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

[Timo Heister]

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
>
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