bluuebunny/crossref_metadata_2025 huggingface dataset:
Water Waves Radiated by Coherent Vortices in a Submerged Jet
Authors: Noblesse Francis , Anthony Douglas G. , Yang Chi
Published: January 2021

The physical problem - We consider a jet of water discharging from a nozzle submerged at a depth D below the free surface of a large body of water at rest, as in the experimental study of Bernal and Madnia (1989). The free surface is taken as the plane Z = 0 and the Z-axis points upward. The X-axis is chosen aligned with the jet and pointing in the same direction. The origin of the X-axis is taken directly above the nozzle, which thus is located at the point (X = 0, Y = 0, Z = -D). The jet is unstable to axially symmetric disturbances, which result in the formation of a periodic train of fairly coherent ring-like vortices. These vortices are created at a frequency co, and are convected downstream with a speed U roughly 70% of the jet exit velocity. The vortices grow in amplitude until nonlinear motions destroy their coherence, within 5 to 8 diameters of the jet exit, as is described in several experimental studies noted in Bernal and Madnia (1989), notably Yule (1978), Zaman and Hussain (1980), Hussain and Zaman (1980), Crow and Champagne (1971), and Hussain and Zaman (1981). An understanding of the free-surface disturbance created by a coherent periodic system of traveling vortices and observed in the experimental study of Bernal and Madnia (1989) is sought.
Efficient Methods for Hydroelastic Analysis of Very Large Floating Structures
Authors: Ertekin R. C. , Riggs H. R. , Che X. L. , Du S. X.
Published: January 2021

The linear hydroelastic response of a very large floating structure (VLFS) consisting of multiple modules is studied theoretically, following a review of the past work on hydroelasticity in fluid-structure interaction. The 3-dimensional Green function method and Morison's equation approach are used to determine the fluid loading in conjunction with two different hydroelastic models. The first method uses a rigid module, flexible connector model in which the hydrodynamic interaction between rigid modules is taken into account. The double composite source distribution method, which is a numerically efficient implementation of the Green function method that exploits double symmetry of the structure in the longitudinal and lateral directions, is used to reduce computations. In the second method, fully elastic modules are considered. In this approach, the fluid loading is obtained by Morison's equation, and the structure is modeled by frame finite elements. The predictions for the rigid-body motions and structural deformations, as well as module-connector loads, obtained by the two different methods are compared. The proposed methods of hydroelasticity have been used to predict the response of a 16-module VLFS, 100 m by 1600 m. Both methods are sufficiently efficient to allow the analysis of even larger VLFS.
Elastic-Plastic Axisymmetric Failure of Swedge-Stiffened Cylindrical Pressure Hull Under External Pressure
Authors: Liang Cho-Chung , Yang Ming-Fung , Chen Hung-Wen
Published: January 2021

An elastoplastic and geometrically nonlinear analysis of two of Ross's swedge-stiffened pressure hulls subjected to hydrostatic pressure loading is presented. Three commonly used swedge forms—trapezoidal, triangular, and sinusoidal-shaped—are considered. Comparisons on the responses of these three swedge-stiffened shapes are included. The load/displacement relations as well as the distribution of stresses with plastic zone spread phenomena are also illustrated.
A Capsizing Probability Computation Method
Authors: Belenky Vadim L.
Published: January 2021

A new method of computing capsizing probability is described. This method is based on a partially linear approximation of nonlinear terms in the rolling differential equation. This substitution permits the retention of the topology of the nonlinear phase portrait and, at the same time, the simplicity of the linear solution. A criterion of stability of a system with partially linear righting moments is suggested. Finally, a practical method of computer-aided calculation of capsizing probability is described.
Note on a Joint Distribution of Successive Wave Periods
Authors: Myrhaug Dag , Rue Havard
Published: January 2021

A joint distribution of successive wave periods is presented. The analysis includes comparisons with observed wave data obtained from measurements in deep water at sea on the Norwegian continental shelf. It appears that the joint distribution of successive wave periods describes the measured data reasonably well in an intermediate wave period range.
Strength Analysis of Sandwich Shell Structures Subjected to Hydrostatic Load
Authors: Cho W. , Papadakis N. A.
Published: January 2021

The strength of a circular, cylindrical sandwich shell subjected to hydrostatic pressure is examined. An energy methodology appropriate for static analysis of sandwich shells is presented. Interactions of the inner and outer shell with the core stiffeners interconnecting these shells are investigated. Numerical results are discussed, together with comparisons between the sandwich shell and alternative designs.
Computation of Ship Interaction Forces in Restricted Waters
Authors: Korsmeyer F. T. , Lee C.-H. , Newman J. N.
Published: January 2021

