Mizutani Natsuki

 This study deals with the process that tsunami flow on land collides with a vertical wall. A tip of the tsunami flow collides with the wall and runs up along the wall and then the water column collapses on the following tsunami flow. In this study, the characteristics of the tsunami impact force on the wall caused by splashdown of the run-up water column were experimentally investigated. From the measurement of the pressure synchronized with the high speed video images, it was found that the local maximum values of the pressure were caused by splashing down of the water column. We assumed this phenomenon was casting up of the water column and then freely falling on the following flow. On the basis of this assumption, the elapsed time from the collision to the splashdown can be explained by using the maximum depth of the passing tsunami flow. The pressure distributions were not a static pressure profile when the local maximum pressure was caused by splashdown of the water column.

Impact tsunami wave pressure acting on near the bottom of a land structure was measured with high speed video images of fluid motion in front of the structure. The value of the maximum impact pressure reached 10 times that of the hydrostatic pressure. The impact pressure changed with some patterns of the fluid motion. The patterns also changed with Fr number. Under low Fr number conditions, the gauge pressure is composed of the dynamic pressure by the maximum of the fluid velocity and the hydrostatic pressure which depends on the height of fluid raising along the front wall. However, Fr number becomes higher, the gauge pressure is getting equal to the dynamic pressure only. The maximum value of the impact pressure can be fit into the function of 0.5 times square of the velocity.

This paper presents model tests on the bearing capacity of foundations during impact loading that simulates the vertical motion of an earthquake. The mechanical behavior of the foundation was measured using strain gauges, the image and PIV analysis, which were performed using super high-speed video camera. The main results of the tests are as follows: The value of settlement of the foundation, when the ultimate impact bearing capacity was generated, was around 0.3mm, and the mechanical behavior of the ground below the foundation was similar to a rigid body. Copyright © 2013 by the International Society of Offshore and Polar Engineers (ISOPE).

This paper describes the engineering performances of Suiken Dike certified as a heritage site by Wakayama Prefecture. The dike was constructed by using excellent construction techniques in Edo period. Especially, the main part of the dike is covered with many rectangular solid stones like a stone wall of a Japanese castle. The soil part is attached with the stone part. However, a functional role of this composite structure is unknown. We carried out the FEM analyses to study interactive effects between these two parts against wave forces as a coastal dike. A Tsunami wave force and a violent wave force due to a typhoon were used as an external force. It was clarified that the soil part effectively functioned to restrain the stone part rotating to the back side.

In this study, impact splitting tests were carried out by loading cylindrical granite specimens horizontally and obtaining the crack propagation velocities by an image analysis using a super-high-speed video camera, which has the fastest frame rate in the world. The relationship between the impulse and the crack propagation velocity indicates that there appears to be a critical state at which the crack propagation velocity does not exceed a certain value even if an increased impulse is applied. The critical value obtained for the granite specimens was 2.6 km/sec. © 2009 IOS Press.

This study presents the results of an experimental investigation of coherent structures generated beneath microscale and large-scale spilling breaking waves. By analyzing PIV velocity data of wind-generated waves that are acquired at three different wind speeds, the similarity and difference in spatio-temporal evolving characteristics of the coherent structures according to the occurrence of the two types of wave breaking are investigated. There have been some researches on the coherent structures that are generated by microscale breaker. However, little has been known about the coherent structures formed underneath large-scale breaking waves. In this study, it is shown that a single strong coherent structure, whose direction of rotation is the same as the wave orbital motion, is formed immediately underneath a large-scale breaking crest. This is different from the processes occurring underneath microscale breakers that produce a series of coherent structures whose direction of rotation is not fixed. However, the overall evolving characteristics of the spatio-temporal evolution of the coherent structures are qualitatively the same between the two wave breaking events. As one of either microscale or large-scale breaker advances across the PIV field of view, the generated coherent structures interact with pre-existing vortices and subsequently disappear or reform in the downwind side of the breaker. (c) 2008 Elsevier Inc. All rights reserved.

In this study, the static and impact splitting tensile tests were carried out by loading the cylindrical mortar/granite specimens horizontally, and the crack propagation velocities were obtained by the image analysis using the ultra high-speed video camera, which is the fastest in the world, developed by Etoh et al.(2003). The correlations between the impulse and the crack propagation velocity indicate that there appears to be a critical state in which the crack propagation velocity does not exceed a certain value even though the increased impulse is applied. The critical value obtained was 2.6km/s for both mortar and granite specimens. Furthermore, crack propagation velocity determined by the image analysis was verified by the dynamic strain analysis.

