R8-a harness: RTX_E3FSI_LOAD_PLANES (the load route on the n mid planes; cost knob, whole span by default)

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

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

@@ -301,7 +301,7 @@ fn fsi2_on_embedded3() {
         if (step + 1) % 50 == 0 || step + 1 == rigid_steps {
             last = fl.loads();
             let (tot, contrib) = &last;
-            let (_, on_flag, on_cyl, _) = distribute(&flag_geo, &rest, contrib, fl.width);
+            let (_, on_flag, on_cyl, _) = distribute(&flag_geo, &rest, contrib, fl.load_width);
             let t = fl.time();
             if let Some(f) = csv.as_mut() {
                 writeln!(
@@ -405,7 +405,7 @@ fn fsi2_on_embedded3() {
             (state, nodal, s.line.clone(), s.c_fluid.clone())
         }
         None => {
-            let (nodal0, _, _, _) = distribute(&flag_geo, &rest, &contrib0, fl.width);
+            let (nodal0, _, _, _) = distribute(&flag_geo, &rest, &contrib0, fl.load_width);
             flag.borrow_mut().set_nodal_forces(&nodal0);
             let state = flag.borrow_mut().rest_state().unwrap();
             // The fluid's own previous line and centreline (its geometry's history).
@@ -467,7 +467,8 @@ fn fsi2_on_embedded3() {
             }
             let tl = std::time::Instant::now();
             let (tot, contrib) = f.loads();
-            let (nodal, on_flag, on_cyl, parts_y) = distribute(&flag_geo, &line, &contrib, f.width);
+            let (nodal, on_flag, on_cyl, parts_y) =
+                distribute(&flag_geo, &line, &contrib, f.load_width);
             t_loads.set(t_loads.get() + tl.elapsed().as_secs_f64());
             let ts = std::time::Instant::now();
             let mut st = flag.borrow_mut();

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

@@ -144,8 +144,10 @@ pub struct E3Fluid {
     pub grid: Grid,
     pub h: f64,
     pub dt: f64,
-    /// The z extent the loads are divided by (per unit span).
+    /// The duct's z extent.
     pub width: f64,
+    /// The z extent the last `loads` integrated over (per unit span).
+    pub load_width: f64,
     pub lines: Arc<RwLock<Lines>>,
     sink: Arc<Mutex<Vec<Contribution>>>,
 }
@@ -284,6 +286,7 @@ impl E3Fluid {
             h,
             dt,
             width,
+            load_width: width,
             lines,
             sink: Arc::new(Mutex::new(Vec::new())),
         }
@@ -324,14 +327,25 @@ impl E3Fluid {
         assert!(previous.is_none(), "a load sink was already installed");
         let mask = self.device.solver.mask().expect("mask");
         let body = self.device.solver.body().expect("body");
+        // `RTX_E3FSI_LOAD_PLANES=n`: the route on the n mid planes only (per
+        // unit span of those planes) — a cost knob for a spanwise-uniform
+        // flow; the whole span by default.
+        let nz = self.grid.nz;
+        let n = (super::env_f("RTX_E3FSI_LOAD_PLANES", 0.0) as usize).min(nz);
+        let f = if n > 0 && n < nz {
+            let k0 = (nz - n) / 2;
+            self.load_width = n as f64 * self.h;
+            mask.cut_wall_force_per_span(body, &self.field, RHO * NU, t, (k0, k0 + n))
+                .expect("cut wall force per span")
+        } else {
+            self.load_width = self.width;
             let f = mask
                 .cut_wall_force(body, &self.field, RHO * NU, t)
                 .expect("cut wall force");
+            [f[0] / self.width, f[1] / self.width, f[2] / self.width]
+        };
         set_load_sink(None);
         let contributions = std::mem::take(&mut *self.sink.lock().expect("sink"));
-        (
-            [f[0] / self.width, f[1] / self.width, f[2] / self.width],
-            contributions,
-        )
+        (f, contributions)
     }
 }