Michlet

Discussion in 'Software' started by fredschmidt, Apr 19, 2010.

  1. fredschmidt
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    fredschmidt Naval Architect

    When I designed my hull with the transom tangent to DWL I noted that when navigating the transom was immersed and we have much eddies behind it.
    What is the cause?
    The dynamical wave formation goes above the DWL and submerged the transom. A know fact.
    My actual design has the transom a little above DWL to prevent this.
    The actual design has also a Cp=0.58 instead 0.55 of the previous.
    The over all length is 1m for both, naturally the previous design has 1m DWL length and the actual has 0.939 for the cause above.
    When I run Michlet I had for previous design:
    V m/s Rt
    0,498580 0.000043
    0,676783 0.000082
    0,854985 0.000132
    1,033188 0.000183
    1,211392 0.000250
    1,389595 0.000334
    1,567797 0.000397
    1,746000 0.000503
    1,924203 0.000602
    2,102405 0.000955
    2,280608 0.001575
    and for the new design:
    V m/s Rt
    0,498580 0.000040
    0,676783 0.000074
    0,854985 0.000126
    1,033188 0.000202
    1,211392 0.000275
    1,389595 0.000407
    1,567797 0.000589
    1,746000 0.000786
    1,924203 0.000789
    2,102405 0.001174
    2,280608 0.001884
    And we see here that only in the three first velocities the actual hull have advantage for the previous.
    This is explained by the fact that the lesser length have benefit, in function of smaller wetted surface, only in very low velocities where the viscous resistance is much more greater than wave resistance.
    Well, but when moving we have the dynamical increase in length.
    By this explanation the questions is:
    Can I affirm that the results from Michlet is not so real because do not analyzes the dynamical effect of the length increase?
    Can I say that this new design is better than previous?
    Since Michlet have the wave form, why do not do the calculations leads in count the dynamical wave formation and consequent increase in length?
    Sorry Leo, more work :)
     
  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I'm not sure what you mean by the increased dynamical length. Do you mean the actual length of the immersed hull? Or do you mean the hull plus a hollow in the free-surface behind the transom?

    Cheers,
    Leo.
     
  3. fredschmidt
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    fredschmidt Naval Architect

    Statical the design length is 0.939 m, when the boat is in motion we have the wave crest reaching the transom, that is above and behind DWL, increasing the wetted length, this new length I call dynamical length and I expect that my dynamical wetted length will be 1 m without transom be immersed.
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I understand.
    No, Michlet does not detect the over-hanging stern.

    Leo.
     
  5. fredschmidt
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    fredschmidt Naval Architect

    But what you think about.
    Dynamically, with the boat dynamical wetted length of about 1m I can expect less resistance?

    Cheers

    Fred
     
  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Sorry, Fred, I don't like guessing, and I don't have time to calculate the possible effect.

    Cheers,
    Leo.
     
  7. fredschmidt
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    fredschmidt Naval Architect

    Thanks Leo

    Cheers

    Fred
     
  8. fredschmidt
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    fredschmidt Naval Architect

    Well, up to now I was waiting for some comments from somebody else.
    The intrinsically problem here is that we have a fact show in Michlet results that contradict a usual naval architect practice - the use of determined Cp values for determined velocity ratio - that can be concised in - hight velocity, hight Cp.
    The designs have the same displacement and the new design has less wetted area than previous. The new design has higher Cp = 0.58 than previous Cp = 0.55. The relative position between LCB and LCF are similar, the new design has less separation between them and are more near wetted mid length. The length of the previous is 1.0 without overhang and new has length 0.939 m with stern overhang with 1 m LOA. The bows are the same.
    Why the stern now has overhang? Because I see that we have much eddies in the astern because it is submerged by the the crest of the dynamical wave (in the vicinity of maximum velocity).
    But the result from Michlet do not confirm this practice - hight velocity hight Cp.
    I read the Michell and Leo's paper about the process of calculating resistance by studying the wave formation and there I do not see anything related to Cp.
    For me, that yet believe in the - hight velocity hight Cp - what I can think?
    For me is intuitive that differents Cp - that is relationed with volume distribution - generate different wave forms and naturally these forms are related with differents velocities.
    A destroyer is totally different from a sailboat. The overhang, if exists, do not produce any effect because is little in function of overall length and the hight velocity ratio - V/L¹/².
    The sailboat work in low ratios and the overhang is do to reduce boundary layer separation effects beside the rating fact that the length is penalized, but this penalized length is the statical length, not the dynamical length, and the designers use the fact that the dynamical wave formation increase the length if the boat has overhang and in reality the boat in movement has a bigger length due wave formation.
    For me the results from Michlet are curious.
     

