# outlet temperature

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 November 14, 2009, 00:29 outlet temperature #1 Member   sarangarajan Join Date: Sep 2009 Posts: 31 Rep Power: 7 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.

 November 14, 2009, 06:17 #2 New Member   anonymous Join Date: Nov 2009 Location: Switzerland Posts: 8 Rep Power: 7 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.

 November 14, 2009, 15:48 hi #3 Member   sarangarajan Join Date: Sep 2009 Posts: 31 Rep Power: 7 but doesnt zero gradient for outlet (say for Temperature) mean that temperature normal to outlet doesnt change??.(correct me if am wrong)

November 15, 2009, 04:42
#4
Member

Primoz Ternik
Join Date: Apr 2009
Location: Maribor, Slovenia
Posts: 65
Rep Power: 8
Quote:
 Originally Posted by sarajags_89 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.

 November 15, 2009, 05:13 hotroom tutorial #5 Member   sarangarajan Join Date: Sep 2009 Posts: 31 Rep Power: 7 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

 November 15, 2009, 05:48 found the mistake #6 Member   sarangarajan Join Date: Sep 2009 Posts: 31 Rep Power: 7 i should have not deleted the files can anyone tell me why is k,mut,alpha zeroGradient at inlet?.

November 15, 2009, 11:34
#7
New Member

Paul Schiefer
Join Date: Sep 2009
Posts: 25
Rep Power: 7
Quote:
 Originally Posted by sarajags_89 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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