[aspect-devel] Far different velocity magnitudes & timestep sizes of the same Ra

[Shangxin Liu]

Hi Timo, John, and others,

I quickly made several new tests using the new Boussinesq approximation
formulation of the higher 1e-7 Stokes linear tolerance and 0.1 CFL number.
The results are compiled in the attachment. 1e-7 higher tolerance and 0.1
CFL number don't help a lot. There is still order-of-magnitude difference
of the velocity statistics and time step size between g 7000&alpha 1, g
70000&alpha 0.1, and g 700000&alpha 0.01. I can further try global
refinement 4 to see but global refinement 3 with quadratic element may be
already enough resolution. Something weird is still happening.

Best,
Shangxin


On Fri, Apr 21, 2017 at 1:44 PM, John Naliboff <jbnaliboff at ucdavis.edu>
wrote:

> Hi Scott, Hi Shangxin,
>
> Shangxin - Thank you for the clarification regarding the models. CFL=0.5
> is certainly more reasonable, but it still might be worth it to try a value
> like 0.1 just to make sure nothing odd is going on there.
>
> Scott - Thanks for the explanation and definitely interested to see what
> solution(s) arise.
>
> Cheers,
> John
>
> *************************************************
> John Naliboff
> Assistant Project Scientist, CIG
> Earth & Planetary Sciences Dept., UC Davis
>
> On 04/21/2017 04:23 AM, Scott King wrote:
>
>
> John;
>
> See the section of the Aspect manual for the 2D incompressible Cartesian
> benchmarks.   This is a trick used to try to circumvent the density term in
> the time derivative of the temperature equation, which is not constant (as
> it would be for Bousinessq).   The small alpha makes that term nearly
> constant while keeping the buoyancy term as Ra.  In 2D the manual shows
> this works up to Ra=7000*1e10, alpha-1e-10.   Trying to use this for 3D
> spherical it breaks around 10^3/1e-3.   It suggests either the 3D spherical
> matrix is more illconditioned to begin with or something about the
> iterations and tolerance levels for the solver is different between 2D and
> 3D.   Or it needs to be different between the two and isn’t.
>
> Scott
>
>
>
> On Apr 20, 2017, at 12:16 PM, John Naliboff <jbnaliboff at ucdavis.edu>
> wrote:
>
> On a side note, I personally have trouble interpreting results that vary
> the Ra number by orders of magnitude through terms other than the
> viscosity. While this is certainly an interesting numerical case study, is
> there a different motivation for varying the Ra number through terms other
> than the viscosity?
>
>
>
>
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