Shell Normals
Use the Surfaces selector arrow on the Shell Definition dialog box to display the normal direction for each shell and to change the direction for one or more shells. Because Creo Simulate sets the normal direction separately for each shell when you create it, the direction of the normals for adjoining shells may end up in opposite or inconsistent directions.
See Normal Direction for Surfaces and Shells for information on how Creo Simulate creates and uses normal directions.
You can fix or flip the shell normals if your shell is not part of a volume, or part of a shell pair in a midsurface compressed model. When you use the Surfaces selector arrow to select the shell on your model, a magenta arrow appears displaying the normal direction. Do one of the following to fix or flip the normal direction:
• Select the shell, and then select the shell again to flip the direction of the arrow that shows the normal direction.
• On the SIM SELECT menu, select Fix Normals or Flip Normals.
Creo Simulate checks if you have fixed normals at certain points in the modeling process.
To reverse the normal direction of a compressed midsurface, select the shell pair on the Model Tree. Right click to select Edit Definition. The Shell Pair Definition dialog box opens. Click 
to interchange the top and bottom surfaces in the shell pair and reverse the direction of the resulting mesh normal.
to interchange the top and bottom surfaces in the shell pair and reverse the direction of the resulting mesh normal.