[aspect-devel] Checkerboard at large scale

Sat Apr 7 10:14:01 PDT 2018

 Hi Juliane,

(sorry for multiple emails -- meant to use "Reply all" the first time)

thank you for the clarification! I'm still confused unfortunately. You're
saying

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
> ...
> So I guess in this case you should check that you conserve volume

But doesn't it contradict what John and Wolfgang are saying (that I should
balance mass, otherwise I'll see anomalous pressure or anomalous
topography)?

Best regards,
Lev Karatun.

2018-04-06 13:20 GMT-04:00 Juliane Dannberg <judannberg at gmail.com>:

> 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>:
>
>>
>>
>> 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
>>
>>
>>
>
> ----------------------------------------------------------------------
> 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/20180407/341c062b/attachment-0001.html>