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Question About Fluid Flow In A Pipe  
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Posted (9 years 9 months 2 weeks 4 days 16 hours ago) and read 1301 times:

say there is a large pipe with a smaller pipe within it, the flow in the smaller pipe is much slower than the outer one. Now say the little pipe ends and empties into the outer pipe which doesn't change it's size.

Is the overall resulting flow moving slower or negligible to the outer pipe's original flow?


The meaning of life is curiosity; we were put on this planet to explore opportunities.
7 replies: All unread, jump to last
 
User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 7
Reply 1, posted (9 years 9 months 2 weeks 4 days 16 hours ago) and read 1290 times:

The average speed would be slightly slower - it's basically the continuity equation. You can of course complicate the problem by considering boundary effects, the wall thickness of the inner pipe, etc., but both these effects would cause reduction of the speed after the flows join.

Pete


User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 2, posted (9 years 9 months 2 weeks 4 days 15 hours ago) and read 1287 times:

So I'm guessing the inner flow would be forced to accelerate? I am familiar with the eqn, just a friend is borrowing my fluids text and their out of state. it is m-dot-1 plus m-dot-2 equals m-dot-3?

By m-dot I think I mean mass flow as in area times velocity times density.

But what I do not know is how to figure where downstream the flow is more or less uniform. If the outer flow is fast enough then I gguess the distance is pretty short.


oh yeah, happy 4th! i totally forgot.



The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offline777236ER From , joined Dec 1969, posts, RR:
Reply 3, posted (9 years 9 months 2 weeks 4 days 12 hours ago) and read 1263 times:

Yeah, it's continunity.

The mass flow rate (dm/dt) is just ρAU, and continunity states that the mass flow rate into a system has to equal the mass flow rate out.

In this case, (ρAU)1 + (ρAU)2 = (ρAU)final. If the fluids are the same, then A1U1 + A2U2 = AfinalUfinal

Which gives, Ufinal= (1/dfinal2)(d12U1+d22U2)


User currently offlineDc10guy From United States of America, joined Feb 2000, 2685 posts, RR: 6
Reply 4, posted (9 years 9 months 2 weeks 4 days 7 hours ago) and read 1237 times:

I would say that PSI would determine flow.


Next time try the old "dirty Sanchez" She'll love it !!!
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 5, posted (9 years 9 months 2 weeks 3 days 7 hours ago) and read 1200 times:

shall I assume U is velocity? We here (accross the pond) use it to mean potential energy, so initially I did not understand your eqn. How do you guys use V?

and d is diameter. Gotcha, thanx!


Optional question, what if the area in the larger pipe approached infinity (got extremely large), does the smaller pipe flow then not affect anything down stream? Imagine a duct outside of a car with no other purpose but to slow air coming in and through it. What happens to that air on the otherside of the duct? Speed up?

Again, that is optional, thanks for the replies.



The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offline777236ER From , joined Dec 1969, posts, RR:
Reply 6, posted (9 years 9 months 2 weeks 3 days 1 hour ago) and read 1184 times:

U is velocity, yeah.

Continuity holds for anything, that is mass flow rate in = mass flow rate out.

If the area of the larger pipe appropached infinity, the mass flow rate in would be infinity + the mass flow rate of the smaller pipe, which is negligible. So the mass flow rate out, downstream, would be infinite (the mass flow rate of the larger pipe.)

Of course none of this takes into account viscous effects etc.

Imagine a duct outside of a car with no other purpose but to slow air coming in and through it. What happens to that air on the otherside of the duct? Speed up?

Effectivly yes, in reality it's so tiny as to be negligible. In reality, turbulence around the pipe would probably slow the air around the pipe down more than continunity would theoretically speed it up.


User currently offlineWellHung From , joined Dec 1969, posts, RR:
Reply 7, posted (9 years 9 months 2 weeks 1 day 14 hours ago) and read 1148 times:

Fluid flow in a pipe? We'll report back to you after this weekend.

Signed,
Jcs17 and Aa61hvy


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