What causes pressure in an noncompressible fluid?

A nice way to look at it is ... it is the container of the fluid that is exerting the pressure! If it is in an open piper there isn't any pressure. As soon as you close the pipe and pump into it you are effectively trying to expand the pipe and the walls of the pipe are providingthe reaction to this force.... hence if you have a rubber pipe you may get upto say 30 PSI if you have steel pipe say 2oopsi.... The difference in the pipe is causing the difference in pressure therefore it must be the pipe that is 'causing the pressure' (whats the prize eh do I get one of Hendrik's GOOZ cards?)

Hi DelGato

Why ?

I know pipe is elastic, in fact we calculate elasticity (volume increase) in a 3000 km 48 inch crude oil pipeline as it adds tremendous 'inventory' to the pipeline that is technically never sold. But in itself how does the elasticity which is squeezing on my fluid, just like the pump did, raising the pressure?

Just to know: The rubber pipe can cause 2000 psig pressure as well as long as it never yields to a point of leaking (pressure drop). We have rubber 'Kelly' hoses on drilling rigs holding back 10,000 psig of mud pressure.

The prize will be a copy of the link to the discussion on "What does engine cc mean" which contains more information that all the encyclopedias of the world combined (and also 3 posts on what cc means).

George

Why ?..I know pipe is elastic,.........But in itself how does the elasticity which is squeezing on my fluid, just like the pump did, raising the pressure?

The fluid merely transmits the pressure...if you take a solid rod and push one end...wow the other end moves! The tube is exerting the pressure...I'm sure you or someone can very adequately explain how the elasticity of rubber (or steel ) pip works!

I feel the Q has been answered...going further is in danger of getting into the realm of philosophy!

Some would argue it doesn't until you measure it! And of course it's much easier to explain how the pressure effects the measuring instrument, as this will have been designed to treat to pressure.

That's a really great prize I shall use all my catly cunning to make sure I get it....prrrr

Del

PS.. My comment is illustrated nicely by a braking system in a car...the fluid is just the meduim to transmit the pressure.

Your foot (servo assisted by atmospheric pressure) provides the pressure...the fluid transmits it (by virtue of its incomressibility) the pipe work contains it, the brake pads exert this pressure onto the discs etc.

Health and safety warning.... On no account let your cat drive.

Since your question is at a molecular level I think those best suited to give an answer would be a physicist or possibly a chemist. At the molecular level I suspect the answer deals with molecular level or subatomic forces.

With the knowledge that no consensus could be reached I am trying this (stupid but hopefully not idiotic) theory. I haven't really thought of why before. Until about 25 years ago I was more concerned of how to get the water there and lately why the water is needed there or if the water should be allowed to go there.

The small effect of gravitational pressure difference is ignored as it do not really have a influence on the theory.

Because water is practically incompressible a big change in pressure will result in a small change of volume.

The molecules of a liquid, too, can be regarded as particles which are continually in motion and of which the average kinetic energy is depended on temperature, pressure and volume. When the pressure is increased the velocity and the pressure will increase releasing some heat energy and with a resultant attempt to increase volume and aggravating the velocity pressure..

By decreasing or violating the comfort zone around each molecule the electrons will fall into a lower energy orbital state to compensate for the loss in space. The molecule still having excessive energy for the reduced orbital state will result in equal pressures in all directions on the surround.

I cannot wait for your and other answers.

This is a really good answer in that it accounts for the pressure effects of both volume and temperature.

Increaseing molecular activity by adding heat at a constant volume raises pressure, decreasing volume in a manner where there is little heat transfer raises pressure.

The bottom line is the energy of the molecule...

Only one rebuttal to your description Steve.

You Said "Increasing molecular activity by adding heat at a constant volume raises pressure"

If the pressure is maintained to the Nth degree at what point will the temperature drop off and thus lose the pressure that is accompanying it.

Ok that was a bit drab lets try again.

If the force applied equals pressure and the flexibility of the vessel were stable if you stop the force applied and the system is allowed to stabilize at a given pressure. Doesn't the heat dissipate and therefore the pressure returns to zero?

Or are you saying that under pressure the molecules stay in an aggravated state and therefore the pressure is maintained?

Increasing liquid temperature + constant volume -> increased pressure?
Not with water below 4-degreesC, for example. Blink gave a reasonable description of why your statement is often true - which also implied the conditions where it is not.

Ok,

Lets start with the definition of pressure. From Dictionary.com, pressure is "the exertion of force upon a surface by an object, fluid, etc., in contact with it."

So the answer of course is that what is causing the pressure to increase in the pipe is that the force upon the pipe's internal surface has increased due to the fluid in contact with it.

Why?

Because on an atomic level water or other fluid is basically a bunch of teeny tiny teensy super balls bouncing about. When they bounce against the pipe wall they exert a force, sort of like when you were a kid and your cousin bounced a super ball into your eye which really hurt.

But why does the pressure go up?

Because every time the piston goes forward it tries to shove more superballs into the pipe. This is sort of like taking a plastic garbage bag and stuffing grass clippings into it, the more you stuff them more the bag stretches, since stuffing more stuff into the bag increases the force on the bag.

Why are super balls like grass?

Sorry, if you had bouncing super balls and tried to stuff them into a plastic garbage bag, then the more you shoved in there the more they would bouence around and the more the bag would stretch.

Why?

Because anytime you try to 50 lbs of super balls into a 10 pound bag there will be some force exerted.

Why?

Because your mother said so, go clean up your room.

First, I know fluids are compressible so this should eliminate 30 replies telling me this known fact, so lets assume a pipe theoretically full of water (or whatever fluid you wish to put in your virtual model) without any air bubbles, blocked on one end and a PD pump on the other (similar to a hydrostatic test of a length of pipe laying on a shop floor)

Are you speaking strictly of liquids when you use the term Fluids. Air is also a fluid and compressible, as you know.

Please enlighten me though on the degree of compressibility of a liquid. Say brake fluid, if it was compressible wouldn't that make our auto brakes somewhat spongy?

I have always understood that a liquid under pressure is as effective as a solid link setup. Maybe I've been mistaken all these years.

