水谷 夏樹

本研究は，陸上遡上津波による鉛直二次元壁への衝突現象を取り扱い，中でも津波の先端が壁面衝突後に打ち上がり，その水塊が後続流の上に落下着水する際に生じる波圧と波力の特性について実験的に検討を行った．高速ビデオの画像と波圧の同期計測から，打ち上がった水塊が落下着水する際に壁面の圧力が上昇し，極大波力を発生させるが，条件が揃えばそれが最大波力となるケースもあった．これらの落下着水現象を，衝突速度を初速度とする鉛直投げ上げと最大打ち上がり高さからの自由落下であると仮定すると，衝突から落下着水までの無次元時間を進行波の最大水位で説明できることが分かった．また，落下着水による極大波力発生時の鉛直波圧分布は静水圧分布にはならず，全作用高さの1/2以下では圧力分布が一様になる傾向を示すことから，構造物への作用波圧は静水圧分布を前提としない対策の必要性が示唆された．

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 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.

本研究は, 風による波・流れの発達過程を明らかにするために, Level set-CIP法を用いた数値風洞水槽の開発を行ったものである. 静止水面に十分に発達した気流乱流を与えて計算を行った結果, 水面に波長3-5cm程度のリップルが発生し, 内部流速場の結果から, それらが波動ではなく風波のごく初期に発生する渦列に伴う水面変動であることが確認できた. これらの結果は既往の実験結果と定性的に一致するものであり, 本計算モデルの妥当性を示すことができた. また, リップルの発生過程における水面形と気圧・水圧分布の特性との関係についても示すことができ, 今後の運動量輸送過程の解明につながる結果を得ることができた.

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.

本研究は風洞水槽実験による吹送流の特性について検討を行った.計測には高速ビデオカメラを用いた画像計測法を採用し, PIVによる風波内部速度場の算出を行った.また波高計との同期計測を行い, 波高計直下における流速の鉛直分布を時系列的に抽出することで時間平均による吹送流の分布を求めた.この実験手法の最大の特徴は, トラフレベル以上の吹送流分布を求めることが可能な点であり, 風波の発達が界面下の吹送流分布に影響を及ぼす点について明らかにすることができた.また, トラフレベル以上で輸送される実質の流量が相当量存在することを示し, 吹送流の予測計算において波浪の存在を無視することができないことも併せて明らかとなった.

風波発生時における気流と水流の相互作用を解明するために, 画像計測を用いた水表面近傍の気流・水流の同時計測を行った. 本研究では, 著者らが独自に開発した画像計測技術を用いて, 風波が生じた条件下で流れ場の計測を行った. 画像計測上問題となる移動する水表面が存在する場合でも, 水表面極近傍まで流速を計測できる粒子追跡速度計測法 (PTV) を適用した. 白波が生じない範囲で, 3つのケースについて計測を行った. 計測された流速分布図より, 渦度分布, せん断変形分布を求め, それぞれのケースにおける流れ場の特徴的な構造を明らかにした.

風波の発達・減衰機構や大気・海洋間の物質輸送等に大きく影響を及ぼす風波のマイクロブレーキング現象を画像解析により解明することを目的としている. 風波のマイクロブレーキング現象は水表面極近傍で生じる微細な現象であり, その流れ場を画像計測するには拡大撮影し, 狭い領域を細かく計測する必要がある. しかしマイクロブレーキングは波動場の状況に大きく左右されるため同時に広く波動場全体を画像計測する必要がある. 本研究では, 複数台のビデオカメラを用いて高解像度に, しかも広い範囲を画像計測する方法を適用し, 高解像度かつ広範囲の画像計測を行った.

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.

風洞水槽を用いた詳細な実験を行うとともに, 浅海波浪推算モデルSWANによる計算結果との比較検討を行った. 推算結果は実験結果に対してスペクトル密度のピーク値でおよそ1/2程度, 全体的に広帯域のスペクトル形状であった.また, スペクトル密度のピーク値及びピーク周波数については吹送距離に依存する量であると推察された. こうしたスペクトル形状についての比較に対し, 有義波高及び有義波周期については極めて良好な一致を示し, これらの結果の推察から, SWANが全エネルギーについては良好に輸送するものの, スペクトル形をシャープにするような非線形エネルギー輸送の取り扱いに不備があるという推察に至った.

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.