LES STUDY ON THE LARGE-SCALE MOTIONS OF WALL TURBULENCE AND THEIR STRUCTURAL DIFFERENCE BETWEEN PLANE CHANNEL AND PIPE FLOWS

The large-scale motions of wall turbulence are investigated numerically using the large eddy simulation (LES) technique. Special attention is paid to how geometrical difference affects the large structures. For this purpose LES of plane channel and pipe flows (Reτ = 395,590,1180) using a large analysis region is conducted. It is found that in the outer layer, the spanwise/azimuthal spacing of the large scales is stable at 1.4 times the boundary layer thickness (roughly the same as the channel half-width or the radius of pipe). Thus, the number of the large motions allocating along the spanwise direction remains the same throughout the outer layer of channel flow, while its number in the pipe along the azimuthal direction changes stepwise against the wall distance. Consequently strong interaction among the large motions occurs in pipe flow and the streamwise size of the large scales are slightly smaller in pipe (2 to 4 times the radius) than those in channel (3 to 5 times the half-height). The very large scales in the log layer remarkably appear at higher Reynolds number case of both channel and pipe flows are also noteworthy.