embedded3 S2-7 VERDICT: the diffusive exchange with SOLID neighbour faces over the axis distance (wall_exchange_axis, S2-5's knob) is the DEFAULT — the oblique wall's h-independent offset closes (four-registration means ≤ 0.005 h at n 32, p_cut halving per rung), DFG 2D generic ladder monotone +0.68/+0.10/+0.014 %, DFG 3D ny 62 / 124 inside the bar (6.1877 / 0.00907 / 0.1691 at ny 124); RTX_E3_WALL_EXCHANGE=h restores every earlier record digit for digit; host suite 21/21, device cut + moving identities green on the new default

Co-Authored-By: Claude Fable 5.1 <[email protected]>

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/mod.rs

@@ -128,9 +128,12 @@ pub struct Parameters {
     pub exchange_convection_off: bool,
     /// The diffusive exchange of a fluid face with a SOLID neighbour face
     /// over the axis distance to the wall, `δ = min(h, d_f/|n_d|)`, and
-    /// implicit — instead of the full `h`, which places the no-slip value
-    /// deeper in the body than the wall (an effective radius ≈ 0.28 h short
-    /// on the DFG 2D-1 ladder; S2-5). `RTX_E3_WALL_EXCHANGE=axis`.
+    /// implicit — instead of the full `h`, which places the no-slip value a
+    /// cell away where the wall is a fraction of a cell away: an O(1) error
+    /// in a wall-normal gradient on the faces next to solid ones, the
+    /// in-plane oblique wall's h-independent offset (S2-7; built in S2-5 and
+    /// judged there under the quarter-cell slip). ON since S2-7
+    /// (`RTX_E3_WALL_EXCHANGE=h` restores the records before it).
     pub wall_exchange_axis: bool,
     /// HOST PROTOTYPE (S2-5): the pressure gradient across cut cells over
     /// the distance between the cells' FLUID CENTROIDS (estimated from the
@@ -200,7 +203,8 @@ impl Default for Parameters {
             wall_advancing: std::env::var("RTX_E3_WALL_ADVANCING").is_ok_and(|v| v == "1"),
             exchange_convection_off: std::env::var("RTX_E3_EXCHANGE_CONVECTION")
                 .is_ok_and(|v| v == "off"),
-            wall_exchange_axis: std::env::var("RTX_E3_WALL_EXCHANGE").is_ok_and(|v| v == "axis"),
+            // ON by default since S2-7 (`=h` reproduces the records before it).
+            wall_exchange_axis: std::env::var("RTX_E3_WALL_EXCHANGE").map_or(true, |v| v != "h"),
             pressure_centroid: std::env::var("RTX_E3_PRESSURE_CENTROID").is_ok_and(|v| v == "1"),
             momentum_volume_tiled: std::env::var("RTX_E3_MOMENTUM_VOLUME")
                 .is_ok_and(|v| v == "tiled"),