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Re: which function space

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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. 

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



--
Anders

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