Ed LaFontaine said:
Since we all like wikipedia, here is their take on fluid mechanics
Water is water even when it is moving, doesn't matter.
Reading from the link I posted earlier on Fluid Mechanics, there is an interesting couple of paragraphs. Page 48, last paragraph starting with"Even if"
Describes the structure and attractive forces in water.
Rather than just throwing text out there as 'support', how about you explain how it helps? I fail to grasp the relevance of the 'Even if...' paragraph in the Fluid Mechanics text...
Please be aware that quoting Fluid Mechanics is not relevant to the discussion at hand. The reason that you consider water to be incompressible for <1GPa is because it makes your transport equations much simpler, not because it is an absolute truth. If you consider it to be compressible you then have to incorporate the effect of pressure upon the volume, which impacts up on the velocity, changing your friction/pressure loss and requiring you to look again at volume, velocity, friction...
For example, there are applications where unless I see a pressure drop of >5-10% I consider gas to be incompressible...I don't suggest that it is, its just that it is for the purposes of the Fluid Mechanics application.
Please be aware that quoting Fluid Mechanics is not relevant to the discussion at hand. The reason that you consider water to be incompressible for <1GPa is because it makes your transport equations much simpler, not because it is an absolute truth. If you consider it to be compressible you then have to incorporate the effect of pressure upon the volume, which impacts up on the velocity, changing your friction/pressure loss and requiring you to look again at volume, velocity, friction...
For example, there are applications where unless I see a pressure drop of >5-10% I consider gas to be incompressible...I don't suggest that it is, its just that it is for the purposes of the Fluid Mechanics application.
MJL21193 said:
Ahem, thats not the same as "water is incompressible"
John, in keeping with my open mind I have another question, also on the theoretical side. In our tank of water that we spoke of earlier, the tank walls wouldn't necessarily have to be completely incompressible to stop sound from traveling through the water, but rather just be made out of a medium that posses a greater resistance to compressibility than water correct?
Because if the tank walls are more resistant to compressibility that the water then the water will not be able to displace them, correct?
-Justin
John,
Here is a decent link which visualizes the propagation of sound in water.
http://www.fas.org/man/dod-101/navy/docs/es310/uw_acous/uw_acous.htm
An excerpt:
I do feel like you do have a intuitive grasp of the physical events transpiring, but I think that where you err is in your attempt to separate pressure and compression. If you look at the sound wave as it is travelling with respect to the medium in which it travels, there will be a local concentration of molecules where the peak pressure (and thus SPL) occurs. It is precisely this local concentration/compression of the medium that creates the pressure which we measure to define SPL (which is by definition a ratio of a local pressure to some defined reference pressure).
Respectfully,
David M
Here is a decent link which visualizes the propagation of sound in water.
http://www.fas.org/man/dod-101/navy/docs/es310/uw_acous/uw_acous.htm
An excerpt:
I do feel like you do have a intuitive grasp of the physical events transpiring, but I think that where you err is in your attempt to separate pressure and compression. If you look at the sound wave as it is travelling with respect to the medium in which it travels, there will be a local concentration of molecules where the peak pressure (and thus SPL) occurs. It is precisely this local concentration/compression of the medium that creates the pressure which we measure to define SPL (which is by definition a ratio of a local pressure to some defined reference pressure).
Respectfully,
David M
MJL21193 said:
a couple of micropascals will compress the water such that the side of a cubic meter of the stuff shrinks by maybe a billionth of an attometer (too lazy to do the math) which I (and probably most of us) am unable to measure. These are linear rules; the beauty of nature. I don't think you're saying that water is only compressible above a certain pressure.
You are right that you can ignore the effect in most practical situations - but water, as all matter, is compressible.
I would say that it is the compression that causes the displacement you talk of.
I still say we are all vehemently in agreement, we're just not very good at reading and writing - and I include myself in that group.
🙂
MJL21193 said:
from this Wikipedia entry under ASSUMPTIONS:
given the choice of air or water in my brake lines, I'll take water.😀
MJL21193 said:
Hi,
What you need to come terms with is people do not need telling how to think.
The above is nonsense and what you should be doing is trying to work out
the flaws is your arguments rather then tediously insisting you are correct.
