Abstract: A series of direct numerical simulation (DNSs) of the turbulent plane Couette flow were performed with various box lengths, Lx = 24h, 32h, 45h and 64h, where h is the channel height. The Reynolds numbers Rew (= Uw h/) of 3000 and 8600 were chosen, where Uw 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 (Lx 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.

Abstract: Direct numerical simulation of heat transfer in a fully developed channel flow has been carried out in a range of low Reynolds numbers from Reτ = 180 down to 60 (based on the friction velocity and the channel half width δ) with emphasis on a puff-like structure. For Reτ ≤ 80 with the largest computational domain of 51.2δ × 2δ × 22.5δ, the turbulent puff is observed and significantly affects the momentum and heat transports. The spatial structure of the equilibrium puff is examined with taking account of two different thermal boundary conditions. It is revealed that there exists a localized strong turbulent region, in which a secondary flow is induced by the puff. In consequence, at the present lowest Reynolds number as low as Reτ = 60, the flow remains turbulent and the larger Nusselt numbers than those without puff is obtained.

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