fenics team mailing list archive

[Kristian Oelgaard <k.b.oelgaard@xxxxxxxxx>]

On 18 June 2010 17:36, Hans Petter Langtangen <hpl@xxxxxxxxx> wrote:
> Fri, 18 Jun Anders Logg wrote:
>> 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.
>
> I agree - in the Poisson demo we skip "flux" and use "normal derivative"
> instead and define it as grad(u).n.

OK, I'll change this in the text and code then.

Kristian

> Hans Petter
>
>
>