[in Japanese]

View full abstract

The rupture of single bubble at water surface was visualized by using a high-speed video camera. Surface tension and viscosity of water are changed by adding ethanol which dissolves into water. A technique is proposed in order to separately evaluate effects of viscosity and surface tension on the droplets generation from bubble eruptions by utilizing the peculiar characteristics of the solution. In the observation of a bursting bubble at water surface, two kinds of bubbles, of which diameters are 2mm and 20mm respectively, are tested. Observation results support the following conclusion; (1) surface tension changes the size and speed of the jet drops of a 2mm bubble bursting, (2) viscosity affects the generation of jet drops of a 2mm bubble bursting, (3) the size, speed and direction of film drops generated from a 20mm bubble are changed by surface tension or viscosity.

View full abstract

First-and second-order accurate 2D numerical models based on finite-volume method for flood flows are developed. The models employ unstructured grids so as to handle a complicated floodplain geometries. The first-order accurate model uses FDS technique. The second-order accuracy, in space and time, is obtained by using MUSCL technique and predictor-corrector time integral sequence in the first-order model. The models are verified against experimental data of dam-break flow in converging-diverging channel as well as 2D flood wave due to partial dam-break under dry-bed condition. The first-order accurate model may be preferred for practical applications when computation time and overall accuracy are considered.

View full abstract

Turbulence measurements in open-channel flows with strong unsteadiness were conducted with a two component laser Doppler anemometer (LDA). The water-surface fluctuations at three points were measured by making use of 3 sets of ultrasonic wave gauges. It was found that the Karman constant κ can be dealt with as a universal constant (κ=0.41) in the case that the unsteadiness parameter α is smaller than 0.002. In contrast, the integration constant A of the log-law changes in a complicated manner. The time-dependent property of each term in the momentum equation (4) was examined on the basis of experimental data. The gravity term is smaller than the pressure and acceleration terms. The pressure term has an effect of increase upon the bed-shear stress τ_b in the rising stage, and vise versa in the falling stage. On the other hand, the unsteady term has the opposite effect to the pressure term.

View full abstract

In curved compound channels, cross-sectional secondary flow is generated by the imbalance of centrifugal force, and horizontal vortices are induced at the junction of a main channel and flood plains by shear instability. Numerical computations using quasi-3D model was performed, and the results were compared with the experiments to know the flow field associated with the secondary flow and the horizontal vortices. As a result, this model was shown to be appropriate to simulate the entire flow field except for the inner junction of the main channel and the flood plains where 3D-turbulence associated with boils is predominant. Based on the results, numerical tests were performed by varying the water depth, the main channel width and the curvature, and their effects on the flow field and the lateral transport of fluid momentum were examined.

View full abstract

At present, the wave-current interaction model proposed by Craik & Leibovich is accepted commonly as a formation theory of Langmuir circulations. However, it leaves some features to be examined and solved. In this study, two types of experiment were carried out on the basis of the idea that the two-dimensional wind-driven current might become unstable owing to small disturbances and change to three-dimensional one with Langmur circulations. One is the case of wind waves and the other is the case of suppressed wind waves. In conclusion, it is shown that the Langmuir circulations are formed in the case of suppressed wind waves. However, the circulations in the case of wind waves are more stable and stronger than those in the case of suppressed wind waves.

View full abstract

The response of a breakwater and the underlying seabed in the presence of waves has been recognized as an important design problem in the field of harbor and coastal engineering. In this research, an analytical model is proposed for the response of a composite-type breakwater and seabed to sinusoidal standing waves. In the proposed analytical model, the problem of the response is decomposed into the wave mode, called a scattering problem, and the caisson motion mode, called a radiation problem. The model is characterized by solving each problem using a complex Fourier series technique. The model is verified, fast, by comparison with an analytical solution of the simpler problem of a caisson on a seabed, and secondly, by comparing it with a finite element solution for the response of a composite-type breakwater (caisson on a rubble mound base). In both cases, the comparisons are in good agreement for pore-water pressure.

