Showing papers in "Journal of the Society of Naval Architects of Japan in 1986"

Ship Maneuvering Motion due to Tugboats and Its Mathematical Model

Abstract: This paper presents results of free-running model tests concerning ship maneuvering motions due to assist tugboats and simulation calculations based on the mathematical model proposed in this paper. The free-running model tests were carried out by the use of wind fans on the model deck which simulate the tug forces. The mathematical model for such maneuvering motions was developed on the basis of captive model tests which consist of the static drift test with whole range of drift angle and the yaw rotating test with several combinations of towing speed and yaw rotating rate. The simulation calculations were compared with the free-running test results for typical maneuvers with tugboats. Through the present study, the following conclusions are obtained.(1) The free-running model tests clarify the basic characteristics of ship maneuvering motions due to tug forces and indicate that such maneuvering motions largely depend on ship forward speed.(2) The simulation calculations show satisfactory agreement with the free-running test results. This indicates that the mathematical model proposed in the present paper is available for describing a so-called low speed maneuvering motion dealt in this study.

On a mean to reduce excited-vibration with the sloshing in a tank

Abstract: This paper deals with experimental and theoretical investigation into a method for reducing vibration with sloshing in a tank. Exciter tests were carried out with a rectangular tank model partially filled with water. The authors also calculated the damping effect of the sloshing, using a theoretical approach in which the sloshing is simplified into mass-spring systems.

Prediction of Wave Forces on a Ship Running in Following Waves with Very Low Encounter Frequency

Abstract: A theoretical method is proposed for computing the wave forces on a ship running in the following oblique waves with very low encounter frequency. It incorporates, in addition to the Froude-Krylov forces, all the terms up to the lowest order of magnitude with respect to the disturbance of the incident waves. They include the interaction effect between the stationary waves generated when the ship running on a calm water and the disturbed incident waves.Several examples of numerical results by the method are presented to test its validity, for example, for predicting the wave yaw moment which was found by Motora et al. to bring about broaching-to phenomenon when it exceeds the course keeping ability of the ship given by the rudder action. The results show that the method proposed is better than the Froude-Krylov assumption for describing the relation between the wave force magnitude and the location of the incident waves relative to the hull.

On a Computer Aided Maneuvering System in Harbours

Abstract: In the present report, an attempt is done to develop a computer aided maneuvering system in harbours. The system contains two functions. The first is to make a plan to lead a ship to a berth and the second is to carry out the plan. To make the plan, an approaching pattern is selected and it is built up as a combination of elemental maneuvering patterns. The trajectory from the initial state to the final one is designed.To assure maneuvering safety, following two concepts are employed ; a false goal and margins of control forces. The first means that a ship approaches to the false goal placed in front of a berth, instead of approaching to the real one. The latter is to design the trajectory with enough margins of control forces.To lead a ship to the berth, both of feedback and feedforward controls are utilized along the designed trajectory. A simplified mathematical model of maneuvering motions are used to determine control forces. The control forces are supplied in quantized forms by tugs.A simulation result is finally reported to show a successful approach along the idea proposed in the present study.

Sub-breaking waves and critical condition for their appearance

Abstract: 2-dimensional steady waves generated by a submerged wing section are studied experimentally to make clear the mechanism of the sub-breaking wave. The sub-breaking wave is a kind of a turbulent free-surface flow taking place around wave-crests. It takes place after a sudden increase of the wave slope. Its appearance brings forth a velocity-defected layer with a significant turbulence intensity beneath the free-surface. The condition for its appearance is derived based on the instability analysis. It is found, for a constant-curvature-wave, that the sub-breaking takes place when the circumferential force is greater than the gravity acceleration ; in other words, the pressure gradient in the vertical direction is positive. Breakings observed in experiments are discussed through this criterion.

