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Abstract: In this paper, the pressure on solid boundaries in 2D sloshing problem for forced rolling motion is numerically investigated using smoothed particle hydrodynamics (SPH) method. The improved SPH model applied in this paper combines some advanced correction algorithms. The Moving Least Squares (MLS) method is used in the density reinitialization to obtain smoother and more stable pressure field in numerical simulation. An improved coupled dynamic solid boundary treatment (SBT) algorithm is implemented, and great improvement in the pressure field near solid boundaries is achieved. To analyze the pressure of specified points on solid walls, a special pressure measurement method on solid walls based on the coupled dynamic SBT method is given. In this method, both the physical pressure of the boundary particle and a correction term based on the repulsive forces are taken into account. Numerical results obtained show good agreement with the experimental ones in the analysis of both the pattern of the flow field and the pressure on the solid boundary. Further conclusions based on these results are discussed. [Copyright &y& Elsevier]

Abstract: This paper describes the experimental validation of non-linear deterministic prediction tools for unidirectional seas, based on the use of two different non-linear wave models: (i) a so called “second order model enhanced”, which is analytic, and (ii) the numerical HOS wave model, which is run at order 3 in our simulations. Both prediction models use time traces of the surface elevation measured at one or several wave probes as an initial condition. Two effective data assimilation processes are used to optimize the model-reconstructed probes measurements, and thus ensure proper initial conditions for the forecast. The validation of our prediction tools on model-generated wave data has been reported in the previous paper (Blondel et al., 2010). Accurate predictions were reported compared to linear predictions. In the present study, the predictions are compared to different sets of laboratory measurements collected in a towing tank. The good agreement between the model-predicted surface elevations and the related experimental measurements indicates that our models can predict irregular long-crested seas more accurately than basic linear approaches with computational costs compatible with the short-term constraint. [Copyright &y& Elsevier]

The launch and recovery of equipment such as remotely operated vehicles (ROVs) is a critical task that defines the operability limits of many marine operations. This paper considers the analysis of control systems that are designed to maximize the operability limits for launch and recovery of a ROV from a small unmanned surface vessel (USV). We use numerical simulation for the analysis, where the method combines recent approaches for wave compensating dynamic positioning, active heave compensation, and positioning control of the ROV with multi-body dynamic simulation of the surface vessel and ROV, including hydrodynamic forces and dynamic interactions from wires that depend on the ROV depth and moonpool. The results show that the choice of control algorithms and their tuning parameters has a significant effect on the system's operability, and should be carefully designed and tuned to optimize the operability limits for any given sea state, weather and operational setup. The results show that numerical analysis with a system's simulation is an effective tool to verify operability and its sensitivity to various parameters for the given ROV recovery application. • The choice of ROV control system, unmanned surface vessel dynamic positioning and active heave compensation control algorithms and their tuning parameters has a significant effect on the system's operability. • Control systems should be carefully designed and tuned to optimize the operability limits for any given sea state, weather and operational setup. • A numerical simulation model for ROV recovery in the moonpool of a small USV is presented, validated and used in analyzing operability. • Numerical system simulation that integrates all physical elements and control systems is an effective method to verify operability from a systems perspective. [ABSTRACT FROM AUTHOR]

This paper describes numerical simulations of the vortex-induced vibrations (VIVs) of a long flexible riser in a step current. We consider the model vertical riser tested at the Delta Flume. The simulation is carried out by our in-house computational fluid dynamics (CFD) solver viv-FOAM-SJTU, which was coupled with the strip method and developed in OpenFOAM platform. The vibration modes in both in-line (IL) and cross-flow (CF) directions are accurately predicted. The numerically predicted maximum mean IL displacement and its location differed marginally from the experimental results. The good agreement between the numerical and experimental results proved that this solver was reliable for predicting the VIV response. A large number of numerical tests were then carried out to study the effects of various parameters on VIV responses further. Three main parameters are considered in this study: current velocity, top tension and mass ratio. The intrinsic relationship between the natural frequency and oscillating frequency was analyzed to explain the occurrence of the dominant mode. Based on the numerical results, the regular characteristics of the VIV response with the reduced velocity were pointed out. The curvatures and the maximum mean offset values were proportional to the squares of the reduced velocities. [ABSTRACT FROM AUTHOR]

