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Article type: Cover
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Published: September 25, 1998
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Article type: Appendix
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App1-
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Article type: Index
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Toc1-
Published: September 25, 1998
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Article type: Index
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Published: September 25, 1998
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Toshihiko KAWASHIMA
Article type: Article
Pages
1-19
Published: September 25, 1998
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To improve the accuracy of model test, fundamental flow properties of measuring section are considered using the advanced circulating water channel of the National Research Institute of Fisheries Engineering. An open channel theory is proposed to express effects to free surface flow due to the friction of boundary layer, bottom and side form of measuring section. A differential calculation of MAC method is tried to investigate the flow properties. Experiment is carried out systematically to examine the flow of measuring section. The fundamental properties of velocity, pressure and wave of flow are discussed. An one-dimensional flow model of measuring section is proposed. Velocity distribution of boundary layer of bottom and side is measured and the characteristics of boundary layer are discussed. Degree of friction is analyzed for aim of use to prove a physical phenomenon of free surface swelling in second paper.
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Toshihiko KAWASHIMA
Article type: Article
Pages
21-36
Published: September 25, 1998
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A physical phenomenon of free surface swelling found out at the circulating water channel of the National Research Institute of Fisheries Engineering is investigated. Form of measuring section is measured and degree of accuracy is discussed. Fundamental properties of effect to free surface flow due to the friction of boundary layer, bottom and side form of measuring section are examined using open channel theory. Physical reason of the phenomenon of flow surface swelling is explained theoretically. Physical characteristics between profile of free surface and form of measuring section are proposed. As an advanced technology, new form of bottom plane to induce free surface flow included 0-inclination condition under all range of velocity is developed. Actual designing is tried for standard type of circulating water channel under designed velocity of 1 m/s and for high velocity type under 5 m/s.
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Tetsuya KAWAKAMI, Katsumi KOTANI, Noriyuki SASAKI, Kyouji MURAKAMI, Ka ...
Article type: Article
Pages
37-43
Published: September 25, 1998
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Ship hull form improvement is tried by means of Mathieu function expansion method for amplitude functions obtained from wave pattern analysis test. Components of wave pattern resistance are calculated from the obtain coefficients of the Mathieu function expansion. The direction of reforming the mother ship is to substruct the components of the sectional area curve corresponding to the larger components of the wave pattern resistance from the mother ship sectional area curve. The method is applied to a middle speed ship at the designed Froude number 0.375 and two new ship forms are designed and tested. The noticeable results are not obtained probably due to the condition that the bow bulb closely beneath the water surface causes the strong nonlinear waves. Nevertheless the present method is concluded to have a possibily of a good method of ship hull from improvement at Frounde number 0.24 to 0.35 because of the fact that the difference of amplitude functions between two defferent ships corresponds well to the difference of the sectional area curves.
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Katsumi KOTANI, Hironori KIKUCHI, Noriyuki SASAKI, Kyouji MURAKAMI, Ha ...
Article type: Article
Pages
45-49
Published: September 25, 1998
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Following the first report Mathieu function expansion method for amplitude function is used to improve the ship hull form of PCC at designed Froude nunmber 0.24. The sectional area curve is reformed from the mother ship so as to reduce the components of the larger values of wave pattern resistance. The ship model newly designed based on the present method has about 20% less wave making and pattern resistances near the designed Froude number and the components of wave pattern resistance for the new ship correspond well with the values predicted by the theory. These show that the present method is available for ship hull form improvement.
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Katsumi KOTANI, Noriyuki SASAKI, Kyouji MURAKAMI, Hajime KIHARA, Katsu ...
Article type: Article
Pages
51-57
Published: September 25, 1998
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In order to supplement the Mathieu expansion method for ship hull form improvement in the previons reports so called discrete wavelet transform of wave height data measured for the longitudinal cut method in wave pattern analysis is applied in the present report. The present analysis gives local wave amplitude functions which show wave making properties of each section along the ship hull. The behavier of the obtained local amplitude function corresponds well with the variation of the sectional area curve. These results show that the present analysis may serve the ship hull from improvement.