Here is a table has some compressibility of fluids. Note that generally as density decreases, compressibility increases. Mercury is less compressible than alcohol, for example. Brake fluid is about double the compressibility of water, but, water corrodes things, boils, and freezes. So brake fluids are typically glycol based so they don't boil at high heat (brakes get bloody red hot), and won't corrode the steel brake lines and components. Look on the back of your glycol based antifreeze bottle and see how the boiling point is raised with glycol concentration. So for simple short run small diameter tubing medium pressure such as brakes, the compressibility of brake fluid is essentially nil. Boiling point and the ability of the fluid to inhibit water absorption from the humidity in the air (which boils easily and makes brakes spongy going down mountain roads) makes certain types of brake fluids better than others DOT 3 vs DOT 4 or DOT 5 for example. If you clear brake fluid turns brown, it likely has absorbed some moisture from the air and will be spongy when it boils during hard braking, like when my wife enters the garage at Mach 2 and slides sideways just before bumping the water heater.

Thank you very much for the link, very interesting, and I learned something new.

Still for all daily, practical purposes liquids such as water and oil in use in most industrial machinery is so close to non-compressible we just accept this it seems.

Where would it be important to know the compressibility factor, and what type of system has this degree of sensitivity to compression of the medium? Now that you pointed this out, I'm sure there are some systems where this would be extremely important.

Thanks again

ietech

Compressibility is a huge factor in "Fluid Mechanics" where things are studied at rocket science levels for things we take for granted. Here is another link and you can Google forever on this subject. Steve S. can likely answer where we see compressibility in our daily lives as he is a type of guru on this. In my world, compressibility is a factor in a 3000 km 48" pipeline going up / down 4000 m altitude mountains where this energy (a type of spring) is released, compressed, released, compressed and I need to anchor the pipe in different ways so it does't blow out of the ground and surprise Juan Valdez picking coffee beans and cause his donkey to set a speed record to the nearest village. It also, interestingly enough, effects the 'inventory' of the pipeline when we detect leaks. Ex: Pipeline leak detection is VERY sophisticated now, so if I have X barrels of oil going in, and X barrels coming out, the difference can be compressibility, or a leak. So if I loose volume in a 200 km run up hill climbing 1500 m, the pressure is quite high to get the oil over the mountain. I compress the oil a tiny bit, so, I loose volume. So I have densitometers and other little black boxes to calculate whats going on so I don't think I just lost a half barrel (leak starting) and send a HAZMAT crew by chopper in thin air where a crash is likely to the top of Mount Unreachable just to learn there was no leak. A more practical application of compressibility is piping runs in refineries or chemical plants where we have pipe running a mile up and down before they come to a valve, and if someone shuts that valve, the fluid starts stacking up (compressing) until we are at zero flow (pressure equalised inside the pipe; therefore no more flow anywhere in this mile long pipe) and the energy released (like a spring) traveling down the pipe, reversing, and going back again is like a "water hammer" effect and rattles the pipes. Light hydrocarbons have low density and compress several times more than heavy liquids. So in a long high pressure pipe run with a low density fluid (including gas) we need to design things differently. An example of water hammer, not really related to compressibility, but interesting none-the-less, is the knocking you get in your house when the clothes washing machine slams the solenoid valve closed. A pressure wave spikes and makes the noise. This is a dumb example but if this was a mile long run of gasoline and I slammed the valve closed that fast, I'd tear the pipes off the pipe racks. In the gasoline example I'm dealing with compressibility which worsens the water hammer effect, plus, the 'other' phenomena that occurs with pressure waves when I change pressures that quick.

Thanks again -- I can see where this is a very important issue in your work. Anything of that magnitude is certainly sensitive to even the most minute fluctuations. I have never had experience with the types of things you must deal with on a daily basis. I sure puts a new light on things in my mind.

I have been living with this "Old Wives Tale" for a long time. Thanks for the new perspective.

I am currently responsible for 17 miles of various conveyor and sorter systems, mostly mechanical, plc, electrical, pneumatic, and some hydraulic so the need for questioning my old way has never come up.

Sometimes I get so angry for spending all that money on school!

Now I feel like I owe you some too!

cr3

I will bet that the change in volume you cite in your examples is mostly due to the elasticity in the pipe or tubing you contain the fluid in and less likely the fluid itself! Case in point would be the home piping. Copper and PVC are very elastic materials and yield easily. Good thing, too, because it reduces the likelihood of bursting when pipes freeze. Automotive brakes are another example where elasticity in the pipes and the tubing will produce a spongy feel to the brakes. Adding moisture does not make it spongier, but trapped air in the fluid will. On the flip side, all fluids compress! However, their compressibility is lower than the vessel walls that contain them.

I really don't understand your initial question. From the initial glance it seems elemental, so I assume you or I are not communicating efficiently. However, several earlier posts have pretty much hit the nail on the head. Thermodynamics, PV= nRT, and atomic forces are the main ingredients that govern fluid pressure. From a molecular level you have molecules in motion that rebound off of each other and the walls of the vessel that contain them. For the most part you can think of these rebounds as perfectly elastic. The rate of movement is a measure of temperature. The more energy that is applied to the molecular Brownian motion, the greater the forces of the collisions and the greater the frequency of those collisions (F=Ma). When your piston bears down on a closed cylinder you increase the rate of rebound and molecules will, to some extent, compress together. Since the collisions are essentially elastic, the total amount of forces applied to the vessel will proportionally increase (as will temperature). The other side of the equation is the forces pushed back by the vessel walls. As the vessel swells it exerts an equal and opposite force on the fluid. QED.

From a molecular standpoint almost all of the rebounding forces of the molecules will be totally elastic. At some point the molecular and atomic structures will yield with increasing force. Ultimately, total collapse is possible with neutron stars and then black holes. However, for your examples the forces of real interest will be Brownian motion and the repulsive forces of the molecules themselves. Molecules never really touch, nor do atoms! There is a repulsive atomic force that keeps that from happening (matter is mostly empty space).

Perhaps the wide varieties of answers you get are due more to the various specialties of the people you pose your question to. Everyone looks at the problem space through the prism of their own field of expertise. Additionally, the way the question is posed raises a lot of questions in my mind. As stated I really do not understand what you want, so I gave you a very broad and drawn out answer.

