Fluid Mechanics help !

  • Thread starter nanunath
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  • #26
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I used the 'gauge pressure' because thats what it is... I really dont kow how I can say that any other way.

Do you know what the difference is between gauge and absolute pressures?

When you say 'applied' pressure, that IS gauge pressure.

The liquid when subjected to -ve pressure....would expand continuously....... as soon as the pressure it is subjected to becomes less than the Saturation pressure at corresponding temperature....
but this pressure can be above the atmospheric pressure...

If you have applied a negative (gauge) pressure, the pressure in the liquid must, by definition, be lower than atmospheric.
 
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  • #27
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According to me...
Pgauge + Patm=Pabsolute...
But..I really dont get even 1% of this statement-"When you say 'applied' pressure, that IS gauge pressure"....why???..I never heard of such thing before...apllied pressure is always gauge..why????????..:confused:
 
  • #28
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When something is resting at standard conditions and you arent doing anything to it, it's at atmospheric (or surrounding) pressure. The moment you apply any pressure (push or pull, positive or negative) to it you are altering FROM atmospheric. If you allpy a positive pressure, you have positive GAUGE pressure.

Lets for example say we have a box of air. At 1 Atm inside it.

What will a pressure gauge read? Nothing.
1 Atm absolute = 1 Atm (atm) + 0 Atm (gauge)

Now lets say we apply 1 Atm pressure to it (compressive). The atmospheric pressure doest change.
What will the gague read? It'll now read 1 Atm.
1 ATm (from atmosphere) + 1 Atm applied (gauge) = 2 Atm total.

So if you apply a negative pressure the absolute pressure MUST be below atmospheric.
 
  • #29
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I think that was alreay clear to me.....
I think you are considering fluid subjected to Patm....
Now consider a fluid under pressure in a container with movable piston....kept in idealised perfect vaccum....the above results are still valid...and any force applied on the piston{away from liquid mass} is what we are trying to say-"fluid subjected to -ve pressure"...right???...
 
  • #30
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No. All the book means when it says a negative pressure. Is negative gauge pressure. As this is fluid mechanics which always refers to gauge unless explicitl stated otherwise.

I cant even begin to try and figure out what you meant by the second part of what you said. Vacuum is the pressure UNDER atmospheric. So a gauge will read the absolute pressure within the cylinder. So its not a good example to use to try and explain your point as its a special case. Cylinders are closed, so the external pressure doesnt really mean anything to the contents inside it only affects the pressure reading.
 
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  • #31
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If you have applied a negative (gauge) pressure, the pressure in the liquid must, by definition, be lower than atmospheric
Why...??.where is this {-ve gauge pressure setting up tensile stresses in fluid} conveyed in the definition of Pressure???..:confused:...2ndly -ve gauge pressure would set up compressive stresses in the fluid not "tensile"

The definition of pressure given in above reply..says "When acting towards the center of mass of fluid" ...so if I say a fluid doesn't sustain -ve Pressure...its -ve because its acting away from the fluid mass..thats all...
I argue on this by the following point:

If the pressure{Absolute} in the fluid can't be -ve that should mean the stresses induced in the fluid aren't tensile(*)..in this case ..the fluid would support{sustain} the external -ve pressure....right???? ..thats contradicting...:confused:

[(*)=Reason :When a fluid is subjected to +ve pressure this sets up compressive stresses in the fluid at any point of time as long as the pressure is +ve....same for -ve pressure...it sets up tensile stresses..right???]


But this doesn't explain it....for that Now thinking in terms of "Gauge" ... the pressure acting on the fluid is still +ve absolute{and the pressure is still directed towards the CM of the fluid}..and it would still induce +ve stresses in the fluid..no way it gives rise to -ve stresses.{because though -ve gauge pressure, it is acting towards the fluid mass ... so that has to induce +ve stresses in the fluid}...which means "-ve gauge pressure" has no role here..its -ve absolute pressure....right????....:frown:

Please note: by fluid here I mean to say Liquids only...I just can't think how do I extend this to gases...:confused:...but thats not our concern as of now...we are interested in liquids subjected to -ve pressure{more specifically our point of discussion is Absolute -ve pressures}
 
  • #32
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Ok, im beginning to have trouble seeing why you are having a problem. I suspect it's with the way I am describing it with the signs. Or whatever, and being honest with you im trying very hard to try to understand what you are saying.

The posts you make are very difficult to read and follow. A conversational writing style over the internet does not work.



Can anyone else have a go at explaining this as i'm starting to go in circles in my head and for the last fwe posts i've been confusing myself.
 
