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Fluids mechanics is also gas mechanics?

  1. Jul 14, 2011 #1

    Femme_physics

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    "Fluids mechanics" is also gas mechanics?

    This bothers me a bit:

    Gases? You include in the chapter of "fluid mechanics" gases? These are two different states. Why not have a field called Gas mechanics? And if you want, call the entire field "Fluid-gas mechanics".
     
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  3. Jul 14, 2011 #2

    Pengwuino

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    Re: "Fluids mechanics" is also gas mechanics?

    By the definition of "fluid", gases are considered fluids. They are not separate states of matter; it's how they interact with the environment that's important.
     
  4. Jul 14, 2011 #3

    Femme_physics

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    Re: "Fluids mechanics" is also gas mechanics?

    From chemistry I learned there are 3 states of matter: Solids, fluids and gases. If they're not separate states, then they shouldn't be given a separate state status!


    I addition, this quote is from wiki:

    But gases don't just take the shape of their container...they move around BOUND by it, but don't "take the shape of it". I'd say it's a false defintion then.
     
  5. Jul 14, 2011 #4

    SteamKing

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    Re: "Fluids mechanics" is also gas mechanics?

    If it will ease your tortured mind, there is a special branch of fluid mechanics call "gas dynamics".

    Under certain circumstances, the equations governing the flow of gases are similar to those governing the flow of liquids. When these circumstances are not present, then the compressibility of gases (liquids are generally incompressible) requires modification to the equations of fluid flow.

    BTW, in addition to the three phases of matter normally encountered on earth, gases which are ionized and at high temperature are called plasmas, and plasmas are considered a fourth phase of matter, because they behave unlike the other three.
     
  6. Jul 14, 2011 #5

    Pengwuino

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    Re: "Fluids mechanics" is also gas mechanics?

    Sorry, I mean fluids are not meant to be part of the gas/liquid/solid/plasma classification.

    No, they do take the shape of it. Maybe you're confused because sometimes you see "heavy" gases that kinda stay at the bottom of a container, but that's simply because there's gravity pulling it down. In vacuum, they'd be taking the shape of the container.
     
  7. Jul 14, 2011 #6

    Femme_physics

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    Re: "Fluids mechanics" is also gas mechanics?

    AHA! So, in fact, the statement that "Fluids include liquids, gases, and plasmas" is false, whereas it should says "fluid MECHANICS include liquids, gases, and plasmas".

    And I rather resent you opening with "if it will ease your tortured mind", as though it's a silly issue to raise. Why I hold accuracy to be an important virtue.

    Interesting.. I'll read on that!

    But another thing on wiki I wonder about

    This is because the fluid sticks to the walls, right?
     
  8. Jul 14, 2011 #7

    Femme_physics

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    Re: "Fluids mechanics" is also gas mechanics?

    I never heard that. Gosh, chemistry and physics are further apart than I thought!

    No, rather, this is what I imagine (the blue inside the container being the gas)

    http://img39.imageshack.us/img39/2663/container11.jpg [Broken]



    Kinda like smoke in a container
     
    Last edited by a moderator: May 5, 2017
  9. Jul 14, 2011 #8

    I like Serena

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    Re: "Fluids mechanics" is also gas mechanics?

    You'll typically see shapes likes this when you mix 2 gases, in this case a blue gas with air (which is transparent of course).

    The air fills the entire container and in time the blue gas will mix more thoroughly with the air, filling the container completely as well.
     
  10. Jul 14, 2011 #9
    Re: "Fluids mechanics" is also gas mechanics?

    Elementary classifications are just that. Elementary classifications.

    When you go into most subjects more deeply they become complicated, the boundaries between classifications blur and new classification categories become necessary.

    The solid / liquid / gas classification is both ancient and elementary.
    It has been found wholely inadequate by modern science and technology.

    Initially fluids (= that which flows) included liquids and gasses, although several famous textbooks have and continue to be published under the title 'hydrodynamics' or even 'Hydraulics'.
    In the latter half of the twentieth century this category also softened as it was realised that the same mechanics applies also to powders, sand, bulk grain and other granular material, mixed state materials such as freshly mixed concrete before it sets. The list is constantly being extended and time is now a factor for instance geologists may regard ice as a fluid.

    Physicists now like to regard plasmas as a separate state from gasses, rather than just gasses made up of charged particles. The mechanics of plasmas is often called magnetohydrodynamics, although water is not involved!

    Equally chemists started distinguishing many states - the dissolved state, the adsorbed state, the disperse state (eg the smoke in your picture) and so on.

    I expect, if you think back, you will notice that the examples for solid/liquid/gas were given of pure substances. These days we tend to reserve that classification for pure substances that obey the 'phase rule' in chemical thermodynamics.

    go well
     
  11. Jul 14, 2011 #10
    Re: "Fluids mechanics" is also gas mechanics?

    Yes you are right Femme ,Fluids mechanics" is also gas mechanics.
    Fluid mechanics, especially fluid dynamics, is an active field of research with many unsolved or partly solved problems. Fluid mechanics can be mathematically complex.
    by Electrician[/PLAIN] [Broken] Leeds
     
    Last edited by a moderator: May 5, 2017
  12. Jul 14, 2011 #11
    Re: "Fluids mechanics" is also gas mechanics?

