12:06 p.m.
I am trying to model Stokes flow at very high viscosities (~10^20), from my results it
looks like I cannot achieve this and am better suited using viscosities of ~10^5 or so. I
know there is this older post [1] which states:
"A word of warning though: SfePy does not
have a dedicated Navier-Stokes solver (the Newton (+ and direct) solver use
that is in the navier_stokes.py example is rather naive, and works for small
problems with only, with viscosity "high enough")"
Has there been a different solver implemented since this was posted? If not, what do you
recommend to work with such large viscosities?
Thank you
[1] "Problem with the mesh while trying to solve the Navier Stokes equations with
sfepy"
5:54 a.m.
Hi Ben,
On 06/06/2018 09:06 PM, Ben Melosh wrote:
I am trying to model Stokes flow at very high
viscosities (~10^20), from my results it looks like I cannot achieve this and am better
suited using viscosities of ~10^5 or so. I know there is this older post [1] which
states:
"A word of warning though: SfePy does not
have a dedicated Navier-Stokes solver (the Newton (+ and direct) solver use
that is in the navier_stokes.py example is rather naive, and works for small
problems with only, with viscosity "high enough")"
Has there been a different solver implemented since this was posted? If not, what do you
recommend to work with such large viscosities?
I have only some general recommendations:
- having a matrix with very different magnitudes of items in different blocks
is problematic - you might need to use some scaling, or choose different
physical units for your input.
- You can try using the FieldSplit preconditioner from PETSc for the Stokes
problem, if it is so large that it cannot be solved using a direct solver
(Umfpack, or, in the git version, also MUMPS). Some of the relevant options of
the solver are mentioned in the docstring of
examples/multi_physics/biot_parallel_interactive.py, see PETSc docs for more, and
examples/multi_physics/biot_short_syntax.py shows how to pass any additional
options to the PETSc solvers from a problem description file.
Does it help?
Cheers,
r.
Thank you
[1] "Problem with the mesh while trying to solve the Navier Stokes equations with
sfepy"
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4:11 p.m.
Thanks for the help Robert,
I will look into these different options. I am wondering what specifically is problematic
about using very different or very large magnitudes of items in different blocks? Is it a
problem with processing power? I am hoping for specifics so I can better understand what
is going on in the model and improve it.
Thanks
10:59 p.m.
On 11/6/18 1:11 AM, Ben Melosh wrote:
Thanks for the help Robert,
I will look into these different options. I am wondering what specifically is problematic
about using very different or very large magnitudes of items in different blocks? Is it a
problem with processing power? I am hoping for specifics so I can better understand what
is going on in the model and improve it.
It is a problem of machine precision - when using the 64 bit float values, 1 +
1e-17 is still just 1, so having a block with values of the order of magnitude
1e20 and another or the order 1e3, the second block is essentially zeros from
the global matrix point of view.
r.
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Ben Melosh -
Robert Cimrman