[aspect-devel] Checkerboard at large scale

[John Naliboff]

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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