## Fluid Compression

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But change of density does not mean compression.

Compression means change of density caused by applied pressure. Not quite the same.

And although there is a slight dilatation (= density change due to temperature), the dominant effect on pressure here remains flow velocity, not dilatation.
bot6
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Joined: 2 Mar 2011

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Ringo, in Turbomachinary air is taken as a compressible fluid.

in free stream fluids its assumed to be incompressible.

Flow under and around F1 cars is not turbomachinary.

Air flow inside an engine is considered under turbomachinary theory i.e. compressible.
Raptor22
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Joined: 7 Apr 2009

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bot6 wrote:But change of density does not mean compression.

Compression means change of density caused by applied pressure. Not quite the same.

And although there is a slight dilatation (= density change due to temperature), the dominant effect on pressure here remains flow velocity, not dilatation.

correct
Raptor22
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Joined: 7 Apr 2009

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Raptor22 wrote:Ringo, in Turbomachinary air is taken as a compressible fluid.

in free stream fluids its assumed to be incompressible.

Flow under and around F1 cars is not turbomachinary.

Air flow inside an engine is considered under turbomachinary theory i.e. compressible.

It's actually more a question of temperature, pressure and speed, all relative to the speed of sound. It is only near or at or above the speed of sound that air is compressible to a sufficiently large extent to modify the flow in any sizable way.

As I recall, F1 cars are quick, but not quite supersonic yet... Hence assuming incompressible flow.
bot6
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Joined: 2 Mar 2011

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Thank you, guys. Real pleasure reading such discussions.
IMHO Ringo relates a general theoretical formulation to a particular practical case and there lies his error.
Sorry, never studied scientific English, difficult to formulate now. Hope I didn't say something very stupid.
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Dragonfly
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Joined: 17 Mar 2008
Location: Bulgaria

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Mosaic for Robert

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Last edited by darq76 on Thu Mar 10, 2011 6:23 am, edited 1 time in total.
darq76
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Joined: 9 Mar 2011

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Dragonfly -> Not at all.

The thing is, Ringo is actually correct, the effects he mentions do happen.

The thing is, they happen on a very little scale at the speeds we are looking at, so generally they are ignored to simplify the problem.

So basically, he's right. He's just too right... if that makes sense...
bot6
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Joined: 2 Mar 2011

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Thanks bot6
I knew I didn't phrase my post right
I think I understand very well what you wrote and, of course, Ringo is right about air/gas compressibility. Maybe it'd be better to say he took the word "incompressible" out of the particular context.
But I am afraid fluid dynamics are a white spot for me, so I stop here.
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Dragonfly
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Joined: 17 Mar 2008
Location: Bulgaria

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bot6 wrote:
Raptor22 wrote:Ringo, in Turbomachinary air is taken as a compressible fluid.

in free stream fluids its assumed to be incompressible.

Flow under and around F1 cars is not turbomachinary.

Air flow inside an engine is considered under turbomachinary theory i.e. compressible.

It's actually more a question of temperature, pressure and speed, all relative to the speed of sound. It is only near or at or above the speed of sound that air is compressible to a sufficiently large extent to modify the flow in any sizable way.

As I recall, F1 cars are quick, but not quite supersonic yet... Hence assuming incompressible flow.

Yes Buddy I understand that. I simply used examples of where the different assumptions are applicable. Propellors in air, axial flow compressors, this is where we deal compressibility. In freestream problems we don't bother because the effect is so small.
Raptor22
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Joined: 7 Apr 2009

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Air is compressible. Full stop.

IT depends on what systems you are studying if you want to assume that it is incompressible.

It is correct to say "Lets assume that air is incompressible."
But air is very compressible, some systems the air is not compressed much at all so some people ignore it.

In the case of the exhaust gasses now... You have to work out whether you can ignore it or not.

What speed and temperature does the air leave the exhaust pipe?
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n smikle
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Joined: 12 Jun 2008

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n smikle wrote:Air is compressible. Full stop.

IT depends on what systems you are studying if you want to assume that it is incompressible.

It is correct to say "Lets assume that air is incompressible."
But air is very compressible, some systems the air is not compressed much at all so some people ignore it.

In the case of the exhaust gasses now... You have to work out whether you can ignore it or not.

What speed and temperature does the air leave the exhaust pipe?

My original statement was: free flow of gases are considered incompressible al low speeds. Air as such is very very compressible. If you have some spare time to go and take a look at quasars, even iron and titanium are very compressible

No experimental data, but quick math (2.4ltr * 18000 /60 /4) gives us for current F1 engine 180 ltr/s at intake, and assuming 300K ambient and 900K after combustion 300 ltr/s to each of pipes after expansion. Assuming incompressibility and pipe with cross-sectional area of 0,00785m^2 (round, 10 cm diameter), it will be about 38 m/s (137 kmh). If the actual temp after combustion is higher, it will be more.
marekk
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Joined: 11 Feb 2011

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Raptor22 wrote:Yes Buddy I understand that. I simply used examples of where the different assumptions are applicable. Propellors in air, axial flow compressors, this is where we deal compressibility. In freestream problems we don't bother because the effect is so small.

That's the thing though. A freestream problem can also be supersonic or close to supersonic (with airplane fuselages for example). So even though you might know that, someone else reading this might misuse your classification, leading to confusion, leading to the usual endless arguments that pollute posts. I just wanted to make it clear to everyone that it's about flow conditions (pressure, temperature, speed), not about the type of problem.

Marekk, thanks for the quick math. I think that pretty much establishes we are nowhere near sound speed, so we can assume incompressibility...
bot6
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Joined: 2 Mar 2011

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marekk wrote:

My original statement was: free flow of gases are considered incompressible al low speeds. Air as such is very very compressible. If you have some spare time to go and take a look at quasars, even iron and titanium are very compressible

Yes, I am invincible some times, but i am mortal at other times. You see why it's wise not to say something like air is incompressible?

You should say the density changes are so small it will be neglected. And those are special cases.

No experimental data, but quick math (2.4ltr * 18000 /60 /4) gives us for current F1 engine 180 ltr/s at intake, and assuming 300K ambient and 900K after combustion 300 ltr/s to each of pipes after expansion. Assuming incompressibility and pipe with cross-sectional area of 0,00785m^2 (round, 10 cm diameter), it will be about 38 m/s (137 kmh). If the actual temp after combustion is higher, it will be more.

Big mistake, an engine calculation is never done with constant air density.

That is why mass flow is used and not volume. Mass cannot vary like density.
Use volume flow only at the beginning with know pressures and temp, just to get the mass. Don't use it again after that, unless you have a chart that shows the density at a certain temperature.
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ringo
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Joined: 29 Mar 2009

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@ringo:
constant air density ?
a chart that shows density at certain temperature?
Did you get your degree on fluid dynamics without reading about ideal gas law ?

My quick math was not meant to design an engine. Just to roughly assess exhaust's speed.
marekk
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Joined: 11 Feb 2011

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Yes you can use the ideal gas law. Geez

Not everyone knows that.
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ringo
46

Joined: 29 Mar 2009

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