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[zhanghuicheng]

[zhanghuicheng]

 

 

 

 

 

 

 

Hello everyone, I am performing a modal analysis of a turbine blade disk/rotor using the Cyclic Symmetry method in ANSYS Mechanical. The model is a cyclic sector of a turbine blade structure. After setting up the cyclic symmetry boundary condition and running the modal analysis, I checked the results for different Harmonic Index / Nodal Diameter values. However, I found that the modal frequencies for the same mode order are almost exactly the same under different nodal diameters. For example, in the result table, several modes with different harmonic index values show identical or repeated frequencies. My understanding is that, in a cyclic symmetric structure, the natural frequencies should generally vary with different nodal diameters, especially for blade-disk type structures. Therefore, I am not sure whether this result is physically reasonable or caused by an incorrect setup. I would like to ask: Why are the modal frequencies identical for different nodal diameters in my cyclic symmetry modal analysis? Could this be caused by incorrect cyclic symmetry settings, such as sector angle, number of sectors, cyclic region definition, or mesh matching? Are there any specific settings in ANSYS Mechanical that I should check for cyclic modal analysis, such as harmonic index range, mode extraction method, or cyclic symmetry boundary pairing? How can I confirm whether the cyclic symmetry modal analysis has been set up correctly? I have attached a screenshot of my modal result table and deformation result. Any suggestions would be greatly appreciated. Thank you!

[ibrahim.karadeniz]

Hi Zhang,

Identical or repeated frequencies in a cyclic symmetry modal analysis are not necessarily wrong. In cyclic symmetric structures, repeated eigenfrequencies can occur naturally, especially because one nodal diameter can have two equivalent mode shapes that are phase-shifted relative to each other. So, seeing two modes with the same frequency is often normal.

However, if the same mode order shows almost exactly the same frequency for many different nodal diameters, then this should be checked carefully. For a blade-disk or bladed rotor, many mode families should usually show some dependency on nodal diameter.

Check this: Make sure the deformation is displayed across the entire rotor, not just in one sector. With different nodal diameters, the circumferential wave pattern should look different. If all the patterns look the same (at different Nodal Diameters), the cyclic symmetry analysis or setup is likely incorrect (as far as i know :))

I have a question myself: How exactly did you mesh your rotor? I also have a compressor rotor that I want to mesh using a structured mesh, but unfortunately I’m not having much success. Do you know which meshing method in Ansys Mechanical is best suited for this, and what I should keep in mind?