Showing papers in "Journal of Ship Research in 2002"

Sloshing in rectangular and cylindrical tanks

Abstract: To validate an existing finite volume computational method, featuring a novel scheme to capture the temporal evolution of the free surface, fluid motions in partially filled tanks were simulated. The purpose was to compare computational and experimental results for test cases where measurements were available. Investigations comprised sloshing in a rectangular tank with a baffle at 60% filling level and in a cylindrical tank at 50% filling level. The numerical study started with examining effects of systematic grid refinement and concluded with examining effects of three-dimensionality and effects of variation of excitation period and amplitude. Predicted time traces of pressures and forces compared favorably with measurements.

A Numerical Study on Sloshing Flows Coupled with Ship Motion—The Anti-Rolling Tank Problem

Abstract: A computational study on the sloshing problem coupled with ship motion in waves is introduced. The ship motion excites the sloshing flow in the ship's liquid cargo, and the slosh-induced forces and moments affect the ship motion in return. This study applies a numerical method to solve the coupling problem of the ship motion and sloshing flow. In particular, it concentrates on the anti-rolling tank, which has the most significant coupling effects of two problems. The three-dimensional sloshing flow has been simulated using the finite-difference method, while the ship motion has been obtained using a time-domain panel method. At each time step, the instantaneous displacement, velocity and acceleration of ship motion have been applied to the excitation of liquid motion, and the corresponding slosh-induced forces and moments have been added to the wave-induced excitation. The computational model is a modified S175 hull, and the computational results have been compared with the experimental data of a supply vessel. Although the two hull forms are not identical, the numerical result for the modified S175 hull shows the same trend of the roll RAOs with experimental data when the anti-rolling tanks are considered. Therefore, the numerical method introduced in this study is expected to be very useful in observing the coupling effects of sloshing and ship motion problems.

Steady Hydrodynamic Analysis of Planing Surfaces

Abstract: The objective of this work is to continue to develop rationally based tools for hydrodynamic analysis of three-dimensional planing surfaces. The small craft naval architect is in need of a computationally efficient method to analyze planing surfaces allowing for sufficient sensitivity to hull geometric details. Planing surface analysis first received attention during the early part of the 20th century. This development was associated with flying boat applications. Von Karman (1929) and Wagner (1932) are considered the first researchers to develop rational models for planing surface analysis. The work contained herein is derived from the extension of Tulin (1957) originally by Vorus (1996) and later by Savander (1997). The model developed allows for general variation in deadrise in both the transverse and longitudinal directions. Hence, buttock line curvature is included in the formulation. The chine beam of the hull can vary longitudinally. Fundamental theoretical and numerical enhancements have been made to Vorus (1996) and Savander (1997). The current model is compared with constant and variable velocity 2-D impact experiments, prismatic planing model tests, Savitsky (1964) prismatic planing model, and deepwater Series 62 model test results (Clement & Blount 1963). The comparisons in many cases are excellent, especially at high Froude number. The comparisons illustrate the utility of the method. Problematic areas are presented and topics for further research are suggested.

An Inverse Hull Design Problem in Optimizing the Desired Wake of a Ship

Abstract: An inverse hull design problem for optimizing the shape of the after hull based on the desired wake distribution is solved using the Levenberg-Marquardt Method (LMM) and the commercial code SHIPFLOW. The desired wake distribution on a propeller plane can be obtained by modifying the existing wake distribution of the parent ship. The surface geometry of the ship is generated using the B-spline surface method, which enables the shape of the hull to be completely specified with only a small number of parameters (i.e., the control points). The advantage of calling SHIPFLOW as a subroutine in the present inverse calculation lies in that many difficult but practical hydrodynamic problems regarding ship design can be solved under this construction. The validity of the present 3-D inverse hull design problem for the after hull of a ship is justified based on the numerical experiments. Results show that optimal hull form can always be obtained based on the required wake distributions.

Implementation of finite-element codes for the simulation of ship-ship collisions

Abstract: The problem of ship collisions may well be considered one of the most complicated impact problems that contemporary engineering research is encountering. The structures involved are very large and include various structural elements. Thus, there is a large number of different modes of deformation that may occur, a fact that introduces a lot of parameters in the investigation of such problems. Finite-element analyses have been used over the past decade to solve several impact problems effectively. Though considerable success has been recorded in many research areas, such as the automotive industry, ship collision simulations still confront difficulties in providing reliable results. This paper attempts to determine the parameters that largely influence ship collisions and to provide tools and guidelines for the effective implementation of finite-element codes.

