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Article type: Cover
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Published: March 25, 1993
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Article type: Index
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Published: March 25, 1993
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Article type: Index
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Published: March 25, 1993
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Katsunori SHIMAZAKI, Yoji HIMENO, Nobuhiro BABA
Article type: Article
Pages
1-10
Published: March 25, 1993
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The importance of CFD validation has been recognized, as needs of CFD has currently increased. Several international workshops and meetings on CFD validation have been held for this purpose. Many researchers discussed general philosophy for CFD validation from the view point of the user, and concluded that the achievement of a high-level confidence in CFD for applying it to practical design problems, in turn, requires experiments focussing the verification of the accuracy of CFD codes. The difference between a measured value of flow phenomena and a computational result by CFD is roughly separated into two kinds of errors ; those are a measurement error and a CFD error that includes a modelling error and numerical errors. Therefore, it is important for CFD validation to estimate the numerical errors. Some kinds of numerical errors may arise in the computation itself such as the errors due to mathematical modelling, discretization, approximate solution algorithm, and digital operation. There are two approaches in numerical error analysis, i.e., sensitivity and uncertainty analyses. The sensitivity analysis deals with the sensitivities of various parameters, i.e., the topology and spacing of grids, etc., to numerical solutions, while in the uncertainty analysis, the total numerical error is estimated by summing up the above elemental errors. The present study follows the latter method. We thus classify numerical errors arising in the processes of computation and propose a procedure to verify the accuracy of a CFD code only by slightly modifying the CFD code itself. The procedure applied to an unsteady incompressible Navier-Stokes equation gives both the solution and the bounds of its errors at the same time. The discretization method used here is based on the finite-volume method with geometric conservation. The bounds of the discretization error are estimated from the contribution of the lowest-order truncated terms on the discretized equation. We also investigated their propagation both in time and space domains. A numerical test is performed for the simulation of viscous flows around a sinusoidally oscillating circular cylinder to demonstrate the validity of this procedure.
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Hiroyoshi SUZUKI, Yasuyuki TODA, Toshio SUZUKI
Article type: Article
Pages
11-18
Published: March 25, 1993
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The viscous flow computation of propeller-rudder interaction is presented through comparisons with available experimental data including flow visualization and mean-flow measurements. The steady flow field is calculated by a viscous flow code coupled with a body-force distribution which represents the propeller. The transport equations are discretized using a staggered grid and the exponential scheme. The velocity-pressure coupling is accomplished based on the SIMPLER algorithm. Qualitative agreement is obtained between the calculations and the mean-flow data. Although the details of the flow field is different because of the laminar flow computation and numerical treatment. The computational results show the main physical feature such as upward movement of propeller slipstream in portside vise versa in starboard side. The streaklines from one blade position are traced and compared with the flow visualization using air bubbles. The results show very similar trends. Those comparisons show the conclusion that the present approach can simulate qualitatively the steady part of the flow field around a rudder in propeller slipstream.
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Taketoshi OKUNO, Shigeru NISHIO, Yukie YOSHINO
Article type: Article
Pages
19-23
Published: March 25, 1993
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A new image measurement method for obtaining time-mean velocity field is proposed. The so-called time-averaged contrast is a statistical parameter of image data. Its characteristics are examined in detail with a simple mathmatical model for particle images. It is found that the variance of image data gives the time-averaged contrast and the velocity of the particles in the case that the averaged intensity of the image data is constant in time. Numerical simulation shows the possibility of practical measurement.
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Toshihiko KAWASHIMA, Hisashi KAJITANI
Article type: Article
Pages
25-36
Published: March 25, 1993
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A fluid dynamical grading-up of a circulating water channel is considered by surveying and controlling the steady wave and the free surface inclination. Their fundamental characteristics and effects on model resistance measurments are studied using the new born circulating water channel of the National Research Inst. of Fisheries Engineering. The evidence is shown that the optimum condition exists to eliminate the steady wave by adjusting systematically the flow volume of the channel and setting the angle of the plate of the wave suppressor appropriate. The longitudinal structure of the free surface profile with its inclination is analized and a method to express the free surface inclination is proposed. The best longitudinal setting position of a ship model for resistance tests is found eventually where the model is free from the effects of the free surface inclination. New parameter of so called "effective free surface inclination", similar to effective/nominal wake, -so that different from former "nominal free surface inclination" -is introduced that is obtained more conveniently from resistance tests through traversing ship model fore and aft, and utilized for diminishing the effects of the free surface inclination upon model resistance evaluation. The flow mechanism of circulating water channel is discussed in relation to the blockage effect and the static pressure distribution.