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Yoji HIMENO, Wen-Xin LIU, Yusuke TAHARA
Article type: Article
Pages
59-64
Published: September 25, 1998
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The present paper describes an attempt of a network transparent computation for a hydrodynamic design problem. The sequential quadratic programming knows as one of the nonlinear programming techniques is coded by using Java programming language, and the Java networking technique is applied to a remote networking computation in which inputs, outputs and executable files in remote computer hosts are treated without serious difficulty using hyper text transfer protocol. As an example the code is applied to a CFD oriented optimization problem of a 2-dimensional tandem foil arrangement under a free surface to find the optimal location and attack angles with a prescribed total lift force. In the optimizing iteration process, the code utilizes a Rankine source CFD code in a remote host computer, and the initial values are also provided from a different remote host. The computational results are compared with that of the same optimization problem developed in a local stand alone system which show quite a good agreement in case of using double precision digits in CFD code.
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Masaki IWASAKI, Kazuo SUZUKI, Hisashi KAI
Article type: Article
Pages
65-74
Published: September 25, 1998
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In the previous study by one of the authors et al., hydrodynamic forces acting on sail boats are calculated by means of PAFS (Panel method Applied to Free Surface flow). In those results, however, some problems are found; for example, ware resistance coefficients estimated by PAFS become negative in the low speed range. In the present study, in order to improve the results, singular characteristics are investigated about the analytical formulae of influence coefficients given by Morino, because it can be considered that the problems of the previous results are raised by these singularities. According to this investigation, a modified scheme of PAFS is suggested. First of all, numerical examples of the free surface flow around KIT-34, a sail boat designed by Masuyama, are given under the condition without leeway. Based on the numerical examples, validations of the present modification are studied. As the next step, hydrodynamic forces acting on sail boat KIT-34 with leeway are estimated, since predictions of hydrodynamic characteristics of a sail boat with leeway are indispensable to the hull form design.
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Koichi MORI
Article type: Article
Pages
75-80
Published: September 25, 1998
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The speed of small water crafts has recently increased more and more. At the same time, however, it seems to be getting important for these crafts to take into account of the environmental and safty aspects. Water jet propulsion might be a possible candidate in future for that purpose, although at present it does not have economical advantage to other existing propulsion systems in most cases. This paper demonstrates the validity of a prediction methods for the hydrodynamic resistance of these high-speed small crafts by using public data and Savitsky's formula, which be useful at the initial design stage of the water jet propulsion system for these crafts.
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Tatsuo NISHIKAWA, Yusuke TAHARA, Koji MASAOKA, Yoji HIMENO
Article type: Article
Pages
81-88
Published: September 25, 1998
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An important factor in the design of lifting membrane is the accurate determination of its aerodynamic behavior. Recent studies related to membrane wings or yacht sails have introduced iterative coupling of two numerical approaches, i.e., vortex-lattice method (VLM) and finite-element method (FEM) for analysis of the flow fields and the membrane deflection, respectively. Although results presented in the works seem to be promising, VLM is not capable for predicting details of flow including viscous effects and three-dimensional-flow separation. This paper presents a numerical approach to predict aeroelastic behavior of lifting membrane in onset viscous flow, where the flow fields are solved by a RaNS equation method. The fully-elliptic RaNS and continuity equations are solved with a zero-equation turbulence model to provide aerodynamic forces acting on the membrane, and FEM considering large deflection theory with membrane-finite elements is used for estimating elastic characteristics of the membrane.
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Kenji OKIMOTO, Yoji HIMENO, Yusuke TAHARA
Article type: Article
Pages
89-99
Published: September 25, 1998
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Many numerical schemes have been developed to reproduce complicated flow fields around ship stern. However, most of them have turned out to be insufficient due to inadequate accuracy of the numerical scheme, grid resolution, and turbulence modeling. The recent workshops stated that turbulence modeling may be the most important issue for the problem. This paper presents a new turbulent energy dissipation model for application to a turbulent flow with longitudinal vortex. The standard εequation is modified so as to provide more realistic time mean flow and Reynolds-stress fields. The fully-elliptic RaNS (Reynolds-averaged Navier-Stokes equation) and continuity equations are solved with an eddy-viscosity turbulence model, where k and εare solved using two-layer technique. Discussions are made concerning the validity of the new turbulence model comparing with existing turbulence models and experiments.
