bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Waves on the Boundary Layer of a Surface-Piercing Flat Plate: Experiment and Theory
Authors: Stern F. , Hwang W. S. , Jaw S. Y.
Published: January 2021

Results are presented from a towing-tank experiment conducted in order to document the effects of waves on the boundary layer of a surface-piercing body. A unique, simple model geometry is utilized which makes it possible to identify and isolate the most important features of the wave-induced effects. Measurements were made of wave profiles as well as detailed boundary-layer velocity profiles for three wave-steepness conditions: zero, medium, and large. The effects of the waves for both the medium-and large-steepness conditions are shown to be significant. In particular, the variations of the external-flow piezometric-pressure gradients cause acceleration and deceleration phases of the streamwise velocity component and alternating direction of the crossflow, which result in large oscillations of the displacement thickness and wall-shear stress as compared to the zero-steepness condition. The magnitude of these trends increases with increasing wave steepness. Wave-induced separation, which is present under certain conditions in the experiments, and other relevant phenomena are discussed. The measurements are compared and close agreement is demonstrated with results from first-order boundary-layer calculations with a symmetry-condition approximation for the free-surface boundary conditions.
Development of a New-Type Hydrofoil Catamaran
Authors: Miyata Hideaki
Published: January 2021

A new-type hydrofoil catamaran has been designed and its resistance and seakeeping properties examined by experiments. This new catamaran is made of two sharp hulls like swords and two rectangular hydrofoils of high aspect ratio that connect them. Almost 90 percent of the weight is supported by the hydrofoils, and the total resistance coefficient is much smaller than that of conventional high-speed craft or similar designs at the design speed of 40 knots for a 200-ton boat. The transfer functions of motions are much smaller than those of planing boats and another hydrofoil catamaran of different configuration.
Keel Design for Low Viscous Drag
Authors: Obara Clifford J. , Dam C. P. van
Published: January 2021

Foil and planform parameters which govern the level of viscous drag produced by the keel of a sailing yacht are discussed. It is shown that the application of laminar boundary-layer flow offers great potential for increased boat speed resulting from the reduction in viscous drag. Three foil shapes have been designed and it is shown that their hydrodynamic characteristics are very much dependent on location and mode of boundary-layer transition. The planform parameter which strongly affects the capabilities of the keel to achieve laminar flow is leading-edge sweep angle. The two significant phenomena related to keel sweep angle which can cause premature transition of the laminar boundary layer are crossflow instability and turbulent contamination of the leading-edge attachment line. These flow phenomena and methods to control them are discussed in detail. The remaining factors that affect the maintainability of laminar flow include surface roughness, surface waviness, and freestream turbulence. Recommended limits for these factors are given to insure achievability of laminar flow on the keel. In addition, the application of a simple trailing-edge flap to improve the hydrodynamic characteristics of a foil at moderate-to-high leeway angles is studied.
Nonlinear Ship Waves at Low Froude Number
Authors: Hermans A. J. , Brandsma F. J.
Published: January 2021

The wave pattern of a thick ship-like object with finite bow and stern angles 0 <β<π/2 is studied. The completely blunt form p = π/2 is excluded. It turns out that the wave pattern is strongly influenced by the nonlinear terms at the free surface. The wave pattern is determined by means of the ray method. The rays are generated mainly at the bow and the stern. A crucial step is the determination of the so-called excitation coefficients. They are constructed by means of an asymptotic evaluation of a distribution of "local"sources at the free surface. It is shown that for small angles β<< 1 the excitation coefficients are the same as the ones obtained by means of an asymptotic expansion for small values of the Froude number of the results of Michell's thin-ship theory. For increasing values of β, the excitation coefficients change asymptotically. The theory herein shows a continuous dependence, nevertheless. Similar changes are observed in the far-field wave pattern.
Nonlinear Free-Surface Flow Around an Impulsively Moving Cylinder
Authors: Wang Keh-Han , Chwang Allen T.
Published: January 2021

Nonlinear free-surface flow around an impulsively accelerating, vertical surface-piercing cylinder is investigated analytically. The exact solutions for the velocity potential and free-surface elevation are derived up to the third order by the small-time-expansion method. The hydrodynamic pressure acting on the cylinder wall is also obtained. A constant horizontal acceleration is considered to illustrate our theoretical results. It is found that, during the initial stage of this impulsive motion, no travelling free-surface waves are present. A surge of water appears on the upwind face and a depression forms on the down-wind face. The second-order free-surface elevation is singular along the contact line between the fluid surface and the cylinder surface. The nonlinear pressure distribution on the cylinder surface has been determined.
Edge Crack in an Elastic Strip on a Liquid Foundation
Authors: Xirouchakis Paul C. , Makrakis George N.
Published: January 2021