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Katsuo SUZUKI, Umesaburo KAWAHARA, Kazuki YABUSHITA
Article type: Article
Pages
37-46
Published: March 25, 1993
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Performance and chracteristics of the circulating water channel newly constructed at the National Defence Academy, the observation part is 5m length, 1.8m breadth, 1m water depth, is reported. The maximum speed reaches above 2.5m/s but somewhat device is desirable to be set for decreasing the air-bubble mixing in the high-speed region. The variance of velocity distribution at a measuring section is about less than ±4 percents and less than ±2 percents in the main part. The heights of the steady waves generated on the water surface are about ±2mm. The mean inclination of the water surface increases as the speed becomes high and the maximum value is less than 1/200. From measuring results of the pressure distribution and the drag for a vertical circular cylinder it is obtained that the critical Reynolds number is 2.2×10^5 and the intensity of turbulence is less than 2 percents. Resistance test and flow observation for a flat plate at zero incidence is conducted to confirm the transition of the boundary layer from laminar to turbulent and the Reynolds number is 1.5×10^6 for full turbulent flow condition. The effect of a trip wire is also confirmed to stimulate the transition to turbulence. The tank is evaluated to have a very high performance for its scale and the construction costs
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Ken TAKAGI
Article type: Article
Pages
47-56
Published: March 25, 1993
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The rankine source method is a kind of the boundary integral equation method, and it uses a simple rankine source as a kernel of the integral equation. The rankine source method has a lot of advantages, however it also has some disadvantages which are mainly due to the numerical error. In order to overcome these disadvantages, the spline interpolation function for the free surface discretization and the second order isoparametric finite element for the hull surface discretization are applied to the rankine source method for the three dimensional unsteady free surface flow with forward speed. Some computational results, such as the hydrodynamic coefficient and the pressure distribution of three dimensional ship hull with forward speed, are verified by comparing with experimental results.
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SAMPAIO Claudio M. P., Kazuo SUZUKI
Article type: Article
Pages
57-64
Published: March 25, 1993
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The main objective in the present work is to have a reliable numerical procedure to solve the two-dimensioanl problem of different shapes entering the flat water surface. A computer program that uses the Boundary Element Technique and the complex velocity potential was implemented. The transient motion and the entry at constant speed of different wedge shapes have been considered. Different arrangements of free surface, body surface and time step discretization have been employed. The importance of the body's surface local angle was verified. For the cases that had been undertaken it was observed that the nodal point distribution and the time step selection have a more significant influence for the entry at constant speed than for the transient motion, at least for the assumed velocities. Some numerical aspects need still to be clarified if this numerical procedure is to be used as a basic solver for approximated solution of the tree-dimensional problem.
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Naoya UMEDA, Toshifumi FUJIWARA, Yoshiho IKEDA
Article type: Article
Pages
65-74
Published: March 25, 1993
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The Japanese Stability Standard and IMO A.562 which were originally established for large ships are occasionally applied to assess the stability quality of small ships. However the validity of the application has not been clarified yet. In the present paper, a method to validate the application of the present stability criteria to small craft on the basis of a risk analysis on capsizing is proposed. In order to compare the results by the present method with experimental results. experimental works on capsizing of a small hard-chine craft model in irregular waves are carried out. Since the comparison between the method and the experiments are in fairly good agreement, the present method is validated. The method is applied to calculate the probability of capsizing of seven kinds of small craft. The results demostrate that the present stability standard is much more severe for a small craft than for a large ship.
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Yutaka TERAO, Kiyomi MINOHARA, Shinichirou SUZUKI
Article type: Article
Pages
75-88
Published: March 25, 1993
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Effectiveness of the newly developed passive-controlled anti-roll tank is studied in system simulation. This tank is deformed U tube, Frahm type ART, that has a butterfly valve at the centerline of the tank and this valve controls the fluid motion in the tank. To get minimum weight of the tank fluid and maximum restoring moment, ART dimensions and configurations are optimized. The weight ratio of the tank fluid per the ship displacement is 2%. The valve control algorism adopted in this system is simple and newly proposed one. Bench test of irregular roll simulation and at the sea trial shows the effectiveness of this algorism and tank system. Maximum roll angle is expected at the ship's natural period, but the measured roll angle transfer function of this system at the sea shows up to 75% roll angle reduction compared to the without ART ship.