On a computer aided maneuvering system in harbors; study on a computer aided manoeuvring system in harbors

Abstract: Concept development for a computer-aided maneuvering system is described. The first function of the system is to plan a ship's approach to a berth. The second is to carry out the plan. Planning involves the selection of an approach pattern, which is built up as a combination of elemental maneuvering patterns. The trajectory from the initial state to the final one is designed. To assure maneuvering safety, the concept of a "false" goal, placed in front of the berth, is introduced, leaving margins for control forces. Feedback and feedforward controls are utilized along the designed trajectory to lead the ship to the berth. Control forces are determined using a simplified mathematical model of maneuvering motions and supplied in quantized forms by tugs. Finally, simulation results are presented in validation of the proposed approach.

On hydrodynamic impact pressure acting upon flat bottomed ships

Abstract: A general expression for the pressure distribution on the inclined flat rectangular bottom of a ship penetrating the water surface is derived by a matched asymptotic expansion on the bases of the shallow-draft and gravity-free assumptions that are essentially equal to those made by Wagner in 2-D theory.

Plastic Collocation Method Considering Strain-hardening Effects

Abstract: Previously, the authors developed the plastic collocation method for elastic-plastic analyses of homogeneous continuum in any geometrical shape. In this method, using ordinary finite elements, plastification is examined at the checking points for plasticity properly selected in the element. Regarding the yield conditions at the checking points as plastic potentials, and applying the plastic flow theory, the elastic-plastic stiffness matrices can be derived without integration over the element. Moreover, suitable selection of the checking points considering characteristics of the stress distributions in the element leads to improvement of the accuracy of the collapse loads.In this paper, expanding the basic theory of the plastic collocation method, a general theory for the analyses considering strain-hardening effects is proposed. In the plastic collocation method, plastic deformations are concentrated only to the nodes. Then, to take into account of strain-hardening effects, it becomes necessary to evaluate the spread of plastic region in the element and condense strain-hardening effects therein to be represented as the strain-hardening rate with respect to the plastic nodal displacements. Applying this expanded theory, several examples are analysed, and the effectiveness of this method is demonstrated.

Basic Studies on the Crashworthiness of Structural Elements Part 1

Abstract: In the present paper a nonlinear finite element code based on the updated Lagrangian approach will be developed for the analysis of the crushing behavior of structural elements and some numerical examples will be presented in order to show the validity of the present algorithm. Several features of the developed code are : 1. The bilinear degenerate shell element with reduced integration is employed.2. The hourglass control stiffness proposed by Flanagan and Belytschko and in-plane rotational stiffness due to Kanok-Nukulchai are added to the element stiffness.3. The virtual work equation is expressed in terms of the updated Green strains and the updated Kirchhoff stresses and the calculated Kirchhoff stress increments are transformed into the Jaumann stress increments at each incremental step. In this transformation the normal strain in the thickness direction is determined so as to maintain the plane stress condition.In numerical examples the post-buckling behavior of axially loaded elastic and elasto-plastic columns are analized and the calculated load-deflection curves are checked against the exact solution and experimental results. The crushing behavior of axially compressed circular cylindrical shells is also calculated, and the obtained results are compared with experimental results.

Wave Forces Acting on Rough Circular Cylinders at High Reynolds Numbers

Abstract: Hydrodynamic forces acting on rough circular cylinders in a harmonically oscillating flow at high Reynolds numbers were experimentally investigated.Especially effects of the roughness height and roughness density were investigated. Two kinds of roughness height-5mm, 10mm and three kinds of roughness density-20, 60, 80% were used in experiments.Experiments were carried out using a large circular cylinder with 1.2m diameter and with 1.5m length at NKK's Tsu Ship Model Basin. The cylinder was fixed to the main carriage. The cylinder was forced to oscillate harmonically with the main carriage. In-line and lift forces acting on cylinders were measured at Keulegan-Carpenter numbers over 6 up to 50 and at two Reynolds numbers-0.5, 1.0×106. Drag coefficients, added mass coefficients, maximum lift coefficients and periods of lift force are obtained.Experiments show that the roughness has a great effect on the hydrodynamic coefficients even at high Reynolds numbers. In the range of the value of roughness parameters used in these experiments, a roughness height has a more considerable effect on the hydrodynamic coefficients than a roughness density.

