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-   -   porousSimpleFoam: oscillating velocity in the porous zone (http://www.cfd-online.com/Forums/openfoam-solving/82729-poroussimplefoam-oscillating-velocity-porous-zone.html)

Se9a December 3, 2010 16:49

porousSimpleFoam: oscillating velocity in the porous zone
 
Hello!
I try to use a porousSimpleFoam solver to simulate a flow in a 2D channel from two equal parts, a free flow part and a porous part (wall's type set in the 'slip' for simplicity):

http://img-fotki.yandex.ru/get/5601/...61536200_L.jpg

Base case is a angleDictImplicit.

My results are a good p, but strange oscillating U.
On the next sctrutured mesh:
http://img-fotki.yandex.ru/get/5106/...c6b86593_L.jpg

I get this solution:
http://img-fotki.yandex.ru/get/5701/...c78ed4bd_L.jpg

Velocity field is:
http://img-fotki.yandex.ru/get/5802/...b0d0a7fc_L.jpg


One can see what velocity is good but slightly oscillated on porous boundary. So, question is:

Are such oscillations caused by a stepwise porous source? If yes, how can I specify smooth porous source?

But more bad situation have place on non-structured mesh.
Mesh is
http://img-fotki.yandex.ru/get/5602/...b921d596_L.jpg

Solution is:
http://img-fotki.yandex.ru/get/6001/...6741e4eb_L.jpg

http://img-fotki.yandex.ru/get/5001/...3b4da787_L.jpg

One can see very oscillating velocity in the porous region.
So, why this happens? Non-orthogonal correction not helped.

Thanks.

Marc10 January 3, 2011 14:28

Hi!

I have exactly the same problem. Did you find a solution for that?

kind regards,

marc

gschaider February 10, 2011 08:44

This problem has been reported to http://www.openfoam.com/mantisbt/view.php?id=134. A fix is also described there

Filankes January 27, 2014 09:27

This is not a problem.


Just change under-relaxation factor to 0.0001, and you will not have this oscillations.

OpenFOAM software is the best software among all CFD, and being unexperienced in CFD, does not meam that OpenFOAM has bugs, that should be reported like that...

gschaider January 28, 2014 06:11

Quote:

Originally Posted by Filankes (Post 471967)
This is not a problem.


Just change under-relaxation factor to 0.0001, and you will not have this oscillations.

This is absolute brilliant: under-relaxation is like alcohol: if you take enough you see no problems anymore. Getting things done will take forever, but who cares?

Filankes January 29, 2014 19:02

Quote:

Originally Posted by gschaider (Post 472107)
This is absolute brilliant: under-relaxation is like alcohol: if you take enough you see no problems anymore. Getting things done will take forever, but who cares?

Hi Bernhard,

You are right it takes a lot of time of calculations.

In industrial application it is not good idea to use it.

However, if you use 10 000 - 17 000 cells (hex) for the case proposed in this topic, and make underrelaxation = 0.0001, it will take 30 minutes to have an Exelent solution.

Unfortunately, tetrahedral cells were used, and seems to me under-relaxation 0.0001 will take a day or two for this simulation.

I feel like it is not industrial application, since it is tetrahedral, and 1-2 days is possible to give to get better velocity profile)) corrected pressure loss))

se9a
btw, I forgot to mension - I always use 7 cells in flow direction of porous zone + 0.0001 underrelaxation.

other regions can have very coarse mesh, since they are of no interest I guess

Bahram August 8, 2014 03:26

Thanks Bernhard

It worked like magic :)

Bahram September 23, 2014 04:18

Quote:

Originally Posted by gschaider (Post 294654)
This problem has been reported to http://www.openfoam.com/mantisbt/view.php?id=134. A fix is also described there

Dear Bernhard

Thanks for the help. I tried it, somehow it works, but as it is also mentioned in there it causes a huge conservation error. Do you have any idea where does this error comes from? any help appreciated!

Best regards
Bahram

gschaider September 23, 2014 14:01

Quote:

Originally Posted by Bahram (Post 511432)
Dear Bernhard

Thanks for the help. I tried it, somehow it works, but as it is also mentioned in there it causes a huge conservation error. Do you have any idea where does this error comes from? any help appreciated!

Best regards
Bahram

Have a look at the link http://www.openfoam.com/mantisbt/view.php?id=134

It is not my claim and I never checked how big HUGE is

Bahram September 24, 2014 03:10

Thanks a lot for quick respond, I have already checked that page but as I said before that idea causes some errors and there is not any clue in there to solve this fluctuating velocity without loosing or gaining some mass!!!


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