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  • #33
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Ok...just read this part:

1]If the pressure{Absolute} in the fluid can't be -ve that should mean the stresses induced in the fluid aren't tensile(*)..in this case ..so they are comprssive ...the fluid would support{sustain} the external -ve pressure....right???? ..thats contradicting...
[(*)=Reason :When a fluid is subjected to +ve pressure this sets up compressive stresses in the fluid at any point of time as long as the pressure is +ve....same for -ve pressure...it sets up tensile stresses..right???]
But this doesn't explain it...
the point is-"-ve gauge pressures set up compressive stresses in the fluid{because the pressure is still directed towards the CM of the fluid}..which the fluid can sustain...so its -ve absolute pressures which the fluid can't sustain"
this is sufficient to convey what I want to say...I think that is clear...if not..I'll try to post more short and ...more clearly

I suspect it's with the way I am describing it with the signs
No..I don't think so...I think [lets hope..;-) ] I do understand your posts..just 1 think left in there-"I just need why '-ve gauge pressure would set up tensile stresses in the fluid...(according to me they set up compressive stresses)' "
 
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  • #34
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Thats the point you are assuming wrongly. I talked about it in the last post also, you are mixing two different concepts of solid & fluid mechanics.

In solid mechanics, stress becomes compressive, when it goes negative. Thats just the sign convention.

In fluid mechanics, negative pressure just means that pressure is below atmospheric pressure. It doesnt mean that the space starts to pull itself outwards, it simply means that it is pushing in less forcefully. Think literally!!
 
  • #35
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Thanks, that was much easier and I understand where we were having a problem now.

Yes you are right. I'm talking in terms of gauge only as thats what you ususally use when referring to fluids, (which now I look at it again is very confusing, so sorry about that). A negative gauge pressure is applied tensile, as its working to decrease the internal pressure within the fluid.

Now if we add atmospheric (to give absolute) to it as well, then we always have that as a compressive stress. Until you get a - 1 Atm gague pressure at which point you are then acutally trying to put the thing into real tension.


So say we have the box of gas again. Its 1 Atm inside.

We apply a force to increase the volume and decrease the pressure to 0.5 Atm.

We are applying a force equivilant to 0.5 Atm pulling the think aparrt. So we have negative gauge pressure applying a tenile stress to it.

Overall though we still ahve 1 Atm compressing it due to the atmosphere.

So althoug hwe are applying a negative gauge pressure, overall the fluid is still inver compression.

So 1 Atm (atm) + -0.5 Atm gauge = +0.5 Atm abs.



EDIT ank, can you check my post for errors. As im totally confusing myself now.
 
  • #36
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Hi...Chris....
Originally posted by Chris
So althoug hwe are applying a negative gauge pressure, overall the fluid is still inver compression.

So 1 Atm (atm) + -0.5 Atm gauge = +0.5 Atm abs
Whats-"inver"????
anyways..as far I get that post...
That should mean its -ve absolute pressures which fluids can't sustain...and not -ve gauge presures...right??
 
  • #37
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Originally posted by ank_gl
In fluid mechanics, negative pressure just means that pressure is below atmospheric pressure. It doesnt mean that the space starts to pull itself outwards, it simply means that it is pushing in less forcefully. Think literally!!

My reply to this:
I wanna ask the Same thing which I posted in one earlier post ...why would -ve gauge pressures induce tensile stresses....???
-ve pressue{abs}..means its pulling things apart...actually....

-ve gauge pressures won't do this....that means it would induce compressive stresses...because it ain't pulling things apart actually...thats why I say "Liquids can sustain -ve gauuge pressures"...Right???
 
  • #38
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It is seriously getting severe now.

Solid mechanics
Stress can either be positive or negative
Negative stress = Compressive stress, denoted with a "-" sign.
--->|||||||||||<---

Positive stress = Tensile stress, denoted with a "+" sign
<---|||||||||||--->

Fluid mechanics
Pressure can be measured either relative to absolute vacuum or atmospheric pressure.

P_abs = P_atm + P_gauge

P_abs can never be less than zero, it is always positive.

P_gauge can be positive or negative. P_gauge is zero when absolute pressure is P_atm, ie P_gauge changes sign at 14.6psi, not at 0psi, which is creating all the confusion.
Negative gauge pressure = Pressure less than atmospheric pressure; DOES NOT start pulling things apart, it still tries to push thing
Positive gauge pressure = Pressure greater than atmoshperic pressure; acts as you know.

