P5-1: RTX_FSI2O_NEWMARK_GAMMA — Newmark dissipation on the flag (β = (γ + ½)²/4); at γ = 0.9 the s = 1 overset march holds 0.3 s from the cached state (789 steps, 1.8 passes, no death) with the physical onset, where γ = ½ ran away in 90 steps: the flag's thickness breathing, resolved at s = 1 and felt by the body-fitted wall, damped at the source

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_march.rs

@@ -149,6 +149,21 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
         max_iterations: 60,
         ..ConvergenceCriteria::default()
     });
+    // `RTX_FSI2O_NEWMARK_GAMMA=γ` (β = (γ + ½)²/4): numerical dissipation
+    // of the flag's high-frequency modes (the thickness breathing the
+    // body-fitted wall couples to at s = 1); the benchmark's average
+    // acceleration (γ = ½) has none.
+    let analysis = match std::env::var("RTX_FSI2O_NEWMARK_GAMMA")
+        .ok()
+        .and_then(|v| v.parse::<f64>().ok())
+    {
+        Some(g) => {
+            let b = (g + 0.5).powi(2) / 4.0;
+            println!("  Newmark γ = {g}, β = {b:.4}");
+            analysis.with_newmark_parameters(g, b)
+        }
+        None => analysis,
+    };
     let flag = RefCell::new(analysis.stepper().unwrap());
     let wetted_dofs: Vec<[usize; 2]> = fluid
         .interface