[FLASH-USERS] Problem with Diffuse module

[Slavin, Jonathan]

Hi Stefano,

The problem that you're running into is that the timestep limitation for
stability of explicit diffusion calculations is typically very short.
However, you can get around this if you use implicit or semi-implicit
diffusion (Crank-Nicholson).  What are you setting for diff_thetaImplct?  I
think the default value is 0.5 which corresponds to Crank-Nicholson
diffusion (0 is explicit and 1 is implicit).  In this case the timesteip
limitation should not really apply and you can set dt_diff_factor to
something large.  That multiplies the dt limit imposed by diffusion by that
factor allowing larger timesteps.  I set it to a value large enough, 1.E10,
so that the dt set by the Diffuse unit doesn't limit the timestep.

There has been some work showing that in some cases using Crank-Nicholson
does not make the simulation unconditionally stable, but in my experience
it generally has been.

Regards,
Jon

On Mon, Nov 27, 2017 at 1:00 PM, <flash-users-request at flash.uchicago.edu>
wrote:
>
> From: ascenzi <stefano.ascenzi at roma2.infn.it>
> To: Flash users <flash-users at flash.uchicago.edu>
> Cc:
> Bcc:
> Date: Mon, 27 Nov 2017 12:14:01 +0100
> Subject: [FLASH-USERS] Problem with Diffuse module
> Dear FLASH users,
>
> I am trying to simulate an expanding optically thick spherical shell of
> matter with a source in the centre (source described by user defined
> boundary conditions).
>
> I tried to switch on the thermal diffusion using a power law prescription
> for the conductivity in the form:   sigma = K0 T**3 rho**(-1) in order to
> reproduce the thermal radiative transfer of the star described by a
> diffusive approach, where the flux is F = -K0 T**-3 rho**(-1) grad T.
>
> Unfortunately when I switch on the thermal diffusion the timestep
> (diffusion dt) becomes extremely small, which makes the simulation
> impossible to handle. Actually I notice that my dt rise monotonically until
> I obtain a warning message (that I had several times also at the beginning
> of the simulation with no diffusion):
>
> [gRSt] afterGeo fallback to order 1 for DIR_X at i,j=           5
>  1  in Block          10 @           0
>  [dR1St] afterGeo   3.3108486768816685        9.2775874623827751E-008
>  275453229333938.16       -86.817733542062342
>
> At this point the dt drops, reaches a minimum and then rise again until it
> reach almost the same value as before and the warning message appears
> again, dt drops and so on in a sort of “saw tooth” fashion. (I notice also
> that in this “saw tooth” fashion there is a very slow drift towards higher
> values of dt, but this drift is still too slow to be useful).
>
> I found that the warning message is probably called by the routine
> “hy_uhd_getRiemannState” and is due to the fact that the pressure is
> negative somewhere.
>
> Do you know how can I solve this warning and why the timestep is so small
> (about 10^-8 s against 1 s of the case with no diffusion)? I put a
> diffusion cutoff at rho=5*10^-12 g/cm^3 .
>
> Thank you very much,
>
> Stefano
>
> P.S. I am using the Unsplit diffusion but also with FluxBased the behavior
> is similar.
>
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>
-- 
________________________________________________________
Jonathan D. Slavin                 Harvard-Smithsonian CfA
jslavin at cfa.harvard.edu       60 Garden Street, MS 83
phone: (617) 496-7981       Cambridge, MA 02138-1516
cell: (781) 363-0035             USA
________________________________________________________
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