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L11-1 ANSYS, Inc. Proprietary
© 2011 ANSYS, Inc. All rights reserved.Release 13.0
September 2011
Introduction to ANSYSMechanical APDL
Customer Training Material
Lecture 11
Postprocessing
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Postprocessing
L11 - 2 ANSYS, Inc. Proprietary
© 2011 ANSYS, Inc. All rights reserved.Release 13.0
September 2011
Customer Training Material
• The MAPDL general postprocessor ( /POST1) has wide range of
capabilities
• This lecture will review post-processing tools that were not
discussed in other lectures
– query picking
– path operations
– Variable Viewer
– Results transformation, error estimation
– Load case combination.
Overview
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Postprocessing
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© 2011 ANSYS, Inc. All rights reserved.Release 13.0
September 2011
Customer Training Material
• /POST1 operates on one set of results at any time
• To access a result set: General Postproc > Read Results
– or use SET command
• Set command activates a set of results data – reads the boundary conditions and results for a solution set from the
rst file into memory and stores them in the database
– by default, the first result set is automatically read
/POST1
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Postprocessing
L11 - 4 ANSYS, Inc. Proprietary
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September 2011
Customer Training Material
• Query picking “probes” the model for results (stresses, displacement,
etc.) at any picked location.
– can also identify 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 results quantity > OK
PowerGraphics
OFF
PowerGraphics
ON
Query Picking
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September 2011
Customer Training Material
• Pick any location in the model to obtain results at that location
– Min and Max will identify 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.
Query Picking
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Postprocessing
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September 2011
Customer Training Material
• All direction-dependent quantities, such as component stresses,
displacements, and reaction forces are reported in the resul ts
coo rdinate sys tem (RSYS).
• RSYS defaults to 0 (global Cartesian).
– /POST1 transforms all results to global Cartesian, including results at
“rotated” nodes.
• In many situations it is more informative to review the results in
another coordinate system (e.g. cylindrical coordinate system for
pressure vessels).
Results Coordinate System
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September 2011
Customer Training Material
• To change the results CS to a different system, use:
– General Postproc > Options for Outp…
• or use 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
Results Coordinate System
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September 2011
Customer Training Material
• The RSYS,SOLU command sets the results coordinate system to
“As calculated.”
– solution coordinate system
• All subsequent contour plots, listings, query picks, etc. report
results in the nodal and element coordinate systems.
– DOF results (displacements, reaction forces, etc.) will be in the nodal CS.
– Element results (stresses, strains, etc.) will be in the element CS.
• orientation of the element coordinate system depends on the element type
and the ESYS attribute of the element. Most solid elements default to the
global cartesian system
– Not supported by PowerGraphics.
Results Coordinate System
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L11 - 9 ANSYS, Inc. Proprietary
© 2011 ANSYS, Inc. All rights reserved.Release 13.0
September 2011
Customer Training Material
• Path operations can be used 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” to visualize how a result item varies along the path
•
Available only for SOLID or SHELL elements.
• Three steps required to produce a path plot:
– Define a path
– Map data onto the path
– Plot the data
Path Operations
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September 2011
Customer Training Material
• Activate the desired coordinate system (CSYS).
–
General Postproc > Path Operations > Define Path – Pick the nodes or WP locations that form the desired path > OK
– Specify a path name.
– Default values are generally used for nSets and nDiv
Define a Path
From
To
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September 2011
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• General Postproc > Path Operations > Map onto Path
• or use PDEF command
– Choose desired quantity, such as SEQV.
– Enter a label for the quantity- used on plots and listings.
– can now display the path if needed.
• General Postproc > Path Operations > Plot Paths
– or issue /PBC,PATH,1 command followed by NPLOT or EPLOT
Map Data onto Path
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September 2011
Customer Training Material
• Path data can be graphed
–
General Postproc > Path Operations > Plot Path Item > On Graph• or use PLPATH command
• or plotted
– General Postproc > Path Operations > Plot Path Item > On Geometry
•
or use PLPAGM command
Plot Path Data
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September 2011
Customer Training Material
• MAPDL allows multiple paths with unique name to be
defined, but only one path can be active at a time.
• Other path capabilities include:
– Stress linearization — used in the pressure vessel industry
to decompose stress along a path into membrane and
bending components.
– Calculus functions — useful in thermal analyses to calculate
heat transmitted across a path.
– Dot products and cross products — useful in low frequency
electromagnetics to operate on vector quantities.
Other Path Operations
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L11 - 14 ANSYS, Inc. Proprietary
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September 2011
Customer Training Material
• The finite element solution calculates stresses on a per-element basis
– stresses are individually calculated in each element.
• Nodal stress plots are smooth contours because the calculated element
stresses are averaged at shared nodes.
• Element stress plots are discontinuous, because the stresses are unaveraged
• Difference between averaged and unaveraged stresses gives an indication of
how mesh “quality”
– basis for error estimation.
Elem 1 Elem 2
avg = 1100
= 1200= 1000
avg = 1200
= 1300= 1100
Error Estimation
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L11 - 15 ANSYS, Inc. Proprietary
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September 2011
Customer Training Material
• Error estimation is is valid only for:
– linear static structural and linear steady-state thermal analyses
– solid elements (2D and 3D) and shell elements
– Full Graphics (not PowerGraphics)
– If these conditions are not met, MAPDL automatically turns off error
estimation calculations.
• To manually activate or deactivate error estimation: General Postproc
> Options for Outp
– or use ERNORM,ON/OFF command
Error Estimation
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September 2011
Customer Training Material
• /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)
Error Estimation
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September 2011
Customer Training Material
• SEPC is a rough estimate of the error (stress, displacement,
temperature, thermal flux, etc.) 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.
