  Streaks in the buffer layer of wall-bounded turbulence are tracked in time to
study their life-cycle. Spatially and temporally resolved direct numerical
simulation data is used to analyze the strong wall-parallel movements
conditioned to low-speed streamwise flow. The analysis of the streaks shows
that there is a clear distinction between wall-attached and detached streaks,
and that the former can be further categorized into streaks that are contained
in the buffer layer and the ones that reach the outer region. The results
reveal that streaks are born in the buffer layer, coalescing with each other to
create larger streaks that are still attached to the wall. Once the streak
becomes large enough, it starts to meander due to the large
streamwise-to-wall-normal aspect ratio, and consequently the elongation in the
streamwise direction, which makes it more difficult for the streak to be
oriented strictly in the streamwise direction. While the continuous interaction
of the streaks allows the super-structure to span extremely long temporal and
length scales, individual streak components are relatively small and
short-lived. Tall-attached streaks eventually split into wall-attached and
wall-detached components. These wall-detached streaks have a strong wall-normal
velocity away from the wall, similar to ejections or bursts observed in the
literature. Conditionally averaging the flow fields to these split events show
that the detached streak has not only a larger wall-normal velocity compared to
the wall-attached counterpart, it also has a larger (less negative) streamwise
velocity, similar to the velocity field at the tip of a vortex cluster.
