  We present 3D hydrodynamics simulations of shell burning in two progenitors
with zero-age main-sequence masses of 22 and 27 $M_{\odot}$ for $\sim$65 and
200 s up to the onset of gravitational collapse, respectively. The 22 and 27
$M_{\odot}$ stars are selected from a suite of 1D progenitors. The former and
the latter have an extended Si- and O-rich layer with a width of $\sim$10$^9$
cm and $\sim$5$\times 10^9$ cm, respectively. Our 3D results show that
turbulent mixing occurs in both of the progenitors with the angle-averaged
turbulent Mach number exceeding $\sim$0.1 at the maximum. We observe that an
episodic burning of O and Ne, which takes place underneath the convection
bases, enhances the turbulent mixing in the 22 and 27 $M_\odot$ models,
respectively. The distribution of nucleosynthetic yields is significantly
different from that in 1D simulations, namely, in 3D more homogeneous and
inhomogeneous in the radial and angular direction, respectively. By performing
a spectrum analysis, we investigate the growth of turbulence and its role of
material mixing in the convective layers. We also present a scalar spherical
harmonics mode analysis of the turbulent Mach number. This analytical formula
would be helpful for supernova modelers to implement the precollapse
perturbations in core-collapse supernova simulations. Based on the results, we
discuss implications for the possible onset of the perturbation-aided
neutrino-driven supernova explosion.