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Tadao YAMANO, Takeshi UEDA, Isao FUNENO, Tetsuro IKEBUCHI
Article type: Article
Pages
101-108
Published: September 25, 1998
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In powering for planing ships with water jet pupm, we have to pay attention especially to the following two points. One, hull attitude of the ships much affects its hull resistance. The other, water jet pump still has some issues to be cleared such as hull and water jet pump interaction. These make the powering for the ships difficult, and the powering method for such ships has not yet been established. This paper proposes a powering method based on model test on each of hull and water jet pump, and its effectiveness is evaluated by applying it to full scale ships.
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Hiroyoshi SUZUKI, Satoshi MIYAZAKI, Toshio SUZUKI, Kiyoshige MATSUMURA
Article type: Article
Pages
109-122
Published: September 25, 1998
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Turbulnce measurements in stern flow fields around ship models were carried out by using triple-sensor hot wire system in the wind tunnel. The mean velocities and turbulent quantities include dissipation ratio were analyzed from the time history data of the 3-component of fluctuating velocities, and detailed stern flow databases were obtained. An evaluation of K-ε turbulence model was carried out using measured data of "Ryuko-Maru" at propeller section. From the investigation of turbulent kinetic energy balance, an assumption of local equilibrium is not effective in the stern flow field. An expression of eddy-viscosity was derived from turbulent kinetic energy transport equation and the distribution was calculated from measured data. It is found that the distribution was very similer to that of Shih-Lumley model. Reynolds stresses estimated by the leading term of Shin-Lumley model show good agreement with measured data. The "standard" transport equations of turbulent kinetic energy and dissipation ratio were examined to substitute measured data into these equations, residuals of the transport equations took large value in the high shear region of axial velocity.
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Hiroyoshi SUZUKI, Satoshi MIYAZAKI, Toshio SUZUKI, Kiyoshige MATSUMURA
Article type: Article
Pages
123-132
Published: September 25, 1998
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To know a turbulent structure of a stern flow field around an oblique ship model is nessesary for developing an estimation method of hydrodynamic force using CFD technique. Turbulent flow around an oblique ship model was meassured by using triple-sensor hot wire system in the wind tunnel. The mean velocities and turbulent quatities include dissipation ratio were analyzed from the time history data of the 3-componet of fluctuating velocities, and detailed stern flow database of a oblique ship model was obtained. Turbulence characteristics of stern flow field around an oblique ship model were investigated as comparison with the data of towing condition. An evaluation of stress-strain relation was carried out by using measured data at propeller section. Shin-Lumley model, one of the argebraic stress models, is effective for estimating Reynolds stresses.
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Tokihiro KATSUI, Kiyoshige MATSUMURA, Shigehisa HAMAGUCHI, Toshio SUZU ...
Article type: Article
Pages
133-140
Published: September 25, 1998
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A simple method of solution based on the adjoint variational principle already presented by the authors is investigated for determination of the unknown wetted surface and also the loading of slender planning ship. Efforts is made focusing on avoidance of the ill definiteness in the slender body theory in which the Kutta's condition, or smooth flowing out condition at the stern end, to determine the circulation distribution has been missed. Any simple trial functions free of slender body approximation of the kernel function in the principle leads to adequate solution as much as appropriate. The adjoint, reversal loading function and circulation distribution as an eigenfunction in the principle are suitable to singularity reduction in the functional, thought their characteristics are physically meaningless as presumed. Comparison of the wetted surface of a plate is made with the experimental one.
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Yusuke TAHARA, Yutaka YOSHIDA, Ryuji NISHIDA
Article type: Article
Pages
141-146
Published: September 25, 1998
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A numerical method is developed for calculating boundary-layer and wake flows around a strut-keel/bulb system of the International America's Cup Class (IACC) sailing yacht. The Reynolds-averaged Navier-Stoked and continuity equations are solved with the Baldwin-Lomax turbulence model, using a body conforming grid, finite-analytic discretization, and a PISO-type velocity-pressure coupling algorithm. In addition, circulating water channel experiments are performed for hydrodynamic force measurements and flow visualization on the body surface. An overview is given for the present numerical approach, and numerical results are presented and discussed for two-types of test strut-keel and bulb models, including detailed comparison with the experimental data.