The propulsion machinery system is one of the major concerns in the design of ships that operate in ice-covered waters. Numerical models and simulations are important tools for a better understanding of the system subjected to the transient ice-related loads. Design loads and design criteria applied in the typical numerical simulations are described by classification societies. However, the rules do not cover the level and type of discretization of the propulsion machinery system is not covered. Therefore, in this paper we study the sensitivity of the system response to: 1) finite-lump vs. finite-mode propeller shaft sub-models; 2) the number of propeller shaft lumps and modes; 3) simplified vs. complex sub-model of the diesel engine. The propulsion machinery system is modeled using the bond graph method. Dynamic torque and angular velocity responses of all elements of the propulsion machinery system are simulated. Simulation results show that the selection of sub-model discretization and fidelity are important parameters for the accuracy of the dynamic response and natural frequencies. Furthermore, energy flow and computational time are used as criteria to identify the most suitable discretization and fidelity of the sub-models in question. [ABSTRACT FROM AUTHOR]

Deepwater tumbler platform (DTP) is a novel semisubmersible concept in offshore industry, which utilizes double tier pontoons to improve heave performance so as to support dry tree systems, and to enable the platform stable unconditionally by installing ballast in lower tier pontoons (LTPs). However, the effectiveness of double tier pontoons on reducing heave motions has not been extensively studied, and additionally, the weight of LTPs brings in extra structural loads on the connections between the upper tier pontoons and LTPs, which make this connection the most critical one in the design of DTP. Therefore, the heave performance and the loads of the connections are the two main concerns in developing the DTP concept. In this paper, the hydrodynamic behavior of DTP and the loads of the connections are analyzed by numerical simulations as well as model tests. Good agreement of the numerical results with the experimental measurements was found and the results of both prove that the design of double tier pontoons greatly improves the heave performance. Besides, considerable amount of forces and bending moments by the motions of LTP were exerted on the connections. Finally, through a sensitivity study on the LTP depth, the relationship among LTP depth, heave performance and connection loads were discovered. This study illustrates the use of double tier pontoons in improving heave performance and provides important design guidance on the DTP design. [ABSTRACT FROM AUTHOR]

The hydrodynamics of the waterjet propelled ships represents a challenging problem due to the complexity of the waterjet system itself and the waterjet/hull interaction. In this paper, the self-propulsion computations of a trimaran were performed by solving the unsteady RANS equations. The waterjet was simulated directly with multi-reference frame model to improve the precision of the numerical simulations. Mesh independence studies of the hull and waterjet were carried out individually. Numerical simulations with/without the waterjet were performed in a speed range of 0.196 < Fr < 0.707. Comparisons between numerical predictions and experimental data showed the numerical method could accurately predict the self-propulsion performance. A numerical tool was developed to obtain the capture area based on tracking the streamlines upstream, which was practical and greatly simplified the post-processing. According to the ITTC procedures and guidelines, the overall efficiency and its components were analyzed in detail. The results showed that the resistance increment was always positive while the waterjet thrust deduction fraction was negative. The overall efficiency of the waterjet was mainly affected by the ideal jet efficiency and the pump efficiency. The analysis of the waterjet powering characteristics can better understand the energy conversion and the interaction between the ship hull and waterjet. • In this paper, numerical simulations and analysis on the self-propulsion performance of a waterjet propelled trimaran were conducted based on a RANS solver. • The pump effect of the waterjet system was simulated directly by using the MRF model to improve the accuracy of prediction. • The validity of the numerical method was proven by comparing the numerical results with the experimental results. • A numerical tool was developed to obtain the capture area based on tracking the streamlines upstream. • The analysis of the waterjet powering characteristics can better understand the energy conversion and the interaction between the ship hull and waterjet. [ABSTRACT FROM AUTHOR]

This paper presents the results of lab-scale experiments with accompanying 3D numerical modeling to parametrically investigate the lateral behavior of finned monopiles under directional loading. A series of scaled tests using monopiles and 2-, 3- and 4-finned piles were conducted. The experiments showed that the lateral load efficiency (i.e. improvement in load capacity at specified deflection) of the finned piles varied from 15% to 98% depending on the number of fins and their orientation to loading direction. The fin-load orientation was quantified by the change in effective bearing area of the pile. The pile head deflection was reduced by more than 65% for all finned piles of different fin orientations and loading directions, except for one two-finned section whose behavior was closer to that of a monopile. Test results also showed that the length of a monopile could be reduced by 30–40% by using fins for the for the same load capacity at a service limit state. The change in effective bearing area influenced the load-deflection behaviors of 2-finned piles more significantly than those of the 3- and 4-finned piles. The 3D numerical simulations of load-deflection agreed well with the experimental findings, thus validating the physical model. The numerical analyses also indicated a limiting fin width, W f and fin length, L f as W f / D p = 1 and L f / L p = 0.45 (where D p and L p are diameter and embedded length of pile, respectively) can optimize performance over a simple monopile. • New findings for load deflection behavior of 2-, 3- and 4-finned piles. • Considers effects of changing direction of loading. • Useful findings for investigations of offshore wind turbine foundations that use finned piles. • Confirms the findings of several previous laboratory and numeric studies of finned piles. [ABSTRACT FROM AUTHOR]

