Daniel --
Thank you, that makes a great deal of sense. On the boundary conditions you
are write about faceNorm; I had misinterpreted the text from
http://www.ctcms.nist.gov/fipy/documentation/USAGE.html
If the gradient normal to the boundary (*e.g.*, [image:
\hat{n}\cdot\nabla\phi]) is to be set to a scalar value of 2, use
>>> var.faceGrad.constrain(2 * mesh.faceNormals, where=mesh.exteriorFaces)
to mean that the gradient boundary conditions was defined normal and
outward of the solution domain. But it is clear from the text above that
this is not so; my bad.
Jamie
On Tue, May 9, 2017 at 1:55 PM, Daniel Wheeler <daniel.wheel...@gmail.com>
wrote:
> On Fri, May 5, 2017 at 5:37 PM, James Pringle <jprin...@unh.edu> wrote:
> > Jon --
> >
> > Thanks for this reply. There is something in it I don't understand.
> Why
> > is it necessary in fipy constrain the tangential derivative to the
> solution
> > to the Poisson equation in fipy?
>
> I don't think that the tangential derivative matters. I think that in
> James's version of "DemoBug.py" multiplying by the "faceNorms" is
> giving the wrong sign in some places from the normal component. By
> removing the "faceNorms", the normal component is correct. I think
> that you can zero out the tangential component and it has no impact.
> For example, the following also works,
>
> ~~~~
> faceNorm_x = faceNorm.copy()
> faceNorm_y = faceNorm.copy()
> faceNorm_x[:] = 0.
> faceNorm_y[:] = 0.
> faceNorm_x[0, :] = 1.
> faceNorm_y[1, :] = 1.
> psiInvert.faceGrad.constrain(psiTrue.faceGrad * faceNorm_y,
> where=yeq0bound)
> psiInvert.constrain(psiTrue.faceValue,where=yeq1bound)
> psiInvert.faceGrad.constrain(psiTrue.faceGrad * faceNorm_x,
> where=xeq0bound)
> psiInvert.faceGrad.constrain(psiTrue.faceGrad * faceNorm_x,
> where=xeq1bound)
> ~~~~
>
> In the above the tangential component is zeroed out and the normal
> component is unity, so the sign of the gradient is unaffected.
>
> --
> Daniel Wheeler
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