Set up a Modal Frequencies analysis

Overview video (4:54)

Workflow of a Modal Frequencies analysis

1. Open or create a model.

   Important: Avoid partial modeling using symmetry, since this method would limit the results to only symmetrical vibration modes.

2. Access the Simulation workspace.

3. From the Setup tab, select Study > New Simulation Study .

4. Set the Study Type to Modal Frequencies.

5. Specify the number of vibration modes to calculate.

   • Optionally, specify the Frequency Range for the calculated modes. Modes outside of the specified range are not output by the solver, even if the specified number of modes to calculate is not yet satisfied.

6. If you plan to preload the structure in a manner that affects the vibration results, activate Compute Preloaded Modes within the study Settings dialog.

7. Optionally, use the Simplify contextual environment to make simulation-specific changes to the model, preserving the production model:

   • Remove unnecessary features that only complicate the analysis and do not provide useful information. See Model Simplification (Defeaturing).
   • Split faces to confine loads or constraints to a only portion of a larger face.

8. Apply materials.

   • Simulation materials may differ from the materials defined in the Design workspace.

9. Optionally, apply constraints.

   Note: A unique characteristic of modal frequencies analyses is that you can successfully run a simulation with no constraints applied to the model. Similarly, you can solve an only partially constrained model. Generally, you should constrain the model to simulate, as closely as possible, the way that the structure is actually supported. However, there are situations when you need to see the free vibration modes and natural frequencies of an unconstrained part or assembly. Six rigid-body vibration modes occur for fully unconstrained models (three translational modes and three rotational modes). There is no deformation of the model for rigid-body modes, only movement. The seventh and subsequent modes involve deformation of the structure. For a partially constrained structure, there are fewer than six rigid-body modes. Of course, there are no rigid-body vibration modes for a fully constrained structure.

10. Optionally, apply loads.

    • Any load that produces tension or compression of a portion of the model affects the natural frequencies.
    • Tension increases the vibration frequencies, and compression lowers them.
    • Point masses affect the vibration frequencies and mode shapes.
    • Pure bending loads do not affect the results.
    Important: Be sure to activate Compute Preloaded Modes (Step 5) or the loads will be ignored by the solver. However, this option is not required for Point Mass loads.

11. Apply contact conditions.

    Important: Only Bonded contact and Offset Bonded contact is permitted for modal analyses. Modal results become indeterminate when parts of the structure can come in and out of contact or freely slide relative to each other.

12. It is a good idea to generate the mesh, though the model is meshed automatically when you solve the simulation. If you are not satisfied with the mesh appearance, adjust the size parameters or apply local refinement. Repeat this step until you are satisfied with the mesh.

13. Solve the simulation.

14. Review the modal results:

    • Natural frequencies
    • Mode shapes
    • Mass participation factors