This paper deals with transom stern flow phenomena around the Froude number where the transom clears the water. Experimental and numerical results for a high-speed hull exhibit a pronounced peak in the resistance coefficient at the clearance speed. This is associated with a maximum in the non-dimensional transom submergence, a minimum in the pressure coefficient at the transom edge and a maximum in the curvature of the flow leaving the edge. Just before clearance, the sum of the hydrodynamic and hydrostatic pressures on the transom yields a force pointing forwards, thereby reducing resistance. This pushing force diminishes abruptly as the transom becomes dry. The total pressure resistance of the rest of the hull increases rapidly just before clearance, mostly due to the increase in hydrostatic pressure resistance, caused by the sudden increase in aft draft. It is the rapid loss of the transom pushing force, together with the rapid increase in pressure resistance of the rest of the hull just before clearance that creates the peak in the total resistance coefficient. [ABSTRACT FROM AUTHOR]

The parametric instabilities of spar platforms with different hull shapes are investigated in irregular waves. Parametric instability is a phenomenon that may cause excessive motion in offshore structures. It occurs when a system parameter varies with time, and the change meets certain conditions. For a spar platform, the parameter that causes parametric resonance is usually the metacentric height. In previous studies, the parametric instability of a spar was mainly analyzed in regular waves. However, real sea conditions are irregular. In addition, some accidents and experiments have shown that parametric resonance also occurs in irregular waves. This paper analyzes the parametric instability property of a spar platform being affected by multifrequency waves, which corresponds to realistic sea conditions. The stability is determined using both an analytic method and numerical simulation. The analytic investigation is carried out using Hill׳s equation. The heave amplitudes of the motion are simulated using the wave spectrum and response amplitude operator (RAO). Different hull shapes cause significant differences when analyzing the parametric instability due to irregular waves because of different RAOs. Stability diagrams containing the boundaries of the stability regions of different hull shapes are given based on the solution of Hill׳s equation. The numerical simulation can not only determine whether the spar is stable, but can also give the maximum pitch angle. The effects of the design parameters of a spar platform and the wave parameters are analyzed by giving the contours of the maximum pitch angle. The effects of time-varying displacement are also considered in this work. Stability diagrams and the contours of the maximum pitch angle are drawn to help with the prediction of the parametric instability. Some suggestions are also given to avoid the parametric pitch of a spar in design by selecting an appropriate design parameter and hull shape. [ABSTRACT FROM AUTHOR]

Abstract: Although many investigations on the drag reduction by microbubbles have been conducted, a general mechanism guiding the design of the drag reduction system has not been achieved yet. The drag reduction by microbubbles was investigated in detail with the Euler–Lagrange two-way coupling method in order to understand the drag reduction mechanism by microbubbles in this paper. The liquid velocity field was solved with direct numerical simulations (DNS), and the bubble trajectory was calculated by Newtonian motion equation. The mutual momentum transfer between gas and liquid phases was bridged by interphase forces. The computational results show that a low drag-reduction rate is obtained, the liquid-phase velocity is slightly increased in the region away from the channel wall (i.e., the channel side containing microbubbles), and the turbulence intensity and Reynolds shear stress of the liquid phase are changed along the whole channel height. The present analysis shows that the drag reduction depends on mutual interactions between microbubbles and the liquid turbulence with the help of interphase forces. [Copyright &y& Elsevier]

Abstract: The overall stability of tidal stream generators can only be assured if the maximum loads expected to arise during the lifetime of the structures are accounted for. These loads typically consist of a predicted maximum wave scenario and a maximum current scenario, which could potentially occur simultaneously. In this paper, combined wave–current offshore conditions are analyzed using a numerical procedure that is based on the unsteady Reynolds-averaged Navier–Stokes equations and incorporates the Volume of Fluid method and Fenton's wave theory. A procedure is introduced that allows for the simulation of non-linear waves in combination with a depth varying current under full consideration of wave–current interaction. The methodology is utilized to investigate the change in the overall loading on a structure subjected to combined wave–current conditions in comparison to a linear superposition of pure wave and pure current loads. It is shown that the peak loads on the structure are significantly higher for the combined load case than those computed for the superimposed individual load scenarios. Finally, the numerical wave channel is applied as part of the design process of a tidal stream generator prototype recently constructed near Jindo Island in South Korea. [Copyright &y& Elsevier]

