[FLASH-USERS] Multiple Species Hydro+MHD Simulation

[Mordecai-Mark Mac Low]

Dear Weston,

Duffin & Pudritz (2008, doi: 10.1111/j.1365-2966.2008.14026.x) did implement a *single-fluid* ion-neutral drift algorithm into Flash (i.e. neglecting ion inertia).  Not sure if that would be sufficient for your problem, but you might want to take a look at it.

Best,

Mordecai


On Jun 21, 2016, at 17:19, Klaus Weide <klaus at flash.uchicago.edu> wrote:

> On Fri, 17 Jun 2016, Weston Lowrie wrote:
>> I'm looking to create a simulation where I have two bulk species
>> (materials?) (i.e. ions and neutrals) where the ions will be bound by the
>> MHD equations, and neutrals by hydro only equations.  Each of these "bulk"
>> quantities will have their individual species.
> 
> Hello Weston,
> 
> What you should do depends on what you need.
> 
> - If your two "bulk species" need to be modeled with each having its 
> own "bulk" velocity - what we might call a "two-fluid" approach - then
> significant code changes in FLASH would be required. Maybe as per your
> option 1) (below), or in some other way that extends the set of variables
> in a way that, AFAIK, has not been done with FLASH so far.
> 
>> It is unclear to me if I need to:
>> 1) Setup two Units, one for the neutrals (hydro) and then another for the
>> ions (MHD) so that they are evolved separately.  If this is the case how do
>> I differentiate between the different densities, temp, velocities, etc?
> 
> The Hydro implementations under HydroMain/unsplit/Hydro_Unsplit and 
> HydroMain/unsplit/MHD_StaggeredMesh (or any of the other implementations) 
> are not meant to be used together; it would require significant changes
> in those implementations to enable this (if it even could be meaningful to 
> do it).
> 
>> or
>> 2) Just use the MHD Unit and define all the different species (i.e. N, N2,
>> O2, O, O+, N2+, O2+, etc.)  By properly defining the species with their
>> respective charge will the hydro advance correctly?  
> 
> That would be the straightforward approach if you do NOT need to track 
> different velocities. Hydro will advance the species (and thus implicitly 
> the ionization levels) correctly, as long as "advancing the species" means 
> just passive advection.
> 
> Any change in the "composition" of your species mixture, including 
> whether chemical (or nuclear) reactions or just ionization / 
> recombination, would have to be implemented in different units.
> 
> You could use FLASH in the 3T mode if you need to distinguish between the 
> temperature of "ions" (including neutrals) and electrons. However, in 3T 
> mode you would normally not represent different ionization levels (say O 
> vs O+ vs O++) as different FLASH "species", but you would have one species 
> (say O_SPEC) per element* whose ionization level is either implied by 
> temperature (if using a table-based Eos implementation) or fixed (for 
> Gamma-like Eos).
> 
> * or per group of elements that you consider a "material"
> 
>> Maybe I also need to
>> use the Ionization unit?
> 
> The Ionize unit in FLASH provides one specific implementation for 
> following the evolution of ionization levels over time, under certain 
> physical assumptions and under the assumption that all ionization levels 
> of several elements are represented as separate FLASH species.
> This may or may not be applicable to the cases you are interested in.
> (Since you appear to be interested in molecular species, I suspect the 
> answer is no.)
> 
> 
>> I appreciate any help on how to think about this in Flash.
> 
> HTH,
> 
> Klaus

--
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