Scaling law of fine scale eddies in turbulent channel flows up to Re_τ = 800

摘要

To clarify the scaling law of fine scale eddies in turbulent channel flows, direct numerical simulations are conducted for Re_τ = 180, 400 and 800. The diameter and the maximum azimuthal velocity of coherent fine scale eddies can be scaled by Kol-mogorov microscale (η) and Kolmogorov velocity (u_k). The most expected diameter and maximum azimuthal velocity are 8-10η and 1.2-2.0u_k, respectively. Near the wall, the most expected diameter increases to 10η from 8η and the most expected maximum azimuthal velocity increases to 2.0u_k from 1.2u_k Strain rates at the center of the coherent fine scale eddies are small compared with the mean strain rate of the whole flow field. The strain rates acting on the fine scale eddies away from the wall coincide with those in homogeneous isotropic turbulence and turbulent mixing layer. However, relatively large strain rates are acting on the near-wall coherent fine scale eddies. The most expected angle between the intermediate eigenvector and the rotating axis of the fine scale eddy is about 15-17°, which is independent of the turbulent flow fields. The probability that coherent fine scale eddies exist in low-speed streaks is higher than that in high-speed streaks. Large scale structures of wall turbulence are visualized by showing spatial distributions of central axes of coherent fine scale eddies.