Takaaki Shigematsu

Numerical modeling of the initial stages of dam-break waves

In this paper, we present a numerical investigation of dam-break waves during the initial stage. The numerical model is based on the Reynolds Averaged Navier-Stokes (RANS) equations with a k-e turbulence closure model (J. Fluid Mech. 359, 1998, 239). Numerical results are compared with existing experimental data (J. Fluid Mech. 374, 1998, 407) as well as the new experimental data. Good agreements are observed. The effects of the water depth in front of the dam and the pull-up motion of dam on the generated waves are then investigated. The turbulence associated with the dam-break waves is also discussed.

Application of three-dimensional hybrid stereoscopic particle image velocimetry to breaking waves

A new technique of three-dimensional hybrid stereoscopic particle image velocimetry (3D hybrid SPIV) for measuring a volumetric spatial distribution of three-dimensional velocity is presented in this paper. The accuracy and uncertainty of this technique are examined by numerical tests. The 3D hybrid SPIV has been found to be able to measure major features of velocity field involving a vortex with turbulence in a shear flow. This technique is used for measuring an instantaneous three-dimensional flow structure under breaking waves. The formation of the three-dimensional vortex structures involving roller and longitudinal vortices in breaking waves is investigated in this study.

DEVELOPMENT OF A NUMERICAL SIMULATION METHOD FOR PREDICTING THE SETTLING BEHAVIOR AND DEPOSITION CONFIGURATION OF SOIL DUMPED INTO WATERS

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

DEVELOPMENT OF A NEW TYPE OF REEF BREAKWATER THEORETICAL AND EXPERIMENTAL STUDY

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.

Development of a High Accuracy Numerical Model for Predicting Motion of A Floating-Type Evacuation shelter from Tsunami with Eccentricity

A floating-type evacuation shelter from tsunami is the facility which is swepted in tsunami. Therefore, for establishment of the shelter on site, it is an important task to predict motion of the shelter with accuracy in terms of strength for tsunami fluid force and safety of evacuees. In this study, a numerical model for prediction of translational and rotational motion of the shelter is presented. According to comparison with laboratory experimental results, calculated results on translational and rotational motion reasonably agree with experimental ones with accuracy.

Distributions of Ulvaohnoi and Currents at the Artificial Tidal Flat of Hannan Second District in Osaka Bay

我々の生活は沿岸域に大きく依存しており,人口や産 業も沿岸域に集中している.沿岸域は比較的水深が浅く, 埋め立ての場として使用しやすい.そのため,高度経済 成長期以降に沿岸域の干潟や砂浜は,都市開発や工場用 地として埋め立てられた.大阪湾もその一つであった.ま た,大阪湾の湾奥部には,淀川,大和川などの大型河川 が位置し,大量の栄養塩や懸濁物が流入する.かつては, 大量の栄養塩や懸濁物は,干潟や砂浜の有する水質浄化 機能によって浄化されていた.しかし,干潟や砂浜が減 少した大阪湾の沿岸域はそれらの機能を失った.そのた め,大阪湾湾奥の港湾域では過養化が問題となっている. そんな中,沿岸域の環境改善策として,近年では干潟の 価値が重要視されるようになり,人工干潟造成事業が行 われている.しかしながら,阪南2 区人工干潟では,夏 にアオサ類(Ulva)が大量発生するグリーンタイドが問 題となっている.阪南2区人工干潟におけるグリーンタイ ドを写真-1に示す.(b)の図中の白いものは,打ち上げ られて数日間,太陽にさらされ脱色したミナミアオサで ある.このアオサはいずれ腐敗するが,腐敗すると悪臭 を放ち,干潟や砂浜が持つ癒し効果などを低下させる.干 潟造成にあたって,片倉ら(2006)は阪南2区人工干潟を 対象に人工干潟の地盤の安定性に関する検討を行ってい る.その他に生物の生息地としての適性,人工干潟の水 質浄化機能および生物生息機能については多くの研究結 果が報告されている.また,グリーンタイドに関して,西 川ら(2009)と芳村ら(2011)はアオサの生理特性を明 らかにし,そこからグリーンタイドの抑制法について検 討を行っている.しかし,地形,流れ等がアオサの分布 に及ぼす影響については十分に検討されていない. 本研究では,阪南2区人工干潟を対象として,地形や流 れとアオサの分布との関係を明らかにし,アオサの大量 蓄積を軽減させる人工干潟造成法について提案する.阪 南2区人工干潟で確認されたアオサはミナミアオサであっ た.以降ミナミアオサもアオサと称する.

