[aspect-devel] Checkerboard at large scale

Fri Apr 6 10:20:47 PDT 2018

Hi Lev,

maybe I phrased that a bit ambiguously, as the model may use different 
densities in different parts of the equations.

If your model is incompressible, we solve divergence u = 0, which 
basically assumes that the density is constant. So from the point of 
view of the mass conservation equation, material entering the model 
always has the same density as material leaving th model. However, you 
might still use a variable density in the buoyancy term, and this will 
also be the density that you see in the visualization output, and that 
is used for postprocessing purposes. So the mass flux postprocessor 
could suggest that the sum of mass in/outflux is not exactly zero if the 
material flowing in and out has different densities. So I guess in this 
case you should check that you conserve volume; the mass flux 
postprocessor can still be useful, as the integrated mass flux should 
still be approximately zero (assuming your density does not change too 
much).

Another idea: From they way you are describing it, the problem is 
essentially 2d (your prescribed velocity at the left/right boundary does 
not change in the 3rd dimension, and all other boundaries are free 
slip). So you could test your setup in 2d, see if you get a reasonable 
solution there, and then go from there. That would make testing much 
faster and simpler.

Best,
Juliane

On 04/06/2018 09:45 AM, Lev Karatun 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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>>
>>
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>>
>>
>>
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>
>     ----------------------------------------------------------------------
>     Juliane Dannberg
>     Project Scientist, UC Davis
>     jdannberg.github.io <https://jdannberg.github.io/>
>
>
>

----------------------------------------------------------------------
Juliane Dannberg
Project Scientist, UC Davis
jdannberg.github.io <https://jdannberg.github.io/>

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