Frequency Options

The Frequency dialog box lets you select the desired options for a frequency study.

Loads affect the modal characteristics of a body. For example, compressive loads decrease resonant frequencies and tensile loads increase them. This fact is easily demonstrated by changing the tension on a violin string. The higher the tension, the higher the frequency (tone). Loads are not used for frequency studies used for linear dynamic studies.

• Number of Frequencies. Lets you set the desired number of natural (resonant) frequencies to be calculated. The default is to calculate the lowest five frequencies. Rigid body modes are calculated by the FFEPlus solver. A body without any restraints has six rigid body modes. Rigid body modes have zero frequencies (infinite period).

If the working scenarios of the model include dynamic loads, it is important to calculate at least one frequency that is higher than the frequency of the load. In most situations, resonance is not desirable because it causes failure. However, some devices exploit resonance to trigger an event while providing measures to control the excessive associated deformation.

• Calculate frequencies closest to (frequency shift). Available for the Direst Sparse solver. Select this option to specify a frequency value of interest. The software calculates the frequencies closest to the specified value. This option is referred to as frequency shift in the literature. You can use this option to avoid calculating rigid body modes.

  In cases where the frequency analysis cannot run due to singularity of the stifness matrix, use the frequency shift option to overcome the singularity issue. Increase the shift value gradually from zero until the Direst Sparse solver succesfully calculates the requested frequencies.

  If you set a higher value for the frequency shift, the Direst Sparse solver selectively calculates the requested number of frequencies, which are clustered around the shift value. Thus, you can avoid the computation of lower range frequencies (including the rigid body modes), which are of no interest to your analysis, and save computational time.

• Upper Bound Frequency. Lets you set the upper bound frequency of the desired frequency range. Use the default value of zero unless you are not interested in natural frequencies higher than a certain value. Entering zero results in calculating the specified number of frequencies.

• Use soft spring to stabilize model. Select this option  to add soft springs to stabilize inadequately supported models.

For the FFEPlus solver, you can specify the Number of Frequencies or the Upper Bound Frequency. For the Direct Sparse solver, you can only specify the Number of Frequencies and specify a frequency shift.

Incompatible bonding options.

• Automatic. If the default surface-to-surface bonding contact slows down the solution considerably, the solver switches to node-to-surface bonding automatically. The automatic option is available for static, frequency, buckling, and linear dynamic studies.

• Simplified bonding contact. The program overrides the default surface-based bonding contact and resumes to node-based bonding contact. Check this option only in cases you run into performance issues when solving models with extensive contact surfaces.

• More accurate (slower). The program applies the default surface-based bonding contact, which results in longer solution time than the node-based contact formulation.

Bonding is most accurate when the mesh is compatible.

Solver. Lets you specify the solver to be used in calculating resonant frequencies and the associated mode shapes. To include the effect of loading on the resonant frequencies, select Automatic or Direct sparse.

• Automatic. The software selects the solver based on the study type, analysis options, contact conditions, etc.

• Direct sparse. Check this option to use the Mode Extraction Routine powered by the Direct Solver when running the study.

• FFEPlus. Check this option to use the FFEPlus solver when running the study.

Results folder. Lets you specify the directory to store the simulation results folder.

Related Topics

Remark

What is Frequency Analysis...