[FLASH-USERS] Problem with Diffuse module

[ERNESTO ZURBRIGGEN]

Stefano,
just to add a tricky detail.
If you are running a simulation using the unsplit hydro solver (
*Hydro/HydroMain/unsplit/Hydro_Unsplit*),
you have to set the runtime parameter addThermalFlux=.false. (.true. by
default), otherwise, you will be adding
the isotropic thermal conduction effect twice.

You might be interested in this old post:

http://flash.uchicago.edu/pipermail/flash-users/2017-August/002388.html

Best!
Ernesto.

2017-11-27 16:20 GMT-03:00 Slavin, Jonathan <jslavin at cfa.harvard.edu>:

> 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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>> flash-users at flash.uchicago.edu
>> http://flash.uchicago.edu/mailman/listinfo/flash-users
>>
> --
> ________________________________________________________
> 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
> ________________________________________________________
>
>


-- 
*Dr. Ernesto Zurbriggen*

*Consejo Nacional de Investigaciones Científicas y Técnicas **(CONICET)**,
Instituto de Astronomía Teórica y Experimental (IATE). *

*Universidad Nacional de Córdoba **(UNC)**, **Observatorio Astronómico de
Córdoba **(OAC)**. *

*Teléfono: +54 0351 4331064-5, interno 222. *

*Laprida 854, Bº Observatorio (CP 5000), Córdoba, Argentina.*
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