Calculation of effective wake distribution of a simple hull form

Abstract: A method for calculating effective wake distribution is developed for a three-dimensional hull form. In the present study the momentum integral equations of the boundary layer and wake are solved by an iterative procedure taking the interaction between propeller and ship wake into account. The velocity field with an operating propeller is divided into effective velocity field and propeller induced velocity field, and the boundary layer approximation is assumed to be applicable to the effective velocity field while the propeller induced velocity field is assumed to be inviscid. To examine the validity of the method the flow measurements were made for a simple hull form, i. e. Wigley's parabolic hull form model. It was found that the method can predict fairly well the velocity field with and without a propeller. The calculated results show that the degree of change from the nominal wake distribution into the effective wake distribution is large in the region where the propeller induced velocity is strong and the velocity gradient of the nominal velocity field is steep.

On crack arrestability by a circular hole based on computational crack path prediction

Abstract: In stiffened plate structures, cracks are often formed at the intersections of the stiffeners, where severe local stress concentrations and welding residual stresses exist. Since the local stress distribution is rather complicated along the crack trajectory, crack curving is one of the essential features of this type of problems. Cracks initiated and growing in the stiffened plate structures are difficult being detected by small leakage of fluid, and the crack arrest design concept which is different from the leak before break (LBB) concept could be introduced in preventing the catastrophic failure of such structures.A computational scheme has been developed for the crack path prediction, which is performed by the step-by-step stress analysis ahead of the crack tip and the prediction of a curved increment of the crack growth by the use of analytically expressed asymptotic crack path in the neighbourhood of the crack tip. As a fracture criterion we use the locally symmetric condition which requires the Mode II stress intensity factor vanishing along the crack path.In this paper, holes are considered as crack arresters and their capability is examined by computational crack path prediction and experiments. We consider the case where a crack is approaching a circular hole. When the crack tip approaches in the neighbourhood of the hole, where the shortest distance between the crack tip and edge of the hole is within the radius of the hole, they attract with each other and the crack penetrates into the circular hole resulting arrest at the hole.

Effective Width of Rectangular Plates Subjected to Combined Loads

Abstract: An analytical expression for effective width of rectangular plates subjected to uniaxial compression is derived taking account of initial deflection and welding residual stresses. To verify the validity of this expression, it is used to evaluate the ultimate strength of compressed plates. An expression is also derived to evaluate the effective width of rectangular plates subjected to biaxial compression and shear, taking initial deflection and welding residual stresses into consideration.

Study of the propeller with small blades on the blade tips (1st Report)

Abstract: A new type propeller derived from “winglets” was investigated. This propeller is also fitted small blades at the blade tips likely to “winglets”, so we named it “bladelets”. These are for the purpose of reducing induced drag from tip vorteces. This paper, as the first report, mainly presents the results of the series tests which are concerned with the arrangement of these small blades. Propeller open test, flow visualization and flow velocity measurements were performed, and one of the best arrangement of “bladelets” were found out. The results looks 1 or 4% better compared the efficiency of “bladelet propeller” with that of the original one within the bound of real working condition.

Plate and stiffened plate units of the idealized structural unit method (2nd report): under in-plane and lateral loading considering initial deflection and residual stress

Abstract: In the previous report, the nonlinear behavior of perfect plates and perfect stiffened plates (without initial imperfections) subjected to in-plane loads were studied and idealized. Basic versions of two idealized elements, the rectangular plate unit and the stiffened plate unit, have been developed. In the current report, the capabilities of these idealized elements are extended so that they can be accurately applied to a wider range of actual structures. Effects of initial imperfections, i.e. initial deflection and welding residual stress, on the behavior of these elements are studied, idealized and included in the formulation. Lateral load is also considered and its interaction with initial imperfections and in-plane load is taken into account.