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Akinori YUMURO
Article type: Article
Pages
89-96
Published: March 25, 1993
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This report describes some problems concerning fundamental characteristics of maneuvering hydrodynamic forces acting on a super-slender twin hull ship advancing with steady drift angle. The contents consist of three parts. Interaction forces between two hulls, flow-direction behind the stern which is closely related to inflow-angle to a rudder, and forward speed effect on hydrodynamic derivatives in mono-hull condition are examined. By representing the ship by the center-plane distributions of normal doublets and sources, the hydrodynamic forces on the twin-hull ship and each of the hulls are calculated theoretically based on the linear lifting-surface theory. It is confirmed that calculated values obtained from the linear treatment agree well with the experimental results. Furthermore, assuming that the local wave effect can be practically neglected, a simplified calculation method for predicting the wave-making effect on hydrodynamic derivatives is developed. The present method gives an approximate estimation of the rate of increase of the linear derivatives due to the forward speed effect.
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Makoto ARAI, Liang-Ye CHENG, Yoshiyuki INOUE
Article type: Article
Pages
97-101
Published: March 25, 1993
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A three dimensional simulation method based on the marker cell concept to analyse liquid sloshing in the tanks of prismatic geometry was developed by the authors. In this study, in order to simulate liquid sloshing inside the tanks with more general geometry, the numerical treatment of the boundary conditions which correspond to the curved tank walls is modified. This improved version of the method is presented in this paper. Liquid slosing inside a cubic and a cylindrical tank is simulated. The time histories of the free surface position and the dynamic pressure are obtained. The accuracy of the present method is verified by comparing the computed results with the experimental ones. These comparisons also show that the present method is able to simulate swirl motion, of the liquid inside a cubic and a cylindrical tank, which is difficult to be analysed by the conventional methods.
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Shoichi HARA, Kenji YAMAKAWA, Kunihiro HOSHINO
Article type: Article
Pages
103-110
Published: March 25, 1993
Released on J-STAGE: April 01, 2018
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According to the statistics of the accidents concerning the marine structures, the number of the accidents during the transportation on the sea is the most. It reaches as high as 54.4% out of the total number of accidents. It has been reported that the drift of the marine structures due to the breakage of the tow line covers 13.6% out of this kind of towing accidents. The establishment of the technical standard on the towing operation as a guide principle has been considered to be very difficult, because a great variety of the marine structures and ships have been involved in the towing operation. Therefore, the evaluation of the safety during the towing operation is quite important. The impulsive tension on the towline during the unstable motion can be an important factor leading to the accidents of the towline breakage in the course of towing of the marine structures. The towline tension during the unstable motion in still water is discussed in this paper as a first step to investigate the towline tension in the case of general single towing using two representative models of the marine structure such as barge and semi-submersible. The hydrodynamic force on the towline due to the current and the deformation of the towline have also been investigated both theoretically using the Imaginary Reaction Method and experimentally.
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Tadao YAMANO, Yasuo SAITO, Kazunori TAGUCHI, Sho TAKEDA, Kenjiro TAKEN ...
Article type: Article
Pages
111-121
Published: March 25, 1993
Released on J-STAGE: April 01, 2018
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Anyone recognizes the importance of grasping actual ship's propulsive performance in a real sea. Nevertheless, enough trials to do so have not been made using the data on an abstract log book (called "ab log data") which are the records of ship's voyage data. The main reason for this seems to lie in the difficulty in dealing with the ab log data. To make the ab log data useful, it is necessary to analyze them properly. The authors have tried to develop an analysis method of the ab log data easily applicable to any ships and able to bring useful results. This paper describes the developement of the analysis method, introduces some examples of its application, and shows its practical effectiveness.
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Masahiko FUJIKUBO, Daisuke YANAGIHARA, Tetsuya YAO
Article type: Article
Pages
123-133
Published: March 25, 1993
Released on J-STAGE: April 01, 2018
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In recent years, a feasibility study about ship structural design which allows panel buckling has been performed to bring out the advantage of using high-tensile strength steels. To optimize the structural components based on this design concept, a nonlinear buckling behavior has to be taken into account. This paper proposes a method of optimal design of stiffened panels allowing local panel buckling, subjected to deflection constraint and initial-yield constraint. The method consists of three parts: elastic large deflection analysis, sensitivity analysis and optimization. The first part is performed with analytical solutions assuming sinusoidal buckling deflections. Hence, any order of design sensitivities can be evaluated simply by direct differentiation. The Sequential Linear Programming (SLP) Technique is employed for structural optimization. The minimum weight design is performed for different panels of mild steel and high-tensile strength steel under inplane compression, taking thicknesses of plate and stiffener, stiffener height and the number of stiffeners as design variables. The empirical formula are used to evaluate the magnitude of initial deflection in panels. The advantages of buckling accepted design are discussed in terms of structural optimization.