Theoretical and computational results are presented for the interaction forces among multiple ships or bodies which are operating near to each other. These computations are made under several assumptions, the most important of which is that the free surface is rigid. An assumption usually made in the analysis of this problem is that the ships are slender, but the approach taken here is fully three-dimensional. The method is applicable to any number of arbitrarily shaped bodies in arbitrary motion in a fluid domain which may be bounded by irregular surfaces. Results are presented for the simple case of two ships passing in a canal of rectangular cross section, for which a particular Green function greatly reduces the computational effort; and for two ships passing in a canal with sloping sides, for which no such economies are available.
A Wind Tunnel Investigation of Yacht Hydrodynamic Side Force and Drag
Authors: Flay R. G. J. , McMillan D. C.
Published: January 2021

Measurements of side force and drag on models of various yacht hull/keel combinations in a wind tunnel are presented. The accuracy of the resulting side force measurements was verified by comparing results for two of the standard configurations with towing tank data from geometrically identical models at the same Reynolds number. The agreement between the results was excellent. Measurements from the models were then compared with the predictions of five different methods for a range of keel to canoe body draft ratios from 2.27 to 5.56. It was found that there was considerable variation in the predictions of the various methods, but that the wind tunnel results fell at the center of the range of predicted values. Effective aspect ratios were estimated from induced drag measurements and were found to be in excess of twice the geometric aspect ratio of the keels, reducing as heel increased to 30 deg.
Using Grooved Surfaces to Improve the Efficiency of Air Injection Drag Reduction Methods in Hydrodynamic Flows
Authors: Reed Jason C.
Published: January 2021

A summary of experiments using grooved surfaces to trap and hold (via surface tension forces) an injected airstream in a low-speed (1.25 to 5 m/s) water flow is presented. The purpose of creating a low-volume near-wall air sheet is to possibly enhance the efficiency of current air injection drag reduction methods in terms of unit gas volume per % drag reduction. Flow visualization and preliminary quantitative data are included for a laminar channel flow, a disturbed laminar channel flow, and a flat plate turbulent boundary-layer flow. A stable convecting low-volume, near-wall gas film is produced in several instances. Groove dimension and the presence of anti-wetting surface coatings are shown to greatly affect the formation and stability of the gas sheet. Deeper, narrower grooves, anti-wetting surface coatings, and shallow-angle gas injection increase the stability of the attached gas layer. Convected disturbances are shown to increase the interfacial instability of the attached sheet. It is not known if a gas sheet can be held under a turbulent boundary layer over 3 m/s, or if the groove sizes needed to do so would become too small to be of use in a practical high-speed hydrodynamic flow.
Acoustic End Effects in Magnetohydrodynamic Submerged Vehicular Propulsors—Part 2: Solitary Waves
Authors: Walker John S. , Brown Samuel H. , Sondergaard Neal A.
Published: January 2021

In a magnetohydrodynamic (MHD) seawater propulsion system for a submerged vehicle, a region of high-uniform magnetic induction (magnetic flux density) inside the MHD propulsion channel is separated from the region of essentially zero magnetic induction outside the channel in the seaway by a region of nonuniform, fringing magnetic induction at each end of the channel. This paper treats the propagation of an a periodic fluid transient (solitary wave) which is produced by an arbitrary unit impulse in velocity or pressure at any cross section in the uniform-field region and which propagates through either fringing field region and into the zero-field region outside the channel. The time scale for the transients is sufficiently short that compressive effects are important, so that the fluid transients are acoustic waves. The channel is a straight, rectangular duct with electrically insulating walls and highly conducting walls perpendicular and parallel to the magnetic induction, respectively. The linearized acoustic equations are averaged over each cross section of the channel to obtain a pair of coupled equations governing the average pressure and average axial velocity as functions of the axial coordinate and time. Together these equations represent a simple wave equation with a retarding force which is proportional to the square of the local magnetic flux density. Results are presented for three values of the acoustic interaction parameter N, which is the characteristic ratio of the electromagnetic body force opposing motions across magnetic-induction lines to the inertial "force" in the fluid transients. An abrupt change in velocity produces a wave front which travels at the speed of sound. Without MHD effects, the entire change in velocity or pressure occurs suddenly as the wave front passes. With MHD effects, only part of each change occurs suddenly as the wave front passes, followed by a gradual evolution to reach the entire change. The split between the abrupt and gradual fractions of the entire change depends on N. In previous work, the authors treated the fundamentally different MHD acoustic problem of the transmission of periodic waves from the channel and the previous paper (Walker et al 1992) is Part 1 of the present work.
Time-Domain Calculations of Drift Forces on Floating Two-Dimensional Object in Current and Waves
Authors: Prins H. J. , Hermans A. J.
Published: January 2021

In this paper the influence of a current on the drift forces acting on a floating body is studied. A theory has been developed which is correct up to second-order in the current velocity. A numerical algorithm has been found which solves the time-dependent equations and is stable for all current velocities. To test this algorithm, a two-dimensional case has been studied, i.e., the drift forces on a horizontal cylinder of infinite length. A computer program has been developed and very encouraging results have been found for the added-mass and damping coefficients and the drift forces. It is shown that especially for higher velocities the second-order terms in the velocity become very important.
A Surface Panel Method for Design of Hydrofoils
Authors: Lee Chang-Sup , Kim Young-Gi , Suh Jung-Chun
Published: January 2021

