This section examines the workflow for a Transient Structural analysis linked to an upstream Modal analysis. Topics include:
Create Analysis System
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Because this analysis is linked to (or based on) modal responses, a Modal analysis is a prerequisite. This linked setup allows the transient system to share resources such as engineering data, geometry, and boundary condition type definitions made in the Modal analysis.
Linking the Modal Solution cell to the Transient Setup cell configures the Transient analysis so that the application selects, by default, the (MSUP) Solution Method.
The Samcef and ABAQUS solvers do not support a linked Modal/Transient Structural analysis. For additional support requirements and notes regarding this analysis configuration, see the references at the bottom of this section.
Note:
For a Mode Superposition Transient Structural analysis, the Modal analysis can be pre-stressed by a Static Structural analysis.
When solving a linked MSUP transient system database from a version prior to the most current version of Mechanical, it is possible to encounter incompatibility of the file file.full created by the modal system. This incompatibility can cause the transient system’s solution to fail. In the event you experience this issue, use the feature and resolve the modal system.
Refer to the Obtain the Mode-Superposition Transient Solution section of the MAPDL Structural Analysis Guide for more information.
From the Toolbox, drag a Modal template to the Project Schematic. Then, drag a Transient Structural template directly onto the Solution cell of Modal template.
Note: You can create a modal environment in Transient Structural system that is already open in Mechanical by:
Selecting the option from the Analysis drop-down menu on the Home tab (or displayed tab).
Setting the Modal Environment property of the Modal (None) object to the Modal system. The Modal object is included under the Initial Conditions object.
Establish Analysis Settings
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For a Transient Structural system linked to a Modal system, the basic Analysis Settings include:
- Step Controls
This analysis is only compatible with constant time stepping. So, auto time stepping is turned off and will always be in read only mode. The user specified substep or time step value is applicable to all the load steps. All of the Step Controls settings applied to this analysis are not step aware. The time integration is turned on by default and will always be in read-only mode. A Time Step value that results in an integral number of sub steps over the load step must be selected.
- Options
You can turn on the following properties:
Include Residual Vectors. Set this property to to execute the RESVEC command and calculate residual vectors.
On Demand Expansion Option: If you set this property to manually or through the setting, the application does not create a result file. Your results are evaluated using the Modal solution data and otherwise calculated “on demand.” This property supports specific result types. See the property's description in the Options section for a listing.
Mode Selection Method: This property enables you to select the modes for the mode expansion. Property options include:
(default): This option expands all the extracted modes from the modal analysis. All the modes participate in the mode-superposition transient dynamic analysis.
: When you select this option, the Significance Threshold property also displays. For the Significance Threshold property value, the application selects the modes that have a ratio for the Modal Effective Mass to the Total Mass that is greater than this value, in all directions, for expansion. That is, only the selected modes participate in the mode-superposition transient dynamic analysis. The application default is
0.001. This feature improves the performance of postprocessing the modal results and reduces the file size.Note: The following loads do not support residual vector calculations:
Nodal Force
Remote Force scoped to a Remote Point (created via Model object)
Moment scoped to a Remote Point (created via Model object)
- Output Controls
These controls enable you to request Stress, Strain, Nodal Force, Reaction, Velocity and Acceleration results to be calculated. To expand reaction forces in the modal solution, set the Nodal Force property to or . The General Miscellaneous property needs to be set to in order to apply a Beam Tool and/or to calculate Beam Results. In addition, this setting is required to correctly produce twisted beam shapes. For better performance, you can also choose to have these results expanded from transient or modal solutions. The Contact Miscellaneous option is not available.
Note: Solver Controls, Restart Controls, Nonlinear Controls and Creep Controls are not applicable to the current analysis.
Define Initial Conditions
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The Transient Structural analysis must point to a Modal analysis in the Modal (Initial Conditions) object. This object also indicates whether the upstream Modal analysis is pre-stressed. If it is a pre-stress analysis, the name of the pre-stress analysis system is displayed in the Pre-Stress Environment field, otherwise the field indicates None. The Modal Analysis must extract all modes that may contribute to the dynamic response.
Note: Command (ADPL) objects can be inserted into Initial Conditions object to execute a restart of the solution process for the Modal Analysis.
