  Under certain conditions, sub-kpc nuclear bars form inside large-scale
stellar bars of disk galaxies. These secondary bars spend a fraction of their
lifetime in a dynamically-decoupled state, tumbling in the gravitational field
of the outer bars. We analyze the flow pattern in such nested bar systems and
find that secondary bars differ fundamentally from their large-scale
counterparts. In particular the gas flow across the bar-bar interface in these
systems can be more chaotic or more regular in nature, and, contrary to
predictions, has no difficulty in penetrating the secondary bars. The outer
parts of both short and long nuclear bars (with respect to their corotation)
appear to be depopulated of gas, while deep inside them the flow exhibits low
Mach numbers and follows ovally-shaped orbits with little dissipation. We find
that gas-dominated and star-dominated nuclear bars avoid the bar-bar interface,
making both types of bars short relative to their corotation. Furthermore, our
earlier work has shown that dynamically-coupled secondary bars exhibit a
similarly relaxed low-dissipation flow as well. Therefore, no large-scale
shocks form in the nuclear bars, and consequently, no offset dust lanes are
expected there. We find that offset dust lanes cannot be used in the search for
secondary (nuclear) bars.
