[FLASH-USERS] MHD staggered mesh (Flash4a)

[Seyit Hocuk]

Dear Flash developers,

I am testing the USM solver a bit in Flash4-alpha and noticed some 
curious features. I use my standard setup, which I normally use with 
split PPM, and do +usm in the setup command. Everything looks and runs 
fine, however, whenever I increase the number of processors, dt_hydro 
seems to drop by an equal amount. I do not use super-time-stepping, 
which I don't understand yet.

Another question I have is that if it is normal that USM MHD requires so 
much more (ram) memory than PPM. It is about 3-4 times more memory 
intensive. I noticed that there are much more (about 3 times more) 
variables than in PPM. These are mainly the scratch and the flux 
variables. So, is it normal that USM requires this much memory?

Lastly, I am new to magnetic field studies and thus not so familiar how 
to implement them. The way I implement magnetic fields is as follow: I 
use my standard setup +usm and in Simulation_initBlock, I give 
reasonable values to center values of MAGX/Y/Z and also do the same for 
the face values of MAG (facex, facey, facez), similar to how it is done 
in the supplied test runs. My run differs from these test runs in the 
fact that I have gravity (and particles) included. I'm curious how the 
magnetic fields will be amplified/weakened over time. I'm also wondering 
why it is not necessary for the code to know the ion/electron abundance. 
Is this assuming some flux freezing state?


Kind regards,
Seyit