Streamwise pseudo-vortical structure in the near-wall region of a wall-bounded shear flow and its structural modeling

Coherent fluid motions near the wall in a fully-developed two-dimensional turbulent channel flow have been visualized by using a sophisticated hydrogen-bubble technique with partially insulated fine tungsten wires, which generate segmental hydrogen-bubbles, at several distances from the wall. These flow visualizations supply not only a direct evidence for the existence of streamwise vortical motions but quantitative data of fluid motions in the wall region. The instantaneous contours of streamwise vorticity thus obtained show pseudo-periodicity in the spanwise direction and also double-layer structure in the direction normal to the wall. The compatibility of the pseudo-vortical structure with what is known about the near-wall turbulence is tested and confirmed by a simple structural model based upon unsteady spanwise-periodic secondary motions.