[FLASH-USERS] confusion regarding abar and zbar

[Jonathan Slavin]

Ah, okay that wasn't clear to me.  That will work for this case without
cooling, but what this is essentially doing is decreasing the mean mass per
particle as a kludge for including electron pressure.  When cooling is
included then, the number density, n = rho/m, will be too high.  I'm
referring to optically thin radiative cooling, e.g. using the Sutherland &
Dopita cooling curve.  In that case what you need is n(H)*n(e).  I suppose
I could kludge that as well...

Jon

On Wed, Sep 16, 2015 at 11:30 AM, Dean Townsley <Dean.M.Townsley at ua.edu>
wrote:

> 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> 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       60 Garden Street, MS 83
>> phone: (617) 496-7981 <%28617%29%20496-7981>       Cambridge, MA
>> 02138-1516
>> cell: (781) 363-0035 <%28781%29%20363-0035>             USA
>> ________________________________________________________
>>
>>
>>
>
>
> --
> ________________________________________________________
> Jonathan D. Slavin                 Harvard-Smithsonian CfA
> jslavin at cfa.harvard.edu       60 Garden Street, MS 83
> phone: (617) 496-7981       Cambridge, MA 02138-1516
> cell: (781) 363-0035             USA
> ________________________________________________________
>
>
>


-- 
________________________________________________________
Jonathan D. Slavin                 Harvard-Smithsonian CfA
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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