How often do you use of CFD (Computational Fluid Dynamics) ?

Discussion in 'Hydrodynamics and Aerodynamics' started by Alan Cattelliot, Feb 14, 2023.

  1. Alan Cattelliot
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    Alan Cattelliot Senior Member

    How useful is CFD in your daily tasks ?

    For what purpose would you use CFD ?

    Do you consider CFD as a decision tools, a double check, or something else ?
     
  2. jehardiman
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    jehardiman Senior Member

    You mean it doesn't stand for Colourful Fanciful Drawings?

    And also do you mean a CFD program (pick your poison...or rather your error), Navier-Stokes, source-sink, or some other hand method? Really isn't any mathematical endeavor to achieve a specific value a "computation"? Or are we just limiting it to computer automation? Lots of early non-visual CFD programs where just digital automation of hand methods. Even today with obsessive need for "detail" in the models it is all JMJ (Just More Junk) that realistically is still subject to GIGO; and nothing that a really-really fast anal-retentive math major couldn't do. Don't forget the reason behind the coke-bottle SWATHs. :rolleyes:

    When I was actively employed in the field, which was throughout the development of "modern" ubiquitous desktop CFD, I used it occasionally to get a bounding condition a little closer to truth than could be given by "classical" methods. In the big scheme of functional ship design it was kinda nice to use as a visualization tool for the non-hydrodynamicists that approve the projects and write the checks; otherwise it was just another tool in the box to measure something that is impossible to measure. In my office, critical hydrodynamic analysis was accomplished by two engineers independently (i.e. one by CFD/custom code of the era and one by other methods) and then checked by someone else. Any large disparity was follow by another independent analysis using another method; small disparities were resolved by choosing the more conservative. Code and methods found to work repeatedly became the go-to method, but were still checked by someone else. The vast majority of projects I was involved with never failed and always worked first time out of the box, and in the end, that is the point when the actual cost of the systems engineering is small compared to the total costs, or even operational day costs, of the whole project. A nuclear submarine is a sunk cost of $300k a day just sitting at the pier, not including crew.

    In the end it is important to remember that CFD, in spite of the "photo realism" achievable today, is actually not real. The fluid is not real, the surface is not real, and several assumptions used in the computational methods are not real. In many ways it is less real than tank or operational data. Personally, based upon my experience, I won't trust any computational answer or collected data better than +/- 5%.

    Here is an example. We needed to know the boundary layer thickness at a specific point for an item. My hand calc said ~9 inches given the operational seawater limits; a CFD from item vendor said 6.xxxx inches. This was critical to the sponsor so they had a lab physically model it in the tank: 8.0 +/- ~1.0 in freshwater depending on time between runs, thinnest layer first run in the morning. Real value? Hell, we don't know! We couldn't measure the real item in its real location in real water.

    In my final opinion, CFD is like any power tool; just another tool suitable for some things and not for others. Both an axe and a chainsaw will fell a tree, but neither determines where it will fall. And like any power tool, CFD can be dangerous in the hands of a fool.
     
  3. TANSL
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    TANSL Senior Member

    To your specific question I must say that I have never used CFD software. But let me ask you the following: when I was active, the large and medium-sized shipyards sent our hulls to a hydrodynamic experience tank (tank test) who, in addition to doing power-speed studies, among others, told us how, how much and where we had to modify the shapes to achieve the intended performances. Do CFD software do the same or do they simply say if some forms are better or worse than others? If they don't do this, I don't think, for the "normal" ships that most of us (*) project, those studies are worth much, no matter how fashionable they are.

    (*) for "us" I mean the majority of the users of this forum.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    It is not.

    I don't.

    Something else.

    That's it in a nut shell... +1 JEH

    The only time CFD has been involved in our designs, was when our client, for a patrol boat, didnt like the tank test results.
    As they were our client we were "forced" to do a CFD analysis, as STRONGLY recommended by the test tank (I wont mention names) ....and hey presto....lots of colours and wow factor.
    Look at the spray, terrible...change the Lines here and here and here....endless.

    What did we do, ignored it...simply redrew the waterline at a slight angle, i.e moved the LCG back by moving the engines and fuel tanks 1 frame bay aft, and "rotated" the attitude of the vessel. No one could tell the differnce once she was built either. Simple! Solved everything...it is basic naval architecture that CFD and those that push buttons, will never understand.

