How to make a closed polysurface

I am looking to import a .stl file into a computational fluid dynamics software and to use the plugin for this software I need to make my geometry file a closed polysurface. I dont believe I can attach my geometry file which might make it more difficult! but any help would be much appreciated

Well, we can’t help you much without letting us know what the geometry is! Even a picture would be useful if you can’t attached a file, a sketch, painting, collage? Anything really. Herein lies the difference between an open and closed polysurface:

Should you not be trying to just translate the mesh into the FEA? 'Cause going from a random STL–where did the STL come from?–to NURBS then back to a mesh for FEA sounds…inefficient.