View full abstract

From the numerical analysis for historical tsunamis, the dispersion effect on the propagation of tsunami in deep sea was investigated. Tsunami time-histories at the point of 200m in depth and spectrum analysis on the initial wave profiles showed that the dispersion effect in deep sea region should be considered even for near field tsunami. The index of dispersion effect considering the spectrum of initial wave profile and the water depth was obtained to determine the critical value where the dispersion term should be taken into account in the simulation. The empirical quation to estimate the index by using the fault parameters and the mean depth in tsunami source was also considered. From numerical simulation considering sea bottom motion, it was shown that the raising time of sea bottom motion affects the dispersion if it is less than 1min. Examinations were carried out on the effect of dispersion in deep sea area on the tsunami behavior in shallow water region for the case of 1983 Nihonkai-chubu tsunami.

View full abstract

This paper has presented the results of numerical and experimental investigations on the flow fields around the base of a vertical circular cylinder subjected to the oscillatory flow. The calculated results from the three-dimensional unsteady Navier-Stokes equation have shown that the development of horseshoe vortex and lee-wake vortices governed by the Keulegan-Carpenter number agree reasonably well with those measured by Particle Image Velocimetory (PIV). Detailed analysis of numerical simulations has indicated that the horseshoe vortex exhibits a unique flow pattern which is different from the vortex topology in steady flow fields. The characteristics of lee-wake vortices near the bed and the bed shear stress beneath those vortices have been investigated in detail.

View full abstract

The dispersion relations and the approximate solutions of solitary-wave type are derived for several depth-averaged and depth-integrated nonlinear dispersive long wave equations. Solutions of depth-averaged equations for solitary wave tend to provide steep wave profile with narrow-width and high-wave hight compared with KdV solution. On the other hand, solutions of depth-integrated equations for solitary wave provide wide wave profile with wide-width and low-bight. On wave deformation and disintegration to the solitons, the difference is quite important. Not only theoretical examinations but also laboratory and numerical experiments show that depth-integrated Peregrine equation and Madsen & Sørensen equation are suitable for numerical analysis of tsunami.

View full abstract

Computer code (FRESH) was applied to a locally high runup phenomenon at Monai district in 1993 Hokkaido Nansei-oki tsunami. The results are summarized as follows. 1) The computed tsunami behaviors temporally and spatially agree well with an experiment result of 1/400 non-distorted scale-model. 2) The computed runup heights agree with tsunami marks including the maximum height (31.7m) at Monai district. 3) Main factors of the locally high runup are the steepness of tsunami wave, angle between coastline and tsunami attack direction and the configuration of the south valley. 4) In runup area, while water level rises, vertical velocity has large value that is not uniform in a vertical direction.

View full abstract

The author derives an analytical solution of tsunamis which are obliquely incident on a offward-finite constant slope. The incident waveforms are assumed to be transient. Based on the solutions obtained, characteristics of transient tsunami propagations on the slope are discussed in terms of the several conditions of incidence such as horizontal incident wave scale, incident angle and incident waveform. When tsunamis are incident almost parallel to the shoreline, multiple reflected waves reveal and are trapped on the slope. The solution indicates that the interference between multiple reflected waves and incoming incident waves could be a contributing cause of conspicuous amplification of tsunamis on the slope.

View full abstract

Field observations on the characteristics of primary production of periphyton were performed at riffles and pools in Tama-river, where many artificial gravels were settled on the bottom. The amount of periphyton growing on them was measured every week for 55 days. It was found that the periphyton growing on the gravels at riffles has larger photosynthetic activity than that at pools. The measurements on the growth of periphyton were conducted in 4 tandemly placed laboratory flumes, in which the environmental conditions except for the hydraulic conditions were controlled to be the same among the flumes in order to estimate the effects of hydraulic characteristics on the primary production of periphyton. The results of the experiments indicate that the primary production of periphyton takes large value for the case where the near-bottom turbulence is large.

View full abstract

Lake Ogawara is a brackish water lake where the most dominant water organism for fishery is Corbicula japonica. There is a mystery about the reproduction of the clams in the lake: The usual salinity concentration of the lake is only 1 psu, which is far lower than the salinity needed for the spawning of the clams. In this study, reason of the mystery is investigated from the standpoint of hydraulics by field measurement, numerical simulation and statistical analysis. The results show the following: The salinity condition favorable to the spawning of the clams is developed in a restricted area near the lake exit in a restricted period of seawater intrusion and the expectation of this condition is considered to be frequent enough for the propagation of the clams. The spawned larva is diffused by wind driven current and can spread over whole area of the lake within floating period of about only 1 week.

View full abstract