  Mixing processes in stars driven by composition gradients as a result of the
Rayleigh-Taylor instability are not anticipated. They are supported only by
hydrodynamic studies of stellar convection. We find that such mixing occurs
below the bottom edge of convection zones in our multidimensional hydrodynamic
shell convection models. It operates at interfaces created by off-center
nuclear burning, where less dense gas with higher mean molecular weight is
located above denser gas with a lower mean molecular weight. We discuss the
mixing under various conditions with hydrodynamic convection models based on
stellar evolutionary calculations of the core helium flash in a 1.25 Msun star,
the core carbon flash in a 9.3 Msun star, and of oxygen burning shell in a star
with a mass of 23 Msun. We simulate the hydrodynamic behavior of shell
convection during various phases of stellar evolution with the Eulerian
hydrodynamics code HERAKLES in two and three spatial dimensions. Initial models
for this purpose are obtained by state-of-the-art stellar evolutionary codes,
namely GARSTEC, STAREVOL, and TYCHO for the core helium flash, core carbon
flash, and oxygen shell burning, respectively. Most of our analysis is
performed for two-dimensional hydrodynamic models of shell convection during
the core helium flash at its peak covering approximately 250 convective
turnover timescales. The mixing manifests itself in the form of overdense and
cold fingers enriched with matter of higher mean molecular weight, originating
from density fluctuations at the lower boundary of the convective shell, and
"shooting" down into the core. They result from the Rayleigh-Taylor instability
at the lower convection zone boundary due to a negative mean molecular weight
gradient. They do not appear when the mean molecular weight gradient is
positive.
