Talk:Compressibility

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Greetings editors: There is frequent confusion between subsonic incompressablility and the effects of compressability as speed approaches and exceeeds the speed of sound.

Below the speed of sound in a gas, the gas behaves as does an incompressible fluid, similar to water. In fact, subsonic airflow can be modeled in a water tank, with adjustments for the diferent properties of the fluids (most significantly, fluid density, viscosity, and model scaling effects (see Reynolds_number). By properly balasting a plastic model aircraft it can be made to glide in a bathtub. In fact, by marking thickly with an apprpriate (old style) "indellible" marking pencil, the marked spots will shed a dye stream into the water, enabling visualization of the water flow and vortex shedding (as is done with smoke generators in wind tunnels).

At the speed of compressiblity (the speed of sound in the gas) a gas, rather than easily displacing and flowing (as does water) around a body, becomes "hard" in a sense (metaphorically speaking), raising compressive pressures at the point of entry and resisting any increase in speed to a degree not seen prior to compressability - the so-called "sound barrier". It is this property that may cause confusion among the casual editor, since water (incompressable - you cannot put significantly more into a rigid container by pushing on it) seems "harder" than normal air (which we can put more of into a rigid container by compressing it), What is occuring at the "sound barrier"" is that the air is now behaving elastically - like a spring pushing back on the moving body. It is this elastisity that creates the so-called "N" wave, the increase-decrease - and return to normal pressure wave observed by a high speed pressure recorder as a shock wave from a supersonic aircraft passes as stationary recording instrument.

Leonard G. 21:26, 6 Sep 2004 (UTC)

Ahhh. Makes much more sense. Note that it would be worth this excellent explanation in the main article, yes? -- Mor

But so wordy! I tend to overwrite and must restrain myself! What is need is some thought about the ovarall article as a whole- its organization and how and when the concepts are introduced. They are counter-intuitive until examined in depth.

Perhaps you could prepare a proposed outline (put it on this page), and we can work together importing and rewriting snippits of the article. It certainly could use some history, the observations of visual effects by early pilots, some Schlieren photography, a nice photo of Glamous Glennis and Chuck Yeager, and the like. Then we would have a genuine encyclopedia article. -- Leonard G. 14:53, 7 Sep 2004 (UTC)

Perhaps we need to dedicate our compressibility topics to more specific articles. There are thousands of topics that relate to compressibility, if we where to dedicate this much material to each on this page it'd be of course slightly to big. Maybe a couple brief examples hear, but it can dominate the article. Piyrwq 18:43, 6 September 2005 (UTC)

Could there be a derivation of compressibility (both isothermal and isentropic/adiabatic) and how it is related to the inverse of the bulk modulus? Dennibr 22:18, 21 January 2007 (UTC)

[edit] Compressibility factor

I've added a section on the theromdynamic definition of the "compressibility factor." However, this means that this article now spans two different topics (one of fluid dynamics and one of thermodynamics). Should this be transformed into a disambiguation page to separate the two definitions? Deklund 28 June 2005 20:29 (UTC)

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Is there some relation between Z and beta compressibility

[edit] Vertical compressibility

The geoscience section has a (potentially useful) table of "Vertical compressibility" coefficient values. If this relates to a volume change, then what (if anything) is the significance of the "vertical" modifier? — DIV (128.250.204.118 08:06, 28 March 2007 (UTC))

The applications of compressibility in the geosciences involve only vertical compressibility, and that goes with the dV too. I think that those coefficients were determined from confined aquifer pump tests, in which case they were measured to be vertical coefficients. I see what your saying, but I don't know whether the coefficients from that test can be assumed to be an isotropic. I'd check with the literature before modifying, but you probably are correct. +mwtoews 08:23, 28 March 2007 (UTC)

No, mea culpa, the "vertical" can stay. I have been doing some reading on this, and (as now makes sense) soils often are isotropic. If they're uniform, then that's the ideal case and they can be described by 2 or 3 (I forget) independent parameters only. Often the soils are cross-anisotropic, with distinct properties horizontally and vertically, which can be described by 5(?) parameters. And 'fully isotropic' soils apparently need about 21 parameters to describe their properties! (This is all confirmed from William Powrie's "Soil Mechanics", 2nd ed'n, p. 355 [—DIV]from memory of something I scanned this afternoon — not to be quoted!).

Nevertheless, I'm looking through the cited paper and have found two things that could be changed.

(1) I don't think the compressibility of water came from the cited reference — I certainly can't see it there — but the article implies that it does. The water really should be cited separately...

(2) ...or not at all. I think it is important to clarify that the values are for the drained case. For the undrained case the compressibility should be much lower, I expect. Also, there is no need for the word "coefficient" — it doesn't appear in the original paper either (which gives values as bulk moduli of drained compression).

I am making the second change, but need someone else to find a citation for water (or agree to remove it).

— DIV (128.250.204.118 08:15, 29 March 2007 (UTC))

I've added a ref for water, and you can move it out into a section and format it as you wish (I've also added the ref to Water (molecule)). The value from before came in a table in a textbook (Physical and Chemical Hydrogeology by Dominico and Schwartz, 1998), which also uses the same data from the original source from Domenico and Mifflin (1965). The numbers of coefficients, I suspect, probably fit in tensor matrices, and depend on the dimensions of analysis, but thanks for checking into that.+mwtoews 17:22, 29 March 2007 (UTC)

Cool, you're really up to speed. I am a bit concerned that the values listed by Domenico and Mifflin (1965) are not really elastic values: QUOTE "the change in height per unit original height decreases with recurrent cycles of equal load, resulting in an apparent increase in the bulk modulus." It sounds to me that at the end of all that cycling there would be no more plastic deformation, only elastic deformation, and thus those higher values would be the true (elastic) bulk moduli!

—DIV (128.250.204.118 02:27, 30 March 2007 (UTC))