[aspect-devel] Checkerboard at large scale

[Juliane Dannberg]

>
>     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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