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Koji OTSUKA, Yoshiho IKEDA
Article type: Article
Pages
135-141
Published: March 25, 1993
Released on J-STAGE: April 01, 2018
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Inertia force acting on a horizontal circular cylinder in waves significantly decreases with in-creasing Kc number at low Kc numbers. The rapid reduction of inertia force is caused by circulating flow around the cylinder. The strength of the circulation around a horizontal cylinder in regular waves is approximately estimated by using a simple vortex shedding model. From this model, it is deduced that the inertia coefficient is given by C_m=2-(2/π^2)Kc^2. The results by this simple formula are in good agreement with experimental ones at Kc less than 2.5. Wave forces acting on a horizontal circular cylinder in irregular waves are also estimated by using the simple vortex shed-ding model.
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Yukio UEDA, Hidekazu MURAKAWA, Dianxiang XIANG, Katsuyuki NABETA
Article type: Article
Pages
143-150
Published: March 25, 1993
Released on J-STAGE: April 01, 2018
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To study a collision problem between a floating body and an offshore structure, the influence of the fluid force acting on the floating body must be considered. The change in the characteristic of the collision phenomenon with the relative speed of collision were investigated in the last report. However, the floating body was assumed to move only in the horizontal direction and the pitching and the heaving motions were neglected. To consider their effects, a new method of analysis using Boundary Element Method is developed. The influence of these motions on the collision phenomenon for both elastic and elastic-plastic responses is examined by comparing with cases assuming only horizontal motion.
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Koji OTSUKA, Futoshi NIWA, Yoshiho IKEDA
Article type: Article
Pages
151-158
Published: March 25, 1993
Released on J-STAGE: April 01, 2018
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Roughness effect by marine growth is one of the most important factors in estimation of wave loads on offshore structures. Most researches on viscous forces acting on oscillating roughened cylinders focus the attention on the relative roughness height K/D which is the ratio of the roughness height (K) to the cylinder diameter (D). In the present study, viscous forces acting on oscillating 10 different roughened cylinders are experimentally investigated. The relation between the relative roughness height and the boundary layer thickness calculated by an oscillating boundary layer theory are also investigated.
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Shigeru NAITO, Munehiko MINOURA
Article type: Article
Pages
159-172
Published: March 25, 1993
Released on J-STAGE: April 01, 2018
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This paper shows the concept of the element wave maker and the experimental result of the divided wave maker which was made according to this concept. The element wave maker dosen't have width vertically or horizontally like a fluid dynamical singularity, and its velocity potential is expressed with a very simple formula. There are two types of wave makers which have been defined as the vertical and horizontal element wave makers. By combining these element wave makers, an arbitrary type wave maker can be expressed. The wave maker superposed with the vertical element wave maker, usually called a snake type wave maker, can generate directional waves and has been researched very well, the one superposed with the horizontal element wave maker can also produce complex motion such as not generating a progressive wave nor a local wave and absorbing an incident waves. The new wave maker-we call it COBRA wave maker-consists of ten horizontal element wave makers which are independently controlled with a personal computer. As the result, the motion of an arbitrary wave maker can be made with them. The comparision between the measurements and calculations for the progressive wave amplitude ratio of various kind of wave makers expressed approximately by combining horizontal element wave maker shows agreement fairly well.
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Horst Nowacki
Article type: Article
Pages
173-184
Published: March 25, 1993
Released on J-STAGE: April 01, 2018
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This paper give an overview of modern, computer-based methods for the systematic variation of ship hull form characteristics and for their evaluation mainly from a hydrodynamic point of view. Form variation is described for curves, surfaces and volume distributions on the basis of variational principles of shape definition. Examples illustrate the variability of the approach. Hydrodynamic evaluation focusses on numerical flow prediction and optimization strategies for flow modification. Perspectives are given for a direct coupling between from variation and hydrodynamic evaluation.
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Masatoshi BESSHO
Article type: Article
Pages
185-204
Published: March 25, 1993
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Article type: Appendix
Pages
205-
Published: March 25, 1993
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Article type: Cover
Pages
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Published: March 25, 1993
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