If you just answer the question "What casuses pressure in an noncompressible fluid", then the answer is whatever force you apply externally.

Intermolecular forces will change if steady state of fluid is changed i.e.specific weight or specific volume at constant temperature.PV=mRT

Repulsive forces will increase with increased compactness of molecules giving resultant pressure.

I am anxious to know Petro power's thinking.

Ratnagiri

This sounds right to me. The molecules in the liquid want to maintain a particular distance from each other, determined by the balance of attractive and replusive forces. When you apply pressure i.e. reduce the volume, you are forcing them to get closer. This destroys the balance and the repulsive force increases. Just like compressing a spring.

Hey . . . . this is why my first marriage failed. Repulsive forces causing pressure rise. Shazammmn.

As fluid is added to the pipe the equilibrium between the molecules is upset and they start moving around (entropy I think) and in doing so impart some energy or reaction with its neighbour resulting in pressure.

regards JD.

"...fluids are compressible." WRONG! Fluids are NOT compressible.

Bob

Ummm, yes fluids are compressible.... A gas is a fluid and it is very compressible, but even water has some compressibility, very very little, but some. The spring analogy used in one of the posts is not bad. We generally assume liquids as incompressible for the sake of making calculations easier.

BTW, the pipe is also elastic, although we generally ignore that as well unless the pressure is very high.

if gases are also categorised as fluid, probably the term to be used should be liquids and not fluids. gases are compressible but not liquids. somebody said fluids, meaning liquids are compressible to a minute extent. i am not sure and i do not know. during water hammer which occurs in long pipelines when a fast shut off is done, momentum of water creates huge pressures expanding the metal or other pipe. when the built up energy is released in the form of a bounce back, the outcome is havoc in the form of burst pipes, pulled out fixtures etc. there are quite a few devices which take care of water hammer effects. from: shankar, bangalore, india

WRONG, what is gas? I think you'll find it's a fluid.

And can we compress gas?

Everything is compressible if you apply enough pressure. (Just go ask Steven Hawking or John Archibald Wheeler).

Regarding a fluid as incompressible simplifies physical models, but such simplifications are simply that: simplifications. They make analysis easier, to a point, and facilitate understanding (particularly by undergrads); but accurate, detailed models take into account fluid, gas, and solid compressibilities and a host of other realities. They're all compressible. Heck, compress ordinary solid matter enough and it turns into a neutron superfluid with about a trillionth the initial volume. It takes a supernova to pull it off, but it happens all the time. Elsewhere, thankfully.

It's really just a matter of degree. Of course, fluid volume changes far less than a gas at a given pressure, but it does change and this change can be measured.

The reality is that both the fluid and its container are deforming slightly, with concomitant changes in volume, under pressure. Yes, the displacements are very slight, even at high pressures, but they're there and can be accurately measured. You can describe the deformation on a microscopic scale as a reduction of intermolecular distances in the case of the fluid, or as an expansion of the IM distances in the container walls, or whatever, but this description isn't strictly necessary, nor does it apply in all cases. Not all vessels are made of "stuff."

In a Tokamak fusion reactor, for example, a tenuous, hot plasma (several million degrees, typically) is constrained by an applied toroidal magnetic field. The field applies pressure to the plasma which hopefully stays put and doesn't contact the vessel walls and cool. But increase the plasma temperature, and the field boundaries change just as if you had raised the temperature or changed the pressure inside a hydraulic vessel. You can accurately model both the hydraulic case and the field case as instances of elastic deformation with accompanying restoring forces. The restoring force requires a displacement, which is supplied by minute changes in fluid volume and expansion of the vessel itself.

FWIW, Del's pragmatic explanation is best, IMO. Mine are always too friggin' wordy.

Hi Bob and thanks for replying. Join up so we can get to know you. Actually, fluids are compressible, that is, their volume changes with pressure, represented by 'k' in equations. As Steve S. points out, it is very small, but, it is there. Even steel is compressible and you can find the equation for this in books.

But for this question I can rephrase: "in an essentially noncompressible fluid . . . .".

Thanks for the reply as I'm normally wrong on most things so it's a good check to be challenged.

George

Liquids (oil, water etc.) are fluids and are not compressible, some fluids are compressible such as gasses and air.

I'll take a quick stab at it. First, let me say that the teacher in me is trained to never answer a question directly, but with another question. So, as a teacher, I would have to ask you why you think pressure exists in incompressible fluids. But I'm also an engineer, so I will attempt an answer. Fluids exhibit many of the properties of solids, such as an elastic modulus. This means that when they are squeezed, their volume changes and they squeeze back. This is essentially the answer to your question. Fluids differ from solids in that their shear strength is close to zero. This is why they transmit internal pressure uniformly in all directions. However, their elastic modulus accounts for the internal pressure. Of course, when you store the fluid in a container under pressure some of the pressure is provided by the elastic properties of the container, but the question was concerned with the properties of the fluid that rendered it compressible.

I'll stick to my answer, but a nice expanation can be found at www.engineeringtoolbox.com/bulk-modulus-elasticity-d_585.html
This web site gives the bulk modulus (the ratio of stress/strain) for some common fluids and illustrates that fluids are compressible the same way gas is (only less).

It does not take a PhD to answer this one.

Force. It is simple a force that is distibuted. P=F/A, F=P*A

If you don't have the force you don't have the pressure, period, end of story.

Now stress on the other hand, That's the pressure that's created when the mind over rules the the body's urge to choke the person who reallt deserves it.

Stress is when my wife catches me replying to my question on this forum Saturday morning instead of cleaning the garage. Then she calls her mother in Argentina and I break out my water cooled Franklin Spanish / English translator trying to catch the high speed words flying around and catch " should have . . . . married . . . . dentist " or something like that. Then we get into the compressibility of human blood as the pressure rises into a throbbing headache.

Very good question!

First, you're right when saying that all fluids are compressible. To have a better view on the water compressibility, I give you this practical example:

Take a low-carbon steel pipe 450' long, 1" ID and .2" thick, plugged at one end and having a free piston very well sealed at the other. If the pipe is filled with water and if the piston is pushed, the maximum travel is 10"! If the water would be completely incompressible, the piston's travel would be only .75" due to pipe's deformation.

