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-   -   Boundary condition (http://www.cfd-online.com/Forums/fluent/41784-boundary-condition.html)

tomik July 19, 2006 13:25

Boundary condition
 
Dear All, I have a question? Which boundary conditon ( wall, pressure inlet, pressure outlet, mass flow rate etc.) I must to set on boundary in order to obtain atmosferic pressure on the boundary domain??? Thanks for help, Best regards, Tomik

cip July 19, 2006 14:58

Re: Boundary condition
 
outflow

Jason July 19, 2006 15:44

Re: Boundary condition
 
I have to disagree there. Outflow assigns zero diffusion flux to all the flow variables at the BC, which is similar to saying the flow is fully developed. It's not universally applicable (which many people seem to forget here) and does not work with compressible flow or pressure inlets (See the fluent manual... chapter 7.10 on Outflow BCs).

If you're trying to get a constant total pressure, then use a pressure inlet. If you're trying to get a constant static pressure, use a pressure outlet. If there is forward motion of the flow, then you have to let Fluent know that, so a Pressure Inlet, Velocity Inlet or Pressure-Far-Field BC would work (VI ONLY WORKS FOR INCOMPRESSIBLE FLOW AND PFF ONLY WORKS FOR COMPRESSIBLE FLOW!!!! but PI works for both). In any of these cases, the BC should be far enough from the geometry so there's little to no iteraction between the effected flow and the BC. Otherwise you risk the BC modifying your flow field and changing the area of interest. Also, with the Pressure inlet and Pressure outlet you will probably get "reversed flow in XXX faces". This means that flow is either leaving through the pressure inlet, or entering through the pressure outlet. It's not a problem, it's just a warning becuase typically an inlet BC is used at an actual inlet where you want flow going in one direction and same with the outlet. But you're talking about using it as an atmospheric bubble, which these BCs will handle just fine. Just make sure to set the Backflow properties if using a pressure outlet.

Check out chapter 7 of the Users Guide for more information, a list of all the BCs, and information on when the BCs are not applicable.

Hope this helps, and good luck, Jason

Albert Franco July 20, 2006 03:25

Re: Boundary condition
 
I agree, I would use presure outlet besides as Jason said outflow does not work with presure inlets.

kp July 20, 2006 07:03

Re: Boundary condition
 
Dear Jason and other guys, I am modelling a bubble column. I want my gas to leave the top of the column where as liquid to stay inside the reactor. So what BC would you suggest in this case. Thanks. KP

Thakorn Paiboonrotrung July 23, 2006 22:58

Re: Boundary condition(Fan)
 
Dear All, I have a question? Which boundary conditon ( wall, pressure inlet, pressure outlet, mass flow rate ,exhaust fan velocity inlet,etc.) I must to set on boundary in order to obtain atmosferic pressure on the boundary domain??? Thanks for help,

Best regards, Thakorn



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