[mesa-users] numerical noise

Bill Paxton paxton at kitp.ucsb.edu
Thu Jun 6 12:49:28 EDT 2013

On Jun 6, 2013, at 6:14 AM, chenhl wrote:

> Dear all,
> I am using MESA for binary evolution. But I found a lot of numerical noise in the calculation. For example, the mass transfer rate curve. Is there any way to reduce the noise and make the curve smooth?


I think it might be a good exercise for you to study the details of how the mdot is set during RLO.
It is a tricky game!  For the "explicit" version, mdot is set as an exponential function of the relative overflow
at the start of the step --- exp((R-RL)/Hp), R=radius of star, RL=roche radius, Hp = scale height.
So if the R has jumped a little, mdot will jump a lot!   The "implicit" version of the binary code
iterates to find an mdot such that at the end of the step it will have produced a new R that is very
close to RL.  It tries different mdots until it finds one that gives the desired match.  But don't
expect this to give a smooth evolution for mdot!  Try it and see.   And spend some "quality time"
reading the routine rlo_check_model and the things it calls in star/binary_systems/rlo_routines.

You might try changing some of the control parameters that I explained yesterday
(email attached below).  E.g., a smaller value of fr should cause the code to use smaller timesteps
during stages of evolution when the relative overflow is changing rapidly.  

Good luck!


Begin forwarded message:

> From: Bill Paxton <paxton at kitp.ucsb.edu>
> Date: June 5, 2013 1:27:30 PM PDT
> To: Jaime Villaseñor <jaime.villasenor at postgrado.uv.cl>
> Subject: Re: [mesa-users] Evolving a pre-CV
> On Jun 5, 2013, at 1:10 PM, Jaime Villaseñor wrote:
>> Ok, great! Everything is working now as I wanted with the rlo test.
>> Just two more questions: Where can I find the angular momentum loss equations that mesa uses?
> The code lives in star/binary_systems/rlo_routines.inc
> Our implementation was originally done with help from Saul Rappaport.
> So you won't be surprised to learn that we use the "classic" scheme from
> rappaport, verbunt, and joss.  apj, 275, 713-731. 1983.
> You can find that plus the GR term and the term for mass ejected from the system
> in the function "get_jdot".   There is even a term for tidal Jdot.
>> and, what is the fr parameter in the 'inlist_rlo' file?
> There are 4 cryptically named parameters that are used in setting timesteps.
> They are specifying target per-timestep fractional changes for various things.
> fm	-- fractional change in mass of envelope
> fa	-- fractional change in binary separation
> fr	-- fractional change in relative gap between R and RL, (RL-R)/R
> fj	-- fractional change in angular momentum
> here are the default values:
>         fm = 0.01d0
>         fa = 0.01d0
>         fr = 0.05d0
>         fj = 0.01d0
> cheers,
> Bill
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