Showing papers in "Journal of Ship Research in 2003"

Benchmarking of computational fluid dynamics for ship flows: the Gothenburg 2000 workshop

Abstract: The Gothenburg 2000 was an international benchmark workshop for computational fluid dynamics applied to ship flows. Test cases were three modern hull forms. One case without a free surface focused on turbulence modeling, whereas wave prediction was of interest for the other two. Of the free-surface cases, one had an operating propeller. For the first time, verification and validation procedures were an integral part of such benchmark efforts in ship flows. The workshop showed that free-surface waves may now be well predicted also away from the hull. There is a general improvement in the computation of the stern flow thanks to better turbulence modeling, but there is still room for improvement. Full-scale viscous flows may be computed without numerical difficulties. Verification and validation procedures should be applied for uncertainty analysis, and there is a discussion of the uncertainty in the predicted integral quantities in the paper. Further detailed conclusions and recommendations are also given based on the comparison of extensive standardized plots of the comparative computations and evaluation of the integral quantities.

Coupling of Sloshing and Ship Motions

Abstract: Sloshing is a violent resonent free surface flow in a container. The main objective of this thesis has been to study sloshing in rectangular and prismatic tanks. The tank may be excited by a harmonic motion or it may move with a ship in waves. In the latter case, the coupled ship motions and sloshing problem is investigated. A nonlinear analytically based sloshing model is used in the solshing calculations. Experiments have been conducedand collected data are utilized in the validation of the sloshing model and computations of interaction between sloshing and ship dynamics. Tank roof impacts are studied. Energy in the impact jet is dissipated and this leads to damping of the sloshing motion. An iterative procedure is applied to incorporate the effect of energy dissipation in the calculations. Damping of the soloshing motion is an important parameter around resonance for the coupled ship motion and sloshing system. The sloshing model is based on a nonlinear theory analysis of two-dimensional nonlinear sloshing of an imcompressible fluid with irrotational flow in a rectangular tank. Infinite tank roof height and no overtuning waves are assumed. The free surface is expressed as a Fourier series and the velocity potential is expanded in terms of the linear natural modes of the fluid motion. The infinite-dimensional modal system is approximated and the result is a finite set of ordinary differential equations in time for generalized coordinates (Fourier cofficients) of the free surface. This theory is not valid for small water depth or when water impacts heavily on the tank roof. The proposed method has a high computational efficiency, facilitates simulations of a coupled vehicle-fluid system and has been extensively validated for forced motions. Experiments with a smooth rectangular tank exited by forced harmonic horizontal oscillations have been performed and the results are used to validate the analytical sloshing model. Transients and associated nonlinear modulation of the waves, beating, are important due to the low level of damping of the fluid motion. The measured parameters are the motion of the tank and the free surface elevation at three positions. Pictures and video are used to study local flow details and the dynamics of the flow. At excitation periods in the vicinity of the first natural period for the fluid motion in the tank, even small motion amplitudes lead to violent sloshing and impacts between the rising water surface and the tank roof. Impacts cause high pressures and forces. The effect of slamming in the tank is included by a local anlysis interacting with the nonlinear sloshing model. A Wagner based mthod is used to find the flow induced by slamming. Hydroelastic effects are ignored. The hypothesis that the kinetic and potential energy in the jet flow coused by the impact is dissipated when the jet flow hits the free surface, enables a rational calculation of the damping effect of impacts on the slishing flow. The Wagner approach requires a small angle between the impacting free surface and the tank roof. A correction by a similarity solution, or alternatively, by a generalization of Wagner's theory valid for larger angles is applied when this is not the case. Since anslytically based methods are used, fluid impact load predictions are robust. A coupled ship motion and sloshing system is studied both experimentally and theoretically. Two-dimensional experiments on a box-shaped ship section excited by regular beam sea have been conducted. The section contains two tanks and can only move in sway. Fluid motion inside the tanks has a large effect on the sway motion response of the section. Simulatons of a corresponding system are performed by assuming a mainly linear external flow and applying the nonlinear sloshing model. The linear external hydrodynamic loads due to body motion are expressed in terms of a convoltion integral representing the history of the fluid motion. detailed numerical study of how to accuratly and numerical sway motion of the ship section is reported. The calculated cooupled motion is sensitive to the damping of the sloshing motion in a certain frequency range where the coupled sloshing and ship motions couse resonant ship motions. A quasi-linear frequency domain analysis is used to explain this by introducing the sloshing loads as a frequency dependent spring.