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Yusuke TAHARA
Article type: Article
Pages
147-152
Published: September 25, 1998
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This paper presents a numerical approach for steady ship-wave problem, which is based on extension and modification of Dawson-type Rankine-source method in order to utilize arbitrary topology of free-surface panels. In this 2nd report, the main emphasis is placed on application of the present method to blunt-nose body and consideration of nonlinear effects into the free-surface boundary conditions. The computational results are presented and discussed for surface piercing NACA0024 foil, and Wigley and Series60 C_B=0.6 ship models including detailed comparisons with experimental data. It appears that the present method adequately predicts free-surface flows around blunt nose, and introduction of nonlinear free-surface effects clearly improves linear solutions especially for forebody wave fields.
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Ritsuo SHIGEHIRO
Article type: Article
Pages
153-164
Published: September 25, 1998
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Over the past few decades a considerable number of studies have been made on course stability of tow and towed vessels. This paper is intended as an investigation of the mathematical model for the maneuvering motions of tow and towed vessels. The mathematical model has some features that distinguish it from ordinary models. The mathematical model consists of two separate procedure: the first part is usual towed motions which keep a tow line under high tension, while the second part is a stopping or hard turning motions which are loose from a tow line. The first part take on a course stability in towed motions. The second part is related to a safety operation between a tow vessel and a towed vessel. The hydrodynamic models are based on MMG (Mathematical Modeling Group in Japan) model. The author conducted the free running model experiments and numerical simulation studies to validate the mathematical model. The calculation results are in good agreement with experiments results.
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Ritsuo SHIGEHIRO, Kohei UEDA, Hiroshi NAKAYAMA
Article type: Article
Pages
165-175
Published: September 25, 1998
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A great deal of effort has been made on course stability of tow and towed vessels. What seems to be lacking, however, is investigation for stopping and hard turning maneuver of tow and towed vessels. It is important maneuverability as well as course stability. This paper is intended as an investigation of stopping and hard turning maneuver of tow and towed vessels. A tow line is loose in such a motion. It is hard to control the motion of a towed vessel. This motion will increase the danger of collision between a tow vessel and a towed vessel. We conducted the free running model experiments in a bay. The model experiments were conducted using a 1/12th scale (1.3m) models. Also we performed the simulation studies to investigate these motions. The results of the experiments made improvements in technique of our numerical simulation. In addition, the calsulation results are in good agreement with experiment results.
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Akinori YUMURO
Article type: Article
Pages
177-183
Published: September 25, 1998
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This report deals with influence of trim quantity on maneuvering hydrodynamic forces in shallow water. Some captive model tests were conducted by changing trim quantity systematically. Here, a tanker type ship, which was called ESSO OSAKA, was employed. Normal force and yaw moment acting on the ship hull moving obliquely were measured at trimmed condition. Moreover, an appoximate calculation method for the hydrodynamic forces is developed on a basis of Bollay's lifting surface theory. it is assumed here that the trimmed ship can be replaced by a trapezoidal flat plate wing. And the shallow water effect is evaluated by using infinite images of the trapezoidal wing. From the results of experiments and calculation, it is pointed out that trim quantity caused by ship's bodily sinkage has a considerable effect on the hydrodynamic force in shallow water. Calculated result is also expressed by the ratio of the hydrodynamic force in trimmed codition and that in even-keel condition. As a result, it is indicated that calculated and experimental results show approximate agreement for the values of the ratio.
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Hajime KIHARA, Shigeru NAITO
Article type: Article
Pages
185-195
Published: September 25, 1998
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The numerical method has been developed for predicting the nonlinear hydrodynamic forces acting on a ship advancing with a constant speed in regular head sea. On the assumption of the slender ship approximation, 2-dimensional boundary value problems in the inner region are formulated and solved at each time step with the higher-order boundary element method. In order to examine the validity of this method, the added-mass and damping coefficients of the modified Wigley model in heave and pitch are calculated. Comparisons with experimental results show good agreement. The present procedure is applicable to the prediction of time-mean wave forces acting on a ship. Numerical results of the added resistance for the modified Wigley models are compared with the experiments and the calculation results based on linear theory. Calculations are carried out for two models with different above water hull forms. Significant effects of the above water forms on added resistance are confirmed.
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Shigeru NAITO, Masaru TSUJIMOTO, Seiji HAMANAKA, Susumu SEGAWA, Kenich ...
