When water flows through broadly graded soil, the small particles within the soil may erode, thereby leading to a narrowing of the grading. An important goal in geo-engineering is to gain a better understanding of the mechanical consequences of this type of event, with attention directed particularly toward the potential for ground deformation or failure due to erosion.
In this study, we focused on the mechanism of internal erosion and, from the viewpoint of instability, defined such erosion as the increase in particle movement and in the extent of macroscopic erosion. On this basis, when a given particle erodes, the pore structures of the rest of the particles should enable exit from a movable void, and their interparticle contact structures should enable egress by seepage force. Pore structure primarily pertains to void size and void continuity. Contact structure is estimated by average coordination number and fabric tensor. To illustrate internal erosion accompanied by changes in grading, we conducted experimental and computational analyses. Through a seepage test with particle erosion, we obtained the grading shaped-induced quantitative difference and trend in the weight of leached particles. With regard to permeability, all the grading values indicate a decreasing trend. Because specimen height was kept constant during the experiment, permeability could have increased given the rise in voids due to particle erosion. On the basis of the decreasing trend, however, the increased permeability can be regarded as a case of particle clogging. The pore structure analysis indicates that at the logarithmic scale, the index shows qualitatively good agreement with the extent of erosion from various grading conditions. As for the contact structure, the average coordination number of the particles and the particle voids also exhibit good agreement with erosion behavior.

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In this study, the effect of shape representation in rockfall simulations using DEM was investigated. A model test and it's simulation were performed. In the simulation, differnt numerical models were prepared by changing shape representation to figure out a quantitative relationship between shape representation and calculation accuracy. Every simulations were done in the same condition as experiment, except the shape representation quality of calculation models. Two non-dimensional indexes, namely ESR and OR, were defined to enable quantification of the shape representation quality. Eventually, the relationship between the shape representation quality and the accuracy of DEM simulation were found through comparison of simulated data with experiment. Based on the results obtained in this study, the framework to enable quantitative design of calculation accuracy by controlling shape representation quality was proposed.

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Site amplification under the strong ground motion is an important factor to quantify the earthquake hazard in a particular region. Recently, the site amplification in the frequency domain has been modeled by a superposition of ground transfer functions for two-layered model based on the existence of the series expansion for L² functions. In this study, I focus on the discrete representation of the series expansion, and discuss its characteristic. In addition, I introdue its application to decompose the major component represented by the two-layered simple transfer function, and its performance.

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This paper presents the implementation and optimization of hybrid parallel programming model to an FEM-based simulation tool for earthquake wave propagation. A prepartitioning method is implemented to avoid hardware memory limitation problem in mesh generation of billion order DOF model. Load imbalance is addressed by developing a load balancing procedure based on two-dimensional irregular grid, and by managing the workload of threads in the implementation of hybrid MPI-OpenMP. For a simple (flat surface) model with 1.82 billion DOF, the improvement in parallel efficiency reached 27%, compared with the simple hybrid implementation. For a realistic crust model with about 5.8 billion DOF, the improved code resulted to speed up of up to 6.8 in using 8 cores per compute node, when 2,304 nodes of K computer were used.

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Due to March 11th, 2011 Tohoku Earthquake Tsunami, both the disaster prevention problem against tsunami and the energy generation came into view. With the aim of solving the two problems, “Coastal Inverse Dam Project” was established led by Kyoto University. Coastal Inverse Dam is the structure designed to store renewable electricity set on the sea bed and has a huge box-like space open to the surface, so the structure is expected to have an effect on disaster mitigation by tsunami damping. However, there has been no examination of the tsunami damping effect. In this study, considering an assumed Nankai Trough Earthquake Tsunami, we calculate and evaluate the effect of tsunami damping according to water elevation behind the Coastal Inverse Dam by applying a 2D3D Hybrid model. We find that the dam has the measurable effect of declining water elevation, and the height on the coast in its vicinity.

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In this study, the repairing method of filling concrete for circular steel bridge piers which have different diameter-to-thickness ratio is to be verified. First, static cyclic loading tests until reaching their predetermined damage were conducted. After filling the concrete inside the specimens, the same static cyclic loading tests were carried out to clarify the seismic performance of repaired piers. Then, the seismic performance has been evaluated by the load-deflection relationship and the damage shape during cyclic loading.