Imagine a cylinder with a piston & an external pressurization setup.
Assume that initial pressure in the cylinder is 1 atm, hence piston is stationary. As the the cylinder is pressurized, the piston starts moving outwards, one can say that the pressure is pushing the cylinder out, but that would not be technically right, it is the pressure differential(P_cylinder - P_atm) that causes the piston to move, or the positive pressure(P_cylinder - P_atm) pushes the piston out.

Now assume that cylinder is depressurized, ie pressure is reduced, therefore net force on piston is(P_cylinder - P_atm)*A, which is negative now, and piston starts to move inwards. That does not mean that negative pressure is pulling piston inwards, it means that the pressure on outside face is more & is pushing in.


@chris, you are right on the money.:smile:
 
  • #39
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Hi...Chris....

Whats-"inver"????
anyways..as far I get that post...
That should mean its -ve absolute pressures which fluids can't sustain...and not -ve gauge presures...right??

Its a typo, it should read 'in' :P

Yes its negative absolute they cant sustain. -ve gauge pressures can be sustained. See post above.

ank_gl said:
@chris, you are right on the money.

Phew!
 
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  • #40
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Originally posted by ank_gl
Fluid mechanics
Pressure can be measured either relative to absolute vacuum or atmospheric pressure.

P_abs = P_atm + P_gauge

P_abs can never be less than zero, it is always positive.

P_gauge can be positive or negative. P_gauge is zero when absolute pressure is P_atm, ie P_gauge changes sign at 14.6psi, not at 0psi, which is creating all the confusion.
Negative gauge pressure = Pressure less than atmospheric pressure; DOES NOT start pulling things apart, it still tries to push thing
Positive gauge pressure = Pressure greater than atmoshperic pressure; acts as you know.

Imagine a cylinder with a piston & an external pressurization setup.
Assume that initial pressure in the cylinder is 1 atm, hence piston is stationary. As the the cylinder is pressurized, the piston starts moving outwards, one can say that the pressure is pushing the cylinder out, but that would not be technically right, it is the pressure differential(P_cylinder - P_atm) that causes the piston to move, or the positive pressure(P_cylinder - P_atm) pushes the piston out.

Now assume that cylinder is depressurized, ie pressure is reduced, therefore net force on piston is(P_cylinder - P_atm)*A, which is negative now, and piston starts to move inwards. That does not mean that negative pressure is pulling piston inwards, it means that the pressure on outside face is more & is pushing in.

Sorry...but I already said what ever I stated isn't for Gases...in an earlier post....

Coming to Gases...
Absolute pressure in gases... I think...can't be negative....and I think thats why you say - "Abs pressure can't be negative"....

But this ain't true for liquids...a liquid would behave differently than gas...
The absolute pressure can be negative for liquids....which induces tensile stresses in the fluid...{Theres no other way you get tensile stresses for a liquid} ...
As soon as the pressure becomes negative...the liquid starts vaporizing....thats what I wanna say....and so gauge pressure has absolutely no role here...

I said earlier I just can't think of how do I extend whatever I said for liquids to gases....:confused:
but our main discussion is the first line-"Liquids normally cannot sustain a tensile (or pulling apart) stress since the liquid would vaporize.Therefore,the absolute pressures used in this book are never negative,since this would imply that the fluid is sustaining a tensile stress" {Plz note the underlined words...this statement is as is by the author....the Author doesn't state anything for the gases....I think...because Pressure in Gases can't be negative{absolute}

...also I don't think that stress and Pressure have different interpretations in fluid mechanics and Solid Mechanics...Pressure and Stress are still defined the same in both...:confused:
 
  • #41
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OK post edited.

Sorry...but I already said what ever I stated isn't for Gases...in an earlier post....

Gases and liquids are both fluids, they behave in the same way according to fluid mechanics.

Coming to Gases...
Absolute pressure in gases... I think...can't be negative....and I think thats why you say - "Abs pressure can't be negative"....

But this ain't true for liquids...a liquid would behave differently than gas...

No they dont.

The absolute pressure can be negative for liquids....which induces tensile stresses in the fluid...{Theres no other way you get tensile stresses for a liquid} ...
As soon as the pressure becomes negative...the liquid starts vaporizing....thats what I wanna say....and so gauge pressure has absolutely no role here...

Absolute pressure can't be negative for the reasons stated. If you get to zero absolute gauge pressure MUST have a role. There is really no debating this.


