Visualization Experimental Investigation on Dark Spot Coherent Structure in Turbulent Boundary Layer

The dark spot coherent structures in the turbulent boundary layer near the wall of a vertical rectangular channel were observed by using hydrogen bubble flow visualization techniques, and the flow Reynolds number Re=3009. Based on the existence of the vortex in the upstream border, the dark spots were divided into two types: Three-vortex dark spots and two-vortex dark spots. The dark spot formation process and its velocity distribution were preliminarily studied. Compared with dark spots in plate boundary layer, the development time of dark spots in rectangular channel boundary is shorter, and its internal fluid velocity increment is weaker.