The behavior of a long elastic strip with an edge crack resting on a liquid foundation is investigated. The faces of the crack are opened by an applied pressure loading. The deformation of the strip is considered within the framework of the linear theory of elasticity assuming plane-stress conditions. Fourier transform techniques are employed to obtain integral expressions for the stresses and displacements. The boundary-value problem is reduced to the solution of a Fredholm integral equation of the second kind. For the particular case of linear pressure loading, the stress-intensity factor is calculated and its dependence is shown on the depth of the crack relative to the thickness of the strip. Application of the present results to the problem of flexure of floating ice strips is discussed.
Roll of a Ship in Astern Seas—Response to GM Fluctuations
Authors: Dunwoody A. Bruce
Published: January 2021

An effect of astern seas on a ship is to produce fluctuations in the metacentric height (GM) of the ship. GM fluctuations are shown to produce an effect analogous to a reduction in the roll damping. Roll damping reductions are predicted for two extreme types of random GM fluctuations: wide-band and narrow-band random processes. Numerical simulations of the roll motion of a ship with realistic random GM fluctuations are used to compare with the predictions of damping reduction. Good agreement is found. Implications of a reduced roll damping are discussed.
System Reliability and Redundancy of Marine Structures: A Review of the State of the Art
Authors: Nikolaidis Efstratios , Kapania Rakesh K.
Published: January 2021

This paper is a survey of state-of-the-art knowledge in system reliability and redundancy. Measures of structural redundancy and reserve strength are presented and their advantages and limitations reviewed. Methods for calculating system reliability are critically reviewed. Emphasis is given to the underlying assumptions and the resulting limitations. Reliability of laminated composite structures is also considered. The potential of system reliability in structural design is emphasized. Recent studies on this subject are reviewed. The most critical problems in each of the above topics are identified and strategies for solving them are proposed.
Dynamics of Compliant Risers
Authors: Patrikalakis N. M. , Kriezis G. A. , Gursoy H. N. , Maekawa T.
Published: January 2021

The objective of this paper is first to formulate the three-dimensional dynamic equations of a compliant riser, idealized as a rotationally nonuniform rod, around a nonlinear static configuration with linearized restoring force and inertial components in the presence of general current and monochromatic wave excitation. Next, to harmonically linearize nonlinear forces such as quadratic drag for the general three-dimensional problem by minimizing the mean square error between the linear approximation and the nonlinear force. Finally, to present an efficient numerical solution method appropriate for nonlinear boundary-value problems with sharp boundary layers such as the problem at hand. Numerical examples and comparisons with time-domain solutions for a catenary riser with a three-dimensional configuration and a steep-wave riser are included. Comparisons of our theoretical predictions with experimental results obtained from a small-scale riser model are also summarized to evaluate our theoretical ability to predict the response of compliant riser systems.
Influence of Draft Variation on Heave Added-Mass and Hydrodynamic Damping Coefficients of Floating Bodies
Authors: Vantorre Marc
Published: January 2021

A general nonlinear theory for solving the radiation problem for floating or immersed bodies in a periodic heave motion, composed of a number of harmonic components, is applied for calculating the influence of draft variations on the linear hydrodynamic coefficients for heave. It is shown that a calculation method for added-mass and damping coefficients of axisymmetric bodies based on a boundary integral equation method can easily be modified to obtain numerical values of the first and second derivatives with respect to draft of the hydrodynamic coefficients as well. The method is illustrated by experimental and numerical data for a floating cone.
Added Mass in Ship Collisions with Structures in Shallow Water
Authors: Ball David J. , Roisin Jean-Philippe
Published: January 2021

The added-mass coefficients for a tanker ship in sway motion are examined experimentally for plastic impact in very shallow water. Marked differences between the elastic and plastic cases are found. Al-though energy and momentum coefficients have similar values in elastic impacts, they differ increasingly in plastic impacts as the duration of impact increases. Unexpectedly, the energy added-mass coefficient does not appear to be sensitive to underkeel clearance, which has a significant effect on the coefficient in elastic condition. There is no difference between current-driven and wind-driven tests for the plastic case. However, in the elastic case there is an increase in the energy added-mass coefficient as the ratio of current velocity to ship speed increases, suggesting added care is needed in designing elastic fendering systems against impact from a drifting ship.
Stochastic Modeling of Maximum Still-Water Load Effects in Ship Structures
Authors: Soares C. Guedes
Published: January 2021

A stochastic model of an alternating renewal pulse process is proposed to describe the time dependence of still-water load effects in ship structures, extending previous studies that dealt only with random variables. The model proposed is applied to different ship types and the probability of occurrence of annual maxima is also determined in those cases. The results of a brief statistical analysis of load duration are also included as a basis to describe the time dependence of the process. Applications of this model to the development of code requirements are also included.
On the Post-Buckling Behavior of Reinforced Composite Shells
Authors: Birman Victor
Published: January 2021

The problem of post-buckling behavior of composite cylindrical shells reinforced in the axial and circumferential directions and subject to axial loading is considered. The equations of equilibrium of an imperfect shell are formulated in terms of displacements. Then the sufficient conditions of imperfection in sensitivity for both static and dynamic buckling problems are formulated. This general analysis is applied to a particular case of axisymmetric buckling of ring-stiffened shells which appear to be practically imperfection-insensitive.
Experimental Study of Diesel Engine Cycle-to-Cycle Variation—Part III
Authors: Jamjang Somsak , Latorre R. , Charnews D.
Published: January 2021