I said earlier I just can't think of how do I extend whatever I said for liquids to gases....:confused:
but our main discussion is the first line-"Liquids normally cannot sustain a tensile (or pulling apart) stress since the liquid would vaporize.Therefore,the absolute pressures used in this book are never negative,since this would imply that the fluid is sustaining a tensile stress" {Plz note the underlined words...this statement is as is by the author....the Author doesn't state anything for the gases....I think...because Pressure in Gases can't be negative{absolute}


You dont need to extend this to gases, as gases and liquids act in the same manner only to different extents due to gases being more compressible.

...also I don't think that stress and Pressure have different interpretations in fluid mechanics and Solid Mechanics...Pressure and Stress are still defined the same in both...:confused:


They do have different interpretations, as sign convention is diffenrent. In solid mechanics positive stresses are tensile. In fluids tensile acts to reduce pressures, and as such are considered nagative as fluid mechanics uses gague pressure as a standard.



NONE OF THE ABOVE IS UP FOR DEBATE, it is what it is. Now if you dont understand WHY something is so, then ask that quiestion. DO NOT just repeat what you have posted before. Otherwise we'll jsut be going in circles and I HATE merrgy go round threads.
 
  • #42
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You are simply arguing now, we've given you the answers and you are constantly repeating the same erroneous claim
No....No....
I think...I'm simply trying to understand something through discussion...JUST A VERY FAIR DISCUSSION....How you even think I was arguing??....{and if you still think so...I'm sorry}
I'm not claiming anything...just trying to understand that very well thought statement by the author...and trying to support what I think I have understood...I'm/anyone is not going to gain anything by claiming/ falsely convincing myself that I've understood a particular thing...
Lastly..thanks a lot for all the time you spent in this thread but I think I'm done onl when I've understood most of the things...and thats why I'm posting...
Anyways....leaving it what so ever....continuing with the discussion...

You can't have negtive absolute pressure in either. Ther eis no debating this WHAT SO EVER. That's you just cant
For liquids its possible ... to the extent surface tension can resist the -ve absolute pressure{something below -1 gauge}...beyond that it vaporises
and
For gases its not possible...

and

You are also uttrly wrong about gague having no effect. At room temperature the vapour pressure of water is about 40 Torr, which is about 0.05 Atomospheres. All you have to do is apply a negative pressure of -.95 Atm gauge to get it to vapourise. In atmosphere to get to zero absolute its THE GAUGE PRESSURE that is doing it.

Ok....I'll think about this again later...with a fresh mind
 
  • #43
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I've edited my post becuase it was needlessly hostile, and I apologise for that. I got slightly frustrated at seeing what I though was the same post again, until I read it more throughtly and realise you'd stated some truths.

Please see my new post for updated answers to your questions.

Edit: I got the vapour pressure of water incorrect (I just looked it up to be sure), its about 20 Torr not 40.
 
  • #44
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Absolute pressure can't be negative for the reasons stated. If you get to zero absolute gauge pressure MUST have a role. There is really no debating this
Then how do I get tensile stresses in the Liquid??
I think...unless the liquid is actually subjected o a pulling apart stress{-ve pressure}..its not possible to induce tensile stresses in the liquid...
 
  • #45
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Hmm, ok now you've got to be very careful now becuase it's going to seem like im contradicting what I said earlier.


If you had say water in a jar and applied enough negative pressure to it, it would vaopurise at about 0.025 Bar. You wouldnt be able to put it in tension.


However if you had a very very thin tube, so you got small enough for surface tension to totally dominate (Which is in not many situations, which is why the book ignores it). In this situations the liquid CAN be in tension. This is achieved by subjecting it to a pressure below its vapour pressure without it vapourising.

Now these suction pressures can be considered to be negative absolute, they had this debate about the xylem in trees. I'm not sure if they are considered negative absolute pressures or not. Someone with more knowledge will have to answer that one.

EDIT: They are deemed to be negative absolute pressures. So negative absolute pressues can exist but only in special curcumstances.


However this is a special case and not a general rule.
 
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  • #46
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If you had say water in a jar and applied enough negative pressure to it, it would vaopurise at about 0.025 Bar. You wouldn't be able to put it in tension
Right....thats what I was trying to say in my earlier posts...I think....provided....
EDITED:
"applied enough negative pressure" this means...
you assume that you are at Patm and then apply -ve pressure just to make it to .025 abs...it doesn't mean -ve abs pressure...right??


Now these suction pressures can be considered to be negative absolute, they had this debate about the xylem in trees. I'm not sure if they are considered negative absolute pressures or not. Someone with more knowledge will have to answer that one.

EDIT: They are deemed to be negative absolute pressures. So negative absolute pressues can exist but only in special curcumstances.

Hhmm....Ok....
Thanks...
 
