Demistifying Froude number

Discussion in 'Hydrodynamics and Aerodynamics' started by gonzo, Feb 2, 2020.

  1. Barry
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    Barry Senior Member

    I never suggested that when you are analyzing flow that Newtonian equations cannot be used in compressible fluids and certainly did not say that air is incompressible as you stated.
    Air has mass, Bernoulli's equations, F=ma, the momentum equations etc can be applied in many cases. To suggest that air compressibility in say a wing can be ignored is not correct.

    I introduced my first post due to the your comments that you made below. One that "fluids" encompassing both incompressible and compressible, refer to a very abrupt increase in density
    and that there is no transfer of the boats kinetic energy to the water though before the water is moved out of the way of the boat it has no velocity or kinetic energy but the water mass
    is given a velocity and hence a kinetic energy

    DC'S comments

    "Shock" in fluid mechanics refers to the very abrupt increase in density" " but that kinetic energy is not transfered from the kinetic energy of the boat generating the waves"


    So a summary
    Fluids exist in two forms, incompressible and compressible.

    Incompressible fluids are liquids and are governed by the basic Newtonian equations for calculations, mass, velocity, accelerations etc. Gas laws are not applied to calculations of incompressible
    fluids

    Compressible fluids are gasses and are governed by the Ideal Gas law and the other 4 or 5 gas laws. When designing a compressible fluid system, compression, ie change in density, will often
    have to be accounted for.

    Over both of the fluids, many other equations can be applied. Bernoulli, momentum, and others
     
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  2. Mikko Brummer
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    Mikko Brummer Senior Member

    A little out of topic, sorry, but demonstrating how a (classic) sailboat is indeed pushing water aside and dragging it behind itself.

     
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  3. DCockey
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    DCockey Senior Member

    When is the compressibility of water or even other liquids taken into account in design or process work? When working with hydraulic higher pressure motors, pumps and rams, this parameter
    has never entered into my calculations.

    The compressibility of water or other liquids can almost always be ignored when considering flow situations.

    Interesting.

    Water waves have kinetic energy as I've said several times. For water waves generated by a moving boat that energy is imparted by the motion of the boat. But it is not related to the kinetic energy of the boat.

    The compressibility of air is routinely ignored when analyzing the flow around sails, subsonic aircraft, ground vehicles and many other situations.

    This is where I disagree. No fluid, gas or liquid, is truly incompressible. Flows can be split into two types; those for which compressibility effects are important which includes some but all flows of gas, and those for which compressibility flows can be ignored which includes some but not all flows of gas and virtually all flows of liquids.
     
  4. Barry
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    Barry Senior Member

    So a question, You have a boat moving with a mass at a specific velocity, and KE is given as 1/2 mvv, and you stop the propulsive force. The boat has a KE and it slows. The main reason, ignoring
    viscous drag, is that the KE of the boat is "I hate to use this word but" absorbed by the water and turned into Kinetic energy. (and PE for wave height differential)

    But you are saying that the boats kinetic energy is not transformed into KE in the water. Then Where or how does the movement of the boat result in KE in the water?
     
  5. DCockey
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    DCockey Senior Member

    If you read my earlier posts you will see that I specifically referred to situations where the boat is moving at constant speed. In that situation the kinectic energy of the boat is not involved with wave making.

    If the boat is moving and the propulsive force is stopped or reduced then kinetic energy of the boat is transfered to the water in several ways including wave making, "added mass" (which I mentioned above) and viscous effects. Kinetic energy is also transfered to the air (exception may be if the tail wind is sufficiently strong).

    Were you talking about boats deaccelerating and not moving at constant speed previously?
     
  6. Erwan
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    Erwan Senior Member

    It is a great demonstration that Modelisation is not far from an Art. It is a great spectacle

    Happy to see the french Dassault System is useful for Finnish smart person,
    still looking forward to see if it could be useful for french designers somedays !

    Thanks for posting.
    EK
     
  7. gonzo
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    gonzo Senior Member

    Very nice video. It is realistic and illustrates the observed behavior of boats.
     
  8. tspeer
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    tspeer Senior Member

    Xflow is a Boltzman-Lattice code, right? It's interesting to see in the last scene how it has problems with retaining air patches on the underwater surfaces, like some RANS codes do as well. Does that stem from using a volume of fluid approach to model the free surface?
     

  9. Mikko Brummer
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    Mikko Brummer Senior Member

    Hi Tom,

    Yes, XFlow is LBM-based. If you are referring to streaks of air entrainment as seen on some RANS free surface sims, I don't think this is about that. I think this is rather just an artefact of visualization in XFlow - what I'm showing in the last scenes is an isosurface of volume of fluid (VOF), and with different fractions of water, results will look different. The free surface is still a work in progress at XFlow, like everything else, and not at a very high priority - visualization of it has even a lower priority, I'm afraid. It probably also has to do with the rather low resolution I'm having on the hull itself - the free surface is computationally very demanding, as you need to assign the same resolution over the whole surface.
     
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