Marine diesel engine excitation can occur from the cycle-to-cycle variation in the engine output. The cycle-to-cycle variation in maximum cylinder pressure (Pmax) is often used to represent this variation. Earlier studies examined the correlation between the cycle-to-cycle variation in the maximum cylinder pressure (Ρmax), pressure impulse (PI) and ignition delay (ID) for the case of constant injection setting. This paper summarizes the results of systematic experiments to determine the cycle-to-cycle variations when the injection timing is changed. In these tests the cycle-to-cycle variations in the maximum cylinder pressure (Pmax), ignition delay (ID), and corresponding pressure impulse (PI) are examined for a single-cylinder test diesel engine at various speeds and compression ratios. Comparisons show that the cycle-to-cycle variations of the diesel engine power output are strongly affected by the injection timing. The best parameter for describing the cycle-to-cycle output variation is shown to depend on the specific engine test condition.
Mean-Flow Measurements in the Boundary Layer and Wake of a Series 60 CB = 0.6 Model Ship With and Without Propeller
Authors: Toda Y. , Stern F. , Tanaka I. , Patel V. C.
Published: January 2021

Results are presented from a towing-tank experiment of propeller-hull interaction conducted in order to provide detailed documentation of the complete flow field appropriate both for explicating the flow physics and validating computational methods. Mean-velocity and pressure field measurements were made for the with-and without-propeller conditions for the Series 60 CB = 0.6 hull form at numerous stations both upstream and downstream of the propeller and in the near-wake region. Surface-pressure distributions and wave profiles were measured for both conditions. Resistance and self-propulsion tests were also conducted. The experimental equipment and procedures are described, and the results are discussed to point out the essential differences between the flows with and without propeller. The results are analyzed to assess the nature of the interaction between the propeller and the hull boundary layer and wake. To this end, use is made of a propeller-performance program with both nominal and effective inflows. It is shown that most features of the interaction can be explained as a direct consequence of the propeller loading resulting from its operation with a three-dimensional nonuniform inflow.
Screws Working in Behind and Prediction of the Performance of Full Ships
Authors: Grigson Christopher
Published: January 2021

Constant speed propulsion tests of full models are investigated. The propulsion factors are found to vary with propeller speed, n. When the tests extend to idling conditions, the nominal wake fraction and the complete propeller-hull behind characteristics ϕ(u/nd) are determined. Fifteen designs of hull and screw are investigated. In some, the coupling between the velocity field of the hull and that of the screw is found to be strong. The behind characteristics depend both on the design of the screw and on the design of the hull. The same design of screw may efficiently power hulls of quite different form. A second kind of behind characteristic, ψ[(1 -ω)u/nd], is introduced. It is obtained from ϕ and it can be compared directly with the open-water characteristics. It is shown experimentally that in a full ship the open-water characteristics are not generally an accurate substitute for the behind ones. Therefore ϕ or ψ ought to be used when predicting ship propeller speed Ν and power. A condition for running the propulsion test is derived in which, after correction for the scale effect of blade friction on torque, the full-scale behind torque coefficient may be found from the model one. Furthermore, in this test condition Ν may be rigorously scaled from n, measured on the model. Thus full-scale performance is determined. Limited tests of the method appear accurate.
A Numerical Solution to the Three-Dimensional Low-Aspect-Ratio Flat Ship Problem
Authors: Cole S. L.
Published: January 2021

This paper defines a numerical procedure for solving the low-aspect-ratio flat ship problem and providesa sample solution using MAPLE, a symbolic manipulation software language, for the class of hulls with slope fx = -(a + bx + cy2 + dxy2 + ex2 + gy4)and parabolic planform y2< xfor 0 < x< 1. The lift and drag are computed as a function of the ship speed and arbitrary hull parameters a through g. To demonstrate the usefulness of these formulas, two simple optimizations are performed for three fixedhull speeds. First, the hull parameters are chosen so as to minimize the drag for the total lift fixed, and second, the hull parameters are chosen so as to minimize the drag for the volume of the hull fixed.
Steady Performance of a Flexible Hydrofoil Near a Free Surface
Authors: Rashad Salwa M. , Green Theodore
Published: January 2021

A linearized cavity-flow theory is used to develop a mathematical model to study the steady characteristics of a flexible hydrofoil near a free surface. The Galerkin method is employed to account for the mutual interaction between the fluid and structure forces. Cheng and Rott's method [1]2 is used to derive general expressions for the deformation characteristics in steady flow of an arbitrarily shaped hydrofoil, with a clamped trailing edge and free leading edge. From the analysis it is possible to determine the lift and drag coefficients, cavity length, and the foil steady deformation for any given specific foil shape, cavitation number, angle of attack, flow depth/chord ratio and rigidity. Sample numerical results are given, and the effects of flexibility and the proximity of the free surface are discussed. Chordwise flexibility tends to increase drag and decrease lift coefficients. This effect is more serious near the free surface. A slight increase of the thickness near the leading edge diminishes the flexibility effects.
Interaction Between Two Bodies Translating in an Inviscid Fluid
Authors: Landweber L. , Chwang A. T. , Guo Z.
Published: January 2021

