Scattering of Water Waves by Dual Symmetric Inclined Floating Porous Barriers Using the DBEM

The scattering of normally incident water waves by two surface-piercing inclined perforated barriers in water with a uniform finite depth is investigated within the framework of linear water wave theory.Considering that thin barriers are zero-thickness,a novel numerical method involving the the coupling of the dual boundary element method(DBEM)with damping layers is applied.In order to effectively damp out the reflected waves,two damping layers,instead of pseudoboundaries are implemented near the two side boundaries of the computational domain.Thus,the modified linearized free surface boundary conditions are formulated and used for solving both the ordinary boundary integral equation as well as the hypersingular boundary integral equation for degenerate boundaries.The newly developed numerical method is validated against analytical methods using the matched eigenfunction expansion method for the special case of two vertical barriers or the inclined angle to the vertical being zero.The influence of the length of the two damping layers has been discussed.Moreover,these findings are also validated against previous results for several cases.After validation,the numerical results for the reflection coefficient,transmission coefficient and dissipation coefficient are obtained by varying the inclination angle and porosity-effect parameter.The effects of both the inclination angle and the porosity on the amplitudes of wave forces acting on both the front and rear barriers are also investigated.It is found that the effect of the inclination angle mainly shifts the location of the extremal values of the reflection and the transmission coefficients.Additionally,a moderate value of the porosity-parameter is quite effective at dissipating wave energy and mitigating the wave loads on dual barriers.

Experimental Study of Air Layer Drag Reduction with Bottom Cavity for A Bulk Carrier Ship Model

Air lubrication by means of a bottom cavity is a promising method for ship drag reduction. The characteristics of the bottom cavity are sensitive to the flow field around the ship hull and the effect of drag reduction, especially the depth of the bottom cavity. In this study, a ship model experiment of a bulk carrier is conducted in a towing tank using the method of air layer drag reduction (ALDR) with different bottom cavity depths. The shape of the air layer is observed, and the changes in resistance are measured. The model experiments produce results of approximately 20% for the total drag reduction at the ship design speed for a 25-mm cavity continuously supplied with air at Cq = 0.224 in calm water, and the air layer covers the whole cavity when the air flow rate is suitable. In a regular head wave, the air layer is easily broken and reduces the drag reduction rate in short waves, particularly when λ/Lw1 is close to one;however, it still has a good drag reduction effect in the long waves.

Effect of Corrosion on the Ship Hull of a Double Hull Very Large Crude Oil Carrier

This study aims to examine and analyze the effect of corrosion wastage on the ship hull of a double hull very large crude oil carrier. To calculate the ultimate bending moment capacity, along with the neutral axis position at the limit state, section modulus, and moment of inertia, the incremental-iterative method is employed. This paper also considered the residual strength checking criteria of ship hull and the ultimate stress behaviors of the representative structural elements. Then, Paik's probabilistic corrosion, which employs two levels of corrosion rate and three different assumptions of coating life time, is applied to assess the corrosion effects. The calculation results obtained through relevant analyses are also presented.

Vibration Reduction Performance of Damping-Enhanced Water-Lubricated Bearing Using Fluid-Saturated Perforated Slabs

As the first link element for the transmission of shaft vibration to the pedestal and even to the hull,water-lubricated bearing plays a key role in suppressing vibration.Although the porous structure is considered as one of the main methods for improving the wideband vibration and noise reduction performance of materials in many industrial fields,the studies in the field of water-lubricated bearing remain insufficient.To enhance vibration reduction performance,a fluid-saturated perforated slab is designed in this study,and via the establishment of a fluid-solid coupled vibration model,the influence law and impact levels were analyzed and verified by simulation and experiments.The results obtained verified that the total vibration amplitude of damping-enhanced stern bearing in the vertical direction was smaller than that of the normal stern bearing,and the reduction amplitude of the characteristic frequency agreed with the optimal value at approximately 0.1 of the volume fraction of the liquid phase when the solid-fluid phase was rubber–water.Additionally,the increase in fluid fraction did not enhance the damping effect,instead,it unexpectedly reduced the natural frequency of the raw material significantly.This research indicates that the design of the fluid-saturated perforated slab is effective in reducing the transmission of the vibration amplitude from the shaft,and presents the best volume fraction of the liquid phase.

