FSI2 overset harness: RTX_FSI2O_DT_SCALE scales the CFL-derived fluid step — P5-3's dt-convergence probe of the first-order Euler fluid steps (the band's placement is excluded: 94.0 mm at a 60 mm patch vs 94.8 at 40 mm; the s = 8 pair moved the amplitude 95 → 60 mm)

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

@@ -303,7 +303,14 @@ 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);
+        // `RTX_FSI2O_DT_SCALE` (1): the fluid step scaled off its CFL-
+        // derived value — P5-3's dt-convergence probe of the first-order
+        // Euler steps (the s = 8 pair moved the amplitude 95 → 60 mm).
+        let dt_scale: f64 = std::env::var("RTX_FSI2O_DT_SCALE")
+            .ok()
+            .and_then(|v| v.parse().ok())
+            .unwrap_or(1.0);
+        let dt_fluid = dt_bg.min(dt_patch) * dt_scale;
         let patch_mesh = initial.unwrap_or(start_mesh);
 
         let shared = Arc::new(RwLock::new(WallMotion {