mapdl intro 13.0 l11 postprocessing

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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



Postprocessing

L11 - 2 ANSYS, Inc. Proprietary


September 2011


• 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



Postprocessing



September 2011


• /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



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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




Postprocessing



September 2011


• 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.




Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• /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



Postprocessing



September 2011


• 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)



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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)



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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



Postprocessing



September 2011


1. Create load cases

2. Read one load case into the database3. Perform the desired operation

Load Case Combination Procedure

P t i



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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


Postprocessing



Postprocessing



September 2011


• 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



Postprocessing



September 2011


• 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


Postprocessing



Postprocessing



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



Postprocessing



September 2011


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



Postprocessing



September 2011


100kg

25kg

k = 36kN/m

F

0,0

0,4000

t

t N F

k = 36kN/m

x

y

Variable Viewer

Postprocessing



Postprocessing


• Refer to your Worksho p Supplement for instructions:

W11A. Connecting Rod

W11B. Spherical Shell

W11C. Axisymmetric Fin with Multiple Load Steps

Workshops

http://localhost/var/www/apps/conversion/tmp/scratch_2/workshops/Intro1_ws13.ppt


















mapdl intro 13.0 l11 postprocessing

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