[FLASH-USERS] Stability and timestep issues with 3D Biermann3T Laserslab simulations

[Reyes, Adam]

Hi Tim,

The time step limitation and subsequently the numerical instability I suspect is due to the requirement of the biermann3T (required for 3D cartesian) switch to also include the hall MHD term. The reasoning is explained a bit in the user guide here <https://flash.rochester.edu/site/flashcode/user_support/flash4_ug_4p7/node103.html#SECTION06235200000000000000>. The gist is that the flux-formulation of the biermann electric field results in a Poynting flux that can advect magnetic energy, that implies some electron enthalpy transport with velocity u_e = (u - J/en_e), hence the hall term.

You might try relaxing this requirement in "source/physics/Hydro/HydroMain/unsplit/Hydro_init.F90” and allow “hy_usehall=.false.”, but I can’t speak to the physical consistency of such a choice.
*********************************************
Adam Reyes


Code Group Leader, Flash Center for Computational Science  
Research Scientist, Dept. of Physics and Astronomy
University of Rochester
River Campus: Bausch and Lomb Hall, 369  
500 Wilson Blvd. PO Box 270171, Rochester, NY 14627
Email adam.reyes at rochester.edu
Web https://flash.rochester.edu
 (he / him / his)


*********************************************



> On Apr 15, 2024, at 8:12 PM, Timothy Mark Johnson <tmarkj at mit.edu> wrote:
> 
> Hi FLASH users,
>  
> I’ve been trying to do some 3D FLASH MHD simulations of laser ablation focusing on the Biermann battery generated magnetic field. I’ve run into issues with stability and a very small time step that make the simulations infeasible. As a test example, I took the Laserslab example.par simulation, moved it to 3D, and added Biermann3T. Attached is the .par file that I’m using. For a CFL of .1, the simulation starts out with timesteps of about 1e-19 and crashes (negative 3T) after about a thousand time steps. It also crashes for CFLs of .05 and .025. 
>  
> Does anyone have advice for dealing with stability issues and small time steps in 3D Biermann3T simulations? I’m hoping to be able to run my simulation for a few nanoseconds.
>  
> For the setup command, I’ve swapped +uhd3t for +usm3t from the Laserslab setup and made the simulation 3D cartesian.
>  
> Best,
> Tim Johnson
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