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| {

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

@@ -100,7 +100,7 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
     // Every setting the acceptance rule reads, printed once: P5-3 lost a
     // day to a floor of 2e-4 against the overnight marches' 1e-6.
     println!(
-        "  coupling: {} (reuse {}, ω0 {}, c1 {}), floor {:.1e}, rtol {:.1e}, stall accept {:.1e}, max subit {}, predictor {}, s = {}, patch offset {} h × {} rows, patch convection {:?}",
+        "  coupling: {} (reuse {}, ω0 {}, c1 {}), floor {:.1e}, rtol {:.1e}, stall accept {:.1e}, max subit {}, predictor {}, s = {}, patch offset {} h × {} rows, patch convection {:?}, bg convection {:?}",
         cfg.coupler,
         cfg.reuse,
         cfg.initial_relaxation,
@@ -114,6 +114,7 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
         std::env::var("RTX_FSI2O_PATCH_OFFSET").unwrap_or_else(|_| "6".into()),
         std::env::var("RTX_FSI2O_PATCH_ROWS").unwrap_or_else(|_| "12".into()),
         super::overset::patch_convection(),
+        super::overset::bg_convection(),
     );
 
     // Phase 1: rigid flag to t_release (`RTX_FSI2O_LOAD=dir` replaces the

crates/specialized/rtx-fsi/tests/turek_hron_fsi2_overset.rs

@@ -413,10 +413,11 @@ fn fsi2_overset_prescribed_motion() {
     let dt = fluid.dt_fluid;
     let (d_rigid, l_rigid) = fluid.measure_force();
     println!(
-        "  PRESCRIBED ny = {ny}: dt {dt:.3e}, rigid drag {d_rigid:.2} lift {l_rigid:.2}, replay from t = {t0} (ramp {ramp_w} s) to {t_end}; patch offset {} h × {} rows, patch convection {:?}",
+        "  PRESCRIBED ny = {ny}: dt {dt:.3e}, rigid drag {d_rigid:.2} lift {l_rigid:.2}, replay from t = {t0} (ramp {ramp_w} s) to {t_end}; patch offset {} h × {} rows, patch convection {:?}, bg convection {:?}",
         std::env::var("RTX_FSI2O_PATCH_OFFSET").unwrap_or_else(|_| "6".into()),
         std::env::var("RTX_FSI2O_PATCH_ROWS").unwrap_or_else(|_| "12".into()),
         fsi2_harness::overset::patch_convection(),
+        fsi2_harness::overset::bg_convection(),
     );
     let mut csv = std::env::var("RTX_FSI2O_CSV").ok().map(|p| {
         use std::io::Write as _;