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-   -   Why the noslip doesnbt work (http://www.cfd-online.com/Forums/openfoam-solving/58611-why-noslip-doesnbt-work.html)

 corenc August 26, 2008 02:50

Hi all, I am a newbie of Op

Hi all,

I am a newbie of OpenFOAM and CFD. I have a question of my first test:

I used potentialFOAM to solve a simple channel flow. The slip bounday condition seems work well. Then I change the boundary to no-slip , ie, set the velocity on boundary to fixedvalue zero.

Then I get a very strange result.

The velocity field is nearly the same as the one of slip condition. I can not see a velocity profile that smoothly ramps down to zero to the boudnaries. Velocity ramps down only in those triangles that has edges (vertieces) on boundaries. So it shoul not be correct.

Could you tell me what's the possible reason leading to this error ?

 michaelh August 26, 2008 03:37

Potential flows are per defini

Potential flows are per definition frictionless. Hence, it is not reasonable to define no slip boundary condition at walls (for that you would need wall friction!)

So you probably need another way of solving your problem (e.g. k-epsilon with wall function or so)

cheers
Michael

 caw August 26, 2008 03:40

Dear Corenc, you should rev

Dear Corenc,

you should review the physical background of potentialFoam.

solving the potential equations requires the flow field to be free of rotation and to have NO friction (no viscosity related effects).

Therefore setting a no-slip BC does not make any sense.

Best regards
Christian

 corenc August 26, 2008 04:37

Hi,Michael & Christian, thank

Hi,Michael & Christian, thank you for your immediately reply. In fact I work in computer animation. I teach Fluid Mechinqiues myself http://www.cfd-online.com/OpenFOAM_D...part/happy.gif So maybe many erros in the followings...

In my understanding, potential flow only requires irrotational. but not non-viscosity. Ok . even it may make no physical sense. Anyway, there is no term of viscosity in the Laplace equation. Why I cant give the no-slip boundary and then solve the laplace?

Thanks

Thank you.

 corenc August 26, 2008 04:50

I also tried the case in Fluen

I also tried the case in Fluent.

And there, I set the viscosity. But I got the same problem as in OpenFoame: non-slip doesn't work. It drives me crazy...

--
Corenc

 alberto August 26, 2008 05:28

Hi Corenc, if you want to s

Hi Corenc,

if you want to solve a steady, fully developed, channel flow in openFOAM, you might want to try simpleFoam.

If you have problems, post the case dictionaries in the forum, so we can give them a look.

With kind regards,
Alberto

 caw August 26, 2008 06:15

Hi Corenc, the potential fl

Hi Corenc,

the potential flow is a simplification of the euler momentum balance which is only valid for high Reynolds-numbers. Therefore friction is neglected --> there is no dissipative term which contains viscosity.

Regarding physical sense: sometimes there is no way to solve a equation which simply does not describe the physics right ;-)

Best regards
Christian

Dear All I would like to down

Dear All
Thank you
Kind Regards
Vijay

 caw August 26, 2008 07:22

Dear Vijay,

http://www.cfd-online.com/cgi-bin/Op...how.cgi?1/8825
You posted your question there by copying the first post (i do not understand why), so why post it again and again?

http://sourceforge.net/project/showf...kage_id=148942

Christian

 michaelh August 26, 2008 07:26

Hi Corenc, here I found a s

Hi Corenc,

here I found a some information about viscous potential flow: http://fluid.ippt.gov.pl/ictam04/tex...FM20_10843.pdf

It seems to be a quite unexplored field.
But normally potential flow is, as also Christian wrote, inviscous. If you set the viscous term in the momentum equation to zero, you end up with a partial differential equation of 1st order. Thus you can apply only one velocity boundary condition and this would be the impermeability condition.

Hope this helps a little more!

Michael

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