Chapter 6: The Time-History Postprocessor (POST26)

Go to the Next Chapter
Go to the Previous Chapter
Go to the Table of Contents for This Manual
Go to the Guides Master Index

Chapter 1 * Chapter 2 * Chapter 3 * Chapter 4 * Chapter 5 * Chapter 6 * Chapter 7 * Chapter 8 * Chapter 9 * Chapter 10 * Chapter 11 * Chapter 12 * Chapter 13 * Chapter 14 * Chapter 15 * Chapter 16 * Chapter 17 * Chapter 18 * Chapter 19

POST26, the time-history postprocessor, enables you to review analysis results at specific points in the model as a function of time, frequency, etc. It has many capabilities, ranging from simple graphics displays and tabular listings to more complex operations such as differential calculus and response spectrum generation. A typical usage of POST26 is to graph result items versus time in a transient analysis, or to graph force versus deflection in a nonlinear structural analysis.

To enter the ANSYS time-history postprocessor, use either of the following:

Command(s):

POST26

GUI:

Main Menu>TimeHist Postpro

6.1 Defining and Storing the POST26 Variables

All POST26 operations refer to variables, which are simply tables of result item versus time (or versus frequency). The result item may be the UX displacement at a node, the heat flux in an element, the force developed at a node, the stress in an element, the magnetic flux in an element, etc. Each POST26 variable is assigned an arbitrary reference number 2 or greater by the user, with reference number 1 reserved for time (or frequency). The first step in POST26, therefore, is to define the desired variables, as explained next. The second step is to store the variable, which is explained on the next page.

6.1.1 Defining Variables

You can use the following commands to define POST26 variables All of these commands are equivalent to these GUI paths:

GUI:

Main Menu>TimeHist Postpro>Define Variables
Main Menu>TimeHist Postpro>Elec&Mag>Circuit>Define Variables

FORCE

SHELL

ESOL

LAYERP26

SHELL

NSOL

ESOL

RFORCE

GAPF

SOLU

For example, the following commands define two POST26 variables:

NSOL,2,358,U,X
ESOL,3,219,47,EPEL,X

Variable 2 is the UX displacement at node 358, and variable 3 is the X component of elastic strain at node 47 of element 219. Any subsequent reference to these result items will be through the reference numbers assigned to them. Defining a new variable with the same number as an existing variable overwrites the existing variable.

See the NSOL and ESOL command descriptions for further information.

6.1.2 Storing Variables

When you define POST26 variables and specifications, you set up pointers to the appropriate data on the results file. Storing means reading the data from the results file into the database. The program stores data automatically when you issue display commands or POST26 data operation commands (including those listed below) or choose their equivalent GUI paths:

Table 6-1 Commands used to store variables

Command

GUI Menu Path

PLVAR

Main Menu>TimeHist Postpro>Graph Variables

PRVAR

Main Menu>TimeHist Postpro>List Variables

ADD

Main Menu>TimeHist Postpro>Math Operations>Add

DERIV

Main Menu>TimeHist Postpro>Math Operations>Derivative

QUOT

Main Menu>TimeHist Postpro>Math Operations>Divide

VGET

Main Menu>TimeHist Postpro>Table Operations>Variable to Par

VPUT

Main Menu>TimeHist Postpro>Table Operations>Parameter to Var

In some situations, you will need to explicitly request storage using the STORE command (Main Menu>TimeHist Postpro>Store Data). These situations are explained below in the command descriptions. If you use the STORE command after issuing the TIMERANGE command or NSTORE command (the GUI equivalent for both commands is Main Menu>TimeHist Postpro>Settings> Data), then the default is STORE,NEW. Otherwise, it is STORE,MERGE as listed in the command description below. This change in command default is required since the TIMERANGE and NSTORE commands redefine time (or frequency) points and time increment for data storage. You have the following options for storing data:

MERGE

Adds newly defined variables to previously stored variables for the time points stored in memory. That is, more columns of data are added to the database. This is useful if you wish to define and then store new variables after some variables have already been stored (default).