Numerical Study on Motion Responses of Two Parallel Ships During Underway Replenishment by the Semi-Analytical HOTP Method

Motion responses of two ships advancing parallel in waves with hydrodynamic interactions are investigated in this paper. Within the framework of the frequency-domain potential flow theory, a semi-analytical higher-order translating-pulsating source(HOTP) method is presented to solve the problems of coupled radiation and diffraction potential. The method employs nine-node bi-quadratic curvilinear elements to discretize the boundary integral equations(BIEs) constructed over the mean wetted surface of the two ship hulls. In order to eliminate the numerical oscillation, analytical quadrature formulas are derived and adopted to evaluate the integrals related to the Froudedependent part of the Green’s function along the horizontal direction in the BIEs. Based on the method, a numerical program is originally coded. Through the calculations of hydrodynamic responses of single ships, the numerical implementation is proved successful. Then the validated program is applied in the investigations on the hydrodynamic interactions of two identical Wigley Ⅲ hulls and the underway replenishment of a frigate and a supply ship in waves with and without stagger, respectively. The comparison between the present computed results with experimental data and numerical solutions of other methods shows that the semi-analytical HOTP method is of higher accuracy than the pulsating source Green’s function method with speed correction and better stability than the traditional HOTP method based on Gauss quadrature. In addition, for two ships with obviously different dimensions,the influence of hydrodynamic interactions on the smaller ship is found to be more noticeable than that on the larger ship, which leads to the differences between the motions of frigate with and without the presence of supply ship.

Wash Waves Generated by High Speed Displacement Ships

It is difficult to compute far-field waves in a relative large area by using one wave generation model when a large calculation domain is needed because of large dimensions of the waterway and long distance of the required computing points. Variation of waterway bathymetry and nonlinearity in the far field cannot be included in a ship fixed process either. A coupled method combining a wave generation model and wave propagation model is then used in this paper to simulate the wash waves generated by the passing ship. A NURBS-based higher order panel method is adopted as the stationary wave generation model; a wave spectrum method and Boussinesq-type equation wave model are used as the wave propagation model for the constant water depth condition and variable water depth condition, respectively. The waves calculated by the NURBS-based higher order panel method in the near field are used as the input for the wave spectrum method and the Boussinesq-type equation wave model to obtain the far-field waves. With this approach it is possible to simulate the ship wash waves including the effects of water depth and waterway bathymetry. Parts of the calculated results are validated experimentally, and the agreement is demonstrated. The effects of ship wash waves on the moored ship are discussed by using a diffraction theory method. The results indicate that the prediction of the ship induced waves by coupling models is feasible.

Homotopy Method for Inverse Design of the Bulbous Bow of A Container Ship

The homotopy method is utilized in the present inverse hull design problem to minimize the wave-making coefficient of a 1300 TEU container ship with a bulbous bow. Moreover, in order to improve the computational efficiency of the algorithm, a properly smooth function is employed to update the homotopy parameter during iteration. Numerical results show that the homotopy method has been successfully applied in the inverse design of the ship hull. This method has an advantage of high performance on convergence and it is credible and valuable for engineering practice.

Effect of initiation manners on the scattering characteristics of semi-preformed fragment warhead

The lethality of a semi-preformed fragment warhead is closely related to the expand velocity and spatial distribution of the fragments from ruptured metal casing. The topic of how to improve the utilization of charge of have been drawing great attention from researchers and designer in this filed. In present paper,in order to investigate the influence of charge initiation manners on the scattering characteristics of semi-preformed fragment warhead, the numerical simulations and experimental test are conducted.Firstly, the influence of grid density on numerical results is investigated, and a proper numerical model with relatively high accuracy and effectiveness is determined. Then. numerical simulations of three kinds of different initiation position of a semi-preformed fragment warhead are carried out. An experimental test of the explosion of a semi-preformed fragment warhead is carried out. By comparing and analyzing the numerical results and experimental data, it is found that the initiation manners have great influence on scattering characteristics of semi-preformed fragment warhead. The researcher work of this paper would provide an effective alternative method to optimize the design of warhead.

Inverse Optimal Control for Speed-varying Path Following of Marine Vessels with Actuator Dynamics

A controller which is locally optimal near the origin and globally inverse optimal for the nonlinear system is proposed for path following of over actuated marine crafts with actuator dynamics. The motivation is the existence of undesired signals sent to the actuators, which can result in bad behavior in path following. To attenuate the oscillation of the control signal and obtain smooth thrust outputs, the actuator dynamics are added into the ship maneuvering model. Instead of modifying the Line-of-Sight (LOS) guidance law, this proposed controller can easily adjust the vessel speed to minimize the large cross-track error caused by the high vessel speed when it is turning. Numerical simulations demonstrate the validity of this proposed controller.

