[FLASH-USERS] confusion regarding abar and zbar

[Dean Townsley]

Hi Johnathan,

The gamma-law EOS is just a simple ideal gas as described in section 
15.2 of the user's guide:
http://flash.uchicago.edu/site/flashcode/user_support/flash4_ug_4p3/node22.html#SECTION06320000000000000000

As you can see from the expressions there, zbar is not used at all. You 
should set abar so that these expressions give what you expect. This is 
what John Zuhone and Tomek I believe are suggesting.

Hopefully that helps clarify,
Dean




On 09/16/2015 10:14 AM, Jonathan Slavin 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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