Author(s): K. Noguchi; I. Nezu; M. Sanjou; T. Nakajima
Linked Author(s): Michio Sanjou
Keywords: Sand dunes; Turbulence; Suspended sediment; Particle concentration; PIV; LES
Abstract: In natural rivers, there are many sizes of sand dunes. The dune shapes of the riverbed affect various significant impacts to the flows, such as the lifting-up phenomena of sediment particles. In the developing stages of sand dunes from a flat bed, turbulence structure is dramatically changed after the reverse flow area occurs behind the dune crest, i.e., a separation-bubble zone appears. This indicates that the generation of the reverse flow makes significant influences to sediment transport mechanism over sand dunes. To reveal such a flow mechanism, we conducted some investigations both experimentally and numerically. The experiments of this study were conducted by a combination of PIV and PTV techniques (see Nezu et al., River Flow 2008). The images which were used in PIV&PTV techniques were taken by the CMOS camera, The number of sediment particles was counted from PTV images to calculate the sediment concentration and sediment flux. Moreover in this study, we also conducted a large-eddy simulation (LES) of turbulent structure over fixed 2-D dunes to examine the instantaneous 3-D structure of kolk-boil vortex. As the results, the turbulence structure was changed significantly with the development of sand dunes to the fully-developed shape. The turbulence intensity and instantaneous Reynolds stress were enhanced by the generation of reverse flow. It was predicted numerically that the spanwise diffusion of suspended particles might be enhanced by kolk-boil vortex.
Year: 2010