Air as a Fluid - Question

Discussion in 'Architecture & Engineering' started by Facial, Dec 6, 2010.

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  1. Facial Valued Senior Member

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    Can an infinitesimal unit mass of air be assumed to maintain kinematic compatibility with the walls of a pipe

    In other words, does air stick to surfaces and walls like water does for fluid flow?
     
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  3. James R Just this guy, you know? Staff Member

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    Yes it does. It sounds like the quantity you need to compare is the viscosity of air and water.
     
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  5. Facial Valued Senior Member

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    Thanks for the response - I asked since an ideal gas is not supposed to have any electrostatic interaction with a solid surface; we know this is not the case for real gases, which exhibit adsorption.

    However, the colliding molecules of gas just beyond the adsorbed layer experience the same shear stress profile (which is theoretically zero at the wall) as a liquid - which has obvious electrostatic interactions and (generally) less solid-liquid interfacial tension.

    But because we know gases too have viscosity, there must be some quasi-static free gas layer just beyond the adsorbed layer. Gases would flow much more readily, for example, having a far greater magnitude of parabolic flow distribution in a pipe.
     
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  7. rcscwc Registered Senior Member

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    Air too sticks to the pipe walls. Since air too has viscousity, the velocity of gas across the pipe cross section is not, cannot be uniform, just like water.
     
  8. scifes heckle the snobs Valued Senior Member

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    yay! at last i'll get to unleash some ScIenCe on all yer asses!
    that's because i'm battling fluid mechanics this semester, i lost 12 to 20, but since the average was 8, i don't think it was that bad.

    to answer your question, all math trying to explain fluid flow just don't turn out correct results when applied experimentally, till a lucky guy had a eurika moment and put an assumption or theory [iow, it came out of nowhere] called the "zero slip condition" which says whenever a fluid is in contact with a solid surface, the small layer of molecules directly in contact with it acquire its velocity, and that fixed all previous equations.

    so, what about the layer of fluid molecules derectly in contact with the first layer[that's stuck with the solid surface]? well it's a bit slower[or faster] than it. and so's the one next to it, till the velocity becomes constant, right there, it's called a boundary layer[between fluid particles affected by the solid surface and those far away not to be affected and have constant velocity.]

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  9. YellowDemon Registered Senior Member

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    Normally in fluid mechanics we always have no slip condition at the contact of fluid and surface encountering flow so yes a layer of air is assumed to be static.

    And do not count much on ideal gas theory, all the postulates of it are wrong in one way or the other
     
  10. Odin'Izm Procrastinator Registered Senior Member

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    You can see that it has a no slip condition from the equation for the Reynolds number, your viscous and density terms change, so the profile is different, but it obviously still acts as a fluid.
     
  11. rcscwc Registered Senior Member

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    Fluid mechanics deals with practical engg aspects of fluid flows. Hyderodynamics is theoretical and hence its results are useless.

    Navier-SDtokes equation brings out the difference, but is cannot be solved analytically.
     
  12. Dywyddyr Penguinaciously duckalicious. Valued Senior Member

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    Huh?
    If you mean hydrodynamics then you're wrong. On both counts.
     
  13. origin Trump is the best argument against a democracy. Valued Senior Member

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    I would just like to add my 'huh?' to Dywyddyr's 'huh?'

    perhaps you would like to elaborate, rcscwc?
     
  14. rcscwc Registered Senior Member

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    Hydrodynamics does not take into account the viscosity, fluidmechanics does.
     
  15. Dywyddyr Penguinaciously duckalicious. Valued Senior Member

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  16. jonte92 Registered Senior Member

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    air is also a fluid and it thus takes to the characteristics of other fluids
     
  17. Michael 歌舞伎 Valued Senior Member

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    Viscosity is related to flow through Poiseuille's Equation (which is why viscosity units are named Poise

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    As for why the boundary layer doesn't move, that's adhesion force with the wall, whereas the fluid itself is cohesive. Loss of kinetic energy due to heat is friction between the layers. Scifes example assumes the fluid is laminar flow. If it were turbulent flow, then that example would be incorrect.
     
    Last edited: Aug 7, 2011
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