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Initial distortion and residual stress are well known as the most important initial imperfection factors and have a significant effect in the decreasing on compression ultimate strength of thin walled structures. The present work focuses on influence of initial imperfection factors to ultimate strength of steel columns especially for welded box section compression members. Beam and developed shell Finite Element (FE) models are used under compression loading. The result comparison between FE model's and ultimate strength formulations in some specifications are used to measure the result accuracy and accomodate the ultimate strength formulations of bridge specifications according to the behavior of current steel compression members. Result of analytical study indicates that both FE models have good similarities with the formulations that have stated in specifications especially JSHB 2012 which means that the specifications have good capability in illustrating behavior of steel compression members.

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We propose a meta-modeling approach for more reliable seismic damage assessment of large pipe networks, and implementation of automated system to realize it. Due to various difficulties in assessing seismic damage of large pipe networks, empirical relations are being used in seismic damage predictions. Though these empirical models are continuously updated, their reliability is low. On the other hand, sophisticated nonlinear FEM models and advanced materials are being used in designing new pipe lines. More rational, and possibly more reliable, damage assessment is possible, if the advanced analysis methods used in the seismic resistance design are employed for damage assessments. However, analysis of large pipe networks with sophisticated FEM models is challenging. Meta-modeling makes it possible to make efficient seismic damage assessment of large networks without loss of accuracy. Meta-modeling uses models of different fidelity for modeling the same structure, depending on the required accuracy. An automated system is developed for analyzing the seismic performance of pipe networks, with meta-modeling approach. The details of the automated system and a simple example to demonstrate the application of the automated meta-modeling system are presented.

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In this paper, we developed an optimization method for disaster recovery process of large-scale road networks using parallel genetic algorithm. Subpopulation-based asynchronously model is used for parallelization scheme. The algorithm is applied to both a model network and emergency routes in Yamanashi prefecture. Numerical results verify the proposed algorithm.

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Intermittent aftershocks can occur in a long period after a strong mainshock. There is a high risk that aftershocks may result in further damage to structures damaged in main-shocks, and hamper retrofitting of important structures.
This paper aims to clarify seismic response of a long-span cable-stayed bridge damaged by a mainshock, subjected to intermittent aftershocks. Seismic analyses of the bridge subjected to a mainshock, and followed artificial successive aftershocks are conducted. Based on the analysis results, the structural safety and serviceability of the bridge are evaluated with the consideration of both the effect of the main-shock and aftershocks. As it is difficult to predict aftershocks as accurately as mainshocks, due to complicated mechanisms to induce aftershocks, the magnitudes of aftershocks are inferred using a simple magnitude evaluation formula in terms of distance, based on the statistical analysis of the aftershocks of the 2011 off the Pacific coast of Tohoku Earthquake.

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This paper presents a large-scale seismic response analysis to estimate the structural damage using solid finite element method. For the problem of the seismic soil-structure interaction analysis based on time-dependent linear elastic body finite element method, we implemented high performance computing techniques such as MPI/OpenMP hybrid parallelization, multigrid/multiple-precision preconditioned CG method, to reduce the computational cost which is able to solve high fidelity model of a structure subjected to ground motion. Also, we have implemented the developed code onto K supercomputer, and carried out it.

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The purpose of this study is to make the control strategy for the sloshing phenomenon happens in the water receiving tank practical. In this study a cross-shaped floating damping device, which is newly proposed, is put into a square section container, a stainless-steel panel tank, in order to verify the utility of the damping device, the experiment was performed with sinusoidal input excitatation and seismic input excitation. During the vibration experiment, the liquid content's response wave height, the fluid pressure change, and the video were used to inspect the utility of the damping device. It's darified that the floating damping device have the equivalent result of height reduction effect, with the fixed damping device used in the prevuous studies.

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With the aid of high-performance computing, we can perform an earthquake disaster assessment based on the seismic response analysis of all individual buildings in a target city, once the building models are constructed from some input data. However, the quantity and quality of single data sources for individual buildings are limited. Thus we need to make the detailed information combining multiple data sources. In order to prepare such information,we have developed a method to automatically combine the 3D shapes and the attributes which are separately recorded in two different types of GIS data. We have applied the method to the real city data, and the results show high reliability of over 80% despite the discrepancy between the locations of the 3D shapes and the attributes.

