[mesa-users] Timestep control

[Bill Paxton]

Hi Tomek,

Good to hear from you.  
I'd be interested to know if we are missing any of the ones you have in mind.
The controls.defaults file has lots of controls for max allowed changes per shell.
I think they already cover the things you mention, but it would be great
if you'd check and let us know if you find anything that needs to be added.

Cheers,
Bill




On Aug 26, 2014, at 10:28 AM, Tomasz Plewa wrote:

> Besides CFL limit, there are several local timescales of interest 
> (nuclear energy, heat, viscosity), All those can be controlled by 
> imposing limits by how much they can change per time step (abundances, 
> pressure, velocity, angular momentum, etc). Perhaps with those in place, 
> advancing through certain violent evolutionary phases will become possible.
> 
> Tomek
> --
> On 08/26/14 13:13, Bill Paxton wrote:
>> 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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