8.3. Keeping Track of Element Faces and Orientations

If your model contains shell elements, and if you apply surface loads, you will need to keep track of the element faces in order to be able to define the proper direction for your loads. In general, shell surface loads will be applied to element face one, and will be positive in accordance with the right-hand rule (following the I, J, K, L nodal sequence, as illustrated below). If you create your shell elements by meshing a solid model area, the normal direction of the elements will be consistent with the normal direction of the area. The area's normal direction can be determined by issuing the ALIST command. The direction of the sequence of lines defining that area will define the normal direction by the right-hand rule.

Figure 8.8: Positive Normal Direction as Defined by the Right-Hand Rule

There are several ways that you can conduct graphical checks:

You can conduct a quick graphical check of the positive normal direction for shell elements by issuing a /NORMAL command followed by an EPLOT command.
You can turn PowerGraphics ON. PowerGraphics displays the "top" and "bottom" of shells with different colors.
You can apply your surface loads with the assumed correct sign, and then verify their direction by turning on the surface load symbol (/PSF,Item,Comp,2) before executing EPLOT.

8.3.1. Controlling Area, Line, and Element Normals

Inconsistent normal directions can lead to problems in your model. For example, if adjacent shell elements have inconsistent normal directions, you could encounter difficulties in postprocessing of stress and strain results. Clearly, if a surface of your model contains both top and bottom shell element faces, averaged nodal stresses and strains could be incorrect. However, PowerGraphics (/GRAPHICS,POWER) accounts for the mismatched normal directions and can produce proper nodal stress plots. (PowerGraphics is the default when the GUI is on.)

The sections that follow describe how you can use these tools to:

Reorient shell element normals so that they are consistent with that of a specified element (ENORM)
Reorient area normals so that they are consistent with that of a specified area (ANORM)
Reverse the normals of existing shell elements (ENSYM)
Reverse the normal of a line (LREVERSE)
Reverse the normal of an area (AREVERSE)

Note: You cannot use the tools described in this section to change the normal direction of any element that has a body or surface load. Ansys recommends that you apply all of your loads only after ensuring that the element normal directions are acceptable.

Caution: Real constants (such as nonuniform shell thickness and tapered beam constants) may be invalidated by an element reversal.

8.3.1.1. Reorienting Shell Element Normals

If you find that your elements have inconsistent positive normal directions, you can reorient them uniformly to match a specified element. (Element coordinate systems, if defined by the I, J, K nodes, might also be reoriented by this operation.)

To use the command method to reorient shell element normals, issue the command ENORM,ENUM:

Use the ENUM argument to identify the number of the element having the normal direction that the reoriented elements are to match.

For example, the command ENORM,3 reorients the normals of all selected shell elements so that they are consistent with the normal direction of element number 3. See the description of the ENORM command for details.

8.3.1.2. Reorienting Area Normals

If a group of areas has inconsistent normal directions, you can reorient them uniformly to match the normal direction of a specified area.

To use the command method to reorient area normals, issue the command ANORM,ANUM,NOEFLIP:

Use the ANUM argument to identify the number of the area having the normal direction that the reoriented areas are to match.
Use the NOEFLIP argument to indicate whether you want to change the normal direction of any existing elements on the reoriented area(s) so that they are consistent with the new normal direction. Specify 0 if you want to make the normals consistent. Specify 1 if you do not.

For example, the command ANORM,5,0 reorients the normals of all selected areas so that they are consistent with the normal direction of area number 5, and also makes any area elements on the areas consistent with that normal direction. See the description of the ANORM command for details.

8.3.1.3. Reversing the Normals of Existing Shell Elements

To use the command method to reverse the normals of existing shell elements, issue the command ENSYM,,,,IEL1,IEL2,IEINC:

Use the IEL1, IEL2, and IEINC arguments to reverse the normals of elements from IEL1 to IEL2 (defaults to IEL1) in steps of IEINC (defaults to 1).

For example, the command ENSYM,,,,1,50 reverses the normals of shell elements numbered 1 through 50.

8.3.1.4. Reversing the Normal of a Line

To use the command method to reverse the normal of a line, issue the command LREVERSE,LNUM,NOEFLIP:

Use the LNUM argument to identify the number of the line whose normal is to be reversed.
Use the NOEFLIP argument to indicate whether you want to change the normal direction of the existing elements on the line so that they are consistent with the reversed line's new normal direction. Specify 0 if you want to make the normals consistent. Specify 1 if you do not.

For example, the command LREVERSE,1,1 reverses the direction of line 1, but does not make any line elements on the line consistent with the new direction.

8.3.1.5. Reversing the Normal of an Area

To use the command method to reverse the normal of an area, issue the command AREVERSE,ANUM,NOEFLIP:

Use the ANUM argument to identify the number of the area whose normal is to be reversed.
Use the NOEFLIP argument to indicate whether you want to change the normal direction of the existing elements on the area so that they are consistent with the reversed area's new normal direction. Specify 0 if you want to make the normals consistent. Specify 1 if you do not.

For example, the command AREVERSE,7,0 reverses the normal direction of area 7 and makes any existing area elements on it consistent with the new normal direction.