General Mechanical

General Mechanical

Modal analysis-low frequencies

    • cpgeagadir
      Subscriber

      Hello community,

      I wanted to do a MSUP transient analysis with a frequency range between 0 and 50 HZ, but the problem that in the modal analysis I find almost zero frequencies (below 0.2 Hz). While to do MSUP analysis I have to choose the frequency lower than 1.5 times that obtained by the modal analysis. Is this correct. If not someone correct me because I am lost on this point.

      Thanks in advance

    • Vinayak Vipradas
      Ansys Employee

      If you perform Free Free Modal Analysis i.e., a Modal Analysis without any boundary conditions, you will get zero natural frequencies as the structure will be allowed to have rigid motion in all 6 DOFs in three-dimensional space without any excitation.
      For MSUP Transient, natural frequency in Modal solution is used for calculation of Time Step Size in Transient analysis. Time Step Size should be small enough to accurately capture the transient response of the structure. Typically, the time step size for MSUP Transient is calculated as 1/10 * period of the highest frequency of interest from the Modal solution. So, if we could excite that frequency, in a time history, we would have 10 time steps, and hence a trace of the response would be smooth.
      Please let me know if this answers your question.

      Regards Vinayak
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    • cpgeagadir
      Subscriber
      Dear Vinayak, I used BCs as you can see in the joint photo. Please I had another question I wanted to model a concentrated load on each wheel with the wheel moving at a speed of 60 km2/h I do explicit analysis but it didn't work. I find that the error due to nodal force that I defined. what should I do at this point?
      Thanks in adavance


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