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Article type: Cover
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Published: March 25, 1992
Released on J-STAGE: April 01, 2018
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Article type: Index
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Published: March 25, 1992
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Article type: Index
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Published: March 25, 1992
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Munehiko HINATSU
Article type: Article
Pages
1-11
Published: March 25, 1992
Released on J-STAGE: April 01, 2018
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This paper presents a method to simulate unsteady viscous nonlinear waves by numerically solving Navier-Stokes equations. The equations were solved by a finite difference method with a free surface fitted coordinate system, which is regrided every time step. The time derivatives were approximated by a time splitting fractional step method. Second order central differences were used to discretize the viscous and pressure terms while the convection terms were discretized by a QUICK scheme. A cell centered stencil was used. In order to check the accuracy of the present scheme, the following test problems were solved numerically: (1) standing wave problems, (2) the propagation and reflection of a soliton, (3) piston-type wavemaker simulations, and (4) the wave generation by a moving topography. The computational results are compared to analytical results when available and good agreements were obtained. The present scheme was thus validated.
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Minoru INOUE, Nobuhiro BABA, Kazuhiro KITAGAWA, Takahiro NAKAGAWA
Article type: Article
Pages
13-20
Published: March 25, 1992
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Three-dimensional finite volume method using a moving grid system is presented for solving the incompressible viscous flows with a free surface. A new technique is developed for the implementation of the nonlinear free surface conditions. In the time-marching solution procedure, the grid points on the free surface are moved in the direction normal to the surface by using the volume flux vector across the temporary cell face. This method holds the conservative properties even in the cells adjacent to the free surface and it allows the moving grids to represent the three-dimensional arbitrary deformation of the free surface without any rearrangement of grid points. Numerical experiments of the collision of the two solitary waves traveling in opposite directions are made to demonstrate the validity of this method. The two waves emerge almost unchanged in profile after their interaction of brief duration. The present method is also applied to the three-dimensional viscous flows about a circular cylinder vertically piercing a free surface. The computed results show the strong interaction of the vortical flows with the free surface waves.
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Tadao YAMANO
Article type: Article
Pages
21-29
Published: March 25, 1992
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The most important characteristic of the wake pattern of full hulls is that its main part is occupied by the wake by bilge vortex. This characteristic makes it difficult to estimate the wake pattern of such hulls. The author has tried to develop a practical estimation method of wake pattern which can be applied even to such hulls. This paper describes its development.
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Katsuo SUZUKI
Article type: Article
Pages
31-43
Published: March 25, 1992
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To visualize potential flow easily, an apparatus of Hele-Shaw flow using service water was designed and constructed. The conditions of the space between two parallel plates and the flow speed are discussed from the theoretical consideration. The apparatus is reported to realize almost exact potential flows past various shapes, circle, inclined plate and so on. The flow is checkedcomparing with the calculated streamlines and velocity around the shape. A new trial is successfully performed to generate circulation around a shape. The flow past an inclined foil is able to satisfy socalled Kutta condition at the trailing edge. The apparatus realizing the potential flow with and without circulation may be utilized to recognize the basis of hydrodynamics.
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Nobuyuki HIRATA, Takanori HINO, Takeshi FUWA
Article type: Article
Pages
45-49
Published: March 25, 1992
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Flow fields around hybrid hulls at high speed have two special features. One is the strongly non-linear phenomena such as spray, ventilation, cavitation, and so on. The other is the interaction among hull components. Firstly in this paper, a submerged body with surface-piercing struts in tandem is considered. A flow field around the hull was simulated by a finite-difference method for the Euler equations with non-linear free surface conditons. The algorithm is based on the MAC method. Since a free surface configuration is expressed by the single-value wave height function, spray cannot be simulated. Ventilation and cavitaion are not also considered in this scheme. The interactions of waves generated by a submerged body and two struts, however, are expressed. Results simulated agree well with experimental data. Secondly, a submerged body with hydrofoils and surface-piercing struts in tandem is considered. The method of calculation is same as that of the former. Computed lifts generated by hydrofoils are compared with the experimental data. Although the agreement is not so well for the aft foil, we got a good agreement for the fore foil. It is not well. The reasons for the disagreement for the aft foil are (1) neglect of the viscosity, ,i.e., viscous wake due to the fore foil is not considered, (2) inconsideration of spray, (3) coarse computational grid and so on. A modelling of high speed flow phenomena and the development of the accurate solver remain as future problems.