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We study the stability of the solutions of the governing equations for soil liquefaction, assuming perturbations in the form of plane wave and of spherical wave. We model the dilatancy effect of soil for stability analysis by setting relevant components of constitutive tensors. Theoretical analysis shows that perturbations in the form of plane wave are always stable without dilatancy and can be unstable when dilatancy ratios exceed a certain critical value. We derive the critical dilatancy ratio explicitly for the plane wave case. For perturbations in the form of spherical wave, numerical simulations reveal similar dependency of stability on dilatancy ratios. As the existence of unstable solution is confirmed, our analysis provides a new perspective on possible initiation of liquefaction: a transition from stable to unstable solutions of the governing equations.

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The tsunami caused by the great east Japan earthquake lead to collapse infrastructures including bridge and breakwater. It is important to reconsider disaster prevention and mitigation techniques towards next millennium tsunami. Currently, most of local governments in Japan are promoting the selection and specification of tsunami refuge buildings for people who live in the difficult area to evacuate within the limited time. However it's difficult to settle the design code of tsunami refuge building because there are so many incertitude accident during tsunami related to water induced impact force, buoyancy force, tumble of the building and so on. Numerical simulation by a particle method has been conducted for the evaluation of tsunami fluid force acting on tsunami refuge building as a first step. Finally, efficiency and adequacy of numerical simulation in the design of tsunami fluid force has been discussed by comparing with the current design code in Japan.

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In the current seismic performace evaluation method, the safety of the road embankments is confirmed by comparing the analyzed sliding displacement with the allowable displacement. The sliding displacment is practically analyzed by the Newmark method. This study proposes a simple evaluation method of sliding displacement which has the equivalent accuracy with the Newmark method. The Newmark method solves the equation of motion consisting of two important parameters, yield and sliding coefficients, and this study propose the regression equations to estimate these two parameters. The regression equations were confirmed to have good accuracy. Futhermore, the new design spectra, namely the sliding displacement spectrum, is proposed which express the effect of the ground motion on the sliding displacement. Simple evaluation method of sliding displacement using the sliding spectra and regression equations are also proposed.

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An agent based model, which includes detailed model of the environment and agents with functionalities to interact with it, is developed for simulating large area evacuations. This model makes it possible to quantitatively estimate the effects of various important factors, like interaction with environment and neighbor agents in low lighting conditions, vehicle-pedestrian interaction in narrow roads, etc., which cannot be modeled with the existing models. The evacuation simulator is coupled with an integrated earthquake simulator to take the earthquake damages into account. To demonstrate the potential of the model, several scenarios, which require detailed modeling of the environment, are presented.

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The mesh-based numerical methods such as finite element, finite difference and finite volume methods are often used, as classical methods, to solve the flow field problems. However, despite their great success, they suffer from limitations in several aspects, such as numerical diffusion, dealing with complex geometries and problems with large deformations of boundaries. Applying mesh-free methods are a remedy for such kind of problems as they have the ability in dealing with problems with large deformation. In this paper, a 3D smoothed particle hydrodynamics method, which is a mesh-free Lagrangian method, was applied to simulate snow avalanche mechanisms. The snow was considered as a Bingham fluid and the snow viscosity was calculated based on the Bingham constitutive model, on the basis of Coulomb's failure criterion. An equivalent Newtonian viscosity is calculated to express the Bingham viscosity into Navier-Stokes equations. A simple and small scale snow avalanche experiment with different types of obstacles was simulated using the SPH method. Numerical results showed that, in the most cases, good agreements were found by the means of leading edge position and travel length.

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General applicability of a classical drag-force model widely used in predicting flows past urban and vegetation canopies is examined against a detailed database of two-dimensional flows past arrays of circular cylinders (Yokojima et al., J. JSCE B1, 70(4), I_829, 2014.). It is demonstrated that the model can, in general, predict the mean flow property fairly well, as long as the bulk drag coefficient of the array can be estimated in a reasonable way. By providing the spatial distribution of the drag coefficient within the array obtained from the database, the model can reproduce the stagnation phenomena at the leading edge of the array more accurately, but it hardly improves the prediction accuracy of the macroscopic flow features.

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This study deals with a numerical method which allows for the hydrodynamic force acting on a natural stone in open-channel flows. The numerical accuracy was verified in comparison with experimental results. We conducted three cases of experiments with a model stone made by reinforced plastic, adjusting the flow discharge in a straight rectangular open-channel with a flat bed. In those experiments, we measured not only the drag force and lift force, but also the vertical velocity distribution of streamwise flow approaching in front of the stone. Addtionally, we carried out numerical simulations for evaluating the forces acting on the numerically-modelled stone. Namely, we approximately generated the numerical stone using tetrahedron elements in terms of stone shape, imitating the actual model stone. For the computation model, we considered the mechanical interactions between fluid motions and the modelled stone which behaves as a rigid body. Results showed that numerical data related to the hydrodynamic forces are consistent with the expmerital measurements. Therefore, the validity of our computation method was demonstrated.