A surface panel method treating a boundary-value problem of the Dirichlet type is presented to design a hydrofoil corresponding to a prescribed pressure distribution. An integral equation is derived from Green's theorem, giving a relation between the total potential of known strength and the unknown local flux. Upon discretization, a system of linear simultaneous equations is formed and solved for an assumed geometry. The pseudo local flux, present due to the incorrect positioning of the assumed geometry, plays a role of the geometry corrector, with which the new geometry is computed for the next iteration. Sample designs for a series of pressure distributions of interest are performed to demonstrate the fast convergence, effectiveness and robustness of the procedure. The method is shown equally applicable to designing two- and three-dimensional hydrofoil geometry.
Acoustic Radiation from a Coupled Planar Semi-Complex Structure in Heavy Fluid
Authors: Atalla Noureddine , Berry Alain
Published: January 2021

A mathematical model is derived to address the vibrations and sound radiation into a dense fluid, of a coupled system consisting of two semi-complex plates (i.e., supporting added mass, stiffeners, and having arbitrary elastic boundary conditions) linked in four points through multistage suspensions elements. The supporting plate is subjected to point, line or surface harmonic excitation, while the radiating plate is excited through the suspensions. Both plates are assumed to be baffled, and the radiating plate is fluid loaded. The model is based on a variational approach for the plates, and a matrix transfer approach is used to handle the coupling between the two-plates. The solution is found using a Rayleigh-Ritz expansion in terms of polynomial trial functions which are shown to allow for the arbitrary elastic boundary conditions and to facilitate the calculation of the radiation impedance matrix. The vibrations and noise design of the system is discussed. The main design indicators are the force transmissibilities between the different excitation and attachment points, the mean square velocity, the radiated power and the radiation efficiency of the radiating plate. Numerical examples are presented to show the effects of fluid loading and different design parameters (plates thickness, boundary conditions, added mass, stiffeners, etc.) on the radiated sound.
Out-of-Plane Solutions and Bifurcations of Submersibles in Free Positive Buoyancy Ascent
Authors: Papoulias Fotis A. , Aydin Ibrahim
Published: January 2021

The problem of motion stability of submersible vehicles in free positive buoyancy ascent is analyzed. Motion is allowed to occur in combined vertical and horizontal planes. Continuation and catastrophe theory techniques are employed to trace all possible steady-state solutions in six degrees of freedom, while local linearization reveals their stability properties. Vehicle geometric properties and control surface deflections are used as the primary bifurcation parameters. It is shown that multiple solutions may exist in the form of pitchfork bifurcation, solution separation, hysteresis, and teardrop branches. Regions in parameter spaces are identified where extreme sensitivity of solutions to geometric properties and hydrodynamic modeling is present.
Contributions to Problems of the Theory of Jets in Ideal Fluids Solved with the Use of Elliptic Functions Theory
Authors: Tselnik David S.
Published: January 2021

A number of plane Invlscid jet flow problems of Interest in naval hydrodynamics require the use of the elliptic functions theory. Generally speaking, finding the general solution to a problem in terms of elliptic functions is not a complicated task. However, finding solutions as rapidly convergent infinite series or as sound asymptotic formulas is often not so easy, and special ways of treatment may prove to be necessary. In this paper—which continues and completes the author's earlier papers (1985,1992)—the source of difficulties (in the case when solutions to problems are expressed as quotients of the elliptic theta-functions) is revealed, and the ways of treatment are discussed.
Inverse Problem in Determining the Normal and Tangential Drag Coefficients of Marine Cables
Authors: Huang Cheng-Hung , Huang Min-Chih
Published: January 2021

The Levenberg-Marquardt method is applied to estimate simultaneously the normal and tangential drag coefficients of marine cable, in a planarly towed cable-body system, from the measurements of the horizontal and vertical tension force components. Because only coefficients are to be estimated, this type of problem belongs to a branch of systems identification called "parameters estimation" (Beck & Blackwell 1988). A statistical analysis is performed to determine the standard deviation and the approximate confidence bounds for estimating the normal and tangential drag coefficients. Results show that the sensitivity and accuracy of the estimated coefficients depend on the type of boundary conditions used in the direct problem as well as on the towed configurations.
The Automatic Functional Design of Hull Surface Geometry
Authors: Lowe T. W. , Bloor M. I. G. , Wilson M. J.
Published: January 2021

The surface geometry of a model yacht hull is generated using the partial differential equation (PDE) method, which enables the shape of the hull to be completely specified using only a small number of parameters while not overlimiting the range of shapes obtainable. This permits a computationally efficient search of the design parameter space for the hull form which optimizes some performance indicator. Here, hulls of low wave resistance are sought subject to certain imposed design constraints. The wave resistance is calculated using a panel method similar to that developed by Dawson. The effects on the hull shape of requiring reductions in approximations to the skin friction and form drags are also considered.
Cobblestone Oscillations of an SES with Flexible Bag Aft Seal
Authors: Steen Sverre , Faltinsen Odd M.
Published: January 2021

