Extreme wall shear stress (WSS) events, usually conditioned to the streamwise WSS, are examined with a further constraint of the azimuthal WSS through direct numerical simulation of turbulent pipe flow at a frictional Reynolds number ${\text{Re}}_{\tau }\approx 500$. In light of the segmented characteristics identified in the statistical distribution of the WSS ${\tau }_{\theta }^{+}$, both extreme positive (EP) events and backflow (BF) events are further categorized into two distinct types, with the help of the proposed local extremum intersection method. The four events each have their own unique probability of occurrence, and those with larger ${\tau }_{\theta }^{+}$ are found to be more intense through the joint probability density function of ${\tau }_x^{+}$ and ${\tau }_{\theta }^{+}$. By conducting a conditional average on the regions of these events, significant differences in their vortical structures and related characteristics have been identified, distinguishing them from previous research findings. On the one hand, the different formation mechanisms of the subdivided EP events' structures are clearly revealed, with the presence of the second vortex on the opposite side of the dominant streamwise vortex in EP leading to a weakening of the spanwise motion. On the other hand, the research on the subdivided BF events demonstrates that the flow characteristics show a close correlation with tilt angle of the dominant oblique vortex, with larger angles tending to induce stronger backflow phenomena. Moreover, a relationship between angles and azimuthal shear stress is established, which roughly follows a power-law distribution. This study highlights that introducing additional new constraints could refine or expand the currently widely accepted classification framework for extreme WSS events, enhancing current understanding of them.

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• Three-dimensional turbulence
• Turbulence
• Turbulence simulations
• Turbulent boundary layers
• Direct numerical simulations