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Kazuyuki OUCHI, Masahiro TAMASHIMA, Hajime KOIZUKA, Kenta ARAI
Article type: Article
Pages
51-59
Published: March 25, 1992
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PBCF (Propeller Boss Cap Fins) is a boss cap with small fins. It recovers the rotational energy in propeller slipstresm by eliminating the hub vortex and improves propeller efficiency. In this paper it was studied how propeller efficiency with PBCF changes by changing pitch distribution of a propeller. Three model propellers, what are called increasing pitch propeller, constant pitch propeller and decreasing pitch propeller respectively, were made changing pitch ratio of each root part. And those were tested in uniform flow and two different non-uniform flows with or without PBCF. On the other hand calculation of propeller performance has made. The results obtained are ; (1) Significant efficiency improvement by PBCF has obtained for all the propellers, and it was 2〜8% in this model test. (2) Increasing pitch propeller, which seems to be adapted to the wake, has also been improved by PBCF. (3) It may be possible that a more efficient propeller is designed considering PBCF and well-matched propeller pitch distribution.
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Eiichi NISHIKAWA, Makoto UCHIDA, Tetsuya FUJIOKA
Article type: Article
Pages
61-65
Published: March 25, 1992
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The measurement of fluctuating pressure on the hull surface just above the propeller and the propeller cavitation observation of the training ship "FUKAE MARU" of Kobe University of Mercantile Marine have been carried out. Her propeller is pitch-controllable and moderately skewed. By the use of those date, this paper examines the validity of the conventional simplified method for estimating the fluctuating pressure in the case of a skewed CPP. The examination makes clear that (1) among 3 methods examined here, that is Takahashi, Johnsson and Holden, the Johnsson's method is most correct, (2) all of them, however, have been developed on basis of the data of FPP, so that they cannot estimate the influence of blade angle especially when the cavitation is occurred, (3) the behavior of the propeller cavitation is seems to be dominantly affected by the attack angle at the blade tip, so that the influence of the blade angle of CPP would be possibly estimated by the attack angle.
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Matao TAKAGI
Article type: Article
Pages
67-75
Published: March 25, 1992
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The dispersion relation of the elementary waves in the 3D Green function of oscillating body with forward speed on water of finite depth is discussed. There is a new finding with respect to the phase velocity of the elementary waves, which accompanies some modifications of the Green function. Furthermore, the possibility of the representation of the above Green function in Bessho's way is discussed from the view-point of the characteristics of the singularities in the Green function.
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Ken TAKAGI, Kazuyoshi HIROTA
Article type: Article
Pages
77-83
Published: March 25, 1992
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In order to discuss the strategy of the numerical calculation of three dimensional unsteady free surface flows, the computer program of the finite element method with adaptive mesh refinement procedure is developed and some numerical results are obtained. This program is composed of several procedures, such as the automatic mesh generation, N-S (Navier-Stokes Eq.) solver by FEM, the adaptive mesh refinement and the band width reduction. Delaunay triangulation algorithm is applied to the automatic mesh generation. Arbitrary Lagrangian Eulerian FEM is applied to theN-S solver. Gibbs-Poole-Stockmeyer and Gibbs-King Algorithms are applied to the reduction of band width. And, the mesh refinement procedure for three dimensional flows is newly developed. Some numerical results are obtained by the above mentioned computer program. A data structure which is sulitable for the vectorization of the computer program is proposed.
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Iwao WATANABE
Article type: Article
Pages
85-94
Published: March 25, 1992
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A computational method for three dimensional hydrodynamic forces on a ship with arbitrary hull form advancing in head sea condition is discussed based upon Hanaoka's thin ship theory. Following the linearization of the problem, it is shown that potential functions of Michell's type can be expressed explicitly and the hydrodynamic forces can be integrated directly from the potentials multiplied by hull form curvature in the vertical direction owing to the thin ship assumption. The finite Fourier series is introduced to express aribtrary hull geometry. Upon substitution of the hull form into the hull form characteristic functions, it is shown that the formula for the forces are expressed in a series with special functions like Bessel, Struve and Weber functions. The computation for the radiation and diffraction forces is done for a container ship and the results show good agreement with a strip theory calculation in lower speed range. Discussion is also given to characteristics of the integrands and integration scheme to show that the present method has advantage over other three dimensional calculation methods in both ease of numerical computaion and the accuracy of the results.
