VLID parameter within AQWA in a round semi-submersible

Discussion in 'Software' started by lyreco32, Jan 7, 2014.

  1. lyreco32
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    lyreco32 Junior Member

    Hi,

    I'm working with a specialized round semi-submersible vessel that has an open centerwell with a diameter in the order of 60m. Compared to model tests the relative elevations within the centerwell of the vessel are over-estimated by AQWA. I'm trying to use the VLID parameter within AQWA to roughly tune the centerwell behavior (I realize anything realistic must be frequency-dependent and that VLID is likely a crude, unrealistic approach, but for my purpose I think it is sufficient). I'm having some issues getting it to work. I'm using both ANSYS WB AQWA and standalone AQWA, v14.5. So far I've been using the Workbench to generate the .dat file for later standalone AQWA processing (I can't find a way to use VLID within WOrkbench).

    Onto my problem:

    I have defined a circular element within the centerwell at the water surface in DesignModeler. After meshing and looking into the Analysis.dat file the element group that defines the circular lid are set as non-diffracting. VLID requires diffracting elements. Within Ansys WB HYDRODIFFRACT, I see options to set Surface Type under the Geometry (defined by a Hull consisting of axes, a point mass, an above water part, a below water part, and the circular lid). The only options are Program Controlled and Non-Diffracting. What can I do to properly define the lid as diffracting so VLID will work?

    Unfortunately the ANSYS help files are rather lacking when it comes to VLID and there is even less info to be found online. I appreciate any advice. Thanks!
     
  2. CWTeebs
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    CWTeebs AnomalyGenerator

    You'll probably have to edit the data file outside of workbench and replace the QPPL/TPPL with QPPL DIFF/TPPL DIFF to make them diffracting panels, if it really requires it (I can't remember if it does or not, physically they are not wave diffracting panels).

    EDIT: Select the geometry, change structure type (not surface type) to 'Abstract Geometry' then abstract type to 'Internal Lid.' See if that yields what you want.

    I tested this with Ansys 14.5
     
  3. lyreco32
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    lyreco32 Junior Member

    Thanks for the response...I just figured that out myself actually but you beat me to an edit. I think it should be "External Lid" which gives the VLID damping and gap parameter options. This creates a .dat file with VLID and properly defined elements for the lid.
     
  4. CWTeebs
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    CWTeebs AnomalyGenerator

    Yeah, I'd have to dig up my notes, the external lid is originally defined for structure to structure interaction. I think it's a little fuzzy when it comes to things like catamarans, moon pools, damping between turbine structure legs, and I can never remember which elements should be used. As you suggested earlier tank tests with some sort of frequency damping be possibly be more accurate.

    If you can share the geometry I've gotten volume of fluid mesh motion working in OpenFOAM's interDyMFoam CFD solver and would be curious to try applying it to a moonpool.

    Anyway good luck with your project, sounds very interesting.
     
  5. lyreco32
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    lyreco32 Junior Member

    Sorry, the geometry is confidential. I'll be doing some work for this project later within STAR-CCM+ but I'm going to use motion inputs from AQWA. I'd love to try CFD for the motions but without validation (the existing model test isn't relevant...complicated problem) it would be a big waste of time and effort. Cheers.
     
  6. CWTeebs
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    CWTeebs AnomalyGenerator

    Understandable. I meant using CFD for finding equivalent (more accurate) damping factors in the areas of interaction where viscous flows can't be neglected.
     
  7. lyreco32
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    lyreco32 Junior Member

    Another question...is it possible to extract relative elevations of points on the sea grid between the vessel and the water surface from a time-domain simulation in Ansys WB?
     
  8. musiice
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    musiice Junior Member

    It is possible by input in deck 1 (node) and deck 18 (motions and/or position relative to water surface).
     
  9. CWTeebs
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    CWTeebs AnomalyGenerator

    I don't know how to do that in Workbench. In GS/batch mode use the ZRWS option (deck 18) before specifying the nodes for printing.
     
  10. lyreco32
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    lyreco32 Junior Member

    Cheers, thanks for the advice.
     
  11. KFreeman
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    KFreeman New Member

    Hi, I am interested in your VLID discussion because I am using AQWA WB to model the water motion around a fixed moon pool structure.

    In order to look at the motion of the water surface, I defined an abstract surface at the water plane which gives field points both within the moon pool and outside it.

    The pool is open and I included a lid which I think should be ILID so as to do frequency suppression, rather than physical damping. The only allowable option for the moon pool geometry seems to be external lid in WB. Does anyone know whether this is the right version or how to check the definitions?

    When I run AQWA with the lid in place, the field points in the same area as the lid disappear. Does the lid behave differently to how I have interpreted it and are the field points doing something other than giving all available information at their position?

    Many thanks.
     
  12. lyreco32
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    lyreco32 Junior Member

    I believe ILID is automatically generated, so I don't know how much control the user has over it.

    I think instead of defining an abstract surface you should define nodes and then use ZRWS in Deck 18 to output the relative vertical distance from the undiffracted surface. That's what I'm working on using.
     
  13. lyreco32
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    lyreco32 Junior Member

    Followup question...I have defined a number of nodes and I've gotten ZRWS working to tell me the relative vertical distance from the nodes to the undiffracted water surface through AQWA-NAUT time domain. I've got a nifty matlab script to extract the relevant node position data from the .PLT files, but to give a complete understanding of the relative distance I also need to incorporate the motion due to radiation and diffraction.

    I'm not sure how to do this within AQWA or if the information can be extracted from the time-domain files...I don't think it exists within them. I am thinking I'll need to do some work to essentially recreate the time-domain analysis in MATLAB based on the AQWA-LINE output by using the per-frequency responses and generate a sea grid that includes the radiated and diffracted components. However, this would not be the same irregular wave as used in AQWA-NAUT because of the random phase angles, and this seems like a tricky and inelegant solution. Any suggestions? I feel like I might be overlooking something much simpler.
     
  14. CWTeebs
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    CWTeebs AnomalyGenerator

    The time domain vessel motions include the linearized radiation and diffraction forces. As far as I know there is no way to get the diffracted wave surface in the time domain modules. There is a post processing module called Aqwa-Flow.exe which can calculate wave elevation and pressure amplitude and phase at selected points in the fluid domain, but as you said you would need to convert this to a moving frame of reference (the moving ship's source's and sinks) and a spectral density function (your sea state with random phase angles).
     

  15. Inventus
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    Inventus New Member

    Dear CWTeebs,

    We are designing a ship with a large harbour section at the stern, and don't know how much viscous damping arises from the bluff edges of the (almost) moonpool. In order to assess the motions of objects floating inside the harbour it is important to get the damping right.

    Are you interested in running our geometry into the volume of fluid mesh motion in OpenFOAM's interDyMFoam CFD solver and see what viscous damping effects result from the moonpool?

    Kind regards,
    Team Inventus
     
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