[FLASH-USERS] confusion regarding abar and zbar

[John ZuHone]

Hi Jonathan,

What I think Tomek is saying (and what I normally do for the Gamma EOS) is define Abar so that it takes into account the contributions from _both_ the ions and the electrons. 

So what he is saying is that you should set eos_singleSpeciesA to abar/(1+zbar), in your notation. 

Best,

John

> On Sep 16, 2015, at 11:14 AM, Jonathan Slavin <jslavin at cfa.harvard.edu> wrote:
> 
> Yes, that's exactly what I said.  How does one get FLASH to use zbar?  I just tried again after running setup with the added argument
> -with-unit=physics/Eos/EosMain/Gamma
> but I still get that d*T/P * kboltz/amu_cgs = 1.28 (= abar) instead of 0.612 (= abar/(1 + zbar)) like I would expect.
> 
> Jon
> 
> On Wed, Sep 16, 2015 at 10:23 AM, Tomasz Plewa <tplewa at fsu.edu <mailto:tplewa at fsu.edu>> wrote:
> Hi Jon -
> 
> It is frequently assumed electron pressure is negligible compared to ions, which is true for a nonionized gas. Obviously, the correct formula should take into account contributions from both ions and electrons. For a fully ionized gas, 
> 
> \mu = \frac{\sum_j n_j A_j }{\sum_j n_j ( 1 + Z_j) }
> 
> Tomek
> --
> 
> On 09/16/15 10:00, Jonathan Slavin wrote:
>> Hi all,
>> 
>> I'm doing simulations aimed at understanding aspects of supernova remnant evolution.  Since the SNR is expanding the general interstellar medium, the proper mean mass per atom (i.e. neutrals and ions) is about 1.28 amu, given 90% H and 10% He by number.  As I understand it, this is set by the runtime parameter eos_singleSpeciesA, which corresponds to abar.  If I want to assume constant, uniform, complete ionization then, as I understand it, I need to set zbar via the runtime parameter eos_singleSpeciesZ, to a value of ~1.091 = 1.2/1.1 = (n(H+) + 2*n(He++))/(n(H) + n(He)).  At least that's what I thought from the docs where at one point it's stated that n(e) = zbar*n(ion).  However, when I look at density*temperature/pressure*kB/amu, I get abar.  So it seems that zbar is not used.  What I expect is that
>> P = (1 + zbar)/abar*kB*density*temperature
>> Where P is the total pressure (electron pressure + atom pressure).  Is this not true?  Do I need to specify some extra unit for zbar to be used (maybe -with-unit=physics/Eos/EosMain/Gamma)?  Am I misunderstanding the meaning of zbar?  Any help would be appreciated.
>> 
>> Thanks,
>> Jon
>> 
>> -- 
>> ________________________________________________________
>> Jonathan D. Slavin                 Harvard-Smithsonian CfA
>> jslavin at cfa.harvard.edu <mailto:jslavin at cfa.harvard.edu>       60 Garden Street, MS 83
>> phone: (617) 496-7981 <tel:%28617%29%20496-7981>       Cambridge, MA 02138-1516
>> cell: (781) 363-0035 <tel:%28781%29%20363-0035>             USA
>> ________________________________________________________
>> 
> 
> 
> 
> 
> -- 
> ________________________________________________________
> Jonathan D. Slavin                 Harvard-Smithsonian CfA
> jslavin at cfa.harvard.edu <mailto:jslavin at cfa.harvard.edu>       60 Garden Street, MS 83
> phone: (617) 496-7981       Cambridge, MA 02138-1516
> cell: (781) 363-0035             USA
> ________________________________________________________
> 

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