[Mesa-users] Dh_div_H_limit

[Hannah Brinkman]

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>:

> 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> 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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