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

[Timo Heister]

Shangxin,

> set Linear solver tolerance                = 1e-3

Is there a reason for choosing such a tolerance? I would have no
confidence that the solution is accurate. Can you re-run with
something like 1e-8.

That said, I think we likely still have a problem with equation
scaling if the gravity is not the same order of magnitude as the
velocity.


On Thu, Apr 20, 2017 at 3:01 PM, Shangxin Liu <sxliu at vt.edu> wrote:
> 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
>> >
>> > _______________________________________________
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>> >
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>



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