CFD Online Discussion Forums

CFD Online Discussion Forums (
-   CFX (
-   -   Problems in pressure when using mass flow rate outlet (

rafiktharwat May 30, 2011 18:12

Problems in pressure when using mass flow rate outlet
hi all,

i faced a problem in the pressure along car park simulation ,
when i use the exhust fans as outlet with mass flow rate

the pressure gradient is increasing by getting closer to the outlet
which is unacceptable solution

i used the supply fans as opening with pressure of 0 [pa]

i think i miss some physical inputs to have the correct solution.

what is the correct setup of the outlet and inlet to have more realistic pressure solution

note: fluid is Air at 25
ref pressure at 1 [atm]

thnx for your time and help

ghorrocks May 30, 2011 18:49

You use a boundary condition setup which matches what you are trying to model. The documentation has some comments about boundary condition choice which is worth reading as well.

And what do you mean by "the pressure gradient is increasing by getting closer to the outlet which is unacceptable"?

rafiktharwat May 30, 2011 19:18

problems with the pressure
i mean that the pressure before the outlet is greater than the pressure after the inlet

which i think its not realistc because the flow moves from the high pressure to the lower pressure not vice versa

my condition is to maintain only the outlet (Mass Flow Rate value)


rafiktharwat June 1, 2011 05:03

hi all

i want to ask if i use for outlet with mass flow rate

if i chose Shift Pressure option

would it help me to have a -ve pressure befor the outlet

and what about pressure profile blend value

thnx for help

ghorrocks June 1, 2011 07:04

Can you post an image of what you are seeing, which you think is not realistic?

kambireddy June 1, 2011 07:13

Usually, i am using mass flow rate as outlet and TP = 0 as inlet for fans.

Does your fan is at open atmosphere? I means to say no pressure drop at in let and outlet. if it is so, you can use inlet Bcs as TP=0 and outlet as SP=0, then frm these you will get mass flow rate.

still it is not your cup of tea, if possible try to explain your problems in more details?

Glenn Horrocks feedback is more helpful...

rafiktharwat June 1, 2011 07:47

sorry for lack of details
1 Attachment(s)
sorry for lack of details
and thanx for trying to help

i attached the car park i am working with

the openings are [ 0 pa]

the outlets are 11 [kg/s] each

the domain is air with pres ref of [0 pa]

in my opinion the pressure before the outlet has to be lower than the pressure after the inlet (opening)

acording to flow direction

sorry for mis understanding of me

and thank you for your help and time

kambireddy June 1, 2011 08:25

Check with Total pressure at both sides.
Apply Bernoulli equation on both sides and check it.

I think what every measuring is static pressure. it was same as flow through diffuser or diverging channel.

ghorrocks June 1, 2011 18:26

Also your reference pressure should be atmospheric pressure, not zero pressure. Unless this is a car park in space that is.

rafiktharwat June 2, 2011 05:04

i am sorry

the domain Reference pressure is [1 atm]

air @ 25 [C]

the opening boundry pressure is [0 pa]


A_Prakash January 2, 2014 10:14

I would have suggested making the Inlets as massFlow specified and outlet as Average Static Pressure of 0 or -1.
Probably your problem is solved by now. Did you manage to figure out what was wrong?

hmasenger January 2, 2014 18:21

I think you need to define a pressure inlet rather than opening.using mass flow BC and an opening sometimes leads to diverged results

All times are GMT -4. The time now is 02:41.