embedded3 flag wake: RTX_E3_FLAG_CYL_SPAN=flag — the cylinder cut to the flag's span with the flag's rounded edges (the whole body finite in the wider ducts); wall to wall by default, slab gate byte-identical

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

crates/specialized/rtx-cfd/tests/embedded3_flag_wake.rs

@@ -256,6 +256,21 @@ fn flag_3d(x: f64, y: f64, z: f64, t: f64, r: f64) -> (f64, (f64, f64)) {
     (outside + q1.max(q2).min(0.0) - r, v)
 }
 
+/// The finite cylinder: the 2D circle cut to `span` in z with the same
+/// rounded edges as the flag (`RTX_E3_FLAG_CYL_SPAN=flag`, 2026-09-23: the
+/// whole body a finite object in the wider ducts); wall to wall otherwise.
+fn cylinder_3d(d2: f64, z: f64, r: f64) -> f64 {
+    let span = flag_span();
+    if span >= duct_depth() || !std::env::var("RTX_E3_FLAG_CYL_SPAN").is_ok_and(|v| v == "flag") {
+        return d2;
+    }
+    let zc = 0.5 * duct_depth();
+    let q1 = d2 + r;
+    let q2 = (z - zc).abs() - 0.5 * span + r;
+    let outside = (q1.max(0.0).powi(2) + q2.max(0.0).powi(2)).sqrt();
+    outside + q1.max(q2).min(0.0) - r
+}
+
 fn inflow(y: f64, z: f64) -> f64 {
     let (hd, d) = (duct_height(), duct_depth());
     16.0 * U_M * y * z * (hd - y) * (d - z) / (hd * hd * d * d)
@@ -355,11 +370,11 @@ fn flag_wake_on_the_device() {
     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)
+        fillet_union(cylinder_3d(cyl(x, y), z, r_edge), flag_3d(x, y, z, t, r_edge).0, r_fillet)
     })
         .with_surface_velocity(move |x, y, z, t| {
             let (df, (vx, vy)) = flag_3d(x, y, z, t, r_edge);
-            if df <= cyl(x, y) {
+            if df <= cylinder_3d(cyl(x, y), z, r_edge) {
                 (vx, vy, 0.0)
             } else {
                 (0.0, 0.0, 0.0)
@@ -378,8 +393,9 @@ fn flag_wake_on_the_device() {
     }
     solver.initialize(&mut field);
     println!(
-        "  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 wake ny {ny} (span {}, cylinder {}; 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(),
+        if std::env::var("RTX_E3_FLAG_CYL_SPAN").is_ok_and(|v| v == "flag") { "cut to the span" } else { "wall to wall" },
         duct_height(),
         duct_depth(),
         if slab_nz > 0 || inflow_2d { "2d" } else { "3d" },