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RAMAN6089 November 27, 2009 05:30

CFD problem, suggest
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:
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

ghorrocks November 27, 2009 05:53

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.

RAMAN6089 November 27, 2009 05:59


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.

ghorrocks November 27, 2009 06:09

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.

RAMAN6089 November 27, 2009 06:16


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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