There are two ways to create hydraulic pressure:

a) using a hydraulic pump that adds volume to the downward system; pressure is the same in all restricted enclosure.

b) through sonicity - using a device like that described in the above experiment and moving the piston harmonically (cam or crank and connected rod or actuator). No volume is added to the system but standing waves are created instead. The pressure is not the same all over but a function of the wavelength of the oscillation. Power can be transported at large distances with negligible losses and standing wave compressors (for refrigeration) are built that way.

Secondly, the volume and density of any material are an expression of its energy.

the piston's travel would be only .75" due to pipe's deformation.

The piston will only travel due to gasses (normally air) present in the liquid, which is the component being compressed, and the deformation of the container, as you stated. All liquids will have air or other gasses present within them in small amounts.

See post 25.

Thanks Ken --- See posts 26, 27, 28

Petro,

I see your dilemma quite clearly. Alas all a humble dabbler as myself might offer is sympathy and support as the answer to a very interesting question comes about.

cr3

As we have a team of gaseous thermodynamicists gathered round this compressibility topic,could they give their opinion on what the state of the contents of a tank of liquid hydrogen,much favoured as automotive fuel by our green friends,would be if the pressure relief valve failed to open and the tank attained ambient temperature.Yes,practically, the tank would rupture but just theoretically assume the tank to be infinitely thick,like me.

Maybe I'm missing something, but wouldn't the liquid under pressure have the molecular distance reduced minutely, reducing its volume slightly. I don't see how it is different from more compressible fluids except the pressure to delta volume ratio is much higher for liquids than for gases.

gasses are composed of molecules that are far apart and the pressure you see is the thermal energy of motion of the molecule in flight. The gas will expand to fill whatever it is in as the speedy little molecules want out.

Cool this down and in time they no longer have enough energy to fill the space and condense into a liquid that occupies about 1/1000th of the space(more or less, some condense to a solid) This liquid has some order and the molecules take up positions that balance their mutual attraction/repulsion. The repulsion is their energy of motion for their temperature, the attraction is their intermolecular bond, van der walls style.

Cool it more and it will either form a glass or a lattice of known crystal array spacing. Glass or lattice is a function of viscosity, chain length etc.

Once you are in liquid or solid phase compression is resisted by the molecules who draw on strong forces to prevent compression. That is why liquids are very slightly compressible. Going from 1 PSI to 100,000 Psi will rproduce only a tiny compression of about 0.3% in water = almost no compression. Other fluids vary.

A gas is not a fluid

From Merriam Webster.com

Main Entry: ¹flu·id
Pronunciation: 'flü-&d
Function: adjective
Etymology: French or Latin; French fluide, from Latin fluidus, from fluere to flow; akin to Greek phlyzein to boil over
1 a : having particles that easily move and change their relative position without a separation of the mass and that easily yield to pressure : capable of flowing b : subject to change or movement <boundaries became fluid>

From Dictionary.com

flu·id /ˈfluɪd/ Pronunciation Key - Show Spelled Pronunciation[floo-id] Pronunciation Key - Show IPA Pronunciation

–noun 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. –adjective 2. pertaining to a substance that easily changes its shape; capable of flowing. 3. consisting of or pertaining to fluids. 4. changing readily; shifting; not fixed, stable, or rigid: fluid movements.

Ladies and Gentlemen, by definition liquids and gasses are fluids. Under enough pressure a solid might even meet the definition.

you run into physics aspects and language aspects.

a fluid is an ordered system with the average distance between molecules varying slightly about a mean. A fluid finds a level in a gravity field.

A gas is a disordered system with the average distance between molecules varying greatly and it does not find a level in a gravity field. ( high enough column will density sort in a g field)

In a language a fluid is what you call a fluid

A gas or liquid is fluid any way you choose to look at it -- it's not a complex thing in it's basic definition of fluid. Moleculular or otherwise fluid is fluid.

A gas .... and it does not find a level in a gravity field.

Rubbish!

Have you never driven through a temperature inversion on a motorbike ?

The cold (denser) air flows to the bottom of a dip or valley, so, on a balmy Autumnal night (in your adolescence) you are driving back from your girlfriend's on your Lambretta and as you go into the dip it suddenly becomes cold...Oh no silly me that must be the ghosts in the haunted hollow.

you can prepare a graded density field in a small space too, if you wish. Climb a mountain and it gets less dense. Drive down the mountains into LA and look at the colored zones of smog you drive through (on a still day)

you can fill a container with a dense gas and it will not fill it, as long as there is air on top. remove the air and it will fill the box, and not find a level..

a gas is a compressible fluid, it is not a liquid,

I fear we are probably just missunderstanding each other... regarding the expression 'find a level'

Gasses of different densities will stratify...as will liquids...(like oil and vinegar). Dense Gas will flow down hill same as a liquid. If you had a U tube filled with dense gas which was visible (chlorine?) I would imagine the two arms of the U tube would show the same level. So I don't really understand the point you are tying to make.

Hmmm...

With oil and vinegar, there are two phenomena at work. 1. One fluid is polar, one is not. Therefore, the two are not at all likely to mix, even if their densities were the same. (It's almost like.... oil and water) Even given Brownian motion, the two will not stay mixed, and even when "mixed" (as in shaken salad dressing) the mixture is cloudy, indicating that the mixture is not a solution, but a suspension of droplets (i.e an emulsion). 2. The other is density, so when the two fluids settle out, the denser one goes to the bottom.

Gases of different densities have a very weak propensity to stratify. Air, for example, is a mixture of gases, and the proportions remain remarkably constant up to about 100km (i.e., in the homosphere). The reason this caught my eye, and the reason I mention it, is because a while ago someone was arguing on CR4 something to the effect that human-created CO2 could not possibly contribute to greenhouse effects, because it would settle out and could not reach a level in the atmosphere where greenhouse effects take place (he was apparently imagining the "glass" on the "greenhouse" quite literally). The obvious fact that such stratification would render all breathing organisms (us, for example) lifeless, seemed lost on him. This guy claimed to have some science education, and was using his lack of understanding to attempt to discredit the notion that human activity might be affecting the rate of global warming.