The two-phase flow measurements using imaging technique for two-dimensional bubble measurements, so called Bubble Tracking Velocimetry (BTV) and PIV were conducted to the surf zone breaking waves. The characteristics of air bubbles in the surf zone such as bubble distributions, bubble size spectra, and turbulent characteristics of fluid velocity were measured with highly temporal and spatial resolutions in the laboratory. The temporal transitions of bubble size spectra were measured and the relationship between the gas and liquid phases were analyzed for the surf zone breaking waves.

The present study describes an experimental investigation of breaking criteria of deepwater wind waves under strong wind action. In a wind wave flume, waves were generated using different wind speeds and measured at different locations to obtain wave trains of no, intermittent, or frequent breaking. Water particle movement and free surface elevation were measured simultaneously using a PIV system and a wave gauge, respectively. For wind waves, not all the waves measured at a fixed location are breaking waves, and the breaking of a larger wave is not guaranteed. However, the larger the wave height, the larger the probability of breaking. In order to take as many breaking waves as possible for the cases of frequent breaking, we used the waves whose heights were close to the highest one-tenth wave height. The experimental results showed that the geometric or kinematic breaking criteria could not explain the occurrence of breaking of wind waves. On the other hand, the vertical acceleration beneath the wave crest was close to the previously suggested limit value, - 0.5g, when frequent breaking of large waves occurred, indicating that the dynamic breaking criterion would be good for discriminating breaking waves under a strong wind action. (C) 2006 Elsevier Ltd. All rights reserved.

The present study describes an experimental investigation of breaking criteria of deepwater wind waves under wind action. In a wind wave flume, waves were generated using different wind speeds and measured at different locations to obtain wave trains of no, intermittent, or frequent breaking. Water particle movement and free surface elevation were measured simultaneously using a PIV system and a wave gauge, respectively. For wind waves, not all the waves measured at a fixed location are breaking waves, and the breaking of a larger wave is not guaranteed. However, the larger the wave height, the larger the probability of breaking. In order to take as many breaking waves as possible for the cases of frequent breaking, we used the waves whose heights were close to the highest one-tenth wave height. ne experimental results showed that the geometric or kinematic breaking criteria could not explain the occurrence of breaking of wind waves. On the other hand, the vertical acceleration beneath the wave crest was close to the previously suggested limit value, -0.5g, when frequent breaking of large waves occurred, indicating that the dynamic breaking criterion would be good for predicting breaking waves under strong wind action.

To make clear the relationship between the wave height decay process and the energy dissipation process, internal velocity field after wave breaking was measured using PIV technique, and vorticity and kinetic energy were investigated on the basis of the measured results. It was indicated that the vortices generated at the breaking wave front were separated from the front and were left behind in the wave propagation. Also, those vortices had not only the turbulent kinetic energy but also the ensemble mean kinetic energy. It was made clear that the mean kinetic energy taken out and retained by those vortices largely contributed to the wave height decay process.

Air-flow field over actual wind waves close to water surface was successfully measured using the PIV technique under a strong wind condition where the mean wind speed exceeds 10 m/s. It was found that the air-flow separation and the large-scale vortex are generated in front of breaking wind waves. A magnitude of the large-scale vortex corresponds to the amplitude of the breaking wind wave. The air-flow separation with inverse flow is generated for incipient wave breaking. Airflow separation with inverse flow was obtained in only 6% of all the studied cases, and progressive breaking wind waves without separation was obtained in about 60% of the cases.

Numerical simulations using a full-nonlinear BIM (Boundary Integral Method) potential-theory wave model are carried out to study the internal velocity and acceleration fields of an solitary wave overturning on a reef with vertical face (submerged breakwater) and their relation to breaker type. The simulations make it clear that the jet size normalized by the incident wave height is uniquely governed by the crown height of the reef, while the jet shape is similar and independent of the size. Further, they reveal that the overall internal kinematics of overturning waves is clearly related to the jet size. As the jet size increases and the breaker type changes from spilling to plunging, the kinematics thus become increasingly different from those of steady waves. Water particles with the greatest velocities or accelerations within the wave converge towards the jet. After the breaking, both of the velocities and accelerations almost simultaneously reach extreme values near locations beneath the jet. Some of the extreme values are closely related to the breaker type and can be uniquely determined by substituting the breaker type index into the regression equations suggested here.