The high-frequency dynamic response of surface effect ships (SES) equipped with a flexible bag aft seal is studied. The results show dominating vertical accelerations in a frequency range important for passenger comfort and crew work ability. The mathematical model for the craft dynamic response accounts for the motions and accelerations in heave and pitch induced by both the unsteady uniform pressure and the spatially varying air cushion pressure. The effect of a flexible bag aft seal and a variable leakage area under the seals is included. The bag dynamic response is treated as linear and quasistatic. It is shown how the flexible bag reduces the spatial pressure resonance frequencies of a 35 m SES. The variable leakage area under the bag is found important for the response of the craft in low sea states.
Extreme Loads and Load Combinations
Authors: Mansour A. E.
Published: January 2021

A simple model for combining extreme responses of correlated load components has been developed in this paper for use in design of marine structures. The combined response has the form fc = f1 + Kf2 for a two correlated load case and fc = f1 + K2f2 + K3f3 for a three correlated load case. The load factors Ki, are determined from probabilistic analysis of the combined response of a multiple component system subjected to common input (waves). Application examples are given and modeling errors are discussed. The model is suitable for use in the usual deterministic design analysis or probabilistic and reliability design procedures. This is the first of a three-paper series on this subject.
Charts for Estimating Nonlinear Hogging and Sagging Bending Moments
Authors: Mansour A. E. , Wasson J-P.
Published: January 2021

This paper presents charts for preliminary estimates of the nonlinearities associated with wave bending moments acting on a ship moving in a stationary sea. Deviation of the actual hull shape above the waterline from "vertical wall" is characterized by a flare coefficient. The charts show the influence of the flare coefficient, among other parameters, on a nonlinearity parameter which may be used to estimate the difference between wave hogging and sagging moments. Three application examples are given at the end of the paper to illustrate how the charts can be used in conjunction with linear strip theory results in order to estimate slightly nonlinear hogging and sagging moments.
Ultimate and Crushing Strength of Plated Structures
Authors: Paik Jeom Kee , Pedersen P. Terndrup
Published: January 2021

An efficient theoretical approach is developed which calculates in the same mathematical process the pre-and post-collapse behavior in addition to the deep-collapse response of plated structures under static/ dynamic compressive loads. In the analysis of complex plated structures, each plate element composing the structure is modeled as one plate unit. Two principal models, one elastic model for analysis of the ultimate strength and the other rigid-plastic model for analysis of the crushing load, for the plate unit subjected to static/dynamic loads are derived theoretically. The models are basically formulated as a quasi-static loading condition, but dynamic effects are included by taking into account the influence of strain rate sensitivity in the material model. The procedure is verified by a comparison of experimental and other theoretical results.
Application of Reynolds-Stress Transport Models to Stern and Wake Flows
Authors: Sotiropoulos F. , Patel V. C.
Published: January 2021

The Reynolds-averaged Navier-Stokes equations are solved to assess the importance of the turbulence model in the prediction of ship stern and wake flows. Solutions are obtained with a two-equation scalar turbulence model and a seven-equation Reynolds-stress tensor model, both of which resolve the flow up to the wall, holding invariant all aspects of the numerical method, including solution domain, initial and boundary conditions, and grid topology and density. Calculations are carried out for two tanker forms used as test cases at recent workshops, and solutions are compared with each other and with experimental data. The comparisons reveal that the Reynolds-stress model accurately predicts most of the experimentally observed flow features in the stern and near-wake regions whereas the two-equation model predicts only the overall qualitative trends. In particular, solutions with the Reynolds-stress model clarify the origin of the stern vortex.
Free-Surface Boundary-Layer Approach to Study the Bow Vortices Generation Ahead of a Semisubmerged Horizontal Circular Cylinder
Authors: Raheja L. R.
Published: January 2021

In the light of experimental observation of a free-surface shear layer, the flow ahead of a semisubmerged horizontal circular cylinder is modeled as a free-surface boundary layer of concentrated vorticity joining the potential flow below it with the aim to study the generation of bow vortices theoretically. The boundary-layer equations are linearized subject to a suitable assumption and are integrated using basically the Kármán Pohlhausen method. It is found that the free surface moves slower than the layer beneath it, but there is more likelihood of bow vortices being generated by instability of velocity profile rather than by separation and backflow. This is confirmed by the nonmonotonicity in the vorticity profile and the fulfillment of the Görtler instability criterion for flow along curved boundaries, near the body upstream. The position of the point of onset of instability, as stipulated from the above observations, compares well with the position of the free-surface separation point as observed in the experiments.
Wave Impact on the Bow End of a Catamaran Wet Deck
Authors: Korobkin Alexander
Published: January 2021