The equations of motion of two bodies in translational motion in an inviscid fluid at rest at infinity are expressed in Lagrangian form. For the case of one body stationary and the other approaching it in a uniform stream, an exact, closed-form solution in terms of added masses is obtained, yielding simple expressions for the velocity of the moving body as a function of its relative position and for the interaction forces. This solution is applied to the case of a rectangular cylinder approaching a cylindrical one, for which the added-mass coefficients had been previously obtained in a companion paper by an integral-equation procedure. In order to compare results with those in the literature, and to evaluate the accuracy of the present procedures, results were calculated for a pair of circular cylinders by these methods as well as by successive images. Very good agreement was found. Comparison with published results showed good agreement with the added mass but very poor agreement on the forces, including disagreement as to whether the forces were repulsive or attractive. The discrepancy is believed to be due to the omission of terms in the Bernoulli equation which was used to obtain the pressure distribution and then the force on a body. The Lagrangian formulation is believed to be preferable to the pressure-integral approach because it yields the hydrodynamic force directly in terms of the added masses and their derivatives, thus requiring the calculation of many fewer coefficients.
Leading-Edge Corrections to the Linear Theory of Partially Cavitating Hydrofoils
Authors: Kinnas Spyros A.
Published: January 2021

In this work, first, the partially cavitating hydrofoil problem is formulated in linear theory in terms of vorticity and source distributions on the projection of the hydrofoil to the free-stream direction. The resulting system of integral equations is inverted and the solution is expressed in terms of integrals of the horizontal perturbation velocity in fully wetted flow, multiplied by weighting functions that are independent of the shape of the hydrofoil. Second, the linearized dynamic boundary condition on the cavity is modified so that the total velocity on the cavity as predicted by applying Lighthill's leading-edge corrections is a constant. This results in a varying horizontal perturbation velocity on the cavity rather than a constant as required by conventional linear theory. The modified system of integral equations is inverted and the solution is expressed in terms of integrals of known quantities. The present linearized theory with the leading-edge corrections included, predicts a finite cavitation inception number as well as the correct effect of foil thickness on cavity size.
Nonlinear Prediction of Vertical Motions of a Fishing Vessel in Head Sea
Authors: Chiu Forng-chen , Fujino Masataka
Published: January 2021

Several years ago the authors developed a practical method for calculating vertical motions and wave loads of a high-speed craft which travels in regular head sea, and verified its validity by comparing the computed motions and wave loads with the results of model tests. In order to clarify further its validity, the method is applied herein to compute the vertical motions of a fishing vessel, and the computed motions are compared with the results of experiments conducted by Bales and others at the David W. Taylor Naval Ship Research and Development Center (DTNSRDC). Also, the vertical motions and shipto-wave relative motions predicted by the method are compared with the numerical results of conventional linear strip theory computations performed at DTNSRDC. As a result, it is found that the present method, which in principle is based on the conventional Ordinary Strip Method synthesis but modified to be able to evaluate nonlinear hydrodynamic impact forces as well as dynamic lift in waves, can be applied to estimate vertical motions and ship-to-wave relative motions of fishing vessels traveling in head sea with enough accuracy for practical use.
Three-Dimensional Nonlinear Dynamics of Nonintegral Riser Bundle
Authors: Vlahopoulos Nickolas , Bernitsas Michael M.
Published: January 2021

The dynamic behavior of a nonintegral riser bundle is studied parametrically. The dynamics of each component-riser is analyzed by a three-dimensional, nonlinear, large deflection, small strain model with coupled bending and torsion. Component-risers are slender, thin-walled, extensible or inextensible tubular beam-columns, subject to response and deformation dependent hydrodynamic loads. The con-nector equations of equilibrium are used to derive the connector forces and moments. Substructuring can thus be achieved even though in three dimensions connectors do not impose linearly dependent deflections at substructure interfaces. The developed time incremental and iterative finite-element computer code is used to analyze the effects of water depth, distribution of connectors, distance between component risers and number of finite elements in the numerical model. The problem of total CPU (central processor unit) time and the advantages of substructuring are discussed by running cases of up to 1094 degrees of freedom.
The Transient Capsize Diagram—A New Method of Quantifying Stability in Waves
Authors: Rainey R. C. T. , Thompson J. M. T.
Published: January 2021

It is argued that a plot of wave steepness against wave period, showing the combinations which cause capsize, is a well-defined measure of the stability of a ship or ocean vehicle in waves, provided the conditions are transient, that is, the vessel is initially in relatively calm water, and is suddenly hit by a train of regular waves. This conclusion is a consequence of recent developments in dynamic systems theory. Such Transient Capsize Diagrams can obviously be obtained by model testing; it is also argued that they could be obtained by computer simulation on contemporary desktop computers, taking advantage of recent developments in nonlinear strip theory.
A Numerical Study of Very High Speed Flat Ship Theory
Authors: McComb Todd
Published: January 2021

