The structure and dynamics of backflow in turbulent channels

A statistical description of flow regions with negative streamwise velocity is provided based on simulations of turbulent plane channels in the Reynolds number range 547 <= Re-tau <= 2003. It is found that regions of backflow are attached and their density per surface area - in wall units - is an increasing function of Re-tau. Their size distribution along the three coordinates reveals that, even though in the mean they appear to be circular in the wall-parallel plane, they tend to become more elongated in the spanwise direction after reaching a certain height. Time-tracking of backflow regions in a Re-tau = 934 simulation showed they convect downstream at the mean velocity corresponding to y(+) approximate to 12, they seldom interact with other backflow events, their statistical signature extends in the streamwise direction for at least 300 wall units, and they result from a complex interaction between regions of high and low spanwise vorticity far beyond the viscous sublayer. This could explain why some statistical aspects of these near-wall events do not scale in viscous units; they are dependent on the Re-tau-dependent dynamics further away from the wall.