General Mechanical

General Mechanical

what is meaning of “harmonic index” in the cyclic symmetric modal analysis?

    • amir_aboodi


      I have done modal analysis on a bladed disk of a steam turbine with 96 blades. In that analysis, I just determined the number of mode shapes. Then Ansys listed the frequencies in order. But then I decided to use cyclic symmetry and modeled a sector with one blade that had 3.75 degrees. In the setting, in addition to a number of mode shapes, it has the number of "harmonic indices". After the calculation, it gives frequencies in each harmonic index. My question is what are these "harmonic indices" and what is the relation between "full model frequencies and mode shapes" with "the cyclic symmetric ones"?

      I really appreciate any comments.

      Thank you very much.

    • Vinayak Vipradas
      Ansys Employee

      Please refer to the following link to read about Harmonic Index and how it is used in Modal Cyclic Symmetry Analysis:
      4.1. Modal Cyclic Symmetry Analysis (

      Regards Vinayak
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    • amir_aboodi

      Thank you very much for your help. I have read that but still I can't understand the meaning of harmonic index. In that text, I recognized the relationship between nodal diameter and harmonic index but why in full model (360┬░) there is no request for determination of harmonic index. I have three questions:
      1. Is 'harminic index' equivalent to nodal circle? ( When nodes are positioned in a circular shape)
      2. How many harmonic index should be considered?
      3. What is the relationship between cyclic symmetric modeling and harmonic index?
      Thanks for any help.
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