[mesa-users] convective boundaries

[David Arnett]

It appears that the simulations would agree with Arlette in the limit of
very rapid mixing and slow burning. This seems to be likely a good
approximation for H burning, although I am still uneasy about He burning,
which is sensitive to the entrainment of new fuel. Late burning is not so
uniformly mixed, and even a very slight composition gradient can affect the
net buoyancy.

A dynamic model gives a similar picture to that which Arlette described,
but with a narrow braking region (negative entropy excess!). Thus instead
of extrapolating to zero, the discriminant is extrapolated to a negative
value, which is a function of the convective velocity and the Brunt outside
the mixing boundary. This is the physical process causing "overshoot"; it
reflects the velocity vector back into the convective region, giving return
flow. In addition, this boundary slowly moves into the radiative region by
mass entrainment; estimation of the entrainment rate is our major challenge
at present.

Most of this seems to be occurring in the "double" zone Arlette mentioned.
We may be able to define jump conditions across this region, which might be
simpler to implement in MESA and friends, and more general.

Dave

-- 
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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