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We have found that cavitation bubbles are generated when a sphere, which is freely falling from a height of several cm above the bottom, is impacting onto a bottom wall. The cavitation bubbles can be categorized into the following three types: 1) a single disk-like bubble generated at the impacting point, 2) multiple spherical bubbles generated away from the impacting point, and 3) multiple bubbles generated at the surface of the sphere and bottom wall. To clarify the generation mechanism of the cavitation bubbles, the following three kinds of experiments are conducted: (1) the experiment of the single free falling sphere impacting onto the flat buttom plate, (2) the experiment of the single sphere pulling up from the flat bottom plate, and (3) the experiment of impact force acting onto a half sphere fixed to a flat bottom plate. The motion of the sphere has been analyzed by using image processing techniques. The result of the experiments shows that the generation mechanisms of the cavitation bubbles are related to the acceleration of the impacting sphere and the shock pressure generated by the impact of the sphere.

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The effects of large roughness group of square columns on the flow structure in a downstream region were investigated experimentally in an open channel flow. Two roughnesses were located in parallel and their interval length was changed, and one roughness was placed backward with 2 kinds of distance on the center line of the front roughnesses. The velocity was measured by using PIV method under two discharge conditions where the roughness was emerged and submerged. In the submerged cases, the effects of lateral and longitudinal distance of each roughness did not so affect the flow structures. In the case of three roughnesses, when the lateral interval length was short, the longitudinal distance between the front and back roughnesses affects significantly to the mean flow and turbulent structures. The long distance setting can keep a low velocity and low turbulence region behind the roughness group. This is caused by the difference of large-scale periodic vortices generated by the roughnesses.

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In bedrock channels, the areal fraction of alluvial cover is generally modeled as a simple function of sediment supply relative to the transport capacity. Other factors are likely to be important, especially the roughness of the underlying bedrock surface. In this study, we report the results of experiments on the interaction between bed roughness and alluvial cover fraction.
Our results show that: 1) the areal fraction of alluvial cover is larger when the surface roughness is larger; 2) when bedrock roughness is rough, the cover fraction gradually increases with sediment supply; 3) low-roughness bedrock requires a relatively large sediment supply before any alluvial patch is formed, and as supply increases, rapidly transitions to a fully alluvial channel.

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Recently, ground degradation induced by soil pollution is reported and becomes one of the most serious environmental problem. The motion of the solute pollution material in the ground is explained in the framework of advection-dispersion phenomena. If the mass of solute material is not negligible in the view from mass of solvent material, the movement of solution becomes complex. Firstly, infiltration equation and advection-dispersion equation are derived from mass conservation law. Next, the fluid head is rewritten to apply the change of the fluid density. And the model to express the motions of the solute material in the change of solute density is established. Devised model is applied to vertical one-dimensional advection-dispersion calculation and the applicability is discussed. Additionally, governing equations are discretized, applied to 2-dementional finite difference method, considered the representation performance. Finally, it is found that the model show the enough performance to express the flow change in the ground derived from density variation of the fluid.

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Transport properties of colloidal particles in a packed bed of collector beads are affected by hydrodynamic and colloidal interactions between particles as well as between the particle and the collector. The colloidal interaction depends on the surface charging of collectors and particles. While many researchers have studied on the colloid deposition and transport in porous media as a function of ionic strength, experimental investigations examining the effect of surface charge are scarce. To figure out the effect of surface charge on the colloid transport in porous media, we have performed colloid transport experiments in a packed bed of spherical beads. In the present experiment, we used sulfate latex with a constant negative charge and carboxylate latex having pH-dependent negative charge as colloidal particles. Zirconia beads with pH-dependent charge and an isoelectric point around pH 7 were adopted as collectors. Colloid transport experiments were carried out in 1 mM KCl as a function of pH and breakthrough curves of colloid were obtained. The experimental breakthrough curves were analyzed by a convection-dispersion equation including colloid deposition, where colloid filtration and dynamic blocking are considered. The results of experiments and analysis show that the maximum surface coverage of sulfate latex on the collector surface is constant against pH change. On the one hand, the maximum coverage of carboxyl latex decreases with increasing pH, indicating that the lateral particle-particle repulsion increased by high charge enhances blocking in later stage of deposition. The result suggests that electro-hydrodynamic interaction also affects the maximum coverage.

