dolfin team mailing list archive

[Patrick Riesen <priesen@xxxxxxxxxxxxxxx>]

Kristian Oelgaard wrote:
> On 12 February 2010 13:14, Patrick Riesen <priesen@xxxxxxxxxxxxxxx> wrote:
> > Kristian Oelgaard wrote:
> > > 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?
> >
> > yes, i tried both, explicit use of -r quadrature and the 'auto' version
> > (doing nothing)
> >
> > > 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
> >
> > once in dolfin0.9.1 or 0.9.3 i could not define the 
> > Test/Trialfunctions for
> > a quadrature element, that was in the form i used for the invariant. when
> > starting with 0.8 i could do that.
> >
> > that differentiation is not
> > > defined.
> > > Does the problem disappear if you use quadrature elements instead of DG?
> >
> > so now i reverted the problem again to use quadrature elements for that
> > viscosity (eta) function. it works as i expect with quadratic convergence
> > for my nonlinear problem when using the optimization flag in  ffc but 
> > else
> > the convergence is poor and seems even a bit ugly..
>
> OK, so now Test/TrialFunctions work for quadrature elements (again)?

yes, now it's like in dolfin0.8 again

> It sounds like a bug for quadrature code generation if you get different 
> results depending on whether or not you use -O.
> Do you have to compile both forms with -O to get i working or is it just 
> the invariant form which is causing the problem?

i tried it, but now the problem seems gone, the convergence results are 
about the same whether or not using the -O flag.

but i must say that i just before i made a new pull for all the packages 
of FEniCS.

A new broom sweeps clean ...

i guess my previous install was somehow broken (but broken very nastily, 
it seemed to work all, but just giving me errors when computing..aiaiai)


> Kristian
>
> > but what the real problem was or is i don't understand.
> >
> > > Kristian
> > >
> > > > if i dont use it, the convergence is poor, i.e. ~linear
> > > >
> > > >
> > > > > --
> > > > > Anders
> > > >
> > > >
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