  We present 3D hydrodynamical simulations of core convection with a stably
stratified envelope of a 25 $\mathrm{M}_\odot$ star in the early phase of the
main-sequence. We use the explicit gas-dynamics code $\texttt{PPMstar}$ which
tracks two fluids and includes radiation pressure and radiative diffusion.
Multiple series of simulations with different luminosities and radiative
thermal conductivities are presented. The entrainment rate at the convective
boundary, internal gravity waves in and above the boundary region, and the
approach to dynamical equilibrium shortly after a few convective turnovers are
investigated. From the results of these simulations we extrapolate to find the
entrainment rate at the nominal heating rate and thermal diffusion given by the
$\texttt{MESA}$ stellar evolution model on which the 3D stratification is
based. Further, to study the effect of radiative diffusion on the thermal
timescale, we perform very long simulations accelerated by 10000 times their
nominal luminosities. In these simulations the growing penetrative convection
reduces the initially unrealistically large entrainment. This reduction is
enabled by a spatial separation that develops between the entropy gradient and
the composition gradient. The convective boundary moves outward much more
slowly at the end of these simulations. Finally, we present a method to predict
the extent and character of penetrative convection beyond the Schwarzschild
boundary. This method is intended to be ultimately deployed in 1D stellar
evolution calculations and is based on the properties of penetrative convection
in our simulations carried forward through the local thermal timescale.