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In this study, mechanisms of suspended sediment transport in a vegetation area in an open channel were investigated by using an experimental flume that was equipped with pile groups. Flow structures in a flume was observed by the Particle Image Velocimetry (PIV) technique for particles of neutral buoyancy and relatively heavy materials. The experimental results show that the deposition of the suspended sediment particles in the vegetation area is influenced by the relative scales between the particle size and the representative scale of the turbulence in the vegetation area.

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Numerical simulations of bed deformation as well as vegetation dynamics in river engineering generally necessitate high-load computations because lots of numerical meshes would be incorporated for high-accuracy calculation. In this study, we investigated performances of GPGPU (General-Purpose GPU) for numerical simulations with the shallow-water model used in river engineering. Especially, we compared the simulation results of GPGPU with those of CPU. Then, we examined the numerical accuracy and time efficiency of GPGPU simulations for the quasi-steady flows in a natural river with movement of the water's edge. Results showed that the numerical accuracy of GPU-accelerated numerical simulations was sufficient for river engineering tasks, although results of GPGPU simulations are not completely consistent with those of CPU simulations. Additionally, it was demonstrated that the numerical efficiency in GPGPU simulations is considerably improved using more numerical meshes within a range of the present numerical conditions.

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Groynes are recognized to have a role on 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 conditions. The experimental results showed that the resistance changes according to the groyne interval and it takes a peak at a certain value of the aspect ratio. If the aspect ratio is small, an isolated separated vortex moves along the junction one by one. However, if the aspect ratio becomes larger, multi vortices can exist together at the junction. The numerical results showed that scale of vortex depends on the momentum thickness. If groyne interval is small, momentum thickness increases along the entire interval. Thus, the vortex is amplified as it goes downstream. If the groyne interval is larger, the momentum thickness takes peak within the interval. Thus, after the point where the momentum thickness takes peak, the vortex becomes attenuated. It is also shown that the momentum exchange rate at the boundary between the mainstream and the groyne region related to scale of vortex, which have close influence on determining flow resistance due to groynes.

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Worldwide climate change has resulted in severe flooding in Japan in the last decade. Some floods were likely caused by the substantial accumulation of driftwood or debris at bridges crossing rivers in mountainous areas. In this study, for a two-dimensional open channel, we conducted a laboratory experiment and numerical simulation with and without driftwood modeling to clarify the physical mechanism of a flood event resulting from driftwood accumulation at a bridge. Open-source software SPHysics, implementing a smoothed particle hydrodynamics method, was calibrated with data of the water level measured near a miniature bridge in the laboratory experiment. The simulation clearly showed that the particle-like flow was scattered by the bridge girder and dammed up by the bridge in the upstream direction. By changing the quantity of driftwood, we analyzed the effect of a flood on a bridge. For the largest quantity of driftwood, the higher pressure pushing against the bridge may damage the bridge.

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Drag in rows of circular cylinders arranged parallel to the main flow in a straight duct is examined by high-resolution numerical simulations based on an immersed boundary method. It is found that shifting the row of cylinders toward the side wall achieves a considerable drag reduction. When a row is allocated along the centerline of the flume, the flow resistance solely depends on the number of cylinders, irrespective of the configuration details. It is also revealed that the flow resistance is maximized at which the surface-to-surface distance between neighboring cylinders is around the cylinder diameter and is closely associated with the lateral mixing of the flow.

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This study examined the morphological behavior of the bed in an open channel flow with bank strength effects by means of a depth-averaged two-dimensional numerical model. We simulated the river morphodynamics under various angles of repose in order to examine whether the aforementioned relations between the angle of repose and characteristics of river morphology can be reproduced numerically. We employed a computational model with depth-averaged two-dimensional shallow flow equations and an equilibrium bedload sediment transport model. In order to take into account the effect of angle of repose, a bank failure model is also incorporated. The numerical results showed that general characteristics of the bed morphology simulated by the present model are in compatible with the features pointed out in the previous papers. Namely, if the angle of repose is larger or smaller, the river becomes meandered or braided, respectively. The present computational results are also compared with the theoretical results by Muller. The agreements between the computations and the theory were generally good. The present computations, not only support the previous empirical and theoretical results on river morphology depending on angle of repose, but also show that the present computational model is a powerful tool to predict river morphology under effects of vegetation.

