On the Large-scale Structure of Flow in Far Layer of Wall Turbulence(Turbulent Folw)

Comparatively high Reynolds number channel flows up to Re_τ=640 are simulated by DNS. Noticeable property of the high Reynolds number flow is large-scale streaks which are larger not only in scale but also in their spanwise separation than those well-known in the buffer layer. It is revealed that the low-speed streaks are the region of high turbulent activity and accordingly of high turbulent shear stress with densely populated elementary vortices. These properties are little influenced by wall condition, such as drag controlled or not. LSE (Linear Stochastic Estimation) technique is applied to extract elementary vortex pairs. Time evolution of these vortices submerged in a laminar flow but having velocity distribution of turbulent one reveals the process of generation of large-scale streaks and accompanying densely populated elementary vortices. The formation of large-scale streaks by the model simulation is also little influenced by the wall condition and this modeled process is expected to mimic that taking place in the near-wall layer of practical channel flow. The existence of the low-speed streaks itself is the key for the formation of the structure, thus suggesting the self-sustenance of turbulence by both large-scale streaks and vortices concentrated area.