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Myung-Soo SHIN, Kuk-Jin KANG, Young-Gill LEE, Seung-Il YANG
Article type: Article
Pages
95-101
Published: March 25, 1992
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The behaviours of the numerical solutions according to the differencing schemes for convection terms in two-dimensional Navier-Stokes equations are investigated. Using the central differencing, the 3rd order upstream differencing (UTOPIA and Kawamura-Kuwahara) and QUICK (Quadratic Upstream Interpolation for Convective Kinematics), pilot computations are carried out for two-dimensional flows in a cavity and around a floating body. The QUICK scheme shows fairly good results in case of flow in a cavity, but the 3rd order upstream differencing schemes show agreeable results in case of flow around a floating body with free-surface.
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Shigeru NAITO, Takeshi UEDA
Article type: Article
Pages
103-113
Published: March 25, 1992
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The authors calculated the added resistance in short wave length by the ray theory and compared it with many experimental results by using the model with several bow forms. Ray theory explained well these results (in case of head wave and oblique wave conditions) and made the effect of the horizontal bow forms on added resistance clear. In this paper authors propose the easy method to estimate the added resistance and the mutual relation between the added resistance and the horizontal bow form in regular short waves and short-crested irregular waves. At first step, the F.P. point and shoulder point of the horizontal bow form are connected by straight line. As the result the bow form is divided into two parts. One is the rectangular part, other is the swollen part. Under zero speed condition, the added resistance acting on the bow can be separated into two components, the rectangular part component and swollen part component. The value of the rectangular component is obtained analytically and that of the swollen part component is simplified by neglecting the higher order value. Approximated formula of the forward speed effect component is proposed based on the many calculated results by the ray theory. The total values of three components are the added resistance. In case of the short-crested irregular waves, the simplified estimated method of the mean added resistance is proposed based on the method mentioned above and using the approximated wave spectrum. The proposal method could be effective on initial disign stage of a ship when a designer wants to know the effect of bow form on added resistance. The behavior of wave motion near the bow corresponds to added resistance strongly. To know the feature of unsteady wave breaking near the bow and unsteady wave patterns, an experiments to take the photo of wave behaviors near the bow are carried out and compared with the theoretical results.
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Masami HAMAMOTO, Takashi ENOMOTO, Hidekazu MANABE
Article type: Article
Pages
115-124
Published: March 25, 1992
Released on J-STAGE: April 01, 2018
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This paper outlines the dynamics and control of a hybrid type hydrofoil craft which is 1.5m length and 11.5kg load suported by surface piercing fore-foil and fully submerged aft-foil with controllable flaps. First, the hydrodynamic lift and drag acting on the foil surfaces are measured by captive model tests and the experimental results are compared with theoretical prediction. Next, the equations of motion required to describe the longitudinal motions of hydrofoil are discussed in detail, together with simulation of the equations. The towing tests of the model are carried out for foil bone condition in still water and regular waves. By comparing measured response with the mathematical model, the velidity of simulation is discussed. Finally, the effect of control flaps on the response are investigated to reduce the heave and pitch motions of hydrofoil craft in longitudinal waves.