Research on stress concentration factor of tube joints under pure bending and torsion loading

For fatigue strength design and evaluation as well as for multiaxial fatigue tests analysis of welded structures, the stress concentration factor(SCF) at the weld must be known. In the present study, two common models of multiaxial fatigue tests, tube-plate(TP) model and tube-tube(TT) model are selected as the research objects. SCFs of the two models with various geometric parameters under pure bending and pure torsion loading have been investigated by conducting a series of three-dimensional(3 D) finite element models based on linear elasticity theory. The effect of weld width(w), plate thickness(t), weld toe notch radius(r), and reinforcement angle(θ) on SCF is assessed. The quadratic parametric equations based on t/r are proposed to predict SCFs of the two models, which reveal satisfactory accuracy.

Scattering of Oblique Water Waves by Two Unequal Surface-Piercing Vertical Thin Plates with Stepped Bottom Topography

Based on linear water-wave theory, this study investigated the scattering of oblique incident water waves by two unequal surface-piercing thin vertical rigid plates with stepped bottom topography. By using the matched eigenfunction expansion method and a least square approach, the analytical solutions are sought for the established boundary value problem. The effects of the incidence angle, location of step, depth ratio of deep to shallow waters,and column width between two plates, on the reflection coefficients, the horizontal wave forces acting on the two plates, and the mean surface elevation between the two plates, are numerically examined under a variety of wave conditions. The results show that the existence of the stepped bottom between two plates considerably impacts the hydrodynamic performances of the present system. It is found that the effect of stepped bottom on the reflection coefficient of the present two-plate structure is evident only with waves of the low dimensionless frequency.Moreover, the influence of the step location on the hydrodynamic performance of the present two-plate structure is slight if the step is placed in between the two plates.

Structural and Acoustic Response of A Finite Stiffened Submarine Hull

After borrowing the idea of precise integration method, a precise integration transfer matrix method （PITMM） is proposed by modifying traditional transfer matrix method. The submarine hull can be modeled as joined conical- cylindrical-spherical shells. By considering the effect of the ring-stiffeners, the field transfer matrixes of shells of revolution are obtained accurately by PITMM. After assembling the field transfer matrixes into an entire matrix, the dynamic model is established to solve the dynamic responses of the joined shell. By describing the sound pressure in fluid by modified wave superposition method （MWSM） and collocating points along the meridian line of the joined shell, finally the structural and acoustic responses of a finite stiffened submarine hull can be predicted by coupled PITMM and MWSM. The effectiveness of the present method has been verified by comparing the structural and acoustic responses of the spherical shell with existing results. Furthermore, the effects of the model truncation, stiffness and thickness on the structural and acoustic responses of the submarine hull are studied.

Numerical study on roll dynamics of damaged ship in beam waves and calm water

Computational fluid dynamics is used to study the roll dynamics of a damaged ship in beam waves with various steepnesses and in calm water.The wave-making method,which combines the velocity-inlet boundary and momentum source with the fifth-order Stokes theory,is employed for wave generation.The SST k-ωturbulence model with a modification to the turbulent viscosity in Reynolds stress is adopted to prevent the over-production of turbulence in the numerical wave tank.The lateral drift restrained model with a combined dynamic mesh strategy is utilized to deal with the coupled heave-sway-roll motions of the ship.First,benchmarking tests are performed,including wave generation and roll response of the damaged ship in regular beam waves.Then,the effects of incident wave steepness on the roll response of the damaged ship are analyzed.For different wave steepnesses,the ship roll motion is dominated by the first-order harmonic component.The second-order component increases with the increase of wave steepness.Finally,the roll hydrodynamic coefficients for different parts of the damaged ship are investigated with different rolling parameters.The added moment of inertia for the whole damaged ship is mainly attributed to the external hull composition and changes slightly with the change of roll amplitude and frequency.The added moment of inertia for the compartments could be negative in particular cases.The damping coefficients of the whole damaged ship and external hull increase with the increase of roll amplitude and frequency,while that of the compartments appears complicated with the change of roll amplitude and frequency.

Dimpling Suppression by Chamfered Rotatable Cubic Punch in Reconfigurable Die Forming

The dimpling defects caused by conventional hemispherical punch in doubly curved sheet metal reconfigurable die forming process were considered.The rotatable cubic punch (RCP) was developed to suppress the dimpling defects more effectively and conveniently.The former punch contacts with the work-piece through a point-surface contact and the latter punch contacts with the work-piece through a surface-surface contact.A series of stamping experiments were carried out using three different punches (hemispherical punch,RCP,chamfered-RCP) with three different loads.Some finite element simulations about the stamping experiments were carried out.The dimple scales were evaluated through the dimple depths.The corresponding data were obtained by 3-D scanning and FE result analysis respectively.A 3-D plate forming machine was developed,in which chamfered-RCP was adopted.Plate forming experiments were carried out on this machine.The stamped samples show a clear basis for the performance of chamfered-RCP.The study provided a means to guide the design of punches for dimpling suppression used in reconfigurable die.

