diff --git a/crates/specialized/rtx-cfd/tests/embedded3_flag_wake.rs b/crates/specialized/rtx-cfd/tests/embedded3_flag_wake.rs
index 82f9dd1..0b22ca9 100644
--- a/crates/specialized/rtx-cfd/tests/embedded3_flag_wake.rs
+++ b/crates/specialized/rtx-cfd/tests/embedded3_flag_wake.rs
@@ -1,6 +1,8 @@
 //! embedded3 S2-3: the free-ended flag with prescribed motion — the first
 //! honest 3D wake. The Turek–Hron channel (2.5 × 0.41) extruded to depth
 //! 0.41 with the cylinder (D 0.1 at (0.2, 0.2)) across the width, the flag
+//! (`RTX_E3_FLAG_HEIGHT` / `RTX_E3_FLAG_DEPTH` grow the duct around the
+//! unchanged body, re-centred: the 2026-09-22/23 explorations), the flag
 //! 0.35 × 0.02 × 0.2 centred in z (z 0.105–0.305), its centreline deflected
 //! as the first clamped-free beam mode with the 2D FSI2 flat-tip record's
 //! tip amplitude 84 mm at 1.930 Hz (motion prescribed, no structure), the
@@ -139,7 +141,7 @@ fn flag_2d_recorded(rec: &Recorded, x: f64, y: f64, t: f64) -> (f64, (f64, f64))
     POLY.with(|cell| {
         let mut c = cell.borrow_mut();
         if c.0.to_bits() != t.to_bits() {
-            c.1 = rec.at(t, CY);
+            c.1 = rec.at(t, body_cy());
             inset_last(&mut c.1, tip_inset());
             c.0 = t;
         }
@@ -179,7 +181,7 @@ fn flag_2d_analytic(x: f64, y: f64, t: f64) -> (f64, f64) {
             for (m, p) in c.1.iter_mut().enumerate() {
                 let s = m as f64 / N as f64;
                 let (d, v) = deflection(s, t);
-                *p = (FLAG_X0 + s * FLAG_LEN, CY + d, v);
+                *p = (FLAG_X0 + s * FLAG_LEN, body_cy() + d, v);
             }
             let inset = tip_inset();
             if inset > 0.0 {
@@ -226,6 +228,19 @@ fn duct_depth() -> f64 {
     env_f("RTX_E3_FLAG_DEPTH", H)
 }
 
+/// The duct's height (the y extent): `RTX_E3_FLAG_HEIGHT` (default H).
+/// The cell size stays `H / ny` (the benchmark's rung definition); the
+/// body is re-centred in the taller duct (2026-09-23, the "big duct").
+fn duct_height() -> f64 {
+    env_f("RTX_E3_FLAG_HEIGHT", H)
+}
+
+/// The cylinder's centre and the flag's rest centreline: `CY` in the
+/// benchmark duct, lifted by half the added height otherwise.
+fn body_cy() -> f64 {
+    CY + 0.5 * (duct_height() - H)
+}
+
 /// The flag in 3D: the extruded capsule cut to the span with edges
 /// rounded to radius `r` (no cut at the full width).
 fn flag_3d(x: f64, y: f64, z: f64, t: f64, r: f64) -> (f64, (f64, f64)) {
@@ -242,8 +257,8 @@ fn flag_3d(x: f64, y: f64, z: f64, t: f64, r: f64) -> (f64, (f64, f64)) {
 }
 
 fn inflow(y: f64, z: f64) -> f64 {
-    let d = duct_depth();
-    16.0 * U_M * y * z * (H - y) * (d - z) / (H * H * d * d)
+    let (hd, d) = (duct_height(), duct_depth());
+    16.0 * U_M * y * z * (hd - y) * (d - z) / (hd * hd * d * d)
 }
 
 #[test]
@@ -253,6 +268,8 @@ fn flag_wake_on_the_device() {
     let periods = env_f("RTX_E3_FLAG_PERIODS", 2.0);
     let h = H / ny as f64;
     let nx = (L / h).round() as usize;
+    // The grid's rows: the benchmark's `ny` unless the duct is taller.
+    let ny_grid = (duct_height() / h).round() as usize;
     // `RTX_E3_FLAG_NZ=4`: a thin slab periodic in z with the 2D inflow (Ū = 1) — the
     // flag as a 2D problem, minutes per rung: the instrument for the load routes' parts.
     let slab_nz = env_f("RTX_E3_FLAG_NZ", 0.0) as usize;
@@ -321,7 +338,8 @@ fn flag_wake_on_the_device() {
     );
     let inflow_at = move |y: f64, z: f64| {
         if slab_nz > 0 || inflow_2d {
-            6.0 * y * (H - y) / (H * H)
+            let hd = duct_height();
+            6.0 * y * (hd - y) / (hd * hd)
         } else {
             inflow(y, z)
         }
@@ -333,7 +351,8 @@ fn flag_wake_on_the_device() {
             (0.0, 0.0, 0.0)
         }
     });
-    let cyl = move |x: f64, y: f64| ((x - CX).powi(2) + (y - CY).powi(2)).sqrt() - R_CYL;
+    let cy = body_cy();
+    let cyl = move |x: f64, y: f64| ((x - CX).powi(2) + (y - cy).powi(2)).sqrt() - R_CYL;
     let r_fillet = root_fillet();
     let body = Body::from_sdf(move |x, y, z, t| {
         fillet_union(cyl(x, y), flag_3d(x, y, z, t, r_edge).0, r_fillet)
@@ -347,10 +366,10 @@ fn flag_wake_on_the_device() {
             }
         });
     solver.set_moving_body(body);
-    let g = Grid::cubic(nx, ny, nz, h);
+    let g = Grid::cubic(nx, ny_grid, nz, h);
     let mut field = Field::new(g);
     for k in 0..nz {
-        for j in 0..ny {
+        for j in 0..ny_grid {
             let u0 = inflow_at((j as f64 + 0.5) * h, (k as f64 + 0.5) * h);
             for i in 0..=nx {
                 field.u[g.uface(k, j, i)] = u0;
@@ -359,8 +378,10 @@ fn flag_wake_on_the_device() {
     }
     solver.initialize(&mut field);
     println!(
-        "  flag wake ny {ny} (span {}; inflow {}, z sides {}; root fillet {:.4} m, tip inset {:.4} m): {nx}×{ny}×{nz} = {} cells, h {h:.4e}, dt {dt:.3e}, {periods} periods = {t_end:.3} s, {} steps",
+        "  flag wake ny {ny} (span {}; duct {:.2} × {:.2} m, inflow {}, z sides {}; root fillet {:.4} m, tip inset {:.4} m): {nx}×{ny_grid}×{nz} = {} cells, h {h:.4e}, dt {dt:.3e}, {periods} periods = {t_end:.3} s, {} steps",
         flag_span(),
+        duct_height(),
+        duct_depth(),
         if slab_nz > 0 || inflow_2d { "2d" } else { "3d" },
         if slab_nz > 0 {
             "periodic"
@@ -443,7 +464,7 @@ fn flag_wake_on_the_device() {
             // recorded mode (until 2026-09-21 this column held the analytic
             // first mode even then — R2's fits use the record directly).
             let tip = match recorded() {
-                Some(rec) => rec.at(t, CY).last().map_or(0.0, |p| p.1 - CY),
+                Some(rec) => rec.at(t, body_cy()).last().map_or(0.0, |p| p.1 - body_cy()),
                 None => deflection(1.0, t).0,
             };
             if sample {