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Fluid Mechanics help !

  1. Aug 11, 2009 #1
    Fluid Mechanics help...plz!


    I've come along these lines from "Fluid Mechanics" - by Streeter,Wylie,Bedford

    These are the lines:[Quoted]
    Section title:[Pg 15-16]
    I just don't understand this....
    more specifically "negative absolute pressures"...whats the matter here...can anyone explain it to me..plz??....:confused:
    Thanks in advance...
    Any views are welcome!...:smile:
  2. jcsd
  3. Aug 11, 2009 #2


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    Re: Fluid Mechanics help...plz!

    Negative absolute pressures? Perhaps it is a type-o in which they should have said negative gauge pressures?
  4. Aug 11, 2009 #3
    Re: Fluid Mechanics help...plz!

    The point about negative pressures is that you simply can't have them. (This isnt fully true as surface tension can support tiny tensions)

    Pressure is the force/area, and sign convention is that positive pressures are when the material is being compressed (squashed).

    Liquids can support loads when they compress, so you canpush on a liquid and it will support you.

    Try pulling a liquid, what happens. There is no resistance, this is because they cant support tension loads.

    I dont think i've explained that very well, can you break down the question into several smaller questions, specifically stating what you dont understand please?
  5. Aug 11, 2009 #4
    Re: Fluid Mechanics help...plz!

    I understand that a liquid would vaporize when subjected to tensile stresses...
    but the statement says..."normally"...here
    Why does "normally" mean..in which cases this doesn't stand true??...:confused:

    moreover note this part:

    I think this shouldn't necessarily hold true for "fluids"...as the statement says...it should say for "Liquids"...am I right??

    and also I didn't get this also:
    I think due to surface tension the fluid surface is in tension...but this tension is in the plane of the surface[I mean the surface remains in tension...because of surface tension..but this all is in the surface's plane]...right??...so I think this tension isn't what the author wants to say about..i.e Tensile stresses...these will be perpendicular to the fluid surface..right....??

    plz help me....:confused:
    ..Thanks a lot for all replies....:smile:
    This statement from the book has taken pretty much of my time ...I utilized for studying today....and still it aint clear to me...so plz help me.......:confused:
    Last edited: Aug 11, 2009
  6. Aug 11, 2009 #5
    Re: Fluid Mechanics help...plz!

    Fluids encompass anything that moves, so gases and liquids. Although thte gas is more extreme than a liquid they both act is a similar manner, in that it doesnt support compressive loads very well either.

    You misunderstand about the surface tension. The fluid wants to stick together because of intermolecular forces, this is what creastes surface tension.

    For the same reason that we have surface tension, the fluid can support a light tension load (giving a negative pressure) because they dont want to be pulled apart. however this is very small when compatred to the compressive load a fluid can withstand so it is ususally just neglected.
  7. Aug 11, 2009 #6
    Re: Fluid Mechanics help...plz!

    A pressure is a force over an area. A positive absolute pressure is a pushing force against an area. In order to have a negative absolute pressure you need to create a pulling force against an area. How can you accomplish this with a fluid?
  8. Aug 11, 2009 #7
    Re: Fluid Mechanics help...plz!

    say in a journal (hrdrodynamic or EHL) bearing, as it rotates the lubrication fluid gets dragged round it creates a pressure profile which is mostly in compression. At the very edges there can be a negative pressure where the fluid is both attached to the inner and outer race. The rotation 'pulls' the fluid.
  9. Aug 11, 2009 #8
    Re: Fluid Mechanics help...plz!

    Ok...I think I get that..
    This is what I understand till now:...
    The fluid is capable of withstanding a small amount of -ve pressure[i.e. tensile stress]...because of surface tension...i.e. due to Surface tension which exists because the fluid has resistance to the inward pulling cohesive forces...and this surface in tension resists tensile forces to a small extent...
    and also, liquids are considered to be almost incompressible[because of very large bulk modulus]..so thats why they can sustain compressive stresses....

    But For gases....[even perfect gas]
    They are compressible ... and surface tension is imp here to a even much much lesser extent here..because of lesser magnitude...though the "-ve pressure sustain part" is still applicable here...right??
    I would like to know "what can we say for the compressive stresses for gases"...as gases are compressible...
    I think for gases we can say that they don't sustain compressive stresses initially..right??..:confused:

    I think you have explained the "-ve pressure" part really well..thanks...:smile:[I wasn't just getting what the author meant by -ve abs pressure..before posting here]

    As fas as I understood what you want to convey...I think your question itself has the answer.....-ve pressure should mean tensile forces..right??:confused:
    Last edited: Aug 11, 2009
  10. Aug 11, 2009 #9
    Re: Fluid Mechanics help...plz!

    Fluids seems ok.

    To be honest i'm not sure what you'd say for gases due to compressibility, i'd have to read up as i've forgotten a lot of what I used to know. The below may not be fully true.

    For gases it depends on the speed of loading, at low speed loading gases dont support compressive loads as the air just moves round the object. Above a certain speeds (Mach 0.3 iirc) compressiblity is a factor and it then begins to support loads.

    EDIT: I've just realised a possilbe point of confustion, the speed is only really critical in open flows such as an aeroplane wing and air. In closed systems such as a piston cylinder, the speed of compression doesnt matter, but the resistance will increase the more the gas is compressed.
    Last edited: Aug 11, 2009
  11. Aug 11, 2009 #10
    Re: Fluid Mechanics help...plz!

    I think I need to think about your last reply properly...
    I will reply as soon as I have understood it properly....

    Thanks anyways..I think the 1st statement I posted[very 1st post] ..that wasn't understood by me earlier....is much clearer to me now...
    Thanks a lot ...for your help..:smile:
  12. Aug 11, 2009 #11
    Re: Fluid Mechanics help...plz!