Bow Wave Dynamics

Abstract: Experimental studies of air entrainment by breaking waves are essential for advancing the understanding of these flows and creating valid models. The present study used experimental simulations of a ship bow wave to examine its dynamics and air entrainment processes. The simulated waves were created by a deflecting plate mounted at an angle in a supercritical free-surface flow in a flume. Measurements of the bow wave geometry at two scales and also for a bow wave created by a wedge in a towing tank are presented. Contact line and bow wave profile measurements from the different experiments are compared and demonstrate the similarity of the flume simulations to the towing tank experiments. The bow wave profile data from the towing tank experiments were used to investigate the scaling of the wave with the flow and the dependence on geometric parameters. In addition, surface disturbances observed on the plunging wave are documented herein because of the role they play in air entrainment. The air entrainment itself is explored in Waniewski et al (2001).

Analytical Expressions of Capsize Boundary for a Ship with Roll Bias in Beam Waves

Abstract: We have considered Melnikov's method for assessing ship safety against capsize in beam waves when there is some steady heel due, for example, to beam wind loading. For a generic restoring lever parameterized with respect to the degree of lever asymmetry (bias) we have derived closed-form relationships characterizing capsizability and linking the critical wave-slope, the amount of bias, and the damping. Qualitative differences in the safe basin characteristics are identified, which depend on whether the bias is small or large. A critical look into the accuracy of the Melnikov formulas is also presented for nearly symmetric and for strongly asymmetric rolling, based on comparisons with numerical predictions.

Nondimensionalized Relationship Between Heating Conditions and Residual Deformations in the Line Heating Process

Abstract: The nature of a line heating process is very complex since a variety of factors affects the amount of residual deformation. A linear relationship between input and output parameters, which has been derived from simple experiments, is successively used. This relationship, however, is very limited since it does not include important parameters and the line heating process is not linear. A rigorous approach is presented here in an attempt to obtain new relationships between input parameters and final deformations during the line heating process. The residual deformations are investigated by using a thermal elastic-plastic analysis based on finite-element analysis (FEA). Experiments are carried out in order to verify the validity of the FEA results. The nondimensional input parameters are then determined by the dimensional analysis. The relationships between the input parameters and the residual deformations are developed by using multi-variate analysis (MVA) and multiple-regression methods. The final form of the relationships is nonlinear and includes relevant information.

Green Function Solutions for the Transient Motion of Water Sections

Abstract: A time-domain boundary element method based on a Green function solution is derived for two-dimensional motions in the presence of a free liquid surface. Particular attention is given to the numerical evaluation of the required Green functions with regard to accuracy and speed of solution by choice of a computational algorithm appropriate to the domain of particular computations. The method is validated with reference to analytic solutions for submerged and floating cylinders in steady, transient and periodic motion. The intended application of the method is in the computation of wave response of slender ships at high Froude number by a fixed frame strip theory where the Green function obviates the necessity to panel the free surface with elements.

On three-dimensional solutions of drift forces and moments between two ships in waves

Abstract: A far-field approach solving the lateral drift forces and moments between two ships in regular waves is adopted. The velocity potentials for diffraction and radiation based on a 3-D-source distribution technique are obtained. Using the Telste & Noblesse algorithm with a series expansion technique for the principal value integral solves the numerical solutions for corresponding Green functions and their derivatives. One pair of ship models is used for numerical calculations and a 2-D method based on the near-field approach is also included for comparisons. Generally the results obtained by the present technique indicate that the interaction effects between two ships have a profound influence on the drift forces and moments, and the direction of incident waves plays an important role. The results also show that the values predicted by the 2-D method are always very much overestimated because of the trapping energy between two ships. Therefore, the 3-D method prediction model developed here is regarded as more physically reasonable than the 2-D one.

Enhanced Heat Transfer with Swirl Duct Under Rolling and Pitching Environment

Abstract: A detailed experimental investigation of heat transfer in a square duct fitted with twisted tape under a rolling and pitching environment is described, with particular reference to the heat transfer augmentation of shipping machinery. This study focuses on the development of an experimental procedure and methods for data processing, the parametric analysis and a selection of measurements that illustrate the manner by which the swinging forces and buoyancy interactively affect the local heat transfer. The swinging Coriolis force and buoyancy influence to a considerable extent the forced convection heat transfer in the swirl duct. Although enhancing the buoyancy level increases the heat transfer as the swirl duct rolls or pitches, the swinging Nusselt number is initially reduced relative to the stationary condition at the weak swinging oscillation, but tends to recover as the swinging force increases. The synergistic effects of harmonic and nonharmonic rolling and pitching oscillations reduce the heat transfer. Hot spots could develop in a swirl duct due to the slow rolling and/or pitching motions if the effect of the swinging oscillations on the heat transfer is not adequately considered. An empirical correlation has been developed for both single-axis and compound swinging conditions which permits the interactive effect of swinging Coriolis and buoyancy forces on forced convection to be quantified and which provides an evaluation of the local heat transfer in a swinging swirl duct.

