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

[Timothy Mark Johnson]

Hi Adam,

Thanks for your reply. Your comment inspired me to look at the source code more. I found that there are two “methods” for doing the Biermann3T flux in the hy_uhd_addBiermannFluxes.F90 file. I’ve tried out method number 2 and I got much better stability and time stepping. Is method 2 alright to use? I see that there are differences in how the electric field is calculated between the methods, but I’m not sure of the significance of the differences.

Best,
Tim Johnson

From: Reyes, Adam <adam.reyes at rochester.edu>
Sent: Monday, April 15, 2024 2:47 PM
To: Timothy Mark Johnson <tmarkj at mit.edu>
Cc: flash-users at flash.rochester.edu
Subject: Re: [FLASH-USERS] Stability and timestep issues with 3D Biermann3T Laserslab simulations

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

[cid:image001.jpg at 01DA9000.B0030A80]
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<mailto:adam.reyes at rochester.edu>
Web https://flash.rochester.edu
 (he / him / his)
[cid:image002.png at 01DA9000.B0030A80]

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On Apr 15, 2024, at 8:12 PM, Timothy Mark Johnson <tmarkj at mit.edu<mailto: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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