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  • #47
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No, surface tension does not provide absolute negative pressure. Infact, there is nothing called negative absolute pressure in context of fluid mechanics.

To clear up nanunath's confusion of assuming surface tension as the source of pulling force, ie so called tensile stress, imagine the following situation.

Imagine a capillary with fluid inside at the middle of the tube.(see the pic attached)

Case#1
If you try to suck out the fluid from one side, a resistance is felt, which is due to the surface tension, the fluid adheres to the tube surface. THIS IS NOT TENSILE STRESS. Surface tension is a surface phenomenon.

Case#2
Now try to blow air into the tube, the fluid is pushed to the other side, but still tries to resist the motion. Would you call this resistance compressive stress? I would say, NO. It is still surface tension
 

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  • #48
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Ank trees can suck up water higher than capillary action at vacuum. There was a massive debate a while ago (apparently) the phenomenon, i've been reading around and it seems as though they are referring to the phenomenon as negative absolute pressure.

In larger trees it's sucking with an equivilant to -2 to -3 Bar abs.

I dont know specifically about the mechanisms at work, so I wouldnt like to comment further until ive read more. But linkys to follow....


Here be dragons...

http://en.wikipedia.org/wiki/Transpirational_pull
http://books.google.co.uk/books?id=...+under+negative+pressure#v=onepage&q=&f=false


Also, is this considerd as putting the liquid in tension as it can flow? Thats another question inhaving issues with.

We've been defining positive pressures (I still keep thinking in terms of gauge, but I suppose any positive pressure is working to compress the fluid) as compressive forces, working to squish the fliud. Surely by extension we have to say that a negative abs. pressure is attempting to put the liquid into tension.



EDIT: Why is nothing ever simple... :cry:
 
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  • #49
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Hi Chris:smile:
Interesting links!!

I am no expert, but I would not treat that case as a proof for negative absolute pressure. I would rather believe that it is a special case where surface tension forces are dominant instead of pressure or inertial force, as is the case with typical engineering problems. Though I will definitely like to see(& will try) some mathematical backup to reach any conclusion.
 
  • #50
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For Liquids :
Liquids have Surace Tension -- Correct. But the Surface Tension as rightly discussed is only a phenomena observed on the surface of the liquid due to the unbalanced cohesive forces acting on the surface molecules from within the subsequent layers below the surface and the adjacent moleccules on the surface.

This does not mean that the liquid can Sustain tensile force/stress. This is actually a pulling force or -ve force applied above the surface. And what it does is creates a pressure lower than the exsisting pressure of the liquid or what we call is partial vacuum.
Under the influence of such pressure the liuqid will simply start to Vaporize if the pressure reaches the required saturation pressure for the given tempreature.

The value of this may vary according to the pressure - tempreature conditions. But ultimately the liquid will Vaporize. This is the effect of a so called -ve pressure on a liquid.

A liquid can only deform under a shear stress and cant be said to sustain tension. Where as under a compressive force it only builds up static pressure but wont deform being Incompressible. ( or deformation is so very negligible, it doesnt count at all).

So we cannot literally say that a liquid sustains a -ve stress or pull. It cant be compared to a solid specimen allowing elastic strain.


For Gases :

Gases when subjected to a compressive force creates a +ve pressure on the gas and tends to compress it reducing the volume and increases the pressure and tempreature. The particular boundary conditions will dictate the type of process and the the value of 'n' in the equation PV^n = C.

As for the confusion you had between gauge pressure and absolute pressure, as mentioned in an earlier post, the relation is

Pabs = Patm + P gauge.

And when a compressive force is applied on a gas P gauge is greate than the P atm ie Pgauge is +ve.

And when a gas is at atmospheric pressure and a we use o force to extend the boundaries of the container holding the gas ( and increasing its volume) we say that a force reduces the pressure of the gas below its original value and the gas expands.

here the pressure we apply can be called as the Pgauge and as it acts in a direction opposite to the compressive ( acting ON the gas) force, it bears a -ve sign.

So, Pabs = Patm + (-Pgauge)

= Patm - Pgauge

Thus there is a reduction in the Absolute pressure and we say that the gas pressure has reduced and it expands.

Thus, the gases immmediately Respond to a change in the external pressure and cant be said to Sustain a pressure but instead React to it. This is because of the molecules being Free to move in space randomly and are not bound to oe and other.( Van Der Vaals corrections not included).

Thus in case of gases we cannot use the term STRESS either Compressive or Tensile but instead we us e the term PRESSURE either +ve (Compressive) or -ve ( gas Expands).

So we say that a gas is subjected to a PRESSURE rather than a Stress
 

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