    Anytime, help is what we are here for.
  13. Aug 19, 2009 #12
    Re: Fluid Mechanics help...plz!

    Hi there... :)
    I think I need to go back and understand what "Sustain -ve Pressure(Tensile stress)" means...
    Like for Liquids...we say it doesn't sustaiin -ve pressures because it would vaporize.....for compressive stresses(Pressure)...Its almost incompressible...so we say it Sustains Pressures(+ve)"..
    ..but how does one talk of this for a Gas..on what basis I say that "A gas is able to/(not able to) Sustain a pressure"..:confused: :confused:
  14. Aug 19, 2009 #13
    Re: Fluid Mechanics help...plz!

    it is simply compressed till the internal pressure becomes equal to the external pressure.(remember gas laws??)
  15. Aug 19, 2009 #14
    Re: Fluid Mechanics help...plz!

    Hi ank_gl..thanks for replying...:smile:
    ya...but what is it that makes me say "The Gas sustains Pressure"...especially during the compression...before its pressure is going to be equal to ext pressure..:confused:...
  16. Aug 20, 2009 #15
    Re: Fluid Mechanics help...plz!

    I don't get the question.

    Anything that has pressure can sustain a 'pressure'.
  17. Aug 20, 2009 #16
    Re: Fluid Mechanics help...plz!

    When you talk about incompressible liquid, it doesnt mean they are not compressed at all, there is a small compression even in incompressible fluids, but it is far less then what would it be for a compressible liquid. I forgot what that term is called, i think its bulk modulus, thats pretty high for incompressible fluids.

    Lets get to the basics. Imagine a piston in a cylinder containing a compressible fluid & atmospheric pressure on the other side of the piston. In this state, pressure on both sides is equal (assuming mass less piston). Lets say now the piston is pushed with a force of 10N, what happens?? The fluid obeys some freaky law & its pressure is increased. Think of it like, when all particles are pushed closer to each other, they try to repel each other more. So anyways, pressure increases, & volume decreases. This change in pressure & volume continues as long as pressure developed inside the cylinder times the cross sectional area of the piston becomes equal to the applied force, ie. 10N. This is how a fluid sustains pressure(or force).

    In case of (so called)incompressible fluids, some other freaky laws are obeyed & change in volume, for the same pressure rise, is negligible.
  18. Aug 21, 2009 #17
    Re: Fluid Mechanics help...plz!

    Hi...ank_gl and Chris....:smile:
    In other words the question can be put as follows -
    Whether the term "Sustain Pressure" has certain specific requirements so that I say "A fluid is sustaining a pressure{+/-ve} ...
    Like consider a liquid first...
    On subjecting to +ve pressures it suffers a negligible change in its volume...the effect of pressure is only a negligible change {so almost incompressible} in its volume with corresponding rise in pressure=Ext {+ve pressure}....
    On subjecting to -ve pressures it would vaporise {Neglecting small "surface tension"}...here the liquid is undergoing a complete change of phase when subjected to -ve pressure...but no such changes were observed during +ve pressures....

    But gases are always compressible{so are liquids but to vvvv small extent}
    ...The term "Sustain" for gases-does it have certain needs..on the basis of which we say if its able to sustain pressure....
    Also ... "Does gas sustain '-ve pressure' ".. I don't think they can...even to the extent the liquids can {because of surface tension}...so does that mean "Sustain" just means that the fluid would resist the ext. pressure{so from above line this pressure has to be +ve...} at some of time as the pressure is applied...
    Fluids generally can't sustain -ve pressures Right..??... :confused:
  19. Aug 21, 2009 #18
    Re: Fluid Mechanics help...plz!

    Very bad choice of words by the author indeed. I believe such analogies should be used for self understanding, & should not be published.

    Technically speaking(in the world of fluid mechanics), negative pressure is pressure below the atmospheric pressure. Pressure is always(not always) measured relative to atmosphere, ie. P[tex]_{actual}[/tex] = P[tex]_{atm}[/tex] + P[tex]_{measured}[/tex]. So if P[tex]_{actual}[/tex] is less than the atmospheric pressure, you get negative gauge pressure.
    P[tex]_{actual}[/tex] can reach 0 ultimately when P[tex]_{measured}[/tex] reaches -P[tex]_{atm}[/tex]. Or in other words, maximum(or minimum??, whatever!!) P[tex]_{measured}[/tex] can be -P[tex]_{atm}[/tex].

    But in no way it means that pressure starts pulling things apart instead of pushing them together.

    Solid Mechanics
    Imagine a cylindrical bar, if at the ends a force is trying to pull apart the bar, the bar is said to be in tension, or positive pressure, or positive stress.
    <------ |||||||||||||||| ------>(positive stress).

    If the force tries to push the bar like this,
    ------> |||||||||||||||| <------
    the bar is in compression, or negative pressure, or negative stress.

    The author has used the analogy between the two subjects to create a concept, which is super stupid in my view.
  20. Aug 21, 2009 #19
    Re: Fluid Mechanics help...plz!

    I think the author was plenty clear in what he meant by that line. I fail to see what was stupid and/or confusing about it. The authors words were perfectly fine. :uhh:
  21. Aug 21, 2009 #20
    Re: Fluid Mechanics help...plz!

    Absolute pressure cannot be negative for a fluid. Even if there are only 2 atoms in the whole galaxy, the pressure would be greater than absolute zero.

    Regarding the evaporation of liquids, you need to understand the concept of vapour pressure. At each temperature, there exists a pressure at which the fluid starts boiling. Refer wiki.

    As soon as the pressure varies, the fluid condenses or evaporates(vaporizes) to maintain the equilibrium, ie. for evaporating a fluid, negative pressure is not necessary, only a decrease in pressure is.
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