[aspect-devel] Checkerboard at large scale

[Lev Karatun]

John, Wolfgang, Phil, thank you for your answers!

I probably didn't quite use the term correctly since the instability is not
cell size scale indeed, I was just trying to describe the general look of
it. I think you're right saying that it's a problem of the setup, not
discretization.

John, let me answer your question one by one.


> 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?

Yes, exactly.

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?

2. Are there similar (or other odd) patterns in the pressure, temperature,
etc fields?
No, not really.
3. What element type?
not sure (I didn't change defaults)
4. Linear or non-linear rheology?
non-linear
5. If non-linear, did the non-linear solver converge to a reasonable value?
no -- this is basically what my problem is.

After switching to constant temperature and constant density the solver
crashed after a mesh refinement step (11 or 21), but without AMR it
converges successfully. In general, it seems that running this model with
higher resolution yields better results, so that's what I'm trying to do
right now.
One more thing that I forgot to mention in my original email: the model
works fine with free surface enabled, and only crashes with free slip at
the top. My understanding was that in general turning free surface on makes
models less stable, but in my case it's the opposite. Should I be alarmed
by it or just run all models of the series with free surface?

Best regards,
Lev Karatun.

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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