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The interactions between the near-wall streaks and the large-scale motions(LSMs) of the outer region of wall-bounded turbulent flows are investigated.The co-supporting hypothesis of Toh & Itano (2005) is checked in full-scalechannels at low to moderate Reynolds numbers, from two points of view. To studythe top-down influence of the outer structures on the spanwise motion of thenear-wall streaks, a method inspired by particle-image velocimetry is used totrack the spanwise position of the streaks. Their spanwise advection velocityis found to be affected by the hierarchy of large-scale circulations in thelogarithmic layer, but their spanwise streak density is only weakly related tothe LSMs. The evidence suggests that a top-down influence exists and drives thedrift of the streaks in the spanwise direction, as suggested by Toh & Itano(2005), but that the hypothesized streak accumulation rarely occurs. Numericalexperiments at Re_tau about 535 are then performed to clarify the role of thenear-wall streaks in the generation and preservation of the outer LSMs. Theresults show that the merger of the near-wall streaks is only weakly correlatedwith the generation of the LSMs, and that removing the near-wall roots of theLSMs does not affect the evolution of their outer region. It is concluded thatthe bottom-up influence from the near-wall streaks is not essential for the LSMgeneration and preservation, also weakening the evidence for the co-supportinghypothesis.

Interaction between near-wall streaks and large-scale motions in turbulent channel flows