Fluids mechanics is also gas mechanics?

  • #1
Femme_physics
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"Fluids mechanics" is also gas mechanics?

This bothers me a bit:

Fluid mechanics
From Wikipedia, the free encyclopedia


Fluid mechanics is the study of fluids and the forces on them. (Fluids include liquids, gases, and plasmas.)

Gases? You include in the chapter of "fluid mechanics" gases? These are two different states. Why not have a field called Gas mechanics? And if you want, call the entire field "Fluid-gas mechanics".
 

Answers and Replies

  • #2
Pengwuino
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By the definition of "fluid", gases are considered fluids. They are not separate states of matter; it's how they interact with the environment that's important.
 
  • #3
Femme_physics
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They are not separate states of matter

From chemistry I learned there are 3 states of matter: Solids, fluids and gases. If they're not separate states, then they shouldn't be given a separate state status!


I addition, this quote is from wiki:

Fluid mechanics
The study of the physics of continuous materials which take the shape of their container.

But gases don't just take the shape of their container...they move around BOUND by it, but don't "take the shape of it". I'd say it's a false defintion then.
 
  • #4
SteamKing
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If it will ease your tortured mind, there is a special branch of fluid mechanics call "gas dynamics".

Under certain circumstances, the equations governing the flow of gases are similar to those governing the flow of liquids. When these circumstances are not present, then the compressibility of gases (liquids are generally incompressible) requires modification to the equations of fluid flow.

BTW, in addition to the three phases of matter normally encountered on earth, gases which are ionized and at high temperature are called plasmas, and plasmas are considered a fourth phase of matter, because they behave unlike the other three.
 
  • #5
Pengwuino
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From chemistry I learned there are 3 states of matter: Solids, fluids and gases. If they're not separate states, then they shouldn't be given a separate state status!

Sorry, I mean fluids are not meant to be part of the gas/liquid/solid/plasma classification.

But gases don't just take the shape of their container...they move around BOUND by it, but don't "take the shape of it". I'd say it's a false defintion then.

No, they do take the shape of it. Maybe you're confused because sometimes you see "heavy" gases that kinda stay at the bottom of a container, but that's simply because there's gravity pulling it down. In vacuum, they'd be taking the shape of the container.
 
  • #6
Femme_physics
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If it will ease your tortured mind, there is a special branch of fluid mechanics call "gas dynamics".

AHA! So, in fact, the statement that "Fluids include liquids, gases, and plasmas" is false, whereas it should says "fluid MECHANICS include liquids, gases, and plasmas".

And I rather resent you opening with "if it will ease your tortured mind", as though it's a silly issue to raise. Why I hold accuracy to be an important virtue.

Fluids include liquids, gases, and plasmas
Interesting.. I'll read on that!

But another thing on wiki I wonder about

Similarly, it can sometimes be assumed that the viscosity of the fluid is zero (the fluid is inviscid). Gases can often be assumed to be inviscid. If a fluid is viscous, and its flow contained in some way (e.g. in a pipe), then the flow at the boundary must have zero velocity

This is because the fluid sticks to the walls, right?
 
  • #7
Femme_physics
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Sorry, I mean fluids are not meant to be part of the gas/liquid/solid/plasma classification.
I never heard that. Gosh, chemistry and physics are further apart than I thought!

No, they do take the shape of it. Maybe you're confused because sometimes you see "heavy" gases that kinda stay at the bottom of a container, but that's simply because there's gravity pulling it down. In vacuum, they'd be taking the shape of the container.

No, rather, this is what I imagine (the blue inside the container being the gas)

http://img39.imageshack.us/img39/2663/container11.jpg [Broken]



Kinda like smoke in a container
 
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  • #8
I like Serena
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You'll typically see shapes likes this when you mix 2 gases, in this case a blue gas with air (which is transparent of course).

The air fills the entire container and in time the blue gas will mix more thoroughly with the air, filling the container completely as well.
 
  • #9
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Elementary classifications are just that. Elementary classifications.

When you go into most subjects more deeply they become complicated, the boundaries between classifications blur and new classification categories become necessary.

The solid / liquid / gas classification is both ancient and elementary.
It has been found wholely inadequate by modern science and technology.

Initially fluids (= that which flows) included liquids and gasses, although several famous textbooks have and continue to be published under the title 'hydrodynamics' or even 'Hydraulics'.
In the latter half of the twentieth century this category also softened as it was realised that the same mechanics applies also to powders, sand, bulk grain and other granular material, mixed state materials such as freshly mixed concrete before it sets. The list is constantly being extended and time is now a factor for instance geologists may regard ice as a fluid.

Physicists now like to regard plasmas as a separate state from gasses, rather than just gasses made up of charged particles. The mechanics of plasmas is often called magnetohydrodynamics, although water is not involved!

Equally chemists started distinguishing many states - the dissolved state, the adsorbed state, the disperse state (eg the smoke in your picture) and so on.

