Evolution of wide backflow via large-scale streak collision in turbulent channel flow

Backflow (BF) events, distinguished by negative wall-shear stress (τx), are rare phenomena occurring in the near-wall region of fully developed wall turbulence. Although these events manifest as small-scale patches of viscous scales, they originate from collisions between large-scale structures (LSSs). Hence, we explore the formation of BF, focusing particularly on interactions with the surrounding LSSs to elucidate the associated inner–outer interactions. We perform direct numerical simulations of turbulent channel flows at Reτ = 180 and 550, including a narrow box simulation at Reτ = 550 to restrict the LSSs. We observe the presence of wide BFs, which are absent at the lower Reynolds number and in the narrow box simulation. These wide BFs have widths significantly larger than the mean size of typical BF regions. Temporal tracking of the BFs with surrounding LSSs and vortical structures reveals that wide BFs result from symmetric collisions between streamwise-aligned high- and low-speed LSSs, whereas narrow BFs stem from asymmetric collisions. In the symmetric collisions, the upstream high-speed structure overrides the downstream low-speed structure, forming a wide shear layer and a significant velocity jump at the interface. This induces a strong prograde vortex near the wall, which elongates laterally and descends owing to the downwash motion of the high-speed structure, ultimately inducing wide BF regions. Conversely, the narrow BF regions develop from the asymmetric collisions occurring at the sides of the spanwise-aligned LSSs, forming narrow, laterally tilted shear layers. The large-scale collisions also induce extreme positive-τx events, particularly noticeable over broad streamwise extents during symmetric collisions. These insights into BF dynamics can inform the development of novel drag reduction strategies by manipulating LSS collisions.