Basic Studies on the Crashworthiness of Structural Elements

Abstract: The crush tests for seven specimens of square tubes under axial compression are carried out, whose results are compared with the existing theoretical solutions and the empirical formulas with respect to the buckling load, the wavelength of the crushing mode and the mean crushing load. And also the finite element analysis is conducted by using the code developed in the previous report. The obtained results can be summarized as follows : (1) As for the buckling load the existing theoretical as well as the finite element solutions agree well with the experimental results.(2) As for the crushing wavelength the theoretical prediction fails, however, the finite element solutions which minimize the buckling loads are in quantitatively good agreement with the experimental values. The theoretical values given by Wierzbicki which minimize the rigid-plastic solutions for the mean crushing stresses also agree well with the experimental results.(3) As for the mean crushing load there is not a good agreement between the experimental results and the existing rigid-plastic solutions in which the strain hardening is not fully taken into account, while the empirical formula given by Magee et al. using the tensile strength instead of the yield stress has sufficient accuracy in spite of its simple form.(4) The complicated crushing deformations can be successfully simulated by the finite element method. The obtained mean crushing loads are about 25% smaller on the average than the experimental values because of the thick-walled effect, however, there is a good agreement between them from a qualitative point of view.

Finite-Difference Simulation Method for Waves and Viscous Flows about a Ship

Abstract: A finite-difference method named WISDAM-II is developed for viscous flow about a steadily advancing ship involving free-surface waves. The Navier-Stokes equations in a rotational form are solved by a time-marching scheme in a boundary-fitted coordinate system which is deformed so as to fit the moving free-surface. Making use of the subgrid-scale turbulence model, the three-dimensional separation and the effect of the free-surface waves on the viscous flow are simulated and discussed.

A Probabilistic Approach to Thickness Effect in Fracture Toughness

Abstract: Fracture toughness values obtained are considerably affected by specimen thickness. Two plausible explanations for such thickness effect are given as follows : (1) Plastic constraint increases as plate thickness increases. And (2) Possibility that the embrittled structure zone exists along crack front increases with increase in plate thickness. In the present paper, probabilistic considerations have been carried out by using various kinds of data obtained in order to clarify the applicability of the weakest link model for evaluating fracture toughness with respect to thickness effect. Moreover, to obtain the data for the above probabilistic analysis, a new statistical method has been studied for determining population of fracture toughness distribution with taking the temperature dependency of fracture toughness into account. Statistical method proposed newly in the present study is sufficiently applicable.In both steel plate and its welds with heterogeneity in fracture toughness, thickness effect in fracture toughness results from a statistical effect and the weakest link model can be adopted to evaluate the thickness effect.

Unsteady Structure of Cloud Cavitation on a Foil Section

Abstract: The structure of flow around unsteady cloud cavitation on a steady two-dimensional hydro-foil was investigated experimentally, using a conditional sampling technique. The unsteady velocity around cloud cavitation was measured by a LDV and matched with the unsteady cavitation appearance taken by a high speed camera. This matching procedure was performed, using data of pressure fluctuation measurement on the foil surface. The velocity fluctuations could be divided into a large-scale (low frequency) component and small-scale (high frequency) one, using Fourier analysis. The unsteady cloud cavitation corresponds with the large-scale structure.In this manner, both large-scale (coherent) and small-scale (incoherent) structure of the flow around the cloud cavitation were clarified and discussed respectively. Main results are given as follows;(1) Cloud cavitation observed at the present research is a kind of large-scale vortex cavitation whose rotation direction is clockwise when main flow direction is from left to right. The velocity of cloud cavitation is much lower than the uniform flow velocity.(2) The small-scale velocity fluctuation is not distributed uniformly in the large-scale structure, but concentrated near the edge of it. One of the reasons might be that cloud bubbles collapse there.(3) Three-dimensional structure of cloud cavitation is a stretched vortex whose configuration looks like a “croissant”.

Motions of floating offshore structures in multi-directional waves

Abstract: For lack of facilities, only a few experimental studies have been carried out exploring the motions of a floating body in multidirectional waves. The authors therefore carried out model basin experiments on the motions of a floating body in generated two-directional waves, checking the linearity of the waves and the body motions. From these investigations, they point out the limitation of linearity and the interesting phenomenon of the slow drift oscillation of a moored floating body in two-directional waves. They also develop an experimental technique for motions in multidirectional waves, deriving theoretical predictions based on a functional polynomial method.

