[aspect-devel] Checkerboard at large scale
Juliane Dannberg
judannberg at gmail.com
Fri Apr 6 11:25:00 PDT 2018
Hi Lev,
to extend on what John said:
>
> If it was me, I would also start by using a uniform mesh
Yes, I would try this!
The reason is this: You prescribe a function for the velocity on both
sides of the model, and it is interpolated using the finite element. So
if your function can not be exactly reproduced by the finite element
shape function, it will be approximated, and you might get different
results for the left and right boundary. So if you have different
element sizes at the different boundaries, you might get a different
mass flux across the boundaries just based on that interpolation, even
if the analytical function that you prescribe would lead to a nonzero
in-/outflux.
>
> If you don’t want to alter the velocity boundary conditions on the
> side, you could simply add inflow or outflow along the model base to
> account for the mass flux difference.
That would be one solution, the easiest way would probably be to use a
prescribed traction at the bottom and just prescribe the initial
lithostatic pressure.
>
> I hope this helps and let us know if you have any additional questions
> or other issues pop up along the way.
>
> Juliane/Rene/Wolfgang/Timo/etc - I believe this points back to the
> issue discussed here: https://github.com/geodynamics/aspect/issues/1872
>
Yes, at least this is what I suspect too (one way to test this would be
to see if the integrated mass flux over all model boundaries is very
small, but nonzero).
> Perhaps this is something to revisit at the Hack or sooner? If the
> composition along the side boundaries varies with time, it may be
> difficult to enforce mass conversation with a free surface. The
> suggestion to modify the boundary conditions slightly to account for
> the mass flux imbalance seems like a the easiest solution.
Yes. Let's think about how to do this best.
Cheers,
Juliane
>
>> On Apr 6, 2018, at 9:45 AM, Lev Karatun <lev.karatun at gmail.com
>> <mailto:lev.karatun at gmail.com>> wrote:
>>
>> Hi Juliane,
>>
>> If density of the material entering the model is different from
>> material leaving it, the b.c. can't be both volume and mass
>> conserving. Am I understanding something wrong?
>>
>> Best regards,
>> Lev Karatun.
>>
>> 2018-04-06 2:18 GMT-04:00 Juliane Dannberg <judannberg at gmail.com
>> <mailto:judannberg at gmail.com>>:
>>
>>
>>>
>>> 1. Are the applied boundary conditions mass and/or volume
>>> conserving?
>>>
>>> Volume conserving. To be honest, I never put too much thought
>>> into this -- how do I determine if I should conserve mass or
>>> volume? Is there some sort of rule of thumb?
>> From what you're saying, my guess would be that the boundary
>> conditions could be the problem (as I think John was implying).
>> You can check if if they conserve mass for example by using the
>> mass flux postprocessor; it will give you the mass flux though
>> each of the boundaries. Usually you should conserve mass (and if
>> the model is incompressible, that would also mean conserving the
>> volume).
>>
>> Alternatively, you can also try leaving the top or bottom
>> boundary open and see if material flows in or out there, that
>> will also give you a clue if the boundary conditions you're
>> prescribing at the sides are reasonable.
>>
>> Best,
>> Juliane
>>
>>
>>> 2018-04-02 12:43 GMT-04:00 John Naliboff <jbnaliboff at ucdavis.edu
>>> <mailto:jbnaliboff at ucdavis.edu>>:
>>>
>>> Hi Lev,
>>>
>>> Typically 'checker boarding' is seen in the pressure field
>>> and arises when using plasticity with low-order (Q1P0)
>>> elements. This looks a bit different and could be related to
>>> any number of issues. However, I have not seen anything like
>>> this specifically. From your description, it sounds like you
>>> would expect convergence in the upper region (lithosphere?)
>>> and a downwelling in the model center that transitions to
>>> outflow in the outflux regions?
>>>
>>> So, a few follow-up questions:
>>>
>>> 1. Are the applied boundary conditions mass and/or volume
>>> conserving?
>>> 2. Are there similar (or other odd) patterns in the
>>> pressure, temperature, etc fields?
>>> 3. What element type?
>>> 4. Linear or non-linear rheology?
>>> 5. If non-linear, did the non-linear solver converge to a
>>> reasonable value?
>>>
>>> Depending on the exact setup, there are a number way to
>>> start going about diagnosing the issue. I would start with
>>> simplifying the material properties and boundary conditions
>>> to make sure you are getting the expected velocity field.
>>> For example, use an isothermal temperature profile and
>>> constant density/viscosity/etc. This could be done in
>>> combination with only applying the imposed inflow/outflow
>>> one one side.
>>>
>>> Cheers,
>>> John
>>>
>>> On 03/29/2018 07:05 PM, Lev Karatun wrote:
>>>> Hi everyone,
>>>>
>>>> I was trying to run some 3-D compression models, but all
>>>> I'm getting is a checkerboard pattern (see screenshot
>>>> attached). Boundary conditions are as follows:
>>>> left, right walls: influx through the top half, outflux
>>>> through bottom.
>>>> all other walls: free slip.
>>>> I tried increasing the resolution but it didn't help. I
>>>> also thought about decreasing the CFL number but the
>>>> instability happens at the first timestep, so it doesn't
>>>> seem relevant. I was wondering if someone faced a similar
>>>> problem in their research? What did you do to overcome it?
>>>>
>>>> Thanks in advance!
>>>>
>>>> Best regards,
>>>> Lev Karatun.
>>>>
>>>>
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>>
>>
>> ----------------------------------------------------------------------
>> Juliane Dannberg
>> Project Scientist, UC Davis
>> jdannberg.github.io <https://jdannberg.github.io/>
>>
>>
>>
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----------------------------------------------------------------------
Juliane Dannberg
Project Scientist, UC Davis
jdannberg.github.io <https://jdannberg.github.io/>
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