NEW

Replaces previously stored variables, erases previously calculated variables, and stores newly defined variables with current specifications.

APPEND

Appends data to previously stored variables. That is, if you think of each variable as a column of data, the APPEND option adds rows to each column. This is useful when you want to "concatenate" the same variable from two files, such as in a transient analysis with results on two separate files. Use the FILE command (Main Menu>TimeHist Postpro>Settings> File) to specify result file names.

ALLOC,N

Allocates space for N points (N rows) for a subsequent storage operation. Previously stored variables, if any, are zeroed. You normally do not need this option, because the program determines the number of points required automatically from the results file.

An example of using the STORE command follows:

/POST26
NSOL,2,23,U,Y	! Variable 2 = UY at node 23
SHELL,TOP	! Specify top of shell results
ESOL,3,20,23,S,X	! Variable 3 = top SX at node 23 of element 20
PRVAR,2,3	! Store and then print variables 2 and 3
SHELL,BOT	! Specify bottom of shell results
ESOL,4,20,23,S,X	! Variable 4 = bottom SX at node 23 of element 20
STORE	! By command default, place variable 4 in memory with 2 and 3
PLESOL,2,3,4	! Plot variables 2,3,4

For more information, see the NSOL, SHELL, ESOL, PRVAR, STORE, and PLESOL command descriptions in the ANSYS Commands Reference.

Notes

You can define up to 10 variables by default. Use the NUMVAR command (Main Menu>TimeHist Postpro>Settings>File) to increase the limit (maximum is 200).
By default, POST26 looks for one of the results files mentioned in Chapter 5. You can specify a different file name using the FILE command (Main Menu>TimeHist Postpro>Settings>File) (RST, RTH, RDSP, etc.).
By default, the force (or moment) values represent the total forces (sum of the static, damping, and inertial components). The FORCE command allows you to work with the individual components.
Results data for shell elements and layered elements are assumed to be at the top surface of the shell or layer. The SHELL command allows you to specify the top, middle or bottom surface. For layered elements, you may use the LAYERP26 command to indicate layer number.
Other commands useful when defining variables are:
- NSTORE (Main Menu>TimeHist Postpro>Settings>Data), which defines the number of time points or frequency points to be stored.
- TIMERANGE (Main Menu>TimeHist Postpro>Settings>Data), which defines the time or frequency range in which data are to be read.
- TVAR (Main Menu>TimeHist Postpro>Settings>Data.), which changes the meaning of variable 1 from time to cumulative iteration number.
- VARNAM (Main Menu>TimeHist Postpro>Settings>Graph or Main Menu>TimeHist Postpro>Settings>List), which assigns a name to a variable.
- RESET (Main Menu>TimeHist Postpro>Reset Postproc), which zeroes out all variables and resets all specifications to initial defaults.
Leaving POST26 with the FINISH command (Main Menu>Finish) erases POST26 variables and specification commands (such as FILE, PRTIME, NPRINT, etc.). Because these are not database items, you cannot save them. The commands, however, are saved on the log file.

6.2 Reviewing the Variables

Once the variables are defined, you can review them via graph plots or tabular listings.

6.2.1 Producing Graph Plots

The PLVAR command (Main Menu>TimeHist Postpro>Graph Variables) graphs up to nine variables on a single frame. The default abscissa (X-axis) is variable number 1: time for static or transient analysis, frequency for harmonic analysis. You can specify a different variable number (representing strain, deflection, etc.) for the abscissa using the XVAR command (Main Menu>TimeHist Postpro> Settings>Graph). Two sample graph plots are shown next.