Local buckling of bracing members in semi-submersible drilling unit (1st report)

Abstract: The local buckling of tubular members after ultimate strength is attained was investigated from both experimental and theoretical viewpoints. First, axial compression tests were carried out on three tubular specimens similar to the bracing member in an existing semisubmersible drilling unit. It was known that : (1) After the ultimate strength was attained, local buckling of a cosine mode first takes place at the compression side of bending. The buckling wave spreads about one half circle in the circumferential direction, but has short wave length in the axial direction.(2) At the same time, load carrying capacity suddenly decreases.(3) Then, the formations of local denting deformation follow at the foot of initial cosine buckling wave.A simplified method was proposed to analyze the elastic-plastic behaviour of a tubular member under axial compression. Two models were newly proposed which simulate the post-local buckling behaviour of a tubular member based on the observed results in the experiments. They are COS model and DENT model.Combining these models with the proposed simplified method, a series of analysis was performed on the test specimens newly carried out and previously done. The results of calculations were compared with the experimental ones, and the validity and the usefulness of the proposed method of analysis were demonstrated.

Effects of Compressive Prestrain on Fracture Toughness

Abstract: It is known that when buckling accident occurs, the buckled part easily separates. In this connection, the deteriorating effects of extremely high compressive prestrain on ductility of mild steel were discussed in the present paper.30mm thick mild steel plates were compressed uniformaly to a few levels of prestrain up to -35%. After prestraining, 2mm V-notch Charpy specimens and compact tension specimens were machined, and tested for evaluating the criteria of Charpy test and fracture toughness. In order to attain to higher compressive prestrain, mild steel plates were deformed by bending to several levels of prestrain up to about -80%. 2mm V-notch Charpy tests were carried out on these platesThe Charpy test results of the materials prestrained by two different ways of plastic deformation were compared, and it was found that there is substantially not much difference between them. The deteriorating effect of compressive prestrain was analysed in terms of prestrain level. Criteria of Charpy test and CT test were found to rise almost linearly against prestrain level. As for the Charpy test criteria, the most sensitive one was vTr15, and rate of rise of it was 3 to 3.7 deg C for one p.c. of compressive prestrain. The results of CT test were found to be more sensitive, and the rate of rise on the stress of yield point of base metal was 3.5 to 4.1deg C.In order to examine the possibility of cracking by brittle fracture from the unnotched part in the case of buckling accident, steel plates were bent to various angle and unloaded. When absolute values of compressive prestrain exceeded the level of ductility of the plate, brittle cracks initiated at the compressive side in the course of unloading. From the fractographic examination, it was found that, at the starting point, fracture surface was covered with not only quasi-cleavage facets but facets of slip planes. After propagation of 2.3mm, fracture surface consisted of cleavage facets.

Basic studies on the crashworthiness of structural elements, part 1: crush analysis by the finite element method

Abstract: A nonlinear finite element code based on the updated Lagrangian approach is developed to analyze the crushing behavior of structural elements. Numerical examples are presented to show the validity of the algorithm.

The idealized structural unit method including global nonlinearities--idealized rectangular plate and stiffened plate elements

Abstract: An elastic-plastic large displacement theory of the "Idealized Rectangular Plate Element" and the "Idealized Stiffened Plate Element" is formulated. In it, both local (inter-unit) nonlinearities (large deflection, buckling and plasticity) and overall nonlinearities are taken into account. On the local level, behavior of plates and stiffened plates is idealized according to the techniques of the Idealized Structural Unit Method. On the global level, plates and stiffened plates are treated as membranes, neglecting their out-of-plane bending contribution to the global structure in comparison with their inplane contribution and the contribution of primary supporting members. Out-of-plane and in- plane large displacements of the global structure are taken into account.