Multidisciplinary Design Optimization of a Naval Surface Combatant

Abstract: Whereas shape optimal design has received considerable attention in many industrial contexts, the application of automatic optimization procedures to hydrodynamic ship design has not yet reached the same maturity. Nevertheless, numerical tools, combining together modern computational fluid dynamics and optimization methods, can aid in the ship design, enhancing the operational performances and reducing development and construction costs. This paper represents an attempt of applying a multidisciplinary design optimization (MDO) procedure to the enhancement of the performances of an existing ship. At the present stage the work involves modeling, development, and implementation of algorithms only for the hydrodynamic optimization. For a naval surface combatant, the David Taylor Model Basin (DTMB) model ship 5415, a three-objective functions optimization for a two-discipline design problem is devised and solved in the framework of the MDO approach. A simple decision maker is used to order the Pareto optimal solutions, and a gradient-based refinement is performed on the selected design.

Wind tunnel tests on flow characteristics of the KRISO 3,600 TEU containership and 300K VLCC double-deck ship models

Abstract: The flow characteristics in the stern and near-wake region of two ship models, the Korea Research Institute of Ships and Ocean Engineering (KRISO) 3,600 TEU containership (KCS) and the KRISO 300K very large crude oil carrier (VLCC) (KVLCC), were investigated experimentally. The double-deck ship models were installed in a subsonic wind tunnel. The freestream velocity was fixed at U o = 25 m/s, and the corresponding Reynolds numbers based on the model length (L pp ) were about 3.3 × 10 6 and 4.6 × 10 6 for the KCS and KVLCC models, respectively. The spatial distributions of mean velocity components and turbulence statistics, including turbulence intensities, Reynolds shear stresses, and turbulent kinetic energy, were measured using a hot-wire anemometer. For both ship models, the stern flow and near-wake show very complicated three-dimensional flow patterns. The longitudinal vortices formed in the stern region dominantly influence the flow structure in the near-wake region. In the region of main longitudinal vortices, the mean velocity deficits and all turbulence statistics have large values, compared with the surrounding flow. As the flow moves downstream, the turbulence statistics increase and have maximum values at the after-perpendicular (AP) plane and then decrease gradually due to the expansion of the shear layer. For the KVLCC model, the spatial distributions of mean velocity components and turbulence intensities behind the propeller plane clearly show hook-shaped contours. These experimental results, especially the turbulence statistics, can be used not only to understand the flows around modern practical hull forms but also to validate the computational fluid dynamics codes and turbulence models. The complete experimental data set is available on the website (http://www.postech.ac.kr/me/efml/data).

Numerical modeling of supercavitating propeller flows

Abstract: A three-dimensional boundary element method is used for the numerical modeling of supercavitating propellers subjected to nonaxisymmetric inflow. The method has been developed in the past for the prediction of unsteady sheet cavitation for conventional propellers. To allow for the treatment of supercavitating propellers, the method is extended to model the separated flow behind trailing edges with nonzero thickness. The convergence of the method is studied. Results from numerical validation, as well as comparisons of predictions with experimental measurements, are provided.

Comparison between numerical computations and experiments for seakeeping on ship models with forward speed

Abstract: The recent progress of computers and numerical algorithms enables us today to use the translating and pulsating Green function in panel methods for seakeeping calculations. Two different methods of calculations of this function, its derivatives and their integrations over panels or segments, are briefly presented and have been introduced into the seakeeping codes Aquaplus developed at Ecole Centrale de Nantes and Poseidon at Laboratoire d'Etudes Aerodynamiques (LEA), Centre d'Etudes Aerodynamiques et Thermiques (CEAT). Both codes interchange the Fourier and boundary integrals on panels or waterline segments, the last part being performed analytically. These methods have been used to compute flows around Wigley or Series 60 model ships. To check the numerical results, an experimental setup has been developed at the CEAT that measures forces and moments on a model in forced harmonic oscillations of pitch or heave. Tests have been performed in the recirculating water channel of Ecole Centrale de Nantes on two L = 1.2 m Series 60 models of C B = 0.6 and 0.8 block coefficients. Unsteady wave patterns have been recorded using a resistive wave probe. The experimental results are compared with the numerical ones.

