Continuous measurement of flow velocity was successfully carried out at the Qiantang River estuary in China using Fluvial Acoustic Tomography System (FATS), which was comprised of a couple of 7 kHz broadband omni-directional transducers. The tidal bore in the Qiantang River estuary is one of the most spectacular bores in the world. The river width and the mean depth at the experimental site, which was located around 90 km upstream from the mouth in the Hangzhou Bay, were 1200 m and around 5.5 m, respectively. The reciprocal sound transmissions were performed between the two acoustic stations located diagonally on both sides of the river; the horizontal distance between the acoustic stations was 3050 m. Heights and celerities of tidal bores were deduced from the cross-sectional average velocity v collected by the FATS. The bore heights deduced from v were comparable to those estimated from water levels near the river banks. Also, the bore celerities deduced from v were in good agreement with those estimated from the water levels.

抄録全体を表示

Analyzing river water samples in a laboratory has been the traditional method used to monitor suspended sediment in rivers. In recent years, however, optical backscatter (turbidity), laser diffraction, pressure-difference, acoustic backscatter, and other surrogate techniques have been adopted increasingly according to the circumstances of each measurement point. The author et al. developed a suspended sediment monitoring system based on ultrasonic attenuation spectroscopy. Using the new system, a test was conducted to continuously measure the concentration and particle size distribution of suspended sediment in the Rokkaku River estuary, and it was verified that ultrasonic attenuation spectroscopy is a promising surrogate technique.

抄録全体を表示

Measurements of flood flow have been conducted intensively at major control sections in Japan for storing reliable hydrological data to use for a long-term river planning. However, such a measurement pays attention only on a flow passing through one cross-section and thus spatial flow features are not available, while river channel changes its feature at every flood event. In this research, we performed concurrent measurements of a snowmelt flood of the Uono River by an image analysis and a radio-controlled ADCP. In the image analysis with STIV (Space-time Image Velocimetry), three video cameras were used to cover a river reach of about 500m by changing their view angles at every location. On the other hand, the boat-mounted ADCP was remotely controlled to form a zigzag trajectory to cover the same reach. The accuracy and limitation of STIV was made clear through a comparison with ADCP data and a spatial distribution of correction factor from surface to depth-averaged velocity was found to have a weak correlation with a large-scale bed slope.

抄録全体を表示

An image-based field measurement method of river flow was applied to a large flood that occurred in the Ibo River and the measurement results were compared with those calculated by two and three dimensional numerical simulation models for evaluating the accuracy of simulations. The target river site has a large bend with a cobble bar on its right in the mid reach of the river. Flow measurements were conducted by taking video images of flood surface flows; i.e. streamwise surface velocity distributions were obtained by the space-time image velocimetry (STIV). It was confirmed from the comparison of velocity distributions at some sections, simulation parameters need some modifications that takes into account of a number of large rocks located near the outer bank. Improvement of simulation accuracy was established by providing direct measurement data by STIV.

抄録全体を表示

In the present study, we examined the fundamental performance of a radio current meter (RCM) for velocity and discharge measurements. For this purpose, we conducted field measurements using RCM, ADCP and ADV (Acoustic Doppler Velocimeter) in the Edogawa River under low- and high-flow conditions. Laboratory experiments in an open channel with wind tunnel were performed to compare velocity and discharge measured by RCM, pitot tube and float. The results indicate that the velocity measured by the RCM was almost comparable to that by floats, showing that the measurement depth of the RCM corresponds to the draft of float (<5 mm). It is also found that the discharge using the RCM and standard coefficient of float (=0.85) were in good agreement with the experimental data.

抄録全体を表示

Authors developed a bed-load discharge measurement system based on ADCP in the experimental flume and applied it to actual river. In order to verify the applicability of this method, flood flow observation by using tethered boat equipped with ADCP and RTK-GPS were conducted in several Japanese rivers with different grain size. The output data were compared with conventional formula and results of 2D bed deformation analysis using observed discharge and water level data. As a result, relationship between non-dimensional shear stress and non-dimensional bed-load discharge estimated by author's method showed similar tendency with conventional formula even though shear velocity calculated from vertical velocity distribution was larger than the other calculated values.

