Generating Frequency-Based Substructures

This section describes how to define individual frequency-based substructures.

Frequency-based substructures differ from conventional substructures (see Generating substructures) in that they use stiffness, inertial, damping, and frequency information to generate a single complex, condensed operator at every specified frequency point.

You can use frequency-based substructures only in direct steady-state dynamic analyses (see Direct-Solution Steady-State Dynamic Analysis).

For information on how substructures are used in a model, see Using Substructures.

This page discusses:

You define a frequency-based substructure using a combination of a direct steady-state dynamic analysis and a substructure generation procedure. The direct steady-state dynamic analysis coupled with retained external degrees of freedom generates the operators corresponding to the frequency-based substructure at the user-specified frequencies. The substructure generation procedure stores both the frequency-based substructure operators and the conventional substructure operators on the merged substructure database. One or the other set of operators is used depending on the analysis type and substructure property settings.

All general behaviors related to substructure generation and usage apply to frequency-based substructures. This section discusses details that are pertinent to frequency-based substructure generation.

Frequency-based substructures work in direct steady-state dynamic analyses in the same way that conventional substructures work in static analyses. Conventional substructures provide exact (up to round-off error) representation of the model in static analyses, while frequency-based substructures provide exact representation of the model in direct steady-state dynamic analyses. When the frequency-based substructure is used at the same frequencies as those used for generation, the dynamic response is exact (up to round-off error). The response at frequencies that do not match those used for generation can be approximate (see Using Substructures).

To define frequency-based substructures, do the following:

  1. Invoke the direct steady-state dynamic procedure.

    1. Define the frequency points at which to generate a frequency-based substructure.

    2. Define the nodes and degrees of freedom to retain as external degrees of freedom when using the frequency-based substructure.

  2. Invoke the substructure generation procedure.

    1. Define the nodes and degrees of freedom to retain as external degrees of freedom when using the frequency-based substructure. These degrees of freedom must be the same as those specified in the direct steady-state dynamic procedure.

    2. Define the rest of the substructure generation procedure (see Generating substructures).

You can define frequency-based substructures using a single analysis or using the restart capability. Because the substructure generation procedure runs only in shared memory parallel (SMP) mode, the performance can be limited when the direct steady-state dynamic analysis and substructure generation procedure are run in a single analysis. You can use the restart capability to run two separate jobs and allow the direct steady-state dynamic analysis to run in distributed memory parallel (DMP) mode to reduce the computational expense, particularly for large models.

 

See Also
Using Substructures
*RETAINED NODAL DOFS
*STEADY STATE DYNAMICS
*SELECT EIGENMODES
*RESTART

Generating Frequency-Based Substructures in a Single Analysis

You define the direct steady-state dynamic analysis as described in Direct-Solution Steady-State Dynamic Analysis. You specify the frequencies at which to generate the frequency-based substructure. You define the retained degrees of freedom as described in Defining the Retained Nodal Degrees of Freedom. You should limit the number of retained degrees of freedom that you specify based on the size of the model because every retained degree of freedom is associated with an internally created right-hand-side vector. Specifying many retained degrees of freedom can affect the computational time and memory consumption in the direct steady-state dynamic analysis.

By default, all previously applied boundary conditions are removed in a direct steady-state dynamic analysis coupled with retained degrees of freedom. If you want to retain the boundary conditions from a previous loading step, you must respecify those boundary conditions in the direct steady-state dynamic analysis. You must ensure that no boundary conditions are specified on the retained degrees of freedom.

When you define a direct steady-state dynamic analysis coupled with retained degrees of freedom, no additional loads or load cases are allowed.

The direct steady-state dynamic analysis coupled with retained degrees of freedom writes the generated frequency-based substructure operators in the jobname_FBS.sim file in the output directory. The substructure generation procedure that immediately follows the direct steady-state dynamic analysis stores the frequency-based substructure operators and the conventional substructure operators in the jobname_Zn.sim file written by the substructure generation procedure (see Generating substructures).

Generating Frequency-Based Substructures Using the Restart Capability

You can run the direct steady-state dynamic analysis coupled with retained degrees of freedom and the conventional substructure generation procedure in separate jobs using the Abaqus restart capability (see Restarting an Analysis). The first job includes the direct steady-state dynamic analysis coupled with retained degrees of freedom, as described in Generating Frequency-Based Substructures in a Single Analysis. In this job, you can apply preloads to the substructure before the direct steady-state dynamic step. This job can run in DMP mode. This job writes the frequency-based substructure operators to the job1name_FBS.sim file.

The substructure generation procedure is then run in a restart job that creates a merged substructure database that includes the frequency-based substructure operators generated in the previous job. Optionally, a frequency step to obtain eigenmodes to add to the conventional substructure can precede the substructure generation procedure. The restart job runs only in SMP mode. The restart job writes the frequency-based substructure operators and the conventional substructure operators together in the job2name_Zn.sim file.

The generated substructure includes both the frequency-based substructure and the conventional substructure and is used in a similar way as the conventional substructure (see Using substructures).

Example: Generating Frequency-Based Substructures Using the Restart Capability

This example shows how to use the restart capability to generate frequency-based substructures. The first job includes the direct steady-state dynamic analysis coupled with retained degrees of freedom.

RESTART, WRITE
STEP
STEADY STATE DYNAMICS, DIRECT
RETAINED NODAL DOFS
…
END STEP

The second job includes the substructure generation procedure.

RESTART, READ
STEP
SUBSTRUCTURE GENERATE, TYPE=Zn
RETAINED NODAL DOFS
…
END STEP

Limitations

  • Frequency-based substructures do not support substructure loads.

  • The boundary conditions in the direct steady-state dynamic procedure coupled with retained degrees of freedom are internally assumed to be defined using OP=NEW.

  • The selected eigenmodes improve the quality only of the conventional substructure and not the frequency-based substructure.