Figure 6-1 POST26 plot using XVAR = 1 (time)

Figure 6-2 POST26 plot using XVAR 0 1

If the abscissa is something other than time, you can display a three-dimensional graph (with time or frequency as the Z dimension). To do so, change from the default X-Y view using one of the following:

Command(s):

/VIEW

GUI:

Utility Menu>PlotCtrls>Pan, Zoom, Rotate

Utility Menu>PlotCtrls>View Settings>Viewing Direction

This type of graph is also available for a nonlinear static or steady state thermal analysis in which substeps are identified as time.

When a variable contains complex data, consisting of real and imaginary parts, the PLVAR command plots the amplitude by default. You can switch to the phase angle, the real part, or the imaginary part by using the PLCPLX command (Main Menu>TimeHist Postpro>Settings>Graph).

Many graphics specifications are available for graph plots. You can perform tasks such as the following by choosing Utility Menu>PlotCtrls>Style>Graphs or via the commands shown below:

Activating a background grid (/GRID command)
Color-filling the area under a curve (/GROPT command)
Limiting the X and Y ranges (/XRANGE and /YRANGE commands)
Defining axis labels (/AXLAB command).
Using multiple Y-axis scales, with an option to project the axes and curves out of the plane (/GRTYP command)

6.2.2 Listing Results in Tabular Form

You can use the PRVAR command (Main Menu>TimeHist Postpro>List Variables) to list up to six variables in tabular form. This is useful if you want to find the value of a result item at a specific time or frequency. You can control the times (or frequencies) for which variables are to be printed. To do so, use one of the following:

Command(s):

NPRINT

PRTIME

GUI:

Main Menu>TimeHist Postpro>Settings>List

You can adjust the format of your listing somewhat with the LINES command (Main Menu>TimeHist Postpro>Settings>List) A sample PRVAR output is shown below.

Sample Output from PRVAR

          ***** ANSYS POST26 VARIABLE LISTING *****

    TIME         51 UX         30 UY      
                  UX            UY        
   .10000E-09     .000000E+00   .000000E+00
   .32000         .106832       .371753E-01
   .42667         .146785       .620728E-01
   .74667         .263833       .144850    
   .87333         .310339       .178505    
   1.0000         .356938       .212601    
   1.3493         .352122       .473230E-01
   1.6847         .349681      -.608717E-01

  

         POST26 SUMMARY OF VARIABLE EXTREME VALUES
 VARI TYPE    IDENTIFIERS   NAME     MINIMUM    AT TIME    MAXIMUM   AT TIME

   1 TIME     1 TIME      TIME       .1000E-09  .1000E-09  6.000      6.000    
   2 NSOL    51 UX        UX         .0000E+00  .1000E-09  .3569      1.000    
   3 NSOL    30 UY        UY        -.3701      6.000      .2126      1.000

When a complex variable consists of real and imaginary parts, the PRVAR command lists both the real and imaginary parts by default. You can work with just one component (real part, imaginary part, amplitude, or phase angle) using the PRCPLX command.

Another useful listing command is EXTREM (Main Menu>TimeHist Postpro>List Extremes), which prints the maximum and minimum Y-variable values within the active X and Y ranges. You can also assign these extreme values to parameters using the *GET command (Utility Menu>Parameters>Get Scalar Data). A sample EXTREM output is shown below.

Sample Output from EXTREM

           POST26 SUMMARY OF VARIABLE EXTREME VALUES
 VARI TYPE    IDENTIFIERS   NAME     MINIMUM    AT TIME    MAXIMUM   AT TIME

   1 TIME     1 TIME      TIME       .1000E-09  .1000E-09  6.000      6.000    
   2 NSOL    50 UX        UX         .0000E+00  .1000E-09  .4170      6.000    
   3 NSOL    30 UY        UY        -.3930      6.000      .2146      1.000

6.3 Additional POST26 Postprocessing

6.3.1 Performing Operations Among the Variables

POST26 performs mathematical operations among previously defined variables. For example, by defining a displacement variable in a transient analysis, you can calculate the velocity and acceleration by taking derivatives with respect to (w.r.t.) time:

NSOL,2,441,U,Y,UY441    ! Defines variable 2 as UY at node 441, name = UY441
DERIV,3,2,1,,VEL441     ! Variable 3 is derivative of variable 2 w.r.t. 
                        !  variable 1 (time), name = VEL441 (velocity)
DERIV,4,3,1,,ACCL441    ! Variable 4 is derivative of variable 3 w.r.t. 
                        !  variable 1 (time), name = ACCL441 (acceleration)

See the NSOL and DERIV command descriptions in the ANSYS Commands Reference for further information.

