fenics team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Fri, Jun 18, 2010 at 04:35:37PM +0200, Hans Petter Langtangen wrote:
> Fri, 18 Jun Anders Logg wrote:
> > 3. Is the normal flux + or - grad(u).n? I think of it as -grad(u).n.
>
> For k*inner(grad(u), grad(v))*dx, -k*grad(u).n is the right flux. It has
> a minus sign. Some people use the word flux for grad(u).n, but that's
> sloppy - normal derivative is then better.
>
> Hans Petter

I agree that -k*grad(u).n is the flux, but perhaps the question is
whether to include the term "flux" at all in the Poisson demo. If it
is included, then we should have a minus sign, both in the definition
of g = -k*grad(u).n and in the form where we get -<g, v>*ds. But if we
don't mention "flux" then it looks strange with two minus signs.

--
Anders

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