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Self-sustaining process at large scale in wall-bounded turbulence

Discovery of very-large-scale motions (large-scale streaky structures) in the outer region of wall-bounded turbulent flow has attracted significant interest of many researchers over the last decade. It was initially proposed that this energetic outer motion is formed by concatenation of large-scale motions (or bulges) given in the form of hairpin vortex packet originating from merger and/or growth of the near-wall hairpin vortices. There is growing evidence, however, that an alternative scenario explains the origin of the long streaky structures: we have recently shown that, by artificially removing active motions in the near-wall and the logarithmic regions using an over-damped large-eddy simulation, large-scale structures in the outer region are able to sustain themselves in the absence of any 'bottom-up' energy transfer. Self-sustaining large-scale structures consist of very-large-scale motions (long streaky motion) and several large-scale motions (quasi-streamwise vortical structures) aligned to it. Their self-sustaining process is remarkably similar to that of the near-wall motions. We finally conclude this talk by showing that an exact solution in the form of travelling wave solutions, which was discovered by Nagata and Waleffe, is embedded to this self-sustaining process and that the real large-scale coherent structures are a reflection of solutions of this kind.