Air flow from large to small tube.What happens to pressure?

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1. Aug 23, 2015

pravi

I would like to know what happens to air pressure when it flows from large tube to small tube. I would like to know the basic science behind this though I am pursuing my B.E. Moreover, does PV=mRT law apply here or some other principle applies here. What happens to temperature and other charactersitics. Please help me with this. Thanks.

2. Aug 23, 2015

Staff: Mentor

Last edited by a moderator: May 7, 2017
3. Aug 23, 2015

SteamKing

Staff Emeritus
You don't say what field your B.E. is in, but questions like this are covered in a course on fluid dynamics.

4. Aug 23, 2015

rowny

Best you can do is to use a fluid simulation software like Fluent or Comsol and see what happens yourself.

5. Aug 23, 2015

Staff: Mentor

How much pressure? How big of a difference between tube sizes? Depending on the answer, this may be more of a thermodynamics question than a simple fluids flow question.

6. Aug 23, 2015

Um, what? This problem is, in all likelihood, easily solvable analytically to within a very good approximation.

7. Aug 23, 2015

rowny

"Other characteristics" he says. For instance, the flow might be turbulent, meaning that velocity and pressure will oscillate in space. When the flow reaches a step-up/step-down obstacle (such as a different pipe), a vortex occurs at the edges. No analytical solution is possible for these.

8. Aug 23, 2015

He also said "basic science" which implies turbulence and flow separation are probably not something to consider just yet.

9. Oct 19, 2015

pravi

10. Oct 19, 2015

BvU

No need to be sad at all. You have to realize that your question is not extremely specific, so you get all kinds of answers. Can be frustrating or disappointing, but that's how it is in PF (and that's also what makes PF interesting for helpers, because they get all kinds of questions that appear easier or more difficult than intended ).

In fluid flow at a constriction the Bernoulli equation is a good start -- see Hyperphysics.

Next steps (e.g. here) bring in more detail -- and as you see life isn't all that simple any more.

Up to you how deep you want to go; now you at least have a start and some useful search terms. Good hunting !

 on the link mfb gave you, under the picture of an airplane wing, the 'accompanying' link makes clear that yes, the gas law applies and the temperature drops. A gas needs energy to expand (because that means it is doing mechanical work !); if it has to take care of that all by itself the temperature drops.

Last edited: Oct 19, 2015
11. Oct 19, 2015

sophiecentaur

Simulation is the last resort when simple analysis gets bogged down with too much complexity. Simulations cannot give to any more than the answer to "what happens when I do this?" question when what you need is to know the relationship between variables.