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Old   December 9, 2012, 13:01
Default Setting velocity at outlet
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Saba Saeb
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Hi all,

I am supposed to simulate a two-phase flow where a fluid enters from the top of a domain initially filled by air and exits the domain from an outlet located at the bottom. I am interested to change the fluid velocity where it is going out of the domain. According to available boundary conditions in fluent, it's not possible to set the velocity at the outlet.
I had a look to previous posts and topics in the forum, but didn't find anything related or useful. This question has been asked before, but no one has replied to any of them yet.
Also, according to the tutorial,
"In special instances, a velocity inlet may be used in FLUENT to define the flow velocity at flow exits. (The scalar inputs are not used in such cases.) In such cases you must ensure that overall continuity is maintained in the domain."
I tried the suggested approach, but didn't work for me.
Any help would be much appreciated.

Cheers,
Saba
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Old   December 9, 2012, 18:11
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Quote:
Originally Posted by saba_saeb View Post
Hi all,

I am supposed to simulate a two-phase flow where a fluid enters from the top of a domain initially filled by air and exits the domain from an outlet located at the bottom. I am interested to change the fluid velocity where it is going out of the domain. According to available boundary conditions in fluent, it's not possible to set the velocity at the outlet.
I had a look to previous posts and topics in the forum, but didn't find anything related or useful. This question has been asked before, but no one has replied to any of them yet.
Also, according to the tutorial,
"In special instances, a velocity inlet may be used in FLUENT to define the flow velocity at flow exits. (The scalar inputs are not used in such cases.) In such cases you must ensure that overall continuity is maintained in the domain."
I tried the suggested approach, but didn't work for me.
Any help would be much appreciated.

Cheers,
Saba
You may change velocity magnitude at outlet by varying exit area.
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Old   December 9, 2012, 18:20
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Quote:
Originally Posted by syavash View Post
You may change velocity magnitude at outlet by varying exit area.
Hi Syavash and tnx for ur reply.
I might face 2 problems doing that.
1. In my case, a polymer is flowing down the inlet which has the diameter of 4 mm. By making outlet smaller, say 1 mm, which is surrounded by wall ( can I replace wall by anything else? ), a great fraction of polymer would hit the wall and remain there ( since the polymer is quite viscose) and doesn't exit the domain at all which makes the simulation far away from the real case!
2. I want to extend the simulation later such a way that I would be able to change the velocity of the fluid at the outlet as time goes by.

Cheers,
Saba
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Old   December 9, 2012, 19:01
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Quote:
Originally Posted by saba_saeb View Post
Hi Syavash and tnx for ur reply.
I might face 2 problems doing that.
1. In my case, a polymer is flowing down the inlet which has the diameter of 4 mm. By making outlet smaller, say 1 mm, which is surrounded by wall ( can I replace wall by anything else? ), a great fraction of polymer would hit the wall and remain there ( since the polymer is quite viscose) and doesn't exit the domain at all which makes the simulation far away from the real case!
2. I want to extend the simulation later such a way that I would be able to change the velocity of the fluid at the outlet as time goes by.

Cheers,
Saba
Hmm..., I see. What kind of B.C. is implemented at outlet?!
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Old   December 9, 2012, 19:10
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Originally Posted by syavash View Post
Hmm..., I see. What kind of B.C. is implemented at outlet?!
I set pressure-outlet just to see whether it works or not, and it did! But it doesn't satisfy what I'm looking for! I need to set the velocity at the outlet and pressure-outlet doesn't do that, clearly!
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Old   December 9, 2012, 19:16
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Originally Posted by saba_saeb View Post
I set pressure-outlet just to see whether it works or not, and it did! But it doesn't satisfy what I'm looking for! I need to set the velocity at the outlet and pressure-outlet doesn't do that, clearly!
Have you tried checking option "target mass flow rate" ?!
I should be reminding have you considered continuity satisfaction?!
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Old   December 10, 2012, 02:25
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Originally Posted by syavash View Post
Have you tried checking option "target mass flow rate" ?!
I should be reminding have you considered continuity satisfaction?!
Thank you again for ur reply.
I tried to implement outlet-vent where "target mass flow" is available, but since I'm using VOF solver, it is not available! When I make VOF az the solver, this option disappears and it's not possible to set it as boundary condition.

How should I consider continuity?