You do not know something the rest of us do not.
It is a very arrogant and ignorant position to take.
A wave has displacement by definition, it propogates.
The propogation speed is determined by the density and elasticity
of the medium. There is no such thing as a real inelastic medium.
The elasticity is directly observable from the propagation speed.
It is far from being unmeasurable : it is staring you in the face.
🙂/sreten.
FWIW the tranverse wave shown above is inapplicable to liquids,
they do not support shear stress, hence no transverse propogation.
Yes indeed. One thing that may help to imagine this is the fact that even the densest matter known in the part of the universe that we live in - is consiting of empty space for the most part.
Regards
Charles
Sreten is correct concerning a liquid being unable to support a shear wave. The image in the above text contained both examples of waves, but that was not to say that transverse waves are supported in water.
"What of water surface waves?" You ask?
Water surface waves appear to be transverse from casual observation, and many will recall the example of a cork bobbing up and down without apparent lateral motion. Water surface waves do contain a transverse component of motion because the molecules move in the axis perpendicular to wave propagation. However they are not purely transverse and as such cannot be called transverse waves. A purely transverse wave will have no longitudinal motion of molecules, and can only propagate through shear force to adjoining molecules. This is why a transverse wave is also known as a shear wave or s-wave.
A water surface wave is best understood by visualization. Below is an animation of the combined transverse and longitudinal motion of a water surface wave. For comparisons sake purely longitudinal and purely transverse waves are also shown. Source for these animations is http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html
Transverse Wave
Longitudinal Wave
Water Surface Wave
Another great site for visualizations:
http://physics-animations.com/Physics/English/wav_txt.htm
Again, the reader should note that the longitudinal wave propagates in the form of densification (compression) and rarefaction of the medium.
For a thought exercise attached to this explanation, consider a SONAR transducer on the ocean floor. This transducer is made of piezoelectric ceramic and is 10mm thick with a maximum displacement of .01mm. When this transducer is excited by a high voltage it elongates in thickness by .01mm causing a local displacement of water at the transducer surface. The displacement lasts for 10 microseconds (100kHz) and then returns to normal.
What happens to this displaced water? Recall that we are on the ocean floor and that there may be hundreds of meters of water above and all around the transducer? The displacement cannot occur instantaneously at all points in the ocean, it must take some time to travel through the medium. And so it does. The water molecules at the transducer surface are displaced into the space occupied by the adjacent molecules. Since these molecules cannot instantaneously move out of the way they must briefly share the same volume of space, resulting in higher pressure for a brief moment in time. This is all we mean by compression; the fact that a region of more dense, higher pressure water is created by the displacement of the transducer.
Regards,
David M
"What of water surface waves?" You ask?
Water surface waves appear to be transverse from casual observation, and many will recall the example of a cork bobbing up and down without apparent lateral motion. Water surface waves do contain a transverse component of motion because the molecules move in the axis perpendicular to wave propagation. However they are not purely transverse and as such cannot be called transverse waves. A purely transverse wave will have no longitudinal motion of molecules, and can only propagate through shear force to adjoining molecules. This is why a transverse wave is also known as a shear wave or s-wave.
A water surface wave is best understood by visualization. Below is an animation of the combined transverse and longitudinal motion of a water surface wave. For comparisons sake purely longitudinal and purely transverse waves are also shown. Source for these animations is http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html
Transverse Wave
An externally hosted image should be here but it was not working when we last tested it.
Longitudinal Wave
An externally hosted image should be here but it was not working when we last tested it.
Water Surface Wave
An externally hosted image should be here but it was not working when we last tested it.
Another great site for visualizations:
http://physics-animations.com/Physics/English/wav_txt.htm
Again, the reader should note that the longitudinal wave propagates in the form of densification (compression) and rarefaction of the medium.
For a thought exercise attached to this explanation, consider a SONAR transducer on the ocean floor. This transducer is made of piezoelectric ceramic and is 10mm thick with a maximum displacement of .01mm. When this transducer is excited by a high voltage it elongates in thickness by .01mm causing a local displacement of water at the transducer surface. The displacement lasts for 10 microseconds (100kHz) and then returns to normal.
