1. Module 14
Postprocessing

2. Postprocessing Overview
There are many ways to review results in the general postprocessor (POST1), some of which have already been covered.
In this chapter, we will explore two additional methods — query picking and path operations — and also introduce you to the concepts of results transformation, error estimation, and load case combination.
We will also introduce two productivity enhancement tools:
Results Viewer
Report Generator
Topics covered:
A. Query Picking E. Load Case Combinations
B. Results Coordinate System F. Results Viewer
C. Path Operations G. Report Generator
D. Error Estimation H. Workshop
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3. Postprocessing A. Query Picking
Query picking allows you to “probe” the model for stresses, displacements, or other results quantities at any picked location.
You can also quickly locate the maximum and minimum values of the item being queried.
Available only through the GUI (no commands):
General Postproc > Query Results > Nodal or Element or Subgrid Solu...
Choose a results quantity and press OK
PowerGraphics ON
PowerGraphics
OFF
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4. Postprocessing ...Query Picking
Then pick any point in the model to see the results value at that point.
Min and Max will show the value at the minimum and maximum points.
Use Reset to clear all values and start over.
Notice that the entity number, its location, and the results value are also shown in the Picker.
Automatically generate text annotation
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5. Postprocessing ...Query Picking
Demo:
Continue from the last multi-load-step solution of rib.db
Plot SEQV for load step 1
Query “Nodal Solu” SEQV at several locations, including MIN & MAX. (Switch to full graphics if needed.)
Switch to PowerGraphics and query “Subgrid Solu.”
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6. Postprocessing B. Results Coordinate System
All direction-dependent quantities that you view in POST1, such as component stresses, displacements, and reaction forces, are reported in the results coordinate system (RSYS).
RSYS defaults to 0 (global Cartesian). That is, POST1 transforms all results to global Cartesian by default, including results at “rotated” nodes.
But there are many situations — such as pressure vessels and spherical structures — where you need to check the results in a cylindrical, spherical, or other local coordinate system.
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7. Postprocessing ...Results Coordinate System
To change the results CS to a different system, use:
General Postproc > Options for Outp…
or the RSYS command
All subsequent contour plots, listings, query picks, etc. will report the values in that system.
Default orientation RSYS,0
Local cylindrical system RSYS,11
Global cylindrical system RSYS,1
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8. Postprocessing ...Results Coordinate System
RSYS,SOLU
Sets the results CS to “as-calculated.”
All subsequent contour plots, listings, query picks, etc. will report the values in the nodal and element coordinate systems.
DOF results and reaction forces will be in the nodal CS.
Stresses, strains, etc. will be in the element CS. (The orientation of the element CS depends on the element type and the ESYS attribute of the element. Most solid elements, for example, default to global Cartesian.)
Not supported by PowerGraphics.
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9. Postprocessing C. Path Operations
Another way to review results is via path operations, which allow you to:
map results data onto an arbitrary “path” through the model
perform mathematical operations along the path, including integration and differentiation
display a “path plot” — see how a result item varies along the path
Available only for models containing 2-D or 3-D solid elements or shell elements.
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10. Postprocessing ...Path Operations
Three steps to produce a path plot:
Define a path
Map data onto the path
Plot the data
1. Define a Path
Requires the following information:
Points defining the path (2 to 1000). You can use existing nodes or locations on the working plane.
Path curvature, determined by the active coordinate system (CSYS).
A name for the path.
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11. Postprocessing ...Path Operations
1. Define a Path (cont’d)
First activate the desired coordinate system (CSYS).
General Postproc > Path Operations > Define Path > By Nodes or On Working Plane
Pick the nodes or WP locations that form the desired path, and press OK
Choose a path name. The nSets and nDiv fields are best left to default in most cases.
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12. Postprocessing ...Path Operations
2. Map Data onto Path
General Postproc > Path Operations > Map onto Path… (or PDEF command)
Choose desired quantity, such as SX.
Enter a label for the quantity, to be used on plots and listings.
You can now display the path if needed.
General Postproc > Path Operations > Plot Paths
(or issue /PBC,PATH,1 followed by NPLOT or EPLOT)
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13. Postprocessing ...Path Operations
3. Plot the Data
You can plot path items either on a graph:
PLPATH or General Postproc > Path Operations > On Graph...
or along path geometry:
PLPAGM or General Postproc > Path Operations > On Geometry...
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14. Postprocessing ...Path Operations
ANSYS allows you to define multiple paths, each with a unique name that you assign. Only one path can be active at a time.
Besides plots and listings, there are many other path capabilities, including:
Stress linearization — used in the pressure vessel industry to decompose stress along a path into its membrane and bending components.
Calculus functions — used in fracture mechanics to calculate J-integrals and stress concentration factors. Also useful in thermal analyses to calculate the heat lost or gained across a path.
