[mesa-users] Timestep control

Bill Paxton paxton at kitp.ucsb.edu
Tue Aug 26 13:13:11 EDT 2014


Hi Mathieu,

Okay: let's add some more controls explicitly based on timescales.
These need to be stated in the form of a ratio of timestep (dt) to timescale.
If the ratio is too large, then the "select timestep" routine reduces the next dt.

[let's not get into debates about order unity prefactors
as long as we are explicit about the definitions we are using.]

"global" timescales for entire star:
dt_div_dt_thermal 
	! dt_thermal = (3/4)*G*M^2/(R*L); Kelin-Helmholtz time
dt_div_dt_dynamic 
	! dt_dynamic = 2*Pi*sqrt(R^3/(G*M))
dt_div_acoustic_radius 
	! acoustic_radius = time for sound from center to photosphere
	! = sum over shells of local sound crossing time dr/csound.
dt_div_dt_mass_loss 
	! dt_mass_loss = -M/Mdot; only applies when Mdot < 0

any other global timescales we should add?


"local" timescales for shells of model:
dt_div_dt_Courant
	! dt_Courant = min over shells of sound crossing time, dr/csound

any other local timescales we should add?


For the next release, I'll add at least these,
and if anyone has other suggestions, please speak up now.

Cheers,
Bill



On Aug 26, 2014, at 6:48 AM, Mathieu wrote:

> Hi everyone,
> 
> I have a (probably silly) doubt on the timestep control MESA does by 
> default.
> If I am correct, in the two-stage process of picking the new timestep 
> there isn't any explicit comparison between the timestep, the thermal 
> timescale, mass loss timescale and other relevant time scales? So 
> nothing forces the timestep to be smaller than, say, the thermal timescale?
> Or is the comparison implicitly done by the digital control-based scheme 
> (I am still a bit confused by this)?
> 
> Thanks,
> 
> Mathieu
> 
> 
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