embedded3 S2-6: the transverse gradient from the face at least 0.2 h off the wall (full neighbours over their own distance along the cut partner's normal) — the first form blew up at DFG ny 61 with the fine floor (coefficient ~ 1/d_f); host = device 7.7e-14 with the closures on; Couette linear exactness kept (<= 0.01 h); RTX_E3_OBLIQUE_N rung knob

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

crates/specialized/rtx-cfd/src/kernels/cuda/e3_cut.cu

@@ -16,6 +16,7 @@
 #define CUT_INERTIA_FLOOR 0.1
 #define CUT_DISTANCE_FLOOR 0.05
 #define CUT_DISTANCE_FLOOR_FINE 0.01
+#define CUT_TRANSVERSE_FLOOR 0.2
 
 struct E3Cut {
     const double *a_u, *a_v, *a_w;      /* apertures per face */
@@ -121,18 +122,31 @@ __device__ double cut_transverse(const E3Params& g, const E3Cut& m, int c, int d
     if (!any) return 0.0;
     double alpha_q, apm_q[3], app_q[3], wall_q[3], distance_q;
     cut_cv(g, m, c, q[0], q[1], q[2], fq, &alpha_q, apm_q, app_q, wall_q, &distance_q);
+    /* Explicit, coefficient ~ 1/d_f: the gradient from the faces at least
+       CUT_TRANSVERSE_FLOOR h off the wall — a FULL neighbour too, over its own
+       distance along the cut partner's normal; only when neither is that far,
+       from the cut faces over the floored distance (cut_predictor.rs). */
+    double h_min = fmin(fmin(g.dx, g.dy), g.dz);
+    double d_min = CUT_TRANSVERSE_FLOOR * h_min;
+    double a0 = sqrt(wall[0] * wall[0] + wall[1] * wall[1] + wall[2] * wall[2]);
+    double aq = sqrt(wall_q[0] * wall_q[0] + wall_q[1] * wall_q[1] + wall_q[2] * wall_q[2]);
+    int cut0 = a0 > 0.0 && alpha < 1.0, cutq = aq > 0.0 && alpha_q < 1.0;
+    if (!cut0 && !cutq) return 0.0;
+    double n0[3], nq[3];
+    for (int e = 0; e < 3; ++e) { n0[e] = cut0 ? wall[e] / a0 : 0.0; nq[e] = cutq ? wall_q[e] / aq : 0.0; }
+    if (!cut0) for (int e = 0; e < 3; ++e) n0[e] = nq[e];
+    if (!cutq) for (int e = 0; e < 3; ++e) nq[e] = n0[e];
+    int far = distance >= d_min || distance_q >= d_min;
     double gr[3] = { 0.0, 0.0, 0.0 };
     double count = 0.0;
-    double a0 = sqrt(wall[0] * wall[0] + wall[1] * wall[1] + wall[2] * wall[2]);
-    if (a0 > 0.0 && alpha < 1.0) {
-        double slope = (u0 - ub) / (distance * a0);
-        for (int e = 0; e < 3; ++e) gr[e] -= slope * wall[e];
+    if (!((far && distance < d_min) || (!far && !cut0))) {
+        double slope = (u0 - ub) / fmax(distance, d_min);
+        for (int e = 0; e < 3; ++e) gr[e] -= slope * n0[e];
         count += 1.0;
     }
-    double aq = sqrt(wall_q[0] * wall_q[0] + wall_q[1] * wall_q[1] + wall_q[2] * wall_q[2]);
-    if (aq > 0.0 && alpha_q < 1.0) {
-        double slope = (uq - ubt[fq]) / (distance_q * aq);
-        for (int e = 0; e < 3; ++e) gr[e] -= slope * wall_q[e];
+    if (!((far && distance_q < d_min) || (!far && !cutq))) {
+        double slope = (uq - ubt[fq]) / fmax(distance_q, d_min);
+        for (int e = 0; e < 3; ++e) gr[e] -= slope * nq[e];
         count += 1.0;
     }
     if (count == 0.0) return 0.0;