Intertidal mudflats are important for land conservation, flooding risk alleviation and biological environment. Mudflat profiles have been measuring monthly along some cross-shore lines exceeding 1,000 m in the vicinity of a river mouth since December 2000. For the mud profile evolution, the annual accretion rate was 4.2 cm/year and 1.6 cm/year on the right and left lines, the seasonal variation in the range -5 cm and 5 cm, and the maximum episodic variation 12.1 cm and 22.0 cm. The wind waves could play many contributions for deposition and erosion of sediment. Sediment budgets based on the monthly bed level and net sediment flux monitoring were estimated to examine the relative contributions of tides and river discharge to sediment transport. The estimated sediment budgets are interpreted using the water mass balance equation and the horizontal tidal current pattern. The intertidal flat accreted during normal discharge conditions are primarily attributable to the alongshore sediment flux toward the river mouth. However, the flat was eroded when the large offshore suspended sediment transport occurred on the flat during the large river discharge. The net alongshore tidal current causes alongshore sediment fluxes toward the river mouth on this intertidal flat adjacent to the river mouth. In observation periods, there were two times of a significant flood occurred on July 12, 2012 with the discharge water rates of 2,300 m3/s and on June 21, 2016 with 1,600 m3/s. Also two earthquakes scaled of 7 on the Japanese scale struck Kumamoto Prefecture on April 14 and 16, 2016, and then about the 0.4 m ground subside was occurred at the present field site. A large amount of sediment from Shirakawa River was discharged into the delta due to the flood. The clinoform along the water route approximately propagated 150 m in the offshore direction and the bed level increased by 1.5 m. The estimated sediment discharge was 1.1x105 m3/yr./km2 during 2014 to 2016, corresponds four times during 1978 to 1997.
Field measurements were carried out in the port of Niigata to elucidate sediment transport processes around navigation channel and turning basin in the port at river mouth. The measurements include salinity, suspended sediment concentration and in-situ mud density at several monitoring points along the navigation channel in the port. The survey successfully captured the formation of three layers structure during a river flood condition with high turbid fresh water in the upper layer and low turbid sea water in the middle layer. Furthermore, high concentrated mud or fluid mud layer was also found in the lowest layer at the deeper dredged channel area. Physical model experiments were also carried out with a circulating flume to elucidate the fluid mud transport on the slope at the edge of step-like bathymetry. The experiments indicate the behavior of near bed turbid water depends on the overlying flow conditions.
The field observations of cross-shore sediment transport from the swash zone to the offshore side of the outer bar using fluorescent sand tracers were conducted in 2014 and 2015 at the Hasaki coast, Japan facing the Pacific Ocean during storm wave conditions. The topography difference between the two years data set is that outer bar shape existed in 2014 and not existed in 2015. During the observations, wave height and wave period were observed, and sand core samplings were conducted. By using the observed wave data and the analyzed results of collected fluorescent sand tracers from the cores, the sediment movements from the offshore side of the outer bar area to the swash zone during storms were investigated. The results indicate that the sediment movements during the storms were affected by the outer bar shape. If the outer bar existed, the sediment of the onshore side of the outer bar tend to move landward, and it also moves to seaward due to undertow until the outer bar location. The sediment at the offshore side of the outer bar tends to move to onshore ward due to bedload sediment transport. However, the transport rate decrease in the trough region. The beach topography without the bar shape, the bed load sediment transport rate at the trough region was higher than that with the bar shape. Thus, the sediments at the offshore side of the bar also could move until the swash zone.
Future beach loss in Japan are reported to be more than 90% due to sea level rise in the worst case. Beaches are affected not only by the sea level rise but also by rainfall and wave characteristic changes and others. This paper reviews past and future beach loss in Japan in a national scale and introduces projection method of future beach loss due to sea level rise and rainfall characteristic change. Even past beach loss and sediment budget are not easy to be quantitatively elucidated in coastal catchments in a national scale, then realistic future beach loss projection is much more challenging. Both further past data analysis and beach profile model improvement/development are necessary for better beach loss projections.
To clarify the mechanism of increase in the power loss for gas-liquid two-phase flow driven by a rotating disk in the cylindrical container, we investigated the influence of kinematic viscosity on torque acting on the surface of the rotating disk. For high viscous liquids, the torque on the disk in the two-phase flow is smaller than that in the single phase which is full of liquid, while for low viscous liquids, the torque in the two-phase flow is larger. We measured the spatial distribution of turbulent intensity via 2D-PIV, and visualized the vortical structure appearing near the free surface and the static casing in the two-phase flow condition via stereo-PTV. We discusse the effect of modifications in flow field on the torque changes in the two-phase flow condition.
This paper shows bubbly flow characteristics measured of standing flow and unsteady flow in a Venturi tube. As for the standing flow characteristics, the distribution of pressure and void fraction are measured. Using the equation of sound velocity in adiabatic isentropic, the shock waves occurs at the point of transition from supersonic flow to subsonic flow, and the bubbles are collapsed at the same area. As a non-steady phenomena, by performing visualization and synchronous measurement of pressure, the presence of propagating pressure wave caused by bubble collapse. The bubble diameters generated by using this device showed a tendency to be further miniaturized under both condition that a shock wave is generated and a pressure wave is generated.
