Simple but not ideal fix for remove sides in ElementSubdomainModifier

Author: dewenyushu

framework/src/userobject/ElementSubdomainModifier.C

@@ -186,6 +186,11 @@ ElementSubdomainModifier::updateBoundaryInfo(MooseMesh & mesh,
         // Otherwise remove this side and the neighbor side from the boundary.
         else
         {
+          // _children_on_boundary is default to False so we cannot remove side from a child element
+          // To set _children_on_boundary=true, we need to add side
+          // This is not ideal solution.
+          // TODO: allow setting _children_on_boundary = true in libmesh
+          bnd_info.add_side(elem, side, _moving_boundary_id);
           bnd_info.remove_side(elem, side);
           unsigned int neighbor_side = neighbor->which_neighbor_am_i(elem);
           bnd_info.remove_side(neighbor, neighbor_side);