  Using hydrodynamic simulations, we investigate the physical properties of
gaseous substructures in barred galaxies and their relationships with the bar
strength. The gaseous medium is assumed to be isothermal and unmagnetized. The
bar potential is modeled as a Ferrers prolate with index n. To explore
situations with differing bar strength, we vary the bar mass fbar relative to
the spheroidal component as well as its aspect ratio. We derive expressions as
functions of fbar and the aspect ratio for the bar strength Qb and the radius
r(Qb) where the maximum bar torque occurs. When applied to observations, these
expressions suggest that bars in real galaxies are most likely to have
fbar=0.25-0.5 and n<1. Dust lanes approximately follow one of x1-orbits and
tend to be more straight under a stronger and more elongated bar, but are
insensitive to the presence of self-gravity. A nuclear ring of a conventional
x2 type forms only when the bar is not so massive or elongated. The radius of
an x2-type ring is generally smaller than the inner Lindblad resonance,
decreases systematically with increasing Qb, and slightly larger when
self-gravity is included. This evidences that the ring position is not
determined by the resonance but by the amount of angular momentum loss at
dust-lane shocks. Nuclear spirals exist only when the ring is of the x2-type
and sufficiently large in size. Unlike the other features, nuclear spirals are
transient in that they start out as being tightly-wound and weak, and then due
to the nonlinear effect unwind and become stronger until turning into shocks,
with an unwinding rate higher for larger Qb. The mass inflow rate to the galaxy
center is found to be less than 0.01 Msun/yr for models with Qb<0.2, while
becoming larger than 0.1 Msun/yr when Qb>0.2 and self-gravity is included.
