ufl team mailing list archive

[Patrick Riesen <priesen@xxxxxxxxxxxxxxx>]

Kristian Oelgaard wrote:
> On 6 July 2010 14:07, Patrick Riesen <priesen@xxxxxxxxxxxxxxx> wrote:
> > Garth N. Wells wrote:
> > > On 06/07/10 13:25, Patrick Riesen wrote:
> > > > hello,
> > > >
> > > > i want to take the gradient of a coefficient with respect to some other
> > > > quantity than the spatial coordinates (i.e. some other coefficient).
> > > >
> > > > i tried to construct a custom operator derived from diff() and the
> > > > SpatialCoordinate/Derivative classes in ufl but after some point beyond
> > > > my knowledge it always gets evaluated to zero by FFC then stops
> > > > compiling with FFC : division by zero!
> > > >
> > > > does somebody of you experts know an approach to this?
> > > Did you try using 'variable'?
> > >
> > > � �. . .
> > > � �v �= variable(v)
> > > � �f �= v*v
> > > � �df = diff(f, v)
> > yes, but if f is a coefficient and not an expression of v, this won't work,
> > i.e.,
> >
> > f = Coefficient(element)
> > f = variable(f)
> > v = SpatialCoordinates(cell)
> > v = variable(v)
> > df = diff(f, v)
> >
> > will end up as Type Zero, although this is equivalent to grad(f). that's the
> > one case, and it v = Coefficient(element) as well, it will also be zero.
> > if would be nice if where not zero :-)
>
> The first example I get although I don't know why it does not work at
> the moment, but it could be a bug. The second case where v is a
> coefficient I don't get.
> If f(x) = x**2, then df/dy = 0, I think that makes sense. 

yes, but if f is a coefficient, i.e., it is a eulerian quantity f=f(x), 
and df/dx is the spatial gradient, ufl computes this from either 
grad(f) or f.dx(i) or Dx(f,i).

but the vector of spatial coordinates x is
hidden in ufl. i would like to do something like f.dx(v[i]), i.e. 
compute df_i/dv_j where v represents some other coordinates, possibly 
given by another coefficient?


Perhaps what
> you want (or is trying to do) is described in this blueprint?
>
> https://blueprints.launchpad.net/ufl/+spec/function-derivatoves
>
> Kristian
>
> > patrick
> >
> > > Garth
> > >
> > > > many thanks and best regards,
> > > > patrick
> > > >
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