STUDY ON COHERENT VORTEX STRUCTURES OVER DUNE TYPE WAVY BED IN AN OPEN-CHANNEL FLOW BY DIRECT NUMERICAL SIMULATION

Abstract

In this study, fully developed three-dimensional turbulent flow over plural solid dune bed in an open-channel was reproduced by direct numerical simulation (DNS). Its coherent structures were also examined using time-line technique and identification of cylindrical vortex tubes.“Boil of the first kind”, which was unique to the flow field on dune-type sand wave, was numerically reproduced. It was found that kolk which causes boil of the first kind can be detected as cylindrical vortex tubes rising from the bed. It was also found that boil of first kind and kolk were generated not only from “hairpin vortices” as conventionally noted but also from “reverse hairpin vortices” with their head near the bed and cyclonic vortices at their foot.