Dynamics of fine scale eddy clusters in turbulent channel flows

To investigate dynamics of vortex clusters and large-scale structures in the outer layer of wall turbulence, direct numerical simulations of turbulent channel flows have been conducted up to Re-tau = 1270. In the outer layer, the vortex clusters are composed of coherent fine-scale eddies (CFSEs) of which diameter and maximum azimuthal velocity are scaled by the Kolmogorov length and velocity. The large-scale structure in the outer layer is composed of these clusters of the CFSEs, which contributes to the streamwise velocity deficit ( i.e. low-momentum region). The CFSE clusters are observed in the low-momentum regions of the outer layer, and the scale of those clusters tends to be enlarged with the increase of a distance from the wall. The dynamics of large-scale structures reveals that the cluster structure generated in the bottom of the logarithmic region moves downstream and its scale increases with the increase of the low-momentum region. The CFSE clusters in the low-momentum regions of u' <= -u(rms) consist of the relatively strong CFSEs, which play an important role in the production of the Reynolds shear stress and the dissipation rate of the turbulent kinetic energy. The process of destruction of the CFSE cluster is also clarified in the outer layer.