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<p class="MsoNormal">Hi Adam,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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.
<o:p></o:p></p>
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<p class="MsoNormal">Best,<o:p></o:p></p>
<p class="MsoNormal">Tim Johnson<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> Reyes, Adam <adam.reyes@rochester.edu> <br>
<b>Sent:</b> Monday, April 15, 2024 2:47 PM<br>
<b>To:</b> Timothy Mark Johnson <tmarkj@mit.edu><br>
<b>Cc:</b> flash-users@flash.rochester.edu<br>
<b>Subject:</b> Re: [FLASH-USERS] Stability and timestep issues with 3D Biermann3T Laserslab simulations<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Hi Tim,<o:p></o:p></p>
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<p class="MsoNormal">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 <a href="https://flash.rochester.edu/site/flashcode/user_support/flash4_ug_4p7/node103.html#SECTION06235200000000000000">user
guide here</a>. 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.<o:p></o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif;color:black">*********************************************<br>
Adam Reyes<o:p></o:p></span></p>
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<img border="0" width="304" height="112" style="width:3.1666in;height:1.1666in" id="Picture_x0020_1" src="cid:image001.jpg@01DA9000.B0030A80"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif;color:black"><br>
Code Group Leader, Flash Center for Computational Science <br>
Research Scientist, Dept. of Physics and Astronomy<br>
University of Rochester<br>
River Campus: Bausch and Lomb Hall, 369 <br>
500 Wilson Blvd. PO Box 270171, Rochester, NY 14627<br>
Email </span><a href="mailto:adam.reyes@rochester.edu"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">adam.reyes@rochester.edu</span></a><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif;color:black"><br>
Web </span><a href="https://flash.rochester.edu"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">https://flash.rochester.edu</span></a><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif;color:black"><br>
(he / him / his)<br>
</span><img border="0" width="304" height="101" style="width:3.1666in;height:1.0555in" id="Picture_x0020_2" src="cid:image002.png@01DA9000.B0030A80"><o:p></o:p></p>
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*********************************************<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal">On Apr 15, 2024, at 8:12 PM, Timothy Mark Johnson <<a href="mailto:tmarkj@mit.edu">tmarkj@mit.edu</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Hi FLASH users,<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">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.<span class="apple-converted-space"> </span><o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">For the setup command, I’ve swapped +uhd3t for +usm3t from the Laserslab setup and made the simulation 3D cartesian.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">Best,<o:p></o:p></p>
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<p class="MsoNormal">Tim Johnson<o:p></o:p></p>
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