Navier - Stokes at high viscosity
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"
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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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
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.
participants (2)
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Ben Melosh -
Robert Cimrman