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Problems on the stability of laminar flow such as Poiseulle flow and boundary layer flow are one of the most traditional problems in the field of fluid mechanics. Though stability of free-surface flow has been studied by a number of researchers, detailed instability diagram illustrating the growth rate of perturbation in the wavenumber-Reynolds number plane has not been obtained so far. In this paper, we perform linear stability analysis of free surface flow by the use of the spectral collocation method with the Chebyshev polynomials and obtain detailed instability diagrams. It is found that the instability diagrams of free-surface flow are characterized by complicated unstable domains compared with those of 2D Poiseulle flow and boundary layer flow.

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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 bed with square ribs and augmented distinctly the flow resistance by comparison with the clear water flow.

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This paper describes the characterisitics of rollwaves in equilibrium condition. Numerical simulations of roll waves are conducted by using shallow flow equations. It is found that rollwaves develop downstream and are finally held in dynamic equilibrium, where they have various wave length and wave height. It is obtained the results that there are some region of cycles where rollwaves are formed at almost constat wavelength and wave height under the conditions that disturbances with constant cycles are given.
Non-linear stability analysis of rollwaves is also carried out, assuming that they are composed of first, second and third order mode of the fundamental wave number. It is showed that the stability domain of wave number appear, when second and third order mode of the fundamental wave number are considerd. It was found that the characterisitics in this domain qualititavely correspond to the results of numerical simulation.

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The article

Keita NAKAMURA and Mamoru KIKUMOTO: CHARACTERISTIC CURVE MODEL FOR WATER-NAPL-AIR THREE-PHASE SYSTEM,
Journal of JSCE, Series A2(Applied Mechanics),Vol.70, No.2, I_831-I_840, 2014.

has been retracted at the authors’ request.

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This paper presents the novel spatiotemporal measuring technique for water surface and bed elevation on flume experiments as movable bed. This method is application of optical cutting method, moving laser beam as a light source at high speed to measure water surface and bed elevation in high spatiotemporal resolution with contactless. The characteristics of this measurement are effective to elucidate physical mechanism of sand waves often show high frequency undulation and unsteady movement. This study applied the measurement to flume experiments, the measured results are well.

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An invariant derived from complex eigenvectors of velocity gradient tensor is applied to analyze the local topology of vortices in vortex identification, as a parameter representing the symmetry of vortical flow.
The eigenvalues and eigenvectors of the velocity gradient tensor specify an intensity and direction of a vortex in the vortex identification. However, it is difficult to specify the symmetry of the vortical flow, which is associated with stability of a vortex. The ratio of norms of real and imaginary vectors of the complex eigenvectors is an invariant and used as a parameter to specify the symmetry of the vortical flow in the swirl plane, which enables to analyze and visualize the symmetry of vortices besides the vortex identification.
The relationships between the intensity and symmetry (the invariant) of vortices in isotropic decaying turbulence show that this symmetry property is high in the core region of a vortex, and that it is associated with the development and decay of a vortex.

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To precisely express contamination phenomena of shallow ground caused by Non-Aqueous Phase Liquids (NAPLs), a multiphase seepage analysis based on a characteristic curve model considering properly a water-NAPL-air three-phase retention property of soil is necessary. In previous research, the model proposed by Lenhard & Perker has been used for water-NAPL-air three-phase coupled seepage analysis. However, regarding soil pollution problems in which a thin layer of NAPL exists in the ground, previous studies showed that the model cannot be applied. Nakamura and Kikumoto solved the problem by the formulation of a new characteristic curve model based on a state variable μ but their new model assumed the unique relationship between suction and degree of saturation. Therefore, it cannot consider hysteretic phe-nomena. In this paper, we propose a new model by introducing a new state variable representing water and liquid phase run to consider the hysteretic phenomena. The model is validated by a simulation of the three-phase column test.

