In this paper, particle image velocimetry and electrochemical measurements were applied simultaneously to examine the relationship between wall shear stress and the three-component velocity field in pipe flow. Moreover, the large-scale motions in the velocity field were separated into very-large-scale motions (VLSMs) and large-scale motions (LSMs) to investigate their respective influences on the wall shear stress fluctuations. The conditional velocity field during negative wall shear stress fluctuations confirmed the existence of the “footprints” of the streamwise and wall-normal velocity components in VLSMs. Further, a pair of counter-rotating roll modes was found to appear in the VLSMs, exhibiting length scales of ${\lambda }_x>3R$, where ${\lambda }_x$ is the streamwise wavelength and $R$ is the pipe radius. In contrast, in LSMs, which exhibit length scales of $0.6R<{\lambda }_x<3R$, counter-rotating vortex pairs were not observed. In this paper, the length-scale threshold for generating counter-rotating vortex pairs in pipe flow was found to be $3R$. This threshold value is higher than the value reported for turbulent boundary layers, which is $1\delta$, where $\delta$ is the boundary-layer thickness. Two-point correlations of wall shear stress, which are associated with the conditional spanwise velocity in the $\theta -x$ plane, revealed the effect of spanwise meandering features on the near-wall region.

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• Boundary layers
• Turbulence