[mesa-users] Proper use of eosDE

[Bill Paxton]

Hi Phil,

Great to hear from you.  I was recently wondering what you were up to these days.

Your questions are an excellent synopsis of why I dislike using density and energy as primary EOS variables in an evolution code.
Given my bias against it, you'll probably get better advice from someone else, for example someone who uses HELM for their hydro code.
The best way to avoid the problems you mention is to use density and temperature as your basic variables so you are in harmony with the input physics.
I can do that with my implicit hydro, including the new version using approximate Riemann HLLC, but it may not be possible for you.  Life is hard.

Good luck - I'm sure you'll find a way to make it work.

Bill




On Jul 22, 2017, at 11:04 PM, Philip Chang wrote:

> Hi,
> 
> I'm integrating the mesa eos module into a hydro code that I am writing/written.  I have lots of experience with Frank Timmes Helmholtz eos (which I got from his website), but for my new code I am going to use the MESA eos.  Anyhow, my hydro code gives me an internal energy and density and I want to get other thermal quantities, i.e., temperature, entropy, adiabatic index, etc.
> 
> The way to do this is using eosDE_get, but I'm not sure what the proper use of this was and was hoping that someone knew offhand. I am using the latest public release 9793.  In particular, my questions are:
> 
> 1. How does one decide on a proper T_guess?  I looked in the mesa-star code, and it appears that one want to use the temperature from the previous step.  I'm wondering how good is this as a shock can move though a cell and totally mess everything up.
> 
> 2. An improper T_guess can usually be corrected, i.e., it produced values that are obviously wrong.  I have tried a guess of 1e2 and 1e8 and usually if one fails then the other gives you a good value. But this is not always the case, for instance, consider this particular pathological case: rho = 794.32840915212967, T=158489.31924611141, solar metallicity.
> 
> For T_guess = 1e7, I find T = 202665.40667059086
> 
> For T_guess = 1e2, I find T = 257198.81215480756
> 
> In this case both are wrong by order unity.  This is a particular bad case as both guess give poor answers.  But there are cases where one guess give a good answer, but the other does not, and it is not obvious which one I should trust.  Any ideas?
> 
> 3. What happens when the internal energy is too low?  For instance, in my previous experience with the Helmholtz equation of state (from Frank Timmes website), small fluctuations can drop the internal energy below the fermi energy.  When this energy is use to try to find a temperature in the Helmholtz equation of state, the code essentially crashed (as it should).  In this case, I prechecked the internal energy to ensure that it is valid.  What happens if I give the eosDE_get an internal energy that is physically too small.
> 
> Thanks in advance for any help.
> 
> Cheers,
> 
> Phil Chang
> 
> 
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