rtx-cfd: PatchConvection::TvdVanAlbada — van Albada deferred correction on the curvilinear predictor (downwind-side linear weight, gradient-ratio r over the face d lengths, far-upwind across the opposite face, boundary faces upwind); annulus MMS orders 2.10/1.69 at 0.24× upwind; cylinder-flag MMS orders 1.98/1.97 (1.06× upwind — diffusion-dominated, recorded); knobs RTX_OVERSET_CFD1_TVD, RTX_OVERSET_MAX_ROUNDS, RTX_CF_SCHEME

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01X2GmJXeQ2njUecEKiJZ1G2

crates/specialized/rtx-cfd/tests/overset_cfd1.rs

@@ -12,7 +12,7 @@ use rtx_cfd::mesh::patch_gen::cylinder_flag_patch;
 use rtx_cfd::solvers::incompressible::{
     AleBoundaries, CurvilinearParameters, CurvilinearPisoSolver, EmbeddedParameters,
     EmbeddedPisoSolver, FlowField, NormalDiffusion, OversetField, OversetParameters,
-    OversetPisoSolver, PatchField, PoissonSolverKind, SideBoundary,
+    OversetPisoSolver, PatchConvection, PatchField, PoissonSolverKind, SideBoundary,
 };
 use rtx_cfd::{CfdConfig, CfdResult};
 
@@ -99,10 +99,18 @@ async fn run_cfd1(ny: usize) -> CfdResult<Cfd1> {
     let dt_patch = 0.4 * (hs * hs / (4.0 * NU)).min(hs / u_peak);
     let dt = dt_bg.min(dt_patch);
 
+    // P4 step 2: `RTX_OVERSET_CFD1_TVD=1` puts the van Albada deferred
+    // correction on the patch (the background stays upwind, as recorded).
+    let convection = if std::env::var("RTX_OVERSET_CFD1_TVD").is_ok() {
+        PatchConvection::TvdVanAlbada
+    } else {
+        PatchConvection::Upwind
+    };
     let mut patch = CurvilinearPisoSolver::new(
         config,
         CurvilinearParameters {
             tolerance: 1e-5,
+            convection,
             normal_diffusion: NormalDiffusion::LineImplicit,
             ..CurvilinearParameters::default()
         },
@@ -127,6 +135,12 @@ async fn run_cfd1(ny: usize) -> CfdResult<Cfd1> {
             .ok()
             .and_then(|v| v.parse().ok())
             .unwrap_or(2),
+        // Cost question (P4): does the second corrector's ~9 rounds buy a
+        // measurable load? `RTX_OVERSET_MAX_ROUNDS=3` caps every corrector.
+        max_rounds: std::env::var("RTX_OVERSET_MAX_ROUNDS")
+            .ok()
+            .and_then(|v| v.parse().ok())
+            .unwrap_or(OversetParameters::default().max_rounds),
         ..OversetParameters::default()
     };
     let mut solver = OversetPisoSolver::new(background, patch, (nx, ny, h, h), params)?;
@@ -274,9 +288,14 @@ async fn cfd1_on_the_overset_against_the_featflow_reference() -> CfdResult<()> {
         let r = run_cfd1(ny).await?;
         let rel = |a: f64, b: f64| 100.0 * (a - b) / b;
         println!(
-            "  CFD1 overset ny = {ny} (h = {:.4}, dt = {:.2e}): wall drag {:.4} ({:+.2}%) lift {:.4} ({:+.2}%); control volume drag {:.4} ({:+.2}%) lift {:.4}; routes differ {:.2}%; [{} steps, {:.0} s, Schwarz rounds mean {:.2}]  reference {REF_DRAG} / {REF_LIFT}; embedded staircase at ny=41: 15.71 / 15.62 (+10%)",
+            "  CFD1 overset ny = {ny} (h = {:.4}, dt = {:.2e}, patch {}): wall drag {:.4} ({:+.2}%) lift {:.4} ({:+.2}%); control volume drag {:.4} ({:+.2}%) lift {:.4}; routes differ {:.2}%; [{} steps, {:.0} s, Schwarz rounds mean {:.2}]  reference {REF_DRAG} / {REF_LIFT}; embedded staircase at ny=41: 15.71 / 15.62 (+10%)",
             H / ny as f64,
             r.dt,
+            if std::env::var("RTX_OVERSET_CFD1_TVD").is_ok() {
+                "tvd"
+            } else {
+                "upwind"
+            },
             r.drag_surface,
             rel(r.drag_surface, REF_DRAG),
             r.lift_surface,