Fluids

Fluids

How to calculate Moments, Added Mass and Damping respect to an arbitrary axis?

    • Medi
      Subscriber

      Dear Ansys guys,

      I am trying to calculate the External Moments, Added Mass and Radiation Damping on a flap respect to a hinge at bottom. I think AQWA gives results respect to the origin which is by default at sea surface (Z=0). Could you please let me know if it is possible to calculate the values respect to the hinge at the bottom or any other axis with Hydrodynamic Diffraction?

      Many thanks,

    • Yu Bi
      Ansys Employee
      Hi, the REFP data record (Aqwa Reference manual Section 25.7) can change the hydrodynamic reference point, but if you have rotations on the new axes (compared to the global coordinate system), I am afraid you need to do the transformation on your own.
    • Medi
      Subscriber
      Thanks,
      I have created the model again to make sure there is no rotation respect to the original global system. The origin is at top of the body on the water surface with a small distance to the body's top. I am looking for a way to evaluate hydrodynamic diffraction results respect to the bottom assuming that the body is bottom hinged. Could you let me know how to find the data category REFP data record - hydrodynamic reference point? is it a data to be specified in a subroutine or can be edited in the AQWA menu?
      Best,


    • Yu Bi
      Ansys Employee
      Hi, if you want to use the REFP data record, you need to manually add and define it in the input *.DAT file and run Aqwa.exe directly with the new *.DAT file. The structure of the *.DAT file is elaborated in the Aqwa Reference manual.
    • Medi
      Subscriber
      Thanks,
      I read the Reference Manual and checked the .DAT file, but there is no field to modify the data category 1.
      In the path (dp0\AQW\AQW\AQ\Analysis) there is ANSYS.DAT file, but it contains only node numbers.
      if you can give me further advice on this issue, or any other alternative to find the values respect to the bottom of the plate, that would be highly appreciated.
      Many thanks


    • Yu Bi
      Ansys Employee
      Hi, you need to add deck 9 to deck 18 to the DAT file, and in deck 16, add the REFP data record. Also, you need to set the RESTART data record from 1 to 5. The coordinate value set in the REFP data record is the coordinate value of the origin of the new reference coordinate which you can measure from SpaceClaim if you don't know the exact value.
      An example of the DAT file modification:

      After solving via Aqwa.exe, you can find the hydrodynamic data with respect to the new reference point in the *.LIS file.
    • Medi
      Subscriber
      Hello again, and thank you very much for your explanation.
      I did that and defined the coordinates of the new REFP via modifications in ANALYSIS.DAT file. However, when I check the results in ANALYSIS.LIS file, I can see the Center of Gravity is moved to the new REFP. This way, the dynamics of the problem will be changed and the moments due to body weight may not be taken into consideration.
      I need to calculate the added mass and damping and moments of wave via Hydrodynamic Diffraction analysis when the body is hinged at an edge as shown in the following figure.
      I was wondering if there is any way to change the REFP, but keep the effects of the body's weight and the Center of Gravity? Is that work if I add a second point mass located at the center of body, in addition to the Center of Gravity which is shifted to the hinge location?
      I would be very thankful if you can give me some advice.
      Many thanks
    • Yu Bi
      Ansys Employee
      Hi, the usage of the REFP won't change the body's weight and centre of gravity.
    • Medi
      Subscriber
      Hi Thanks again. I change the new REFP in the .DAT file according to the Reference Manual as shown here, If I am right!
      Could you let me know how I can make sure if the changes in reference point is made?
      To run the aqwa.exe, I can find two different paths in C:\Program Files\ANSYS Inc. There are two folders v195 and v212 through both of which I can find aqwa\bin\winx64 and aqwa.exe. Could you let me know which one I should run, if there is any difference.
      And finally, is it possible to see the results plotted in workbench after changing the REFP in .DAT file? Or I can just see the new results as .LIS file.
      Many thanks
    • Yu Bi
      Ansys Employee
      v212 is the more latest version of Ansys, so please use it. New reference point results cannot be viewed in the workbench, but you can see them in the new result LIS file. If you can find the hydrodynamic data (e.g. added mass) with respect to the new reference point, then it means the changes of REFP have been made successfully.
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