抄録全体を表示

The present study aims to clarify the temporal variations of pick-up rates in a river under flood conditions. For this, an acoustic Doppler velocimeter which can measure three-dimensional velocities and echo intensity closely related to turbidity, was applied to measure the pick-up rates in the Oohori River, flowing into Lake Teganuma. The results indicated that 1) the echo intensity measured by the ADV was found to be highly related to the turbidity, 2) the hysteresis between water level and turbulent quantities under floods was dominant in the flood with relatively higher unsteadiness parameter, and 3) the correlations between the pick-up rates and bottom shear stress were influenced by API(Antecedent Precipitation Index) and flood magnitude.

抄録全体を表示

The authors of this paper have examined to establish an automatic water-discharge measurement system with a fixed type Non-Contact Current Meter, especially Radio wave type. In this system, it is important to establish the velocity index to convert the water surface velocity into the cross-sectional mean velocity. To establish the velocity index, ADCP traverse measurements were conducted in some actual rivers. From the result of the measurement, it was shown that the velocity index was constant in rivers without river-bed change, while the index showed clear duration in rivers with river-bed change during flooding. The authors proposed the system which can calculate the changing index in real-time during flooding and showed the system was useful under certain conditions.

抄録全体を表示

A lot of sediments are often captured in the dead water zone such as an embayment. It is well known that a velocity difference between the main-channel and the side cavity induces a shear instability. Additionally, large-scale horizontal circulations are also formed in the whole cavity region. These multiscale turbulent vortices contribute to mass transfer through a mainstream / embayment boundary and a local sedimentation in the cavity. When the bed configuration varies in space and time, they give striking impacts on hydraulic characteristics. However, the relation between the local sedimentation and the distributions of mean velocity and turbulence is still poorly understood. In the present study, velocity measurements are conducted in a smooth bottom condition and movable one in order to consider how the turbulence structure and exchange properties changes.

抄録全体を表示

The necessity to understand and predict flows that carry large suspended sediment and wash loads has become acute in the Shirakawa River Basin where significant erosion and siltation associated with hyperconcentrated flood give rise to many river problems. Mud flows, debris flows or slurries, made up of a large amount of clay and/or silt particles suspended in water, often show non-Newtonian properties but remain poorly understood concerning the impacts of their rheological properties on fully developed turbulent structure.
In this paper, we experimentally investigated resistance and momentum transport of hyper- concentrated sediment laden-flow in an open channel with two-dimensional square ribs by using Particle Image Velocimetry(PIV). The results showed that rheological properties significantly dissipated the turbulent flow fluctuations over the dune bed and augmented the flow resistance by comparison with the clear water flow.

抄録全体を表示

Boulders are sometimes placed in rivers to enhance an ecological function by making diverse bed configuration. However, large-scale roughness causes water level rising in flood events. It is necessary for flood control measures to evaluate resistance characteristics of rivers with large boulders. In this study, water level was measured in an open channel with various patterns of cube groups. The resistance to flow of each case was evaluated as a water level rising ratio and Manning's roughness coefficient calculated from 1D numerical calculation. Then, 2D depth-averaged equations were applied to estimate the water surface profiles in these experiments. For improving prediction accuracy it is proposed to set the wake flow region behind roughness and the impinging area in front of roughness as the dead water zone.

抄録全体を表示

Understanding of hydraulic mechanism on and around the side weir is essential for the facilities to be designed and to be maintained well. Although past studies have advanced our ability to estimate the overflow discharge from side weir, detailed study of the tree-dimensional turbulent structure in an open channel flow with overflow from a side weir has been lacking. Authors have carried out an experimental study of the three-dimensional flow structure in an open channel flow of an aspect ratio two with overflow from a side weir. In this study, we carried out similar study on the case of open channel flow with an aspect ratio of three. The study found some differences in secondary flow characteristics between the aspect ratio two and the aspect ratio three.

