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

[Shangxin Liu]

Hi Timo and John,

Before making further comments, there is a typo in my first email. I just
checked my prm files for these three cases. The CFL number of them is all
0.5 instead of 1. And I use 3 global refinement, 0 adaptive refinement,
with fixed mesh.

Yes. I can further try decreasing CFL number or increasing the grid
resolution. But the reason why we use 3 global refinement, i.e., 49152
cells globally is that Zhong et al., 2008 paper uses 12*32^3 (393216) cells
globally, which is equivalent to global refinement 4 in ASPECT. However,
since ASPECT uses quadratic element while CitcomS uses linear element, we
suppose the lower global refinement 3 of ASPECT is equivalent to higher
global refinement 4 in CitcomS.

John, the reason why we modify alpha and gravity to fix Ra is that
previously before the true Boussinesq approximation was merged into ASPECT,
we use a very small alpha to mimic constant reference density in energy
equation. Now we further try these series with the new ASPECT with Boussinesq
approximation formulation and found this confused issue.

Timo, I'm using free slip boundary condition for velocity on both top and
bottom boundary.

The attachment is the prm file for my first case that uses 7000 gravity and
1.0 alpha. You can simply change gravity to 70000/700000 and alpha to
0.1/0.01 to get case2/case3. Let's see whether we can get the
same tilmestep size and velocity magnitudes of the three cases.

Best,
Shangxin



On Thu, Apr 20, 2017 at 1:09 PM, Timo Heister <timo.heister at gmail.com>
wrote:

> Shangxin,
>
> can you post your .prm? Are you using free slip boundary conditions
> for the velocity?
>
> On Wed, Apr 19, 2017 at 10:17 PM, Shangxin Liu <sxliu at vt.edu> wrote:
> > Hi;
> >
> > Recently I'm trying the new Boussinesq approximation formulation of
> ASPECT
> > to test the 3D spherical shell time-dependent convection. I tried running
> > the non-dimensional cases and set the start time to 0 and end time to 1.
> > Through a series of tests, I found that the same Ra but different gravity
> > and alpha have far different results of velocity magnitude and the
> tilmestep
> > size. For example, I fixed the Ra to 7000 and run three cases:
> >
> > Case1: gravity 7000, alpha 1
> > Case2: gravity 70000, alpha 0.1
> > Case3:  gravity 700000, alpha 0.01
> > (All other parameters of these three cases in prm file are the same:
> > constant viscosity, CFL number=1, same solver tolerance, same spherical
> > shell geometry,etc..)
> >
> > Case1 takes only 13 timesteps to reach the end time and the end RMS/Max
> > velocity is 0.00182/0.0055;
> > Case2 takes 20 timesteps to reach the end time and the end RMS/Max
> velocity
> > is 0.00558/0.0352;
> > Case3 takes 141 timesteps to reach the end time and the end RMS/Max
> velocity
> > is 0.0479/0.305.
> >
> > They all ran on one node of our cluster but the running time is also
> > different:
> > case1: 999s
> > case2: 1.48e03 s
> > case3: 3.86e03 s
> >
> > The end min/avg/max temperature and heat flux of upper/lower boundary of
> the
> > three cases are all the same.
> >
> > From ASPECT manual, I noticed that the time step size is inversely
> > proportional to the max velocity of the cell so such a huge difference of
> > tilmestep size must be caused by the difference of the velocity
> magnitude.
> > But in the non-dimemsioanl problem, the same Ra should have the same
> > magnitude of velocity. Why in ASPECT, the same Ra can have such different
> > velocity magnitude? I've realized that ASPECT uses the total pressure and
> > total density in its momentum equation, so larger value of gravity can
> > introduce a larger rho_0*g_0 term in the momentum equation. I'm not sure
> > whether this will cause the solver matrix of momentum equation going
> crazy.
> > Did anyone else run into this similar confusion? Wolfgang, Timo, Rene,
> > Juliane, any insight on this?
> >
> > Best,
> > Shangxin
> >
> > _______________________________________________
> > Aspect-devel mailing list
> > Aspect-devel at geodynamics.org
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