[aspect-devel] Checkerboard at large scale

Fri Apr 6 11:07:21 PDT 2018

Hi Lev,

If the boundary conditions conserve mass, then they should also conserve volume. However, as you state you could have a situation where volume is conserved while mass is not conserved if the density along the boundaries are different.

I suspect this is the issue, as using a free surface (verse free-surface) top boundary apparently ‘fixes’ (or improves) the model behavior. This makes sense as deformation of the top boundary can account for the ‘mass imbalance’ introduced by the velocity boundary conditions.

If you want to use a free-slip top boundary, you should use velocity boundary conditions that conserve mass. As Juliane mentioned, you can see the mass flux through each boundary with one of the post processors.

My suggestion would still be to start with an isothermal and isoviscous model. With this setup, your density field should only contain variations related to composition and you can do one time-step (only need 1 non-linear iteration) to quickly check whether your boundary conditions are conserving mass via the post-processors. 

Once your setup ‘works', then start systematically starting adding complexity back in. If it was me, I would also start by using a uniform mesh and even turn off gravity. 

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.

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 <https://github.com/geodynamics/aspect/issues/1872>

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. 

Cheers,
John

> On Apr 6, 2018, at 9:45 AM, Lev Karatun <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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