Masahiro Tamai

A silt curtain is commonly used in a reclamation work to reduce outflow of turbidity from construction area. However it's quite difficult to get an appropreate drawing plan of the curtain since there is little information about a series of real phenomena which occurs under the sea, with the exception of some laboratory experiments with small scale in which we can't remove scale effect. In this study, we carried out two-dimensional numerical simulation with a scale of the construction site using the two-fluid model with the k-ε turbulence model to investigate turbidity transport generated by direct dumping of soil using a hopper barge with bottom doors. We briefly discussed characteristics of turbidity transport and effectiveness of the silt curtain.

In order to quantify direct interactions between solid particles and turbulence in solid and liquid two-phase flows, we investigate an interaction between turbulence generated by uniformly falling particles and the ocsillating grid turbulence. A hydraulic experiment shows that modulation of turbulence intensities of fluid phase which have been pointed out in the preceding studies occurs even in the oscillating grid turbulence and that the modulation of turbulence intensities is anisotropic. A simple model which is proposed to interpret the direct interaction reproduces tendency of the experimental results well.

記事分類: 建設工学--河海工学--河川・運河 ; 応用流体力学

An initial stage of diffusion of turbidity in direct dumping of soil using hopper barges with bottom doors was studied both through laboratory experiments and theoretical consideration. The two-dimensional experiments were carried out to observe turbidity clouds as a mechanism of the initial diffusion process by a flow field generated by a swarm of soil particles falling vertically in water. A semi-empirical model based on results of the experiments was developed and applied to the experiment.

When a swarm of solid particles is dumped instantaneously from a point source in stagnant water, its falling behaviour and induced flow field are investigated both through a hydraulic experiment and through a theoretical analysis. The experiment shows that characteristics of the falling behaviour of the swarm varies both with particle size and with dumped amount from turbulent thermal-like to free settling-like. The theoretical model on the basis of the turbulent thermal theory which can represent the variation of the falling behaviour and aspects of the induced flow field is developed.

Three types of models, i. e. one fluid model, two fluid model and particlefluid model, are generally applied to investigate the characteristics of two phase flows. In each model, the careful consideration on the structures of governing equations and the constitutive relations employed should be needed. Essential point to be discussed in this paper is how the effects of interactions between two phases are taken into account in these basic equations.

In order to study turbulence modulation in solid-liquid two-phase flows, in particular effects of slip velocity between the two phases on turbulence structure of the fluid phase, interaction between turbulence generated by settling particles and the oscillating grid turbulence is calculated using the turbulence model based on the one-point closures. The calculated values either for the turbulence due to settling particles or for the oscillating grid turbulence agree well with the experimental data in the previous studies. And the model predicts that mixing of particles decays turbulence of the fluid phase.

Some critical discussions are made on the structures of governing equations which are applied to one fluid model, two fluid model and fluid-solid particle model. Special concerns are paid on the interaction force in the conservation equations for moment and energy. It is shown that there are two conflicting concepts in evaluating the interaction force between liquid and solid phases; one is that the force should not be introduced positively in governing equations, as a form like body force, but as in stress tensors, and the other emphasizes the force is reasonable in present formulas.

The purpose of this investigation is to clarify the turbulence properties of particle plumes which are generated by relatively coarse and nearly monodisperse particles (the particle diametersare 1 and 3mm) falling with constant volume fluxes in the stagnant water bath. The main results are as follows: In the case that 1mm diameter particles are used, the mean and turbulence properties are quite similar to that of the single phase plume or jet. In the case of 3mm diameter particles, the flow properties are chiefly dominated by vortex shedding of each particle.

Recently, groundwater contamination by chrolinated organic compounds (trichloroethylene, tetrachloroethylene etc.) is broadly observed and its treatment technology is urged in contaminated areas. Physical process of dissolution of such compounds by the groundwater stream should be discussed to understand the mechanism of early stage of contamination near the contaminant source. So, we need some experiments on the movement of contaminants in subsurface and solution process into groundwater. This paper discusses experimentally the dissolution from interface between water and chlorinated organic compounds.

A numerical simulation of the vortex-street model for the coherent structures in a turbulent plane jet and plume is described. Attension is focussed on the topological features of the coherent structures detected experimentally. The model formulation includes the streamwise development of von Karman-type vortex street pattern in terms of the similarity-scaling relationships. The model was found to predict the lateral profiles of mean flow and turbulence quantities including the Reynolds stress, in particular, the differences of their quantities in a jet and a plume reasonably well. The non-symmetric arrangement of coherent structures is the essential mechanism for the production of the Reynolds stress and the entrainment.

Recently large-scale projects of land reclamation, such as Kansai International Airport, have been on the rapid increase. Direct dumping of soils with bottom-dump barges enables us to reclaim lands in short time economically. The method of construction, however, includes very serious technological problem to be solved, that is diffusion of turbidity. The purpose of this study is to clarify the diffusion process of turbidity especially by the flow field which falling-behavior of soils induces through hydraulic experiments. The results show that the induced flow plays an important part of initial diffusion of turbidity.

Experimental results on the mean and turbulent properties, notably the Reynolds shear stress and turbulent heat fluxes of turbulent plane jets and plumes are presented. All the quantities have been measured using two-component hot-film anemometry and resistence thermometry in the developed flow region of hot water issuing into low speed coflowing stream from a sharp-edged rectangular slot of 0.4cm wide. The results show that buoyancy increases the turbulence levels in plumes when compared with jets. For example, the maximum Reynolds shear stress is nearly 60% higher in plumes. The constant eddy viscosity model is found to be applicable to describe the turbulence structure in both flows. Furthermore, the differences in turbulent kinetic energy balance between both flows are examined. The buoyant production of turbulent energy is about 30% of the shear production in plumes.