I expect, if you think back, you will notice that the examples for solid/liquid/gas were given of pure substances. These days we tend to reserve that classification for pure substances that obey the 'phase rule' in chemical thermodynamics.

go well
 
  • #10


Yes you are right Femme ,Fluids mechanics" is also gas mechanics.
Fluid mechanics, especially fluid dynamics, is an active field of research with many unsolved or partly solved problems. Fluid mechanics can be mathematically complex.
by Electrician[/PLAIN] [Broken] Leeds
 
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  • #11
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From chemistry I learned there are 3 states of matter: Solids, fluids and gases. If they're not separate states, then they shouldn't be given a separate state status!
[..].

Probably this is just about words and definitions. :tongue2:

In physics we say that there are 3 (4) states of matter: solid, liquid and gas (and plasma).

Compare http://dictionary.reference.com/browse/fluid :

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.

Cheers,
Harald
 
  • #12
Dale
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From chemistry I learned there are 3 states of matter: Solids, fluids and gases.
You are confusing "fluid" and "liquid". The 3 states of matter are solids, liquids, and gasses. Both liquids and gasses are fluids.

Fluid means that the material continually deforms under shear stress. I.e. the shear rate is proportional to the shear stress. This definition covers both liquids and gasses.

A liquid is a fluid which is incompressible, and a gas is a fluid which is compressible. But they are both fluids.
 
  • #13
Femme_physics
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You'll typically see shapes likes this when you mix 2 gases, in this case a blue gas with air (which is transparent of course).

Ah, I see!

The air fills the entire container and in time the blue gas will mix more thoroughly with the air, filling the container completely as well.

I fully accept your explanation :approve:

You are confusing "fluid" and "liquid". The 3 states of matter are solids, liquids, and gasses. Both liquids and gasses are fluids.

Fluid means that the material continually deforms under shear stress. I.e. the shear rate is proportional to the shear stress. This definition covers both liquids and gasses.

A liquid is a fluid which is incompressible, and a gas is a fluid which is compressible. But they are both fluids.

I fully accept this explanation as well :approve;


Thanks to the others for their replies. I got nothing else to say other than I accept your explanation :smile:


Can anyone though answer me for what I asked before


Similarly, it can sometimes be assumed that the viscosity of the fluid is zero (the fluid is inviscid). Gases can often be assumed to be inviscid. If a fluid is viscous, and its flow contained in some way (e.g. in a pipe), then the flow at the boundary must have zero velocity
This is because the fluid sticks to the walls, right?
 
  • #14
Andy Resnick
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Gases? You include in the chapter of "fluid mechanics" gases? These are two different states. Why not have a field called Gas mechanics? And if you want, call the entire field "Fluid-gas mechanics".

From chemistry I learned there are 3 states of matter: Solids, fluids and gases. If they're not separate states, then they shouldn't be given a separate state status!
.

Liquids and gases are both considered "fluids", as has been pointed out. The relevant parameter in your question is the Knudsen number:

http://en.wikipedia.org/wiki/Knudsen_number

For Kn >>1, the continuum approximation breaks down and we instead model the fluid as a dilute gas using statistical methods.

Can anyone though answer me for what I asked before

This is because the fluid sticks to the walls, right?

Gases are not inviscid! In fact, accounting for the difference in density (the kinematic viscosity, measured in Stokes ), air is as viscous as water. Fluids don't "stick" to walls (exempting adhesion/bonding/chemical interactions)- the no-slip condition arises simply by demanding the stress tensor be finite.
 
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  • #15
Femme_physics
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Gases are not inviscid!

The quote said "fluid" which can also mean liquid! As you probably already know.
 
  • #16
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FP,
How about moving on from the argument and telling us what aspects of fluid mechanics you are studying?
 
  • #17
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Fluids aren't really inviscid. I guess you can think of it as "sticking" to the wall if it helps you, but I think Resnick answered the question very succinctly. We assume inviscid flow a lot of times to simplify the equation when we know (or think) that viscosity doesn't play a large part in the problem we're trying to solve. However, when dealing with certain types of problems (like in aerodynamics), you can't assume inviscid flow.

Hope that helps.
 
  • #18
Femme_physics
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FP,
How about moving on from the argument and telling us what aspects of fluid mechanics you are studying?


Oh I'm just trying to enrich myself before the semester starts :smile:
 
  • #19
cjl
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Liquids and gases are both considered "fluids", as has been pointed out. The relevant parameter in your question is the Knudsen number:

http://en.wikipedia.org/wiki/Knudsen_number

For Kn >>1, the continuum approximation breaks down and we instead model the fluid as a dilute gas using statistical methods.
Oh dear.

The Knudsen number has absolutely nothing to do with any of the concepts discussed in this thread. The Knudsen number describes whether a gas is sufficiently rarefied such that individual molecular effects must be taken into consideration when modeling the flow. It comes into play when modeling either very low density flows (such as satellite drag or reentry) or very small scale flows (such as the flow around a hard disk drive's head, or flows around some MEMS and NEMS devices). While the Knudsen number is tremendously useful in some cases, it's completely irrelevant for this discussion.


