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

TL;DR

This study investigates the interaction between near-wall streaks and large-scale motions in turbulent channel flows, providing evidence that bottom-up influence from streaks is weak and questioning the co-supporting hypothesis.

Contribution

It offers a detailed analysis of the influence of large-scale structures on near-wall streaks and challenges the idea that streaks significantly support outer large-scale motions.

Findings

Spanwise advection of streaks is affected by large-scale circulations.

Weak correlation between streak merger and large-scale motion generation.

Removing near-wall streak roots does not impact outer large-scale motions.

Abstract

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-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 (2005), but that the hypothesized streak accumulation rarely occurs. Numerical experiments at Re_tau about 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.