抄録全体を表示

Groynes are recognized to have role of preventing bank erosion because they act as flow resistance and decrease velocity near banks. In general, water surface is elevated under larger flow resistance. However, if groynes are installed, river bed at the main channel part is scoured. So, groynes has those two opposite effects on affecting water elevation. Interaction between those two opposite effects seem to have key factor for determining water level though such features have not yet been fully discussed. In this study, we try to clarify the effects of groyne interval on flow resistance by both experimental and numerical approaches under fixed bed condition. The results showed that the apparent roughness changes considerably according to the groyne interval and it takes a peak at a certain value of the aspect ratio. It is also shown that the structure of vortex between groynes can be classified into three different types, which also have close influence of determining flow resistance due to groynes.

抄録全体を表示

This study aims to clarify the mechanism of ice jams on ice-covered rivers. We focused on the phenomenon whereby river ice of various sizes accumulates in narrow spaces over rivers. Before conducting an ice jam hydraulic experiment, we developed a calculation model for the occurrence of ice jams formed from river ice of mixed sizes under one-dimensional unsteady flow. The occurrence of ice jam was found to depend on the ratio of river width to ice size. The calculation model created in this study was found to be able to reproduce the water level of the ice jam experiment.

抄録全体を表示

The effect of the vegetation patch on the downstream flow and turbulence is investigated, including the effect of a length of a vegetation-patch on the development of mean and turbulence characteristics. Turbulence measurements were intensively conducted in partly vegetated-open-channel flows using Particle Image Velocimetry (PIV). The vegetation patch consists of emergent, rectangular array of cylinders in shallow flow. The mechanisms which control the development of coherent structure around the vegetation patch are also discussed. The results indicated that an increase in the vegetation patch length causes acceleration of the development of flow velocity and turbulence characteristics and increases the length of wake region downstream of the patch where its effect is significant.

抄録全体を表示

In this study, mechanisms of woody debris transport on the unsteady flow around a group of spur dikes were investigated by performing a flume experiment with the Particle Tracking Velocimetry (PTV) technique for physical models of woody debris and Particle Image Velocimetry (PIV) technique for particles of neutral buoyancy. The particle based numerical model that treats woody debris as combined particle elements was additionally applied in order to investigate the details of the debris motions. The experimental results show that the woody debris concentrated to the regions where the vorticity was relatively low. Meanwhile, woody debris diffused while they were transported donwstream as their length is longer. The calculation results imply that the diffusion of woody debris was caused by the filter-like effect by their spatial features.

抄録全体を表示

The high water level, HWL. for a design flood is the most important parameter in flood control and management. It is a critical issue how to determine Manning roughness coefficient in a vegetated reach, since channelized rivers tend to be densely vegetated in last decades. Although there are several sophisticated hydrodynamic models to analyze flows in vegetated channels, flow resistance due to vegetation is usually formulated in terms of a drag force coefficient which is not always a convenient parameter in flood control management. The HWL. is usually determined from a one dimensional nonuniform flow analysis in which Manning roughness coefficient is required in describing channel perimeter's roughness such as vegetation. In this study, an equivalent Manning roughness coefficient, nv, for a vegetated reach was theoretically estimated by using a two-dimensional two-layer (2D2L) flow model that was developed by the authors. The model's performance was already confirmed by laboratory and field experiments. A functioal dependency of nv on vegetation properties such as discharge, profiles of vegetation, stem diameter, tree height, etc. was investigated. It was found that nv was kept constant independently of water depth or discharge after the vegetation being submerged.

抄録全体を表示

Characteristics of secondary flow structure near side wall region of backward-facing step in open channel flow are investigated using velocity measurements and flow visualization techniques. The secondary flow structure near the side wall region before the step is composed of two rotational flows that the rotation direction is different each other on the side wall and the bottom wall. Whereas after the step these rotational flows disappear and change to lateral and downward flow and then the rotational flow on the side wall regenerates and develop faster than one of the bottom wall. The results of flow visualization indicate that streamwise vortex is formed on the side wall and the bottom wall before the step and the scale of streamwise vortex on the side wall becomes small according to diminish of streamwise vortex on the bottom wall after the step. Furthermore, the result of DPTV (Dye-streak pattern Particle Tracking Velocimetry) shows that streamwise vortex contributes to generating instantaneous secondary flow.

