
+            for (f, _) in start_mesh.cell_faces(c) {
+                if start_mesh.is_sface(f) {
+                    let d = start_mesh.faces()[f].d;
                     hs = hs.min((d[0] * d[0] + d[1] * d[1]).sqrt());
                 }
             }
@@ -273,6 +273,7 @@ impl OversetFluid {
         let dt_bg = 0.25 / (2.0 * u_peak / h + 4.0 * NU_F / (h * h));
         let dt_patch = 0.4 * (hs * hs / (4.0 * NU_F)).min(hs / u_peak);
         let dt_fluid = dt_bg.min(dt_patch);
+        let patch_mesh = initial.unwrap_or(start_mesh);
 
         let shared = Arc::new(RwLock::new(WallMotion {
             polygon: interface
@@ -417,7 +418,11 @@ impl OversetFluid {
         }
         std::fs::write(
             dir.join("meta.txt"),
-            format!("t {:.17e}\nns {ns}\nnn {nn}\n", self.solver.time()),
+            format!(
+                "t {:.17e}\nns {ns}\nnn {nn}\ndt {:.17e}\n",
+                self.solver.time(),
+                self.dt_fluid
+            ),
         )
         .expect("meta");
         Ok(())
@@ -444,12 +449,14 @@ impl OversetFluid {
         let meta = std::fs::read_to_string(dir.join("meta.txt")).expect("meta");
         let mut t = 0.0;
         let (mut ns, mut nn) = (0usize, 0usize);
+        let mut dt_saved: Option<f64> = None;
         for line in meta.lines() {
             let mut it = line.split_whitespace();
             match (it.next(), it.next()) {
                 (Some("t"), Some(v)) => t = v.parse().expect("t"),
                 (Some("ns"), Some(v)) => ns = v.parse().expect("ns"),
                 (Some("nn"), Some(v)) => nn = v.parse().expect("nn"),
+                (Some("dt"), Some(v)) => dt_saved = v.parse().ok(),
                 _ => {}
             }
         }
@@ -483,6 +490,9 @@ impl OversetFluid {
         fluid.field.patch.flux = read("flux");
         fluid.solver.set_time(t);
         fluid.last_d = d.clone();
+        if let Some(dt) = dt_saved {
+            fluid.dt_fluid = dt;
+        }
         Ok((fluid, d, t))
     }