Proper boundary conditions

zavid · March 7, 2012, 11:53

Dear CFX users,
I faced a problem in setting boundary conditions of a pure free convection problem. A box has two openings: one is located at higher elevation and another is located lower. Just guess the cavity flow with two openings. My question is, What should be the boundary condition at those openings? I set opening (entrainment) boundary conditions with zero relative pressure and 25C static temperature (ambient). But my adviser said its wrong. He said that I should not set any temperature and pressure at those openings. It might be calculated from the internal flow. Anyway as I know, we set pressure and temperature at the opening as the ambient conditions. If anyone has better experience, please kindly share. Thanks.

evcelica · March 7, 2012, 18:54

If your adviser knows what is "wrong" he should be able to suggest what is "right".
You cannot just omit the temperature if there is inflow though, it sound to me like your adviser should rethink his statement. Do both of the openings have only inflow or only outflow?

ghorrocks · March 7, 2012, 19:39

You define the boundaries at places you know the conditions. If you know the pressure then define that. If you know the velocity/flow rate then define that. You cannot define a boundary at a location you know nothing - you must define either the pressure or velocity. Also you cannot define both pressure and velocity, that is over specifying the boundary.

Светлана · October 25, 2013, 01:05

Did you figure it out?

March 7, 2012, 11:53	Proper boundary conditions	#1
zavid New Member Join Date: Mar 2012 Posts: 1 Rep Power: 0	Dear CFX users, I faced a problem in setting boundary conditions of a pure free convection problem. A box has two openings: one is located at higher elevation and another is located lower. Just guess the cavity flow with two openings. My question is, What should be the boundary condition at those openings? I set opening (entrainment) boundary conditions with zero relative pressure and 25C static temperature (ambient). But my adviser said its wrong. He said that I should not set any temperature and pressure at those openings. It might be calculated from the internal flow. Anyway as I know, we set pressure and temperature at the opening as the ambient conditions. If anyone has better experience, please kindly share. Thanks.

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March 7, 2012, 18:54		#2
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,167 Rep Power: 23	If your adviser knows what is "wrong" he should be able to suggest what is "right". You cannot just omit the temperature if there is inflow though, it sound to me like your adviser should rethink his statement. Do both of the openings have only inflow or only outflow?

March 7, 2012, 19:39		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	You define the boundaries at places you know the conditions. If you know the pressure then define that. If you know the velocity/flow rate then define that. You cannot define a boundary at a location you know nothing - you must define either the pressure or velocity. Also you cannot define both pressure and velocity, that is over specifying the boundary.

October 25, 2013, 01:05		#4
Светлана Senior Member Svetlana Tkachenko Join Date: Oct 2013 Location: Australia, Sydney Posts: 407 Rep Power: 14	Did you figure it out?