[BMCombustor]

Hi

I have a complex geometry which has been generated by stereolithography (STL file). The thing is that the whole geometry is just one surface while I need to define some parts as inlet and outlet. What I have in my mind is to put some vertices on those related faces of my interest and create a plane (or face) and then move on toward the rest of it; but the problem is that I don't know the right coordination to put the vertices. I have meshed my surface and am sure that my face of interest (shown with a green circle and arrow in the attached image) is completely planar and I could tangent a surface on it but I do not know how! Does anyone have an idea?

Thanks