    Probably this is just about words and definitions. :tongue2:

    In physics we say that there are 3 (4) states of matter: solid, liquid and gas (and plasma).

    Compare http://dictionary.reference.com/browse/fluid :

    1. a substance, as a liquid or gas, that is capable of flowing and that changes its shape at a steady rate when acted upon by a force tending to change its shape.

    Cheers,
    Harald
     
  13. Jul 14, 2011 #12

    Dale

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    Re: "Fluids mechanics" is also gas mechanics?

    You are confusing "fluid" and "liquid". The 3 states of matter are solids, liquids, and gasses. Both liquids and gasses are fluids.

    Fluid means that the material continually deforms under shear stress. I.e. the shear rate is proportional to the shear stress. This definition covers both liquids and gasses.

    A liquid is a fluid which is incompressible, and a gas is a fluid which is compressible. But they are both fluids.
     
  14. Jul 14, 2011 #13

    Femme_physics

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    Re: "Fluids mechanics" is also gas mechanics?

    Ah, I see!

    I fully accept your explanation :approve:

    I fully accept this explanation as well :approve;


    Thanks to the others for their replies. I got nothing else to say other than I accept your explanation :smile:


    Can anyone though answer me for what I asked before


    This is because the fluid sticks to the walls, right?
     
  15. Jul 14, 2011 #14

    Andy Resnick

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    Re: "Fluids mechanics" is also gas mechanics?

    Liquids and gases are both considered "fluids", as has been pointed out. The relevant parameter in your question is the Knudsen number:

    http://en.wikipedia.org/wiki/Knudsen_number

    For Kn >>1, the continuum approximation breaks down and we instead model the fluid as a dilute gas using statistical methods.

    Gases are not inviscid! In fact, accounting for the difference in density (the kinematic viscosity, measured in Stokes ), air is as viscous as water. Fluids don't "stick" to walls (exempting adhesion/bonding/chemical interactions)- the no-slip condition arises simply by demanding the stress tensor be finite.
     
    Last edited: Jul 14, 2011
  16. Jul 14, 2011 #15

    Femme_physics

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    Re: "Fluids mechanics" is also gas mechanics?

    The quote said "fluid" which can also mean liquid! As you probably already know.
     
  17. Jul 14, 2011 #16
    Re: "Fluids mechanics" is also gas mechanics?

    FP,
    How about moving on from the argument and telling us what aspects of fluid mechanics you are studying?
     
  18. Jul 14, 2011 #17
    Re: "Fluids mechanics" is also gas mechanics?

    Fluids aren't really inviscid. I guess you can think of it as "sticking" to the wall if it helps you, but I think Resnick answered the question very succinctly. We assume inviscid flow a lot of times to simplify the equation when we know (or think) that viscosity doesn't play a large part in the problem we're trying to solve. However, when dealing with certain types of problems (like in aerodynamics), you can't assume inviscid flow.

    Hope that helps.
     
  19. Jul 14, 2011 #18

    Femme_physics

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    Re: "Fluids mechanics" is also gas mechanics?


    Oh I'm just trying to enrich myself before the semester starts :smile:
     
  20. Jul 14, 2011 #19

    cjl

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    Re: "Fluids mechanics" is also gas mechanics?

    Oh dear.

    The Knudsen number has absolutely nothing to do with any of the concepts discussed in this thread. The Knudsen number describes whether a gas is sufficiently rarefied such that individual molecular effects must be taken into consideration when modeling the flow. It comes into play when modeling either very low density flows (such as satellite drag or reentry) or very small scale flows (such as the flow around a hard disk drive's head, or flows around some MEMS and NEMS devices). While the Knudsen number is tremendously useful in some cases, it's completely irrelevant for this discussion.


    True, gases are not inviscid. However, their viscosity is small enough that in some cases, the viscosity can be ignored and useful solutions still arise (which is actually true for some liquids as well). As for air vs water viscosity? Kinematic viscosity isn't really the relevant parameter - dynamic viscosity is the much more common parameter, and using that measurement, air is far less viscous than water (as would be expected).

    As for the no-slip condition? No, it does not arise from the need for the divergence of the stress tensor to be finite. It arises from the tendency of the flow to stick to the surface. At a molecular level, individual surface reflections tend to be diffuse, which means that the outgoing angle of an individual molecule after impacting the surface tends to be independent of the incoming angle, and statistically distributed. When averaged over large numbers of molecules, this means that the reflected fluid is stationary with respect to the surface, aside from the velocity away from the surface (that comes from the fact that we are only considering reflected molecules). These reflected molecules then interact with incoming molecules, and the net result is that the fluid adjacent to the surface is stationary for any flow in which the molecular interaction length scale is substantially smaller than the object's length scale (such that the reflected molecules can interact with and slow down the incoming molecules).
     
  21. Jul 14, 2011 #20

    cjl

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    Re: "Fluids mechanics" is also gas mechanics?

    Actually, in aerodynamics, inviscid flow is frequently assumed, as inviscid flow plus a couple of small corrections (the biggest one for aerodynamics is the Kutta condition) can give surprisingly accurate results for high Reynolds number flow. You do need viscosity to correctly model the skin friction drag and boundary layer behavior, but induced drag and lift don't really need viscosity to determine an accurate solution.
     
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