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 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:
Invoke the direct steady-state dynamic procedure.
Define the frequency points at which to generate a frequency-based
substructure.
Define the nodes and degrees of freedom to retain as external degrees of freedom when
using the frequency-based substructure.
Invoke the substructure generation procedure.
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.
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.
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).
Input File Usage
Use the following options to define a frequency-based substructure using a direct
steady-state dynamic analysis coupled with retained degrees of freedom and a
substructure generation step with the same retained degrees of freedom in a
single analysis:
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 1: 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.
Example 2: Generating Frequency-Based Substructures Using the Restart Capability
You can also generate frequency-based substructures using a three-job workflow.
The first job (job1name) specifies the model
definition and frequency step and runs in SMP mode.
The second job (job2name) restarts from the
first job, consists of a direct steady-state dynamic step with retained
degrees of freedom, and runs in DMP mode. The second job writes the
frequency-based substructure operators to the
job1name_FBS.sim
(the name of the first job is used for file output).
The third job (job3name) consists of a
substructure generation step with the same retained degrees of freedom
as those specified in the direct steady-state dynamic step in the second
job and runs in SMP mode.
The example below demonstrates this three-job workflow.
The first job includes the model definition and the frequency step.
The third job includes the substructure generation step with the same retained degrees of
freedom as in the direct steady-state dynamic step in the second job.
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.