The Recovery of Rotational Energy in Propeller Slipstream by Fins installed after Propellers

Abstract: At first, the fundamental equation of the energy distribution in the flow field around a propeller and fins are derived. Next, the theoretical and experimental studies on the recovery of the rotational energy in the propeller slipstream show the following results : a) In the propeller slipstream between 0.125 D and 2.0 D behind propellers (D ; propeller diameter), the dissipation of the rotational energy is very weak and the rotational flow is same at every cross sections. b) The fins after propellers decrease the rotational energy in the propeller slipstream and convert the most of the energy into the work done by fin thrust.c) With increasing the number of blades, fin improve the rotational energy recovery. The increase of fin drag, however, makes the optimum blade number finite. It mainly depend on propeller loading conditions and fin chord length.d) The rudder with fins recovers the rotational energy more efficiently than without fins.

A study on the prediction of propeller slipstream and the numerical analysis of lifting surface

Abstract: In recent years, a highly advanced lifting surface method with high accuracy has been required in order to design the various types of propellers which demand high efficiency and to reduce ship vibration and noise.In this paper, an improved lifting surface procedure based on the vortex lattice method is presented. For the determination of a trailing vortex wake geometory, a numerical iterative procedure is described. The numerical results on propeller slipstream give a good agreement with the measured values by laser doppler velocimeter.The present method is applied to calculate the open water characteristics of a propeller. And a quasi-steady technique based on the present method is adopted to calculate the fluctuation of propeller forces and cavitation patterns behind the ship's wake field.The advantages of the present method are shown by comparing the numerical results with the experimental data of conventional and highly skewed propeller.

Vortex-Induced Vibration Experienced on Sea Chest

Abstract: A rare kind of structural vibration induced by flow was experienced on sea chests of high speed ships. Model experiments in cavitation tunnel revealed the cause of vibration and the mechanism of excitation. The results showed that the free shear layers flowed into the cavity of sea chest were rolled up periodically into vortices and the vortex excitation resonated with the natural frequency of the vibration system composed of the sea chest structure and the water involved in it. Countermeasures reducing the excitation were also obtained by the model test.Energy method was applied for the estimation of the natural frequency of sea chest vibration system. The calculated results by this method were verified to correlate well with the results of vibration test on sea chest model, together with the measured results on actual ship.

Fundamental Studies on Underwater Sound Radiated from a Vibrating Ship Hull

Abstract: In preceding reports, authors studied on the two dimensional sound radiation problem and sound scattering problem from a vibrating infinitely long cylindrical elastic shell. Introducing the numerical analysis, we clarified the property of these phenomena.However an actual structure is three dimensional so that we can not exactly estimate the deflection of the structure and the propagation of underwater sound from two dimensional treatment.In this report, developing former procedure, we show the radiation problem, the sound scattering problem, and the reciprocal theorem between these problem from a time harmonically vibrating three dimensional elastic structure. Two calculation approaches are introduced. The one is the analytical approach for which the classical method of separation of variables. By using this approach, we calculate the radiation and scattering problem from vibrating spherical elastic shell in an infinite fluid domain and confirm the reciprocal relation numerically. The other approach is that the finite element analysis of the structure is matched at the structure-fluid interface with the boundary element analysis of the exterior fluid domain. This approach can be applied to any complex three dimensional structure. We also discuss the property of the singularity of the kernel function in the boundary ele ment analysis and show an accurate numerical method of integration. To verify its usefulness and accuracy, some numerical examples are shown for submerged elastic spherical shell subjected to time harmonic exciting force.

A study on performance and cavitation of propellers for high speed crafts including effect of boss, 1st report: analysis in uniform flow

Abstract: In the first report the authors discussed analysis for uniform flow. Using the same discrete model, the analysis described in the present report was performed for the oblique case. In addition to the effect of boss, propeller slipstream asymmetry was incorporated into the model. Experiments were performed by inclining the propeller shaft with respect to the flow direction in the cavitation tunnel. Predicted cavity extents were found to agree relatively well with observed ones when values were advanced by sixty degrees. In the calculation of pressure distribution on the blade, it was found that there was a considerable drop in pressure at about the 60% chord position of the blade near the boss and this position has been found to agree well with the observed position of root cavitation erosion.