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no-slip condition no obeyed?

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Old   August 23, 2013, 04:03
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The result for laminar mass flow inlet , pressure outlet is attached , Re=300
I am doing pressure inlet , mass flow outlet for the same case .
To some length after the inlet , the profile has the same that is shown in the image.
Still the question is there , the profile for the velocity shown here can not be validated with the experiment .

Please let me know your idea ,

Thanks
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File Type: jpg laminar.jpg (63.7 KB, 12 views)
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Old   August 23, 2013, 06:41
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Glenn, I hope you dont mind if I participate in this lecture about fluid dynamics.

mejahan, you havent shown any experiment of the development of a laminar pipe flow. The picture in post number 38 is taken from the transition to turbulence of a pipe flow, which is something completely different.
As Glenn already said, the other schematic images (including that in post 38) are plain wrong.
The last simulation result you show is correct for the development of a laminar pipe flow.
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Old   August 23, 2013, 12:25
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Dear Glenn and Alex,

I really appreciate your comments , I learnt a lot from your comments.

But some questions;

- If the last post picture about laminar flow velocity profile is correct , then what about Pressure inlet BC , it is not showing the same profile that we see on the mass flow inlet BC with a little velocity increase at the corner after BL.
- What is the difference between the case that we have laminar and turbulent flow in matter of flow profile that we are discussing.
- I would thank you if you refer me to any references that shows the profile that you are discussing before fully developed in a pipe , laminar and turbulent . I could not find any rather the image on the post 38.


Thank you in advance.

Mehdi
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Old   August 25, 2013, 03:54
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Dear Glenn and Alex,

Your comments are really valuable for me , I rearnt alot from your comments.
I have found some articles about the flow profile and are studying them, you were wright about the profile .

But still some questions,

- why I can not see the same profile when using the pressure inlet, I know that this BC uses zero gradient for momentum eq. But why it is not showing the same profile befor fully developed .
- is there any specific BC that is more suitable for using low Re turbulence modeling like SST , mass flow inlet or pressure inlet? Flow is imcompressible .

Thank you in advance

Mehdi
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Old   August 25, 2013, 19:39
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No problem Alex, glad for the help. Yes, this thread has become a bit of a fluid mechanics lesson.

The choice of boundary condition is driven by what most closely matches your experimental conditions. If you cannot match your experimental conditions (or do not know what they are) then move the boundary further upstream (for inlets) or downstream (for outlets) until either you do know the boundary or it is far enough away that it makes no difference.

This trend of the developing flow having flow peaks away from the centreline is universal, so the type of upstream BC does not matter. The only way the centreline can accelerate is by shear from the adjacent fluid pushing it faster, and the only way that can happen is if it is going faster than the centreline (at least during the flow development).
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Old   August 26, 2013, 05:26
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Dear Glenn and Alex,

I am really grateful for your helps and valuable comments,
I learnt a lot from your answers.

Mehdi
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