An Experimental Investigation on Bow Water Shipping

Abstract: The water on deck caused on a restrained ship model without forward speed in head waves is studied experimentally by using a transient-test technique. A single watershipping event is induced by the wave packet, and the severity of the interaction is controlled by the wave-packet steepness. Three different bow geometries are considered. Two of them are analytical hull forms, and the last is the ESSO-Osaka tanker. The models are equipped with a transparent-material deck to study the flow-field evolution by image analysis. A vertical wall is placed at a certain distance from the forward perpendicular to mimic the presence of deck structures. Velocity of the shipped water along the deck, pressure field on the deck, and horizontal impact force on the wall are measured. The main fluid-dynamic aspects of the green-water phenomenon are highlighted. For the tested cases, water shipping starts always with the free surface exceeding the freeboard, plunging onto the deck, and forming complex cavities entrapping air inside. The geometry of the air cavity depends on the hull form and the wave steepness. Then the water propagates along the deck. In general, the water front is strongly three dimensional because of the water entering along the deck contour. The interaction of the shipped water with the vertical structure consists of impact, run up-run down cycle, and backward plunging of the water onto the deck, still wetted. The evolution of the pressure field follows that of the water front. Pressure peaks are associated with the impact against the vertical wall, and by the backward plunging of the water on the deck, at the end of the run up-run down cycle of the water. It is shown that both these stages can be of importance from the structural point of view.

A NURBS panel method for three-dimensional radiation and diffraction problems

Abstract: A B-spline panel method is developed to solve a boundary integral equation for three-dimensional potential flow problems. In particular, a nonuniform rational B-spline (NURBS) surface is introduced to accommodate a precise geometric description of a given body. For the unknown (potential) description over the surface, uniform B-spline basis functions are used. A collocation approach is adopted for numerical computations, and influence coefficients are evaluated in a robust manner over the surface defined by NURBS. Convergence characteristics of the present method have been investigated in the numerical experiments on the unbounded problem of a sphere, the radiation problem of a floating hemisphere, and the diffraction problem of a submerged spheroid. It is shown that the numerical results are in excellent agreement with analytical solutions in all cases considered. It is concluded that the NURBS panel method is superior to existing panel methods in geometric description of body surface, and convergent solutions can be achieved rapidly with a small number of panels.

A B-Spline Higher-Order Panel Method Applied to Two-Dimensional Lifting Problem

Abstract: A higher-order panel method based on B-spline representation for both the geometry and the solution is developed for the solution of the flow around two-dimensional lifting bodies. The influence functions due to the normal dipole and the source are separated into the singular and nonsingular parts; then the former is integrated analytically, whereas the latter is integrated using Gaussian quadrature. Through a desingularization process, the accuracy of the present method can be increased without limit to any order by selecting a proper numerical quadrature. A null pressure jump Kutta condition at the trailing edge is found to be effective in stabilizing the solution process and in predicting the correct solution. Numerical experiments indicate that the present method is robust and predicts the pressure distribution around lifting foils with far fewer panels than existing low-order panel methods.

Prediction of unsteady effective wake by a Euler solver/vortex-lattice coupled method

Abstract: A fully three-dimensional Euler solver, based on a finite volume approach, is developed and applied to the prediction of the unsteady effective wake for propellers subject to non-axisymmetric inflows. The Euler solver is coupled with an existing lifting-surface vortex-lattice method for the computation of unsteady propeller flows. The coupled method is validated against the uniform inflow case, in which ideally the uniform flow should be recovered as the effective wake. The predicted total velocity field correlates very well with that measured in the water tunnel experiment. Lastly, the unsteady effective wake predicted by the present method is compared with the steady effective wake predicted by the authors 1 previous steady method.

Influence of trapped air on the slamming of a ship

Abstract: We describe a theoretical approach to a distorted plate penetrating calm water surface as a flow model of the water impact in rough seas. Further simplifications are employed so that the structure of ship is modeled by a tandem mass and spring system and a sequence of circular hollows is used as a bottom shape of the body instead of the surface shape of short crested waves. The results show that the model-scale ship experiences much larger stress at the local structure because of the influence of trapped air. Some results for the full-scale ship show that the three-dimensional effect, that is, the shape of sea surface deformation, is dominant for the cushioning of the impact force, and the trapped air affects some of this effect according to the magnitude of β and the natural period of the local structure.

