Visualization of Open-Channel Surface Flow by Lagrangian Superposition of Multiple Images

Water surface velocity field is affected by the three dimensional flow structures under the water. Typically, some of the separation vortices generated near the bottom ascend up to the water surface which causes the diversion of surface floating particles. These vortices, usually called the boil vortices in the field of river engineering, can be commonly observed in natural streams with relatively large roughness. In order to make clear the characteristics of such boil vortices appeared on the water surface, horizontal flow fields at the water surface of flows with several strip roughness are visualized by tracer particles. The visualized images are processed by multiply superposing the images viewed from a frame moving with the surface velocity. It is shown that superposed images clearly emphasize the flow features showing artistic patterns generated by floating tracers.