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

Streak instability in turbulent channel flow: the seeding mechanism of large-scale motions