Study on Crack Closure Considering Welding Residual Stress Under Constant Amplitude Loading and Overloading

Welding residual stress in the engineering structure has a non-negligible influence on crack propagation,and crack closure is a significant factor affecting the crack propagation.Based on the elastoplastic finite element method and crack closure theory,we studied crack closure and residual compressive stress field of butt-welded plates under constant amplitude loading and overloading regarding the stress ratio,maximum load,overload ratio,and number of overloads.The results show that the welding residual tensile stress can decrease the crack closure because of a decrease in the residual compressive stress in the wake zone,but the effect is gradually reduced with increased stress ratio or maximum load.And the combined effect of welding residual tensile stress and overload can produce a stronger retardation effect on crack propagation.

Tool for Predicting the Ultimate Bending Moment of Ship and Ship-Shaped Hull Girders

The present paper presents a historical review associated with the research works on hull girder strength of ship and ship-shaped structures.Then,a new program is developed to determine the ultimate vertical bending moment of hull girder by applying direct method,stress distribution method,and progressive collapse analysis method.Six ships and ship-shaped structures used in the benchmark study of International Ship and Offshore Structures Congress(ISSC) in 2012 are adopted as examples.The calculation results by applying the developed program are analyzed and compared with the existing results.Finally,the roles of the developed program and its further development are discussed.

Theoretical and Numerical Approximation Methods for Predicting Bending Characteristics of Bimodulus Sandwich Structures

Sandwich materials are widely used in marine structures because of their excellent comprehensive properties.However,the solution of bimodulus is challenging.Therefore,the theoretical and numerical approximation methods for the analysis of load-bearing characteristics of bimodulus sandwich structures are put forward comprehensively in this paper.Based on the superposition principle,a theoretical method for calculating the neutral surface position of bimodulus sandwich plate is derived,and the corresponding bending control equation is obtained.The proposed theoretical approximation method can fully consider the sawtooth deformation between the plate and the core,as well as the sawtooth deformation inside the core at the tension–compression interface.Moreover,a finite element model is established for complex sandwich structures to analyze the influence of bimodulus.Numerical examples show that the theoretical approximation model proposed in this paper has higher calculation accuracy.

Structural and acoustic response of a finite stiffened conical shell

In this paper, a precise transfer matrix method is presented to calculate the struc- tural and acoustic responses of the conical shell. The governing equations of conical shells are written as a coupled set of first order differential equations. The field transfer matrix of the shell and non-homogenous term resulting from the external excitation are obtained by precise integra- tion method. After assembling the field transfer matrixes, the whole matrix describing dynamic behavior of the stiffened conical shell is obtained. Then the structural and acoustic responses of the shell are solved by obtaining unknown sound pressure coefficients. The natural frequencies of the shell are compared with the FEM results to test the validity. Furthermore, the effects of the semi-vertex angle, driving force directions and boundary conditions on the structural and acoustic responses are studied.

Complete Solution of Large Plastic Deformation of Square Plates Under Exponentially Decaying Pulse Loading

The rigid-plastic assumption has greatly simplified the theoretical analysis of dynamic plastic response of structures.Within this framework,a common tool is the modal technique using approximate independent yield criteria,which leads to upper-and lower-bound solutions,but usually with poor accuracy.In this paper,by utilizing the membrane factor method(MFM),the large-deflection dynamic plastic response of square plates subjected to exponentially decaying pulse loading is analyzed by taking both the transient response phase and the exact yield criterion into account.Based on the combination of saturation analysis(SA)and MFM,the complete solutions and regressive formulae of saturated deflection and saturated impulse are obtained.As the dynamic behavior of plates under rectangular pulse loading serves as a benchmark of pulse-equivalent techniques,the large plastic deformation of square plates under short-duration rectangular pulse is also analyzed in detail.Moreover,by comparing the SA results of pulse-loaded square plates with different boundary conditions,it is found that the saturated deflection and saturated impulse of the fully clamped and simply supported square plates both increase linearly with the pulse amplitude,and the slopes are approximately the same,so the conversion between the SA quantities of plates with different boundary conditions can be easily achieved.

Shallow-water sloshing motions in rectangular tank in general motions based on Boussinesq-type equations

Based on the highly accurate Boussinesq-type equations in terms of velocity potential, the shallow-water sloshing in a two-dimensional rectangular tank is studied. The rectangular tank in harmonic sway, heave and roll motions with small excitation amplitudes is considered. The total velocity potential is divided into two parts： the particular solution and the remaining part to be determined by the Boussinesq-type equations. The Stokes-Joukowski potential is adopted in the particular solution for the roll excitation motion. The comparisons of the numerical results indicate that the shallow-water sloshing motions in a rectangular tank can be predicted well based on the Boussinesq-type equations.