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  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Fred:
    Michell's integral is not an empirical formula, therefore it doesn't require Cp as an input. It only requires the offsets, or in other formulations, the (longitudinal) hull slopes.

    Leo.
     
  10. fredschmidt
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    fredschmidt Naval Architect

    I understand.

    Is two visions of the same physical phenomenon:

    One by the side of the effect: waves made by hull displacement.

    Other by the originator.

    And they need agree.

    The influence of Cp is not empirical. Is the fact searched.

    If you do not take into account the real hull slope what I can think?
     
    Last edited: Apr 21, 2010
  11. daiquiri
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    daiquiri Engineering and Design

    Premise (knowing how these things have ended up before): this is just a letter of praise to our friend here, not a criticizm of any kind towards anyone, ok?

    Fredschmidt,

    There is nothing that should really make you confuse about the results you are obtaining, imho. Michlet is a software made upon a theory which has it's scope and limitations, as any other theory and as any other software (and as any experimental investigation, on the other side). If you use it within said boundaries, you will obtain reasonable results. Rick W's boats are an example of designs which operate inside the boundaries comprised by Michlet (or vice-versa) as well as many other practical hulls probably are.
    But it appears that with your latest modification you have stepped outside of that boundary. So, you have to either accept these limits and eventually find out the ways to by-pass them while still using Michlet (if possible), or to try other roads and analysis methods.

    But what I really wanted to underline is this - I find that the effort you are putting into the critical analysis (http://www.wordwebonline.com/en/CRITICALANALYSIS) of the results is out of ordinary, and has to be praised. Not many people outside of academical and professional ambients bother to analyse results from automated numerical calculations to the level you have ventured into - also because it is often not easy to find a known and validated data-set to which the numerical results can be compared. Your observations have also led to an improvement of a major design software (Delftship).
    Your method of continuous verification of results makes you a designer who can be relied upon, that is the impression you have left on me. My respect for that. I just felt like saying that (and I have not been paid :) ).

    Slavi
     
  12. fredschmidt
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    fredschmidt Naval Architect

    Hi Slavi

    My principle of life is respect and learn.

    Rick kindly introduced me to Michlet and I started to use and I only find somethings that it seems interesting.

    How I read in the Ciberyad site that Design.net is the appropriate place to learn about Michlet, I cast my doubts here to find someone that can help me. Not necessarily Leo.

    My findig take me to think that how I can use a statical hull slope to study a dynamical hull slope?

    We do not have waves on immobile boat.

    With the movement initiate waves appear and the hull slope change and naturally the waves form change, is a continuous modification.

    By this reason is strange for me that overhang, wave forme in function of velocity is not taken in consideration in hull slopes.

    My doubt is a single look to a physical phenomenom and I only want learn about this and perhaps help with an a more microscopic view of the phenomenon. .

    Cheers

    fred
     
  13. sottorf
    Joined: Sep 2007
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    sottorf member

    Michlet & Rhino

    Leo,
    I have been wondering if you would be interested in creating a plugin for Rhino so that it is possible to run the Michlet routines directly in Rhino and then also display the free surfaces, optimised hull lines etc. directly in Rhino. I investigated this for some different CFD software and established it would cost about USD500 to get the code written.

    If there is interest in this we could all chip in and contribute to the cost. What do you think?
     
  14. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Heartfelt thanks for the suggestion and offer, but my main focus is on "Flotilla" at present. Michlet has too many limitations e.g. far-field waves only, no squat calculations, no above-water components, no weight, powering and range estimates, no ACV and SES capabilities etc. I have also just developed a new transom stern model which I don't have time to incorporate it into Michlet.

    Your $500 quote sounds reasonable for someone who knows Rhino well, but I have never used nor seen Rhino, so it would take me a few days to program a plug-in.

    All the best,
    Leo.
     

  15. daiquiri
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    daiquiri Engineering and Design

    So Flotilla will be able to calculate a near-field waves too? That's very interesting. Does it mean something like waves between cat's hulls, for example?

    Btw, just few days to create a plugin for Rhino? You are becoming a monster of computer programming, Leo! :)
     
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