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As a step toward obtaining aeroelastic characteristics of suspension bridges by numerical simulations instead of costly wind-tunnel or on-site tests, fluid-structure interaction analysis of a girder bridge section in a uniform wind has been conducted and its accuracy and reliability as a design tool for wind stability of bridges have been examined. The two-degree motion of rigid-body girder sections undergoing heaving and torsional vibrations has been simulatd by a Large-eddy Simulation (LES) method. The LES methods which is based on the non-inertial coordinates fixed on moving body, had been validated in analyzing aerodynamic forces and lateral vibration of a square cylinder. Comparing with corresponding wind-tunnel test results, it has been found that the differences in vibration characteroistics due to different on-coming wind speeds, different section geometries and the effects of various appendicies are reproduced well. Some descrepancies of the simulated results from the experiments are found but they are mostly due to the inadequacy of computational grid resolution, which can be improved by using finer grids and more powerful computing system.

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The mechanical behavior of corroded steel members is of interest by the increase of aged steel structures. Though there exists considerable literature on the tensile behavior of corroded steel plates, relatively few studies have been reported on the buckling behavior, which in turn, the accumulation of experimental data is still not enough. Therefore, we have applied the three dimensional measurement method by stereo image processing to obtain the out-of-plane deformation during the buckling experiment, and have proved that the method satisfies both of low cost and high accuracy. The method is expected to help researchers to obtain the valuable data easily. Then, buckling experiments have been conducted using the corroded steel specimens taken from three different steel bridges, and the out-of-plane deformation has been measured. It is found from the measuring results that the minimum average thickness, which is defined as the average thickness at the minimum cross section, is the influencing factor of the buckling behavior, and the same knowledge has been obtained from the finite element analysis.
In addition, we have investigated the applicability of the existing buckling formula, by determining the effective thickness as the minimum average thickness. It is clarified from the comparison with the experimental and finite element results that the existing buckling formula can evaluate the residual buckling strength well.

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In this study, we conducted fundamental study of structural health monitoring by using 3D smart acceleration sensors and high resolution FE model of a pedestrian bridge.Vibration measurement is performed at a pedestrian bridge having an actual damage by using Imote2 smart sensors. Several vibration modes are identified and compared to eigenvalue analysis of detailed FE model. In FE modeling, stairs restrain girders of the pedestrian bridge and its modal shapes are affected by the connection between stairs and girders. Vibration characteristics of updated FE model are improved in both natural frequencies and mode shapes.

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As a simple repair technique for steel members suffering from corrosion attacks and fatigue damages, bonding patch plate have been used. In the patch plate bonding repair, the prevention of delamination of patch plate is one of the main concerns. Therefore, some verification is strongly required to prevent the delamination of patch plate. For the evaluation of brittle failure like debonding, the energy release rate is often used. However, the energy release rate previously proposed does not take into account the delamination mode I and II. In this study, mode I/II energy release rates by using normal and shear stresses in adhesive layer are derived for cracked steel plate repaired with patch plates on both side. By using high accuracy solution of normal and shear stresses in adhesive layer, both energy release rates almost agree with the results of FEM analysis.

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This paper presents an image analysis on the flexural behaviors of CFRP box beams made of cross ply laminate. Image analysis is conducted in order to measure the deformation field around the center span of CFRP box beams, and strains, principal strains, principal stresses, ant Tsai-Wu failure index are calculated consequently. In the present study, four times more measurement points are employed, and measurement area is enlarged, compared to a previous study. This high density image analysis leads to a clear picture of localized features of strains and stresses around a fracture origin. It is successfully shown that Tsai-Wu failure index obtained through image analysis explains possible fracture origins that mostly match with the origins observed experimentally.

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The authors have been developing a vibration characteristic estimation method using the accurate small vibrator. In this method, acceleration responses excited by the accurate small vibrator are measured with accelerometers and converted to displacement time series. The displacement time series and the force function of accurate small vibrater are described by an ARX (Auto-Regressive eXogeneous) model. The frequency transfer function and vibration characteristics are estimated by solving the ARX equations with least-squares method.
In this research, We apply the prototype system to a road sign and estimate the vibration characteristics. In this experiment, a weight of 1.5kg is fixed to the road sign and the change of eigen frequency of the mode with the largest amplitude is accurately determined. However the eigen frequency of the mode with second largest amplitude is less accurately estimated. The estimation accuracy is verified by analyzing a simulation data of two degree of freedom system and an experimental data obtained in a laboratory scale vibration test using a steel simple beam.

