[FLASH-USERS] How to properly calculate sound speed

[Mike Zingale]

One additional comment, gamma_1 is that derivative (dlogP/dlogrho) at 
constant entropy.  The link to the FLASH manual seems to omit what is held 
constant.

On Tue, 2 Feb 2010, Carlo Graziani wrote:

> 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)
> University of Chicago      -------------------------------------
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>


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Assistant Professor

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