Another example would be to break a complex variable from a harmonic analysis (a+ib) into its real and imaginary parts (a and b) and then to calculate the amplitude and the phase angle (arc tangent (b/a)):

REALVAR,3,2,,,REAL2     ! Variable 3 is real part of variable 2, name = REAL2
IMAGIN,4,2,,,IMAG2      ! Variable 4 is imaginary part
PROD,5,3,3              ! Variable 5 is square of variable 3
PROD,6,4,4              ! Variable 6 is square of variable 4
ADD,5,5,6               ! Variable 5 (reused) is sum of variables 5 and 6
SQRT,6,5,,,AMPL2        ! Variable 6 (reused) is the amplitude
QUOT,5,3,4              ! Variable 5 (reused) is the variable (b/a)
ATAN,7,5,,,PHASE2       ! Variable 7 is the phase angle

(This could be saved as a macro for future use.)

For more information, see the REALVAR, IMAGIN, PROD, ADD, SQRT, QUOT, and ATAN command descriptions in the ANSYS Commands Reference.

Notes

You can also create your own POST26 variables using either of the following:
- The FILLDATA command (Main Menu>TimeHist Postpro>Table Operations>Fill Data); either fills data into a variable by a ramp function.
- The DATA command, which reads data from a file. This command has no GUI equivalent. The file to be read must contain the DATA command on the first line, a format statement in parentheses on the second line, and the data to be read on subsequent lines. You then read the file in with the /INPUT command (Utility Menu>File>Read Input from).
Another way to create your own POST26 variable is with the VPUT command, which allows you to move an array parameter into a variable. The reverse operation, VGET, moves a POST26 variable into an array parameter. For the GUI equivalents to these commands, see Section 6.1.2.

6.3.2 Generating a Response Spectrum

This feature allows you to generate a displacement, velocity, or acceleration response spectrum from a given displacement time-history. The response spectrum can then be specified in a spectrum analysis to calculate the overall response of a structure.

The RESP command in POST26 is used to generate the response spectrum, use either of the following:

Command(s):

RESP

GUI:

Main Menu>TimeHist Postpro>Generate Spectrm

RESP requires two previously defined variables: one containing frequency values for the response spectrum (field LFTAB) and the other containing the displacement time-history (field LDTAB). The frequency values in LFTAB represent not only the abscissa of the response spectrum curve, but also the frequencies of the one-degree-of-freedom oscillators used to generate the response spectrum. You can create the LFTAB variable using either the FILLDATA command or the DATA command.

The displacement time-history values in LDTAB usually result from a transient dynamic analysis of a single-DOF system. You can create the LDTAB variable using the DATA command (if the displacement time-history is on a file), the NSOL command (Main Menu>TimeHist Postpro>Define Variables). A numerical time-integration scheme is used to calculate the response spectrum.

Go to the beginning of this chapter

Command	GUI Menu Path
PLVAR	Main Menu>TimeHist Postpro>Graph Variables
PRVAR	Main Menu>TimeHist Postpro>List Variables
ADD	Main Menu>TimeHist Postpro>Math Operations>Add
DERIV	Main Menu>TimeHist Postpro>Math Operations>Derivative
QUOT	Main Menu>TimeHist Postpro>Math Operations>Divide
VGET	Main Menu>TimeHist Postpro>Table Operations>Variable to Par
VPUT	Main Menu>TimeHist Postpro>Table Operations>Parameter to Var