Apply Boundary Conditions
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The following loads are allowed for the linked analysis:
Acceleration as a base excitation
Force (applied to a face, edge, or vertex)
Note:
For the Force and Pressure loading conditions, the default setting for the Applied By property is Direct.
When you have a supported topology (geometric or mesh entity) selected on your model, and you insert a Remote Force or a Moment, the application automatically creates (promotes) a corresponding Remote Point for the specified load and scopes the load to that Remote Point. This promotion feature is only available when none of the analyses in your project have been solved.
Support Limitations
Note the following limitations:
If the Reference Temperature is set as By Body and that temperature does not match the environment temperature, a thermally induced transient load will result (from the thermal strain assuming a nonzero thermal expansion coefficient). This thermal transient loading is ignored for Transient Structural Analysis using Linked Modal Analysis System.
Remote Force is not supported for vertex scoping.
During a linked MSUP Transient analysis, if a Remote Force or Moment scoped to an internal remote point is specified with the Behavior property set to , the boundary conditions cannot be scoped to the edges of line bodies such that all of their nodes in combination are collinear.
Remote Force and Moment applied to a rigid body is not supported.
Moment is not supported for vertex scoping on 3D solid bodies because a beam entity is created for the load application. The beam entity changes the stiffness of the structural component shared and solved by the preceding modal analysis.
Joint probes, Energy Probe, and Strain Energy results are not supported when expanded from a Modal solution.
The Force Reaction and Moment Reaction probes are not supported for a either a Harmonic Response analysis or a Transient structural analysis using the method Solution Method when 1) the Damping Ratio property is defined with a non-zero value and 2) if the Analysis Settings property Expand Result From is set to or .
Cyclic symmetry models are not supported for a Transient Structural Analysis that is using a linked Modal Analysis System.
Spring probe only supports Elastic force result when expanded from modal solution where as it supports both Elastic force and Elongation results when expanded from transient solution. The Elastic force results include the spring damping effect if the Reduced method is selected from Modal Solver controls, and Store Complex Solution is set to .
Standard Earth Gravity is not allowed in conjunction with the Acceleration load.
Elemental Triads results are not available on solid bodies.
When the Step Controls are defined by Substeps, the time step value sent to the solver is based on the settings for the first load step. For the load steps greater than one, you may notice an inconsistent value of the number of sub-steps in the Details View or the Worksheet View.
For the Samcef and ABAQUS solvers, Hydrostatic Pressure and Pipe Pressure are not supported.
Analysis Notes
Remote Force and Moment loading combined with the Rigid contact behavior is allowed when the loading is scoped through a Remote Point Application.
To obtain the most accurate results, it is recommended that you specify as the contact Type and set the contact Formulation to in the Details for the Contact Region. See the Contact Definition and Contact Advanced Category for more detailed information about these settings.
When the result is expanded from Modal Solution or when Reaction Object is scoped to a Contact Region, the Reaction Object requires both Nodal Forces and Calculate Reactions Output Controls settings to be turned . If they are not set, the error message "A result is invalid with current output control settings" displays. For other cases, the Reaction Object requires only the Calculate Reactions Output Controls setting to be turned .
The default value of Numerical Damping is different for full and mode-superposition transient structural analyses. So, the results comparison of a model must be done by matching the Numerical Damping value settings in the Damping Controls section.
File Management Options
When solving a Transient Structural analysis that includes an upstream linked Modal analysis, the application can reference the prerequisite files by specifying the full path of their location (refer to RESUME and MODDIR commands) instead of making copies. Referencing improves solution time and disk usage. The application will perform referencing once the following conditions and settings are performed:
You must perform the solution locally (called "In Process").
The On Demand Expansion Option property of the Options category of the Analysis Settings of the Modal analysis must be set to manually or through the setting.
Important: Make sure that Modal and the downstream Transient analysis request the same result data in the Output Controls. Otherwise, you may receive unexpected results.
Note: If you specify material damping, the Eqv. Damping Ratio From Modal property (for the Transient Structural analysis) must also be set to .
Limitations
Also note the following limitations for referencing. The Transient structural analysis cannot include:
Cyclic Symmetry
Line Pressure
Pressure or Force applied using the surface effect elements created on the top of the scoped geometry (Applied By property set to Surface Effect)
Remote Force or Moment not scoped to global remote points
Commands (APDL) object