Gases are not inviscid! In fact, accounting for the difference in density (the kinematic viscosity, measured in Stokes ), air is as viscous as water. Fluids don't "stick" to walls (exempting adhesion/bonding/chemical interactions)- the no-slip condition arises simply by demanding the divergence of the stress tensor be finite.

True, gases are not inviscid. However, their viscosity is small enough that in some cases, the viscosity can be ignored and useful solutions still arise (which is actually true for some liquids as well). As for air vs water viscosity? Kinematic viscosity isn't really the relevant parameter - dynamic viscosity is the much more common parameter, and using that measurement, air is far less viscous than water (as would be expected).

As for the no-slip condition? No, it does not arise from the need for the divergence of the stress tensor to be finite. It arises from the tendency of the flow to stick to the surface. At a molecular level, individual surface reflections tend to be diffuse, which means that the outgoing angle of an individual molecule after impacting the surface tends to be independent of the incoming angle, and statistically distributed. When averaged over large numbers of molecules, this means that the reflected fluid is stationary with respect to the surface, aside from the velocity away from the surface (that comes from the fact that we are only considering reflected molecules). These reflected molecules then interact with incoming molecules, and the net result is that the fluid adjacent to the surface is stationary for any flow in which the molecular interaction length scale is substantially smaller than the object's length scale (such that the reflected molecules can interact with and slow down the incoming molecules).
 
  • #20
cjl
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Fluids aren't really inviscid. I guess you can think of it as "sticking" to the wall if it helps you, but I think Resnick answered the question very succinctly. We assume inviscid flow a lot of times to simplify the equation when we know (or think) that viscosity doesn't play a large part in the problem we're trying to solve. However, when dealing with certain types of problems (like in aerodynamics), you can't assume inviscid flow.

Hope that helps.

Actually, in aerodynamics, inviscid flow is frequently assumed, as inviscid flow plus a couple of small corrections (the biggest one for aerodynamics is the Kutta condition) can give surprisingly accurate results for high Reynolds number flow. You do need viscosity to correctly model the skin friction drag and boundary layer behavior, but induced drag and lift don't really need viscosity to determine an accurate solution.
 
  • #21
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The point I was making was more about the ability/inability to assume inviscid flow. I knew that there were certain problems in aerodynamics that you could assume inviscid flow, but I know there are certain ones you can't. I couldn't come up with another case in other areas of fluids where you could assume inviscid flow off the top of my head.
 
  • #22
Andy Resnick
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The quote said "fluid" which can also mean liquid! As you probably already know.

Eh? The quote within your quote mentioned 'gases":

<snip>

Can anyone though answer me for what I asked before

[Similarly, it can sometimes be assumed that the viscosity of the fluid is zero (the fluid is inviscid). Gases can often be assumed to be inviscid. If a fluid is viscous, and its flow contained in some way (e.g. in a pipe), then the flow at the boundary must have zero velocity]

This is because the fluid sticks to the walls, right?
 
  • #23
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The air fills the entire container and in time the blue gas will mix more thoroughly with the air, filling the container completely as well.

You may see this as a quibble, but I should like to take issue with this statement. In kinetic gas theory and statistical mechanics it is considered a matter of some importance that most gases do NOT completely fill their container.

For instance, a container of air at NTP is 99.9% nothingness and only 0.1% gas molecules. The air is a long, long way from "filling" the container. I prefer to say that the probability of a gas molecule being in any volume of that container is the same for any similarly-sized volume. The actual number of molecules in any given volume will vary considerably from volume to volume an any given instant; and will vary from instant to instant for any given volume. It's all a matter of probability.

I believe the "filling" language is a holdover from the days when gas molecules were believed to be able to expand indefinitely in size so as to "fill" a container.
 
  • #24
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You may see this as a quibble, but I should like to take issue with this statement. In kinetic gas theory and statistical mechanics it is considered a matter of some importance that most gases do NOT completely fill their container.

I don't think that is quite what is meant by filling the container or taking on the shape as mentioned earlier.

Take a container.

The difference in behaviour as regards 'filling' between a gas and a liquid is simple here.

For a gas there is no part of the volume that is not available to the gas molecules to occupy.
Yes there is only a finite probability of finding a gas molecule in any given region at any given time, and yes the distribution will be uneven and vary with time, but unlike a liquid, there is no boundary or surface.

A liquid on the other hand has a surface. All the liquid molecules occupy space on one side of the surface only.
Yes there may be vapour molecules escaped from the surface in the rest of the container, but these are no longer liquid.

go well
 
  • #25
Femme_physics
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Eh? The quote within your quote mentioned 'gases":

Then in that case you have an argument to pick against wiki, not me!
 

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