    Just as my footer notes...
     
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  5. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Thanks for your very interesting answers.

    Are we really able to get indications, directly from the tank test results, of how much and where shapes are to be modified to achieve targets ? For instance, let's consider a boat that should be powered by a 75kW engine, maximum operating speed at 13tks. Here are the tank test results. How should the shape be modified in order to achieve targets ? Could a naval architect anticipate the answer ?

    upload_2023-2-15_8-30-55.png


    In your case, it seems that the CFD simulations were not trustable enough by themselves because CFD was executed in parallel with another method. It seems also that, even though the CFD was not trustable enough, it provided the decision maker with a justification of his choice.

    Thanks for the order of magnitude.


    :):):)

    So it seems that CFD could not outperform experience in naval architecture, isn't it ? This is a quote from an ORCA3D Marine CFD developper :

    ORCA3D Marine CFD allows the naval architect to :
    - Avoid/minimze expense and time associated with model and full scale test programs
    - Evaluate the effects of specific unique design features
    - Perform high fidelity design evaluations in earlier stages of design when decisions have the largest impact
    - Discover and avoid potentially costly design flaws before the design is built (bolt and underline by the quoted engineer himself)

    What do you think of these arguments ?
     

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  6. TANSL
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    TANSL Senior Member

    That is a very poor response to all the studies that are done in a tank test on the bottom of a ship. Designers and shipyards pay very high amounts to test their designs because the results obtained are much better than what is shown in that simple Excel table. Those results, almost, almost, can be obtained by doing the calculations by hand.
    I think CFD is a great tool that, at least from what I've seen, few are interested or able, at the moment, to fully explore and exploit. I have no practical experience with this tool but I have consulted with people who make their living selling CFD studies. And what I say here is the opinion that they have allowed me to obtain. At the moment I think it is a very good marketing tool that, like many others, the modern designer should take advantage of. As benefits as, for example, a good rendering can be.
     
  7. jehardiman
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    jehardiman Senior Member

    Going to get out my 2x4 explaining stick right now. The table you provided IS NOT the tank test results. It is the extrapolation, by hopefully a consistent and approved method, of the (hopefully correctly) reduced data to full sized vessel. Without the original tank data, including any video, what was presented is pretty meaningless for analysis; it could be a curve fit of a Rorschach chart for all I know. Letting a computer or some else do your work does not make anyone a better engineer and definitely not a better Naval Architect. SHOW THE RAW DATA!

    Did you ever take a technical writing course in university? I see three egregious discussion points in those bullets that would give me, as an engineer pause. First, only one of the 4 bullets addresses any technical use. Second, the first bullet is not addressed to the engineer, but to the accountant and patron. Finally, the last bullet, which should be the strongest technical one, is almost just fear mongering. I have a feeling that the paragraph was written by someone other than the SME for the program.

    EDIT: X-post with TANSL.
    2nd EDIT: If you want to see a well written CFD manual, take a look at the WAMIT documentation. It is online.
     
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  8. Alan Cattelliot
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    Alan Cattelliot Senior Member

    I'm afraid that raw data couldn't be posted. You've cleverly guess that these data points, shown in the excel snapshot, are extrapolations of tank test results, obtained by Team New Zealand, during the tank test campaigns realized between years 1996 and 1997. The upright resistance values have been carefully post-processed and extrapolated, from the 1:4 model to full scale model, to be feed into the VPP.

    If these results are "meaningless for analysis", without having the original tank data and associated video, does it mean that the visual representations, associated with CFD results, are somehow helpfull for a Naval Architect ? As TANSL said, "these results can certainly be obtained by doing the calculations by hand". So what additionnal informations give the tank tests ? Is CFD able to provide the same informations ? Could CFD simulation, as the ORCA3D marine consultant said, really "avoid/minimize expense and time associated with model and full scale test programs" ? What is the opinion of an engineer, or a Naval Architect about this point ?

    Here is the link, were can be found the arguments developped about ORCA3D Marine CFD (around 7'52''). I should precise that I'm not working with ORCA3D, and have no relation with this company.
     
  9. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Please, allow me one more question. +/- 5 % in comparison with what ? Full test campaigns, in real conditions ?
     
