Swept Volume Theory

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Aug 2, 2022.

  1. Sailor Al
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    Sailor Al Senior Member

    You presented the graph in #117 in response to my appeal for experimental evidence of velocity increase, and I dismissed that on the basis that the caption clearly stated the experimental data was pressure distribution not velocity.
    I really don't want to distract the discussion further by arguing over the contents of a textbook.
    Whether or not Bernoulli's Theory applies to air over a sail is not essential to the theory. It is not even part of the theory.
    I added those 16 Q&As to the Appendix to demonstrate that I had not developed the theory in a vacuum (excuse the pun), but had fully explored the landscape of the subject.
    I should have realised how distracting that entry would be and handled it more diplomatically.

    Please, can we put aside our disagreement on this subject and see if a discussion of the actual theory can develop?

    So far there have only been a few comments on the theory itself and I don't think they have been fully resolved.

    Can we, for example, work together to see if the theory can provide explanations for some of the principles of sail trim that I raise in the Conclusion?
     
  2. Sailor Al
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    Sailor Al Senior Member

    In an attempt to revitalise this discussion, I have visualised a possible pressure distribution around a sail based on Fossati's figure.
    It's only the mainsail, I don't know how the jib would affect the pressure.
    The AWA is set to 25° which is a reasonable value for a performance keelboat in 10-15 kts.
    The second boat is in the dirty air of the first boat. Maybe the low pressure extends much further to leeward?
    windShadowPlusHiPressure2.png
    I'm struck by how different it is from the CFD images.
    I know there are measurements of pressure distribution over the sail's surface, but I don't recall seeing any experimental data on the pressure distribution on the surrounding region.
     
  3. mc_rash
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    mc_rash Junior Member

    How did you visualize the pressure distribution? Did you use any calculation? Is this how you expect the pressures will be?

    There is a second boat visible, why is there no pressure distribution shown?

    Yes, the first boat will effect the second boat.

    What are you trying to say with "Maybe the low pressure extends much further to leeward?"?

    What are the difference from CFD? Did you run a CFD simulation with the same conditions etc? Have you something you compare your "visualized" data (from somewhere) with?

    Re experimental data, ever seen a windtunnel test?
     
  4. philSweet
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    philSweet Senior Member

    It looks nothing like that whatsoever.
     
  5. Sailor Al
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    Sailor Al Senior Member

    Maybe not, I did say it was "possible" but what would you suggest?
     
  6. gonzo
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    gonzo Senior Member

    You demand experimental data because you don't trust calculated results. However, you then complain about experimental data and make the absurd statement that experimental values were calculated.:eek:
     
  7. Sailor Al
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    Sailor Al Senior Member

    Correct.
    I think you will find that the experiment measured pressure, not velocity. The reported values (v/V)^2 were calculated.
     
  8. gonzo
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    gonzo Senior Member

    All graphs are the results of calculations; even the ones you post.
     
  9. Sailor Al
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    Sailor Al Senior Member

    I requested experimental evidence of velocity. This graph is derived from an experiment measuring pressure, not velocity.
    I'm sure our readers will see the difference.
     
  10. gonzo
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    gonzo Senior Member

    Experimental evidence is based on the theory of cause and effect. For example, we accept that a voltage change on the output of a sensor equals a change in pressure. However, we accept that the change in voltage makes the needle in an analog meter move. All calculations and theories.
     
  11. CT249
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    CT249 Senior Member

    Now I understand why the boats Sailor Al trims go slower than the boats other people trim.
     
    Doug Halsey likes this.
  12. Sailor Al
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    Sailor Al Senior Member

    Ah, cause and effect eh? Yes, and, as you may have deduced, I have doubts about the use of Bernoulli's Theorem which links gas pressure differences with velocity.
    So you invoke cause and effect to calculate (v/V)^2 from pressure?
     
  13. Sailor Al
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    Sailor Al Senior Member

    We don't do too badly, but could do better, that's why I'm trying to understand a bit better. I'm pretty sure fluid dynamics isn't going to help.
     
  14. gonzo
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    gonzo Senior Member

    I don't invoke anything. I point out that in science causes are deducted or calculated based on theories. We don't have direct input, but use instruments that out theories say output data from some physical phenomenon.
     

  15. Barry
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    Barry Senior Member

    So just for clarification

    Are you willing to state that your position is that an increase in velocity of a fluid,-liquid- in the first case, results in a reduction in the static pressure within the stream?

    Are you willing to state that your position is that an increase in velocity of a fluid, - a gas- in this case, does not result in a reduction in the static pressure within the stream?

    Are you willing to state that for a wing that Bernoulli's theorem does not apply?

    The conditions above is that the stream is a wide constant flow / velocity regime prior to being influenced by the airfoil/wing
    I had to clean up the prior wording as it was not clear


    I know that you have hung your hat on your belief that because Bernoulli wrote a book whose title included Hydrodynamics that his theorem applies only to liquids and excludes gases
     
    Last edited: Nov 5, 2022
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