Joint Distributions of Successive Wave Crest Heights and Successive Wave Trough Depths for Second-Order Nonlinear Waves

Abstract: Joint distributions of successive wave crest heights and successive wave trough depths for nonlinear waves are presented. Two different approaches are used in order to derive the probability distributions. The first method includes the effect of second-order Stokes-type nonlinearity on successive wave statistics in finite water depth, and the second method is a parametric model only for crest heights based on second-order simulations. The theoretical distributions are compared with observed wave data obtained from field measurements in the central North Sea.

Fatigue Damage of Structural Joints Accounting for Nonlinear Corrosion

Abstract: Analysis of the influence of corrosion on the stress concentration factors of typical ship structural details is presented. While traditionally constant stress concentration factors are adopted, it is proposed here to use time-varying stress concentration factors, which result from the progress of corrosion in the structure. Three-dimensional finite-element models are adopted to obtain the detailed stress distribution at different times. Linear and nonlinear models of the effects of corrosion wastage on the plate thickness reduction are considered and stress concentration factors and fatigue damage are calculated as a function of time. It is concluded that the stress concentration factors have a nonlinear dependency with the time and this leads to a significant difference of the fatigue damage of structural components subjected to corrosion as compared with the traditional predictions.

An Algorithm for Optimized Design of Maneuvering Experiments

Abstract: Properties of an adjusted version of Mitchell's algorithm for synthesis of D-optimized experimental designs for response estimation were studied numerically for a third-order polynomial linear regression model widely used in ship maneuvering. Numerical experiments for 2, 3, and 4-factor cases were conducted for different numbers of levels in the classic full-factorial plan used as a set of candidate points. Selection of the runs to be included in the design under construction was made after an exhaustive search. Initial designs were obtained with the pseudo-random or quasi-random generation. A method of assembling an optimized design with the given degree of redundancy is proposed. An important conclusion is that relatively coarse grids can be used for discretizing the factor space that can facilitate substantially the synthesis of the designs without any significant loss of quality.

Pitch-heave dynamics of a segmented skirt cushion

Abstract: The formul ation and experimental val idation of a mathematical model of the nonlinear pitch-heave dynamics of a segmented skirt cushion system is described. This cushion is uncompartmented so that it re1 ies on surface contact to attain static stability in pitch and roll. The formulation i ncludes a dynamic model of lift fan b ehavior, the effect of seg~nent flexi bi 1 i ty on both effective cushion capacitance and hovergap, and hysteretic surface c ontact forces. Predictions of l inear stability and nonlinear response to initial condition pitch input d isturbance predictions are made for two skirt materials, the first generating c onsiderable hysteresis in pitch stiffness, and the second having much greater e xtensibi 1 ity but negl igi hle hysteresis. These predictions are compared with experimental results obtained from a 900 kg test model. The analysis predicts correctly the general stabi 1 ity characteristics including a nonlinear pitch-heave instability that appears to occur because of coupling between pitch and heave motion and because there is insufficient dampi ng associated with t he modulation of the cushion volume by pitch motion. However there remains an unaccounted source of cushion damping. The results are also shown to be sensitive to flexure of the skin panels that form the model air supply plenum. Nomenclature:

Analysis of Global Load Effects in Catamarans

Abstract: Relevant stresses and deformations resulting from longitudinal bending and torsion are calculated for a 60 m catamaran by a global finite-element model. The results are compared with results from prismatic beam theory modified to account for the effect of wide flanges and significant window openings typical for a catamaran hull. It is found that the global stresses are predicted with a reasonable accuracy for the initial design stage. In the present case, the effect of warping on shear stress due to torsional loading is moderate and can be neglected. However, this effect is strongly dependent on the vessel design and can be important for a vessel with narrower hulls and a superstructure that is lower relative to the total height.

Resistance reduction by increased beam for displacement-type ships

Abstract: The resistance of a conventional hull with a parallel middlebody was compared with its modified version having no parallel middlebody. The modified hull was obtained by parabolizing the waterlines, that is, by extending the waterlines continuously about the middlebody, which results in an increased beam and elimination of shoulders. Numerical and experimental results suggest that there could be a significant reduction in the ship's EHP by parabolizing the waterlines at modest Froude numbers. Also, a theoretical justification based on Michell's integral is presented.

