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raghu.tejaswi May 2, 2012 05:57

Couple outlet to inlet to achieve a fully developed flow.
Hello FLUENT users,

I am trying to simulate a simple 2D FULLY DEVELOPED flow (for both temp and velocity) between parallel plates in steady state. The working medium is water. To achieve a fully developed flow, the length of the domain becomes too long according to equations for turbulent entry lengths. Hence I would like to couple the values from outlet to inlet and solve both the flow and energy equations until convergence is reached for a short length of the domain.

For this I would like to write a UDF. I am very new to UDfs. My idea is to initialise velocity and temp in a udf. Use the value of velocity and temp from outlet and set it as inlet. This needs to continue until convergence is reached.

I understand periodic BC does exactly the above for flow equations. I am not sure if I can have a thermally developed flow too with periodic BC.

I just need to know how I can map the outlet surface to inlet in FLUENT using udf.

Blader May 3, 2012 14:21


I have the same problem. I need to link outlet and inlet. Can anyone help us ?

Thanks in advance :)

raghu.tejaswi May 4, 2012 02:06

Hi Blader,

I think I have found a solution. Use periodic BC in FLUENT. But pls make sure you meet all the necessary criteria before beginning the simulation. Run the simulation till residuals converge (normally one would monitor the outlet temp but in periodic BC case, the outlet is a shadow of the inlet..)

Pls try this for your case and do let me know if you are successful.


Blader May 4, 2012 02:55

I will try to do the simulation with periodic BC and I will comment the results.

Thank you very much.

raghu.tejaswi May 10, 2012 03:30

Hi Blader,

I hope periodic BC worked for you coz it worked for me pretty well. The flow profile is fully developed and the conditions to attain the fully developed thermal profile is achieved..

My problem is solved, atleast temporarily... :)


Blader May 10, 2012 04:38


In my case doesn't work very well.

I make a brief summary of my problem:

I want to simulate a closed loop pipe. I don't found a way to do it. I decided to do it with a open pipe linking the output to input. However, I just want to link the temperature of the output to input.

It really works with periodic BC properties?

thanks in advance !

raghu.tejaswi May 10, 2012 05:31

The concept of Periodic BC used by me is to achieve a fully developed flow. Since I use water as the working medium (whose Pr = 7.01), theoretically the entrance length is very high. I need a huge computational domain to achieve this. Instead I couple inlet to outlet to achieve. I need both velocity and temp to achieve fully developed condition. But here the outlet parameters are made as the inlet and this you see creates a loop...

Im really not sure about periodic BC in your case.

claudia.h July 18, 2013 08:49


i am looking for a way to couple the outlet with the inlet because i want to use the outlet parameters (velocity, temperature) for the inlet boundary for each iteration. Then i found your posting and hoped to get an input how to solve such a problem but unfortunately the link doesn´t work. It would be a great help for me if you could give me an hint how to solve such a problem.

Thanks in advance !

raghu.tejaswi July 18, 2013 09:04

Hi Claudia,

Did you try periodic BC?? It should work. This is how I got my results and they seemed to match quite good with the theoretical values and I was also able to give a Logical Explanation.

I was looking at the second condition explained here. Are you trying to achieve the same? The reason for choosing the periodic BC is explained in my previous Posts.


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