[FLASH-USERS] How to properly calculate sound speed

[Carlo Graziani]

On 02/03/2010 10:13 AM, Klaus Weide wrote:
> On Wed, 3 Feb 2010, Seyit Hocuk wrote:
> 
>> Very basic question maybe, but does one have to use Helmholtz eos to get that
>> gamma? 
> 
> Seyit,
> 
> If you are not using the Helmholtz EOS then I assume you are using either 
> the Gamma EOS or the Multigamma EOS.  In both of those cases, the 
> adiabatic index gamma already has a well-defined meaning, so it would not 
> make sense for the FLASH Eos unit to provide a different (and conflicting)
> way to compute gamma.
> 

In other words: if you're using Multigamma, you already have gamma_c defined
in the way you want.  If you look carefully at
source/physics/Eos/EosMain/Multigamma/Eos.F90, and trace a few of the
calls, you'll see that gamma_c is defined according to Eq.14.7 of the
User's Guide
(http://flash.uchicago.edu/website/codesupport/flash3_ug_3p2/node21.html#SECTION06220000000000000000).
That formula is precisely what you get for (dlog(P)/dlog(rho)) at constant
entropy, assuming an admixture of ideal gases with different (constant)
specific heats --- that is, different fixed gammas.  So it is still
precisely what you want to compute the sound speed.

Cheers,

Carlo

-- 
Carlo Graziani                                 (773) 702-7973 (Voice)
Department of Astronomy and Astrophysics       (773) 702-6645 (FAX)
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