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Incompressible flow velocity: outlet faster than inlet

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Old   October 29, 2015, 13:03
Exclamation Incompressible flow velocity: outlet faster than inlet
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Xuekun Lu
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Hi,

I just tried a simple constant diameter pipe gas flow.

I set the gas as incompressible (constant density), velocity inlet, pressure out at 1 bar.

Based on continuity equation, I guess the mass-in should be equal to mass-out. But the simulation results show that, the outlet velocity is larger than inlet, as a consequence, the mass-out is larger than mass-in since the density is constant. Why this can occur? It breaks the continuity equation.

Also, the velocity is larger at the outlet than inlet. But I expect it should be no larger than the inlet since the outlet pressure is 1 bar.

Why there is no mass flow rate and mass flux in the fluid domain?

Can anyone help me with this ? I feel quite confused.

Thanks !
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Old   October 29, 2015, 13:06
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Quote:
Originally Posted by Xuekun View Post
Hi,

I just tried a simple constant diameter pipe gas flow.

I set the gas as incompressible (constant density), velocity inlet, pressure out at 1 bar.

Based on continuity equation, I guess the mass-in should be equal to mass-out. But the simulation results show that, the outlet velocity is larger than inlet, as a consequence, the mass-out is larger than mass-in since the density is constant. Why this can occur? It breaks the continuity equation.

Also, the velocity is larger at the outlet than inlet. But I expect it should be no larger than the inlet since the outlet pressure is 1 bar.

Why there is no mass flow rate and mass flux in the fluid domain?

Can anyone help me with this ? I feel quite confused.

Thanks !

without details about the formulation, discretization, etc, it is not possible to answer ....

just as first try, have you checked you really reached the convergence in the solution?
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Old   October 29, 2015, 13:22
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Originally Posted by FMDenaro View Post
without details about the formulation, discretization, etc, it is not possible to answer ....

just as first try, have you checked you really reached the convergence in the solution?
Thanks a lot for your responce . The model physics are : Laminar, constant density, segregated flow, steady. I just left the discretization as default. This is the most basic example from the tutorial.




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Old   October 29, 2015, 13:25
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Originally Posted by FMDenaro View Post
without details about the formulation, discretization, etc, it is not possible to answer ....

just as first try, have you checked you really reached the convergence in the solution?
I am trying to show you the result as the pictures. The residual is 1e-7 by the way.

Thanks !
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Old   October 29, 2015, 13:26
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Originally Posted by Xuekun View Post
I am trying to show you the result as the pictures. The residual is 1e-7 by the way.

Thanks !





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Old   October 29, 2015, 14:38
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did you set uniform velocity at inlet?

how about the mass difference between inlet and outlet?
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Old   October 29, 2015, 17:35
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Originally Posted by FMDenaro View Post
did you set uniform velocity at inlet?

how about the mass difference between inlet and outlet?

Yes, the inlet velocity is constant for the whole boundary surface.

There is no mass value from the results, as you can see it is totally grey.
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Old   October 29, 2015, 18:20
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how did you estimate the difference between flow rate at inlet and outlet?
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Old   October 29, 2015, 18:43
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the fact that the centerline velocity at outlet is greater than at inlet is correct, the acceleration is due to the viscous effect that produce the boundary layer. That does not mean you have different flow rate at the outlet
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Old   October 29, 2015, 18:46
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how did you estimate the difference between flow rate at inlet and outlet?
Hi, I didn't estimate that. The mass flow rate is what I want to get out from the results. At the inlet, I just gave it a constant velocity, so I guess the mass flow rate can be calculated by the solver itself at the boundary using the velocity, the diameter and density. I wonder why there is no flux shown in the result.
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Old   October 29, 2015, 18:48
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Originally Posted by FMDenaro View Post
the fact that the centerline velocity at outlet is greater than at inlet is correct, the acceleration is due to the viscous effect that produce the boundary layer. That does not mean you have different flow rate at the outlet
Thanks a lot for your explanation. Based on what you said, does it mean the average velocity at inlet and outlet is the same despite the difference at centerline?
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Old   October 29, 2015, 23:19
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Quote:
Originally Posted by Xuekun View Post
Thanks a lot for your explanation. Based on what you said, does it mean the average velocity at inlet and outlet is the same despite the difference at centerline?
The average velocity must be the same based on conservation of mass. Since the density is constant and area is the same at the inlet and outlet. The average velocity at the inlet and outlet must be equal.

You don't even need the code to calculate the mass-flow rate for you, since you already know what it is (you specified it actually when you specified the inlet velocity).
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Old   October 30, 2015, 03:38
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Thanks a lot for your explanation. Based on what you said, does it mean the average velocity at inlet and outlet is the same despite the difference at centerline?

it is just a simple math issue, you can have the same integral value with infinite different functions ....
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Old   October 30, 2015, 14:52
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Originally Posted by Xuekun View Post
Thanks a lot for your explanation. Based on what you said, does it mean the average velocity at inlet and outlet is the same despite the difference at centerline?
Simple example: If you have a garden hose and you cover the tip with your thumb the velocity gets high.

The product rho*A*V would be the same throughout the hose.
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