# Talk:Compressibility

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What is "ß" in the formula given within the article's "Thermodynamics" subsection? Either a link or explanation is needed here.

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)

## 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)

Is there some relation between Z and beta compressibility

## 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))

## Fluid Section Complaint!

With repect the fluid section is rubbish! Some of the examples are not relevent to compressible aerodynamics. Terminology is incorrect as well; pitch, not trim. Get the terms right. Can someone with "more" academic knowledge fix this please? Proper explaination (with maths if viable) of how compressiblity afffects aircraft aerodynamics properties would be helpful! Thanks. En51cm 17:19, 4 July 2007 (UTC)

I think I broadly agree with what you're saying. The current "Fluid Dynamics" section goes on at length about aeronautical issues. It needs to be accurate, but moreover it needs to be balanced by other applications within the broader ambit of the section heading. I've demoted the material to a L3 heading.
— DIV (128.250.204.118 06:00, 27 September 2007 (UTC))

## Bulk modulus (B) compressibility (K)

Sorry to the person who did the bulk of the writing on this page, but you've got the symbols backward. B is for bulk modulus and K is for compressibility. When I get some time, I will try to repair the damage, but it's going to take a lot of work as the error is spread across several pages. Elert 20:50, 2 December 2007 (UTC)elert

The symbols may not be the best choice, but they are not in error. Ive seen bulk modulus mostly as K as (Atkins-Physical Chemistry, Landau and Lifshitz-Theory of Elasticity) sometimes as B (Symon-Mechanics). Ive seen compressibility always as a Greek letter sometimes as β sometimes as κ. When faced with these options, I think the bottom line should be IUPAC. IUPAC specifies "k" for the isothermal compressibility, but does not specify a symbol for the bulk modulus. (Perhaps its not a commonly used chemical parameter). I would be in favor of changing the symbol for compressibility to "k" but go slow on changing bulk modulus to "B". PAR 00:08, 3 December 2007 (UTC)

## Scope of article

The section "Aeronautical Dynamics" substantially strays from the scope of "compressibility" as it discusses the handling characteristics of specific aircraft and doesn't really tie it in to compressibility. The article is more thermodynamic and fluid dynamic in nature and content regarding specific aircraft should be moved to Aeronautic dynamics or other suitable locations. Iepeulas (talk) 14:49, 5 June 2008 (UTC)

Disagree. Compressibility was the name of a huge problem in aircraft development during World War II and it had everything to do with fluid dynamics. I don't have a problem with both theoretical and practical elements sharing the same article. Binksternet (talk) 16:19, 5 June 2008 (UTC)
I agree that aircraft development has everything to do with Fluid dynamics, however, the article is about compressibility, which is a thermodynamic material property. The content on aircraft in the article simply state several problems and design flaws of aircraft, not how compressibility effects the planes. The aircraft content should explain how compressibility effects aircraft dynamics, and even then, the content would be more suited to Compressible flow (which is more dynamic than compressibility) or an aerodynamics article. Iepeulas (talk) 00:35, 6 June 2008 (UTC)
If the aircraft section were moved to Compressible flow, we would lose readers who were looking specifically for the word "Compressibility". That word was used extensively in the 1940s and 1950s in describing the near-Mach problems facing aircraft designers. Binksternet (talk) 15:12, 21 June 2008 (UTC)
Easily resolved by either
• moving the contentious text to a page with a more relevant title and putting a flag at the top of this article (or elsewehere) to redirect readers looking for that information, or
• creating a diambiguation page and splitting the article into Compressibility (aeronautics) and Compressibility (materials science)
I have already indicated my agreement with User:Iepeulas that the contentious material strays from the topic indicated by the article title. —DIV (128.250.80.15 (talk) 11:21, 25 July 2008 (UTC))
P.S. User:Leonard_G also highlighted this problem above —DIV (128.250.80.15 (talk) 11:26, 25 July 2008 (UTC))
This is slightly off-topic, but if you do create a disambiguation page, the mathematics community would greatly appreciate it if you would add a link to the page concerning the mathematical notion of compressibility, currently found here: Incompressible surface. Otherwise, perhaps the top of the page could also have, "For compressibility (mathematics), see Incompressible surface. Adammanifold (talk) 13:52, 15 May 2009 (UTC)

