fsi2 harness: RTX_FSI2O_PATCH_STRETCH knob (the patch's across stretch, 4.0 in every recorded run; printed in both headers) — P5-3's wall-normal resolution test at fixed h: the rows ladder does not build (the overlap needs outer patch cells ≈ h/2 or larger), the stretch refines the wall cell 0.227h → 0.144h → 0.088h at 4/8/16 while the outer cells grow

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01LzcjQX7tvgn87CQCyg9Cfr

crates/specialized/rtx-fsi/tests/fsi2_harness/overset.rs

@@ -45,6 +45,19 @@ fn patch_offset_h() -> f64 {
         .unwrap_or(6.0)
 }
 
+/// The patch's across stretch (`RTX_FSI2O_PATCH_STRETCH`, the geometric
+/// ratio of the outer to the wall cell, 4.0 in every recorded run): with
+/// 12 rows over 6 h the wall cell is 6 h (r − 1)/(r¹² − 1), r¹¹ = stretch —
+/// 0.227 h / 0.144 h / 0.088 h at 4 / 8 / 16 — P5-3's wall-normal
+/// resolution knob at fixed h (the rows ladder does not build: the overlap
+/// needs outer cells ≈ h/2 or larger).
+pub fn patch_stretch() -> f64 {
+    std::env::var("RTX_FSI2O_PATCH_STRETCH")
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(PATCH_STRETCH)
+}
+
 /// The background's convection scheme (`RTX_FSI2O_BG_CONVECTION`: `tvd`
 /// (default, van Albada), `upwind`) — P5-3's wake-dissipation knob on the
 /// fixed-motion replay (the patch's scheme was excluded as the h-term's
@@ -276,7 +289,7 @@ impl OversetFluid {
             FILLET,
             patch_offset_h() * h,
             patch_rows(),
-            PATCH_STRETCH,
+            patch_stretch(),
             sweeps,
         )?;
         // With the warm-started regeneration (`RTX_FSI2O_WARM_SWEEPS`), the
@@ -299,7 +312,7 @@ impl OversetFluid {
                 FILLET,
                 patch_offset_h() * h,
                 patch_rows(),
-                PATCH_STRETCH,
+                patch_stretch(),
                 20_000,
             )?;
             println!(
@@ -777,7 +790,7 @@ impl OversetFluid {
             FILLET,
             patch_offset_h() * self.h,
             patch_rows(),
-            PATCH_STRETCH,
+            patch_stretch(),
             if self.warm_sweeps > 0 && self.warm_base.is_some() {
                 self.warm_sweeps
             } else {