Hi @Anonymous
The only situations that I have seen that is when using plate (or shell) elements and the "Normal Point" is not located on the proper side of the elements. (The "Normal Point" is entered in the Element Definition and tells the software which side of the plate element has a positive pressure.)
Is this your situation? If so, try entering a normal point that is located "inside" the tunnel for every individual element. See "Why is pressure on Plate elements pointing in inconsistent or wrong directions in Simulation Mechanical".
John Holtz, P.E. Global Product Support
Autodesk, Inc. If not provided, indicate the version of Inventor Nastran you are using.If the issue is related to a model, attach the model! See What files to provide when the model is needed.