What happens to this displaced water? Recall that we are on the ocean floor and that there may be hundreds of meters of water above and all around the transducer? The displacement cannot occur instantaneously at all points in the ocean, it must take some time to travel through the medium. And so it does. The water molecules at the transducer surface are displaced into the space occupied by the adjacent molecules. Since these molecules cannot instantaneously move out of the way they must briefly share the same volume of space, resulting in higher pressure for a brief moment in time. This is all we mean by compression; the fact that a region of more dense, higher pressure water is created by the displacement of the transducer.
Regards,
David M
John's problem is not with the idea that the pressure has ups and downs with space and time, he is not seeing that the same is true of the density. If I understand his argument, he is positing that it's local temperature variation, with the density remaining constant at all times and all points.
MJL21193 said:
One is the inverse of the other. Water is always compressible, ignoring it results in very small experimental error most of the time. Relativistic effects are ALWAYS there but ignoring them when seeing how fast a bowling ball falls from a tree results in no measurable error. Without them GPS could never work. There seems to be no point in posting examples, but for instance lab experiments involving cetrifugal separation can have considerable errors when ignoring the compressibility of water.
You seem to be hung up on these "tiny" forces vs "huge" barriers. Why not get a good USN text on sonar, and get some facts on wave energy propagation under water and put some numbers on the forces and mechanical impedances involved you would probably find that everything works out.
BTW "shockwave" refers to cavitation, in air it's when the "negative" peak would go below a vacuum (can't happen). Same thing happens under water, ask any sonar guy.
Just another quick thought your argument taken to extreme would probably predict particle displacement at low frequencies and high SPL of several feet.
MJL21193 said:
Reliable sources be damned. This is a question of reason.
MJL21193 said:
Yes. If transmission is by displacement alone, why is it's speed not instantaneous?
After all, with steel balls, push the first one, and the last in line moves simultaneously. Or does it? And if it doesn't, why doesn't it?
I'm afraid your argument doesn't hold water.
w
If transmission were by displacement then the effor required to excite a wave would increase as the distance it was required to travel (the size of the body of water) increased, since more molecules would require to be moved. - Nonsense.
Daveze said:
I put the reference there for you all to read - it has something important to say about liquids.
Fluid mechanics has no bearing on the subject? That's a fresh idea.
The biggest problem I see here is you all want to join in and have your say (mostly to say how I am wrong, over and over again), but you are not willing to read and try to understand what I am saying.
I take the time to read through everything that you all have posted. I don't respond to the same old, same old.
Read what I have wrote, try to understand and comment on the specifics please.
despotic931 said:
Justin, where did I say water is incompressible?
The tank walls would need to absolutely contain the volume of water. This means it would need to be strong enough to stop the water from expanding in volume.
The material of the tank would need to be incredibly stiff and rigid, to fully contain the pressure increase.
gtforme00 said:
Hi David,
I have seen all of the "visualizations" of how sound wave travel, but thanks anyway.
I'll do some more writing here that will be glanced at and then ignored:
There will always be compression with the application of pressure. The amount of compression is what I'm talking about. I say that when a sound wave moves through water, the primary mechanism of energy transfer is not compression, but displacement. Displacement causes a volume increase due to a temperature increase.
Water is dense. The bond forces between molecules are strong, in both attraction and repulsion. If the molecules are free to move, as in a liquid in an open system, then the force required to compress two molecules closer together will need to be strong enough to overcome the bond forces between molecules AND the molecules inertia (resistance to change in motion). IF the applied force is not strong enough, it will merely push the first which will push the second and so on, without any reduction in the physical distance between the molecules.
When a sound wave introduces energy into the water, motion is the product. Motion, in this universe, creates friction. Friction creates heat. Heat makes the temperature of the water higher. Higher temperature means less density. What is the opposite of compression? Expansion. That's what happens to water when sound passes through it. Very slight, but there non the less.
sreten said:
I'm daft, remember?
Is it possible for you to contribute here without resorting to personal criticism? To be honest, I don't care what your assessment of my intellect or personality is.
Now, you are arguing with someone else, because I didn't say that water is "inelastic".