  10. fastwave
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    fastwave Senior Member

    Not all CFD methods are equal
    Not all CFD Software are equal
    Not all CFD engineers are equal
    Not all Naval architects are equal

    I have seen people create wonders with the same tools that other people have failed miserably.

    the tool is rarely the problem
     
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  11. tlouth7
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    tlouth7 Senior Member

    I would say that the "computation" in CFD refers specifically to using a numerical approximation rather than trying to fully analyse a complex geometry. Of course with Navier-Stokes any solution is going to be numerical rather than analytical, but I doubt anyone does that by hand for anything other than the small number of standard geometries.
     
  12. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Thank youFASTWAVE, to recall this important fact. That said, I understand that CFD is indeed of some help, despite the issue in making a suitable adequation between methods, software, engineers and architects. So it seems that CFD may be good at something. From the previous comment of Jehardiman, it may provide a double check for naval architects, or engineers, and could also serves as a justification of the quality of the work, whatever the contribution of the CFD itself in the design loop.

    What could possibly go wrong with the CFD or its use in the design process ? Let's take the example of the ORMA trimanran Gitana X
    Le Trimaran Gitana 10 dévoilé https://www.multiplast.eu/fr/actualit%C3%A9s/presse/103-actualit%C3%A9s/newsletters/archives-composite/178-le-trimaran-gitana-10-d%C3%A9voil%C3%A9.html.
    upload_2023-2-15_13-45-55.png
    The conception of this trimaran is the result of the collaboration of Sébastien Schmidt, Duncan Maclane et and the Gilles Ollier Design Team, The engineering has been made by HDS for the structure and the ECN hydrodynamic laboratory, the two best places at this time where top level experienced PhD and engineers could be found. Still, the track record of this boat is the worst of the entiere class, despite 3 famous sailors have take command of this boat. Of course, as you imagine, intensive CFD and tank tests have been conducted. So, how is it possible that this boat could not achieve at least an average ranking, given the respective contributions of the best CFD methods, the best software, the best engineers and the best naval architects ?
     
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  13. Ad Hoc
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    Ad Hoc Naval Architect

    Sales tools, nothing to do with naval architecture.

    Sales blurb and fear mongering again.
    This is not naval architecture.

    Read above....and it also sounds like a classic case of too many chefs!

    Naval architecture is not an absolute. That is what those with little knowledge and experience seek, absolutes to justify their output and their "skill".
    Naval architecture is all about "trends"... and best guesses. It is never about absolutes.

    And 10 years experience takes...yup... 10 years.
    Not 10 seconds on Google search or 10 seconds of tap tap into a programme.
     
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  14. Ad Hoc
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    Ad Hoc Naval Architect

    We did a project with a very well known company (shall remain nameless).

    They used a Cray computer to analysis their hull they said we must use, with CFD and trusted every decimal place that came out of it...they spent over $1m doing their CFD testing.

    We spent one afternoon doing a simple model test that costs peanuts, with a basic model. Proved them wrong..bcos we didn't trust their data. .
    They trusted their CFD not our "simple" physical testing bcos it was done on a Cray computer with the best CFD they could buy.

    Suffice to say... they spent the next 5 years in court fighting why their data was wrong as the boat didn't perform as per their CFD.
     

  15. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Thank you TLOUTH7. This should be taken into consideration when speaking of CFD, I think. It could be that the disapointement of some about CFD may come from the relatively large investment needed to benefit from it, compare to what can be obtained with simpler approximations, that have already prooved themselves to be in sufficiently good agreement with experiences. CFD vendors may have promise extraordinary results to naval architects, that couldn't be achieved, perhaps due to the lack of knowledge of the state of art in Naval Architecture. It may explains why the CFD is so widely used in some industries, while so little in the naval field. So far, it seems that CFD users of this particular field are seen as "youngsters", inexperienced "roockies" or worst, as incompetent salesmans, trying to abuse everyone with good renderings.

    Still, I'm wondering why so many companies propose some CFD packages, dedicaced to the naval industry, and why so many engineers and researchers do publish so many papers on the subject. And, for now, I cannot find any answer or even the shadow of a clue, in the comments already received, without putting in question the seriousness of these companies, these engineers and researchers.

    Is CFD really nothing more than a mere sham ?
     
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