The plane unsteady problem of a wave impact on the bow end of a catamaran wet deck is considered. The wet deck is modeled by a Euler beam. At the initial instant of time, the wave crest touches the beam edge. In order to describe the beam deflection and the liquid flow, the method of normal modes and the Wagner approach are used. The present analysis focuses on the method of treating this coupled problem and the estimations of its applicability. Preliminary numerical calculations reveal that the interaction of the beam deflection and the liquid flow may lead to hydrodynamic pressures which are higher than those for a rigid wet deck.
Determination of Three-Dimensional Body Forms from Given Pressure Distribution Over Their Surfaces
Authors: Pashin V. M. , Bushkovsky V. A. , Amromin E. L.
Published: January 2021

A method for solving inverse three-dimensional problems in hydromechanics is proposed which makes it possible to fit desired pressure distributions within design constraints immediately in the course of calculations. Examples of the method of application are given for bodies of revolution in flows at nonzero drift angles. These flows are not axisymmetric. Bodies of revolution in them are very handy examples of demonstrations of the method, and these examples have many technical applications.
Approximate Assessment of the Ultimate Longitudinal Strength of the Hull Girder
Authors: Gordo J. M. , Soares C. Guedes , Faulkner D.
Published: January 2021

A method is presented to estimate the ultimate moment based on a simplified approach to represent the behavior of stiffened plate columns. The assessment of the strength of a very large crude carrier is performed and compared with the moment at failure in hogging estimated by other methods applied to the same case. The proposed method allows the prediction of the degradation of the strength due to corrosion and residual stresses. It also allows the evaluation of the strength of the hull at several heeling conditions. Finally, an analysis of the efficiency of the high tensile steel is carried out.
Prediction of the Dynamic Roll Stability of Hard-Chine Planing Craft
Authors: Lewandowski Edward M.
Published: January 2021

A method is presented to predict the roll restoring moments acting on hard-chine craft operating at planing speeds, which can be used to evaluate the dynamic roll stability of these vessels. Expression are developed for the static and dynamic contributions to the restoring moment. Tests were conducted to measure the restoring moment on five different planing hull models. The predictions for these hulls compare well with the experimental results.
Influences of Charge Air Humidity and Temperature on the Performance and Emission Characteristics of Diesel Engines
Authors: Lin Cherng-Yuan , Jeng Yuan-Liang
Published: January 2021

The effects of humidity and temperature of intake air on the performance and emission characteristics of diesel engines were systematically investigated in order to improve their design and operations. A fourstroke diesel engine with a displacement volume of 3856 cc was employed as the test engine. The intake air was conditioned to various temperatures and humidities via an air-conditioner before entering the intake port of the engine. This study shows that the air consumption rate, brake torque, and nitrogen oxides decrease, while the brake specific fuel consumption, carbon monoxide, and sulfur dioxide increase with both the temperature and humidity of the charge air. However, the effects of the humidity on the formation of the carbon monoxide and sulfur dioxide emissions are more significant at a higher temperature of the charge air. Furthermore, charge air temperature tends to increase the percentage of C02 in the exhaust while decreasing the percentage of excess oxygen.
Experimental Validation of Two-and Three-Dimensional Numerical Analysis of Partially Cavitating Hydrofoils
Authors: Peallat Jean Marc , Pellone Christian
Published: January 2021

This paper presents an original panel method to treat partially cavitating two-and three-dimensional hydrofoil problems. The method uses a velocity-based boundary element method (VBEM) but its originality lies in the deformation process. Indeed, the cavitation number is imposed, and the cavity shape is calculated in the course of iterations (free cavity length method). In fact, considering the boundary layers, a normal velocity is observed on the cavity surface which supplies the new stream-surface shape. For any imposed closure condition, the cavity shape (thickness and length) is adapted in such a way that it closes on the hydrofoil. The use of parabolic dipoles, constant source (panel method minimization process) allows a good description of the pressure field. The two-and three-dimensional results obtained are compared with experimental measurements and also with the results of earlier former methods.
Nonlinear Shallow-Water Flow on Deck
Authors: Huang Zhenjia , Hsiung Chi-Chao
Published: January 2021

Euler's equations have been used for nonlinear shallow-water flow on deck. The equations are simplified under the shallow-water assumption to obtain the governing equations. The Flux-Difference Splitting method is devised to solve this shallow-water flow problem. The flux-difference in the governing equations is split based on characteristic directions. The Superbee flux limiter is employed in the algorithm to make the finite-difference scheme of second order with high resolution. For two-dimensional decks, numerical results are presented to reveal the characteristics of shallowwater flow on deck. For three-dimensional decks, the Flux-Difference Splitting method together with the Fractional Step method are used, so that solutions of the shallow-water equation can be obtained by solving two sets of one-dimensional differential equations. Numerical results are presented to show the wave patterns for different modes of motion excitation. Velocity vectors in the flow field are also given to help understand the flow properties.
Analytical Expressions of the Stability and Bifurcation Boundaries for General Spread Mooring Systems
Authors: Garza-Rios Luis O. , Bernitsas Michael M.
Published: January 2021

