On the outer large-scale motions of wall turbulence and their interaction with near-wall structures using large eddy simulation

Large-scale structures typically observed at or above the logarithmic layer of fully developed turbulent channel flows were numerically studied. The potential validity of large eddy simulation (LES) to the large-scale analysis was focused on, and its applicability was investigated for the first time by carrying out an intensive grid resolution study to determine the minimum grid spacing necessary to properly capture such flows. It was found that rather fine grid spacing sufficient to resolve the near-wall streaky motions represented by λx+×λz+∼1000×100 is required to reproduce the typical spectral features of large structures in the outer layer. Subsequently, the Reynolds-number scaling for such structures and their interaction with buffer-layer turbulence were examined. It was observed that the large structures in the outer layer remarkably appear only in the streamwise velocity fluctuation, basically obeying the outer scaling, and their spanwise size is approximately twice as large as the boundary layer thickness, independent of the Reynolds-number range tested here. It was also found that they penetrate deep into the buffer layer, where small streaky structures obey the inner scaling. These results clearly demonstrate that mixed inner–outer scaling instead of simple inner scaling for the streamwise velocity root mean square in the near-wall region is reasonable.