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