# Boundary condition for UDS

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 October 30, 2007, 08:15 Boundary condition for UDS #1 Tong Wu Guest   Posts: n/a Hi: Does anyone know how to difine zero normal gradient boundary condition for UDS? The boundary condition for the fluid is pressure outlet. The buildin options for UDS boundary are specific value and specific flux which are different from my case. Any suggestions or examples are very appreciated. Thank you very much.

 February 7, 2016, 16:28 #2 New Member   Join Date: Feb 2016 Posts: 20 Rep Power: 2 would you please help me? I'm using UDS for simulation of mixing of two fluids (with concentration of 1 at one inlet and concentration of 0 at another inlet) but I dont know how can I specify uds in pressure outlet boundary condition?

 February 8, 2016, 10:02 #3 Senior Member   Join Date: Nov 2013 Posts: 919 Rep Power: 12 Open the boundary condition and click 'UDS'. Where else?

February 9, 2016, 17:43
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 Originally Posted by pakk Open the boundary condition and click 'UDS'. Where else?
i mean that which amount is appropriate for uds in pressure outlet ?

 February 10, 2016, 05:09 #5 Senior Member   Join Date: Nov 2013 Posts: 919 Rep Power: 12 How did you find out which pressure is appropriate for the pressure outlet? (What I mean to say: use the condition that your problem requires... If your problem requires a value of 0, use a value of zero. If your problem requires a flux of 0, use a flux of 0. We can not say that...)

February 11, 2016, 10:05
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 Originally Posted by pakk How did you find out which pressure is appropriate for the pressure outlet? (What I mean to say: use the condition that your problem requires... If your problem requires a value of 0, use a value of zero. If your problem requires a flux of 0, use a flux of 0. We can not say that...)
my problem is the article that i am modeling just shows pressure outlet for outlet boundary condition. and said nothing about amount of user scalar.
i sent you the pic of my question location
Attached Images
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February 11, 2016, 10:14
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Quote:
 Originally Posted by pmechz my problem is the article that i am modeling just shows pressure outlet for outlet boundary condition. and said nothing about amount of user scalar. i sent you the pic of my question location
Your pictures shows that you are looking at the correct location. But I can not guess which boundary condition was used in the article that you are using. There are two 'standard' conditions:

1. Assume there is no scalar at this boundary, as if the boundary is completely 'clean'. In this case, set value to zero.
2. Assume there is equilibrium at this boundary, so there is no net flow of scalar across the boundary. In this case, set the flux to zero. (change "specified value" into "specified flux".)

But it is just as physical to set the value to "1.3712e-3" or the flux to "4.293e+2" or whatever values. It is just representing a different problem.

February 11, 2016, 10:37
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 Originally Posted by pakk Your pictures shows that you are looking at the correct location. But I can not guess which boundary condition was used in the article that you are using. There are two 'standard' conditions: 1. Assume there is no scalar at this boundary, as if the boundary is completely 'clean'. In this case, set value to zero. 2. Assume there is equilibrium at this boundary, so there is no net flow of scalar across the boundary. In this case, set the flux to zero. (change "specified value" into "specified flux".) But it is just as physical to set the value to "1.3712e-3" or the flux to "4.293e+2" or whatever values. It is just representing a different problem.
sorry. may i have your private email to sent my article to you and help me more, if it's possible?
thanx

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