[FLASH-USERS] Help with configuring Flash to run with astronomical Units

[Hansen, Eddie]

Most of FLASH (especially the extended MHD routines) is coded to work in a scaled CGS system (the code’s default UnitSystem = “none”). You are free to use large numbers for runtime parameters like ‘xmax’ and ‘tmax’ that would equate to parsecs and years, but they still need to be in cm and s. You can define scaling factors as you suggested for convenience to use inside a Simualtion_initBlock with custom ‘sim_’ initialization runtime parameters, but whatever goes into the data array (often named ‘solnData’) in Simulation_initBlock still needs to be in CGS.

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Eddie Hansen
Applications Group Leader
Flash Center for Computational Science


From: flash-users <flash-users-bounces at flash.rochester.edu> on behalf of Sumit Banerjee <sumitb at g.clemson.edu>
Date: Sunday, July 20, 2025 at 10:30 AM
To: flash-users at flash.rochester.edu <flash-users at flash.rochester.edu>
Subject: [FLASH-USERS] Help with configuring Flash to run with astronomical Units
Flash users,

I need your help in configuring Flash to run astronomical units (on scales of about a few parsecs and over a few thousand years to start with) instead.

I scanned through the FLASH documentation to get some cues on how it is done but couldn't get anything. I also tried adding parameters like unit_length, unit_density and unit_time to the par file but they are ignored during runtime and the simulation falls back to the base units.

Any help on this would be appreciated. I am looking forward to hearing from you all.

Clear Skies,
Sumit (Sam)
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