16.1.11.3. Modal Acoustics and Modal Analysis Options

For Modal Acoustics and Modal analyses, the Options group includes the following properties:

Max Modes to Find

Specifies the number of natural frequencies to solve for in a modal analysis. The default value is 6. Set the property to zero (0) to specify a value of All (find all frequencies). In order to specify this value, you must set the Limit Search to Range property to Yes and then set the Solver Type explicitly to either Direct, Unsymmetric, or Subspace or when the Program Controlled option selects one of these solvers.

Limit Search Range

Allows you to specify a frequency range within which to find the natural frequencies. The default is set to No. If you set this to Yes, you can enter a minimum and maximum frequency value. If you specify a range the solver will strive to extract as many frequencies as possible within the specified range subject to a maximum specified in the Max Modes to Find field.

On Demand Expansion Option

Options for this property include Program Controlled (default), Yes, and No. When set to Program Controlled, an additional read-only property, On Demand Expansion, displays. This property only displays for the Program Controlled option and the application determines the value for the property, either Yes or No.

When the On Demand Expansion Option property is set to Yes, either manually or from the Program Controlled setting, the application creates the result file optimally. That is, the application evaluates the results using the Modal solution data and calculates any other results “on demand.” This improves solution performance and reduces file size.

Important:  Once the analysis is solved, if you make changes and those changes do not clear the current solution, the application does not reevaluate the setting of the property for the Program Controlled option (Yes or No). The previous setting is used.

If the application detects any of the following conditions, it automatically sets the On Demand Expansion property to No:

  • The system is a standalone modal analysis (no downstream systems).

  • You specify any Output Controls.

  • A downstream system does not support an On Demand Expansion Option property setting of Yes.

  • A downstream system has the On Demand Expansion Option set to either No or Program Controlled (No).

  • Damping is present in the analysis.

  • Symmetry present in the analysis.

When the On Demand Expansion Option property is set to Yes, either manually or from the Program Controlled setting, the application automatically changes the settings of the following properties to improve solution performance and reduces file size.

  • The Store Modal Results property is set to No to reduce mode file size

  • Solution files are not recombined when the Distributed solution option is active.

  • Expansion Settings are suppressed because on demand expansion is done "on the fly" to plot displacement results on Condensed Parts.

Note:

  • The application automatically removes mode shapes from the result files. You can change the setting of this process using the On Demand Mode Shapes preference, also contained under the Options (Modal, Harmonic and Transient Mode Superposition) category of the Analysis Settings and Solution category of the Options dialog.

  • You can change the default setting (Program Controlled) of this property using the On Demand Expansion Option preference contained in the Options (Modal, Harmonic and Transient Mode Superposition) category of the Analysis Settings and Solution category of the Options dialog.

In addition, when the On Demand Expansion Option property is set to Yes, either manually or from the Program Controlled setting, the property automatically modifies the following properties in any linked downstream Harmonic Response or Transient structural analysis to improve its performance.

  • On Demand Expansion is set to Yes.

  • Expansion Results From is set to Harmonic/Transient.

  • Expansion Settings are suppressed.

Also note that if you solve the analysis when the On Demand Expansion Option property is set to Yes, either manually or from the Program Controlled setting, and you change the setting of the property to No manually or as a result of the application setting the On Demand Expansion property to No because you change a property or a required condition, the application automatically clears the solution data.

Furthermore, there are specific result types (contour, frequency response, and probes) supported when this option is active:

  • Total and Directional Deformation

  • Maximum, Middle and Minimum Principal Elastic Strain

  • Maximum Shear Elastic Strain

  • Elastic Strain Intensity

  • Normal Elastic Strain/Shear Elastic Strain

  • Equivalent Stress

  • Maximum, Middle and Minimum Principal Stress

  • Maximum Shear Stress

  • Stress Intensity

  • Normal Stress/Shear Stress

Important:

  • Response Spectrum, Random Vibration, and Structural Optimization analyses cannot be linked to an upstream Modal analysis that has the On Demand Expansion property set to Yes, either manually or from the Program Controlled setting.

  • Stress and strain cannot be evaluated on Condensed Parts.

  • Spring annotations do not update when viewing results.

  • You can change the default setting of this property under the Options (Modal, Harmonic and Transient Mode Superposition) category of the Analysis Settings and Solution group in the Options dialog.

Spin Softening

Enables you to specify whether to include or exclude Spin Softening effect in the linear perturbation analyses. This field is available when performing a Pre-Stressed Modal analysis. The options include Program Controlled (default), Yes, and No. The Program Controlled setting selects either to include or exclude Spin Softening in the solution based on whether the rotating reference frame attached to the body is fixed (Yes) or in motion (No). The Yes option is invalid if the Coriolis Effect property (Rotordynamics Controls) is set to On, Stationary Reference Frame.

For more information, refer to the PERTURB command from MAPDL Command Reference Guide.