The inaccuracies in the above quote prove that you haven't taken the time to read what I have written.
SY said:
Not quite. I am saying that pressure increases (due to increased molecule excitation or motion) and pressure decreases (due to reduced molecule movement) do not result in a density increase. The opposite it true actually. The density will decrease as more sound energy is dissipated in the water, thereby raising the temperature and increasing the volume. Higher volume = lower density - conservation of mass.
scott wurcer said:
My position is that the compressibility of water can be ignored in this particular case. It takes a "huge" amount of force to compress even a small amount of water (in relative terms) and a sound wave, being "tiny" in this regard will not do it. It has enough energy to displace, and that's the point I'm trying to make here.
I have gone to great lengths to explain my thoughts, and it's all there in previous posts. I don't want to repeat myself over and over again.
MJL21193 said:
What of the example above with a SONAR transducer on the ocean floor? When the SONAR transducer expands in thickness, where do the molecules that were previously in the space occupied by the transducer go?
They must go somewhere, and that somewhere is into the adjacent fluid. Unless the transducer is displacing the entire water column above it (which would have to occur instantaneously at the surface of the ocean and at the transducer surface) then the molecules have to be compressed into the adjacent space next to the transducer for a short amount of time. The tendency for the molecules to want to equalize pressure into the surrounding region is what causes the pressure wave to radiate out from the source.
Regards,
David
...with emphasis added
Ahem, those two things are mutually exclusive. That line of thought is flawed.
If there is no reduction in distance, ie., the molecules move together, there is no relative motion to create friction.
Please entertain us some more.
gtforme00 said:
Thanks David, this is an interesting look at the situation. Refreshing to be able to discuss the subject reasonably.
I'm not well versed in SONAR transducers, therefore I don't know how they are constructed. It seems reasonable that it would have to be a driving surface that is surrounded by water, otherwise the immense pressure from the water (at ocean depth) would push the driving surface into, say an air space, such as a normal loudspeaker box (crude example but I think you get the picture).
With that assumption, much of the driving surfaces forward motion would result in displacement around it - water displaced from the front would push water in at the back. This is an impedance mismatch between the driver and the medium, similar to a drivers cone and the air - A loss of efficiency.
The "sound" wave that results from the driving surfaces forward motion is as I have described - increased molecule movement. The displacement for this motion would not necessarily happen in a straight column or line, but bloom outwards in three dimensions. The displacement seems like a difficult thing to do, but it's not. An open body of water is not a closed system and minute volume changes can happen with little force.
Ed LaFontaine said:
Ed, Ed, Ed...
Your thinking is too linear. Too 1-dimensional.
Where does the temperature of the water come from if not heat from friction due to molecule motion? Except for absolute zero, molecules are always in motion, creating heat.
Furthermore, water molecules are not all lined up in neat little rows, one after the other. Also, water has low elastic properties - that's to say that once moved, a molecule may not (probably won't) return to it's original position.
Sound through a medium will start with the source and expand outwards in 3 dimensions. It looses energy along the way, due to the energy being dissipated in the form of heat and being diluted in the spreading out.
"The clown with his pants falling down
Or the dance that's a dream of romance
Or the scene where the villain is mean
That's entertainment"
MJL21193 said:
MJL21193 said:
Hi,
1) I'm critising what you are saying, if you choose to take it
personally that is up to you, it does not change anything.
An academic publishing such musings would get slaughtered.
And it would be the end of said whatever academic careeer.
2) Which one of us cannot read ? Or infer ? If you can
effectively ignore compressibility for a particular case
you are treating the medium as inelastic, which is very
sensible in the case of hydraulics.
3) The principles of sound wave propagation in
any gas, any liquid and any solid are well known.
There is nothing special about water.
4) You are repeating the same thing over and over again.
( nigel .... huh ? ... .... hmm..... ..... it goes to eleven.)
It it is not an argument, it is a position.
5) Consequently the answer is always the same. The elasticity
of any medium is fundamental to sound wave wave propogation.
You can try convincing yourself otherwise as much as you like.
I have no problem with it whatsoever, neither do the textbooks.
6) Prove 5) to be wrong for the general case, gas, liquid or solid.
🙂/sreten.
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