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To reduce the period of calculation has been a key when the digital image correlation method is applied to the reinforced concrete (RC) structures having a large size. This paper proposed the algorism for reducing the time of calculation at the digital image correlation method, which is one of the image analysis methods to obtain the deformation of targets. The proposed algorism reduced the time for `coarse search' by using the shape functions. According to the proposed method, the strain generated at the surface of cementitious materials was measured. The specimen was deformed through drying and being loaded. The employed time of the analysis using the proposed `coarse search' algorism reduced significantly the time compared with the full-checked type `coarse search' algorism. The paper concluded that the proposed analysis evaluated the crack propagation of cementitious composite materials, even being difficult to capture visually.

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Early judgment of damage level of infrastructures is required after the earthquake. Human-based inspections are carried out mainly, however it takes long time and is expensive. On the other hand, accelerometer is relatively inexpensive and is installed easily. But a method to detect the state-change of structures from only the acceleration information in real-time has not been established. In this study, by applying wavelet transform to acceleration data of several large shaking table tests, we tried to detect the state change of structures. First, in the analysis of the acceleration data measured on the top of the RC column which had damage by shaking table test, it was possible to detect singularity points of the acceleration waveform by wavelet transform. To confirm whether the singularities were caused by damage, we checked strain of rebar, hysteresis and video records. As a result, it was shown that most of singularities in acceleration waveform occurred when RC columns had damages such as cracks and rebar yielding. We usually measure acceleration with sampling frequency 100 ∼ 200 Hz in seismic monitoring. However, there is a possibility that the occurrence of damage affects the higher frequency components of acceleration, so we conducted measurement with high frequency sampling (10 kHz). As a result, on the points which have been detected as singularities in 200 Hz sampling, we found high frequency components existed. In addition, it was suggested that such as the frequency of these high-frequency components varies due to the damage conditions of RC columns, there is a difference among the detected singularities due to damage. From these results, it is considered to be possible to detect damage from singularities which are extracted by wavelet transform. We consider that these results give us merits for developing efficient inspection techniques of actual RC structures.

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In order to detect the damages of PC sleeper in the ballast track which cannot be inspected visually, the damage detection system using vibration mode characteristic of PC sleepers was developed in this paper. As a result of the vibration measurement test that conducted in parallel to the bending test, it was confirmed that the natural frequencies begin to fall when the load of 1.2 times or more of crack load acted. And, it was confirmed that the natural frequenciy of 3rd mode in which the influence of damage appears greatly and influence of ballast or rails don't appear is suitable for detection of the opening crack of the bottom of rail place from the vibration measurement test results of PC sleepers which have real damage and the vibration measurement test results in the examination railway line. Furthermore, towards practical use, the damage detection by single point measurement and simple signal processing was considered, and it was shown that the same detection accuracy as multipoint measurement is securable.

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The purpose of this study is to develop an optimization method of maintenance planning of bridges for the asset management. It is realistically difficult to keep the safety of all bridges within a limited budget. Furthermore, the increase of the number of bridges treated in an optimization significantly increases the number of combination of solution candidates and the difficulty of optimization. Because an optimization of maintenance planning for multiple bridges is effective to decrease an annual cost, it is necessary to overcome this problem. In this paper, an attempt is made to propose a method making optimal plans in response to various objectives such as the cost saving, the improvement of safety and the adaptation to uncertainties. This method improves the flexibility of plan against the change of schedule by basing on the preventive maintenance. In the proposed method, various candidates of plan for each bridge are created in consideration of the preventive maintenance. Then, a combination of candidates corresponding to the purpose of planning is searched. A numerical experiment was presented to demonstrate that the proposed method could develop an optimal plan based on a safety level different from bridge to bridge.

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This paper investigates the influence of mechanical nonlinearity at the interface between the sleeper and ballast on the critical velocity of moving lateral load acting on an axially stressed rail. To this end, a mathematical model consisting of an elastic half-space and an infinite beam is considered. Taking into account the nonlinear stiffness in the interaction system, the critical velocity is derived via dispersion corves drown in the wavenumber-frequency space. The sensitivity of the critical velocity to the disturbance represented by a lateral moving load is revealed through theoretical discussion. Furthermore, in order to verify this phenomenon, steady-state solutions governed by the hysteresis property of the ballast material are obtained numerically for a compressed beam subjected to a moving load.

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In order to realize a floating offshore wind turbin system, it is necessary to examine the dynamic response characteristics of the floating foundation. In this study, we present a method for estimating the dynamic stability of a spar platform in regular waves by applying the Hsu's method (1963). The presented method enables us to estimate the wave frequency at which the parametric resonance may occur as a multiple degrees of freedom system. In addition, we investigate the validity of this method by numericalcal analysis and experiments.

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