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-   -   UDS help? (http://www.cfd-online.com/Forums/fluent-udf/105245-uds-help.html)

eh65 July 26, 2012 01:40

UDS help?
 
1 Attachment(s)
hi
I am beginner in fluent. I have a problem that have a convective B.C in outlet as follows:
http://www.cfd-online.com/Forums/dat...AAAElFTkSuQmCCAttachment 14765


that http://www.cfd-online.com/Forums/dat...AAAElFTkSuQmCC is a independent variable p,v and T.
can anyone help me for this case??
thanks ..

eh65 July 26, 2012 01:42

1 Attachment(s)
sorry . the equation is :

Attachment 14766
http://www.cfd-online.com/Forums/dat...AAAElFTkSuQmCC

Amir July 26, 2012 02:42

Dear friend,

you can consider phi as a scalar and solve a modified UDS equation for that easily and set a Dirichlet BC instead, but I think you meant that phi is a dependent variable.

Bests,

eh65 July 26, 2012 04:19

thanks amir
but phi is a independent variable. but i dont know that what to do and begin from which step?
this is the problem of channel that air passes through it.

Amir July 26, 2012 04:22

Quote:

Originally Posted by eh65 (Post 373638)
thanks amir
but phi is a independent variable. but i dont know that what to do and begin from which step?
this is the problem of channel that air passes through it.

What's the governing equations and what is phi variable?!

eh65 July 26, 2012 04:41

(d/dt)(phi)+ U(d/dx)(phi)= 0
phi is pressure, tempreture and avreaged x-velocity (U)

Amir July 26, 2012 04:51

Quote:

Originally Posted by eh65 (Post 373647)
(d/dt)(phi)+ U(d/dx)(phi)= 0
phi is pressure, tempreture and avreaged x-velocity (U)

Ah, sorry I think this is sth different at first. Ok, here note that for some BCs you cannot set both velocity and pressure. But you have 2 choices:
1) using a separate UDS equation but it may face some difficulties in its own BCs
2) write a UDF like this:
phi_new=phi_old-U(d/dx phi)*dt
which is a approximation.

Bests,

eh65 July 26, 2012 05:04

thanks
in situation 2 i have to use flux uds?

Amir July 26, 2012 05:17

Quote:

Originally Posted by eh65 (Post 373652)
thanks
in situation 2 i have to use flux uds?

No! in this situation you wouldn't need any UDS. you're setting phi directly. (note that you can access gradient operator in UDF)

eh65 July 26, 2012 11:57

thanks amir


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