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Old Feb 12th 2017, 05:02 PM   #1
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Swirl Flow in a conical diffuser

Hello everybody,
I have a simple question (maybe too dummy… forgive me).
I simulated a swirl flow in a conical diffuser.
Along the axes, obviously the velocity decreases and the pressure increase
But along a radius of divergent why does the pressure increase even if the velocity increase? The swirl velocity in fact is proportional to radius...
I really don't know why...
Attached Thumbnails
Swirl Flow in a conical diffuser-velocity.jpg   Swirl Flow in a conical diffuser-pressure.jpg  
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Old Feb 13th 2017, 06:38 AM   #2
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I guess that you are expecting to see the simple relationship between velocity and (static) pressure as indicated by <Bernoulli's Principle>.
However this only holds for incompressible, constant temperature, irrotational flows.

Also I note that your static pressure is going negative,
can I assume that this is static pressure relative to some other pressure (e.g. atmospheric)?
If not then there is something wrong with your basic setup of your CFD input files, (rubbish in, rubbish out).

Last edited by Woody; Feb 13th 2017 at 06:42 AM.
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Old Feb 13th 2017, 12:08 PM   #3
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Originally Posted by Woody View Post
I guess that you are expecting to see the simple relationship between velocity and (static) pressure as indicated by <Bernoulli's Principle>.
However this only holds for incompressible, constant temperature, irrotational flows.

Also I note that your static pressure is going negative,
can I assume that this is static pressure relative to some other pressure (e.g. atmospheric)?
If not then there is something wrong with your basic setup of your CFD input files, (rubbish in, rubbish out).
Naturaly, the static pressur is relative to atmospheric pressure... and today I discovered that it's correct because I found a pdf that show the effectiveness of diffuser with swirl flow. It reports the pressure distribution (attached image).
Therefore the simulation is not so bad as I thought.
But now, I'm not able to understand what you want to say...
The pressure is higher where the velocity increases...How is it possible?
What do I have to apply if not Bernoulli's theorem? ù
The flow is incomprissible...
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Swirl Flow in a conical diffuser-isobare.jpg  
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