This paper uses an asymptotic procedure to generate analytic solutions to the low-aspect-ratio problem of flat ship theory, in the limit for a very fast ship. The first two terms of the solution are worked out for hulls of parabolic planform and with 20 arbitrary constants in the expression for the draft. Optimizations are then performed for lift and drag on a smaller class of hulls. Analytic solutions were found by using symbolic computation, and the results are discussed. Optimal hulls are presented for various values of the ship's speed, optimized with both total lift and static lift held fixed. The optimization solution in the limit as the ship's speed goes to infinity gives independence of some constants in the expression for the hull.
Stern Flows at Full-Scale Reynolds Numbers
Authors: Ju S. , Patel V. C.
Published: January 2021

A numerical method for the solution of the Reynolds-averaged Navier-Stokes equations for three-dimensional flow of an incompressible fluid, used previously for calculations at model-scale Reynolds numbers, has been applied to the flow over the stern of an axisymmetric body and a representative ship over a range of Reynolds numbers up to five billion. High Reynolds number experiments on axi-symmetric bodies strongly support the use of wall functions in the turbulence model. Comparisons with traditional ship wake scaling laws suggest that such laws do not properly describe the scale effects on the flow in the propeller plane.
Modified Strip Model for Analyzing the Line Heating Method-Part 1: Elastic Plates
Authors: Shin Jong Gye , Moshaiov Amiram
Published: January 2021

The line heating method can be most useful in the automation of plate forming and for removal of undesired residual distortions during shipbuilding. The three-dimensional transient temperature and stress fields complicate the problem of simulating the line heating process. In past research efforts, a strip model taken perpendicular to the heated line was used. However, recent experimental and theoretical works have demonstrated the need for a different model. Recognizing that the existing strip model cannot be used effectively in the analysis of the stress-strain field during the line heating process, a modified strip model is suggested. In Part 1 of this paper, the concept of a modified strip is introduced by developing a theory for an elastic modified strip. The results of selected examples show that the modified strip model gives excellent results for elastic plates in comparison with a regular strip model. In Part 2 (to be published in a later issue of JSR), the theory is extended to the thermo-elastic-plastic modified strip. The concept of a modified elastic strip may have other useful applications and therefore is highlighted separately.
Discussion on Venkat & Spaulding: “Numerical Simulation of Nonlinear Free-Surface Flows Generated by a Heaving Body of Arbitrary Cross Section”
Authors: Papanikolaou Apostolos
Published: January 2021

A method has been presented recently by Venkat and Spaulding to solve the nonlinear boundary-value problem of oscillating two-dimensional cylinders of arbitrary cross section on the free surface of a fluid. The method relies on a second-order finite-difference technique with a modified Euler method for the time domain and a successive over-relaxation procedure for the spatial domain. The authors compare their numerical results with those of other authors (theoretical and experimental), as they have published data for specialized forms like a wedge, circular cylinders, and ship-like sections in forced heave motion (references [4] to [7] and [22], [23] of the paper).
Beam Theories for Torsional-Bending Response of Ship Hulls
Authors: Pedersen P. Terndrup
Published: January 2021

A consistent one-dimensional finite-element procedure for analysis of the coupled torsional-bending response of thin-walled beam structures such as ship hulls is presented. At each element end there are three translations, three rotations and one classical Vlasov warping degree of freedom plus possibly N degrees of freedom associated with higher order generalized warping deformation modes. These higher order warping modes are generated from an eigenvalue problem associated with the homogeneous plane stress equilibrium condition for the individual beam cross sections. The assembly of the beam elements to the global model is performed by use of transition matrices which assure compatibility between the elements in the sense of least squares. Numerical examples are included which demonstrate the accuracy of the mathematical model and the applicability of the proposed analysis procedure for calculation of torsion-horizontal bending response of a containership hull. Even if the higher order warping modes are not included in the finite element formulation it is found that the mathematical model is quite accurate for overall response analysis of hull structures.
Ship-Hydrodynamic Free-Surface Problems Without Waves
Authors: Tuck E. O.
Published: January 2021

This paper consists of two parts. In the first part, I shall attempt to survey without too much mathematical detail the general problem of wave elimination in ship hydrodynamics, both in two and three dimensions. In the second part, I shall concentrate attention on a particular mathematical approach to this problem in two dimensions. The problem is reduced to that of solving an integro-differential equation, and the properties of this equation and of a simple "model" ordinary differential equation are discussed.
Viscous Effects on Turbulent Boundary-Layer Noise of Ship's Sonar Dome in a Water Tunnel
Authors: Rao V. Bhujanga , Kumar P. V. S. Ganesh , Gupta P. K.
Published: January 2021

