[mesa-users] Timestep control

Tomasz Plewa tomekplewa at gmail.com
Tue Aug 26 18:39:20 EDT 2014


Hello Bill -

Thanks, I will defintiely take another look into that.

I agree with your other comment it would be great to eliminate most if 
not all criteria of this kind, but it may not be optimal. In fact, I do 
not know of any application code solving a reasonable set of PDEs that 
would operate in such a way. There might be a a good reason for that. ;)

Tomek
--
On 08/26/14 13:45, Bill Paxton wrote:
> 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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