GUI Path Command
Option Dynamics Options
Main Menu> Solution> Load Step Opts> Time/Fre-quenc> Time Integration
TINTP Transient Integration Paramet-ers
Main Menu> Solution> Define Loads> Apply> Load Vector> For Mode Super
LVSCALE Load Vector
Main Menu> Solution> Load Step Opts> Time/Fre-quenc> Damping
Main Menu> Solution> Load Step Opts> Other>
Change Mat Props> Material Models> Structural>
Damping
Section 5.5: Performing a Mode Superposition Transient Dynamic Analysis
GUI Path Command
Option
Main Menu> Solution> Load Step Opts> Time/Fre-quenc> Time-Time Step
DELTIM Integration Time Step
Output Control Options
Main Menu> Solution> Load Step Opts> Output Ctrls>
Solu Printout OUTPR
Printed Output
• Dynamics options include the following:
– Transient Integration Parameters (TINTP)
Transient integration parameters control the nature of the Newmark time integration technique.
The default is to use the constant average acceleration scheme; see your ANSYS, Inc. Theory Refer-ence for further details.
– Load Vector (LVSCALE)
The load vector option allows you to apply a load vector created by the modal solution as one of the loads. You can use such a load vector to apply element loads (pressures, temperatures, and so on) on the model.
– Damping
Damping in some form is present in most structures and should be included in your analysis. You can specify five forms of damping in a mode superposition transient dynamic analysis:
→ Alpha (mass) damping (ALPHAD)
→ Beta (stiffness) damping (BETAD)
→ Constant damping ratio (DMPRAT)
→ Material-dependent beta damping (MP,DAMP)
→ Modal damping (MDAMP)
Remember that, as described earlier in Section 5.5.2: Obtain the Modal Solution, any damping that you specify in the mode superposition transient analysis is ignored if you used the QR damped mode-extraction method. Constant material damping coefficient (MP,DMPR) is not applicable in transient analysis. See Section 5.10.3: Damping for further details.
• The only valid general option for the first load step is integration time step (DELTIM), which is assumed to be constant throughout the transient. By default, the integration time step is assumed to be 1/(20f), where f is the highest frequency chosen for the solution. The DELTIM command is valid only in the first load step and is ignored in subsequent load steps.
Note — If you do issue the TIME command in the first load step, it will be ignored. The first solution is always a static solution at TIME = 0.
• The output control option for the first load step is printed output (OUTPR). Use this option to control printout of the displacement solution at the master DOF.
6. Write the first load step to a load step file (Jobname.S01) by issuing the LSWRITE command.
Command(s): LSWRITE
GUI: Main Menu> Solution> Load Step Opts> Write LS File
7. Specify loads and load step options for the transient loading portion, writing each load step to a load step file (LSWRITE).
• General options include the following:
– Time Option (TIME)
This option specifies time at the end of the load step.
– Load Vector (LVSCALE)
The load vector option allows you to apply a load vector created by the modal solution as one of the loads.
– Stepped or Ramped Loads (KBC)
This option indicates whether to ramp the load change over the load step (KBC) or to step-apply the load change (KBC,1). The default is ramped.
• Output control options include the following:
– Printed Output (OUTPR)
Use this option to control printed output.
– Database and Results File Output (OUTRES)
This option controls the data on the reduced displacement file.
The only valid label on these commands is NSOL (nodal solution). The default for OUTRES is to write the solution for every fourth time-point to the reduced displacement file (unless there are gap con-ditions defined, in which case the default is to write every solution).
8. If you used either the Block Lanczos (default) or subspace option for the modal analysis (MODOPT,LANB or MODOPT,SUBSP), you may use a nodal component with the OUTRES,NSOL command to limit the displacement data written to the reduced displacement file Jobname.RDSP. The expansion pass will only produce valid results for those nodes and for those elements in which all of the nodes of the elements have been written to the .RDSP file. To use this option, first suppress all writing by invoking
OUTRES,NSOL,NONE, then specify the item(s) of interest by invoking OUTRES,NSOL,FREQ,COMP. Repeat the OUTRES command for any additional nodal components that you want to write to the .RDSP file.
Only one output frequency is allowed - ANSYS uses the last frequency specified by OUTRES.
9. Save a backup copy of the database to a named file.
Command(s): SAVE
GUI: Utility Menu> File> Save as 10. Start the transient solution.
Command(s): LSSOLVE
GUI: Main Menu> Solution> Solve> From LS Files 11. Leave SOLUTION.
Command(s): FINISH
GUI: Close the Solution menu.
Section 5.5: Performing a Mode Superposition Transient Dynamic Analysis
The mode superposition transient solution is written to the reduced displacement file, Jobname.RDSP, regardless of whether the subspace, Block Lanczos, reduced, PowerDynamics, or QR damped method was used for the modal solution. You will therefore need to expand the solution if you are interested in stress results.
5.5.4. Expand the Mode Superposition Solution
The procedure for the expansion pass is the same as described for the reduced method (see Section 5.6.3: Expand the Solution (Expansion Pass)). Jobname.TRI is needed only if the reduced method was used for the modal solution. The output from the expansion pass includes the structural results file, Jobname.RST, containing ex-panded results.
