Time-history operations deal with variables, 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, or the magnetic flux in an element, for example.
You assign unique identifiers to each of the variables. Up to 200 such variables can be defined.
TIME is reserved for the time value. FREQ is reserved for the frequency value. All other identifiers must be unique, and can be made up of 32 letters and characters.
If you do not provide a unique identifier, the program assigns one.
In addition to the unique identifiers, the program uses numerical indices (reference numbers) to track and manipulate the variables. These numbers can be used interchangeably with the identifiers at the command level, and in some interactive operations. The numerical index is displayed, along with any name you choose in the data properties dialog box.
Defining variables is actually a two-part operation consisting of defining and storing:
Define a variable according to the result item in the results file. To do so, set up pointers to the result item and create labels for the areas where that data will be stored.
You can define up to ten variables by default. The maximum number can be increased up to 200 (NUMVAR).
TIME (time) or FREQ (frequency) is always variable 1.
Example 8.1: Defining Time-History Variables Two, Three and Four
NSOL,2,358,U,X,UX_at_node_358 ESOL,3,219,47,EPEL,X,Elastic_Strain ANSOL,4,101,S,X,Avtg_Stress_101
Variable two is a nodal result defined by the NSOL command. It is the UX displacement at node 358.
Variable three is an element result defined by the ESOL command. It is the X component of elastic strain at node 47 for element 219.
Variable four is an averaged element nodal result defined via the ANSOL command. It is the X-component of averaged element nodal stress at node 101.
Any subsequent reference to these results items occurs via either the reference numbers or the labels assigned to them.
Defining a new variable with the same number as an existing variable overwrites the existing variable.
The following commands are available for defining variables:
ANSOL -- Specifies averaged element nodal data to be stored from the results file in the solution coordinate system.
ESOL -- Specifies element data to be stored from the results file.
NSOL -- Specifies nodal data to be stored from the results file.
RFORCE -- Specifies the total reaction force data to be stored.
SOLU -- Specifies solution summary data per substep to be stored.
These commands define the location of the results:
Other useful commands for defining variables are:
NSTORE - Specifies the number of time points or frequency points to be stored.
TIMERANGE - Specifies the time or frequency range in which data are to be stored.
TVAR - Changes the meaning of default variable 1 TIME to a cumulative iteration number.
VARNAM - Assigns a name (up to 32 character) to a variable.
RESET - Removes all variables and resets all specifications to initial defaults.
The force (or moment) values by default represent the total forces (sum of the static, damping, and inertial components). The FORCE command allows you to work with the individual components. (The FORCE command affects only the output of element nodal forces.)
Results data for shell elements and layered elements are by default 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, use the LAYERP26 and SHELL commands to indicate layer number and surface location, respectively.
Issue the STORE command to store a defined variable. Storing means reading the data from the results file into the database.
The program also stores data automatically when you issue display commands (PLVAR and PRVAR) or time-history data operation commands (ADD, QUOT, etc.).
Example 8.2: Using the STORE Command
/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 PLVAR,2,3,4 ! Plot variables 2,3,4
In some situations, it is necessary to explicitly request storage. If you issue the STORE command after issuing TIMERANGE or NSTORE, the default is STORE,NEW. Otherwise, it is STORE,MERGE as listed in the command descriptions below.
This change in the STORE command default is required because the TIMERANGE and NSTORE commands redefine time (or frequency) points and time increment for data storage.
The following STORE command options are available for storing variable data:
MERGE -- Adds newly defined variables to previously stored variables for the time points stored in memory, useful if you wish to store data using one specification (FORCE, SHELL, LAYERP26) and store data using another specification, as shown in the prior example.
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. Issue the FILE command to specify results file names.
ALLOC,
N-- Allocates space for N points (N rows) for a subsequent storage operation. Previously stored variables, if any, are cleared. Typically, this option is unnecessary because the program determines the number of points required automatically from the results file.