Mechanical APDL Basic Analysis Guide

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 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
program calculates a percent error in energy norm (SEPC for structural percent error,TEPC for thermal
percent error).

Note

Error estimation is based on stiffness and conductivity matrices that are evaluated at the

reference temperatures (TREF). Err or estimates, therefore, can be incorrect for elements with

temperature-dependent 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 im-
proved 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 indicat es the rel-
ative 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 Guide. For theoretical details about error estimation, see the Mechanical APDL
Theory Reference.

7.2.7. Using the Results Viewer to Access Results File Data

The following links correspond to the three basic control areas on the Results Viewer:

For the Main Menu, see Main Menu (p. 177)

For the Toolbar, see Toolbar (p. 178)

For the Step/Sequence Data Access Control, see Step/Sequence Data Access Controls (p. 179)

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The General Postprocessor (POST1)