Add extra check for multicut cell (AMReX-Codes#3087)

For some edge EB geometry cases when building the faces during an EB
iteration, nodes can go from regular to covered. Additional check needs
to be added to label appropriate cells as a `multicut` cell.

Src/EB/AMReX_EB2_3D_C.cpp

@@ -46,14 +46,22 @@ void set_eb_data (const int i, const int j, const int k,
 
     // Check for multiple cuts
     // We know there are no multiple cuts on faces by now.
-    // So we only need to check the case that there are two cuts
+    // Firstly, we need to check the case that there are two cuts
     // at the opposite corners.
     bool multi_cuts = (axm >= 0.5_rt && axm < 1.0_rt &&
                        axp >= 0.5_rt && axp < 1.0_rt &&
                        aym >= 0.5_rt && aym < 1.0_rt &&
                        ayp >= 0.5_rt && ayp < 1.0_rt &&
                        azm >= 0.5_rt && azm < 1.0_rt &&
                        azp >= 0.5_rt && azp < 1.0_rt);
+    // Secondly, we also need to check if area fractions became 0
+    // at any opposite face when building the faces
+    if ((axm == 0.0_rt && axp == 0.0_rt) ||
+        (aym == 0.0_rt && ayp == 0.0_rt) ||
+        (azm == 0.0_rt && azp == 0.0_rt)) {
+        multi_cuts = true;
+    }
+
     if (multi_cuts) {
         set_covered(i, j, k, cell, vfrac, vcent, barea, bcent, bnorm);
         is_multicut = true;