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How to separate flow in two outputs as 70:30?

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Old   December 27, 2013, 20:48
Question How to separate flow in two outputs as 70:30?
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Hi. I have a 2d tube. In this tube with diameter D0 i have as inlet a constant velocity U. I call this tube Mother tube.
Then this tube is separated in two smaller tubes (daughter tubes) of different sizes (say D1=D0/2 and D2=D0).
Each output is= "Type:Pressure-outlet" with zero Gauge Pressure and the Backflow direction is set to default "Normal to Boundary".

How can i have a constant flow ratio 70:30 in the outlets of the daughter tubes, of D2(70) and D1(30)??

I try to do it like that in the first daughter tube:
I clicked the "Target Mass Flow Rate" and then in the "Target Mass Flow" box i input the number: 0.3*(A*U*m)
where:
A= area of mother tube
U=Velocity of the fluid in the mother tube
M=mass (Density) of the fluid in mother tube.
And for the other daughter tube i did:0.7*(A*U*m)

Is this the correct way to tell Fluent that i want a flow ration of 70:30 at the two outputs?
Thank you
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Old   December 28, 2013, 14:30
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François Grégoire
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Hi Spiros,

What are the results? Did you obtain the targeted mass flow rates? What pressures Fluent calculated at the outlets? I think your approach is ok, and depending on the shape of your geometry, Fluent will adjust the pressure at the outlets in order to obtain the targeted flow rates.
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Old   December 28, 2013, 15:10
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I think by using velocity outlet BC, you can separate them easily. just need to define two Velocity outlet BC with 0.7 and 0.3 outflow rate
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Old   December 28, 2013, 15:34
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Quote:
Originally Posted by macfly View Post
Hi Spiros,

What are the results? Did you obtain the targeted mass flow rates? What pressures Fluent calculated at the outlets? I think your approach is ok, and depending on the shape of your geometry, Fluent will adjust the pressure at the outlets in order to obtain the targeted flow rates.
The results at the outflows where as expected but i get some disturbances (very high velocities) at some points in the beginning of the bifurcation on the site of the smaller tube but i think that's normal. But i was not sure if i my approach of the problem was wright or wrong. Thank you for the reply!
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Old   December 28, 2013, 15:36
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Quote:
Originally Posted by mrji8011 View Post
I think by using velocity outlet BC, you can separate them easily. just need to define two Velocity outlet BC with 0.7 and 0.3 outflow rate
Thank you for the reply! I'm running the simulation now and i will see whether that worked for me or not.
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Old   December 29, 2013, 05:38
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If you are sure that the flow is fully developed and you are working with incompressible fluid you may use outflow BC with mass weightening (without any pressure level specification at the Boundaries)
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