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inlet flow rate

Posted Sep 11, 2012, 1:54 p.m. EDT Fluid & Heat, Computational Fluid Dynamics (CFD) Version 4.2a, Version 4.3 5 Replies

Abbie
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Hi,

Anyone knows how to implement "flow rate" as inlet boundary condition in FSI or fluid flow physics, where flow should come in as plug profile? It is noted that the inlet boundary is allowed to move horizontally (prescribed mesh displacement in z-axes=0) in 2D axisymmetric.

As far as I know, "flow rate" can be implemented as inlet boundary condition by selecting Inlet>"Laminar inflow>flow rate", however this laminar flow rate produces flows with parabolic profile at the inlet.

Any suggestion/ idea to implement inlet flow rate with plug profile, in a moving inlet boundary?


Thanks!



Abbie



5 Replies Last Post Sep 12, 2012, 3:57 a.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Sep 11, 2012, 3:09 p.m. EDT
Hi

there is something I do not really follow in your explanations (but I'm no CFD specialist ;), for me "plugflow" is related to highly turbulent flow, while parabolic profiles is related to gentle laminar flow. So if you have any plug flow conditions, you should be in turbulent mode too then you can define also a constat input velocity, no ?

--
Good luck
Ivar
Hi there is something I do not really follow in your explanations (but I'm no CFD specialist ;), for me "plugflow" is related to highly turbulent flow, while parabolic profiles is related to gentle laminar flow. So if you have any plug flow conditions, you should be in turbulent mode too then you can define also a constat input velocity, no ? -- Good luck Ivar
Abbie
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Posted: 1 decade ago Sep 11, 2012, 9:35 p.m. EDT
Hi Ivar,

The plug flow that I meant is a laminar plug (flat) profile, the fluid is in laminar flow but not yet fully developed into parabolic profile.
In COMSOL>Laminar Flow module, by selecting Inlet>"Velocity" produces laminar flat profile while Inlet>"Laminar inflow" produces laminar parabolic profile.
Hi Ivar, The plug flow that I meant is a laminar plug (flat) profile, the fluid is in laminar flow but not yet fully developed into parabolic profile. In COMSOL>Laminar Flow module, by selecting Inlet>"Velocity" produces laminar flat profile while Inlet>"Laminar inflow" produces laminar parabolic profile.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Sep 12, 2012, 3:02 a.m. EDT
Hi
You are right in laminar you have the two options, the only thing that I get often wrong, is that if you use the laminar inflow, and presure driven flow, COMSOL considers the pressure drop of the extra laminar inflow length, while I mostly forget that one, and want to consider the pressure at my true inlet.

But anyhow the parabolic shape comes very quickly (at least so far for my analysis), therefor I never consider a "plug flow" for laminar cases as this ony makes the solver take longer. But you might have a special fluid for which this flow case goes on for longer ...

But then I did not understand what is the problem to use a constant veocity input, in aminar case, it takes only longer to get the solver to converge, and often one must amp the velocity slowly to allow the solver to follow

--
Good luck
Ivar
Hi You are right in laminar you have the two options, the only thing that I get often wrong, is that if you use the laminar inflow, and presure driven flow, COMSOL considers the pressure drop of the extra laminar inflow length, while I mostly forget that one, and want to consider the pressure at my true inlet. But anyhow the parabolic shape comes very quickly (at least so far for my analysis), therefor I never consider a "plug flow" for laminar cases as this ony makes the solver take longer. But you might have a special fluid for which this flow case goes on for longer ... But then I did not understand what is the problem to use a constant veocity input, in aminar case, it takes only longer to get the solver to converge, and often one must amp the velocity slowly to allow the solver to follow -- Good luck Ivar
Abbie
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Posted: 1 decade ago Sep 12, 2012, 3:29 a.m. EDT
Hi

I noticed that flow profile is affecting the flow phenomena result that I am investigating, therefore i have to implement laminar flat profile instead of laminar parabolic profile. In my case, the total inflow volume is a constant while I have a moving (expanding) inlet boundary. If using velocity as inlet boundary condition, together with a moving inlet, the flow rate obtained would be exceed the desired constant volume.

I guess I can use an equation (velocity=flow rate/ inlet area) in the Inlet>"Velocity" to conserved the flow rate while having a laminar flat profile flow. However I am not sure how to determine the area of inlet as it is only set to move freely in horizontal axes (prescribed mesh displacment, dz=0) in 2D axisymmetry.
Hi I noticed that flow profile is affecting the flow phenomena result that I am investigating, therefore i have to implement laminar flat profile instead of laminar parabolic profile. In my case, the total inflow volume is a constant while I have a moving (expanding) inlet boundary. If using velocity as inlet boundary condition, together with a moving inlet, the flow rate obtained would be exceed the desired constant volume. I guess I can use an equation (velocity=flow rate/ inlet area) in the Inlet>"Velocity" to conserved the flow rate while having a laminar flat profile flow. However I am not sure how to determine the area of inlet as it is only set to move freely in horizontal axes (prescribed mesh displacment, dz=0) in 2D axisymmetry.
Amir Fadel
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Posted: 1 decade ago Sep 12, 2012, 3:57 a.m. EDT
Hi,

You'd better to use an expression involving the local variable s, which ranges from 0 to 1 following the direction of the boundary.

So if you have a 2D-axisymmetric geometry, you may write an expression like Uin = Umax *(s^(1/a) - 1/a*s)

where Umax is the maximum velocity of your flow, and a is a constant which determines how steep your plug flow is. For example a turbulent flow may be approximated by a=7, so I would say that it is a good starting point for a developing velocity profile.

Cheers
Hi, You'd better to use an expression involving the local variable s, which ranges from 0 to 1 following the direction of the boundary. So if you have a 2D-axisymmetric geometry, you may write an expression like Uin = Umax *(s^(1/a) - 1/a*s) where Umax is the maximum velocity of your flow, and a is a constant which determines how steep your plug flow is. For example a turbulent flow may be approximated by a=7, so I would say that it is a good starting point for a developing velocity profile. Cheers

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