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Old   January 30, 2011, 13:40
Default Boundary Condition in CFX
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Ricky Chen
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Hi all,

I'm trying to make a head pond model for a run-of-river project. The inlet is where the river is, outlet is where the penstock will be.

The inlet boundary condition is normal speed = 2m/s and I set the outlet condition as static pressure = Pres (where Pres is the water pressure due to gravity) (Please see the first picture for details). To simulate the gravity effect on the water, I added a sub-domain with general momentum source.

The result seems fine (please see the second diagram). But when I tried to change the inlet boundary to 0.2 m/s, the model starts to have problem with convergence. And the result doesn't make sense at all, especially the streamline from outlet (see third diagram). I'm thinking that there is something wrong with the outlet boundary condition, but I'm not sure where it is.

It would be great if anyone can provide some advices. Any suggestions would be welcome.

Thanks in advance!!

Ricky Chen
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Last edited by ssbear; January 30, 2011 at 14:13.
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Old   January 30, 2011, 17:50
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Glenn Horrocks
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Quote:
To simulate the gravity effect on the water, I added a sub-domain with general momentum source.
Argh! How many times do I have to tell you that this is a bad idea and not required!

Your pictures are weird. Why are velocity vectors going outwards at wall boundaries?

Your outlet boundary condition will have problems as the flow rate changes. That, coupled with the strange behaviour which your "gravity" source term will generate is going to mean bizarre things will happen.
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Old   January 30, 2011, 21:34
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Originally Posted by ghorrocks View Post
Argh! How many times do I have to tell you that this is a bad idea and not required!

Your pictures are weird. Why are velocity vectors going outwards at wall boundaries?

Your outlet boundary condition will have problems as the flow rate changes. That, coupled with the strange behaviour which your "gravity" source term will generate is going to mean bizarre things will happen.

ghorrocks,

Thanks for taking your time into my problem and sorry for my weird questions. I've removed the gravity source term and does that make more sense now?

Looking forward to your reply. Thanks!!
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Old   January 31, 2011, 07:15
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Unless you have variable density from something (or are using the bousinessq buoyancy model) then you will have no need for gravity and no need for static head. Then you can define your outlet boundary as a constant pressure boundary and things are much easier.

And yes, now you are not getting impossible flows across walls so keep that momentum source term off.
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Old   January 31, 2011, 14:19
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Originally Posted by ghorrocks View Post
Unless you have variable density from something (or are using the bousinessq buoyancy model) then you will have no need for gravity and no need for static head. Then you can define your outlet boundary as a constant pressure boundary and things are much easier.

And yes, now you are not getting impossible flows across walls so keep that momentum source term off.

Thanks very much!!
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Old   February 7, 2011, 02:10
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Originally Posted by ghorrocks View Post
Unless you have variable density from something (or are using the bousinessq buoyancy model) then you will have no need for gravity and no need for static head. Then you can define your outlet boundary as a constant pressure boundary and things are much easier.

And yes, now you are not getting impossible flows across walls so keep that momentum source term off.

ghorrocks,

This initial model works out well. My manager is happy with the results. Thanks again for your help.
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