Article type: Article
Pages
197-203
Published: September 25, 1998
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A long term prediction method of seakeeping performance and ship responses at sea is developed by FUKUDA. The method is commonly used at the initial design stage because of its simple formulation. However it has problems to have to be more investigated as follows. (1) accuracy of wave statistics, (2) assumption of wave height distribution and (3) assumption of constant ship speed. For settling these questions, following subjects are executed. Firstly for investigating accuracy of wave statistics, sensitive analysis on wave statistics is conducted. Secondly in place of the Rayleigh distribution which is commonly applied as wave height distribution, the Crest-to-Trough wave height distribution is applied to the long term prediction method. That wave height distribution is well known as representing measured wave height distribution. Thirdly in actual seas ships change their speed at all times due to natural force or deliberate handling of ships. Therefore instead of the assumption of constant ship speed, probability distribution function of ship speed is applied to the long term prediction method, which function are obtained by author's method.
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Tsugukiyo HIRAYAMA, Yasuhiro SAITO, Kiyoshi MIYAKAWA, Takehiko TAKAYAM ...
Article type: Article
Pages
205-213
Published: September 25, 1998
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For ocean going ships, some times anti rolling systems are indispensable for carrying out her mission or obtaining comfortable life at sea. Those ideas for anti rolling were mostly proposed during the end of 19th century. Among those, bilge keels, anti rolling tanks and weight stabilizers are used until today. Those anti rolling systems use damping system by energy dissipation or motion or force compensation system. Opposite to those existing ideas, here, we proposed a new anti rolling system for high speed container ship, using anti moment by wing stabilizer and gyradius stabilizer and these idea were confirmed by numerical simulations and model tests.
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Toshihiko FUNAKI, Shigehiro HAYASHI, Naruyoshi IZUMI
Article type: Article
Pages
215-221
Published: September 25, 1998
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ISO6954 (Guidelines for the overall evaluation of vibration in merchant ship), which was adopted by ISO (International Standard Organization) in 1984, is generally used as the guideline for evaluation of vibration in a superstructure. It is necessary to design the superstructure, as the vibration that occurs to the superstructure should be satisfied with the evaluation curve of this guideline. In this paper, the word of 'Linear Grade' is defined as amounts of vibration level corresponding to the ISO6954. Then the linear grade and the linear grade's sensitivity of design variables are formulated by using sensitivity analysis at the peak frequency. Furthermore, by combining the linear grade and the linear grade's sensitivity with the optimization system of vibration level, it is verified that optimization of the vibration level corresponding to the ISO6954 is possible.
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Takashi TSUBOGO, Tsuyoshi YAGI, Hiroo OKADA
Article type: Article
Pages
223-230
Published: September 25, 1998
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Recently many studies on the elastic response behavour of very large floating structures have been done. Such a very large structure is relatively flexile compared with existing floating structures like large ships. For estimating the dynamic response behaviour of the structure, it is important to consider the deflection wave propagation based on fluid-structure interaction analysis. This paper deals with the dynamic response of a pontoon type floating structure in regular waves experimentally. In order to explain fluid field under the pontoon type floating structure in regular waves, it is necessary to know the elastic response or deflection behaviour of the structure which is the boundary condition of fluid field. By measuring bending strain time history on a pontoon type floating model in regular waves, the dispersion relation of deflection wave is experimentally investigated. From comparison between above experimental results and those obtained in theoretical analyses, the validity of the estimation method considering the deflection wave propagation is also discussed.
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Naohiro YONEDA, Setsuo IWATA, Masayuki TANIGAWA, Masao ARAKI
Article type: Article
Pages
231-239
Published: September 25, 1998
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Results from an experimental and theoretical study of the cylinder with U-type longitudinal stiffeners on buckling and ultimate strength are reported. In the experimental investigation, combined loading (axial, moment, and shear) tests were conducted on longitudinally stiffened cylinders with 60 U-type stiffeners. The thickness of the cylinder and the stiffeners are 1mm, 1.2mm respectively, and the diameter of the models are 2500mm. In the theoretical investigation, FEM analyses by use of the computer program MSC/DYTRAN for the ultimate strength of the stiffened cylinder were conducted. The calculated results were in good agreements with experimental ones. From the parametric FEM analyses, it was clarified that the thickness of the cylinder and the stiffener exert decisive influence upon the ultimate strength.
