CFD-ACE+ can utilize many different types of elements when solving stress simulations using the built-in finite element stress module. This note gives you a brief overview of the currently existing element types and some tips on how and when to use each type.

By default, the elements created in a standard grid system, e.g. triangles, hexahedrals (bricks), etc., are taken as first order elements, which means that the dependent variables are interpolated linearly between the nodes.

Alternatively you may choose second order elements. The stress solver will create second order elements by inserting mid nodes along each edge. Thus, a 3-noded triangle becomes a 6-noded triangle; an 8-noded hexahedral (brick) becomes a 20-noded hexahedral (brick), etc. With second order elements, variables are interpolated quadratically using the three nodes along an edge, greatly increasing the accuracy in most cases. Second order elements should be used with caution, however, because in addition to increasing the accuracy, the memory and computational requirements are also increased.

In order to improve the accuracy of first order elements we have added two element types in V2003. You now have the option of activating enhanced first order brick elements or solid shell brick elements. Enhanced first order brick elements are almost as accurate as second order elements with the advantage that they require roughly the same memory as first order elements. Solid shell brick elements are more accurate than standard elements for high aspect ratio grids (Also new for V2003 is the ability to stack solid shell elements in multiple layers). Please note that these options can only be applied to 3D hexahedral elements.

The following example illustrates the different solutions that could be obtained with the different element types. This case is a simple cantilever beam with an applied pressure of 200000 Pa to the upper side of the beam. The applied pressure causes the beam to deflect downward. The analytical result for this case indicate a maximum displacement of 0.0146 m. The results are shown below along with normalized CPU time and memory usage for each element type.

How to Choose Your Element Type?

First order elements are the most robust and efficient and can be used for many simulations. However, here are a few guidelines that might help:

• If you have a bending dominated problem (like the bending of a plate or beam) then second order elements (or enhanced first order bricks) should be used.

   

• If (nearly) incompressible behavior is present (e.g., in linear elastic materials with Poisson's ratio greater than 0.49 or nonlinear elastoplastic materials) then enhanced first order elements or even solid shell elements will perform better than standard first or second order elements. Keep in mind that these element types are only available for 3D hexahedral grid systems.

   

• If you have high aspect ratio hexahedral grids (as in modeling a large plate structure) then you should use solid shell brick elements.
  

How to Set the Stress Element Type?

First Order Elements

Second Order Elements

From the CFD-ACE-GUI Model Options (MO) -> Stress page, set the Element Order to "Second".

Enhanced First Order Brick Elements or Solid Shell Brick Elements

• From the CFD-ACE-GUI Model Options (MO) -> Stress page set the Element Order to "First" (this is the default setting)
• If Solid Shells are desired then you must activate the "Element Conversion" option (this is not required for Enhanced Brick option).
• Pick a volume condition region that consists of hexahedral cells.
• Change the VC setting mode to "Stress"

• Change the Brick Element Option to either Enhanced Brick or Solid Shell.
• If you are using Solid Shell elements then you also need to set the shell surface direction by selecting one BC for each solid that represents the side which will undergo the most bending (usually the surface with the largest area) and activating the "Shell Surface" option.

(Recall that the enhanced and solid shell options can only be applied to 3D hexahedral grid systems).

If you have any questions about this topic or would like us to discuss some other topic in the future, please let us know.

Amit Saxena
Applications Engineer
CFDRC Customer Support