  It has often been proposed that the formation of large-scale motion (or
bulges) is a consequence of successive mergers and/or growth of near-wall
hairpin vortices. In the present study, we report our direct observation that
large-scale motion is generated by an instability of an `amplified' streaky
motion in the outer region (i.e. very-large-scale motion). We design a
numerical experiment in turbulent channel flow up to $Re_\tau\simeq 2000$ where
a streamwise-uniform streaky motion is artificially driven by body forcing in
the outer region computed from the previous linear theory (Hwang \& Cossu, J.
Fluid Mech., vol. 664, 2015, pp. 51--73). As the forcing amplitude is
increased, it is found that an energetic streamwise vortical structure emerges
at a streamwise wavelength of $\lambda_x/h\simeq 1-5$ ($h$ is the half-height
of the channel). The application of dynamic mode decomposition and the
examination of turbulence statistics reveal that this structure is a
consequence of the sinuous-mode instability of the streak, a sub-process of the
self-sustaining mechanism of the large-scale outer structures. It is also found
that the statistical features of the vortical structure are remarkably similar
to those of the large-scale motion in the outer region. Finally, it is proposed
that the largest streamwise length of the streak instability determines the
streamwise length scale of very-large-scale motion.

Fluid Dynamics