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Kohei MATSUMOTO, Toshimasa SAITOH
Article type: Article
Pages
241-246
Published: September 25, 1998
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Analytical method of ship vibration response which are utilizing at design stage were reviewed for various external forces. Database method proposed by authors were compared with ordinary empirical formula. Database method to estimate Accelerance of ship hull vibration which means Acceleration per unit exciting force at ship end were described. For 6 kinds of combination of vibration variables, effect of factors upon vibration response were investigated by histogram and regression analysis. Results of analysis were largely scattered due to complicity of phenomenon and few number of data. It is found that correlation of Accelerance and displacement of ship is stronger than that of Accelerance and ship logarithmic decrement.
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Hidekazu MURAKAWA, Zhengqi WU
Article type: Article
Pages
247-255
Published: September 25, 1998
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A new computer simulation method has been developed in order to simulate the fracture phenomena such as crack growth and interface stripping in composite material which can be considered as the formation of new surface with the crack extension. Based on the fact that surface energy must be supplied for the formation of new surface, a finite element method in which a potential function representing the surface energy is introduced is developed. The Rennard-Jones type potential commonly used in molecular dynamics is chosen to describe the surface energy in the present study. The proposed method is applied to model-I crack propagation problems and its validity is examined.
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Sadayoshi CHIAKI, Akio SAKURAI, Norio FUKIAGE
Article type: Article
Pages
257-262
Published: September 25, 1998
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In order to know the safety margin of an aged FRP boat, we have been performing a series of experimental test program. The program includes tension, compression, bending, fatigue, and impact tests on specimens cut out from the bottom and the siede plates of a typical FRP boat used over 17 years. Among the tests, the static ones; those are tension, compression, and bending tests, were completed. In this paper, the test results are compared with the calculations based on Strip Method and a long term prediction. After that the safety margin is discussed.
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Masakazu SHIBAHARA, Hidekazu MURAKAWA
Article type: Article
Pages
263-272
Published: September 25, 1998
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In this report, the effect various factors on welding deformation is investigated in order to predict the transverse deformation in high speed FCB one side butt welding of large steel plates. To calssify the problem with respect to the size and the welding condition, the welding on small laboratory specimen and FCB welding of large plate are compared based on governing parameters derived from similarity rule. Further to clarify the effect of the governing parameters on the welding deformation, serial transient thermal-elastic-plastic FEM analyses are performed. In case of small specimen, the validity of the empirical formula proposed by Satoh and Terasaki is examined. Based on the computed results, the effects of the welding conditions, the size of plate and the tab are summarized.
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Shinsuke AKAGI, Fumio NAKAHATA, Kikuo FUJITA
Article type: Article
Pages
273-282
Published: September 25, 1998
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The interest in exploiting fast ships as cargo vessels has grown in recent years. This paper examines the economic viability of such vessels by deriving operating cost estimates for outline design solutions relating to three kinds of fast freighters, i.e., a monohull , a catamaran, and a trimaran. Thier hullforms are then optimized so as to minimize each of the total resistance. The total transport cost is also caluclated for each vessel option in an assumed operating scenario, which allows an assessment of the potential competitiveness of fast freighters with conventional containerships and air transports, operating for several Asean trade routes including Taiwan, Hongkong and Singapore.
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Ken TAKAGI, Akiko YAMADA, Katsuyuki KOHARA, Tetsuro IKEBUCI, Shigeru N ...
Article type: Article
Pages
283-290
Published: September 25, 1998
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Statistical properties of slamming pressure acting on a Very Large Floating Structure (VLFS) has been investigated experimentally in this research. The bottom pressure, the displacement and the acceleration at the tip of the elastic model were measured in irregular waves. The elastic model is 1/150 scale of a VLFS which is planned in the Mega-Float project in Japan. The spectrum analysis and the statistical analysis of the measured data have been carried out. Results shows that the responses in irregular waves are almost linear and slamming can be treated as a separable phenomena. the statistical analysis shows that the bottom pressure in the sea of return period 200 years is small enough compared with the value of a conventional ship. On the contrary, the acceleration at the tip is not negligible.
