[Mesa-users] Dh_div_H_limit
Bill Paxton
paxton at kitp.ucsb.edu
Wed Jun 17 11:54:08 EDT 2020
think of it in terms of a limit on the magnitude of decrease in H (dH) and another limit for the relative change (dH/H).
as it says in controls.defaults
dH_div_H_limit = Limit on magnitude of relative decrease in any cell hydrogen abundance.
dH_limit = Limit on magnitude of decrease in any cell hydrogen abundance during a single timestep.
dH(k) = max(0d0, mass fraction H in cell k at start of step - mass fraction H in cell k at end of step) ! > 0 for decrease
H(k) = mass fraction H in cell k at end of step
the controls with names starting “dH_div_H_limit” are for the relative changes.
the controls with names starting with just “dH” are for the absolute ones.
for H, there is also an option to consider both increases and decreases in setting the limits.
! If true, then only consider decreases in abundance.
dH_decreases_only = .true.
> What confuses me is the bold-faced part.
> dH_div_H_limit_min_H = 1d-3
> !### dH_div_H_limit
> ! If max `dH_div_H` is greater than this, reduce the next timestep by `dH_limit/max_dH`.
> dH_div_H_limit = 0.5d0
That a bug in the documentation! cut-and-paste strikes again.
What it should say if the max value of dH(k)/H(k) for all cells k is > dH_div_H_limit
then reduce the next timestep by dH_div_H_limit/max of (dH(k)/H(k))
Here’s what it looks like now in controls.defaults - see any problems with this?
!### dH_div_H_limit
! If max `dH_div_H` is greater than this, reduce the next timestep by `dH_div_H_limit/max dH_div_H`.
dH_div_H_limit = 0.9d0
i’m sure there are still more confusing ambiguities in the comments. thanks for reporting this one. since i never read documentation of mesa myself, it took me longer than it should have to notice what you were pointing out.
b
> On Jun 17, 2020, at 8:30 AM, Hannah Brinkman <brinkmanhe at gmail.com> wrote:
>
> Dear Bill,
>
> Thank you for your reply.
>
> I found the same part in the controls.defaults of the MESA version I use (10398). What confuses me is the bold-faced part.
>
> ! Limit on magnitude of relative decrease in any cell hydrogen abundance.
> ! `dH_div_H` here is `abs(xa(h1,k) - xa_old(h1,k))/xa(h1,k)`
> ! considers all cells except where have convective mixing.
> ! `dH_decreases_only` applies to `dH_div_H` also.
> !### dH_div_H_limit_min_H
> ! `dH_div_H` limits only apply where xa(h1,k) >= this limit.
> dH_div_H_limit_min_H = 1d-3
> !### dH_div_H_limit
> ! If max `dH_div_H` is greater than this, reduce the next timestep by `dH_limit/max_dH`.
> dH_div_H_limit = 0.5d0
> !### dH_div_H_hard_limit
> ! If max `dH_div_H` is greater than this, retry with smaller timestep.
> dH_div_H_hard_limit = 1d99
>
> This seems to refer to a control higher up in the same section, where dH_limit is defined:
>
> ! Limit on magnitude of decrease in any cell hydrogen abundance during a single timestep.
> ! dH here is `abs(xa(h1,k) - xa_old(h1,k))` for any cell k.
> ! Considers all cells except where have convective mixing.
> !### dH_limit_min_H
> ! dH limits only apply where xa(h1,k) >= this limit.
> dH_limit_min_H = 1d99
> !### dH_limit
> ! If max dH is greater than this, reduce the next timestep by `dH_limit/max_dH`.
> dH_limit = 1d99
> !### dH_hard_limit
> ! If max dH is greater than this, retry with smaller timestep.
> dH_hard_limit = 1d99
> !### dH_decreases_only
> ! If true, then only consider decreases in abundance.
> dH_decreases_only = .true
>
> Even after reading this, I still have some questions left. If I understand correctly, this limiter shows up when the hydrogen abundance between two non-convective cells changes too much between timesteps. The questions I have left now, are;
> - What determines the value of max_dH?
> - If dH_limit = 1d99, wouldn't this lead to a very large decrease of the timestep?
>
> I hope you can clarify this for me.
>
> With kind regards,
> Hannah
>
>
> Op di 16 jun. 2020 om 18:25 schreef Bill Paxton <paxton at kitp.ucsb.edu <mailto:paxton at kitp.ucsb.edu>>:
> The trick is to copy the string from the terminal output and search for it in star/defaults/controls.defaults
>
> Turns out there are 2 different limits related to dH for absolute and relative changes.
>
> ! 'dH' dH_limit
> ! 'dH/H' dH_div_H_limit
>
> You want to then look at the info about dH_div_H_limit
>
> ! Limit on magnitude of relative decrease in any cell hydrogen abundance.
> ! `dH_div_H` here is `abs(xa(h1,k) - xa_old(h1,k))/xa(h1,k)`
> ! considers all cells except where have convective mixing.
> ! `dH_decreases_only` applies to `dH_div_H` also.
>
>
> !### dH_div_H_limit_min_H
>
> ! `dH_div_H` limits only apply where xa(h1,k) >= this limit.
>
> dH_div_H_limit_min_H = 1d-3
>
>
> !### dH_div_H_limit
>
> ! If max `dH_div_H` is greater than this, reduce the next timestep by `dH_limit/max_dH`.
>
> dH_div_H_limit = 0.9d0
>
>
> !### dH_div_H_hard_limit
>
> ! If max `dH_div_H` is greater than this, retry with smaller timestep.
>
> dH_div_H_hard_limit = 1d99
>
>
>
>> On Jun 16, 2020, at 9:06 AM, Hannah Brinkman via Mesa-users <mesa-users at lists.mesastar.org <mailto:mesa-users at lists.mesastar.org>> wrote:
>>
>> Dear All,
>>
>> When checking some of my recent models, I see a lot of times that the timestep is limited by DH/H. After checking this in the controls, I noticed that when this limiter appears, the timestep is decreased by DH_limit/max_DH. However, DH_limit is 1d99 by default. Wouldn't this lead to an enormous decrease of the timestep each time this control limits the timestep? Or do I misinterpret this limiter? I cannot find what determines max_dH.
>>
>> I hope someone can explain to me what is happening in the star when this limiter shows up.
>>
>> With kind regards,
>> Hannah
>>
>>
>>
>>
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