Mixtures of gases are very nearly perfect analogues to solutions. Brownian motion alone (even without the mixing effects of wind) keeps things mixed in a gas, just as Brownian motion keeps your scotch from settling out into a layer of water and a layer of alcohol. In air, the mixing is so effective that getting individual components back out, separately, is not a simple task.

But in your previous post you said "A gas is not a fluid" (that was copy and paste by the way), and now you say "a gas is a compressible fluid, it is not a liquid." (also copied and pasted by the way).

That is what set off this side discussion. We all agree that a gas is not a liquid, but both gasses and liquids are fluids.

I suspect that maybe english is not your first language and we are getting confused in translation perhaps?

no, just hurried comments

My apologies then for taking us off track.

There's a track? Oh crap is that a train? I wanna know who got me on track. The stinkers!

cr3

Flubber! He has invented Flubber!!!!

No really,

The molecular force of replusion of like electromechanical energy contained in the molecules of the 'uncompressable fluid', Like two similar magnetic forces opposing each other but forced closer together, hence the return force or pressure.

Is that thinking small enough for the question?

A few answers here keep pointing to elasticity of the pipe (or other container) causing the pressure. Yes, pipe is elastic. But the elasticity, in my uneducated simple mind (just ask my ex-wife) is a pressure maintaining device, not a pressure creating device. I think to evaluate things in extremes to keep my thinking straight. Imagine a pipe made out of Super-Unobtainium with a wall thinkness 1 meter thick and a hole 5 mm diameter. If I created one barg (14.504 psig) pressure inside with my PD pump, the elasticity of this pipe (I suppose) would have little direct effect on the theory behind what caused the pressure FIRST that has tried to yield the pipe SECOND. Correct? Del ? Steve? Anybody save me from myself !

May I suggest adding a 'perfecto non return valve' (the top of the range one made of unbreakium) so that once the pump has built up pressure it can be dissconnected...this will stop me saying.. 'it's the pump!'

Hey I'm arguing with myself now...no I'm not...etc.

It's those pesky molecues pushin' and a shovin'

(If Mrs Cat gave 'em on of her 'teacher's stares' they'd soon stop it and there would be no pressure )

For every action there is an equal and opposite reaction. So it is a bit of a chicken and egg argument.

I like Dells answer, its that blasted pump which is the culprit, if it would quit shoving 50 pounds of stuff into a 10 pound bag there would be no pressure...

The pump creates more volume in the same space causing higher density than the molecular medium is stable at. The liquid atoms are compressed closer than the energy field allows without pressing back, like the magnet analogy.

Please explain how my theory is incorrect so I can sleep at night.

I'm sorry, but if you are talking to me, I agree with you. Your model is valid in my opinion.

Sorry for my stupid question: what exactly is a PD pump?

Not stupid at all. PD stands for positive displacement. This is a pump that increases pressure via a piston as opposed to a centrifugal pump that imparts velocity to the fluid via a rotating impeller.

Positive displacement. Things like piston pumps (assuming there are no leaks around the piston) are positive displacement. In a general sense, if you block the output of a positive displacement pump, the driving motor must stop. A fan or turbine is not positive displacement -- block the output, and the motor keeps turning.

Perhaps a good analogy for a liquid (which we will call essentially non-compressible for the time being) is a load of BB's. The BB's are the molecules. [Although they are darn near vacuums, in terms of the space between the spinning particles involved inside the BB's (relatively, about 100 times less dense that our solar system) BB's seem solid to us.] So let's call them solid. In a similar way, molecules of a liquid seem pretty solid too, in terms of their propensity for not wanting to occupy space already occupied by another molecule. Interatomic forces and intermolecular forces prevent compression in somewhat the same way that repelling magnets prevent compression -- the effect is simply more dramatic: magnets feel spongy, hydraulic fluid does not.

Stick your finger in a full bowl of BB's and the bowl overflows, just as it would if water were in the bowl. Put a cover on the bowl, with a hole sized to your finger. The BB's will then push on every surface of the bowl, trying to get out as you try to poke your finger in. You'll fail to get your finger in, but you could measure the pressure of the BBs against all the surfaces of the bowl and lid. Push harder, more pressure.

Replace your finger with a PD pump, and the bowl with closed pipes. Push (motor on) and pressure increases. Stop pushing, (motor off) pressure drops. The motor torque, combined with the leverages ramps and other mechanics of the pump can tell you exactly how hard the pump pushes against a one square inch chunk of fluid.

If you play with a manual hydraulic press, (pressing against something solid) you can see the mechanical relationship, and can calculate the force applied by knowing the leverage and the ratios of areas. (It's easy to imagine a very large hydraulic press using BBs as the working fluid.) As you pull harder on the pump lever, you can see the pressure increase. As soon as you release force on the lever, pressure stabilizes (and sould fall, if it weren't for the check valve.) It's pure simple mechanics: a longer lever could replace the difference in areas. Even the molecular end of it can be thought of as pure mechanics: the molecules are working as non-compressible BB's.

Of course BB's are compressible, as are liquids. But for most applications we can ignore the compressibility.

Suppose you pump fluid through a check valve, into a pre-filled capped pipe. The check valve will retain pressure in the pipe, when you turn the pump off. Part of that retained pressure is due to fluid compression, and part is due to pipe stretch. If you open a valve connected to the pipe, you find (in ordinary small hydraulic systems, with rigid piping) that the amount of fluid that comes out to equalize pressure with ambient is just a drop or two: piping and hydraulic fluid are very stiff. (The couple of drops required to expand the bourdon tube in the gauge will equal the couple of drops due to fluid compression and pipe stretch in a stiff system.) If the working fluid were instead air, a very large volume would come out.

Everything is a spring: some things just have a very very high spring rate.

The law in mechanics says action equals reaction. The forces that the fluid is exerting to the pipe wall is the action. As a reaction, the pipe wall is exerting the same order of forces to the fluid which increases the pressure in the fluid.