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Keiichi KARASUNO, Jiro MATSUNO, Tomoyuki ITO, Kazuyuki IGARASHI
Article type: Article
Pages
125-135
Published: March 25, 1992
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This paper describes the physical mathematical model of ship hydrodynamic forces in turning motion that occur with drifting, The mathematical model has been derived also from the physical mathematical model of ship hydrodynamic forces acting on a hull moving in an oblique direction. The hydrodynamic forces in the mathematical model consist of six elements of fluiddynamic forces. They are as follows : ideal force, lift and induced drag forces due to viscosity, cross flow drag force and cross flow lift force due to fore and aft asymmetry, and frictional drag force. At the leading and trailing edges of the ship's hull, viscous lift, induced drag and cross flow lift forces occur. These forces connect with drifting angles at both edges. Furthermore, it is assummed that the coefficient of cross flow drag force is changed by drift angle βx, C_D=C_<D90>・√<u'^2+(v'+x'・γ')^2∣sinβ_x∣・(1+p・cos^2β_x). This function is determined from the coefficient of induced drag force, which is derived from the assumption that no stall effect in Iongitudinal force occurs within the range of small drift angles. Experimental data of hydrodynamic forces generated from a turning ship model of a pure car carrier (P.C.C.), were incorporated with the mathematical model constructed above and resulted in the following mathematical model.[numerical formula]
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Akinori YUMURO, Kyuichi YAMAMOTO
Article type: Article
Pages
137-143
Published: March 25, 1992
Released on J-STAGE: April 01, 2018
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This paper deals with scale effect on direction of a flow behind a ship's hull in oblique flows. This problem is related with scaled effect on rudder forces during maneuvering motions. The flow directions were determined experimentally, employing three geosim tanker models of 2m, 4m and 7m in length. The measured results were expressed by the coefficient of flow-straightening effect of a ship body, which was defined as the ratio of a flow direction angle to a drift angle. In addition, on the basis of the assumption to decompose the free stream velocity into lengthwise and crosswise components, calculation for the scale effect on flow-straightening effect was examined. In this calculation method, downwash due to vortex distribution correspnding to hydrodynamic forces on the ship's hull was taken into consideration. Through the study, the authors found that the coefficient becomes larger as ship model length increases. But on the contrary, calculated results indicate that the coefficient decreases with increment of ship model length. Therefore, the problem concerning the prediction method for the scale effect on the characteristics of the flow direction has been left unsolved.
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Hyo jae JO, Hisaaki MAEDA, Shogo MIYAJIMA
Article type: Article
Pages
145-151
Published: March 25, 1992
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It is very important to consider low-frequency motion, in designing moored systems for a floating structure. Many researches have been studied about them. Still there remain various problems left related to uncertainties which were simplified or assumed in both theoretical calculations and model tests. Here, we try to explain how directional waves affect on the behaviour of moored offshore structures. We developed a simulation program to calculate low-frequency motions of moored floating structures in directional waves using the time domain analysis. We investigate the effects of steady drift force due to viscous drag force on the low-frequency motions. The results of theoretical calculations are validated through the comparison with model tests carried out at a square basin which can generate bi-directional waves. There are good agreements between the numerical simulations and experiments. Thus, we can investigate the effect of the directionality of waves on low-frequency motions of moored floating structures by the present simulation program.
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Isao SUZUKI, Kenjiro MIKI, Takanobu YAGI
Article type: Article
Pages
153-162
Published: March 25, 1992
Released on J-STAGE: April 01, 2018
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In a previous paper, the authors have shown the supercritical open-channel flow model for the flow of an oil layer around an advancing skimming vessel and have verified that the model is characterized by the densimetric Froude number, using models of mono-hull vessels. Tank tests have been carried out on the flow of an oil layer between hulls of advancing skimming twin-hull vessels by means of the previous paper, using geosim models and different large-sized models with the geosim models. The geosim models with different entrance angles have been tested, at varying ship speeds and oil layer thicknesses. The test results of the geosim models show that the supercritical open-channel flow model and the law of similarity are valid at low densimetric Froude numbers. However, it is very difficult to separate shock wave from the effect of blocking up the oil layer flow between twin hulls. The large-sized models have been investigated for the purpose of improving the test accuracy. It has been verified that the interference between shock wave and negative disturbance lines behind the stem exists. Comparison between theoretical results and experimental ones shows good agreement for the behavior of an oil layer using the large-sized models.
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Kohichi OKAMOTO, Hideaki NAOI, Yuhkichi TAKAOKA
Article type: Article
Pages
163-167
Published: March 25, 1992
Released on J-STAGE: April 01, 2018
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Hydrofoils with water jet propulsion systems often have local vibration on their stern-end shells caused by the fluctuating water pressure induced by the water jets gushing into sea water in hull-borne conditions. At the structural design stage, it is important to estimate the fluctuating water pressure for the prevention of structural problems caused by such local vibration. In this study, the fluctuating water pressure, local vibration and stresses in the stern-ehd shell were measured by using an actual hydrofoil. From the results of these measurements, a method for estimating the fluctuating water pressure and structural responses is newly proposed for the structural design of such hydrofoils. It is then shown that this method will be useful at the design stage of such hydrofoils to estimate the local vibration and the fatigue damage to these stern-end structures.
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Article type: Appendix
Pages
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Published: March 25, 1992
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Article type: Cover
Pages
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Published: March 25, 1992
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