  We perform two-dimensional axisymmetric hydrodynamic simulations of matter
mixing in aspherical core-collapse supernova explosions of a 16.3 $M_{\odot}$
star with a compact hydrogen envelope. Observations of SN 1987A have provided
evidence that $^{56}$Ni synthesized by explosive nucleosynthesis is mixed into
fast moving matter ($\gtrsim$ 3,500 km s$^{-1}$) in the exploding star. In
order to clarify the key conditions for reproducing such high velocity of
$^{56}$Ni, we revisit matter mixing in aspherical core-collapse supernova
explosions. Explosions are initiated artificially by injecting thermal and
kinetic energies around the interface between the iron core and the
silicon-rich layer. Perturbations of 5% or 30% amplitude in the radial
velocities are introduced at several points in time. We found that no high
velocity $^{56}$Ni can be obtained if we consider bipolar explosions with
perturbations (5% amplitude) of pre-supernova origins. If large perturbations
(30% amplitude) are introduced or exist due to some unknown mechanism in a
later phase just before the shock wave reaches the hydrogen envelope, $^{56}$Ni
with a velocity of 3,000 km s$^{-1}$ can be obtained. Aspherical explosions
that are asymmetric across the equatorial plane with clumpy structures in the
initial shock waves are investigated. We found that the clump sizes affect the
penetration of $^{56}$Ni. Finally, we report that an aspherical explosion model
that is asymmetric across the equatorial plane with multiple perturbations of
pre-supernova origins can cause the penetration of $^{56}$Ni clumps into fast
moving matter of 3,000 km s$^{-1}$. We show that both aspherical explosion with
clumpy structures and perturbations of pre-supernova origins may be necessary
to reproduce the observed high velocity of $^{56}$Ni. To confirm this, more
robust three-dimensional simulations are required.
