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

[Shangxin Liu]

Hi Timo,

Just to confirm. Are you plotting the total density at the first time step?


Best,
Shangxin

On Mon, Apr 24, 2017 at 11:09 AM, Timo Heister <heister at clemson.edu> wrote:

> 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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