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-   -   outlet temperature (https://www.cfd-online.com/Forums/openfoam/70092-outlet-temperature.html)

sarajags_89 November 14, 2009 00:29

outlet temperature
 
hi i am trying to do a flow through a cube

i have specified T at inlet
inlet
{
type fixedValue;
value uniform 280;
}

now for outlet
{ type ?
}

what type should i give for "type"?.
it would be great if you can explain me , I am new to cfd. :)

muellea November 14, 2009 06:17

The most natural choice would be zeroGradient, since:
  • in most cases you have no idea of the outlet temperature (you want to compute)
  • and the assumption that T won't change much close to the wall is often sufficient. (Same with velocity)
Try to compute it with these BCs and proof if your assumption was fine. If not, may be fixedGradient would be an alternative.

sarajags_89 November 14, 2009 15:48

hi
 
but doesnt zero gradient for outlet (say for Temperature) mean that temperature normal to outlet doesnt change??.(correct me if am wrong)

ternik November 15, 2009 04:42

Quote:

Originally Posted by sarajags_89 (Post 236273)
but doesnt zero gradient for outlet (say for Temperature) mean that temperature normal to outlet doesnt change??.(correct me if am wrong)

You are right - by using zero normal gradient (a.k.a. Neumann) boundary conditions one assumes "no change" (in normal direction) of particular flow variable. This is quite O.K. type of b.c., but one should assure and model long enough (in the flow direction) section, where such a flow conditions actually occur!

Hope this helps,
Primoz.

sarajags_89 November 15, 2009 05:13

hotroom tutorial
 
hi
i modified the hot room tutorial
1. i deleted mut,alpha,K to make it simple
2. i changed one face as the inlet and the other as outlet
3. b.c's
for inlet
velocity is specified ,pressure is zero gradient
for outlet
pressure is specified,velocity is zero gradient
4.inlet termperature fixedValue 280
outlet temperature zero gradient

now in the controldict file start time is zero and end time is 1000
when i type ./Allrun only '0' is written. which step did I go wrong

sarajags_89 November 15, 2009 05:48

found the mistake
 
i should have not deleted the files :)
can anyone tell me why is k,mut,alpha zeroGradient at inlet?.

pauls November 15, 2009 11:34

Quote:

Originally Posted by sarajags_89 (Post 236302)
i should have not deleted the files :)
can anyone tell me why is k,mut,alpha zeroGradient at inlet?.

There are two reasons for this:
  1. Because you should understand the basics of CFD before you use OpenFOAM.
  2. Because these are given as boundary conditions.


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