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-   -   Why is the SimpleFoam Laminar solver Valid for Re3000 (https://www.cfd-online.com/Forums/openfoam-solving/199221-why-simplefoam-laminar-solver-valid-re3000.html)

walakaka February 28, 2018 14:54

Why is the SimpleFoam Laminar solver Valid for Re3000 [solved]
 
Hi everyone,

I am currently validating a pipeflow simulation (Single-phase). I aim to validate laminar flows first (RAS turned off in pimpleFoam) for Re300 to Re2400.

When the Re gets high enough (approximately past Re2000), I expect the results to stop converging (with RAS turned off) as the flow is no longer laminar.

However, I obtained excellent results when compared to the H-P equation for my flow at Re3000.

My pipe is of length 20m, Diameter 0.0563m. Any if this result is expected?

Regards
Shafik

walakaka March 1, 2018 08:56

R3500 is valid against H-P too? What's wrong
 
1 Attachment(s)
Hi everyone!

My velocity profile for Re3500 is also valid against the H-P equation using laminar PIMPLE solver in openfoam. Does anyone have any idea what I'm doing wrong?

Pipe is 20m long and 0.0563m Dia.

Attached is the image of the developed velocity profile!

Regards
Shafik

Tobi March 7, 2018 09:26

Hi,

first of all. Please read the purpose of the subforums.
Why shouldn't pimpleFoam be not valid for such Reynolds numbers? If you solve in 'laminar' mode, you are using the Navier-Stokes equations without any assumptions. It is like DNS. The fluid itself follows the Navier-Stokes equations so why should there be a problem. If you mesh if fine, you probably resolve everything.

walakaka March 7, 2018 15:55

Quote:

Originally Posted by Tobi (Post 684207)
Hi,

first of all. Please read the purpose of the subforums.
Why shouldn't pimpleFoam be not valid for such Reynolds numbers? If you solve in 'laminar' mode, you are using the Navier-Stokes equations without any assumptions. It is like DNS. The fluid itself follows the Navier-Stokes equations so why should there be a problem. If you mesh if fine, you probably resolve everything.

Thank you for your reply. And I do apologize, I should've posted this in the validation section of the forum.

Thanks again!

Tobi March 7, 2018 17:58

Actually you should not put it to the validation cases :) read the purpose of that forum :)

I confirm, it confuses a bit :)

Santiago March 8, 2018 07:31

Quote:

Originally Posted by walakaka (Post 683362)
Hi everyone!

My velocity profile for Re3500 is also valid against the H-P equation using laminar PIMPLE solver in openfoam. Does anyone have any idea what I'm doing wrong?

Pipe is 20m long and 0.0563m Dia.

Attached is the image of the developed velocity profile!

Regards
Shafik

Just for the purpose of clarification before deleting this post: high Reynolds numbers do not generate turbulence, it only changes the character of NS as a quasi-linear system. Instabilities, and high enough Re, trigger turbulence. This is true in the "real world" as it is in the "numerical world"

kalle March 8, 2018 08:30

From https://en.wikipedia.org/wiki/Lamina...ent_transition

Quote:

Reynolds found that the transition occurred between Re = 2000 and 13000, depending on the smoothness of the entry conditions. When extreme care is taken, the transition can even happen with Re as high as 40000. On the other hand, Re = 2000 appears to be about the lowest value obtained at a rough entrance.

Tobi March 8, 2018 08:47

Quote:

Originally Posted by Santiago (Post 684389)
Just for the purpose of clarification before deleting this post

The post is not deleted :) I just moved it. Thus, your answer will be here forever :)

Santiago March 8, 2018 09:01

Look at me, I'm citing myself
 
Quote:

Originally Posted by kalle (Post 684396)


"If not much care is taken, when running your simulation, you can arrive to Re=10^8 and still not get turbulence"

-- Santiago

:)


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