Cheers,
Saba
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Old   December 10, 2012, 06:18
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Hi Saba,

Why don't you just set (in Fluent) a "velocity inlet" boundary condition to your outlet and a "pressure outlet" bc to your inlet? Then type a negative velocity at your "velocity inlet" to ensure that the fluid flows out of the domain.
You will get warnings of reversed flow at your pressure outlet, but that doesn't matter, because you actually want the fluid to enter there.
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Old   December 10, 2012, 09:19
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Quote:
Originally Posted by RodriguezFatz View Post
Hi Saba,

Why don't you just set (in Fluent) a "velocity inlet" boundary condition to your outlet and a "pressure outlet" bc to your inlet? Then type a negative velocity at your "velocity inlet" to ensure that the fluid flows out of the domain.
You will get warnings of reversed flow at your pressure outlet, but that doesn't matter, because you actually want the fluid to enter there.
Hi Rodriguez,

It doesn't solve my problem since I need to specify the velocity of the fluid at inlet as well as outlet. However, as you suggested, I set velocity-inlet boundary condition to both inlet and outlet while putting negative sign for outlet, but the results are just strange! It seems the fluid tends to move back to the domain instead of exiting it at the outlet!

Bests,
Saba
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Old   December 10, 2012, 09:27
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Quote:
Originally Posted by saba_saeb View Post
Hi Rodriguez,

It doesn't solve my problem since I need to specify the velocity of the fluid at inlet as well as outlet. However, as you suggested, I set velocity-inlet boundary condition to both inlet and outlet while putting negative sign for outlet, but the results are just strange! It seems the fluid tends to move back to the domain instead of exiting it at the outlet!

Bests,
Saba
Dear Saba,

Try to adjust the outlet gauge pressure (pressure outlet B.C.) in order to get desired flow rate, and feed response back.
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Old   December 10, 2012, 09:32
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Quote:
Originally Posted by syavash View Post
Dear Saba,

Try to adjust the outlet gauge pressure (pressure outlet B.C.) in order to get desired flow rate, and feed response back.
Thanks Syavash,

It seems the only way to set velocity at outlet is to play with pressure instead of setting the exact desired velocity, as you just mentioned. I will have a try using pressure-outlet and will let you know about the results.

Bests,
Saba
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Old   December 10, 2012, 09:33
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I don't get you. Do you want to specify the velocity at both the inlet and the outlet?
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Old   December 10, 2012, 09:34
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Quote:
Originally Posted by RodriguezFatz View Post
I don't get you. Do you want to specify the velocity at both the inlet and the outlet?
Yeah, exactly!
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Old   December 10, 2012, 09:36
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So you use compressible air or what?
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Old   December 10, 2012, 09:40
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You can't specify the velocity at both the inlet and the outlet. One has to be a pressure BC (inlet or outlet). This is because you need to conserve mass. So unless there is some degree of compression this isn't possible.

What exactly is the nature of the problem? Have you got a diagram? Is there gravity?
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Old   December 10, 2012, 09:55
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Originally Posted by jamesg3373 View Post
You can't specify the velocity at both the inlet and the outlet. One has to be a pressure BC (inlet or outlet). This is because you need to conserve mass. So unless there is some degree of compression this isn't possible.

What exactly is the nature of the problem? Have you got a diagram? Is there gravity?
There wouldn't be any problem if I set velocity at both boundaries I guess, since both air and water are exiting the domain at outlet. This lets the fluid diameter to be changed in order to satisfy continuity.
I did quite similar simulation using openFoam and setting velocity at both inlet and outlet didn't make any problem at all!

Bests,
Saba
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Old   December 10, 2012, 09:57
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What do you mean by "fluid diameter"?
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Old   December 10, 2012, 10:04
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Mass flow rate is kg/s.

What this means in terms of dimension is that the amount of fluid passing though your BC is determined by the velocity of the fluid, the density of the fluid and the area of the BC.

Your total mass flow rate into a system should be the same as the total mass flow rate out of the system. The only way I can see your velocity at each BC being different is if the area of the BCs are different or if the density of the fluid has changed.

If you want more help on this model you're going to have to provide more information in the way of diagrams and explanation. At the minute it's very difficult to imagine the problem you're trying to solve.
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Old   December 10, 2012, 10:07
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Quote:
Originally Posted by RodriguezFatz View Post
What do you mean by "fluid diameter"?
Here is a picture which might help



By fluid diameter, I mean the polymer thickness or the diameter by which fluid is exiting the domain!
According to the picture, fluid is coming down the inlet which has a diameter of 4 mm. As polymer gets closer to exit, it gets narrower. I have to mention that outlet diameter has been set to 2 mm. But, it is clear that "fluid diameter" is much less than 2 mm at outlet and air is exiting the domain through outlet as well as a consequence. So continuity can be satisfied since the fluid is wholly free to adjust its diameter or "thickness"!
I hope the picture makes it clearer!

Bests,
Saba
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Old   December 10, 2012, 10:10
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So what you're saying is that you want to model the outlet as elastic? So the diameter changes according to the pressure at the outlet?

EDIT: I've just reread your last post and in hindsight I don't think that's what you meant, but I'm still struggling to understand the problem. Is there a real life situation you can liken it to?
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