Numerical simulations are powerful tools to study tsunami impacts on building structures. We have developed a CFD code for free-surface flows interacting with floating debris by using Lattice Boltzmann Method (LBM) and Discrete Element Method (DEM). Both methods are suitable for GPU computing and large-scale simulations because they are explicit time-integration schemes. In order to improve the accuracy and the stability of flow computation, the cumulant LBM model has been employed and coupled with the conservative Allen-Cahn equation for the purpose of free-surface capturing. A moving boundary approach based on the interpolated bounce-back scheme is utilized at liquid-solid interfaces. Rigid body dynamics of floating debris is computed by using DEM. A model constructed by multiple small spheres represents complex shapes of debris. We measure the weak scalability on multiple GPUs of the TSUBAME3.0 supercomputer, and the code achieved 83.4% parallel efficiency when scaled 16 to 256 GPUs. As a test case, we demonstrate a tsunami flow with driftwoods using 200×600×3000 lattice nodes and 24 GPUs Tesla P100, and the simulation has completed within 24 hours. The impact force of the driftwoods acting on the wall is about 15 times larger than only water. It shows that effects of driftwoods are important to evaluate the tsunami damage on building structures.
Gabor holography has a problem of image reconstructed elongation in the optical axis direction. It is known that the elongation length depends on the diameter of the recorded particle and the wavelength of light. In this study, the phase retrieval holography with two holograms is used for reconstruct particle images in numerical simulations and performance test. In numerical simulation, we find the correlation between particle diameter and elongation by measuring the elongation of reconstructed image of particles with given diameters. In the performance test, we record and reconstruct holograms of the particles with the diameter distribution, and measure the elongation of reconstructed particles. Then, we applied the result of numerical experiments to the elongation lengths, and estimate the droplet diameter distribution. As a result, it is shown that this method can estimate particle diameter distribution close to the given distribution.
Highly accurate measurement of thermal properties, reaction rates and mass transfers is necessary to grasp thermal flow characteristics of nuclear reactor plant because these properties are unsteadily sensitive to concentration, temperature, pressure and scale. On the real scale of the plant, however, there are problems such as the complexity of three-dimensional flow and the effect of convection. In this study, a novel technology that measures mass transfer and reaction rate of nuclear application material with high accuracy and high speed by applying microchannel which makes one-dimensional laminar flow, has developed. In this paper, a micro flow control system that can adjust flow with a minimum volume of 1 μL/min. by a pressurized pump, was fabricated. In order to observe concentration diffusion of the flow field in real time, a near infrared spectroscopic visible observation device was developed and combined with the flow system. The mutual diffusion coefficient was measured for hydrazine hydrate, which is added for the purpose of preventing corrosion of secondary piping in nuclear power plant. It revealed that the absorption depends on the concentration near 1555 nm for Fourier transform near-infrared spectrophotometer. The diffusion coefficients of 5, 10 and 20 wt% for hydrazine hydrate were quantified with the luminance profiles in the absorption peak wavelength.
This paper derives a nonlinear wave equation for plane progressive quasi-monochromatic waves in an initially quiescent compressible liquid containing many spherical microbubbles. Main assumptions are as follows: (i) The wave frequency is larger than an eigenfrequency of single bubble oscillations; (ii) the compressibility of the liquid phase is incorporated; (iii) the phase velocity is larger than the speed of sound in a pure liquid; (iv) the effect of viscosity in the gas phase, heat conduction in the gas and liquid phases, phase change across the bubble wall, and thermal conductivities of the gas and liquid, are neglected. The governing equations for bubbly flows are composed of the conservation equations of mass and momentum in a two-fluid model, equation of bubble dynamics, equations of state for gas and liquid phases, and so on. By using the method of multiple scales with an appropriate choice of set of scaling relations of nondimensional parameters with respect to speed, length, and frequency in terms of nondimensional wave amplitude, the nonlinear Schrödinger (NLS) equation with a dissipation term and some correction terms can be derived from the governing equations, which describes a long-range wave propagation with dissipation and dispersion effects. The decrease of the group velocity in a far field is clarified.
Ultrasound effects on the motion of a falling sphere in water (i.e. Newtonian fluid) and in PolyAcrylAmide (PAA) solution (i.e. non-Newtonian shear-thinning fluid) are experimentally investigated via an image processing technique. The falling speed in water is confirmed to be almost irrespective of the frequency or amplitude in the applied ultrasound, while that in PAA solution is found to be higher with increasing the ultrasound intensity. In consideration of pseudo-plastic factors estimated by various velocity and length scales involved in the system, we consider that the mechanism to enhance the falling speed has the influence of viscosity in the acoustic boundary layer and its thickness.
A weakly compressible scheme for low-Mach number gas-liquid two-phase flows have been developed for a full-explicit time integration to avoid solving Poisson equation for large-scale two-phase flow simulations. To describe the gas-liquid interface the conservative Allen-Cahn equation, which is one of the phase-field models, is introduced and combined with the continuum equation. The accuracy of numerical results of two-phase flow strongly depends on the mesh resolution near the interface. The AMR(Adaptive Mesh Refinement) method greatly reduces the computational cost to assign high-resolution mesh to the region around the moving interface. We have developed a GPU-code of the tree-based AMR. We choose the node-centered configuration and the interpolation can be closed within a leaf in refining the leaf. We successfully have carried out two-phase flow simulations including very thin liquid film efficiently.
In industrial washing process, using chemicals causes increase of environmental load and processing costs. In this study, we focus on ice jet washing technology with ice particles generated in a converging-diverging nozzle. The purpose is to clarify the mechanism of the ice formation phenomena in the nozzle and to evaluate washing capability in ice jet washing technology. By visualization in the nozzle, we verified that shape of the nozzles was important factors to ice formation. Second, we constructed a numerical calculation system of quasi-one-dimensional stationary air single phase flow. It was indicated that a supersonic flow was formed in the diverging section and the pressure and temperature decreased in the ice formation area. Furthermore, a reverse pressure gradient was appeared with the generation of pseudo-shock waves and cooling of water was promoted. Finally, washing tests using fine particles simulated dirt was conducted and high washing capacity of the ice jet was verified. Improvement of washing efficiency was expected with the design of the optimal nozzle to ice generation.