"Fluids mechanics" is also gas mechanics?


by Femme_physics
Tags: fluids mechanics, mechanics
Femme_physics
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#1
Jul14-11, 12:48 AM
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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".
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Jul14-11, 12:50 AM
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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.
Femme_physics
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Jul14-11, 12:59 AM
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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.

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Jul14-11, 01:15 AM
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"Fluids mechanics" is also gas mechanics?


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.
Pengwuino
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Jul14-11, 01:18 AM
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Quote Quote by Femme_physics View Post
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.
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Jul14-11, 01:20 AM
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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?
Femme_physics
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Jul14-11, 01:28 AM
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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)





Kinda like smoke in a container
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Jul14-11, 01:40 AM
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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.
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Jul14-11, 02:50 AM
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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
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Jul14-11, 03:20 AM
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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.
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harrylin
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#11
Jul14-11, 04:16 AM
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Quote Quote by Femme_physics View Post
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.

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
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Jul14-11, 06:21 AM
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Quote Quote by Femme_physics View Post
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.
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Jul14-11, 06:37 AM
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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

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


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?
Andy Resnick
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Jul14-11, 08:17 AM
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Quote Quote by Femme_physics View Post
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".
Quote Quote by Femme_physics View Post
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.

Quote Quote by Femme_physics View Post
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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Jul14-11, 09:21 AM
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Gases are not inviscid!
The quote said "fluid" which can also mean liquid! As you probably already know.
Studiot
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Jul14-11, 09:49 AM
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FP,
How about moving on from the argument and telling us what aspects of fluid mechanics you are studying?
timthereaper
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Jul14-11, 10:04 AM
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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.
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Jul14-11, 10:21 AM
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Quote Quote by Studiot View Post
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


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