COHERENT FINE SCALE EDDIES IN THE LOGARITHMIC REGION OF TURBULENT CHANNEL FLOWS

Coherent fine scale eddies (CFSEs) play an important role in generating streak structures and Reynolds shear stress not only in the near-wall region but also in the logarithmic region. The diameter and maximum azimuthal velocity of the CFSEs in the near-wall, logarithmic and wake regions can be scaled by Kolmogorov length η and velocity uk. The most expected diameter and maximum azimuthal velocity of the CFSEs are about 10 η and 2.0 uk in the near-wall region (y + < 40). In the logarithmic region (40 <y + < 200 ∼ 300), they become about 8.5 η and 1.7uk, respectively. These features are independent of Reynolds number up to Reτ = 1270. Large-scale structure in turbulent channel flows is organized by the CFSEs in the logarithmic region, which contribute to the streamwise velocity deficit (i.e. low-momentum region). By visualizing spatial distributions of the CFSE axes, it is made clear that the probability that the CFSEs exist in low- momentum regions is higher than that existing in high-momentum regions. The low-momentum regions of the logarithmic layer are composed of the CFSEs which have narrower diameter and stronger azimuthal velocity.