抄録全体を表示

Characteristics of turbulent structure of open channel flow with flexible vegetation model on the bottom wall were investigated using velocity measurements and flow visualization techniques. The velocity measurements indicate that vertical and horizontal mean primary velocity shears(∂U/∂y,∂U/∂z) and maximums of turbulence intensity and vertical Reynolds shear stress are formed in the vicinity of the top of the flexible vegetation model. The results of flow visualization indicate that large scale streamwise vortex is formed over the vegetation model and a pair of vortex structure having vertical axis is formed behind of the vegetation model. The results of DPTV, which is combined dye injection method and Particle Tracking Velocimetry, show that the streamwise vortex contributes generating instantaneous Reynolds stress directly.

抄録全体を表示

Willows are one of the most predominant species vegetated in many channelized rives. Willows tend to sprout and grow mostly along shoreline of the main channel. Since the riparian zone is exposed to river flow, willows vegetated there could be frequently destructed by flood flows depending on channel's geomorphology and flood frequency. If a river restoration would be properly carried out, overgrowth of riparian willows could be efficiently controlled and minimized by using natural flow forces. Therefore, it is a key issue to understand hydrodynamics of riparian willows for a labor-saving river management. In this study, a numerical analysis and laboratory experiment were performed in order to examine hydrodynamics of an open channel with compound cross section vegetated along shoreline.

抄録全体を表示

Understanding of the propagation mechanisms of the discharge and water level hydrographs in river channels is important for proper river management. It has been known that discharge hydrographs of flood flows passing through channel contractions are distorted by longitudinal difference of the storage volume of water in the channel. In this case, propagation mechanisms of the discharge and water level hydrographs are different, unlike in the case of the flood flow through a uniform channel. This paper characterize a part of the storage volume of water that causes the distortion of a discharge hydrograph as retarding storage volume of water and its evaluation method is proposed. We apply this method to a large flood through the valley of the Kitakami River and propagation mechanisms of the discharge and water level hydrographs are explained with relation to the storage volume and retarding storage volume of flood water.

抄録全体を表示

This study investigated the basic hydraulic characteristics of a bending river section with a cut-off channel by flume experiment. Contents of measurements included the longitudinal profile of water surface, flow velocity in cross sections by an electromagnetic current meter, and the surface flow velocity by using PIV methods. In addition, tendency of bed sediment motion was observed by bottom tracer experiment. Installation of cut-off channel resulted in a wide dead water area at the side of divergence in the main stream and low-velocity area at the center of the confluence. The point of surface flow which attacked the bank moved upstream in the main stream of distribution. Then the secondary flow became stronger, and the direction of bottom flow was strongly skewed in a transverse direction. Moreover, the bottom tracers moved and accumulated rapidly from the outer bank towards the inner bank at the curve of distributaries downstream. It suggested the occurrence of scour and sedimentation.

抄録全体を表示

In the event of tsunami during winter, damage can be incurred by tsunami intrusion into rivers such as collision between ice floes and river facilities, ice-jamming caused by deposited ice floes and bank overtopping of ice floes and consequent house damage in residential areas. The objective of this study is to clarify the ice floe behaviors during tsunami and to understand the mechanism which causes river ice-jamming. As the result, it was confirmed that ice-jamming occurred during the time of tsunami undertow. It was also revealed that ice floes were transported at the speed twice higher than in normal cases immediately downstream the ice-jamming point. Analysis by the authors succeeded in establishing a hydraulic experimental model to reproduce tsunami induced ice-jamming in rivers.

抄録全体を表示

Authors have applied the CRD scheme to the open channel flow, and shown validity as compared with the experiment-with-a-model result or the theoretical value. However, there was a case where a flow error arose at the place where river width changes rapidly. The cause is considered to be dry-wet boundary condition. In the paper, in order to make a discharge error small in a steep gradient river, the dry-wet boundary condition was improved. The improved model was applied to the Toyohira River which is a torrent municipal river. In this calculation, as compared with the past calculation, the discharge error became small.

抄録全体を表示

In a meandering river, the interaction of flow and bed deformation intensifies the stream shift. It is an important problem how to describe this process in the modeling of meandering river. This paper presents the method for simulating the time evolution of river bed topography in one dimension on the basis of existing meandering flow model. First, the fitting method for the bed topography of channel experiment data is discussed for the model calculation. The result of stream shift calculation under the condition of the fitted bed topography agrees roughly with the experiment data. After that, this paper mentions how to calculate the bed deformation, adding the continuity equation of bed load in one dimensional flow model. Although the result partially agreed with the experiment data, the phase lag of bed undulations to channel curvature was underestimated. Therefore, the method how to correct the equation and express the phase lag well is also discussed.

