Statistical properties of coherent fine eddies in wall-bounded turbulent flows by direct numerical simulation

The characteristics of coherent fine eddies in wall-bounded turbulent flows are investigated by direct numerical simulation (DNS). The results show that when coherent eddies scaled with Kolmogorov microscale, η and root mean square (rms) of velocity fluctuations, u′rms, it is found that its average diameter is about 10η ∼ 12η and average maximum azimuthal velocity is about 0.5 ∼ 0.6u′rms. Mean azimuthal velocity of coherent eddies follows the profile of Burgers' vortex. Circulations of coherent vortices at different wall locations collapse in similar patterns and show power law behavior. A theoretical description of coherent eddies can be made based on the Burgers' vortex approximation. The diameter and maximum azimuthal velocity of coherent eddies are well approximated at different circulations of Burgers' vortices. It is observed that coherent eddies, those having an average diameter about 10η ∼ 12η, possess maximum azimuthal velocity and intensity ranges as far as 3u′rms. It is shown that the diameter and velocity of coherent eddies are strongly correlated. The simulation results may provide important insight into better understanding of the behaviors of coherent eddies in wall-bounded turbulent flows.