fsi2 harness: RTX_FSI2O_BG_CONVECTION knob (tvd default / upwind) on the background, printed in both headers beside the patch's — the patch scheme was excluded as the h-term's carrier (upwind ladder +1.52 vs TVD +1.73 W/m from ny 41 to 62), the background's wake dissipation is the registered next probe

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 +45,17 @@ fn patch_offset_h() -> f64 {
         .unwrap_or(6.0)
 }
 
+/// 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
+/// carrier: both ladders move +1.5–1.7 W/m from ny 41 to 62).
+pub fn bg_convection() -> ConvectionScheme {
+    match std::env::var("RTX_FSI2O_BG_CONVECTION").as_deref() {
+        Ok("upwind") => ConvectionScheme::Upwind,
+        _ => ConvectionScheme::TvdVanAlbada,
+    }
+}
+
 /// The patch's convection scheme (`RTX_FSI2O_PATCH_CONVECTION`: `tvd`
 /// (default, van Albada), `upwind`, `none`) — P5-3's near-wake dispersion
 /// knob on the fixed-motion replay.
@@ -244,7 +255,7 @@ impl OversetFluid {
                 },
                 poisson_solver: PoissonSolverKind::Multigrid,
                 poisson_precision: MgPrecision::F64,
-                convection_scheme: ConvectionScheme::TvdVanAlbada,
+                convection_scheme: bg_convection(),
             },
         )?;
         background.set_boundary_velocity(move |x, y, t| {