抄録全体を表示

This paper describes the basic characteristics of tsunami invasion processes over land after overtopping seawall. These processes are divided into two stage. In early stage, flow propagates over land as supercritical flow, soon after overtopping seawall. Inertia-pressure region appears in this stage. The effect of bottom-friction increases with time. As the result, hydraulic jump is established. After that, pressure-friction region appears. In order to discuss both regions, the self-similarity distribution analysis is applied and similarity solutions in both regions are derived. Theoretical findings are verified by comparing the numerical simulation results with experimental result. Especially, the numerical simulation results are in good agreement with the theoretical findings.

抄録全体を表示

Air entrainments by vortices at water surfaces inside surface intake facilities of electrical power plants cause some problems, such as deterioration of power generation, destruction of blade in a turbine, etc. To suppress air entrainments by the vortices, some vortex-preventing devices have been proposed and have been used in many electrical power plants. The vortex-preventing devices used in the intake facilities are working effectively in many cases. However, the hydraulic properties of flow structure by the vortex-preventing devices are still unclear.
In this paper, the effects of the vortex-preventing devices in surface intake facilities have been investigated experimentally by using the particle tracking velocimetry. The results show that the vortex-preventing devices suppress the turbulent intensity and the vorticity inside the facility.

抄録全体を表示

This paper describes the applicability of Boussinesq equation to air cavity advancing into a closed conduit filled with water. Hosoda et al. previously proposed the numerical method to track an interface between pressurized flow and free surface flow, using both the hydrostatic momentum equation and Boussinesq equation. But, the applicability of two models is not clear in view of the reproduction of air-cavity profile and free surface profile near an air cavity. In this study, numerical simulations were conducted under the conditions of previous experiments including free over flow, lock exchange flow and controlled buoyancy flux cases to identify the proper setting of computational methods related to the front speed of air-cavity, pressure drop near the cavity front, etc. It is pointed out that simulated air-cavity profiles obtained using Boussinesq eq. are in good agreement with experiments for all cases if the proper computational methods are used. It is also pointed out that the solitary wave solution is compatible with air-cavity profile of experiments.

抄録全体を表示

The phenomenon of injection into a parallel slot under laminar flow condition is investigated analytically and numerically in this study. A comprehensive mathematical model capable of simulating the injection phenomenon from the initial stage until late stage is developed. It is shown that a tank model is necessary in order to reproduce the initial stage characteristic of the penetration in the slot. Numerical model with higher scheme is used to validate the mathematical model. The model is extended to model an experiment setup where injection in a cylindrical pipe under constant pressure was carried out. It is shown that the mathematical model is capable of reproducing the flow characteristics and thus justifies the need to include a tank model to comprehensively model the injection or grouting phenomenon.

抄録全体を表示

An urban storm water runoff phenomenon in which manhole covers in a storm sewer pipe system are blown away has occurred frequently during heavy rain. This phenomenon occurs due to the compression of air in manholes and sewer pipes, and the compression is caused by the rising storm water surface. In this paper, an urban storm water runoff simulation model that can simulate the air movement and temporal and spatial variations of air pressure in the pipe system is presented. At first, basic equations for air flow and storm water flow in the pipe system are derived. Secondly, a numerical simulation method for the basic equations that is a characteristic grid method of specified time intervals is proposed. Finally, the applicability of the simulation model is examined by applying the model to runoff experiments. As a result, it is concluded that the model is a useful and practical simulation model, which can precisely simulate the air movement in the sewer pipe system during a heavy rain.

抄録全体を表示

The skimming flows in stepped spillways are classified into the nonaerated and aerated regions. For a large discharge and/or a small dam height, the nonaerated region may occupy a large portion of the flow in stepped spillways. However, most experimental studies for stepped channels have focused on aerated skimming flows, and the characteristics of nonaerated skimming flows have not yet been clarified. In this paper, empirical equations for the estimation of the flow depth and the boundary layer thickness are proposed in the nonaerated flow region. The velocity profile and the boundary layer development are shown, and the relative specific energy E/d_c (d_c=critical flow depth, E=specific energy) with the energy coefficient is obtained along the stepped channels. The effect of the channel slope and the relative step height on E/d_c is clarified on the basis of the boundary layer development.

