  Isolated turbulent bands observed in transitional channel flows have
downstream heads and inclined bulks at a characteristic angle. In the
large-scale mean flow, a $\nu$-shape vortex found at the head elongates into
the bulk part, forming a pair of counter-rotating vortex tube structures. It is
revealed numerically and theoretically that the head and the bulk convection
velocities reflect the obliquely forward and the backward self-induced
velocities of the $\nu$-shape vortex and the vortex tube pair, respectively,
and the difference between these convection velocities provides a restoring
angular momentum to retain the characteristic inclination angle through a
self-adjustment process.
