dolfin team mailing list archive

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

On 12 February 2010 12:51, Patrick Riesen <priesen@xxxxxxxxxxxxxxx> wrote:
> Anders Logg wrote:
> > On Fri, Feb 12, 2010 at 12:02:53PM +0100, Patrick Riesen wrote:
> > > Anders Logg wrote:
> > > > On Fri, Feb 12, 2010 at 11:36:33AM +0100, Patrick Riesen wrote:
> > > > > Anders Logg wrote:
> > > > > > On Fri, Feb 12, 2010 at 10:42:14AM +0100, Patrick Riesen wrote:
> > > > > > > Anders Logg wrote:
> > > > > > > > On Fri, Feb 12, 2010 at 10:25:52AM +0100, Patrick Riesen wrote:
> > > > > > > > > hi all of you,
> > > > > > > > >
> > > > > > > > > in dolfin0.9.6 i'm having a hard time with the following problem:
> > > > > > > > >
> > > > > > > > > i'm solving for a variable viscosity, depending on the power x of
> > > > > > > > > the strain-rate tensor's 2nd invariant.
> > > > > > > > >
> > > > > > > > > the invariant i compute in a separate form, and then i compute the
> > > > > > > > > power by iterating through all the dofs, extracting/reinserting the
> > > > > > > > > values in the vector of the function inside the code.
> > > > > > > > >
> > > > > > > > > this was all fine for dolfin < 0.9.6 but now i think i have a
> > > > > > > > > problem with choosing the right function space for the viscosity
> > > > > > > > > coefficient in my nonlinear problem, since for those i used before
> > > > > > > > > (Quadrature, DG) i have serious problem which i do not understand
> > > > > > > > > (they all worked before dolfin0.9.6)
> > > > > > > > >
> > > > > > > > > my problem is a mixed problem with space definitions:
> > > > > > > > > #--------------------------------------
> > > > > > > > > velocity = VectorElement("CG" , geom, 2)
> > > > > > > > > pressure = FiniteElement("CG" , geom, 1)
> > > > > > > > > ME = MixedElement(velocity, pressure)
> > > > > > > > >
> > > > > > > > > # viscosity element
> > > > > > > > > visc = FiniteElement("DG", geom, 2)
> > > > > > > > >
> > > > > > > > > # placeholder variable viscosity
> > > > > > > > > eta = Coefficient(qe)
> > > > > > > > > #---------------------------------------
> > > > > > > > >
> > > > > > > > > using this has worked well before, but i don't follow why it
> > > > > > > > > shouldn't work in dolfin0.9.6/ffc0.9. Or else somebody can propose
> > > > > > > > > me another approach.
> > > > > > > > >
> > > > > > > > > regards,
> > > > > > > > > patrick
> > > > > > > >
> > > > > > > > Is the problem the quadrature element for eta? What is the problem
> > > > > > > > exactly?
> > > > > > >
> > > > > > > yes i think so, in dolfin0.8 i had a quadrature-element for eta,
> > > > > > > which was perfect, then basis-functions on quadrature elements were
> > > > > > > not supported anymore, and i changed it to a DG element the same
> > > > > > > order like velocity. that worked as well in dolfin0.9.2-0.9.5 but
> > > > > > > now this gives me a few negative values occuring in eta, when i
> > > > > > > solve for the invariant.
> > > > > >
> > > > > > Quadrature elements should still be supported. Try to isolate the
> > > > > > problem to a small and simple test case and submit it as a bug report
> > > > > > for FFC through Launchpad.
> > > > > ok, i'll try first something else: how can i tell ffc to use
> > > > > specific quadrature options, i.e. i want to make the terms involving
> > > > > eta and velocity to be computed on the same points and degree.
> > > >
> > > > If there is a quadrature element in the form (for eta), then FFC
> > > > should automatically choose the number of quadrature points to be the
> > > > same as for eta.
> > > >
> > > > Compile with the option -d (for debugging) and see what FFC says about
> > > > quadrature degree.
> > > ok, it says
> > >
> > >  Found quadrature element(s) with the following degree(s): [3]
> > >  Selecting quadrature degree based on quadrature element: 3
> > >  quadrature_degree: auto --> 3
> > >
> > > which is fine, i choose 1 order higher for the quadrature element
> > > than the velocity element.
> > >
> > > i now got the code now running again with the quadrature element,
> > > but the convergence of the newton solver is linear or less. i'm sure
> > > that my nonlinear function F and the jacobian J and how i compute
> > > the variable viscosity are correct, do you have a guess what of
> > > dolfin/ffc could affect this?
> >
> > I have no idea.
>
> ops, i found something:
>
> if i use the optimization flag -O in ffc, i get what i want: quadratic
> convergence.

That must be the first time ever using -O has actually fixed a problem. You use quadrature representation I assume?
I don't get why you say that using quadrature elements for the basis functions is not supported. Quadrature elements can be used exactly like any other element with the only difference that differentiation is not defined.
Does the problem disappear if you use quadrature elements instead of DG?

Kristian

> if i dont use it, the convergence is poor, i.e. ~linear
>
>
> > --
> > Anders
>
>
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