[FLASH-USERS] How to properly calculate sound speed

[Carlo Graziani]

Hi Seyit.

To amplify a bit on Tomek:  as documented in the User's Guide, at
http://flash.uchicago.edu/website/codesupport/flash3_ug_3p2/node21.html#SECTION06220000000000000000
gamma_c is in fact a calculated thermodynamic variable, emitted by
helmoltz_eos, defined as (rho/P)(dP/drho).  It is definitely not a
constant, and in fact the local speed of sound is correctly calculated
using the formula c^2 = dP/drho = gamma_c * P / rho.

Cheers,

Carlo

> Seyit -
> 
> The sound speed is one of the quantities defined by the equation of 
> state, and so the gamma(s) in general are as well. Constant gamma is 
> essentially a fairly good model applying only to relatively simple 
> gases. As you already implied, the value of sound speed does not depend 
> on physics processes per se but rather on the thermodynamic state of matter.
> 
> Also,
> 
> C^2 
> Reviewing a few chapters in the thermodynamics textbook might be most useful, and you may want to take a look at
> 
> http://en.wikipedia.org/wiki/Speed_of_sound
> 
> Tomek
> ---
> 
> Seyit Hocuk wrote:
> > Hi all,
> >
> > Within Flash, the speed of sound is calculated by "C > a constant predefined gamma. However, if you have additional physics 
> > like heating, cooling, or radiative transfer effects, gamma should vary. 
> > Hence, the sound speed is then calculated wrong. Gamma is in fact 
> > "dlog(P)/dlog(rho)" and the sound speed is actually "dP/drho".
> >
> > How can one approximate, if not solve, the sound speed more carefully?
> >
> > One basic idea I had was to derive 'C' for a zone using a neighboring 
> > zone (P2-P1/rho2-rho1) and to take the average of 'C' for an entire 
> > block. That doesn't work very well.
> >
> > Kind regards,
> > Seyit
> >   

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