抄録全体を表示

The applicability of MPS method with a modified model for non-uniform particle diameter and a method to suppress numerical pressure fluctuation, as an assessment tool for flow regimes in installing a current water wheel in an open channel as one of the development of micro-scale hydropower generation in irrigation systems, was examined. As a result, the turbulent flows around water wheel were calculated with stable condition. Furthermore, the simulated water depths at upstream side of water wheel were about 0.005m larger than the experimental results, at any given runner depths and rotation speeds. This error was equal to the distance between water particles at initial arrangement, and was only 3 % of water depth at the condition with no water wheel.

抄録全体を表示

Features of numerical errors of Tsunami simulation using unstructured triangular meshes with finite volume methods investigated quantitatively. A series of test simulations of the Tsunami wave propagating in thedee(are) p area shows that the numerical diffusion errors strongly depend on the spatial size of mesh, kind of the adopted numerical scheme and the travel distance to be solved. Furthermore, introduction of an Adaptive Mesh Refinement (AMR) technique to the triangular unstructured mesh system is investigated. By conducting numerical simulation of the Tsunami propagation for 2011 Tohoku Earthquake, it is found that the AMR technique can suppress the numerical errors and reduce the required computational loads.

抄録全体を表示

We developed a new RANS turbulence model for simulating flows with large scale vortex formations due to the shear instability. In this model, the eddy viscosity is set locally larger where the turbulence level is increased due to vortex shedding. The eddy viscosity is expressed by the summation of two terms, i.e., (i) effect of turbulence in a base flow and (ii) contribution by the vortex formations. The first term is set in proportion with the product of the local depth and the local friction velocity. For the second term, we considered two dimensionless parameters, i.e., Froude number and the bed friction number. The second term is modelled with the previous theoretical studies on the effects of those parameters on instability. The present model is validated through the comparison with the experimental data by Chu and Babarutui (1988) in parallel shear flows. Computational results showed that the present approach could capture well the velocity profiles around the shear layer even if a coarser computationlgrid is employed.

抄録全体を表示

In this paper, a three-dimensional numerical model using the Weighted Essentially Non-Oscillatory (WENO) scheme is developed. In the model, the Cartesian coordinate system was adopted, and the Fractional Area/Volume Obstacle Representation (FAVOR) method was introduced into the governing equations. The Highly Simplified MAC (HSMAC) method on the collocated grid system was employed in order to solve the Poison's equation of anomaly pressure. The model was verified against experimental data of flows around a submerged spur-dike on flat fixed bed and on fixed bed with local scour. By comparing the simulated results with the experimental data, the model was found to reproduce the flows around a submerged spur-dike with sufficient accuracy.

抄録全体を表示

Numerical models for 2D free-surface flows and sediment transport are developed. The models are based on finite volume method using HLL (Harten, Lax and van Leer(1983)) numerical flux, which are based on approximate Riemann solver. These models deal with the system equation formed by the 2D shallow water equations and the Exner equation as a coupled or decoupled system. These models are verified against two experimental data of unsteady dam-break flow over erodible bed. It shows that the models can reproduce the complex behavior of the flows and sediment transport with reasonable accuracy.

抄録全体を表示

In this study, a computational method was developed to predict thermal interactions between immiscible two fluids and complicated-shaped solid objects on the basis of the multiphase model. In our numerical method, thermal and mechanical interactions between fluids and complicated-shaped solids are predicted accurately. Phase-averaged governing equations are discretized with a finite volume method (FVM) and SMAC method is used as a numerical algorithm. The present method was applied to natural convections in two immiscible fluids and porous media with which the voids are filled by two immiscible fluids. As a result of computations, velocity and temperature distributions are reasonably predicted.