PetroPower: First you ask what creates pressure in a noncompressible fluid, then you state fluids are compressible, which is true. Excluding aeration that can lead to cavitation or to diesel effect, I assume, for consistency, that you ask what creates pressure when the threshold of compressibility of a fluid is already achieved.

Gravity creates pressure in a hydraulic system, output work of a PD pump creates pressure if it's not obstructed, but you know these and you want some other answer.

When increasing pressure is applied to a confined fluid using, let's say a PD pump, the translational movement of the molecules is progressively restrained up to zero which is probably the threshold of compressibility for that particular fluid. If pressure continues to raise, the vibrational movement of molecules is slightly affected, possibly leading to a phase transformation from fluid to solid. The measure and expression of these energies (translational and vibrational) is called heat.

On the other hand, heat applied to a confined fluid creates pressure. Thermal dilatation of fluids can generate enormous pressures that can crack accumulators, cylinders, hoses, etc. For this reason, when designing or troubleshooting a hydraulic system, heat is traced throughly. That's why some systems have heat exchangers and faulty components are identified with IR cameras or IR thermometers and hydraulic fluid grade is carefully chosen according to the environmental temperature in order to preserve its viscosity and lubrication property.

So again, my answer to your question is HEAT.

Regards,

Michael

Hi Michael

I titled this 'noncompressible' to accomplish two things. The purists would immediately open this to tell me fluids are compressible, and that gets them 'inside' for a comment (so to avoid that I had my first sentence to keep the comment on track), and if I had typed 'compressible' a lot of other answers would be 'because of the compression like in a balloon or car tire' or the 'fluids are not compressible' one liners without further comment. It was very intentional.

********* Heat - OK let me comment *******

Firstly, I don't know this answer. I assumed the answer as an uneducated person and I've used the answer in my pipeline class for the past 5 years (and it might be wrong, thus this query). An executive who took this class asked me this so I included an answer, went out to the industry experts shown in my query to confirm my theory and was surprised at the inconsistency of answers to what I felt was a simple question.

So, if I build up pressure in a pipe far below the yield of the pipe, hold it (without leaks) at 25*C, come back 24 hours later and providing the temperature is 25*C the pressure will be the same. So, if I cool it, the pressure will go down, and heat it, up, without any other 'mechanical' means adjusting the pressure. Just temperature control regulating the new pressure. Right?

If yes, then the 'root' of my query is "Why?" (and this in fact is the basis of my classroom answer). Why does heat vary the pressure at a molecular level? I am assuming that molecules do not grow or shrink with heat; i.e. displaced volume remains the same, so hot or cold they take up the same space. Right? Your answer to this may very well match my answer.

"Why does heat vary the pressure at a molecular level? I am assuming that molecules do not grow or shrink with heat; i.e. displaced volume remains the same, so hot or cold they take up the same space. Right? Your answer to this may very well match my answer."

With increases in temperature the molecular level increases and I suspect that their reaction to pressure changes to be more compressible due to increased spacing within.

<So, if I build up pressure in a pipe far below the yield of the pipe, hold it (without leaks) at 25*C, come back 24 hours later and providing the temperature is 25*C the pressure will be the same. So, if I cool it, the pressure will go down, and heat it, up, without any other 'mechanical' means adjusting the pressure. Just temperature control regulating the new pressure. Right?>

It depends on the relative rates of volumetric expansion of the fluid, and of the solid enclosing it. On most occasions this premise will be correct.

I am assuming that molecules do not grow or shrink with heat; i.e. displaced volume remains the same, so hot or cold they take up the same space. Right? Your answer to this may very well match my answer.

No, molecules (as a group) take up more space when warmer: in other words the space between them (both atomic and molecular space) increases. That's how we measure temperature. If the particular molecules are in a lattice (a solid) the expansion with temperature is small. If the molecules are free to move (a gas) then the expansion is dramatic, with volume* increasing directly with temperature relative to absolute zero. (See Boyle's Law)

(* or if volume is restrained, then pressure)

The following is from this site. You'll have to wade a little, but this whole page is worth reading. I've just extracted one paragraph:

NOTE: The atoms in a solid are held together in a three-dimensional periodic lattice by spring-like interaction forces. The potential energy for a pair of neighboring atoms depends on their separation r, and has a minimum at r = r0. The distance r0 is the lattice spacing of a solid when the temperature approaches zero. The potential energy curve is not symmetrical around r = r0; it rises more steeply when the atoms are pushed together (r < r0) than when they are pulled apart (r > r0). The average separation distance at a temperature above the absolute zero will therefore be larger than r0. A solid with a symmetric potential energy curve would not expand.

Temperature can be said to be a measure of the space molecules take up. This can be shown dramatically by lighting the fuse of a firecracker. Or a little less dramatically by starting your car. Or less dramatically yet by watching a thermometer.

Pressure, incidentally, is the sum total of the force of all the billions of collisions that occur with a fluid and its container. Take an equal number of molecules and put them in a smaller container, and these billions of collisions must act on a smaller area, meaning that pressure (collisions per square inch) must increase.

Thanks Ken

I guess I was speaking about the atoms themselves in the molecule (growing in volume), not the space between, coming back to the BB example. But, interesting article so thanks for that.

Two things have happened:

1) My wife caught slowing putting my finger in / out / in / out of a bowl of BBs and laughing wildly when 1 or 2 spilled out shouting "Ken Ken lookie lookie". Again, she called her mother in Argentina.

2) The last paragraph of your last post matches my 'theory' so I need to know how to attach a few power point slides from my class to this post to finally give my answer (and put it up for pot shots). Maybe I need to convert the slides to a picture format ??

Yes, that works. You can just copy the slide via clipboard into Paint, save it in Paint, and then attach it using the "camera" button. I haven't been able to make a simpler method work consistently.

Petro Power: Thanks for the invitation.

I'm an ME but from what little I know about such things I suspect it is a matter of molecular structure of the fluid under consideration. The denser the fluid the less it can be compressed and vice versa. SS

Perhaps PHYS can elucidate on the matter.

What causes pressure in an noncompressible fluid?

Compresssible and non compressible are relative terms and generally refer to external forces applied to the fluid. The do not exert any inherent internal pressure in and of themselves but react to external forces.

