How to eliminate modes in Ansys modal analysis

• Mohamed_Wael
In summary: However, doing this manually is beyond the scope of this document.In summary, it is not possible to formulate a problem in ANSYS so that only the transverse modes are output. To do this would require intimate knowledge of the eigenvectors that need to be eliminated.
Mohamed_Wael
I am studying the modal analysis of a mechanism and I am only interested in the transverse modes, I wonder if there is a way to formulate my problem so that the results show only the transverse modes without the torsional and longitudinal ones, this would help in the post-processing a lot.

While I am not an ANSYS expert in any sense, I doubt that this will be possible as long as you work in ANSYS. One of the "features" of the finite element method is that it captures all types of deformation.

As an alternative, if you avoid ANSYS, and simply formulate your problem as a conventional eigneproblem, you can choose what coordinates you use in the formulation; these will be the only ones that appear in the final result. You can use an eigensolver in Mathmatica, Maple, etc to get your modal results.

The best way to do this [in FEA] is through boundary conditions. If you apply boundary supports that prevent motion in certain directions, you won't see any modes in that direction.

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It is certainly true that particular boundary conditions will exclude specific modes. To use this idea, however, requires an intimate knowledge before the fact of the eigenvectors that are to be eliminated. This is usually not practical.

Dr.D said:
It is certainly true that particular boundary conditions will exclude specific modes. To use this idea, however, requires an intimate knowledge before the fact of the eigenvectors that are to be eliminated. This is usually not practical.

I meant for excluding certain mode shapes in ANSYS. If you aren't interested in modes of a certain kind in an FEA model, one way to exclude them is to apply boundary conditions which don't allow motion in certain directions.

Take for example a square plate with mounting holes at the corners. By constraining the plate at the holes you will still get high-order modes which allow deflection in the middle of the plate between the mounting holes, which might not happen if the plate is bolted to a rigid structure. So to constrain these modes in the FEA model you might apply a boundary condition to the supported surface area rather than just the holes.

There is an option to write an APDL code for post processing. Perhaps the code could check the directional deformations of each mode and
determine modes of interest.

1. How do I define the modes to be eliminated in Ansys modal analysis?

To eliminate modes in Ansys modal analysis, you can use the "Modes" option under the "Analysis Options" tab. This allows you to specify the range of modes to be eliminated based on their frequencies or mode numbers. You can also use the "Exclude" option under the "Modes" tab to manually select the modes to be eliminated.

2. Can I eliminate multiple modes at once in Ansys modal analysis?

Yes, you can eliminate multiple modes at once in Ansys modal analysis by selecting the "Multiple" option under the "Modes" tab. This allows you to enter a list of mode numbers or frequencies to be eliminated.

3. What is the purpose of eliminating modes in Ansys modal analysis?

The main purpose of eliminating modes in Ansys modal analysis is to reduce the complexity of the model and improve the accuracy of the results. By eliminating unwanted modes, the analysis can focus on the important modes and provide more accurate results.

4. How do I know which modes to eliminate in Ansys modal analysis?

The decision to eliminate modes in Ansys modal analysis depends on the specific problem being analyzed. Generally, it is recommended to eliminate modes with low frequencies or modes that do not contribute significantly to the overall response of the system. You can also refer to the mode shapes to identify which modes are important for your analysis.

5. Can I re-run the analysis after eliminating modes in Ansys modal analysis?

Yes, you can re-run the analysis after eliminating modes in Ansys modal analysis. However, it is important to carefully consider which modes to eliminate and how it may affect the results. It is recommended to perform a sensitivity analysis to determine the impact of eliminating certain modes on the overall results.

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