Tadanobu Nakayama

Space-time correlation structures of horizontal coherent vortices in compound open-channel flows by using particle-tracking velocimetry

In compound open-channel flows, it is very important to clarify an interaction mechanism between the mainchannel and flood-plains induced by coherent vortices, so-called the “intermittent upward secondary currents” and the “large-scale horizontal vortices”, as shown in Figure 1. In the present study, time-averaged structures of secondary currents were revealed experimentally by making use of a two-component fiber-optic Laser Doppler Anemometer (LDA). Next, a new quantitative visualization technique called the “Particle-Tracking Velocimetry (PTV)” was used to investigate evolutionary patterns of horizontal large-scale vortices, which are the ultimate interest in a shallow water flood-plain. By this technique, instantaneous velocity components in arbitrary horizontal plane were obtained at all grid points of laser-light-sheet (LLS) from continuous four images of one particle, and space-time correlation structures of coherent vortices between the main-channel and flood-plain were investigated by using a cond...

Relationship between Turbulence Structure near the Free-Surface and Surface-Wave-Fluctuations

It has been pointed out that a turbulence structure near the free-surface has a close relationship with surface-wave fluctuations and the Froude number. In particular, a critical flow occurs when the Froude number is close to one, and the turbulence structure may become unstable. This phenomenon can be seen in the complexity of turbulence redistribution and the undulation property of surface-wave fluctuations. In this study, turbulence measurements were three-dimensionally conducted in open-channel flows by using two sets of fiber-optic LDA system, and simultaneously, the surfacewave fluctuations were measured by an ultrasonic depth-meter instrument. The aim of this study is to clarify the relationship between the turbulence structure near the freesurface and the surface-wave fluctuations

Comparison of 3-D Experimental Database with Direct Numerical Simulation

In an open-channel flow, a turbulent structure is affected by the presence of a free surface, and therefore, its characteristics are quite different from those of duct flow near the free surface. Many researchers have suggested that the turbulence redistribution changes complicately near the free surface. In this study, turbulence measurements were three-dimensionally conducted in open-channel flows by using two sets of twocomponent fiber-optic LDA. Some comparisons of the experimental LDA data with a direct numerical simulation (DNS) were conducted. Then, the peculiar characteristics in open-channel flows were clarified in detail