A Model for the Simulation of Steady Spilling Breaking Waves

Abstract: A numerical model for the simulation of two-dimensional spilling breaking waves is described. The model is derived from Cointe and Tulin's theory of steady breakers (Cointe & Tulin 1994), although some important changes have been introduced in order to obtain a stable algorithm when coupled with steady-state Reynolds averaged Navier-Stokes equations (RANSE) solvers. In particular, the shape of the breaker and its relation with the following wave height differ from the original model, and moreover, additional conditions for the tangential stress and the turbulent viscosity are proposed. The model has been implemented in a RANSE code, developed for the study of ship flows, through a modification in the free-surface boundary conditions below the breaker. This yields a simple but effective way to reproduce the breaker influence on the underlying flow. The algorithm was used for the simulation of the flow past a submerged hydrofoil. The numerical results are compared with the experimental data by Duncan (1983).

Conceptual hull design using a genetic algorithm

Abstract: A genetic algorithm is used to search the design variable space of a model ship hull for forms having specified values of various primary and secondary geometric parameters. A representative of each distinct cluster of the forms found is identified and presented as a candidate hull design having the required geometric characteristics. This could form the basis of a prototype conceptual design tool enabling the generation of hull forms satisfying specified requirements. The geometric parameters considered include the displacement, waterline length, waterline beam, and waterplane coefficient together with the locations of the centers of flotation and buoyancy. The hull surface is represented using the partial differential equation method, which permits a wide range of fair hull forms to be accessible using a relatively small number of design variables.

Hybrid Reliability Predictions of Single and Advanced Double-Hull Ship Structures

Abstract: A hybrid approach based on the combination of the Monte Carlo simulation (MCS) technique and the first-order reliability method (FORM) is developed to quantify the small probability of failure of a hull girder under longitudinal bending. The ultimate strength (ULTSTR) bending solver, developed by the U.S. Navy, is integrated with the hybrid probabilistic analysis framework. Different from the conventional approach, where the probability distribution of the hull capacity is preassumed, the probability distribution of the ultimate bending strength is determined by propagation of the uncertainties associated with hull geometric and material property parameters into ULTSTR via a small number of Monte Carlo simulations. The FORM-based solution module is then applied in the reduced random variables space to compute the probability of failure. Both the computational efficiency and numerical accuracy of the hybrid approach are demonstrated through its application to a Navy frigate, designated Ship A. The validated hybrid approach is then applied to a Navy combatant, designated Ship B, in both its single- and double-hull configurations. The reliability results reveal the benefit in using the advanced double-hull configuration, assuming its design load is the same design load encountered by the corresponding single hull during its service life.

Effect of fracture on crushing of ship structures

Abstract: This paper is concerned with loads and energy absorption during crushing of ship structures. Particular focus is on the effect of fracture of welds or parent material on the energy absorption of typical structural subassemblies of ships during deep collapse. The paper presents experiments and theories on the crushing response of typical strength elements. The theories are derived for an infinitely ductile material response and then consistently modified to include the effect of fracture. Theoretical formulas are compared with results of large-scale experiments performed at the Technical University of Denmark. The experimental series included 24 X and T aluminum and steel specimens scaled according to geometrical similarity and with a plate thickness varying between 2 and 20 mm. Theories and experiments demonstrate that the effect of fracture may be very significant for the loads and energy absorption in axial crushing of typical ship structural components. This effect of fracture has been neglected in previously published studies of bow crushing mechanics.

Wave Reduction by S-Catamaran at Supercritical Speeds

Abstract: ∗† The S-catamaran is a catamaran with twin hulls that are slightly curved in an S-form and arranged at a mean yaw angle but mirror symmetric to their common longitudinal centerplane. Theoretical studies (Chen & Sharma 1997, Chen 1997) show that by proper choice of the sectional area curve, separation, curvature, and yaw the waves generated by the component hulls cancel each other at a supercritical design speed, and consequently the wave wake and wave resistance can be substantially reduced. Theoretically, an almost complete elimination of the waves would be conceivable for an S-catamaran. To verify this theory, a model experiment with an S-catamaran was recently carried out in the VBD. The S-catamaran (Chen & Sharma 1997) was designed to have the same length and displacement as the VBD model series M601, which was developed and tested much earlier in the VBD by Heuser (1973). Despite certain deviations from the ideal form for practical reasons, the wave resistance of the new curved-yawed-hull catamaran with and without skeg was numerically found to be less than that of an equivalent straight-unyawed-hull catamaran by 50% and 30%, respectively. Now, the new design, albeit without skeg, has been validated by the model experiment. In comparison with a reference catamaran of the series M601, up to 28% wave-resistance reduction was achieved in the experiment, although not in the originally designed configuration but at a reduced yaw angle found by trial and error.

