[FLASH-USERS] Regarding FLASH simulation basics

[phz238490]

Dear Flash users,
My name is Mohammad Yasir, and I have recently started my doctoral 
studies. As part of my work, I am looking to simulate laser-matter 
interactions and FLASH is going to be part of my repertoire.

I have been going through the FLASH user manual and while the running 
process itself is straightforward, I am unable to understand how to 
setup my own simulations. Here us what I know so far:
1. To setup a simulation, there are a number of files I must define, 
including config file, a few f90 files and flash.par file. Then compile 
the program and execute. That sounds straightforward.
2. I know that the config file contains information about required 
headers, runtime parameters, etc. I know what each f90 file is supposed 
to do. But that is where it gets muddy for me. I would be obliged if you 
could help me understand some things.

1. I can figure out what units to use depending on the physics I 
require. But what about the runtime variables, etc. I went through the 
test Sedov problem. It defines   sim_rhoAmbient and such variables. But 
why and where is this used? Theoretically, what's stopping me from 
defining a variable called my_dummy_variable, giving it a value. And if 
I did define it, how would FLASH know what to do with it?
2. To setup a simulation, which variables SHOULD I define?
3. Once defined, how does FLASH know what these variables are, how they 
evolve, and what to do with them?

As someone who is more accustomed to PIC simulations using EPOCH/SMILEI, 
this is new territory for me, and I apologize if the questions don't 
make sense. If someone has a resource that explains the process better, 
or someone has ideas on how to make it simpler, kindly let me know.


Regards,
Mohammad Yasir
Research Scholar
Department of Physics