A Study on Effect of Urban Heat Release into an Enclosed Sea on Water Temperature Using the Regional Ocean Modeling System with Two-Way Nesting Method

(1)数値モデルの概要 検討には,ROMS_AGRIFを用いた.ROMS_AGRIFは, Regional Ocean Modeling System(ROMS)に双方向ネス ティング手法を導入したオープンソースである.ROMS は水平面に対してはカルテシアン座標系を,鉛直方向に はσ座標系を用いて空間を離散化し,さまざまなコンパ ートメントや物理モデルを組み合わせて,計算対象領域 の地形や境界条件に適合した海域流動の計算を行うもの である.既に,森ら(2008,2010)は,ROMSを用いて 都市排熱を海域に放出した場合の影響評価について検討 を行っている.しかし,水平方向の空間解像度が大阪湾 全域で一様であったために海岸に接するグリッドにおけ る水深の精度が十分ではなく,そのため,鉛直方向の空 間解像度が粗かったうえに,水平方向の空間解像度も放 出口近傍の流動を計算するには十分な精度であるとは言 えなかった.そこで,本研究では,ROMS_AGRIFを用い て放出口近傍の空間解像度を向上させて計算を行った. (2)検討対象領域とネスティング 計算対象領域は,図-2に示すように,大阪湾全域を対 象とする大領域(空間解像度 500m),港湾スケールの 5 .0km× 5 .0kmの中領域(同 100m),放出口付近の 1.1km×1.0kmの小領域(同20m)の2段階ネスティング を行った(図-2).海底地形は,海上保安庁の深浅測量デ ータ(日本水路協会)を基に作成した.それぞれの領域 土木学会論文集B2(海岸工学) Vol. 68,No. 2,2012,I_1146-I_1150

Characteristics of Oxygen Consumption by the Bottom of Port and Harbor under Anoxic Environment

For effective restoration of enclosed coastal zone under anoxic environment such as Osaka bay, it is necessary to understand characteristics of oxygen consumption by the bottom sediment of port and harbor. In this study, in-situ measurement method of oxygen consumption by using a convenient chamber was developed. From results of field investigation particular characteristics of the sediment oxygen demand in port and harbor under anoxic environment were presented.

An Experimental Study on the Effect of Reynolds Number on Turbulence induced by Porous Media

The effect of Reynolds number on the properties of turbulence generated by a porous media was investigated by a hydraulic experiment. Velocity field inside and around the porous media was measured with a high resolution temporally and spatially using the Super-resolution KC method and a refractive index matching method. It is presented that some properties of turbulence can be normalized by a characteristic velocity and a diameter of the sphere which constitute the porous media.

EXPERIMENTAL STUDY ON GENERATION AND DISSIPATION MECHAMISIMS OF TURBULENCE INCIDE AND IN THE VICINITY OF POROUS MEDIA

A measuring method of fluid motion inside and in the vicinity of porous media made of silicon spheres is presented. Sodium iodide solution is used as the refraction index matching fluid. In order to capture pictures, a CCD camera and YAG laser are used. The velocity datum are obtained by analyzing the pictures using the superresolution PIV method. Some experimental results on the velocity field and turbulence kinematic energy are presented. Structure of the flow filed inside and in the vicinity of the porous media is discussed. Moreover generation and dissipation mechanisms of the turbulence are discussed.