[Mesa-users] wd_diffusion doesn't work?

[Aaron Dotter]

Hi Michael,

The diffusion solver will break a MESA timestep up into some number of
substeps (for the reason you mention), as explained in controls.defaults

         ! Each stellar evolution step can be divided into many substeps
for diffusion.
         ! The substep timescale is set by rates of flow in and out for
each species in each cell.
         ! The substep size, dt, is initially set to
`timescale*diffusion_dt_div_timescale`.

You should have no problem taking MESA timesteps that are larger than the
expected diffusion timescale.  Perhaps you don't need to limit the timestep
to 1000 years?

Aaron

On Mon, Oct 2, 2017 at 2:42 PM, Zhang, Zhaoxi M. (Michael) <
mz at astro.caltech.edu> wrote:

> Hello everyone,
>
>
> In MESA 10000, I ran wd_diffusion in the test suites.  It worked, but its
> timesteps were almost 100 million years, which I thought was far higher
> than the diffusion timescale.    So I decreased the timesteps to see how
> the elements diffuse initially:
>
>
> max_years_for_timestep = 1d3
>
>
> and the test stopped working.  It struggles to converge, and fails with:
>
>
> stopping because of problems dt < min_timestep_limit
> terminated evolution: cannot find acceptable model
>
> Is it expected behaviour for this test to fail due to smaller timesteps?
> Why would having a smaller timestep create convergence problems?
>
>
>
> Cheers,
>
>
> Michael
>
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