Tail-like vortical structures in extreme turbulent kinetic energy dissipation events of wall turbulence

Structural environment of extreme turbulent kinetic energy dissipation events in turbulent channel flows is studied at friction Reynolds numbers Reτ = 180, 395, and 640 using direct numerical simulation. Extreme events were identified by applying a connected-component labeling framework with spatiotemporal tracking, ensuring unbiased sampling. The vortical motions surrounding these events were characterized using the Rortex criterion, which suppresses the shear contamination and provides directional information about local rotation. The analysis revealed a previously unreported tail-like vortical structure that consistently accompanies extreme dissipation events. This structure exhibits a preferential orientation, negative spanwise Rortex dominance, and a strong association with sweep motions characterized by positive streamwise and negative wall-normal fluctuations. The results refine the classical view of quasi-streamwise vortices by demonstrating their entanglement with secondary tail structures during intermittency.