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

[Max Rudolph]

>
> 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.
>

measured in what units?
i.e. does gravity in m/s/s have to be of the same order of magnitude as a
plate velocity in cm/year?



>
> 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
> >> >
> >> > _______________________________________________
> >> > Aspect-devel mailing list
> >> > Aspect-devel at geodynamics.org
> >> >
> >> > https://urldefense.proofpoint.com/v2/url?u=http-3A__lists.
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>
>
> --
> Timo Heister
> http://www.math.clemson.edu/~heister/
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