[FLASH-USERS] combining MHD and thermal conduction

[Tomasz Plewa]

It might be worth noting that Crank-Nicolson does not automatically 
guarantee the solution will be physically correct. The solution might be 
stable, but positivity is not guaranteed for arbitrary large time steps.

http://www.sciencedirect.com/science/article/pii/S0377042712004128
https://www.encyclopediaofmath.org/index.php/Crank-Nicolson_method

Tomek
--
On 08/14/17 17:50, Slavin, Jonathan wrote:
> ​Hi Ernesto,
>
> I maybe should have mentioned that my runs have been in 2D 
> (cylindrical).  So have you done such runs?  Also for the conduction, 
> I'm using unsplit diffusion with electron thermal conduction and power 
> law conductivity. You mention being careful with split vs. unsplit 
> diffusion, but you don't say which one works - or what that means to 
> be careful.​
> ​ Or is it just that parameter setting addThermalFlux? I'll try 
> setting that to False to see if that helps.
>
> To answer your question, I'm not using super-time-stepping.  Didn't 
> seem necessary since with Crank Nicholson diffusion you have 
> unconditional stability.  So I haven't looked into how to use that.
>
> Thanks for the suggestions.
>
> Jon ​
>
>     ---------- Forwarded message ----------
>     From: ERNESTO ZURBRIGGEN <ezurbriggen at unc.edu.ar
>     <mailto:ezurbriggen at unc.edu.ar>>
>     To: flash-users at flash.uchicago.edu
>     <mailto:flash-users at flash.uchicago.edu>
>     Cc:
>     Bcc:
>     Date: Mon, 14 Aug 2017 12:18:31 -0300
>     Subject: Re: [FLASH-USERS] combining MHD and thermal conduction
>     Hi Jon!
>
>     >It made me wonder if there is some numerical way that the
>     implementations of thermal conduction and MHD interfere with each
>     other.
>
>     I have used MHD+(isotropic) Conduction without any problem of that
>     sort. One confusing thing is that you can select Diffusion/split
>     and Diffuse/unsplit. Be careful with that.
>
>     Something curious is that if the simulation include unsplit/MHD +
>     (regardless of split or unsplit) Diffusion you have to also set
>     the runtime parameter addThermalFlux=.false. (.true. by default)
>     Otherwise, you are including the thermal conduction effect twice,
>     i.e., heat diffusion via the HYPRE libraries decoupling the heat
>     equation, and adding the thermal flux to the total energy flux in
>     the Hydro unit.
>     I think the most consistent configuration would be unsplit/MHD +
>     Diffusion/unsplit + addThermalFlux=.false.
>
>
>     >I tried reducing the conductivity further and found that only by
>     reducing it by many orders of magnitude (~8) was I able to get
>     runs that didn't crash
>
>     Maybe you can try reducing the parameter dt_diff_factor.
>
>     Are you using super-time-stepping?
>
>     Best!
>     Ernesto.
>
> -- 
> ________________________________________________________
> Jonathan D. Slavin                 Harvard-Smithsonian CfA
> jslavin at cfa.harvard.edu <mailto: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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