Hello all and thanks for replying. Now I show you what is in my class, 3 slides below. And based on what I've learned from all of you I conclude, as my ex-wife has, that I am a moron !

Well, remember that these slides shown are for a simple class, not for Fluid Mechanics Professors or Physicists. Ahemmm. So during class I speak to these slides by indicating that increased pressure squeezes the molecules closer, they react by getting excited and repelling each other >>> I'm not sure why but perhaps it has something to do with imbalance attempt trying to share electrons in valence rings shoved together, protesting this forced attempt (like Ken's BBs and Steve's super balls - Hmmm . . . that doesn't sound very good does it?).

For my class, suggested wording to be more clear or more accurate is appreciated, but, keep in mind this is to explain a VERY basic idea without getting into a physics argument if my wording isn't completely absurd. Short and simple was the idea.

So I'm gleaning from the replies that those that mentioned velocity, vibration, heat, repulsion, etc . . . . of the molecules were more / less in alignment with my simple understanding (however your descriptions were more scientifically accurate and eloquent than my blue collar wording). Do you like my little pressure guage ?

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Well, now it is clear what you are looking for. I think you have the "meat" of the principle in your slides. Well done! The answers you got were very good, but I think people had a hard time understanding exactly what you wanted to know. In addition you probably now realize how much more there is to know on the subject. So, speaking intelligently about the principles of pressure on vessel walls when questions are asked by students is a bit of a challenge.

I might recommend some investigative work on your own to help round out your understanding. Both chemistry and physics are the chief disciplines you need to focus on. You will need a grasp of atomic structure, thermodynamics, Newtonian physics (as far as force and energy are concerned), and in particular, chemical bonds.

First, you must realize that everything is in motion. Even atoms jiggle around. There is a relationship with the amount of jiggling and the temperature of that particle. When temperatures reach a point called Absolute Zero, all jiggling motion stops. There is an energy associated with the jiggling and the more jiggling, the higher the energy and the higher the temperature.

Molecules are composed of atoms. Atoms bond together covalently to form molecules and molecules will bind together to form larger structures. You should explore covalent bonds, ionic bonds, metallic bonds (although these are limited to metal atoms), hydrogen bonding, and van der Waals forces. There are other forces involved in the composition of matter, but these are some of the big players and they work in complex ways. There are responsible for binding matter together and they give matter its shape and properties.

In the case of a fluid, molecules are doing a dance (jiggling and moving). That dance is balanced by the attractive and repulsive forces listed above. The repulsive forces between molecules are important in describing how fluid pressure works. Under external force you can push molecules together, but they will also push back as they begin to crowd together (like invisible springs). The degree of repulsive force you get is a function of the square of the distance between molecules. The closer together, the higher the repulsive force.

In a fluid, much like a gas, molecules are free to drift around. When a molecule gets close to another molecule, the two molecules rebound like pool balls with 100% rebound, sending the molecules off in new directions. You can see this by taking a glass of still clear water and adding a few drops of food coloring. Eventually the food coloring and the water will completely mix on their own by a process called diffusion (Brownian Motion (look it up)).

As you compress a fluid a few interesting things happen. The intermolecular distance between molecules decreases and the intermolecular repulsive forces increase (again, by the square of the distance). The second important thing that happens is that the temperature of the fluid goes up. Remember the relationship between temperature and jiggling speed? Compression imparts more energy into the system and the temperature goes up as does the movement of molecules and their velocity. The net result is more elastic collisions between molecules and more Newtonian force (Force = mass * acceleration) applied to the vessel walls. It is important to note that molecules never really touch each other. They are repelled from each other much like two like poles of magnets do when brought in proximity to each other. Essentially, the rebound of these near misses sends the molecules skittering away like a set of billiard balls on a pool table. The greater the number of elastic collisions, the greater the force or pressure the fluid exerts on its neighboring vessel walls. Also, the speed at which these molecules travel will increase.

If you research this enough you will realize that matter is composed almost entirely of empty space! If you dive into atomic physics and then quantum physics you will find that matter is entirely empty space. It just depends how far you want to take the model when describing the nature of things.

One last thing. Your initial question stated that you wanted to skip the diatribe on how fluids actually are compressible. This brought a flurry of replies stating that fluids are compressible, but there wasn't much explanation as to why this was so fundamental to the answer you sought. I will try to answer that. The compressibility of a fluid is important when you consider what actually takes place at a molecular level. Your contained fluid acts like a spring. Even steel blocks will compress, but they just act like a stiffer spring. The elastic nature is due to the repulsive intermolecular forces at work. If the intermolecular distances did not change you would not see a change in pressure.

You should also study gasses and the laws that govern their behavior. In particular is something called the Ideal Gas Law ( PV = nRT ). http://en.wikipedia.org/wiki/Ideal_gas_law Fluids behave similar to a gas, so the principles overlap and it will help round out your understanding.

If you do the homework I outlined for you, you will be able to both dazzle your students and feel comfortable about the subject. Additionally, you will be embarking on a new journey of knowledge. What I have given you so far is not the real answer to your question, but merely some sign posts that should point you in the right direction. So, put on your hiking shoes and get hiking! Have fun!!!

This Guest seems to have rounded out what I was thinking. As much as I hate to agree with Ken ;-), he was correct with Boyles Law and this guest is pointing that out with a reference to the ideal gas equation.

I started thinking about fluid statics and the basic relationship between depth of a liquid and the pressure and remembered some old equations. The whole question goes to the intermolecular forces, and even though we don't want to, we have to go back to the fact that all fluids are in some measure compressible. So, take Boyles Law and throw in a bit of those differential equations we are all trying so hard to forget and the change in pressure is a result of the change in density, which is a result of the particles coming closer together:

∫ dP = - g ∫ ρ dz

With the dz term becoming constant because we said the system is in a static state and assuming g to be pretty constant; this leads to a direct relationship between the change in density and the change in pressure which is based on the spacing (or density) of the molecules.

Am I close here?

Stephan

I read this comment as implying that the pressure in a liquid is transferred primarily by momentum transfer. Unfortunately , that is plain wrong.

Comment made in view of what I see as inappropriate positive votes.