Turbulent boundary-layer (TBL) wall pressure fluctuations of a body measured in a water tunnel need correction to obtain unbounded free-field values. Besides blockage effects in a tunnel which are easily accounted for, viscous effects on TBL noise are to be evaluated to quantify this correction. An analytical method using suitable wave vector spectrum modeling to estimate the correction needed due to viscous effects is presented. A sonar dome body is considered as a typical example.
Acoustic End Effects in Magnetohydrodynamic Submerged Vehicular Propulsors
Authors: Walker John S. , Talmage Gita , Brown Samuel H. , Sondergaard Neal A.
Published: January 2021

This paper treats the effects near the ends of the channel on the transmission and reflection of periodic acoustic waves generated at some cross section inside a magnetohydrodynamic (MHD) seawater propulsion system. A region of high uniform magnetic field inside the MHD submerged vehicular propulsor is separated from the essentially zero magnetic field outside the channel by a nonuniform, fringing magnetic field at each end of the channel. The channel configuration chosen here is that of a straight, rectangular duct with electrically insulating top and bottom walls perpendicular to the magnetic field and highly conducting sidewalls parallel to the field. In particular, the mathematical analysis focuses on determining the percentage of the incident wave which is reflected by the fringing-field region back into the uniform-field region and the percentage which is transmitted through the fringing-field region into the zero-field region. The key parameter is the acoustic interaction parameter N, which is the characteristic ratio of the electromagnetic body force opposing motions across magnetic field lines to the inertial "force" in the acoustic wave. Solutions are presented for the fundamental, plane acoustic mode for arbitrary values of Ν and for all acoustic modes for Ν < 1. The amplitudes of the reflected and transmitted waves depend on the wave frequency, the length of the fringing-field region, N, and the type of wave mode. The magnetic field introduces a strong anisotropy with strong damping of modes involving transverse motions across magnetic field lines and with weak damping of modes involving transverse motions along field lines. This is the third in a series of articles on MHD marine propulsion from the David Taylor Research Center MHD propulsion program [Brown et al (1990), Tempelmeyer (1990)].
Dynamics of Two-Line Ship Towing/Mooring Systems: Bifurcations, Singularities of Stability Boundaries, Chaos
Authors: Chung Jin-Sug , Bernitsas Michael M.
Published: January 2021

A methodology is developed for analysis and design of two-line towing and mooring (TLT/M) systems based on their nonlinear, horizontal-plane, slow-motion dynamics. The time evolution of the corresponding autonomous dynamical system is described in a six-dimensional state space. Branching diagrams identify supercritical pitchfork bifurcations and turning points which result in static loss of stability. TLT/ Ms also exhibit Hopf bifurcations which result in dynamic loss of stability and periodic solutions. Stability charts are developed to show catastrophe sets and regions of qualitatively different dynamics. Thus, regions of hazardous dynamics and line breaking—such as unstable equilibrium, unstable limit cycle, and chaotic dynamics—are identified. Theoretical conclusions are verified by numerical simulations of tanker and barge TLT/Ms. Based on this methodology, only a limited number of simulations is needed for analysis and design of TLT/Ms. Lines can be selected then to complement—rather than countereach other and allow the system to attain a safe equilibrium.
A Potential-Based Panel Method for the Analysis of a Two-Dimensional Super-or Partially-Cavitating Hydrofoil
Authors: Lee C.-S. , Kim Y.-G. , Lee J.-T.
Published: January 2021

A potential-based panel method is presented for the analysis of a super-or partially-cavitating two-dimensional hydrofoil. The method employs normal dipoles and sources distributed on the foil and cavity surfaces. It is shown that the source plays an important role in positioning the cavity surface through an iterative process. The cavity closure condition is found very effective in generating the cavity shape. Upon convergence, the method predicts the cavitation number together with the lift, drag, and surface pressure distribution for a given cavity length. Systematic convergence tests of the present numerical method show fast and stable characteristics. Good correlations are obtained with existing theories and experimental results for both partially-and supercavitating flows.
Stability and Bifurcations of Towed Underwater Vehicles in the Dive Plane
Authors: Papoulias Fotis A.
Published: January 2021

The problem of static and dynamic loss of stability in the vertical plane in steady towing of underwater vehicles is considered. Bifurcations of steady-state equilibria are studied using singularity theory techniques and all qualitatively different bifurcation diagrams that occur locally are revealed. Analytical conditions for stability of straight-line motion are derived. Bifurcations to periodic solutions are analyzed and shown to provide paths to complicated dynamics. The incorporation of the techniques used in this work, with related studies in cable dynamics, can lead to a design methodology for safer and more efficient operations.
Recipes for Computing the Steady Free-Surface Flow Due to a Source Distribution
Authors: Hendrix Dane , Noblesse Francis
Published: January 2021

This study provides complete and precise rules for evaluating the steady velocity potential of a piece-wise-constant distribution of Kelvin-Havelock sources on flat triangular hull panels and straight waterline segments with three digits of accuracy. The recipes yield a reliable and practical basis for a numerical method to compute steady flow about a ship using a distribution of Kelvin-Havelock sources.
Mean-Flow Measurements in the Boundary Layer and Wake and Wave Field of a Series 60 CB = 0.6 Ship Model—Part 1: Froude Numbers 0.16 and 0.316
Authors: Toda Y. , Stern F. , Longo J.
Published: January 2021

