R4-d: the dt²-scaled floor uses the COUPLED step (the first form scaled with the fluid substep); ny 62 check identical to the 3e-5 run

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

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

@@ -95,22 +95,23 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
     let mut fluid = OversetFluid::build_case(case, cfg.ny, cfg.flag_nx, cfg.sweeps, cfg.max_rounds)
         .expect("overset fluid");
     let dt_fluid = fluid.dt_fluid;
+    let dt = dt_fluid * cfg.subcycle as f64;
     // R4-d (2026-09-21): the acceleration-scaled floor. The loop converges
-    // the displacement; the fluid feels its second difference over dt², so
-    // a fixed floor's admissible acceleration noise quadruples per rung
-    // (R4: the FSI3 force spikes). `RTX_FSI2O_TOL_FLOOR_DT2=<floor at the
-    // ny 62 fluid step 3.44e-4>` sets floor = value · (dt_fluid / 3.44e-4)².
+    // the displacement; the fluid feels its second difference over the
+    // COUPLED step dt², so a fixed floor's admissible acceleration noise
+    // quadruples per rung (R4: the FSI3 force spikes).
+    // `RTX_FSI2O_TOL_FLOOR_DT2=<floor at the ny 62 coupled step 3.44e-4>`
+    // sets floor = value · (dt / 3.44e-4)².
     if let Some(f0) = std::env::var("RTX_FSI2O_TOL_FLOOR_DT2")
         .ok()
         .and_then(|v| v.parse::<f64>().ok())
     {
-        cfg.tol_floor = f0 * (dt_fluid / 3.44e-4).powi(2);
+        cfg.tol_floor = f0 * (dt / 3.44e-4).powi(2);
         println!(
-            "  floor scaled with dt² (R4-d): {:.3e} at dt {:.3e} (from {f0:.1e} at 3.44e-4)",
-            cfg.tol_floor, dt_fluid
+            "  floor scaled with dt² (R4-d): {:.3e} at the coupled step {dt:.3e} (from {f0:.1e} at 3.44e-4)",
+            cfg.tol_floor
         );
     }
-    let dt = dt_fluid * cfg.subcycle as f64;
     let zero_d = vec![0.0; 2 * fluid.interface.wetted.len()];
     // Every setting the acceptance rule reads, printed once: P5-3 lost a
     // day to a floor of 2e-4 against the overnight marches' 1e-6.