Maneuvering of Surface Vessels Using a Fuzzy Logic Controller

Abstract: A fuzzy logic controller is developed for maneuvering control of surface vessels. The fuzzy controller uses a vessel's heading, yaw rate, distance from a reference point, and the velocity of the vessel relative to the reference point as inputs to generate the control outputs. The control outputs include rudder angle, increase in propeller thrust, and lateral bow thrust. The design of the fuzzy controller is simple and does not require a mathematical modeling of the complicated nonlinear system. The core of the fuzzy controller is a set of fuzzy associative memory (FAM) rules that correlate each group of fuzzy input sets to a fuzzy output set. A FAM rule is a logical if-then type statement based on one's sense of realism, experience, and expert knowledge. The effectiveness and robustness of the fuzzy controller are demonstrated through the numerical time-domain simulations of path tracking and dynamic positioning of a Mariner-class hull with use of nonlinear maneuvering equations of motions.

Calculation of deep-water wash waves using a combined Rankine/Kelvin source method

Abstract: This paper presents a method for computation of far-field wash waves in deep water. The method combines a non-linear Rankine source method in an inner domain with a Kelvin source method for the far-field waves in an outer domain. Kelvin sources are distributed on a vertical matching wall, positioned at the outer edge of the inner domain. These sources are used to specify a boundary condition for the disturbance velocity potential on the matching wall. The boundary condition is used in the Rankine source solution of the inner domain. The size of the inner domain can be reduced in the transverse direction compared to a method using Rankine sources only, as the wave reflections at the edge of the inner domain are eliminated. Further, the far-field waves can be computed using the solution on the matching wall together with the the Kelvin source distribution. The verification of the present method includes a comparison for a single Kelvin point source and a comparison to a Rankine source method at intermediate distances for the Wigley hull and for a catamaran. A grid dependence study for the position, size and panel density on the matching wall is included for the Wigley hull. Computed and measured longitudinal wave cuts are compared for a catamaran both in the inner and the outer domain. Good agreement is obtained.

Efficient and Simplified Time Domain Simulation of Nonlinear Responses of Ships in Waves

Abstract: In this paper, a nonlinear time-domain strip theory is developed to predict nonlinear vertical ship motions and structural responses in severe waves. The effects of bottom impact, bow flare slamming, and green water on bending moments have been simulated. The flexible modes of the ship hull girder are accounted for by a Timoshenko beam theory. To validate the predicted responses, a model test was conducted for a ship with large bow flare and low bending rigidity, in both regular and irregular waves. The agreements between the calculated results and the model test are fairly good. The coupling effect between higher-order harmonic and the whipping components of vertical bending moments are verified by numerical calculations. Comparative studies with test and other theoretical results are also carried out for an S-175 containership with two kinds of bow flare forms. The causes of whipping and the variance in theoretical results are discussed. The good performance and high efficiency will make it possible to use the theory and its code for direct calculation of nonlinear bending moments in a long-term period and to develop a rule formula of design wave loads in the future.

Experimental investigation of the flow around a fast displacement ship hull model

Abstract: The aim of the present paper is the description of the flow around a fast displacement ship hull model with particular attention on the mechanisms responsible for the generation of streamwise vortical structures along the hull boundary layer. The collected data contribute to a surface-ship resistance and propulsion model-scale database for computational fluid dynamics validation as part of an international cooperative project between Istituto Nazionale per Studi ed Esperienze di Architettura Navale (INSEAN), Iowa Institute of Hydraulic Research (IIHR), and David Taylor Model Basin (DTMB) on experimental and computational fluid dynamics and uncertainty assessment for a combatant geometry (Stern et al 2000). A cross validation of the present results has been carried out through the ones obtained at DTMB and IIHR. Uncertainty assessment of the results has been evaluated following the AIAA Standard S-071-1995.

