Auto-generation in wall turbulence by the interaction of weak eddies

For channel flow, we explore how commonly found weak eddies can still auto-generate and produce new eddies. Before, only strong eddies (above a threshold strength) were considered to auto-generate. Such strong eddies are rarely observed in actual turbulent flows however. Here, the evolution of two weak conditional eddies aligned in stream-wise direction is studied. The numerical procedure followed is similar to Zhou et al. (1999). The two eddies are found to merge into a single stronger eddy when the initial upstream eddy is taller than the downstream eddy, which further auto-generates when the initial stream-wise separation is small (<120 wall units). It is also observed that non-merging cases with small initial stream-wise separation auto-generated. The rapid lift-up of an eddy by ejection events is found to play a role in the onset of auto-generation. This lead to a modified interpretation at the later stages of auto-generation mechanism where blockage of mean flow and shear layer deformation are considered instead of vortex dynamics.