Normal Direction for Surfaces and Shells
The normal is always perpendicular to a surface or shell, except if a surface belongs to one and only one volume. These surfaces already have a natural consistent normal that points out of the volume.
If your shell model is a midsurface compressed model, the normal direction is from the red side to the yellow side of the midsurface.
For 3D models, if a solid face lies on a surface or shell, the surface or shell determines the normal direction of the face. If a solid face is not coincident with a surface or shell, the normal direction of the solid face always points away from the center of the solid.
The normal direction has these two purposes:
• Pressure loads always oppose the normal of a surface, midsurface shell, or any other element. However, for quilts, pressure load direction always coincides with the shell normal.
• The top of a surface or shell is always in the normal direction. The surface or shell displayed on the screen represents the midsurface of the element. You can view results for stress quantities at the top or bottom of a surface or shell.