[mesa-users] Convergence

Robert Farmer rjfarmer at asu.edu
Fri May 20 12:55:02 EDT 2016

> Also is there a way to figure out after the runs have completed to
determine how well the convergence was in reality?

If you haven't already found it, there is this option in controls:

report_hydro_solver_progress = .true.

Which will tell give you information about the newton iterations as the run
progresses and how good the acceptance was.

On Fri, May 20, 2016 at 9:24 AM, Bill Paxton <paxton at kitp.ucsb.edu> wrote:

> On May 20, 2016, at 9:05 AM, Kenny Van wrote:
> One question I had was that does tol_max_correction guarantee that
> conversion is always no worse than that? Also is there a way to figure out
> after the runs have completed to determine how well the convergence was in
> reality?
> Hi,
> First we need to be careful about the meaning of "convergence"
> 1) we speak of the newton iterations converging to a solution for the new
> model at the end of a timestep
> and
> 2) we also use convergence to mean the final results of a run converging
> to (roughly) the same values as the tolerances are tightened forcing more
> timesteps and more zones.
> the 1st kind is done at each timestep, the 2nd is done before publishing
> (or sooner!) by expert users to check the chance that their results are
> numerical artifacts of inadequate time or space resolution.   there is of
> course the danger that by reducing timesteps, you'll open up new physics
> that you would rather stayed hidden (surface pulsations for example).   so
> this process cannot be pushed too far, but it also should not be neglected.
> for newton iterations, i try to avoid the term "convergence" and talk
> about "acceptance" instead.  the newton generates a series of trial
> solutions.  each is checked for how well it satisfies the equations
> ("residuals") and how small the difference is from the previous trial
> sotution ("corrections").
> acceptance must happen within a specified number of trials, or the effort
> is stopped and the system must do a retry with a smaller timestep.
> both the residuals and the corrections can be considered in deciding
> whether or not to accept a trial solution.
> in many cases, it is desirable to stop checking residuals after a
> specified number of iterations and just use corrections to decide
> acceptance.
> these options are given in controls.defaults, so that's where you need to
> go next.  search for "solver controls" and read on.
> -Bill
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