Dry tank testing. Can there be such a material?

Discussion in 'Sailboats' started by Omeron, Jan 30, 2008.

  1. water addict
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    water addict Naval Architect

  2. Yeah thats fine for non critical flow regimes like for prop.s or ships since you are not looking to minimise resistances, look a wetted areas, trim angles and wave formation etc, for an accurate yacht test you need to know exactly the speed of the flow, which is quite difficult to control due to boundry layers etc in the case of moving the water, air is fine obviously though.
    If you were trying to create a circulation tank you would need different types of flow straightners in the form of vanes/fins etc. to eliminate 'swirl' caused by the bend in the tunnel and the pump depending on what type youre using, you may also have a problem with free surface effects waves...
    A simple enought panel coded (potential theory) CFD package, (quite cheap - PALUSIPAN) would be a good investment and will prob be more accurate than the circulation tank in terms of resistances, but the absence of free-surface modelling is a drawback.
     
  3. water addict
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    water addict Naval Architect

    Let's face it.
    The reality is you ain't gonna get an accurate home built test tank to predict resistance. It's a bit nutty to think about it. So if what you want it a toy to mess around with, the link I put above is a good example.
     
  4. Wolczko
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    Wolczko Navarc Student

    I agree with Water Addict. Especially with something up to 4' long... You'd have to use something a good bit bigger than the rolling hills research flow channel posted above, and even then, there are significant problems with the use of flow channels for resistance testing. Aside from the problem of turbulent flow in the channel, as well as a flow velocity gradient across the entire channel (which may or may not be easily measured), there will be problems arising with eddies in the flow which will change pretty much every component of resistance in such a way that would be very difficult if not impossible to account for accurately.

    There could be some possibility of non-scalable comparative testing in a flow channel, supposing (probably - I'm not sure) that the hulls tested are similar and that there are a great deal of test runs conducted on each hull. But, like Water Addict was saying, I think that trying to get any resistance data which could be applied to a full scale boat from flow channel testing is probably futile. Besides, if you've got a swimming pool handy, you can always just do some old school testing in that. ;) I seem to remember Ben Franklin rigging up some sort of weight-driven towing apparatus in his attic which you could at least have a good time designing and building. And again, comparative testing could probably achieve some level of relevance in that setting, so long as you remember to turn off your circulation pumps and buy a nice stopwatch.

    :) Wolczko
     
  5. FarmerColin
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    FarmerColin Junior Member

    Given all the difficulties of scaling - gravitational wave considerations need the flow speed to be reduced by the square root of the scaling factor while Re number considerations need it to be increased by the scaling factor!! - I can't help feeling you would get more accurate indicators of the relative performance of two hull-forms by numerical methods
     
  6. fastwave
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    fastwave Senior Member

    In the right hands you will get accuracies in the order of 1%, but it is not cheap.
     
  7. nico
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    nico Senior Member

    Yes, i dont know if a fluid with these characteristics can be found.

    CFD is for sure the way to go, it's getting more accurate every day (computers are getting faster and bigger). You get far more insight into the physics. It's cheaper. And it's already more accurate that towing tank test; america's cup team are already moving to only cfd.
     
  8. fastwave
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    fastwave Senior Member

    Cfd Cost

    Just to put some numbers into context, because there are a lot of people saying CFD is cheap.

    To do free surface CFD with a RANS code and expect good results these are the following costs.

    16 CPU workstation 5,000euro
    commercial CFD licenses 3000euro+/month
    CFD specialist 5000+euro/month

    so total 13,000 euro for probably 30 points.


    so 400euro per point.


    Panel codes can be 50euro per point or cheaper if you do it yourself.
     
  9. Guest625101138

    Guest625101138 Previous Member

    I expect you are considering drag of efficient displacement hulls. In this case Michlet will give results within 5% and it is free.

    Rick W.
     
  10. fastwave
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    fastwave Senior Member

    I belive that since this is posted under the sailboat section that most real boats do not fall under the L/B of michlet and also sail are not symmetric while sailing.

    Also, what about lift??
     
  11. No Cup teams used purely CFD for anything, and none did a complete analysis of fully appended boat, appendage designers use it but none do or ever will use purely CFD, you will not get 1% accuracy ever, even with an infanitley detailed mesh which will cost infinite computing! Towing tanks will always be around, Its very cheap compared to tank testing considering a 1:3 scale model of cup boat will cost roughly 10,000 euro and a testing programe up to 30,000 depending on test matrix and tank used...
     
  12. fastwave
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    fastwave Senior Member

    I am afraid that is not true. Many team analized fully appended boats. In addition the accuracy that can be achieved is very good. Within the experimental error or the tank, which inherently has scaling errors.

    Computational resources are not an issue. Caclulations can takes a couple of days on 40+ CPUs but that is not an issue.

    CFD can be cheap depending on how far you want to push it otherwise it gets expensive.
     
  13. Guest625101138

    Guest625101138 Previous Member

    What L/B is being considered? Michlet will give useful results at least down to 5.

    As far as lift goes I would expect leeway to be slight if it is an efficient design with deep keel. As a first approximation you can add the induced drag of the hull for the determined angle of leeway.

    I have not tested the pitch or roll settings in Michlet but it would be worth trying as it gives very good results in level trim. I have adjusted trim on hulls but imported the trimmed hull into Michlet. I did not use the internal settings in Michlet. You could ask Leo about the limitations. He has a number of comparisons against CFD and the real world results usually compare better with Michlet than the CFD.

    Rick W.
     
  14. nico
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    nico Senior Member

    Ok, thanks for that; you seem to be well informed. :)


    make it 150 points; 80euros/point. And compare to tank test.


    There are also a few comments about Michlet, it's a great tool and probably a good introduction to CFD; but it has a very very limited application to yacht design.
     

  15. I was at the RNLI lecture at Uni Southampton given by Andy Claughton, former design co-ordinator of Emirates team NZ and now for Team Origin who said that no teams did fully apended CFD analysis, they did for just keel and bulb and also for keel hull joint.
    I dont know what Michlet is, but CFX or Fluent are your best bet for accurate(comparitave)results.
    Quoting from Wolfson Unit, yacht experimental techniques lectures for MSc given by Ian Campbell former cheif experimentalist for Luna Rossa:
    CFD
    Potential flow or RANS or LES:

    •“Numerical experiment”
    •Qualitatively correct
    •Numbers NOT generally correct
    •Not enough cells/discretisation
    •Good for understanding what is
    happening in flow

    Ian carried out a tank testing regieme for Luna Rossa of 57 scale models over their campain for the 32nd cup and achieved a 0.5% accuracy compared to 10-15% accuracy of CFD results...
     
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