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HelicopterOff Topics › Need help in Physics
05-02-2004 06:59 PM  13 years agoPost 1
Kristjan

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Estonia, Tallinn

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What is the speed of air flow in a pipe that has inner diameter of 15 mm ( 0.590 inch) and pressure is 350 PSI = 23,8 ATM = 24,1 BAR?

Ideas?

Kristjan

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05-02-2004 07:05 PM  13 years agoPost 2
Davo

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You can't figure it out from that information. Are you saying that the pressure reservoir is at 24.1 bar and it is venting to atmospheric conditions? If so the flow will undoubtedly be choked (i.e. sonic). You can work out this velocity using the equation a = SQRT(gamma * R * T), where a is the speed of sound, gamma is the ratio of specific heats, R is the universal gas constant and T is the temperature in Kelvin.

On top of this you need info on pipe skin friction and the length of the pipe to determine laminar/turbulent flow to estimate the associated velocity profiles.

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05-02-2004 07:11 PM  13 years agoPost 3
Davo

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Hey prophanger1, if you're going to become a pilot as it says in your profile you'll need to learn some of this stuff (I think)!!! Go get a copy of Anderson's "Fundamentals of Aerodynamics" and be enlightened.

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05-02-2004 07:30 PM  13 years agoPost 4
Kristjan

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I made a drawing in 5 seconds.

Air comes in left. Barometer is in the exit side. Isn`t this enough to determine the speed?

Kristjan

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05-02-2004 09:30 PM  13 years agoPost 5
Doug

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The missing parameter is temperature. Since "any" flow will be choked at about 1.8/1 pressure ratio the resulting velocity will be dependent on the starting and ending temperatures as well as heat flow into or out of the pipe. ( the expansion will lower the temp the friction will raise it).

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05-03-2004 07:14 AM  13 years agoPost 6
Kristjan

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Temperature would be 300 Kelvin in the environment where the pipe is situated.

I can`t say anyting about friction. What is the frictional force between air and smooth aluminum for example (higher speeds, higher force)?

By choking are you saying that no matter what pressure, the air velocity won`t exceed some critical point?

K.

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05-03-2004 05:44 PM  13 years agoPost 7
Allora

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OOOOOwwwww. I just remembered how much I hated Fluids class....

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05-03-2004 05:49 PM  13 years agoPost 8
Doug

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The velocity can never exceed the local speed of sound and that is totally dependent on the fluid and the temperature. if you increase the pressure further although the velocity does not change the mass flow does because you flow at ever increasing densities

For your 300K the answer would be approx. 347.604 M/S for dry air. This would be toward the center of the pipe . and would likely be less due to skin friction. As the air approaches the "end" of the pipe the local velocity would increase due to expansion.

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05-05-2004 11:01 AM  13 years agoPost 9
Hawk4flyer

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It would be easier to determine the volume of air than the speed.

Following Bernuli's principle, speed would be different at each point along the tube. The farther along the tube you get the higher the pressure, thus the lower the speed. This is of course assuming that the tube is straight (without converging or diverging ducts).

I have to agree, you don't have enough info to complete the problem.

Is the tube vented to ambient air?
What is the moisture content of the air entering the tube?
What is the length of the tube?
What is the temp of the air entering the tube?
At what pressure altitude is the air vented into?

Blah, blah, blah.....

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05-05-2004 01:29 PM  13 years agoPost 10
Doug

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Following Bernuli's principle, speed would be different at each point along the tube. The farther along the tube you get the higher the pressure, thus the lower the speed. This is of course assuming that the tube is straight (without converging or diverging ducts).
I don't think so.....

As the flow is "choked" the downstream pressure is irrelevant (as long as choked flow is maintained). That pressure would only affect the mass flow. Assuming a sharp edged pipe the normal shock wave would be at the entrance, the flow down stream would be turbulent and the pressure would be about half of the head pressure. The velocity would again rise at the exit and would accelerate supersonic after it left the pipe. there would be a large temp drop at the exit and the "free" air stream would be markedly cooled by the expansion.

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05-05-2004 02:01 PM  13 years agoPost 11
Dahflyboy1

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Fellas I think your making this to complicated. This is a physics problem, not an aero problem. Stick to the basics, I don't think he has to account for friction or shock waves or any of that stuff. I sure didn't learn that in physics.

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05-05-2004 02:27 PM  13 years agoPost 12
Doug

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Please refer to my answer to the original question +_ 348 ms

And remember "most" Physics problems use ideal fluids, incompressible flow and friction less interface, hardly the case here

This is a fluid dynamics question not a physics question.

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05-05-2004 02:30 PM  13 years agoPost 13
Davo

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This is a physics problem, not an aero problem
Since when was aerodynamics disassociated from physics?

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05-05-2004 02:32 PM  13 years agoPost 14
Doug

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When you use ideal fluids and incompressible flow.

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05-05-2004 02:40 PM  13 years agoPost 15
Davo

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It's still aerodynamics/ fluid dynamics, no matter how much idealisation you apply. Aero is a part of physics same as electromagnetism or springy things. Guess I'm just being pedantic

I think I may have missed the point of that post somewhat.

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05-05-2004 02:56 PM  13 years agoPost 16
Doug

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The answer is simply the "air" will flow at the "local" speed of sound which is totaly dependent ont the gas and the temp.

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05-05-2004 03:55 PM  13 years agoPost 17
Jagboy69

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Miami

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Who said you have to be able to work this stuff as a pilot??? I know Seneca and myself would find just find someone else to work this problem...
Problem solved.

Jason /// Sceadu50/9chp WWW.Jagboy69.com

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05-05-2004 04:13 PM  13 years agoPost 18
Davo

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Point taken. I meant that as a pilot you might need to know that lift is a function of air density, velocity and angle of attack. I guess sonic pipe flow is pretty irrelevant unless you're flying Concorde through a tunnel

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05-06-2004 02:14 AM  13 years agoPost 19
Jagboy69

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Miami

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you got it man... as long as I know that high density altitudes suck and can be deadly... It's all good....

Jason /// Sceadu50/9chp WWW.Jagboy69.com

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05-06-2004 03:11 AM  13 years agoPost 20
mcatech

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Mount Gambier SA Australia

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Just give me 5 minutes Captain
I need to re-align the Di-Lithium Crystals to stabilise the warp core

sorry guys interesting stuff though
very heavy
I will read it again when I am not so tired

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