Timo,
Thank you for the quick answer.
It is incompressible two-component flow, with viscosity dependent in a
nonlinear fashion on the concentration of a polymer in water.
The irregularities start out very small, about 1e-13 relative to the
values, but soon after they appear, they grow into large structures
(viscous fingers). That is the nature of the problem being solved: a
small irregularity grows into a big one very fast. I am not having a
problem with numerical errors per se -- it is the irregularity of those
errors over y that causes the problem. Numerical errors over x are not a
problem.
I will play with Assembly.NumericDifference.BaseEpsilon to see if that
helps, but I do not put much hope on it because, again, as inexact as
the Jacobian entries may be, they are equally inexact over y, as long as
the flow is 1d.
I did not know that DuMuX can do quadruple precision. How do I enable it?
Best regards,
Dmitry
On 11.11.2020 20:16, Timo Koch wrote:
On 11. Nov 2020, at 17:32, Dmitry Pavlov <[email protected]> wrote:
Hello,
I am running a fairly simple 1p porous flow simulation which is on a 2D rectangular grid. In a particular
case I came at, the initial and boundary conditions, as well as spatial parameters, do not depend on the
"y" coordinate, only on "x". There is no gravity and the flow is designed to be strictly
horizontal. So I naturally expect the results to be essentially 1-dimensional, i. e. the solution at any
moment in time should be a function of x and not of (x,y). But after some time, I get irregularities over the
"y" axis, that I think come from the linear solver. They are small, but they cause further
inconsistencies.
Hi Dmitry,
we’ll probably need some more information to be able to help.
What are you simulating exactly? A compressible flow problem? Seems to be
non-linear since you use a Newton solver I guess?
In case you are solving some kind of problem with density differences, there
could be also physical instabilities but I assume you ruled that out by your
setup.
What do you mean by irregularities along y? Are they significant? Much more
than numerical precision? What’s the magnitude of pressure in your problem and
what are the differences in the y-axis?
I tried AMGBiCGSTABBackend, ILU0BiCGSTABBackend and UMFPackBackend and the
problem persists. I tried to play with the convergence criteria
(LinearSolver.ResidualReduction, Newton.MaxAbsoluteResidual,
Newton.ResidualReduction, Newton.MaxRelativeShift). Some of them helped to
mitigate the problem, but did not eliminate it. Strictening the criteria too
much kills the convergence.
UMFPack is a direct solver, so this is as much accuracy as you get I guess. You
can try adjusting the Epsilon of the numerical derivatives
(Assembly.NumericDifference.BaseEpsilon, see
https://dumux.org/docs/doxygen/master/a00005.html). If you are fancy you can
also switch from double to quad precision floats and see if this reduces the
error.
Are there any options that I can set for solvers to force them to respect the
symmetry of the problem over one axis?
No. You would have to write your own solver.
Timo
(I know that I can use a 1D grid and have a consistent solution, but that will
not help in 2D situations where I have part of the system depending on (x,y)
and the other part depending only on x.)
Best regards,
Dmitry
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