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

[Timo Heister]

Shangxin,

I looked a little bit more into your example. Some observations:
1. You are using the "simple" model for a nondimensional computation.
It is probably a better idea to use "nondimensional" instead. I will
try to see if that makes a difference.
2. I haven't quite figured out how to quantify "don't make gravity too
large", but as I expected, the difference increases the larger the
gravity is. I think our pressure scaling or linear solver tolerance
needs to take the size of the gravity into account but it currently
doesn't.
3. A finer solver tolerance is likely important (see 2).
4. It looks like the buoyancy term can not be resolved adequately on
the current mesh (if you plot T or rho, you can see that it jumps from
0 to 0.5 within a single cell. See attached.
5. If you plot RMS over time, you can see that the timesteps are quite
large (especially for alpha>0.1). I am not sure if this is connected
to 4) or not.

Anyways, I will get back to you when I figure out more.

On Fri, Apr 21, 2017 at 6:00 PM, Shangxin Liu <sxliu at vt.edu> wrote:
> 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?
>>
>>
>>
>



-- 
Timo Heister
http://www.math.clemson.edu/~heister/
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