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Old   May 18, 2011, 16:34
Question 2 questions about boundary conditions
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Mateus Palharini Schwalbert
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Hi. I'm starting to use OpenFOAM, and I'm not very used to it yet, so I believe my doubts are really simple. I've had some difficulties about setting some boundary conditions:

(1) Is there a way of setting a BC for p in an inlet to be extrapolated from the internal field, for the use of a solver for incompressible flows, like icoFoam?

(2) Is there a way of setting a given expression as a BC (a given velocity profile, for example)?
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Old   May 18, 2011, 17:26
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Nima Sam
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hi
1) extrapolated from the internal field?? for what?
2) groovyBC
http://openfoamwiki.net/index.php/Contrib_groovyBC
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Old   May 19, 2011, 08:17
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Mateus Palharini Schwalbert
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groovyBC, I'll check it.

I want to set BCs to be extrapolated from the internal field in cases where there's no physical consistency in setting other BCs. Then p should be extrapolated from the last volumes near the boundary...
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Old   May 19, 2011, 12:24
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Bernhard Gschaider
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Quote:
Originally Posted by mateusps View Post
I want to set BCs to be extrapolated from the internal field in cases where there's no physical consistency in setting other BCs. Then p should be extrapolated from the last volumes near the boundary...
That sounds like zeroGradient to me
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Old   May 19, 2011, 12:31
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Mateus Palharini Schwalbert
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what I want is similar to zeroGradient, but zeroGradient uses only one point to extrapolate (equals the value on the face to that on the last volume center), and I want some BC that uses more points to extrapolate (it could use the last two volume centers to extrapolate to the face using a linear straight, or the last three volume centers to extrapolate to the face using a quadratic function...).


Another doubt: is it possible to use groovyBC (or something similar) to create the inicial internalField???????
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Old   May 20, 2011, 01:51
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Bernhard
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If there is no physical consistency, then what is the reasoning in doing it like this? Can you comment on the physics you're trying to simulate?


For the initial field you can use funkySetFields.
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Old   May 20, 2011, 11:14
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Mateus Palharini Schwalbert
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thanks for the tip for the initial field. I'll take a look.

the reason for setting a BC to be extrapolated is that it's a better choice for when there's no physically consistent choice of BC applicable. Sometimes the physics of the problem just specifies one BC, but the algorithms of solution require that you specify the BC at all boundaries, so the extrapolation from the internal field is a better choice than others.

Right now I don't wanna simulate anything specifically, but in general such a BC would be useful. For instance, in the simple incompressible laminar flow in a tube, mathmatically it would be necessary just the specification of one BC for p in the axial direction (the inlet OR outlet, usually), but in openFOAM you must specify the inlet AND the outlet. What's usually done is specifying a fixedValue at the outlet and a zeroGradient at the inlet, but that's physically untrue (the gradient is NOT ZERO in the real problem), what causes a bigger entrance length than it would be required. This is a typical case where setting the BC to be extrapolated from the internal field would do a better simulation.

A case in which this is really significant is the simulation of viscoelastic fluids flow, for example. It's necessary to specify BCs for the tension tensor at all boundaries, and, without a better solution, it's usually used zeroGradient, but in this case that's a really bad approach, cause the tension gradient is indeed big and far from zero at the walls. That's a case in which the tension at the walls could be extrapolated from the internal field like a better approximation than the zeroGradient.

Do you understand the problem?
Tushar@cfd and mgg like this.
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Old   February 2, 2013, 11:19
Default OpenFoam BC hoe to define an extrapolation BC
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Samir
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Hi All
I like to ask how I can define an extrapolation BC at my channel outlet using
OpenFoam BC.
regards,
Samir
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Old   August 6, 2014, 02:29
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Lianhua Zhu
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Hi, mateusps

Have you find answer for you problem 1?

Best,

Lianhua
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