Dot products and cross products — used widely in electromagnetics analyses to operate on vector quantities.
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15. Postprocessing ...Path Operations
Demo:
Continue with rib postprocessing…
Plot nodes, then switch to CSYS,1 if desired
Define a path using nodes
Map SX or SEQV or other data onto path
Plot the path itself
Plot the path item on graph and on geometry
Define a second path elsewhere in the model and show how to toggle between the two.
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16. Postprocessing D. Error Estimation
The finite element solution calculates stresses on a per- element basis, i.e, stresses are individually calculated in each element.
When you plot nodal stress contours in POST1, however, you will see smooth contours because the stresses are averaged at the nodes.
If you plot the element solution, you will see unaveraged data, which shows the discontinuity between elements.
The difference between averaged and unaveraged stresses gives an indication of how “good” or how “bad” the mesh is. This is the basis for error estimation.
savg = 1100
s = 1000
s = 1200
Elem 1
Elem 2
s = 1100
s = 1300
savg = 1200
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17. Postprocessing ...Error Estimation
Error estimation is available only in POST1 and is valid only for:
linear static structural and linear steady-state thermal analyses
solid elements (2-D and 3-D) and shell elements
Full Graphics (not PowerGraphics)
If these conditions are not met, ANSYS automatically turns off error estimation calculations.
To manually activate or deactivate error estimation, use
ERNORM,ON/OFF
or General Postproc > Options for Outp...
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18. Postprocessing ...Error Estimation
POST1 calculates the following error measures.
Stress analysis:
percentage error in energy norm (SEPC)
element stress deviations (SDSG)
element energy error (SERR)
maximum and minimum stress bounds (SMXB, SMNB)
Thermal analysis:
percentage error in energy norm (TEPC)
element thermal gradient deviations (TDSG)
element energy error (TERR)
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19. Postprocessing ...Error Estimation
Percentage error in energy norm (SEPC)
SEPC is a rough estimate of the stress error (or displacement, temperature, or thermal flux) over the entire set of selected elements.
Can be used to compare similar models of similar structures subjected to similar loadings.
SEPC is shown in the legend column of deformed shape displays. You can list it manually using PRERR or General Postproc > List Results > Percent Error.
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20. Postprocessing ...Error Estimation
As a general rule of thumb, look for SEPC to be 10% or less. If it is higher, then:
Check for point loads or other stress singularities and unselect elements in the vicinity.
If it is still higher, plot the element energy error. The elements with high values of energy error are candidates for mesh refinement.
SEPC = 34.5
SEPC = 10.2
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21. Postprocessing ...Error Estimation
Element stress deviations (SDSG)
SDSG is a measure of the amount by which an element’s stress disagrees with the stress averages at its nodes.
You can plot SDSG contours using PLESOL,SDSG or General Postproc > Plot Results > Element Solu...
A high value of SDSG is not necessarily bad, especially if it is a small percentage of the nominal stresses in the structure.
For example, this plate-with-a-hole model shows only a 1.5% stress deviation in the region of interest.
SDSG at location of interest = ~450 psi, which is ~1.5% of ~30,000 psi nominal stress
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22. Postprocessing ...Error Estimation
Element energy error (SERR)
SERR is the energy associated with the stress mismatches at the nodes of the element. This is the basic error measure from which the other error quantities are derived. SERR has units of energy.
To plot SERR contours, issue PLESOL,SERR or General Postproc > Plot Results > Element Solu...
Generally, the elements with the highest SERR are candidates for mesh refinement. However, since SERR will always be highest at stress singularities, be sure to unselect those elements first.
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23. Postprocessing ...Error Estimation
Stress bounds (SMXB and SMNB)
The stress bounds can help you determine the potential effect of mesh discretization error on the maximum stress.
They are displayed on stress contour plots in the legend column as SMXB (upper bound) and SMNB (lower bound).
The bounds are not estimates of the actual maximum and minimum, but they do define a “confidence band.” Without other supporting verification, you have no basis for believing that the true maximum stress is below SMXB.
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24. Postprocessing ...Error Estimation
Caution: If you don’t unselect elements near stress singularities, the stress bounds are meaningless, as shown below.
SMXB = 18,102
SMXB = 4,773
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25. Postprocessing E. Load Case Combinations
Whenever you solve multiple load steps, the results of each load step are stored as separate sets on the results file (identified by load step number).
A load case combination is an operation between two sets of results, which are called load cases.
The operation occurs between one load case in the database and the second load case on the results file.
The result of the operation — the combined load case — is stored back in the database.
