resonance on AQWA RAO

Discussion in 'Software' started by Ptinou, Nov 10, 2013.

  1. Ptinou
    Joined: Nov 2013
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    Location: le Havre, France

    Ptinou Junior Member

    Hi,

    I m working on a mooring solution for a wave energy convector and I need calculate my RAO on AQWA to work thereafter on Orcaflex.

    My problem is that I don't trust the result give by AQWA particulary for the heave RAO that you can find below.
    I would like to know if the heave RAO peak is usual (factor 11)
    I noted that Added Mass and Radiation Damping have also a peak but on a differant frequency.
    If no how can I correct the values give by AQWA ?

    Regards
     

    Attached Files:

    • RAO.PNG
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  2. musiice
    Joined: Sep 2013
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    musiice Junior Member

    Hi Ptinou,

    Are you sure that frequency is in Hz? Looks more like it is in rad/s.
    Is the peak of resonance within a range of frequencies you expect? Check typical values for heave RAO peak of resonance and compare to what you have.

    How are you computing the RAO?
    If you are running regular waves with unit amplitude for different frequencies remember to make the analysis long enough so the response is stable, or regular. Also don't forget to cut-off the beginning of the series.

    Is your model 1:1 or a scale model? What elements are you using only panels/morison elements or combination of both?

    That peak of resonance with 11 m/m shouldn't be usual, but you can get any kind of bad output when the input is incorrect.

    Best regards.
     
  3. Ptinou
    Joined: Nov 2013
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    Ptinou Junior Member

    Hi musiice,

    Thanks for you answer.

    For the frequency it in Hz.
    I actually work with a scale model (1:30) so device is more and less a cube of 60cm size.

    The peak of heave is excepted around this value of frequency but less high.

    I m running with regular wave but I can chose the length of analysis, in the same way I do not see where I can chose between panel or Morisons elements (I think it's panel).

    I would like to know if it s because i m working on ANSYS AQWA and not directly on AQWA.
    If not where can I change the setup that you proposes.

    Best regards.
     
  4. musiice
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    musiice Junior Member

    Hi Ptinou,

    It doesn't have an influence that you work with ANSYS AQWA, in workbench I guess or AQWA. But, to complete all the stages and do certain runs you will need go to on text editor level.

    If you modelled a surface only then it is only panels, but if you included lines+(cross-sectional area)(tubes) and/or disc then you have a combination of both. Important to account for drag.

    You may need to refine your mesh, make a smaller size. How many elements do you have now? Have you check your natural frequencies in LIBRIUM or ran a decay test to verify they are correct? Start by refining the mesh and putting the maximum number of frequencies (50) to run in LINE. This is important regarding convergence.

    However, I would recommend you to go for a full-scale model and once you verify it is up and running do the scale one.

    I am also a bit concern that AQWA would not suit you if you want to test a scale model. You can find this in the manual AQWA Reference searching for Convolution, under the section Stage 1 / Theory.

    Best regards.

    EDIT:

    By the way depending on the symmetry of your structure you will need more or less directions. You can see in your plot how the RAOs are 'symmetric' and you are duplicating results. So for a cube with all sides = 60 cm you will need for example 0º/45º/90º to have covered the whole structure, or if you want more detail 0º/22.5º/45º/67.5º/90º. But for that structure you are considering, in my opinion, is not going to be relevant to compute 5 directions instead of 3.

    Hope this helps.
     
    Last edited: Nov 12, 2013
  5. Ptinou
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    Ptinou Junior Member

    Hi musiice,

    I join a capture or the model for have a idea of the wave energy converter.
    ogWAVE by Ocenalinx
    http://www.oceanlinx.com/technology/products/ogwave-2

    I chose to modeling the device only with panel on the workbrench because it seem more easy to analyse.

    There is already decay test done one this wave energy converter (one a same size scale model). We find the same frequency for the peak of heave the test but not so high(>2(m/m)).

    I m already near to the maximum of mesh (around 10500 / 12000).
    The number of modeling rule violation is lower than 300 (2% of element).
    I have already use 50 frequency just around the peak but is don t change the height of the peak.

    Actually I don t really see what can I change into the input to have a better analysis.

    Against thanks a lot
    Best regards.


    Thanks for the symmetry comment, i ll try to fix that to make the calculation faster.
     

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  6. Ptinou
    Joined: Nov 2013
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    Ptinou Junior Member

    Hum

    I just find a mistake in the inertia of the point of mass. Probably the cause of the problem.
    Answer after 10h of run...
     
  7. musiice
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    musiice Junior Member

    Hi Ptinou,

    Now that I see your geometry, I think that maybe that is not the best approach to model what you want.

    I cannot send you a private message but If you want to send me one with your email details we can speak further about it.


    Best regards.
     
  8. Ptinou
    Joined: Nov 2013
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    Ptinou Junior Member

    Hi musiice,

    Heave RAO below after inertia correction, it s not realy better
    my mail
    antoinedenijs#hotmail.com (#=@)

    Best regards.
     

    Attached Files:

    • RAO.PNG
      RAO.PNG
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  9. Ptinou
    Joined: Nov 2013
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    Ptinou Junior Member

    On the precedent graph of heave RAO, the two first peak one come from resonance of the device in heave and another probably link to surge resonance.
    the last peak with a normal high come from added mass resonance.

    So the problem do not really change, the two fist peak are probably on the good frequency but too much high

    Regards,

    Ptinou
     

  10. musiice
    Joined: Sep 2013
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    musiice Junior Member

    Hi Ptinou,

    Not sure you should have two peaks.

    I sent you an email.

    Regards
     
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