Spread mooring systems (SMS) are labeled as general when they are not restricted by conditions of symmetry. The six necessary and sufficient conditions for stability of general SMS are derived analytically. The boundaries where static and dynamic loss of stability occur also are derived in terms of the system eigenvalues, thus providing analytical means for defining the morphogenesis that occurs when a bifurcation boundary is crossed. The equations derived in this paper provide analytical expressions of elementary singularities and routes to chaos for general mooring system configurations. Catastrophe sets are generated first by the derived expressions and then numerically using nonlinear dynamics and codimension-one and -two bifurcation theory; agreement is excellent. The mathematical model consists of the nonlinear, third-order maneuvering equations without memory of the horizontal plane, slow-motion dynamics—surge, sway, and yaw—of a vessel moored to several terminals. Mooring lines can be modeled by synthetic nylon ropes, chains, or steel cables. External excitation consists of time-independent current, wind, and mean wave drift forces. The analytical expressions derived in this paper apply to nylon ropes and current excitation. Expressions for other combinations of lines and excitation can be derived.
Fairing Spatial B-Spline Curves
Authors: Pigounakis Kostis G. , Sapidis Nickolas S. , Kaklis Panagiotis D.
Published: January 2021

Three-dimensional curves are playing an increasing role in ship-hull modeling and many other areas of computer-aided design (CAD). The problem of evaluating and improving the fairness of such a curve is considered and three solutions (algorithms) are proposed representing all major methodologies currently pursued by CAD researchers: local fairing by knot removal, and local/global fairing based on "energy" minimization. The performance of the algorithms is studied for both cubic and quintic B-splines using realistic test cases. Finally, a comparison with existing techniques is presented and some visualization tools for spatial-curve fairing are briefly discussed.
Effect of Tripping Laminar-to-Turbulent Boundary Layer Transition on Tip Vortex Cavitation
Authors: Pichon T. , Pauchet A. , Astolfi A. , Fruman D. H. , Billard J-Y.
Published: January 2021

It is by now well established that, for Reynolds numbers larger than those corresponding to the conditions of laminar-to-turbulent boundary layer transition over a flat plate (≈0.5 × 106) and for a variety of wing shapes and cross sections, desinent cavitation numbers divided by the Reynolds number to the power 0.4 correlate with the square of the lift coefficient. In the case of foils having an NACA 16020 cross section and for Reynolds numbers below or close to those leading to transition over a flat plate, the results are very much different from those obtained for well-developed turbulent boundary layer conditions. Thus, a research program has been conducted in order to investigate the effect of boundary layer manipulation on cavitation occurrence. It consisted in determining the critical cavitation numbers, the lift coefficients, and the velocities in the tip vortex of foils having either a smooth surface or tripping roughness (promoters) near the leading edge. Tests were performed using elliptical foils of NACA 16020 cross section having the promoters extending over 60, 80 and 90 percent of the semi-span. The region near the tip was kept smooth in order to distinguish laminar-to-turbulent transition effects from tip vortex cavitation inhibition effects associated with artificial roughness at the wing tip. Results obtained at very low Reynolds numbers, ≥ 0.24 × 106, with the foil tripped on both the pressure and suction sides collapse rather well with those previously obtained at much larger Reynolds numbers with the smooth foil, and correlate with the square of the lift coefficient. The differences between the tripped and smooth foil results are due to the modification of the lift characteristics through the modification of the wing boundary layer, as shown by flow visualization studies, and as a result of the local tip vortex intensity.
Boundary-Integral Representation of Linear Free-Surface Potential Flows
Authors: Noblesse Francis , Yang Chi , Chen Xiao-Bo
Published: January 2021

A new potential-flow representation, which defines the velocity field ∇ϕ in a potential flow region explicitly in terms of the velocity distribution (u,v,w) at a boundary surface E, is given for the case of wave diffraction-radiation by a ship. This flow representation does not involve the potential ϕ at ϵ (unlike the usual Green identity which expresses ϕ within a flow domain in terms of boundary values of ϕ and ∂ϕ/∂n) and defines the velocity field ∇ϕ directly (i.e., not via numerical differentiation of ϕ). The new flow representation can be useful for extending a given near-field flow into the far field, and for coupling a near-field nonlinear viscous flow calculation method and a far-field linear potential-flow representation.
A Benchmark Study on Crushing and Cutting of Plated Structures
Authors: Paik Jeom Kee , Wierzbicki Tomasz
Published: January 2021