• Can be manually listed using
– General Postproc > List Results > Percent Error.
• or with PRERR command
Percentage error in energy norm (SEPC)
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Customer Training Material
• As a general rule of thumb, SEPC should
be less than 10%
• If it is higher:
– unselect element near point loads or other
stress singularities.
–
plot the element energy error. Elementswith high values of energy error are
candidates for mesh refinement.
SEPC = 35.149
SEPC = 3.484
SEPC
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L11 - 19 ANSYS, Inc. Proprietary
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Customer Training Material
• SDSG is a measure of the amount by which an element’s stress differs
from the average stress at its nodes.
• To plot SDSG contours: General Postproc > Plot Results > Contour
Plot > Element Solu...
– or use PLESOL,SDSG command
• A high value for SDSG is not necessarily bad, especially if it is a small
percentage of the nominal stresses in the structure.
– for example, the 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
Element stress deviations (SDSG)
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L11 - 20 ANSYS, Inc. Proprietary
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September 2011
Customer Training Material
• SERR is the energy associated with the stress mismatches at the
nodes of the element. It is the basic error measure from which the
other error quantities are derived.
– has units of energy.
• To plot SERR contours: General Postproc > Plot Results > Contour
Plot > Element Solu – or use PLESOL,SERR command
• Generally, the elements with the highest SERR are candidates for
mesh refinement, unless it occurs near stress singularities.
Element energy error (SERR)
P i
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L11 - 21 ANSYS, Inc. Proprietary
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Customer Training Material
• The stress bounds (SMXB and SMNB ) can help assess the 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.”
– if elements near stress singularities are active, the stress bounds may be
meaningless
– unselect elements near stress singularities to achieve realistic stress
bounds
Stress bounds (SMXB and SMNB)
P t i
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L11 - 22 ANSYS, Inc. Proprietary
© 2011 ANSYS, Inc. All rights reserved.Release 13.0
September 2011
Customer Training Material
• Whenever multiple load steps are solved, the results of each load step
are stored as separate sets on the results file
– identified by load step number and referred to as load cases
• A load case combination is an operation between two sets of results
– operation occurs between the load case currently stored in the database
and another load case stored on the results file.
– result of the operation (the combined load case ) is then stored in the
database.
Load case in database
(computer memory)
Load case
on results fileCombined load case in database
overwrites previous contents
Load Case Combinations
P t i
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L11 - 23 ANSYS, Inc. Proprietary
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September 2011
Customer Training Material
1. Create load cases
2. Read one load case into the database3. Perform the desired operation
Load Case Combination Procedure
P t i
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L11 - 24 ANSYS, Inc. Proprietary
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September 2011
Customer Training Material
• A load case simply acts as a pointer to a set of results.
• Requires two pieces of information:
– a unique ID number
– the results set it represents (load step and substep number)
• Use General Postproc > Load Case > Create Load Case – or use the LCDEF command
Create Load Cases
P t i
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L11 - 25 ANSYS, Inc. Proprietary
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September 2011
Customer Training Material
• To read a load case into the database (memory), identify
the results set by its load case number using
– General Postproc > Load Case > Read Load Case.
• or the LCASE command
• or use any of the standard “Read Results” choices in the
postprocessor – use the SET command
Load Case Combinations
Postprocessing
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L11 - 26 ANSYS, Inc. Proprietary
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September 2011
Customer Training Material
• Many load case operations are available (see menu)
• To access: General Postproc > Load Case >
– or use the LCOPER command
• The results of the operation are stored in the database
as load case 9999.
Load Case Operations
Postprocessing
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L11 - 27 ANSYS, Inc. Proprietary
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September 2011
Customer Training Material
• Two options available to save the combined load
case:
– Write a load case file
– Append the load case to the results file
• To write a load case file use: General Postproc >
Write Results – creates a file that is similar but much smaller than
the rst file.
• or use LCWRITE command
• To append load case to rst file use: GeneralPostproc > Load Case > Write Load Case
– adds combined load case to results file and
identifies it with a given load step number and time.
• or use RAPPND command
Load Case Combinations
Postprocessing
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L11 - 28 ANSYS, Inc. Proprietary
© 2011 ANSYS, Inc. All rights reserved.Release 13.0
September 2011
Customer Training MaterialVariable Viewer
• The Variable Viewer (time history post-processor or /POST26) can be
used to post-process results with respect to time.
• The Variable Viewer can be started by:
– opening the Time History Postprocessor or
– Main Menu > TimeHist Postproc > Variable Viewer
Postprocessing
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Postprocessing
L11 - 29 ANSYS, Inc. Proprietary
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September 2011
Customer Training Material
1 2 3 4 5 6 7 8 9 10 11
12
17
Add variable button1
Delete variable button2
Graph variable button3
List variable button4
Properties button5
Import data button6
Export data button7
Export data type8
Clear Time History Data9
Refresh Time History Data10
Variable name input area
11
Expression input area14
Defined APDL variables15
Defined Post26
variables16
17 Calculator
Variable Viewer
16
Real/Imaginary Components
Variable list
14
12
13
13
15
Postprocessing
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L11 - 30 ANSYS, Inc. Proprietary
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September 2011
Customer Training Material
100kg
25kg
k = 36kN/m
F
0,0
0,4000
t
t N F
k = 36kN/m
x
y
Variable Viewer
Postprocessing
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Postprocessing
Customer Training Material
• Refer to your Worksho p Supplement for instructions:
W11A. Connecting Rod
W11B. Spherical Shell
W11C. Axisymmetric Fin with Multiple Load Steps
Workshops