# CFD problem, suggest

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 November 27, 2009, 05:30 CFD problem, suggest #1 New Member   Ramandeep Singh Join Date: Nov 2009 Posts: 20 Rep Power: 10 i'm an HVAC Designer and a new user for CFD also. i need ur experience while solving a fluid problem relating to air ducts. i have designed one ducting Plenum(air box), i'm checking the feasibilty for the same. conditions are: Air is supplied to the ducting system by fan at 9.14 m/s (it's a big) to fulfill the capacity of 38000 Cfm(or 64600 m3/hr) each(there are two fans) two inlets in plenum and two outlets from plenum my conditions for Pre are: Inlet: normal speed = 9.14 m/s at outlet i'm making it as opening and static pressure= 250 Pa or 25mmWc i'm doubtful how this static pressure is gonna effect coz there is sepration in the ducting afterwards. and if i give velocity at opening than it will forcefully withdraws the air through it. suggest

 November 27, 2009, 05:53 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,190 Rep Power: 109 Read the documentation about tips for obtaining convergence. Putting an outlet boundary through a recirculation zone is a bad idea. You should move it downstream to somewhere the flow is simpler.

 November 27, 2009, 05:59 hi #3 New Member   Ramandeep Singh Join Date: Nov 2009 Posts: 20 Rep Power: 10 gharock, i know about the wall condition which got created when we give the outlet condition. i opted not to go further with downstream as the velocity is high, in place i took it as an opening. suggest me about calculating the static pressure at the opening.

 November 27, 2009, 06:09 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,190 Rep Power: 109 Read the documentation - if you put the boundary through a recirculation the static pressure (or any other variable for that matter) is not constant so defining a constant pressure boundary will lead to an error. The speed of the flow is not important for setting a boundary location. If the flow is simpler with no recirculations then move it downstream as the error will be much reduced by a constant pressure boundary.

 November 27, 2009, 06:16 gharocks #5 New Member   Ramandeep Singh Join Date: Nov 2009 Posts: 20 Rep Power: 10 gharocks, it's not that, i'm not getting results. i have checked the flow is smooth at oulet, i have also given an opening condiyion for the same. can u find me the particular example u have gone trhu, i'm finding it really difficult to get the perfect answer. inlet and static pressure related

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