A benchmark study on several closed-form solutions for the mean crushing strength and the cutting resistance of plated structures during collision or grounding is carried out by comparing theoretical solutions with experimental data. Based on expressions which have been derived for unstiffened structures, an extension of the methods is proposed for longitudinally and/or transversely stiffened structures. Dynamic effects on the crushing and cutting response are discussed, and applicability of the quasistatic formulations to analyze the crushing and cutting damage of the structure in the dynamic situations is investigated.
Application of a Numerical Optimization Technique to the Design of Cavitating Propellers in Nonuniform Flow
Authors: Mishima Shigenori , Kinnas Spyros A.
Published: January 2021

High-speed propulsor blades often experience moderate to substantial amounts of unsteady cavitation, and up to now have been designed via design methods for noncavitating blades combined with methods for the analysis of cavitating flows in a trial-and-error manner. In this paper, a numerical nonlinear optimization algorithm is developed for the automated, systematic design of cavitating blades. The method is first applied to the design of propeller blades in uniform flow. The blade mean camber surface is defined via a cubic B-spline polygon net in order to facilitate the handling of the geometry, and to reduce the number of the design parameters. Noncavitating blade geometries designed by the present method are directly compared with those designed via an existing lifting-line/lifting-surface design approach. Finally, the optimization algorithm is applied to the design of cavitating blades in nonuniform flow. The objective of the design is to obtain maximum propeller efficiency for given conditions by allowing controlled amounts of sheet cavitation. Several constraints on the unsteady cavity characteristics, such as the area of cavity planform and the amplitudes of the cavity volume velocity harmonics, are incorporated in the optimization technique. The effect of the constraints on the efficiency of the propeller design is demonstrated with various test cases.
Effects of Ship Hull and Propeller on Rudder Cavitation
Authors: Shen Young T. , Remmers Kenneth D. , Jiang Chen W.
Published: January 2021

Ship trial observation and drydock inspection of a surface ship combatant show that rudder cavitation can be a real problem in terms of ship operation and maintenance. A project has been initiated by the Navyto identify fleet rudder cavitation problem,to investigate experimentally and numerically hull/propeller effects on rudder cavitation, andto develop methods to incorporate propeller/rudder interaction into the future rudder design for cavitation improvement.

Experiments were carried out at the Navy's Large Cavitation Channel (LCC) with a geosim model of a fleet rudder, propeller and ship hull of a surface combatant. Two-component Laser DopplerVelocimetry (LDV) was used to measure the field velocity and inflow angles in the propeller slipstream. A dynamometer and pressure taps were used to measure rudder lift, drag and pressure distributions. Influence of the ship hull and propeller on rudder forces, pressure distribution, and cavitation performance are compared with numerical calculations. The resemblance between the model rudder cavitation pattern in the LCC and full-scale cavitation erosion pattern observed in the dry dock is discussed.
A Note on the Computation of Maximum Roll Amplitudes in Regular Beam Seas
Authors: Jones J. C. Peyton , Qankaya I.
Published: January 2021

Generalized methods for deriving the harmonic balance equations are illustrated using a ship roll model with angle dependent cubic damping, and cubic stiffness terms. The balance equations are then solved subject to a phase constraint which identifies the resonant solution only. The amplitude of the response at resonance, and the frequency at which resonance occurs, can therefore be obtained without needing to compute the response over a wide frequency range. This provides an efficient tool for investigating the dependence of the resonant response on the level of input excitation, with results which agree well with more time-consuming simulation methods.
Fatigue of Ship Structural Details—Technical Development and Problems
Authors: Xu Tao
Published: January 2021

During the past five years, a joint industry-government sponsored research project titled the Ship Structural Maintenance Project (SMP) has been conducted at the Department of Naval Architecture & Offshore Engineering, University of California at Berkeley. As a part of this project, the fatigue damage of ship structural details has been extensively studied. This paper summarizes the technical development in fatigue assessment of ship structural details developed during this project. In the fatigue resistance model, the fatigue damage evaluation of structural details was based on a stress range/number-of-cycles-to-failure (SN) approach in which the nominal stress procedure and the hot spot stress procedure were employed. The fatigue assessment of cracked structural details was based on a cracked SN approach developed by a hybrid SN-FM (fracture mechanics) methodology. This cracked SN approach was further developed to incorporate load-shedding effects. In the fatigue loading model, a new formula for the damage correction factor associated with a wide-banded load process was developed. The random loading sequence for ship service was addressed. Fatigue reliability models were then reviewed and the different fatigue reliability updating procedures were evaluated. These techniques were employed in the fatigue analysis of structural details in three tankers. Analysis results from these three ships were presented to illustrate the technical developments and problems associated with fatigue assessment of ship structural details.
Wavemaking Resistance on a Submerged Sphere in a Channel
Authors: Wu G. X.
Published: January 2021

The hydrodynamic problem of a submerged sphere moving in a channel is analyzed based on the velocity potential theory. The solution is obtained by using the multipole expansion which satisfies the boundary conditions on the free surface, at infinity, and on the sidewalls. Numerical results are obtained for the resistance and lift on the sphere, and for the attraction force by the sidewall. Results are also provided for the free-surface waves generated by the sphere.
Nonlinear Ship Loads: Stochastic Models for Extreme Response
Authors: Torhaug Rune , Winterstein Steven R. , Braathen Arne
Published: January 2021

