# outflow boundary

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 August 10, 2004, 23:21 outflow boundary #1 Zhihua Guest   Posts: n/a Hi, What does the "outflow" boundary mean in Fluent? Does it mean the flow is fully developed, dU/dx=0? In transient simulations, is it the same as: du/dt+U(du/dx)=0? Can anybody help? Thank you very much! Zhihua

 August 11, 2004, 14:02 Re: outflow boundary #2 Amir Guest   Posts: n/a Hi yes ,this is true. when you havnt any information about outlet flow (its pressure,...)you can use this BC. but your flow shuld be fully devloped. if your flow isnt fully devloped(or when you have recirculation) and you used outflow BC results dont converge. hope,it help you . best regards, Amir

 August 11, 2004, 17:50 Re: outflow boundary #3 Zhihua Guest   Posts: n/a Amir, Is it the same as "convective boundary condition"? I am simulating flow over a blunt body. If I defined the computational domain long enough behind the body, can I use this BC? Thank you very much! Zhihua

 August 12, 2004, 03:43 Re: outflow boundary #4 Amir Guest   Posts: n/a Hi outflow BC is used for imcompressible flow. pressure far-feild BC is usually used for external flow around any body(infinity condition. (like airfoil). pressureoutlet BC is used when you know outlet statice pressure, and when secondary flow appears in outlet. if it isnt enough please more explaine about your problem. hope help you, Amir

 August 12, 2004, 12:35 Re: outflow boundary #5 Nish Guest   Posts: n/a Amir and Zhihua, I have a similar problem. You guys might be able to help. I have a geometry which is essentially a box with inlet and outlet. The BC at the inlet is specified as the velocity inlet. However, at the outlet, I want to specify a pressure outlet. The gauge pressure inside the box is positive (i.e. 50Pa ) to the atmosphere. My question is how and what do I specify as the outlet boundary condition. Do I specify the operating pressure as 101KPa and then the Gauge pressure on the outlet pressure boundary as 50Pa? Your advice will be much appreciated as I am unsure of the Fluent terminolgy here. Regards, Nish

 August 12, 2004, 14:20 Re: outflow boundary #6 Zhihua Guest   Posts: n/a Nish, I think your way of doing it (specify operating pressure = 101 KPa, and gauge pressure 50 Pa at outlet) is correct. It is stated in Fluent manual that the gauge pressure is a value relative to the operating pressure. Good luck! Zhihua

 August 12, 2004, 15:52 To Amir Re: outflow boundary #7 Zhihua Guest   Posts: n/a Amir, In Fluent, the pressure far field BC is used for compressible flow. Now I am comparing my results from "outflow B.C." and "pressure outlet B.C.". They seem to give me similar global drag coefficient. I am gonna do more comparison on local velocity and drag. Thank you very much for your advice. Zhihua

 August 16, 2004, 15:56 Re: To Amir Re: outflow boundary #8 Zhihua Guest   Posts: n/a Results: For flow across a circular cylinder, the outlet boundary is 30D away from the cylinder center. My simulated drag and heat transfer rate are exactly the same for both boundary conditions.

 August 17, 2004, 10:25 Re: outflow boundary #9 Nish Guest   Posts: n/a Thanks Zhihua, It seems to work fine. Regards, Nish

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