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

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 (J. Fluid Mech., vol. 524, 2005, pp. 249-262) is checked in full-scale channels at low to moderate Reynolds numbers, from two points of view. To study the top-down influence of the outer structures on the spanwise motion of the near-wall streaks, a method inspired by particle-image velocimetry is used to track the spanwise position of the streaks. Their spanwise advection velocity is found to be affected by the hierarchy of large-scale circulations in the logarithmic layer, but their spanwise streak density is only weakly related to the LSMs. The evidence suggests that a top-down influence exists and drives the drift of the streaks in the spanwise direction, as suggested by Toh & Itano (J. Fluid Mech., vol. 524, 2005, pp. 249-262), but that the hypothesised streak accumulation rarely occurs. Numerical experiments at Re-tau approximate to 535 are then performed to clarify the role of the near-wall streaks in the generation and preservation of the outer LSMs. The results show that the merger of the near-wall streaks is only weakly correlated with the generation of the LSMs, and that removing the near-wall roots of the LSMs does not affect the evolution of their outer region. It is concluded that the bottom-up influence from the near-wall streaks is not essential for the LSM generation and preservation, also weakening the evidence for the co-supporting hypothesis.

Automated summary

The study found that the large-scale motions in the outer region of wall-bounded flows have a top-down influence on the spanwise motion of near-wall streaks. However, the density of streaks is weakly related to LSMs. Numerical experiments also showed that near-wall streaks merging is not strongly correlated with the generation of LSMs.