Performing Frequency Analysis

To perform frequency analysis:

  1. Create a frequency study. Right-click the top icon in the Simulation study tree and select Properties. Define the Properties of the study to set the number of frequencies.
    The program calculates the lowest five modes by default. If the program detects rigid body modes, it lists them first. In this case, you need to increase the number of modes to let the program calculate the non-rigid body modes.
  2. Define material for each solid, shell, and beam. To define a material for a solid, shell, or beam, right-click its icon in the Simulation study tree and select Apply/Edit Material.
    Note that density is a required material property.
  3. Define appropriate restraints, if applicable.

    Both the FFEPlus and Direct Sparse solvers detect rigid body modes automatically for unrestrained or not adequately supported models. Rigid body modes, if detected, are counted among the requested number of modes.

    For example, if you are interested in the first five elastic modes for an unsupported solid model, you need to set the required modes to 11: six for the rigid body modes and five for the elastic modes.

    If you use the Direct Sparse solver, you must apply adequate restraints to stabilize your model, or activate the Use soft spring to stabilize model option. However, note that improper application of restraints can over-stiffen the model, and adversely affect the elastic modes.

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

    If you set a higher value for the frequency shift, the Direct 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.

  4. Define loads if desired.

    Loads are not required but are used if defined. The Direct Sparse solver is required to solve frequency problems with defined loads. Note that you can define thermal and fluid loading by right-clicking the study icon, selecting Properties, and clicking the Flow/Thermal Effects tab.

    For assemblies and multibody parts make sure to define the proper contact settings. Contact conditions other than free and bonded are not allowed in frequency studies.
  5. Mesh the model and run the study.

    Before running the study, you can use the Result Options to request generating plots for all mode shapes automatically.

    If you run a study before meshing it, the program meshes the study automatically before running it. You can also request to run the study by checking Run analysis after meshing in the Mesh PropertyManager.
  6. View the results:
    • Double-click an icon in a Results folder to display the associated plot.
    • Right-click the Results folder and select List Resonant Frequencies to list the requested number of resonant frequencies in addition to any rigid body modes.
    • Right-click the Results folder and select List Mass Participation to list the mass participation of the calculated modes.
    • To define a mode shape, right-click the Results folder and select Define Mode Shape/Amplitude Plot.
    • To animate an active plot, right-click its icon or right-click in the graphics area and select Animate.
    • To generate a report, right-click the Report folder and select Define.
    Click here for more result viewing options.
    When you run a study that does not have any result folders, the software creates the folders and plots specified in the result options for the study type. If result folders are present, the software updates the existing plots.