[aspect-devel] Checkerboard at large scale

[Lev Karatun]

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>:

>
>
> 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>:
>
>> 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.
>>
>>
>> _______________________________________________
>> Aspect-devel mailing listAspect-devel at geodynamics.orghttp://lists.geodynamics.org/cgi-bin/mailman/listinfo/aspect-devel
>>
>>
>>
>> _______________________________________________
>> Aspect-devel mailing list
>> Aspect-devel at geodynamics.org
>> http://lists.geodynamics.org/cgi-bin/mailman/listinfo/aspect-devel
>>
>
>
>
> _______________________________________________
> Aspect-devel mailing listAspect-devel at geodynamics.orghttp://lists.geodynamics.org/cgi-bin/mailman/listinfo/aspect-devel
>
>
> ----------------------------------------------------------------------
> Juliane Dannberg
> Project Scientist, UC Davis
> jdannberg.github.io
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.geodynamics.org/pipermail/aspect-devel/attachments/20180406/39e1c0d7/attachment.html>