[FLASH-USERS] Multiple Species Hydro+MHD Simulation

[Klaus Weide]

On Tue, 21 Jun 2016, Weston Lowrie wrote:

> Hi Klaus,
> 
> Thanks for the helpful response.  I think I understand the issues better
> now.
> For my option (1) it would probably involve creating a new Unit, but I
> gather it may be pretty complicated to tie all the other existing
> Units/physics such that they can work together?

Yes.

[This message was written before I saw Mordecai Mac Low's response 
pointing to the implementation of Duffin & Pudritz for ambipolar 
diffusion. I am not familiar with that implementation, the below applies to 
the unmodified MHD implementations of FLASH.]

> Option (2) or the 3T mode seems like it would be the easiest path.  Do you
> know how the MHD unit works with different species states?  For instance
> would only the ionized species be affected by the magnetic field?  I
> imagine the "bulk" density of all species is advanced in the momentum
> equation, and therefore the field impacts the whole fluid?

Yes to the latter. There is only one "fluid", with just one macroscopic 
velocity field, in the MHD approximation. And MHD does not care how the 
fluid is made up of ions of different charge states, all it cares about 
are macroscopic quantities, including the electron number density (ONE 
scalar field for the whole fluid). So in this approximation, there is no 
way to describe different macroscopic behavior of the ionized part of the 
fluid vs the part consisting of neutrals. What matters to MHD is the 
resulting electron density.

All this is pretty much implied by the system of equations that FLASH's 
MHD solver purports to solve (as documented in the U.G. and other places).

You should find it easy to just add some species to an existing test - 
come up with some reasonable initial condition, and implement it in 
Simulation_initBlock; use a Multispecies Eos - and visualize how this 
changes the behavior of the other quantities (or not).

Klaus