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

[John Naliboff]

Hi Shangxin,

Thanks for sending this question in.

Before thinking about your results in terms of the equations, I have two questions:
1. Do the values for RMS/Max velocity vary significantly if you systematically decrease the CFL number?
2. Likewise, do the values change significantly if you increase the grid resolution?

If the grid resolution is low and with the CFL number near the upper limit of what should be used, it’s possible you may get some divergence in the results. Worth a check before diving into other potential issues.

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?  

Cheers,
John

*************************************************
John Naliboff
Assistant Project Scientist, CIG
Earth & Planetary Sciences Dept., UC Davis


> On Apr 19, 2017, at 7:17 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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