In this study we focus on stochastic analysis methods for selective simulations, and we consider the extreme midspan moment of a fast-moving ship subjected to random Gaussian waves. We concentrate on analysis within a stationary sea state and our purpose is to accurately estimate hourly maximum ship response (compared with the correct result per hour) within a sea state with as little computational resources as possible. We consider how the use of a limited number of short simulations with "critical wave episodes" (short wave segments which are likely candidates to produce extreme response in the simulated hour-long history) reduces the cost of nonlinear time-domain ship response analysis.
The Application of S-N Curves Considering Mismatch of Stress Concentration Between Test Specimen and Structure
Authors: Petinov Sergei V. , Thayamballi Anil K.
Published: January 2021

This paper presents a procedure for the adjustment of S-N diagrams for noncontinuous welded joints in the fatigue analysis of structural details. The adjustment takes into account the effects of differences in geometry between the test specimen underlying the S-N curve and the hull structural detail it is applied to, considering the inelastic response of material at the anticipated crack origin, multiple modes of loading, and their statistical correlation. The basis of the approach is a comparison of relative fatigue lives for the structural detail of interest and the corresponding baseline specimen, using the local strain approach to fatigue, with extensions to the high-cycle regime. An S-N diagram so corrected is used in the conventional manner to predict fatigue damage or to establish related allowable stresses. The procedure developed is thus entirely compatible with present fatigue life assessment methods. The developed procedure is illustrated through numerical examples.
On a Remarkable Phenomenon by Partial Optimization of Lifting Surface Systems
Authors: Sparenberg J. A.
Published: January 2021

We consider a system of lifting surfaces which has to deliver a prescribed force action. When for each lifting surface we can choose freely the (possibly time dependent) shape of the profiles, it is in a linear theory not difficult to characterize the shed vorticity in the optimum case of minimum kinetic energy losses. However, when part of these surfaces has prescribed profiles an essentially more complicated problem arises of which a curious aspect will be discussed which can be important for numerical computations.
Ship Seakeeping Through the t = 1/4 Critical Frequency
Authors: Kring D. C.
Published: January 2021

This study demonstrates that a bounded, physically relevant solution does exist at the so-called T = Uω/g = 1/4 resonance in the linear seakeeping problem for a realistic ship with forward speed, U, frequency of encounter, ω, and gravitational acceleration, g. The solution of the seakeeping problem by a linear, three dimensional, time-domain Rankine panel method, validated through numerical analysis, testing, and comparison to physical experiments, supports this claim. The solution can also be obtained with equal validity through frequencies both above and below the critical frequency.
A Simplified Method to Predict the Added Resistance of a SWATH Ship in Waves
Authors: Fang Ming-Chung
Published: January 2021

A simplified method called phase transfer is used to calculate the added resistance of a SWATH ship advancing in regular waves. Using the method, the total added resistance of the twin-hull body can be obtained by calculating the corresponding hydrodynamic forces of a single body only. There is no need to calculate the force on each body twice, and the hydrodynamic interactions are not included. The strip theory is still used and the nonlinear force prediction is based on the theory for a monohull ship advancing in waves. Two additional assumptions are also made:the hull body is a "weak scatterer" andthe second-order potential is small and can be neglected.

Practically, the present method is suitable for predicting the corresponding hydrodynamic properties of a SWATH ship with high speed because the hydrodynamic interactions are not significant. The results obtained in the paper mostly agree with the experimental data and show that the present idea is reasonable. It is also found that the inclusion of the interaction between two hulls may lead to some unreasonable negative added resistance in bow waves. Therefore, the method developed here can be considered as a simpler tool to analyze the added resistance for SWATH ships advancing in waves, especially for higher speed.
Ship Hull Ultimate Strength Reliability Considering Corrosion
Authors: Paik Jeom Kee , Thayamballi Anil K. , Kim Sung Kyu , Yang Soo Hong
Published: January 2021

The aim of the present paper is to develop and demonstrate a procedure for assessment of ship hull girder ultimate strength reliability taking into account the degradation of primary members due to general corrosion. The probabilistic model for ultimate hull girder strength is established on the basis of an analytical formula that considers corrosion related time dependent strength degradation in the various failure modes. Corrosion rates and their probabilistic characterization are based on available studies using gauging data. Applicable extreme hull girder loads are calculated using a simplified direct method for wave load calculation together with the IACS design guidance formula for stillwater bending moment. The variability in strength, corrosion rates and loads are accounted for in the second order reliability method (SORM) based on calculations of the time dependent reliability index. The procedure developed is illustrated by application to both tankers and bulk carriers. For a given set of renewal criteria, apart from trends of hull girder section modulus, ultimate strength and the reliability index as a function of vessel age, the probability of steel renewal due to corrosion is also predicted.