Formation of wide backflow events via large-scale streak interactions in turbulent channel flow

This study investigates the formation mechanism of wide backflow events using direct numerical simulation data of turbulent channel flow at Re_τ = 180 and 550. By changing the spanwise size of the computational domain, we observe wide backflow events whose width exceeds 0.1h (where h is the channel-half height), which is an order of the characteristic width of large-scale structures. It is found that wide backflow events originate from a collision between large-scale high-speed and low-speed structures. Due to the difference in convection velocities, the large-scale high-speed structures climb over the low-speed structures. When the incoming high-speed structure aligns with the trailing edge of the low-speed one, a relatively wide shear layer forms near the wall, which subsequently leads to the formation of a strong vortex. As the process evolves, this vortical structure elongates in the spanwise direction with a length scale over 0.1h, which induces a wide backflow region. The collision between large-scale structures and the associated formation of wide backflow events is found to contribute to turbulence production in the near-wall region.