Load case in database
(computer memory)
Load case
on results file
Combined load case in database
overwrites previous contents
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26. Postprocessing ...Load Case Combinations
Typical procedure:
1. Create the load cases
2. Read one load case into the database
3. Perform the desired operation
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27. Postprocessing ...Load Case Combinations
Create Load Cases
A load case simply acts as a pointer to a set of results. It requires two pieces of information:
a unique ID number
the results set it represents (load step and substep number)
Use the LCDEF command or General Postproc > Load Case > Create Load Case
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28. Postprocessing ...Load Case Combinations
Read One Load Case into the Database (memory)
Simply identify the results set by its load case number using LCASE or General Postproc > Load Case > Read Load Case.
Or use one of the standard “Read Results” choices in the postprocessor (SET command).
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29. Postprocessing ...Load Case Combinations
Perform the Desired Operation
Many operations are available as shown in the menu here.
Use the LCOPER command or General Postproc > Load Case > Add, Subtract, etc.
Remember that the results of the operation are stored in the database (memory). The combined load case is identified on plots and listings as number 9999.
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30. Postprocessing ...Load Case Combinations
There are two useful options to save the combined load case:
Write a load case file
Append the load case to the results file
Writing a load case file (LCWRITE or General Postproc > Write Results) creates a file that is similar to, but much smaller than the results file.
The Append option (RAPPND or General Postproc > Load Case > Write Load Case) allows you to add the combined load case to the results file and identify it with a given load step number and time value.
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31. Postprocessing F. Results Viewer
The Results Viewer is a specialized postprocessing menu and graphic system.
Fast graphics for large models or models that have many time steps
Easy to use menu system for quick results viewing
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32. Postprocessing … Results Viewer
Can be created two different ways …
Use the POUTRES command before solving to write a jobname.pgr file during solution.
Solution > Output Cntrls > PGR file…
Hold CTRL key for multiple selection
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33. Postprocessing … Results Viewer
Use the PGWRITE command after solution to write a jobname.pgr file.
General Postproc > Write PGR File…
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34. Postprocessing … Results Viewer
Open the Results Viewer from the General Post Processor.
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35. Postprocessing ... Results Viewer
Nodal/element/vector/trace results plots
Query Results
Element Plot
Animate using PNG files
List Results
Capture/print image
Results Set Locator
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36. Postprocessing ... Results Viewer
The graphics window becomes “Context Sensitive”.
Context-sensitive graphics window
Right Click on Model
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37. Postprocessing ... Results Viewer
Context-sensitive graphics window
Right Click on Contour Legend
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38. Postprocessing ... Results Viewer
Context-sensitive graphics window
Right Click on Contour Bar
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39. Postprocessing G. Report Generator
A time consuming part of any analysis is documenting the model and results. This procedure has been partially automated through the implementation of the ANSYS Report Generator.
The Report Generator allows the user to quickly capture pictures, listings, tables, and other pertinent information.
It also facilitates the creation of an HTML formatted file ready to be distributed to colleagues or posted to a web site.
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40. Postprocessing … Report Generator
Launching the Report Generator will shrink the graphics window and set the background to white.
The “Capture Tool” will be opened allowing the user to grab plots, listings, and tables.
Utility Menu> File> Report Generator… or
~eui,’euidl::report::toolbar::create’
Capture Tool
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41. Postprocessing … Report Generator
Capture Tool
Table Capture
Animation Capture
(multiple PNG files)
Image Capture
(single PNG file)
Settings
The Log File records all captures!
HTML Report Assembler
Listing Capture
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42. Postprocessing … Report Generator
HTML Assembler . . .
A tool that allows quick organization of ANSYS graphics, tables, listings .
Log file can be used as an HTML template.
utilize parameter substitution
The generated HTML file can be used with Netscape Composer, Microsoft FrontPage, or any other HTML editor to finalize the report.
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43. Postprocessing … Report Generator
Insert TEXT
Insert any HTML FILE
possibly created outside ANSYS
Insert an IMAGE
possibly one from an external source like a digital photo
Insert DYNAMIC DATA
information specific to your current ANSYS run like version, run time, etc.
Insert a Report Heading
Including your name, analysis title, date, and company name
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44. Postprocessing … Report Generator
Insert information grabbed using the Capture Tool
Report Images
Contour plots, element plots, volume plots, graphs
Report Tables
Material properties, reaction forces, etc.
Report Lists
Stress along a path, constraints, etc.
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45. Postprocessing … Report Generator
Preview the report
Delete sections of the report
Move sections of the report up or down
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46. Postprocessing … Report Generator
In the Dynamic Data section, the ANSYS revision number is obtained using the following set of commands:
/nopr
*get,revision,active,0,rev
*VWRITE,revision
(f3.1)
Dynamic Data
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47. Postprocessing H. Workshop
This workshop consists of five exercises:
W12A. Connecting Rod
W12B. Spherical Shell
W12C. Axisymmetric Fin with Multiple Load Steps
W12D. Results Viewer
W12E. Report Generator
Refer to your Workshop Supplement for instructions.
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