  We run self-consistent simulations of Milky Way-sized, isolated disk galaxies
to study formation and evolution of a stellar bar as well as a nuclear ring in
the presence of gas. We consider two sets of models with cold or warm disks
that differ in the radial velocity dispersions, and vary the gas fraction
$f_{\rm gas}$ by fixing the total disk mass. A bar forms earlier and more
strongly in the cold disks with larger $f_{\rm gas}$, while gas progressively
delays the bar formation in the warm disks . The bar formation enhances a
central mass concentration which in turn makes the bar decay temporarily, after
which it regrows in size and strength, eventually becoming stronger in models
with smaller $f_{\rm gas}$. Although all bars rotate fast in the beginning,
they rapidly turn to slow rotators. In our models, only the gas-free, warm disk
undergoes rapid buckling instability, while other disks thicken more gradually
via vertical heating. The gas driven inward by the bar potential readily forms
a star-forming nuclear ring. The ring is very small when it first forms and
grows in size over time. The ring star formation rate is episodic and bursty
due to feedback, and well correlated with the mass inflow rate to the ring.
Some expanding shells produced by star formation feedback are sheared out in
the bar regions and collide with dust lanes to appear as filamentary interbar
spurs. The bars and nuclear rings formed in our simulations have properties
similar to those in the Milky Way.
