rtx-cfd: overset_cfd23 RTX_OVERSET_CFD23_ND=explicit — the patch's across-diffusion explicit (dt limit then includes the n-spacing) for the 10 mm damping-floor probe; field tags carry it

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_0116sg1Qz1gMv9hdcKP1XUam

crates/specialized/rtx-cfd/tests/overset_cfd23.rs

@@ -83,11 +83,16 @@ fn overlap_rows() -> usize {
 fn field_tag(case: &str, ny: usize) -> String {
     let rows = overlap_rows();
     format!(
-        "{case}_ny{ny}_tvd{}",
+        "{case}_ny{ny}_tvd{}{}",
         if rows == OversetParameters::default().overlap_rows {
             String::new()
         } else {
             format!("_rows{rows}")
+        },
+        if std::env::var("RTX_OVERSET_CFD23_ND").is_ok_and(|v| v == "explicit") {
+            "_explicitnd"
+        } else {
+            ""
         }
     )
 }
@@ -150,10 +155,11 @@ impl Composite {
             4.0,
             500,
         )?;
+        let explicit_nd = std::env::var("RTX_OVERSET_CFD23_ND").is_ok_and(|v| v == "explicit");
         let mut hs = f64::INFINITY;
         for c in 0..mesh.cell_count() {
             for (f, _) in mesh.cell_faces(c) {
-                if mesh.is_sface(f) {
+                if mesh.is_sface(f) || explicit_nd {
                     let d = mesh.faces()[f].d;
                     hs = hs.min((d[0] * d[0] + d[1] * d[1]).sqrt());
                 }
@@ -169,7 +175,16 @@ impl Composite {
             CurvilinearParameters {
                 tolerance: 1e-5,
                 convection: PatchConvection::TvdVanAlbada,
-                normal_diffusion: NormalDiffusion::LineImplicit,
+                // `RTX_OVERSET_CFD23_ND=explicit`: the 10 mm damping-floor
+                // probe (§5.11) — is the line-implicit across-diffusion the
+                // wake's damping?
+                normal_diffusion: if std::env::var("RTX_OVERSET_CFD23_ND")
+                    .is_ok_and(|v| v == "explicit")
+                {
+                    NormalDiffusion::Explicit
+                } else {
+                    NormalDiffusion::LineImplicit
+                },
                 ..CurvilinearParameters::default()
             },
             mesh,