General Mechanical

General Mechanical

Theoretical doubt on Forced Response

    • Marco501
      Subscriber

      Dear all,

      I am performing a FSI using ANSYS Fluent and Mechanical on a flow in a channel.

      I solved for the unsteady pressure field and imported the pressure field at a certain number of time steps. Then, I found the frequency content of the system and I extract the Fourier excitation modes, in order to import them into Mechanical for the harmonic response.

      Using this procedure, the system shows very high amplitude of von Mises stresses around the first natural frequency.

      My question is: if I import the excitation at (for example) 100 Hz and the natural frequency is at 300 Hz, what I focus on is the amplitude/stress at 100 Hz in the frequency response right?

      At this point, what if the Fourier mode around 300 Hz (continuing the same example of before) has very low amplitude but then superposing it with the structural mode at 300 Hz, I get very high oscillating stress?

      What can we conclude about the system safety in this case? In reality, we are not exactly exciting the system at a sinusoidal oscillation at 300 Hz, but still the stress value might not be acceptable. Have you faced such an issue previously?


      Thanks a lot for helping

      Marco

    • Chandra Sekaran
      Ansys Employee
      if I import the excitation at (for example) 100 Hz and the natural frequency is at 300 Hz, what I focus on is the amplitude/stress at 100 Hz in the frequency response right?
      In the harmonic response the structure responds at the same frequency as the excitation. So if your excitation is at 100 Hz the response will only be at 100 Hz. And if this frequency is well away from the natural frequency you may just get the equivalent of a static response to that load.
      On the other hand if you have the Fourier component at 300 Hz from CFD and apply this as harmonic load and the natural frequency of the structure is also 300 Hz you will get high oscillating stress. can you elaborate on your question?
    • Marco501
      Subscriber
      To elaborate more on the question, I will be more detailed with my case.
      This is for example the forced response with excitation at 140 Hz: as you can see, the response is almost equivalent to the static response. Moreover, you can see the stress amplitude peak around 360 Hz (first natural frequency) is around 30 MPa. The Fourier component around 360 Hz has very low amplitude, but still the static response is high. In my case, this value of osciallating stress is a bit too high considering also the mean component.
      My question is very basic (and maybe a bit silly): can I conclude that the structure is not safe in that flow condition since the component has stress (mean + oscillating) higher than the yield stress? Because the forced response is in theory considering a sinusoidal oscillation at 360 Hz, while in reality that excitation has low amplitude and it is not exactly sinusoidal....


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