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-   -   Open boundary condition (http://www.cfd-online.com/Forums/fluent/43039-open-boundary-condition.html)

suvash December 5, 2006 20:12

Open boundary condition
 
Hi, I am using fluent and now facing some problem with boundary conditions! My geometry is a triangular cavity. I would like to give one side hot and other two open boundary so that I can see the boundary layer and that boundary layer is not affected by any other flow.

Could you please help me how can I use open boundary condition.

Regards Suvash

Ralf Schmidt December 6, 2006 13:39

Re: Open boundary condition
 
Hi,

i didn't understood your problem right. I think, you may use pressure outlet BC at the two faces that are "open".

Ralf

Phil December 6, 2006 15:11

Re: Open boundary condition
 
You might also try to use the far field pressure condition if you boundaries are sufficiently far away in comparison to your flow features. Phil

suvash December 6, 2006 21:20

Re: Open boundary condition
 
Thanks Ralf, I got the following in the fluent documents. Since I am simulating the natural convection then how do I do the pressure. Should I use zero pressure or operating pressure (101325 pascal which is default). Please help me.

"Pressure inlet boundary conditions are used to define the fluid pressure at flow inlets, along with all other scalar properties of the flow. They are suitable for both incompressible and compressible flow calculations. Pressure inlet boundary conditions can be used when the inlet pressure is known but the flow rate and/or velocity is not known. This situation may arise in many practical situations, including buoyancy-driven flows. Pressure inlet boundary conditions can also be used to define a "free'' boundary in an external or unconfined flow."


Ralf Schmidt December 7, 2006 03:41

Re: Open boundary condition
 
Hi!

you should NOT use the operating pressure for pressure out/inlet! For your case, with no resistance for the fluid to flow out (like you have e.g. in a pipe-system) then there is zero pressure at the pressure out/inlet.

The pressure far field BC is only working with some special settings - I don't have good experience with it concerning convergence.

Ralf

suvash December 7, 2006 04:10

Re: Open boundary condition
 
Thanks a lot Ralf, I have some more questions!!! Suppose it is a square cavity. I want to set up the left wall HOT and inside cold. Bottom, top and right surfaces are open. Is it ok if I set up bottom -> pressure inlet, top-> pressure outlet and right -> pressure inlet? What will be the temperature of backflow. I want to set up it as the initial interior temperature (cold). Is the direction of the flow normal to the surface? If I incline the cavity on right side, will the setup be the same?

Sorry to ask you so many questions!! If possible response me..

many many thanks.. regards Suvash

Ralf Schmidt December 7, 2006 07:53

Re: Open boundary condition
 
Hi Suvash,

the difference between pressure inlet and outlet is not very large, actually, I don't know it.

The backflow temperatur is the temperatur of the fluid, that flows from outside of your domain through the face that is difined as pressure out/inlet. That might happend, when an eddy developes close to the boundary that "sucks" fluid from outside inside.

The direction of velocitys from massflow inlet or vel.inlet can be set. They are ether normal to bondary or you can set the components of the vel (x,y,z)

Hope, it helps

Ralf

suvash December 8, 2006 06:24

Re: Open boundary condition
 
Thank you very much. It will definitely help me. Suvash


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