抄録全体を表示

In open-channel turbulence with relatively large Reynolds number, water surface is subject to various types of fluctuations depending on the strength of turbulence. The level of fluctuation varies also with the Froude number and several researches have been conducted with respect to the root-mean-square value of surface oscillation. However, the mechanism that creates such water surface features has not been executed in detail in terms of turbulence vortex structure especially in rough-wall turbulence. This research aims at extracting a vortex structure that directly affects the water surface phenomenon by using an experimental technique with PIV and a sophisticated numerical technique with LES. A three-dimensional vortex structure that has an impact on surface features was successfully extracted from the LES results by using a conditional sampling technique.

抄録全体を表示

Numerical simulations of a two-dimensional uniform flow past arrays of 50 circular cylinders of uniform diameter D have been performed for solid volume fraction \phi=0.349, 0.196, 0.126, 0.087, and 0.049 at a Reynolds number Re_D=U₀D/\nu=180, where U₀= inflow velocity and \nu= kinematic viscosity. Three types of array configuration, a lattice-type square pattern, a staggered pattern, and a random distribution are adopted. As \phi is increased, the bulk drag coefficient for the array of cylinders tends to decrease and becomes less sensitive to the details of the array configuration. Hydrodynamic force acting on a cylinder in an array can markedly increase or decrease depending on the allocation pattern of the cylinders. The maximum drag force acting on a cylinder is found to reach up to seven times as high as the time average, indicating the importance of the fluctuation properties of hydrodynamic parameters.

抄録全体を表示

It is important to predict the movements of multiple solid objects driven by free surface flows in case that the flood and Tsunami occur. In this study, essential improvement was performed for our multiphase parallel computational method, in which multiple complicated-shaped solid objects are transported by the pressure and viscous effects of the predicted multiphase model. Our parallel computational method is based on the domain decomposition method. Thus, the improvement was achieved by dynamically adjusting only the ``block-computational domain'' according to the number of blocks included in them. On the other hand, the ``fluid-computation domain'' is fixed throughout the computation. As a result, we found 64-process simulation of water column collapse, including 12,800 objects in the water, exert 24.52 speedup compared to sequential execution.

抄録全体を表示

Elements of a Large Eddy Simulation (LES) method are examined emphasizing application to practical real-scale open-channel flows. Though LES methods have been validated by developers as specific methods and models are proposed or applied, most of these validations are not intended for hydraulic applications. We validate a LES method for analyzing open-channel flows in real applications such as design of hydraulic structures. It is shown that proper treatment of wall boundary condition is necessary and a simple but practical wall stress model is sufficient to obtain accurate mean-flow and the instantaneous large-scale turbulence structure with moderate grid resolution. It is suggested that the turbulent stresses are determined more by shear-stress carrying large motions than by energy-dissipating small scale motions. Specific examples include very high Reynolds number flows in smooth and rough surface channels with three-dimensional mean flow and large changes with separation and recirculation.

抄録全体を表示

A retarding basin, that temporarily stores stormwater to reduce downstream flow rate, is one of the most effective countermeasures to mitigate existing downstream flooding problems. Effectiveness of the basin is strongly dependent on design of an overflow levee. Experimental works have been commonly employed for the overflow levee design, because the flows around the levee are very difficult to be handled by theoretical and/or numerical methods. The objective of this study is to demonstrate that a numerical model, which comprises the finite volume method based on unstructured grid using FDS technique along with appropriate boundary conditions, is effective to calculate the stage-discharge relationship of the Mutabe retarding basin in the Rokkaku River.

抄録全体を表示

A retarding basin, that temporarily stores stormwater to reduce downstream flow rate, is one of the most effective countermeasures to mitigate existing downstream flooding problems. Effectiveness of the basin is strongly dependent on design of an overflow levee. Experimental works have been commonly employed for the overflow levee design, because the flows around the levee are very difficult to be handled by theoretical and/or numerical methods. The objective of this study is to demonstrate with the use of data of scale model experiments that the numerical model along with appropriate boundary conditions is effective to calculate the stage-discharge relationship, that solves simultaneously flood flows in a channel and inundation flows in a flood plain with the dynamic model, which comprises the finite volume method based on unstructured grid using FDS technique.

