[FLASH-USERS] MHD staggered mesh (Flash4a)

[Aaron Froese]

All the species that you define are considered fully ionized according to the Z that is given in Simulation_initSpecies.F90.  In order to simulate partial ionization, a new species is necessary for each ionization state.  If you try the NeiTest example, you can see how different ionization states are handled by a unit that uses SimulationComposition.

As I understand it, the electron abundance is not a mass scalar that the Riemann solvers can act upon, but is just a scratch variable to store the result of N_e = sum( Z * N_i(Z) ) for convenience.  It is only necessary for units such as Bremsstrahlung cooling that benefit from having the electron abundance precalculated.

Aaron
________________________________________
From: Seyit Hocuk [seyit at astro.rug.nl]
Sent: Friday, July 15, 2011 3:39 AM
To: Aaron Froese
Subject: Re: MHD staggered mesh (Flash4a)

Dear Aaron,

Thanks for your reply. I still don't understand it completely, please
bear with me. I initialize only two species (H and He), which are
neutral. So it is not important what species there are. In the limit
that you mentioned, is there an assumption that it is fully ionized or
something, or does it not matter anymore?

What if I want to add more species, electron abundances, that change in
time. For example, I want to use rate equations that calculates the
abundances of the species. Things might become more ionized/neutral in
time due to some radiation field.

Kind regards,
Seyit



Aaron Froese wrote:
> 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
>