Fyz

Fluids flow eg gases, liquids and molten metals

"Liquid" historically meant something fluid at human-tolerant temperature and pressure but now science can create liquids under exotic conditions incompatible with human life.

Your liquid molecules have stored energy arising from their mass and velocities - this is kinetic energy - some kinetic energy is released as heat energy at the container wall and compression increases collisions (pressure) and thus additional heat is released.

Hey Petro,

I wonder if you need this sort of microscopic detail? If so, there are some issues with the illustration. The low pressure vs high pressure picture seems to suggest a gas, with a large difference in the number of molecules present. Also, you definitely would not want to say force = pressure. You could say "force/area = pressure" or force per unit of area = pressure.

Maybe a demonstration would be better. I guess we never really asked who your audience is and what you are trying to convey. For most technician/engineering audiences, an understanding at a more general and pragmatic level might be better. I think almost everyone intuitively understands the relative incompressibility of liquids, vs the considerably compressibility of a gas, and a simple demonstration would reinforce that. Also a simple demonstration would show how pressure acts in all directions in a container, and how a liquid conforms to its container. A hydraulic jack with a pressure gauge on it might be a good prop.

But the unasked and unanswered question: who is your audience?

What you have drawn is an excellent illustration of the behaviour of a rather good vacuum containing just a few gas (or vapour) molecules. Although it in no way represents what happens in a liquid, it is actually rather a good starting point for this level of empirical description. What follows is roughly as taught many years go to me in my class of fifteen-year-olds, so it should be suitable (with minor modifications) for your class*. (I've added the reference to superfluids, which were not accessible when I was learning, but I think they help with the picture).
*Our school-master was a capable physicist and a very good teacher - even though we didn't appreciate this at the time, because we spent our lessons correcting his sums and algebra when he worked on the blackboard.

If you are to see behaviour that resembles a gas, you need to have more collisions with other gas molecules than with the containing walls.

But that's nothing like a liquid. A far better analogy that your class will be familiar with is dry sand. This flows, but you don't get horizontal surfaces; and if you want to push your finger into the pile you often need to wiggle it to move the sand out of the way. That is because there is friction between the sand particles - remove the friction, and you will get horizontal surfaces, and you can push your finger in easily. Is that like a liquid - no! Try pouring it - the sand particles fall independently, not as drops of liquid.

So we need for the particles to hold to a reasonably uniform distance while they are in close proximity, but slip past each-other without resistance. Is that a liquid - not exactly! It would behave more like an unusual state of matter called a super-fluid.

So what is different about a liquid? Adjacent molecules do not simply slide past each other, but join temporarily to form groups (or complexes) of molecules with 'fixed' relative locations; that's very similar to what you get in a solid. What makes it a liquid is that thermal vibrations are sufficient to break up the groups, and the molecules then slide past each-other with reasonable freedom before rejoining in a new configuration. That means that over a number of "break-and-rejoin" times, the liquid can settle to any shape you like. (The time-scales are extremely short in human terms). However, this process is largely responsible for the viscosity of practical liquids.
BTW, the same joining and breaking takes place with the materials of the container. A very rough description of wetting: if the molecules of the liquid attach faster and/or spend more time attached to the surface of the container than to other liquid molecules, the liquid will wet the container; otherwise, otherwise.
Regarding pressure: the behaviour, whether within the fluid or between the liquid and the walls, is far closer to the distortion of a spring than the kinetic theory you illustrated.

If you want details at the molecular level, see Blink's and/or my contributions for oversimplified descriptions that apply to some cases.

Finally, give whichever professor or other colleague that gave an answer equivalent to this a Ph.D., even if he already has one. I'm far too old to need any more technical qualifications

Fyz

Assuming non-compressible fluid. With it seating inside a close pipe we can say pressure is 0. With pump turn on to try adding more fluid into the pipe, force is acting on the fluid. That force will be transmit to the pipe through the fluid. That will result in pressure increase. So basically the pressure is result of the pump. Pressure will increase until the pump stop, unable to flow the fluid or pipe can't hold the pressure.

Do I get a PhD now?

Pineapple

How comprehensive an answer do you want? If it were a gas you could make the approximation that you have particles bouncing off each other.

In a liquid, there is an element of this, but it is nowhere near dominant*. On a not-fully-QM approximation, you are looking at the distortion of the fluid; this results in:
. increased overlap of outer-shell electron clouds (at this level you could think of them as orbits) which (roughly speaking) attract magnetically, and:
. reduced distance between the inner shells which have an overall positive charge. The uncompressed state represents an equilibrium point where the attractive and repulsive effects balance over time. When you reduce the spacing, the repulsion due to electrostatics increases faster than the attraction due to the interpenetration of the magnetics.

But, as already pointed out by Hendrik, the location and distortion of the electron clouds in the outer shell are determined by QM. I would not even attempt to explain this here, even if I could claim to understand it fully (I admit that, although I have modelled the elasticity of crystals, what actually happens at the underlying level remains a conceptual mystery to me - though the Maths seems to work out OK). The best I could do is recommend you start with Born's book, and work from there.

*If collision frequency were dominant, the compressional behaviour of liquids would resemble that of gases. In practice volumetric compression of less-compressible liquids is remarkably similar to that of solids.

Fyz

I sense harmonics at work here; Particularly, the Thirteenth.

There's a nasty disruption of the coumpounding tri-angular oscliations, when the molecules compress at a sudden moment; at double rate, at a poorly performing note. This is accepted to cause an abnormal frequency. (Beware of B-flat, especially at a chord or shrill octave).

Sound can't travel far, without a tuned condiut, or can it? If we cannot sense it, might it be preseant, none the less?

Probably a damn good thing, in my opinion.

Do you employ a method of tuning the distance of the pipe, while simultaniously montoring with real-time vibronic resonance sensor?

Do your pipe-lines also incude vibration dampeners, every so often? How close do you require them in order to have some assurance of safety?

Can't you just coat the inside of the pipe-line with corrosion resistant tri-polymer resins; in order to prevent these vibrations over a greater distance? Why not use nano-particle suspension within the pipe-line as a sensor?

Maybe some day we will have reactive skin and armor, in much the same way.