抄録全体を表示

Vegetated flow in open channel is studied by many researchers. However, there are few studies about steep slope flows with vegetation. This study aims to predict turbulence property of this flow so as to estimate the degree of erosion at levees when flood flow occurs. Two models and two coefficients are used in this study. There are significantly different results due to the difference of coefficients which is not optimized from those which is optimized. Therefore, it is important to optimize coefficients to be applied. The prediction of turbulence property was improved by optimized coefficients in this study.

抄録全体を表示

We propose a simple numerical method to describe the air-water gas transfer at high Schmidt number. A numerical simulation is carried out in terms of the advection-diffusion equation using interfacial velocities obtained from a direct numerical simulation of open-channel turbulent flow. The present numerical method can describe the high Schmidt number air-water gas transfer driven by turbulence dynamics of the open channel flow. The numerical results show the surface divergence on the air-water interface to promote effectively the interfacial gas flux. Characteristics of surface-renewal eddies are found on the basis of cross-correlation coefficients between the interfacial gas flux and turbulent quantities such as the surface divergence and the vorticities.

抄録全体を表示

Schmid & Henningson discussed the point disturbance in the homogeneous medium of piecewise linear boundary layer using the method of stationary phase. In addition to this study, we include the density profile to consider the inhomogeneous boundary layer like atmospheric boundary layer. We assume first that the fluid is inviscid and incompressible, density being convected but not diffused. Then the fundamental equations are the Navier-Stokes equations, continuity equation and incompressibility equation and we derive the 3-dimentional Taylor-Goldstein equation. Under the Boussinesq approximation, we obtain the exact solution about the point disturbance propagation on the arbitrary horizontal plane. Using internal Froude number, we explain the characteristics of the solutions.

抄録全体を表示

This study proposes a new depth-averaged two-dimensional convection-dispersion model to investigate the seawater intrusion in well-mixed estuary. The new model considers both barotropic salinity flux and the baroclinic flux caused by weak salinity stratification. Liner salinity profile was assumed and physically meaningful baroclinic flux model was constructed base on hydrodynamic consideration. Then, model verification was carried out by simulating seawater intrusion in Takase River. In the new model, the salinity front arrived earlier and the peak value became stronger, which agrees better with the field data measured at Takase Bridge.

抄録全体を表示

Very heavy rain on July 28th in 2013 in Yamaguchi and Shimane prefectures caused severe damages of bridges. It is considered that not only the extremely high flood discharge but also accumulation of large amount of driftwoods caused the failure. In this study, at first possibility of obtaining a force acting on the bridge using three-dimensional flow analysis was examined. Then hydrodynamic forces acting on the bridge under different water level condition and with or without driftwoods accumulation were also investigated. The numerical results show that the force acting on the bridge can be estimated using three-dimensional flow simulation. And it is clarified that the force acting on the bridge under over flow condition of bridge with driftwoods accumulated state increases ten times larger than ordinary flood discharge state

抄録全体を表示

In summer, localized heavy rainfall in a short time on Kofu Basin is caused by a cumulonimbus cloud. We analyzed the characteristics of distribution and preceding surface conditions of cumulonimbus clouds appeared on Kofu Basin using X-MP radar and surface observation data from July to September in 2012. The frequency of the cumulonimbus clouds appeared on Kofu Basin was low. However, the rainfall amount brought by such a cumulonimbus cloud was large. The first appearance of cumulonimbus clouds concentrated from east side to center of Kofu Basin. Before the appearance of a cumulonimbus cloud, south-southwesterly wind with warmish and moist air from the inlet of the valley on southwest side of Kofu Basin and southeasterly wind with warmish and semimoist air crossing the mountains on east side of Kofu Basin co-existed at surface on Kofu Basin with warm and low moist air. The winds formed the convergence and the airs formed the conditional instability on Kofu basin, which formed the environment of the appearance of a cumulonimbus cloud on Kofu Basin.

抄録全体を表示

We have developed a method to combine extrapolating model and meso-scale meteorological model to improve the accuracy of nowcasting model until a few hours ahead. In the developed model, outputs obtained by extrapolating model and meso-scale meteorological model are combined with the optimum combination factor. The optimum combination factor in prediction lead time is estimated from error variance and bias error of latest outputs from each model. We applied the method to a precipitation event in Kinki region. The results showed higher accuracy than those of each model.

抄録全体を表示