Mechanical APDL Basic Analysis Guide - Ansys

7.2.6. Estimating Solution Error
One of the main concerns in a finite element analysis is the adequacy of the finite element mesh. Is the
mesh fine enough for good results? If not, what portion of the model should be remeshed? You can get
answers to such questions with the ANSYS error estimation technique, which estimates the amount of
solution error due specifically to mesh discretization. This technique is available only for linear structural
and linear/nonlinear thermal analyses using 2-D or 3-D solid elements or shell elements.
In the postprocessor, the program calculates an energy error for each element in the model. The energy error
is similar in concept to the strain energy. The structural energy error (labeled SERR) is a measure of the discontinuity
of the stress field from element to element, and the thermal energy error (TERR) is a measure of
the discontinuity of the heat flux from element to element. Using SERR and TERR, the ANSYS program calculates
a percent error in energy norm (SEPC for structural percent error, TEPC for thermal percent error).
Note
7.2.7. Using the Results Viewer to Access Your Results File Data
Error estimation is based on stiffness and conductivity matrices that are evaluated at the reference
temperatures (TREF). Error estimates, therefore, can be incorrect for elements with temperaturedependent
material properties if those elements are at a temperature that is significantly different
than TREF.
In many cases, you can significantly increase program speed by suppressing error estimation. This improved
performance is most evident when error estimation is turned off in a thermal analysis. Therefore, you may
want to use error estimation only when needed, such as when you wish to determine if your mesh is adequate
for good results.
You may turn error estimation off issuing ERNORM,OFF (Main Menu> General Postproc> Options for
Outp). By default, error estimation is active. Since the value set by the ERNORM command is not saved on
Jobname.DB, you will need to reissue ERNORM,OFF if you wish to again deactivate error estimation after
resuming an analysis .
In POST1 then, you can list SEPC and TEPC for all selected elements using the PRERR command (Main
Menu> General Postproc> List Results> Percent Error). The value of SEPC or TEPC indicates the relative
error due to a particular mesh discretization. To find out where you should refine the mesh, simply produce
a contour display of SERR or TERR and look for high-error regions.
Using this error estimation technique, you can set up an automated scheme whereby the mesh is automatically
refined in high-error regions. This is called adaptive meshing. See "Adaptive Meshing" in the Advanced
Analysis Techniques Guide. For theoretical details about error estimation, see the Theory Reference for the
Mechanical APDL and Mechanical Applications.
7.2.7. Using the Results Viewer to Access Your Results File Data
The following links correspond to the three basic control areas on the Results Viewer:
For the Main Menu, see The Results Viewer Main Menu (p. 162)
For the Toolbar, see The Results Viewer Toolbar (p. 163)
For the Step/Sequence Data Access Control, see The Results Viewer Step/Sequence Data Access Controls
(p. 164)
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