Froude and planing

Discussion in 'Hydrodynamics and Aerodynamics' started by sandhammaren05, Feb 26, 2017.

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  1. DCockey
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    DCockey Senior Member

    The statements quoted above contain fundamental errors as can be found by study of any aerodynamics textbook which includes compressibility effects.

    Gas flow is not "incompressible so long as the speed is subsonic and the temperature is above the condensation point." The speed below which the effects of compressiblity can be typically be ignored are much less than the speed of sound. Around ninety years ago Glauret published what is now know as the Glauert-Prandtl rule for thin/slender bodies which cause small disturbances to the flow and the velocity is not too high. That rule relates the compressible flow to the equivalent incompressible flow by the ratio 1/sqrt(1-M^2) where M is the free stream Mach number. The Glauert-Prandtl rule has been found to be a good approximation for many purposes as long as the free stream Mach number is low enough that the flow does not become locally supersonic. For more details consult a text or reference on aerodynamics. Chapter 9 of Theory of Wing Sections by Abbott and Von Doenhoff (1949, 1959) is on compressibility effects and includes experimental results showing the deviation at higher speeds from low speed "incompressible" results.

    For higher speed flows in liquid where cavitation occurs the flow in the liquid phase can continue to be considered as incompressible, as long as the local Mach number is not too high which would be true in any marine related application.
     
  2. sandhammaren05
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    sandhammaren05 Senior Member

    One irrelevant post after another:100ft/s/11ft/s<1/10! The Mach nr. is irrelevant here (60 mph=88 ft/s).
     
  3. DCockey
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    DCockey Senior Member

    Another interesting and informative response when a repeated fundamental error in knowledge of aerodynamics is pointed out, even if irrelevant to primary discussion.
     
  4. sandhammaren05
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    sandhammaren05 Senior Member

    What error?
    I'm talking about boat speeds. Why do you bring up the Mach nr.-it has nothing to do with boat speeds. Why should anyone care about compressibility under these conditions? Another red herring thrown into the pot.


    red_herring1.png
     
  5. DCockey
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    DCockey Senior Member

    The fundamental error concerning compressibility in your posts in which you introduced the speed of sound. Perhaps the relevance to the overall discussion is contained in your comments in those posts about the knowledge of others.

     
  6. sandhammaren05
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    sandhammaren05 Senior Member

    There is no error on my part. The focus in this thread is on boat speeds, not in breaking the sound barrier! I have therefore not nitpicked about irrelevant parameters like Mach. That only confuses others.

    red_herring1.png
     
  7. gonzo
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    gonzo Senior Member

    All gasses are compressible. Simplified models consider it incompressible to make the math easier. However, since you dismiss models, the simplification is not acceptable.
     
  8. patzefran
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    patzefran patzefran

    Sorry Guys
    A flow is said incompressible as long as the Kinetic energy ( related to ordered flow velocity) is negligible relating to the internal energy (thermal energy related to mean thermal random velocity). In this case the gas or liquid density variations due to pressure variations Drau/rau (which always exists !) are very little and can be neglected. The Mach number is also the ratio of Kinetic energy to internal energy.
    This figure on every textbook on aerodynamics or hydrodynamics.
    I don't understand your Crusade, read any textbook !
     
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  9. Joakim
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    Joakim Senior Member

    Yes there is a development of the starting vortex, which is what is seen on the Prandtl photo and which can of course be simulated with a RANS software (sorry, not interested in finding a link for that). And there is backflow around the trailing edge in the very beginning, BUT that does not mean there would be no lift, the lift is just less than it would be later at constant speed. Here is a paper about a flat plate being accelerated to a rather low speed (Re 60 000). The flow field and lift are measured as a function of distance the plate travels from start. Note there is lift from the very beginning of the movement. Naturally there is no lift at zero, since the plate is not moving yet. The plate reaches constant speed at 0.6 c (chord length) and at that point the lift is already close to the steady state value.

    Where do you see onset of lift in that paper? Note that Re 60 000 means a very slow speed for a airplane or a boat.
     
  10. Joakim
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    Joakim Senior Member

  11. Joakim
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    Joakim Senior Member

    I used to work for more than 20 years with RANS CFD doing simulations in many different fields. Have been doing completely different things lately (electronics, mainly highly accurate inclinometers e.g. for inclining tests).

    I haven't written a CFD software (except for some parts during a CFD course for PhD students) and haven't really studied the formulation detailed enough. But for me "incompressible flow" means a flow situation in which the acoustic waves can be ignored and thus no sharp changes of pressure are to be expected. That has actually nothing to do with the compressibility of the fluid. Also water is compressible and needs to be modeled as such when acoustic waves are important.

    I did a lot of modelling using an incompressible solver, but with density defined as a function of pressure, temperature and composition in each computational cell. Even large changes in pressure can be solved as "an incompressible flow" accurately. E.g. a compressor or flow through a valve with a lot of pressure drop.

    Out of personal interest I even modeled a planing boat during spare time. It was just a quick test, but results were promising when compared to measurements.

    For actual work I did a lot of simulations of propellers, but not for propulsion. These were used for mixing. In some cases there were three phases (solids, liquid and gas) and obviously gas compressed due to hydrostatic pressure. Chemical reactions, mass transfer from phase to phase etc. was sometimes included. Still with "incompressible flow" methods.

    Here is a short summary of different approaches used in CFD.
     
  12. Joakim
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    Joakim Senior Member

  13. sandhammaren05
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    sandhammaren05 Senior Member

    Sounds like interesting work for you. I've boated and raced since an early age. My main interest is surface piercing propellers, since I was encouraged in that direction years ago by an former OMC head of engineering after winning two national championships back to back. It looked too hard for me in 1980, I made no headway until I picked it up again in 2015. Du Cane's old book was a great help in getting me to focus (preceded by a prior private remark by Baeckmo that one should focus on the Froude nr. in replacing Crouch's incorrect formula). I'm not sure why you worry about accoustics unless you were doing prop research for submarines. For propeller theory (not worrying about submarines) mass conservation with constant density (incompressible flow) gives us potential flow and vortex theory, and that's enough. I'm pretty sure you understand that. I'm not a fan of computer programs that other people have written because error growth is almost always ignored. When we simulate anything (finance markets models, stochastic processes, nonlinear dynamics, turbulence) we write our own programs-with knowledge of error growth due to discretization, roundoff, and truncation. I would like to see the link on RANS and lift. In 1980 I was given access to Baumann Propeller in Houston where I built my own props for competition, sometimes in between teaching physics classes at the university.
     
    Last edited: Apr 15, 2018
  14. sandhammaren05
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    sandhammaren05 Senior Member

    I don't dismiss models, I dismiss ridiculous remarks made off the cuff. With no real content. Like yours.
     

  15. sandhammaren05
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    sandhammaren05 Senior Member

    Yes, the paper by Kristoffersen et al is interesting but not surprising. All who understand hydrodynamics know that vorticity begins in the boundary layer, that's their focus. One can look at that kind of detail, but once we know that the vortex pair develops we don't need it-ideal fluid theory is still the tool for describing lift, unless you want to understand the reduction of lift in a turbulent flow. Even then, a free stream approximation will have to be made for the average flow in the averaged NS pdes. So I see what you're worrying about, the details of generating vorticity in the b.l.
     
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