[FLASH-USERS] MHD staggered mesh (Flash4a)

[Aaron Froese]

Hi Seyit,

MHD is a single-fluid approximation for frequencies that are much slower than the electron plasma frequency.  Therefore, the electrons are assumed to be in quasineutral equilibrium with the ions; their abundance does not evolve independently of the ions.  The quasineutral assumption is not valid in the sheath, where the plasma is in contact with a solid surface or expanding into a vacuum, but both these situations can typically be avoided for most astrophysical problems.

I have not made the jump to Flash 4 yet, but I have a related question for someone more knowledgeable.  Why do the ionization and multitemperature units, for example, need an ELEC species defined?  Surely it's mass abundance is always zero?

Aaron


Message: 1
Date: Thu, 14 Jul 2011 12:06:34 +0200
From: Seyit Hocuk <seyit at astro.rug.nl>
Subject: [FLASH-USERS] MHD staggered mesh (Flash4a)
To: flash <flash-users at flash.uchicago.edu>
Message-ID: <4E1EBFAA.6000504 at astro.rug.nl>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

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