  We present hydrodynamic simulations of gas clouds inflowing from the disk to
a few hundred parsec region of the Milky Way. A gravitational potential is
generated to include realistic Galactic structures by using thousands of
multipole expansions that describe 6.4 million stellar particles of a
self-consistent Galaxy simulation. We find that a hybrid multipole expansion
model, with two different basis sets and a thick disk correction, accurately
reproduces the overall structures of the Milky Way. Through non-axisymmetric
Galactic structures of an elongated bar and spiral arms, gas clouds in the disk
inflow to the nuclear region and form a central molecular zone (CMZ)-like
nuclear ring. We find that the size of the nuclear ring evolves into ~240 pc at
T~1500 Myr, regardless of the initial size. For most simulation runs, the rate
of gas inflow to the nuclear region is equilibrated to ~0.02 M_sun/yr. The
nuclear ring is off-centered, relative to the Galactic center, by the lopsided
central mass distribution of the Galaxy model, and thus an asymmetric mass
distribution of the nuclear ring arises accordingly. The vertical asymmetry of
the the Galaxy model also causes the nuclear ring to be tilted along the
Galactic plane. During the first ~100 Myr, the vertical frequency of the gas
motion is twice that of the orbital frequency, thus the projected nuclear ring
shows a twisted, infinity-like shape.