Cyclostationary Random Vibration of a Ship Propeller

Abstract: A special class of nonstationary processes with periodically varying statistics, called cyclostationary (CS), is investigated. This process is encountered in many engineering problems involving rotating machinery, such as turbines, propellers, helicopter rotors, and diesel engines. The objective of this paper is to show that a CS model of the flow-induced excitation on a propeller can describe the physics of the problem more accurately than a traditional stationary model. The mean values of the hydrodynamic forces are calculated using the vortex panel method and the vortex theory of propellers. Considering the randomness in the axial and the tangential components of the wake velocity, we calculate the covariance matrix of the forces. This analysis shows that the hydrodynamic forces acting on the propeller are CS processes. Then we calculate the standard deviation (root mean square [RMS]) of the blade response. We show that the CS model predicts the time-wise variation of the statistics of the excitation and the response (e.g., the RMS), including their peaks. A traditional stationary model cannot provide this information because it assumes constant statistics. Finally, a parametric analysis is performed to demonstrate the effects of the correlation structure of the velocity field behind a ship hull on the RMS of the blade deflection.

Investigation of Ship Structural Vibration and Underwater Radiation Noise

Abstract: Underwater radiation noise is a very important factor for most ships, such as fishing boats, warships, and so forth. The magnitude of its energy depends on the vibration of the hull in contact with water. The vibration of the hull caused by the power plant, while the vessel is cruising, is the dominant source of underwater radiation noise, which is the subject of our investigation. In this paper, the coupled finite element/boundary element method is used to investigate ship structural vibration and underwater radiation noise. The finite element method (FEM) is employed to analyze modes and vibration responses of an entire ship for different kinds of excitations in consideration of fluid-structure interaction. The boundary element method (BEM) is used to analyze the underwater radiation noise. A FEM model is first constructed by using 30 geometric parameters and five kinds of finite elements. Then, the reduced matrix method is used to eliminate the local modes in order to obtain the overall bending and torsional modes of the ship. Last, vibration displacements of the hull are treated as the velocity boundary condition of BEM to calculate underwater radiation noise. Numerical results show that (1) the calculated sound-pressure levels of underwater radiation noise are in a good agreement with experimentally measured results; (2) although the vibration isolator is used, the propulsion diesel engine is the dominant source of the underwater radiation noise among all machines in the engine room and the maximum sound-pressure levels increase as the sailing speed of the ship increases; (3) the underwater radiation noise of the ship with gearbox excitation is greater than that of the ship with diesel generator set excitation, which should be noticed by the ship designers during the design stage.

Reliability-Based Specification of Welding Distortion Tolerances for Stiffened Steel Panels

Abstract: The purpose of this study is to analytically derive a simple and reasonably accurate expression for the maximum allowable unfairness tolerance of longitudinally stiffened panels in ship structures. The stiffened panels under consideration are typical of those found in the deck, bottom, or side shell of longitudinally stiffened ships. They are assumed to be under still water and wave-induced loads, resulting in predominantly compressive loads. A plate-stiffener combination model is used as representative of the stiffened panel. Ultimate strength is determined based on a strut approach taking into account the effects of initial stiffener deflection and welding residual stresses in the stiffener. A series of stiffener reliability analyses relative to the ultimate failure strength of the stiffener for varying proportions of column slenderness ratios is carried out. Based on the computed results, a simple expression for predicting the maximum allowable unfairness tolerance of the stiffener is derived. The developed expression, expressed in terms of the stiffener slenderness ratio, can be useful for the assessment of fairness limits of plating with frames, or as a design guideline in ship structures during construction.

Limit loads for laterally loaded I-section beams with consideration of shear

Abstract: The paper examines an effect of shear forces on limit load for I-section beams carrying lateral loads. The problem is solved on the basis of a physical model, which enables one to take into account the effect of a resistance of beam flanges to the plastic shear strain in the web of the beam. The physical model for the evaluation of limit loads was verified using nonlinear finite element analysis. An engineering technique for the calculation of limit loads for ship hull beams subjected to large shear forces was developed using this model. As illustrative examples, the paper shows the application of the proposed technique to obtain closed-form solutions for the prediction of limit loads.