On the Large-Scale Structure of Flow in Far Layer of Wall Turbulence.

Abstract

Comparatively high Reynolds number channel flows up to Reτ=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 buffer layer. It is revealed that 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 vertices 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 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 low speed streaks itself is the key for the formation of the structure, thus suggesting the self-sustenance of turbulence by large-scale streaks and vortices concentrated area.