How to Define the Number of Frequencies/Modes to be Solved
In the Analysis Parameters dialog (right-click Analysis Type in the tree view and choose Edit Analysis Parameters or use Setup 
Model Setup Parameters).
- Specify the number of frequencies/modes to solve for in the Number of modes to calculate field.
- The Cut-off frequency field is used to terminate computations if all eigenvalues below the specified frequency have been found. To avoid having such a limit, enter a value of zero. Note that this option is not available when the Type of Solver is set to AMG-Subspace under the Solution tab.
- Activate the Rigid body modes are expected option if the model is not statically constrained and you wish to solve for rigid body vibration modes.
Controlling the Data in the Text Output Files
The following input and intermediate calculations can be output to the summary text file. Whether these options are activated or not will not affect the results that are available in the Results environment.
- Equation numbers data: Output the equation number for each node and degree of freedom.
- Element stiffnesses: Output the stiffness matrix in terms of element coordinates.
- Global stiffnesses: Output the stiffness matrix in terms of global coordinates.
- Intermediate modal results:
Advanced Settings
In the Other tab of the Analysis Parameters dialog, you can choose between two mass representations in the Mass representation drop-down box, Lumped and Consistent. Lumped will place the mass of the element at the nodes; consistent will distribute the mass of the element throughout the volume. It should be noted that the computation of the effective load vector at each solution step is considerably longer when the consistent mass option is requested. The extra effort involved in a consistent (as opposed to the lumped) mass analysis may not be justified in many situations.
The Ratio of allocated memory space for data storage field can be used to specify the ratio of reserved RAM memory storage to total available RAM memory in arrays internal to processor. The analysis may request a large value for this entry upon execution.
Solution Options
Use the Solution tab to set the type of solver to use. (See also Types of Solvers Available for background information.)
The Type of solver has the following options available:
- Automatic: If this option is selected, the processor chooses the type of solver to use based on the size of the model and the number of frequencies to be found. For smaller models or for solutions of many eigenvalues, the BCSLIB-EXT sparse solver is used. Conversely if the model is larger and you request a relatively small number of frequencies, then the Subspace-AMG solver is used. The equation for determining which solver to use is:
If #DOF / #Freq > 10,000, then use the Subspace-AMG solver. Otherwise, use the BCSLIB-EXT sparse solver.
Where:
- #DOF is the number of degrees of freedom (equations)
- #Freq is the requested number of frequencies to be found
Hence, for the default number of frequencies (5), models with greater than 500,000 degrees of freedom will use the Subspace-AMG solver.
- BCSLIB-EXT (Boeing sparse solver, supported on Windows and Linux): This option forces the usage of the Boeing sparse solver regardless of model size or the requested number of modes.
For Windows only, note that the BCSLIB-EXT solver may write temporary files to the folder specified by the environment variable USERPROFILE. By default, this variable is set to the folder %SYSTEMDRIVE%\Users\Username where %SYSTEMDRIVE% is the drive on which the operating system is installed, typically C:. The error numbers -701 or -804 returned from the BCSLIB-EXT solver means that it ran out of hard disk space for storing the temporary files. If this occurs, change the USERPROFILE variable to a directory that can provide sufficient hard disk space. (See the Windows Help and Support for documentation on changing environment variables.)
- The Subspace-AMG option forces the usage of this solver, regardless of model size or the requested number of modes.
Solver memory allocation: This field controls the amount of memory to use during the sparse matrix solution for the BCSLIB-EXT solver. In general, allocating more memory should result in a faster analysis. The default value is 100%. This setting is grayed out when the Subspace-AMG solver is selected.
As listed above, both solvers take advantage of multiple threads/cores when available on the computer. The drop-down Number of threads/cores menu controls how many threads/cores are used. Use all of the threads/cores available for the fastest solution. Alternatively, choose to use fewer threads/cores in reserve if you need some computing power to run other applications at the same time as the analysis.
Subspace Iteration Section
The Subspace Iteration section is only applicable to, and is only available if you are using, the Subspace-AMG solver.
- The Maximum number of iterations field controls the number of iterations allowed in the attempt to reach the convergence tolerance. For the default value of zero, the solver will limit the number of iterations to 100.
- Number of vectors used in solution: The number of vectors to be used in the analysis can be specified in this field. For the default value of zero, the processor will use the
minimum
of the following two choices for the number of vectors:
- Number of frequencies requested + 8, or
- 2 * number of frequencies requested.
If you specify a positive, non-zero number of vectors, the processor will use the maximum of the following two choices for the number of vectors:
- The requested number of vectors, or
- The number of frequencies requested.
- Frequency shift: This input is not supported currently and has no affect on the solution.
- The option selected in the Accuracy of last frequency drop-down list is used to trade off solution speed versus solution accuracy for the higher modal frequency results. The choices are:
- Fair (fast) - Use this setting for the quickest solution, when you are not as concerned about the accuracy of the higher frequencies as compared to the first few vibration modes. This is the default setting.
- Good - This setting balances performance and accuracy of the higher frequencies.
- Better (slowest) - This setting provides the best accuracy of the higher frequency results but also requires the greatest solution time.