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/curvilinear_mms.rs

@@ -81,7 +81,7 @@ async fn march(
         normal_diffusion: diffusion,
         ..CurvilinearParameters::default()
     };
-    let convecting = convection == PatchConvection::Upwind;
+    let convecting = convection != PatchConvection::None;
     let mut solver = CurvilinearPisoSolver::new(config, params, mesh)?;
     solver.set_boundary_velocity(|x, y, _t| (u_exact(x, y), v_exact(x, y)));
     solver.set_momentum_source(move |x, y, _t| source(x, y, convecting));
@@ -242,6 +242,45 @@ async fn skewed_annulus_with_upwind_is_first_order() -> CfdResult<()> {
     Ok(())
 }
 
+/// P4 step 2 gate (ii): the van Albada deferred correction on the skewed
+/// annulus beats upwind at every rung (upwind: 1.151502e-1 / 5.445770e-2 /
+/// 3.119486e-2) with orders >= 1.4.
+#[tokio::test]
+async fn skewed_annulus_with_tvd_beats_upwind() -> CfdResult<()> {
+    let upwind = [1.151502e-1, 5.445770e-2, 3.119486e-2];
+    let mut errs = Vec::new();
+    for (&ns, &u) in [32usize, 64, 128].iter().zip(&upwind) {
+        let m = march(
+            annulus_skewed([0.0, 0.0], 0.5, 1.5, ns, ns / 4, 0.3, 3.0)?,
+            PatchConvection::TvdVanAlbada,
+            NormalDiffusion::Explicit,
+            1e-6,
+        )
+        .await?;
+        println!(
+            "annulus tvd ns={ns}: L2 {:.6e} (upwind {u:.6e}, ratio {:.2}), max div {:.2e}, {} steps",
+            m.l2_velocity,
+            m.l2_velocity / u,
+            m.max_div_rel,
+            m.steps
+        );
+        assert!(m.max_div_rel < 1e-9, "divergence {:.3e}", m.max_div_rel);
+        assert!(
+            m.l2_velocity < u,
+            "TVD {:.4e} not below upwind {u:.4e}",
+            m.l2_velocity
+        );
+        errs.push(m.l2_velocity);
+    }
+    let o = orders(&errs);
+    println!("annulus tvd orders {o:?}");
+    assert!(
+        o.iter().all(|&x| x >= 1.4),
+        "tvd orders {o:?} (gate >= 1.4)"
+    );
+    Ok(())
+}
+
 #[tokio::test]
 async fn snapshot_restore_rerun_is_bit_identical() -> CfdResult<()> {
     let mesh = annulus_skewed([0.0, 0.0], 0.5, 1.5, 24, 6, 0.3, 2.0)?;