If the air if flowing from larger to smaller pipe, as you say, there
shouldn't really be a whirlpool effect.  If anything, there would be eddy
currents along the sides of the pipe. Basically, the air backs up because it
can't all go into the pipe at the same time and it curls back on itself.

What happens in the reduction is that the air has to speed up to maintain a
constant flowrate, I really don't think it would swirl like you are talking
about.

Is there anyway you can put something in the flow path and watch it to see
what the air is doing?

Hope this helps,

Jim

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of John Pearson
Sent: Friday, November 21, 2003 09:34
To: PICLIST@MITVMA.MIT.EDU
Subject: [OT:] Fluid dynamics question


I have an air flow going from a 4" pipe into a 2" pipe. I am convinced, but
not positive that the air is swirling, like a whirl-pool, inside the 4" pipe
as it enters the 2" pipe. This is not desirable because it leads to false
static pressure readings. I can imagine that it looks very much like water
flowing out of a soda-pop bottle when turned upside down.

I have greatly reduced this effect with baffles, but, I would rather not use
baffles.

Is there transition shapes that induce whirl-pool action. I currently have a
trumpet like transition.

Oh, and I know nothing about fluid dynamics.

Thanks

John

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