Hey I'm working on developing several fluids articles and the compressible fluids page is one of them. I plan on doing major revamps on it as currently, it is pretty crappy. I plan on stealing away most of the aerodynamics section as it really has no place in this article. I would like to insert a section on fluid dynamics compressibility giving a basic description of what constitutes a compressible fluid, maybe describe shock waves generally, and then add a redirect to the compressible fluids page. This seems to make the most sense as the argument that people looking for world war 2 compressibility references should come to this article is ridiculous. The term compressibility, at least to my understanding, is simply a measure by which one can tell whether compressible fluids equations need to be used over incompressible equations to characterize a fluid. It is still used today, but this does not by any means mean that all compressible fluids interests should be diverted here. Add to this that the airplane information is more a result of applied compressible phenomena rather than the pure science of it, I'm still toying with the idea of throwing it into a control theory article, placing an aerodynamic section in the compressible fluids article linking to it and leave it at that. Any opinions? Iron_Engineer (talk) 03:08, 2 July 2009 (UTC)

Can you throw out some possible names for the article that the WWII fighter people will be referred to? Just curious about what you're thinking. Preferably, it would have compressibility in the name. Binksternet (talk) 04:09, 2 July 2009 (UTC)
Looking through wikipedia, there actually is no existing article to cover the effects described as far as I can find. This actually really surprises me since it is a pretty well established phenomena. He is slightly erroneous in his reasoning however. The affects only last within the transonic regime. As the air accelerates over the top of the wing at ambient air speeds approaching mach 1, local regions of the flow will go supersonic. This begins at the region of highest curvature according to the rules of aerodynamics. Below Mach 1 for the plane though, the air has to be shocked on the wing surface to slow down to subsonic speeds before leaving the tail edge. Having this shock sitting on top of the control surfaces rendered them useless. Nowadays, swept wings improve this but this is a reason why today most planes don't fly within the transonic region below mach 1. You typically fly below transonic or blast through it to supersonic. As the shock moves to the rear of the control surfaces, and off the back at mach 1, control is restored. This was experienced in the bell X-1. It was just that before this plane, the drag from the plane peaked before mach 1 so you'd max your air speed and have no control.
I would nominate a home for it at flight control surfaces. Lose some of the examples and go more theory, as I stated above and it would make a nice addition to the theory. It should really be tied in with the control reversal article though as they come from the same phenomena. As Yeager stated about the rocket planes he flew, first you would have no control, then before mach 1 you get some control back. But you had to go against your every instinct as a pilot and turn the controls in the opposite direction to make the plane fly correctly. To my knowledge, this affect has been better controlled since then and tends not to happen unless a plane is designed for it, such as the X-53.
So I've convinced myself. New article like control reversal and bring small sections of them both in to flight control systems, where a mention of the reversal article belongs but is missing.
Iron_Engineer (talk) 05:36, 2 July 2009 (UTC)
Too much talk, not enough action. I've moved most of that content to Aerodynamics where it fits better. 121.45.196.67 (talk) 01:27, 21 April 2012 (UTC)

## Subscripts of derivatives

How about s instead of S, because T and s are intrinsic (that is, specific) properies, while S is an extrinsic property. Alternative, something like n could be added to the subscripts. —DIV (128.250.80.15 (talk) 00:50, 28 July 2008 (UTC))

## That last paragraph's a doozy!

The last paragraph in the Aeronautical dynamics section ("Dissociation absorbs a great deal of energy...") seems jarringly out of place. It looks like it belongs up in the Thermodynamics section. Reading it after reading through the paragraphs before it is like reading a Scientific American article and then suddenly being hit in the face with a thermodynamics textbook. Ouch!

It seems like it should be moved up to the Thermodynamics section, or else rewritten to be a little easier to understand for those who haven't taken a college level thermo course recently.

KeithHearn (talk) 22:26, 1 July 2011 (UTC)