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zero gradient pressure B/C

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Old   June 29, 2010, 16:47
Default zero gradient pressure B/C
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Srinath Madhavan (a.k.a pUl|)
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Can someone kindly help in explaining why setting the component of "pressure gradient" normal to the wall zero is an appropriate Boundary condition for walls?

Put simply, why should the pressure gradient (i.e. grad (p)) be orthogonal to the outward normal to the wall?
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Old   June 30, 2010, 14:49
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Anybody?? Does setting "grad(p) dot nhat = 0" mean that from the wall onward up till beyond the wall boundary (i.e. the region occupied by ghost CVs), there is no component of the "pressure gradient" in the direction of the outward wall normal?

In other words, physically speaking, we are enforcing the condition that there is no fluid flow through the wall?? Am I even close???
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Old   August 6, 2010, 00:52
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Srinath Madhavan (a.k.a pUl|)
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Found some free time. Thought I could use this to close my query. A little too embarrassing actually, but after a sharp reprimand from one of my profs. for not knowing something so trivial I have the answer:

http://www.ualberta.ca/~madhavan/zer...nt_at_wall.pdf
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Old   August 7, 2010, 01:16
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Robert
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Thank you for sharing Srniath.
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Old   August 7, 2010, 01:26
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Glad you found it useful Have a good weekend!
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Old   August 7, 2010, 15:22
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Claus Meister
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Okay, I have mixed up x and n! I'm gonna to study this again, because it seems very interesting. Sorry for the rigor, but as a mathematician, one is compelled to investigate things more critically.

Cheers and have a nice one!
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Old   August 7, 2010, 16:16
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Srinath Madhavan (a.k.a pUl|)
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No problem. I for one welcome any criticisms. After all we are all here to share and learn
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Old   September 28, 2010, 16:31
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Is there a patch/method, in OpenFOAM, which extrapolates the pressure to the wall from the adjacent normal cells rather than forcing the approximation of dp/dy = 0 ?

That's my first question, my second being is it easily achievable to prescribe dp/dy = nu*d2v/dy2 as a custom patch?

I ask in the context of using the icoFoam solver on 2-D laminar boundary layers.

Any suggestions would be greatly appreciated. Thanks!
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Old   November 11, 2010, 10:58
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I'm also interested in this topic. If you have a look on the cavity tutorial, the pressure-gradient normal to the movingWall is certainly not zero.

Did you already come to a result, steph79?
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Old   November 15, 2010, 06:45
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Adam James
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Quote:
Originally Posted by AsafPa View Post
I'm also interested in this topic. If you have a look on the cavity tutorial, the pressure-gradient normal to the movingWall is certainly not zero.

Did you already come to a result, steph79?
Yes, PRESTO! pressure discretisation is what you're looking for but whether that can easily be implemented in OpenFOAM I'm not sure. Could someone with more experience please comment on that?

Thank you.
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Old   December 13, 2010, 20:43
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Hi, I have a question.
In page 2, line 6 where you say dv/dy=0. then d2v/dy2=0, I cannot see why is that? for other points, you have that because you can compare 2 points on the wall ( which is x direction) but for y direction, I don't know how to say that. Any way, thank you for this calculation, really helps understand what is going on in OpenFOAM.
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Old   December 14, 2010, 10:50
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I think there is another reason for zeroGradient pressure BC at walls. It comes from numerics. When you implement PISO or SIMPLE methods finally a Poisson equation is assembled for pressure. This equation requires at least a one point with prescribed pressure which is searched from BC. If there is not a prescribed pressure BC then the pRefValue is taking into account. Since pressure is not known at walls, the BC is set to take the value from the nearest point within the fluid, or in other words taking the zeroGradient perpendicularly to the wall.

Just my 2 cents.

Regards.
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