Part 1 of this two-part paper presents results from a towing-tank experiment conducted in order to explicate the influence of wavemaking by a surface-piercing body on its boundary layer and wake and provide detailed documentation of the complete flow field appropriate for validating computational methods. Mean-velocity and pressure field measurements were performed for Froude numbers 0.16 and 0.316 for a 3.048 m Series 60 CB = 0.6 hull form at numerous stations from the bow to the stern and into the near wake. For Froude number = 0.316, free-surface effects are very significant, whereas for Froude number = 0.16, they are negligible, except near the bow, such that comparison of the results enables the identification of the salient features of the wave-induced effects. Wave profiles and local and global elevations were also measured. In addition, resistance tests were conducted. The experimental equipment and procedures are described and the results are discussed to point out the essential differences between the flows at low and high Froude number. On the forebody, the differences are primarily in the outer (inviscid) flow, except at the bow, whereas on the afterbody and in the near wake, both the inner (viscous) and outer flows are altered. The results are discussed to assess the nature of the interaction between wavemaking and the boundary layer and wake. Most of the interaction can be explicated as a result of the wave elevations, wave-induced pressure gradients, and the displacement effects of the boundary layer. Part 2 concerns scale effects on near-field wave patterns and comparisons between the experimental results and inviscid theory.
Collapse of Inverted Hemi-Ellipsoidal Shell Domes Under Uniform Pressure
Authors: Ross C. T. F. , Rotherhamn N.
Published: January 2021

Nine hemi-ellipsoidal shell domes were tested to destruction, under uniform pressure acting on their concave surfaces. The shell domes varied from flat oblate hemi-ellipsoids to long prolate hemi-ellipsoids. The experimentally obtained collapse pressures showed that the flat oblate hemi-ellipsoidal domes and the long prolate hemi-ellipsoidal domes were stronger than the hemi-spherical and near hemi-spherical domes. The theoretical analysis was based on the finite-element method, and it showed that the theoretically obtained stresses were, in general, larger in magnitude than the experimentally obtained stresses.
Mean-Flow Measurements in the Boundary Layer and Wake and Wave Field of a Series 60 CB = 0.6 Ship Model—Part 2: Scale Effects on Near-Field Wave Patterns and Comparisons with Inviscid Theory
Authors: Longo J. , Stern F. , Toda Y.
Published: January 2021

Part 2 of this two-part paper presents additional results from a towing-tank experiment conducted in order to explicate the influence of wavemaking by a surface-piercing body on its boundary-layer and wake and provide detailed documentation of the complete flow field appropriate for validating computational methods. In Part 1 (Journal of Ship Research, Dec. 1992), wave profile, local and global wave-elevation, and mean-velocity and pressure field measurements for Froude numbers 0.16 and 0.316 for a 3.048 m Series 60 CB = 0.6 hull form are presented and discussed to point out the essential differences between the flows at low and high Froude number and to assess the nature of the interaction between wavemaking and the boundary layer and wake. In Part 2, scale effects on the near-field wave patterns are examined through wave profile and local and global wave-elevation measurements for 1.829 and 3.048 m models and Froude numbers 0.316, 0.3, and 0.25. The bow-wave amplitude and divergence angle are larger and the stern waves smaller for the smaller model. The latter scale effect is well known, but the former one is a new and unexpected result. Also, comparisons are made between the experimental results and those from a wavy inviscid-flow method, which provides an evaluation of the capabilities of the computational method. Although the computations predict the gross features of the wave system and velocity and pressure fields, they do not simulate the complex details of either the wave system or the flow field, especially close to the hull and wake centerplane.
Time-Domain Second-Order Wave Radiation in Two Dimensions
Authors: Isaacson Michael , Ng Joseph Y. T.
Published: January 2021

This paper presents a time-domain second-order method to study the nonlinear wave radiation problem in two dimensions. A time-stepping scheme is adopted to obtain the resulting flow development which satisfies the nonlinear free-surface boundary conditions and the radiation condition to second order, and the numerical procedure utilizes a boundary integral equation method based on Green's theorem to calculate the field solution at each time step. The body surface boundary condition is expanded about the mean body position to second order by a Taylor series. The method is applied to the cases of a semi-submerged circular cylinder and a rectangular cylinder undergoing sinusoidal sway, heave and roll motions. For the case of the circular cylinder, comparisons of the computed hydrodynamic forces at first and second order are made with previous theoretical and experimental results and a favorable agreement is indicated. The importance of second-order effects in the calculation of the hydrodynamic force is discussed.
Aspects of Modeling Partially Cavitating Flows
Authors: Rowe A. , Blottiaux O.
Published: January 2021