Rankine and Fourier-Kochin representation of near-field ship waves

Abstract: New fundamental analytical representations of the near-field potential flow that corresponds to a given flow at a surface bounding a potential-flow region are given for three classes of free-surface flows in deep water: diffraction-radiation of regular water waves by an offshore structure, steady ship waves, and time-harmonic ship waves (diffraction-radiation with forward speed). These near-field flow representations, called Rankine and Fourier-Kochin representations, define the flow in terms of distributions of Rankine singularities and Fourier-Kochin distributions of elementary waves over the boundary surface and its intersection with the mean free surface. The Rankine and Fourier-Kochin near-field flow representations involve only simple ordinary functions. These flow representations extend the previously given Fourier-Kochin representations of waves.

Free Vibration Analysis of Orthotropic Circular Cylindrical Shell Under External Hydrostatic Pressure

Abstract: An analysis is presented for the free vibration of an orthotropic circular cylindrical shell subjected to hydrostatic pressure. Based on Flugge shell theory, the equations of free vibrations of an orthotropic circular cylindrical shell under hydrostatic pressure are obtained. For shear diaphragms at both ends, the resulting characteristic equations about pressure and frequency are given. These two parameters are calculated exactly. The effect of the shell's parameters (L/R, h/R) and material properties on the free vibration characteristics are studied in detail. Differences between Love-Timoshenko, Donnell equations and that of the Flugge theory are examined as well.

Stochastic Still Water Response Model

Abstract: In this study a stochastic field model for the still water loading is formulated where the statistics (mean value, standard deviation, and correlation) of the sectional forces are obtained by integration of the load field over the relevant part of the ship structure. The objective of the model is to establish the stochastic load field conditional on a given draft and trim of the vessel. Emphasis is given to container vessels. The formulation of the model for obtaining the stochastic cargo container load field is based on a queuing and loading policy that assumes containers are handled by a first-come-first-serve policy. The load field is assumed to be Gaussian. The ballast system is imposed to counteract the angle of heel and to regulate both the draft and the trim caused by the possible uneven distribution of the cargo load and the bunker load over the system. Finally the calculated second moment statistics of the sectional forces in a container vessel in a full load condition are reported. The obtained statistics for the maximum still water bending moment is compared to statistics from available regression formulas. It is found that the suggested model predicts a coefficient of variation of the maximum still water bending moment that is a factor of three to six times lower than that obtained by use of the regression formula. It turns out that an important parameter of the stochastic cargo field model is the mean number of containers delivered by each customer.

Hydrodynamic Characteristics of Wings in Circular Motion

Abstract: Hydrodynamic characteristics of wings in circular motion are determined at present very approximately, as a rule, according to the solution of the problem on streamlining the wing by a small curvature nonviscid potential flow. However, in some practical cases the radius of the wing trajectory is of the same order of magnitude as the wing chord. That is why there is a necessity for a more correct determination of wing hydrodynamic characteristics, accounting for the relatively large curvature of the wing trajectory and the fluid viscosity. This paper presents a description and results of our theoretical and experimental investigation of the hydrodynamic characteristics of wings in a wide range of relative radiuses of their motion.

A stochastic stillwater response model

Abstract: In this study a stochastic field model for the stillwater loading is formulated where the statistics (mean value, standard deviation, and correlation) of the sectional forces are obtained by integration of the load field over the relevant part of the ship structure. The objective of the model is to establish the stochastic load field conditional on a given draft and trim of the vessel. The model contributes to a realistic modeling of the stochastic load processes to be used in a reliability evaluation of the ship hull. Emphasis is given to container vessels. The formulation of the model for obtaining the stochastic cargo container load field is based on a queuing and loading policy that assumes containers are handled by a first-come-first-serve policy. The load field is assumed to be Gaussian. The ballast system is imposed to counteract the angle of heel and to regulate both the draft and the trim caused by the possible uneven distribution of the cargo load and the bunker load over the system. Stability is not explicitly accounted for. Finally, the calculated second moment statistics of the sectional forces in a container vessel in a full load condition are reported. The obtained statistics for the maximum stillwater bending moment is compared to statistics from available regression formulas. It is found that the suggested model predicts a coefficient of variation of the maximum stillwater bending moment that is a factor of two to three times lower than that obtained by use of the regression formula. It turns out that an important parameter of the stochastic cargo field model is the mean number of containers delivered by each customer.