5.5.5. Review the Results
Results consist of displacements, stresses, and reaction forces at each time-point for which the solution was ex-panded. You can review these results using POST26 or POST1, as explained for the full method (see Section 5.4.11:
Review the Results).
Note — In the mode superposition or reduced methods, only static force is available with the FORCE command.
5.5.6. Sample Input for a Mode Superposition Transient Dynamic Analysis
A sample input listing for a mode superposition transient analysis is shown below:
! Build the Model
! Obtain the Modal Solution
/SOLU ! Enter SOLUTION
! Obtain the Mode Superposition Transient Solution /SOLU ! Reenter SOLUTION
ANTYPE,TRANS ! Transient analysis
TRNOPT,MSUP,... ! Mode superposition method LVSCALE,... ! Scale factor for element loads
---! Loads, etc. for 2nd load step
TIME,... ! Time at end of second load step
LSSOLVE ! Initiate multiple load step solution FINISH
! Review results of the mode superposition solution /POST26 ! Enter POST26
! Expand the Solution
/SOLU ! Reenter SOLUTION
! Review the Results of the Expanded Solution /POST1
---! Other postprocessing as desired
---FINISH
See the ANSYS Commands Reference for discussions of the ANTYPE, MODOPT, M, TOTAL, ACEL, TRNOPT, LVS-CALE, MDAMP, DELTIM, TIME, KBC, OUTRES, LSSOLVE, FILE, SOLU, NSOL, PLVAR, PRVAR, EXPASS, NUMEXP, OUTRES, PLDISP, PRRSOL, PLNSOL, and PRERR commands.
5.6. Performing a Reduced Transient Dynamic Analysis
The reduced method, as its name implies, uses reduced matrices to calculate the dynamic response. It is available in the ANSYS Multiphysics, ANSYS Mechanical, and ANSYS Structural products. You should consider using this method if you do not want to include nonlinearities (other than simple node-to-node contact) in the analysis.
The procedure for a reduced transient dynamic analysis consists of these main steps:
1. Build the model.
2. Obtain the reduced solution.
3. Review the results of the reduced solution.
4. Expand the solution (expansion pass).
5. Review the results of the expanded solution.
Of these, the first step is the same as for the full method, except that no nonlinearities are allowed (other than simple node-to-node contact, which is specified in the form of a gap condition instead of an element type). Details of the other steps are explained below.
5.6.1. Obtain the Reduced Solution
By reduced solution, we mean the degree of freedom solution calculated at the master DOF. The tasks required to obtain the reduced solution are explained in the following sections. For the following tasks, you need to first enter the SOLUTION processor.
Command(s): /SOLU
Section 5.6: Performing a Reduced Transient Dynamic Analysis
GUI: Main Menu> Solution
5.6.1.1. Define the Analysis Type and Options
These are the same as the analysis options that are described for the full method (in Section 5.4.3: Set Solution Controls and Section 5.4.4: Set Additional Solution Options) except for the following differences:
• You cannot use the Solution Controls dialog box to define analysis type and analysis options for a reduced transient dynamic analysis. Instead, you must set them using the standard set of ANSYS solution commands (which are listed in Section 5.4.3: Set Solution Controls and Section 5.4.4: Set Additional Solution Options) and the standard corresponding menu paths.
• Restarts are not available (ANTYPE).
• Choose the reduced method of solution (TRNOPT).
• When you specify a reduced transient analysis, a Solution menu that is appropriate for that specific type of analysis appears. The Solution menu will be either “abridged” or “unabridged,” depending on the actions you took prior to this step in your ANSYS session. The abridged menu contains only those solution options that are valid and/or recommended for reduced transient analyses. If you are on the abridged Solution menu and you want to access other solution options (that is, solution options that are valid for you to use, but their use may not be encouraged for this type of analysis), select the Unabridged Menu option from the Solution menu. For details, see Section 3.11.1: Using Abridged Solution Menus in the ANSYS Basic Analysis Guide.
• Nonlinear options (NLGEOM, SSTIF, NROPT) are not available.
5.6.1.2. Define Master Degrees of Freedom
Master DOF are essential degrees of freedom that characterize the dynamic behavior of the structure. For a reduced transient dynamic analysis, master DOF are also required at locations where you want to define gap conditions, forces, or nonzero displacements. You can list the defined master DOF or delete master DOF as well. See Sec-tion 3.14: Matrix ReducSec-tion for guidelines to choose master DOF.
Command(s): M, MGEN, TOTAL, MLIST, MDELE
GUI: Main Menu> Solution> Master DOFs> User Selected> Define Main Menu> Solution> Master DOFs> User Selected> Copy Main Menu> Solution> Master DOFs> Program Selected Main Menu> Solution> Master DOFs> User Selected> List All Main Menu> Solution> Master DOFs> User Selected> Delete
5.6.1.3. Define Gap Conditions
Define any gap conditions.
Command(s): GP
GUI: Main Menu> Solution> Dynamic Gap Cond> Define You can also list the defined gaps and delete gaps.
Command(s): GPLIST, GPDELE
GUI: Main Menu> Solution> Dynamic Gap Cond> List All Main Menu> Solution> Dynamic Gap Cond> Delete
5.6.1.3.1. Gap Conditions
Gap conditions can only be defined between two master degree of freedom (DOF) nodes or between master DOF nodes and ground, as shown in the following figure.