  We perform ideal MHD high resolution AMR simulations with driven turbulence
and self-gravity and find that long filamentary molecular clouds are formed at
the converging locations of large-scale turbulence flows and the filaments are
bounded by gravity. The magnetic field helps shape and reinforce the long
filamentary structures. The main filamentary cloud has a length of ~4.4 pc.
Instead of a monolithic cylindrical structure, the main cloud is shown to be a
collection of fiber/web-like sub-structures similar to filamentary clouds such
as L1495. Unless the line-of-sight is close to the mean field direction, the
large-scale magnetic field and striations in the simulation are found roughly
perpendicular to the long axis of the main cloud, similar to 1495. This
provides strong support for a large-scale moderately strong magnetic field
surrounding L1495. We find that the projection effect from observations can
lead to incorrect interpretations of the true three-dimensional physical shape,
size, and velocity structure of the clouds. Helical magnetic field structures
found around filamentary clouds that are interpreted from Zeeman observations
can be explained by a simple bending of the magnetic field that pierces through
the cloud. We demonstrate that two dark clouds form a T-shape configuration
which are strikingly similar to the Infrared dark cloud SDC13 leading to the
interpretation that SDC13 results from a collision of two long filamentary
clouds. We show that a moderately strong magnetic field (M_A ~ 1) is crucial
for maintaining a long and slender filamentary cloud for a long period of time
~0.5 million years.
