[mesa-users] lnE_flag

[Bill Paxton]

Hi Jing,

The lnE_flag is related to the new alternative way of calculating eps_grav in mesa/star.
Previously, we only provided the form equivalent to
	eps_grav = -T*dS/dt
Now, you have the alternative of setting use_lnE_for_eps_grav = .true. to use this form:
	eps_grav = (dE/dt + P dV/dt), where E is specific internal energy and V = 1/rho
This form is better in that it includes composition changes, but you need to be careful
where you use it -- in some situations such as nova bursts the dE/dt and P dV/dt terms
both become large but with opposite signs, i.e., eps_grav should be nearly zero as
the result of the cancellation of two large values.  That's a very nasty situation numerically,
so you can get bad convergence.  For cases like that, you are better off using the first
form of eps_grav (i.e., the -T*dS/dt form).

Normally, the new, more accurate, form calculates dE/dt by taking E from the eos.
But I've been experimenting with adding lnE as another primary variable for use with
higher order solvers.  That's still a work in progress that isn't ready for general use
(and may never be!).   So please leave lnE_flag .false. for now.

-Bill





On May 10, 2012, at 7:37 PM, jingluan at caltech.edu wrote:

> Hello :-)
> 
> I use mesa4028 and try to build a 2.5Msun star from pre-MS. I set lnE_flag
> to be true in my inlist_project. And mesa is stuck at some point in
> pre-MS. I find that the problem is in eps_grav calculation in
> star/private/hydro_vars.f;
> 
> When I force hydro_vars.f to call do_eps_grav_using_E rather than
> do_eps_grav_using_E_var by revising Line 813 in hydro_vars.f into
> 
> if (s% lnE_flag .and. .false.) then
> ,
> 
> mesa can then happily evolves the star from pre-MS to MS. I attach my
> inlist_project and hydro_vars.f.
> 
> May some one understand and kindly explain what's wrong with
> do_eps_grav_using_E_var please?
> 
> And I do not really understand what specific purpose lnE_flag=.true. is
> for. Is it for some special case, e.g. some special phase of stellar
> evolution or some special stars?
> 
> Thank you so much :-)
> 
> Sincerely,
> Jing<hydro_vars.f><inlist_project>------------------------------------------------------------------------------
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