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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