[mesa-users] Platform dependencies

David Arnett wdarnett at gmail.com
Mon Nov 25 13:32:40 EST 2013


Hi Bill,

Don't stop trying. We need to understand where this comes from, as software
producers.

There is another shoe dropping. It is now clear that convection in the late
stages is fundamentally different from the mixing-length/diffusion
algorithm that MESA, KEPLER and most other codes use. 3D simulations at as
high resolution as now feasible show that convection is strongly turbulent
and time dependent. It is chaotic in the precise sense used in dynamical
systems theory. This seems likely to be a robust result. The physics is
similar to that Rob Izzard referred to in meteorology; we too may have to
run several versions to make an ensemble average.   It may be that we will
choose to use an analytic averaging procedure (a sort of super MLT)
directly in our convection algorithm. But we should not loose sight of the
existence and importance of large scale fluctuations inherent in these late
stages. The fluctuations are real, not a numerical artifact, and will cause
real physical differences.

I think that there are three issues: (1) open source software should
produce results independent of platform, (2) We do not yet have a strong
culture of cross-checking of stellar codes (but MESA is helping), and (3)
the real world can be chaotic, and we have to learn to deal with that.

-- 
David Arnett
Regents Professor
Steward Observatory
University of Arizona

Facts are stubborn, but statistics are more pliable.  Mark Twain
Facts do not cease to exist because they are ignored.  Aldous Huxley
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