Merge r8e-coupled-dt-stability (R8-e: the coupled 3D FSI dt instability = the harness's structure-to-fluid mapping, fix RTX_E3FSI_KIN=surface; default-off). Merge fixes with R8-h: the surface station list over stations() (the flat tip's station A = the mean of the tip corners); the impulse test's capsule-only flat_tip(). Merged-tree gate (r6/merge_gate7): slab default / check / all-host / plate byte-identical to main's base, suites 2/2 3/3, the default coupled smoke = R8-a's march row for row, a KIN=surface + TIP=flat + LOADS_DEVICE coupled run clean

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/specialized/rtx-fsi/tests/fsi2_embedded3.rs

@@ -18,7 +18,11 @@
 //! `RTOL` (1e-3), `FLOOR` (1.5e-7 on the centreline vector), `MAX_SUBIT`
 //! (12), `STALL_ACCEPT` (5), `GAMMA` (0.7), `SPEED` (3.0 m/s, the band's
 //! surface-speed bound), `CSV` (per-step series), `TRACE` (steps whose
-//! passes are printed).
+//! passes are printed); R8-e: `KIN` (`centre` = the mid-plane nodes drive
+//! the fluid's centreline; `surface` = the mean of the bottom and top edge
+//! nodes, the nodes the loads land on — the transfer-consistent pair that
+//! removes the small-dt instability), `LINE_CSV` (the committed centreline
+//! and the per-station mid/bottom/top y displacement per step).
 //!
 //! `RTX_E3FSI_NY=62 RTX_E3FSI_CSV=<path> RTX_CUDA_ARCH=sm_120 cargo test --release -p rtx-fsi \
 //!  --features cuda --test fsi2_embedded3 -- --ignored --nocapture`
@@ -88,6 +92,9 @@ struct Flag {
     /// (reference y, wetted index), ascending (corners included).
     tip: Vec<(f64, usize)>,
     a_node: NodeId,
+    /// R8-e: the bottom and top edge nodes at each station's x (the wetted
+    /// surface the loads land on).
+    surface: Vec<[NodeId; 2]>,
 }
 
 impl Flag {
@@ -128,6 +135,13 @@ impl Flag {
             top,
             tip,
             a_node: find(0.6, 0.2),
+            // Over stations(): with the flat tip, station A (x 0.60) takes the mean of the tip's two corner nodes (merge of R8-e with R8-h).
+            surface: (0..stations())
+                .map(|k| {
+                    let x = X0 + 0.01 * k as f64;
+                    [find(x, 0.19), find(x, 0.21)]
+                })
+                .collect(),
         }
     }
 }
@@ -308,6 +322,11 @@ fn fsi2_on_embedded3() {
         .unwrap();
         f
     });
+    // R8-e `RTX_E3FSI_LINE_CSV`: the committed centreline per coupled step
+    // (t, then x, y displacement per station) — the unstable mode's shape.
+    let mut line_csv = std::env::var("RTX_E3FSI_LINE_CSV")
+        .ok()
+        .map(|p| std::fs::File::create(p).expect("line csv"));
     let start = std::time::Instant::now();
 
     // Phase 1: the rigid flag (target 1: the rest state vs CFD2 136.7 / 10.53).
@@ -401,8 +420,27 @@ fn fsi2_on_embedded3() {
         })
         .collect();
     let a_dofs = flag.borrow().node_dofs(flag_geo.a_node);
+    // R8-e `RTX_E3FSI_KIN=surface`: the fluid's centreline is the mean of the
+    // bottom and top edge nodes at each station (the nodes the loads land on)
+    // instead of the mid-plane nodes (default `centre`).
+    let kin_surface = std::env::var("RTX_E3FSI_KIN").is_ok_and(|v| v == "surface");
+    let surface_dofs: Vec<[usize; 4]> = flag_geo
+        .surface
+        .iter()
+        .map(|[b, t]| {
+            let (db, dt_) = (flag.borrow().node_dofs(*b), flag.borrow().node_dofs(*t));
+            [db[0], db[1], dt_[0], dt_[1]]
+        })
+        .collect();
     let extract = |s: &DynamicState| -> Vec<f64> {
         let mut c = vec![0.0; 2 * stations()];
+        if kin_surface {
+            for (k, d) in surface_dofs.iter().enumerate() {
+                c[2 * k] = 0.5 * (s.displacement[d[0]] + s.displacement[d[2]]);
+                c[2 * k + 1] = 0.5 * (s.displacement[d[1]] + s.displacement[d[3]]);
+            }
+            return c;
+        }
         for (k, d) in centre_dofs.iter().enumerate() {
             c[2 * k] = s.displacement[d[0]];
             c[2 * k + 1] = s.displacement[d[1]];
@@ -624,6 +662,16 @@ fn fsi2_on_embedded3() {
             )
             .unwrap();
         }
+        if let Some(f) = line_csv.as_mut() {
+            // Then the y displacement of the mid-plane, bottom and top nodes per station.
+            let mut c: Vec<String> = c_fluid_n.iter().map(|v| format!("{v:.6e}")).collect();
+            for (k, d) in surface_dofs.iter().enumerate() {
+                let yc = flag_state.displacement[centre_dofs[k][1]];
+                let (yb, yt) = (flag_state.displacement[d[1]], flag_state.displacement[d[3]]);
+                c.extend([yc, yb, yt].map(|v| format!("{v:.6e}")));
+            }
+            writeln!(f, "{t_new:.6},{}", c.join(",")).unwrap();
+        }
         let at_end = step + 1 == coupled_steps;
         if let Some(dir) = save_dir
             .as_ref()