[aspect-devel] Checkerboard at large scale

Sat Apr 7 10:20:40 PDT 2018

Hi John,

thank you for the detailed answers.
So if I was trying to conserve mass, my understanding is that I'd have to
integrate over density over outward velocity, similar to how it's done in
the mass_statistics.cc? And if I want to keep the prescribed velocity at
the sides constant, I would then then do the same thing for the bottom
boundary, and set a very small outflux or influx to compensate for the
sides. Does that sound right?

Best regards,
Lev Karatun.

2018-04-06 14:07 GMT-04:00 John Naliboff <jbnaliboff at ucdavis.edu>:

> 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
>
> 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>:
>
>>
>>
>> 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.
>>>
>>>
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>>
>>
>>
>> _______________________________________________
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>>
>>
>>
>> ----------------------------------------------------------------------
>> Juliane Dannberg
>> Project Scientist, UC Davis
>> jdannberg.github.io
>>
>>
>>
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