DNS of turbulent Couette flow with emphasis on the large-scale structure in the core region

A series of direct numerical simulation (DNSs) of the turbulent plane Couette flow were performed with various box lengths, L-x = 24h, 32h, 45h and 64h, where h is the channel height. The Reynolds numbers Re-w(= U(w)h/nu) of 3000 and 8600 were chosen, where U-w is the relative wall speed between top and bottom walls and. is the kinematic viscosity. In the core region, the meandering of the large-scale structure (LSS) has been captured with a long box size (L-x >= 32h). Correspondingly, the streamwise two-point correlation decreases and becomes negative at half the box length. The effect of the computational box size on the statistics, e. g. turbulence intensities, is also examined. Significant Reynolds-number dependence is observed in the streamwise LSS. For the higher Reynolds number, both the visualized instantaneous flow field and the pre-multiplied energy spectra show that the LSS can be present, with a finite wavelength of 21 - 32h in the streamwise direction and a spanwise spacing of 2.1 - 2.5h. The structure in the wall-normal direction is also discussed using the two-dimensional two-point correlations.