Tetgen (called from Gibbon) can mesh the tube but results in missing some elements in defining the no slip wall

[alibehrang]

Hi, I am working on generating a bifurcated tube (from scratch) using Gibbon/Matlab and performing FEA and CFD and eventually FSI on it. I have completed the FEA part (mesh and analysis) and I am having trouble with the CFD mesh that is generated by Tetgen from Gibbon. The mesh looks ok, but I cannot define wall elements for no-slip boundary condition. The problem is that Tetgen generates some elements on the inner surface with all 4 nodes (1st order tetrahedral) being on the surface. In other words, I have two faces of an element on the wall and thus I am missing some elements when I am defining (list of elements) this wall no matter how I re-orient those elements. Do you know how I can prevent this? Image 1 is showing some of the missing elements and image 2 shows that those elements have two faces on the surface.