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..