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Kaori GOTOH, Naoki NAKATANI, Taketoshi OKUNO
Article type: Article
Pages
291-297
Published: September 25, 1998
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The water quality in Osaka Bay has been chronically bad because of the typical enclosed sea of the high pollution load. Environmental simulation model is necessary to construct an environmental management system for not only sea but also land area. In this research, we develop an ecosystem model for the enclosed sea with reference to the results of field investigations at Yura harbor area in Osaka bay. The results of the numerical simulation clarify the changes in water quality in Yura area, and also show some problems of environmental simulation for coastal area.
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Nobuhiro BABA, Shoji OKAMURA
Article type: Article
Pages
299-304
Published: September 25, 1998
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The structure of a gravity current advancing into a stratified fluid is investigated in the experiment and the computation of a lock-exchange flow. A finite volume of homogeneous fluid was released instantaneously into stratified fluid of constant density gradient in an open rectangular channel when a lock gate was opened. Visualization experiments were made by the shadowgraph method and with dye to trace the gravity currents of the homogeneous and stratified fluides. The homogeneous fluid released into stratified fluid advances at constant speed at the initial stage at the depth where the density of the released fluid coincides with that of the stratified fluid. At the same time the stratified fluid separated to upper and lower side advances in the opposite direction. In the following stages, several patterns of the penetration and the advancement of the fluids have been observed according to the density difference ratio. To study the effect of the stratification and the structure of the gravity current, the computation was also made in the same condition as the experiments. The incompressible Navier-Stokes equation for an inhomogeneous fluid together with the transport equation for solute was solved by the finite volume method. The computational results show the same patterns of the penetration as the experiment, and it is confirmed that the advancing gravity current changes remarkably the shape and the structure according to the density difference ratio.
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Nobuhiro BABA, Tetsuya FUKUBA
Article type: Article
Pages
305-312
Published: September 25, 1998
Released on J-STAGE: April 01, 2018
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Density fluctuation due to the passage of a gravity current advancing in an open water channel was measured to study the structure of the head and the mixing across the density interface quantitatively. Some open-type conductivity probes with different shape and size were developed and tested for resolution, accuracy, and response. A small probe with its censor 3mm long showed enough quick response and high accuracy to measure the density fluctuation in a lock-exchange flow by mounting it on the bottom of the rectangular channel. Flow visualization with dye was conducted by the shadowgraph method in the same time to know the relative position of the probe to the advancing gravity current. For comparison of these experimental results the computation was also performed by solving the Navier-Stokes equation for an inhomogeneous fluid, the continuity equation, and the transport equation for solute by the finite volume method. The results indicate that the head of the gravity current has maintained a sharp density interface even in the continuous process of the strong mixing caused by the relatively large-scale entrainment across the upper interface. This entrainment of ambient fluid starts to occur behind the head at the early stage of the evolution and continues in the developing gravity current. The region of the ntrainment is extended downstream, however the mixing is still vigorous just behind the head. These result from conductivity probes compared with the flow visualization as well as the computation, show that this large-scale mixing is attributable to the deformation and the roll-up of the upper density interface.
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Hisafumi YOSHIDA, Shuichi NAGATA, Hiroshi ISSHIKI, Takashi FUJITA
Article type: Article
Pages
313-319
Published: September 25, 1998
Released on J-STAGE: April 01, 2018
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Mooring simulation is very important to design mooring system of a very large floating structure of 5km in length. Hydrodynamic forces, wave exciting forces and wave drift forces are necessary for this simulation, however, source distribution method, commonly used to calculate hydrodynamic forces for conventional floating structure, requires huge number of source panels on the surface of a floating structure and is time consuming for the computation. In this paper, computed results of hydrodynamic forces, wave exciting forces and wave drifting forces acting on a very large floating structure are shown. These forces are computed by three-dimensional velocity potential continuation method, because this method need much less memory and CPU time compared with the source distribution method. Results of mooring simulation of a very large floating structure using the hydrodynamic forces obtained are also shown here, and characteristics of mooring simulation results are studied.
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Article type: Appendix
Pages
App2-
Published: September 25, 1998
Released on J-STAGE: April 01, 2018
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Article type: Cover
Pages
Cover2-
Published: September 25, 1998
Released on J-STAGE: April 01, 2018
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