Ansys aqwa error

Discussion in 'Software' started by Joeri, Oct 3, 2017.

  1. Joeri
    Joined: Oct 2017
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    Location: Delft

    Joeri New Member

    Dear All,

    I'm currently trying to simulate a Spar foundation for a floating wind turbine. When I just do a hydrostatic solve everything is fine, but when I try to run a full diffraction solve I get an error. I added a screenshot of the error. I also added a picture of the values I used for the model.


    I did a manual input of the mass(7466330 kg, z position -89m), and a direct input of inertia.

    Does anyone know what I did wrong?
     

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  2. TANSL
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    TANSL Senior Member

    If I am not mistaken, inertia should not be given in "kg.m2" but in "m4".
     
  3. Joeri
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    Joeri New Member

    Hey Tansl,

    Thank you for replying, but these are the inertia properties I added to the point mass in AQWA, which are in Kg.m2?
     

    Attached Files:

  4. TANSL
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    TANSL Senior Member

    Well, I'm probably wrong. For me, inertia means the second moment of the area, so, since it is not a mass, but an area, it must be in m4.
     
  5. Joeri
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    Joeri New Member

    I found the problem, when I aligned the global axis with the Z-axis instead of the X-axis my model seems to run fine.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Yup...incorrect as it is not the whole picture

    What is inertia in the context of the OP's question? It is the inertia mass when calculating the periods of motion of a vessel, about a radius of gyration. Thus the inertia, is the kinetic energy, which has its units of kg.m^2 and is correct. The concept of inertia remains the same, but one is either in a 2D plane of reference or a 3D plane of reference. Inertia being the "resistance" or "reluctance" of an object to move, when forces act upon it. In the equations of motion of a vessel which has 3 components to it (mass moment of inertia - 3D -, the linear damping, and finally the restoring moments - 2D geometry), the constants for the restoring force "c" are based upon the inertia of the waterplane, these being the waterplane 2nd moment of inertia, that you refer too. Thus there is more to a vessel than simply the waterplane moment of inertia.!
     
  7. TANSL
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    TANSL Senior Member

    Thank you, as always for your explanations, Ad Hoc.
    Yes, it is clear that I spoke of the moment of inertia and the OP speaks of another "inertia" different from mine. However, if the "inertia" is the same as, or similar to, the kinetic energy, as you explain, the units should be kg (m / sec) ^ 2. (m.v ^ 2), and further, kg mass, not kg force.
    Now I understand less than before. But it is quite possible that I am not able to understand what is totally obvious.
    On the other hand, the nomenclature "Ixx", "Iyy" or "Izz" can be equivocal since it corresponds to what is usually understood as moment of inertia with respect to each of the coordinate axes. Because the waterplane area, even if it is 2D, has inertia with respect to the three axes, right ?. Perhaps it is referring to the components of the inertial force with respect to those axes whose origin of coordinates is, where ?.
    In the end, everything seems a bit confusing.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    On object that is rotating about a fixed axis (in this case a vessel about its own x/y/z axis) has mass and a distance to a centre of rotation of said mass. The kinetic energy, linearly is 1/2.m.v^2 but the kinetic energy about a rotating plane is simply 1/2.I.w^2, where w is the angular velocity and I is the inertia. The I, in the kinetic energy has the units kg.m^2.
     
  9. TANSL
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    TANSL Senior Member

    It is clear, this is the rotational moment of inertia. Thanks, again, Ad Hoc.
    I was thinking of linear movements on the waves, not rotations.
     

  10. Heri Setyawan
    Joined: Dec 7, 2017
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    Heri Setyawan New Member

    hello all

    Right now Iam doing my final projet use ANSYS AQWA. My final project focus to show difference between ship with bilge keel and ship without bilge keel.
    But, the result of my running in ANSYS AQWA only show small diferrent between them, its graph very close. Is anyone have a same problem with me?

    Thanks
     
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