CFD state of the art

Discussion in 'Hydrodynamics and Aerodynamics' started by Mikko Brummer, Dec 17, 2015.

  1. Mikko Brummer
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    Mikko Brummer Senior Member

    1 person likes this.
  2. daiquiri
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    daiquiri Engineering and Design

    Excellent find, thank you for the link. :)
     
  3. Doug Lord
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    Doug Lord Flight Ready

    Thanks, Mr, Brummer-appreciate the links!
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks for a magnificent set of data and predictions, Mikko.

    All the best to you for the Heavier-Wider Season and the New Year!
     
  5. Remmlinger
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    Remmlinger engineer

    Thanks!!!

    Mikko, what a fantastic Christmas present.
    None of my family will be topping this.
    Instead they will get angry at me, because I will be blocking all the computers in the house during the holidays.
    Have a great time!
    Uli
     
  6. baeckmo
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    baeckmo Hydrodynamics

    Really interesting to see a well designed validation test between the different CFD-codes. Thanks for sharing Mikko!
     
  7. Mikko Brummer
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    Mikko Brummer Senior Member

    Don't thank me, thank Jim Teeters & Steve Benjamin & al, who have done a great job with SYRF. There's more than these projects, if you look at the library there's nearly all of the existing sailboat papers available for free - most of them have been available for money only earlier. And on the project side, for instance the Volvo 70 transom immersion, which could shed some light on the transom drag discussion in the next thread. Or the Atlantic test program, with nice comments by Olin Stephens.
     
  8. Remmlinger
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    Remmlinger engineer

    Rhino-model

    I would be interested, if someone has already worked with the CAD-model.
    Using the hydrostatic function in Rhino, the displacement at the DWL of 4.6 m does not match the given weight of 197 kg for the canoe body.
    The alternatives are:
    Waterplane z=_0 mm______10 mm
    CB draft______0.0985 m___0.1085 m
    Bwl__________0.8887 m___0.9101 m
    Lwl__________4.596 m____4.738 m
    Volume_____167.5 kg____197.4 kg

    Looking at the photos of the tank-model at rest, it could be that the actual waterline is 10 mm above the DWL
    Either the model is heavier than intended, or the Rhino-model is not accurate.
    Any ideas?
    Uli
     
  9. Mikko Brummer
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    Mikko Brummer Senior Member

    I haven't had the time to look at the model, but if the CFD runs were done with Z=0 (volume 167,5 kg), that would explain why all the predictions were 5-7% under the tank... It would be very unlikely, though, that they would make such an obvious mistake.
     
  10. Mikko Brummer
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    Mikko Brummer Senior Member

    Looking at the Star-CCM results, I can see that Fz was in the first few runs about 1920N, so they were using the correct volume 197 kg.
     
  11. Windvang
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    Windvang Yacht Designer

    With Orca 3d I get almost the same hydrostatic results as you at o mm.
     
  12. Mikko Brummer
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    Mikko Brummer Senior Member

    I discussed the problem pointed out by Uli with the SYRF and here's the answer:

    "With regards to the 3D model displacement question in the forums; the model was run unappended at a displacement of 197kg and appended at 215kg, so the 3D model was sunk to equilibrium in both cases. However, this question made me aware that we did not notate that freshwater was used at 20 degrees C to match the conditions at the QinetiQ Tank. Therefore the unappended sink is actually .011m from the DWL. I've made a note of the test conditions on the project page.

    Myles Cornwell"
     
  13. Windvang
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    Windvang Yacht Designer

    Seahorse magazine suggest the model is a model of Ran2. The ORC measurement certificate however states:

    RAN 2, GBR 7236
    Loa : 21.974
    Beam: 5.805
    DSPL 17620 Kg.
    Draft 4.793

    These do not scale down to the model size.
    Any other suggesties of the original design?

    [​IMG]
     
  14. Remmlinger
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    Remmlinger engineer

    In the Excel-file from the Wolfson Unit there are two sheets, one with "raw data" and one with the "corrected drag".
    The measurements were taken at 18.5 deg C in fresh water. After that, the measured drag was corrected to the standard condition of 15 deg C. The corrected drag is higher than the raw data, because the viscosity at 15 deg and hence the drag is higher than at 18.5 deg. It seems that the ITTC-friction coefficient was used for the correction.
    The density of the water is practically the same at these two temperatures, the difference comes when changing from sea- to fresh-water.

    The CFD-studies stated the conditions for the comparison correctly with 15 deg C and fresh-water.

    For those interested I have attached a diagram based on the raw data. I have corrected the measured values only for the tank blockage and the drag of the (b.l.-trip) studs. It is amazing how well the Delft-method predicts the resistance.
    Uli
     

    Attached Files:


  15. DMacPherson
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    DMacPherson Senior Member

    Uli:

    Does this mean that the Miles Cornwell statement about test conditions being FW at 20C is incorrect?

    Don MacPherson
     
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