[mesa-users] convective boundaries

[Arlette Grotsch]

Thanks for this return, Dave!

When I insist on finding the zero of grad_rad - grad_ad (or grad_Ledoux), it is for the simple case of the local MLT theory for which the convective velocity must be zero at the boundary. The coherency of the models is very important for asteroseismology since the Brunt-Väisälä frequency is affected. When confronting models and observed oscillation modes, the encountered difficulties will hopefully show that things are definitely different from MLT. 

Indeed for red giants in core helium burning phases, asterosesimology is rather strict on the extent of the mixed (convective and …) region.

Your last sentence is indeed a great hope for « easy » model improvement.
Cheers,
Arlette


Le 16 juin 2014 à 11:36, David Arnett <wdarnett at gmail.com> a écrit :

> 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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