[FLASH-USERS] Problems with 2.5D cylindrical disk using FLASH 4.2.2 unsplit solver

[Andrea Derdzinski]

Hi Sean,

It turns out the issue was in boundary conditions--particularly the upper z
boundary was not set to provide pressure support against gravity. I
set fixed boundary conditions to ensure hydrostatic equilibrium, and
although there are some spurious velocities in low density regions (they
are minimal) the disk stays steady after several dynamical times.

I noticed that FLASH has an option to set hydrostatic boundary conditions,
and that there are two versions (Flash2HSE and Flash3HSE) but I couldn't
find any documentation that described the difference between these two
versions. Can anyone explain why the Flash3 implementation of hydrostatic
boundary conditions may be better than the Flash2 one?
Do they both use the same method described in Zingale et al 2002?
http://adsabs.harvard.edu/abs/2002ApJS..143..539Z)?
While the current HSE boundary conditions assume uniform gravity, I am
interested in implementing hydrostatic boundary conditions that work with
spatially dependent gravitational acceleration.

Thanks
Andrea


On Tuesday, April 14, 2015, Sean Couch <smc at flash.uchicago.edu> wrote:

> Hi Andrea,
>
> I have used the 2.5D UHD approach and it seems to work for me.  Behavior
> is as expected, though I guess I have not done an extremely thorough job of
> testing the angular momentum conservation.  Do you have a simple test
> problem that could be used?
>
> Sean
>
>
> -----------------------------------------------------------
> Sean M. Couch
> Theoretical Astrophysics Including Relativity
> California Institute of Technology
> Mail Code 350-17
> Pasadena, CA  91125
> (626) 395-4282
> www.tapir.caltech.edu/~smc
>
> On Apr 13, 2015, at 12:42 PM, Andrea Derdzinski <
> aderdzinski at astro.columbia.edu> wrote:
>
> Hi everyone,
>
> I'm trying to simulate an isothermal disk with in 2.5D cylindrical
> geometry (r,z) around a point mass (with the computational domain outside
> of the origin). To my understanding I need to use the unsplit Hydro solver
> in order to conserve angular momentum (
> http://flash.uchicago.edu/pipermail/flash-users/2012-September/001163.html).
>
>
> There is no viscosity, so this is essentially a rotating fluid. Initially
> I want the fluid to be steady, with no radial velocity (v_r = 0) and no
> vertical velocity (v_z = 0), only a rotational phi velocity. I solve for
> the initial conditions analytically to ensure that there is both radial and
> vertical support, but at the first time step there seems to be a lack of
> vertical pressure support and the gas develops a negative vertical
> velocity.
>
> Analytically the gravitational force is balanced by the pressure gradient
> force, but this doesn't seem to be the case in the simulation. The issue
> does not go away with higher resolution. I've checked that the isothermal
> condition is enforced in the EOS unit, and that the gravitational
> acceleration is implemented correctly.
>
> Has anyone had success running a simulation in 2.5D cylindrical geometry
> using the unsplit solver? I'd appreciate hearing about any issues other
> users have ran into or any advice.
>
> The unsplit setup:
> order = 3
> slopeLimiter = "mc"
> RiemannSolver = "HLLC"
> conserveAngmom = .true.
> use_gravhalfupdate = .true.
> fluxcorrect = .true.
>
>
>
> Thanks,
> Andrea
>
>
>
>
>
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