A method for calculating partially cavitating flows is presented. This method respects the impermeability condition on the profile in the vicinity of the cavity. The difficulties inherent in a scheme which gives a solution depending on the internal field organization, when the cavity is open, are analyzed. Several closure models are compared with the experimental results. This comparison shows the great variety of models that would have to be considered in order to give a proper account of the t(ac) law for three types of geometry. The pressure recovery study for one of the three geometries shows that pressure recovery can be simulated by a distribution of sinks distributed immediately downstream of the cavity, followed by a positive flux zone farther downstream. Validated by means of a finite-element calculation, the method proves its capability to take into account the effect of nonparallel confining walls placed very close around a foil of very small relative thickness.
General Instability of Swedge-Stiffened Circular Cylinders Under Uniform External Pressure
Authors: Ross C. T. F. , Palmer A.
Published: January 2021

A theoretical and experimental investigation into the general instability of nine swedge-stiffened circularcylinders under uniform external pressure is described. The investigation found that most of the vesselssuffered plastic general instability, and that initial out-of-roundness played a significant role in the magnitude of the elastic knockdown. The theoretical analysis was based on the finite-element method, and a thinness ratio is proposed from which one can determine a plastic reduction factor when more experimental results are available.
A New Interpretation of Linear Theory in the Calculation of Ship’s Wave Resistance
Authors: Amromin E. L. , Lordkipanidze A. N. , Timoshin J. S.
Published: January 2021

A modification of linear theory is suggested based on the existence of maximum amplitudes and energy density of surface waves. The value of a density is derived from a calculation test for the Wigley ship model and used in the Havelock formula modification. Such an interpretation of linear theory, as shown in the paper for different types of ships, results in a large reduction of discrepancies between the calculation and experimental data.
Asymptotic Analysis of Wave Resistance of a Submerged Body Moving with an Oscillating Velocity
Authors: Kuznetsov N. G.
Published: January 2021

An asymptotic study is made of potential and tree-surface elevation due to forward motion of a submerged body with an oscillating velocity. The latter is supposed to be a short period. As the nondimensional period ε 1, the singular perturbation technique is applied. Using two-term expansion for the potential, the principal terms of asymptotics for instant and mean values of wave resistance are obtained. The mean value (up to a term 0(e)) is the sum of two addends, the first of which is the wave resistance of the same body moving at the mean speed. The second addend is proportional to the dispersion of velocity with the coefficient depending on the form of the body. This coefficient vanishes if the body is symmetric with respect to the mid-section. Numerical examples for the two-dimensional problem show that there exist cylinders with the following property: the absolute value of wave resistance decreases when passing from motion at the mean speed to motion with oscillating velocity.
On Seakeeping of Conventional and High-Speed Vessels
Authors: Faltinsen Odd M.
Published: January 2021

The focus is on numerical and theoretical seakeeping problems for conventional and high-speed vessels. Wave-induced motions, accelerations and added resistance in waves of nonplaning high-speed monohulls and catamarans are discussed. Emphasis is on the interaction with the steady flow and transom stern effects. The "cobblestone" effect and the speed loss of a surface-effect ship (SES) in a seaway are discussed. The importance and possibility of predicting the influence of flow separation on the vertical motions of conventional ships are studied. Numerical methods that accurately describe slamming on hull sections are discussed. Correlations with published model test data for bow flare slamming are presented.
A Study of Jones’ Equation for Buckling of Laminated Composite Cylinders Under External Hydrostatic Pressure
Authors: Perry Thomas , Miller Zan
Published: January 2021

A classical solution derived by Jones (1968) is used to evaluate the buckling performance of unstiffened generally orthotropic and quasi-isotropic laminated Graphite/Epoxy (GREP) composite cylinders subjected to external hydrostatic pressure. The results of the analysis are compared to finite-element analysis results. Hydrostatic testing to failure of several 12-ply T300/5208 GREP cylinders demonstrated that the classical buckling solution is quite accurate. The finite-element results showed good agreement with both Jones' solution and test data, with several notable exceptions. Evaluation of strain gage data via Southwell's (1932) method indicates that the test cylinders were fabricated very nearly true. A postiori buckling predictions using Southwell plots all compared quite favorably with the Jones' equation predictions. This work demonstrates that a relatively simple analytical solution can reliably evaluate the performance of composite materials in pressure hull applications.
Boundary-Integral Computation of Hydrodynamic Interactions Between a Three-Dimensional Body and an Infinitely Long Cylinder
Authors: Guo Zhi , Chwang Allen T.
Published: January 2021

Hydrodynamic interactions between a three-dimensional body of revolution and an infinitely long circular cylinder in an inviscid fluid are studied numerically by the boundary-integral method. The added-mass coefficients and their derivatives are computed in terms of the solutions of four integral equations of the second kind. A numerical technique based on variable transformations is developed to evaluate integrations over steep peaks. Integrations over the cylindrical surface are properly computed by mapping the infinite region onto a finite region and regularizing the ill-behaved kernels with sharp peaks. The discrete added masses and their derivatives are fitted by the least-squares approximation on the basis of Legendre polynomials. As a practical example, the moving trajectories of a sphere conveyed by a uniform flow around a fixed circular cylinder are computed and presented.