Abstract: A three-dimensional (3D) numerical model is established to simulate the flow field around fishing plane nets in a current. The realizable k–ε turbulence model was chosen to describe the flow, and the governing equations are solved by using the finite volume method. In this model, the net is modeled using the porous media model. The unknown porous coefficients are determined from the hydrodynamic forces on the net and from the flow velocities and the attack angles using the least squares method. In order to validate the numerical model, the numerical results were compared with the data obtained from two physical model tests. The comparisons show that the numerical results are in good agreement with the experimental data. This numerical model is applicable for the simulation of the flow field around a fishing net. Using the proposed method, this paper presents the flow field around a plane net with different plane net inclination angles, plane net heights, spacing distances between two plane nets and plane net numbers. This study provides information about the flow field around the fishing net cage. [Copyright &y& Elsevier]

Abstract: In the last decade much research works has been carried out in order to improve the operation and speed of the underwater vehicles. In this study important supercavitation features including formation, evolution and loss of supercavitation and partial cavitation for an underwater moving body are both experimentally and numerically investigated. To achieve this goal the natural supercavitation experiment was conducted through a moving body and the trajectory of the body and the shape of supercavity profile were recorded by a high speed camera. For the numerical simulation of the underwater moving body the unsteady Reynolds averaged Navier–Stokes equations are coupled to a six-degree-of-freedom (6DOF) rigid body motion model. The numerical results are validated by comparing the predicted trajectory and velocity of the body with presented experimental data. A good agreement between the predicted and experimental values ensures the accuracy of the prepared numerical algorithm that used in the present study. The results reported in this paper illustrate that the above experimental method and numerical algorithm can be one of the most powerful and beneficial tools for the supercavitating underwater vehicles optimization and performance analysis. [Copyright &y& Elsevier]

Abstract: This paper presents a numerical simulation and simplified methods for estimating the residual strength of a damaged double bottom. A typical double-bottom structure from a shuttle tanker was modelled. The structure was idealised to a small degree. Imperfections were introduced to the whole structure according to design rules. The most severe situation of the fully loaded condition was investigated. Hull girder bending was considered. Damage was caused by a variety of indenters that were specially designed to obtain a desired damage profile. A total of 16 numerical-calculation cases were investigated using the explicit commercial code LS-DYNA 971. A simplified method was derived. This method includes elastic plus rigid-plastic analyses of a 3-span, single-stiffener model. Unlike Smith''s method, the present approach takes damaged stiffened panels into account in the residual strength assessment. The elastic and rigid-plastic analyses were combined to provide the load-end shortening curve for the damaged stiffeners. A simplified damage mechanism for the tripping of stiffeners is presented, as is a 3-span single stiffener model with proper boundary conditions. Both the analytical and single stiffener models were used to estimate the residual strength of a damaged double bottom. Both methods were validated with a non-linear, finite-element analysis (NLFEA) simulation carried out using LS-DYNA 971. [Copyright &y& Elsevier]

Abstract: Phase control may substantially increase the power absorption in point-absorber wave energy converters. This study deals with validation of dynamic models and latching control algorithms for an oscillating water column (OWC) inside a fixed vertical tube of small circular cross-section by small-scale testing. The paper describes experimental and numerical results for the system''s dynamics, using simple and practical latching control techniques that do not require the prediction of waves or wave forces, and which will be relevant to any type of point-absorbing devices. In the experimental set-up, the upper end of the tube was equipped with an outlet duct and a shut-off valve, which could be controlled to give a latching of the inner free surface movement. The pressure drop through the open valve is used as a simplified measure of the energy extraction. The control was realized by using the real-time measurement signals for the inner and outer surface displacement. A mathematical model of the system was established and applied in numerical simulation. In the case the OWC''s diameter is much smaller than the wavelength and the wave amplitude much smaller than the draft, the free surface movement inside the tube can be described as an oscillating weightless piston. For this hydrodynamic problem an analytical solution is known. In addition, the mathematical model includes the effects of viscous flow losses, the air compressibility inside the chamber and the pressure drop across the valve. Experimental results were used to calibrate some of the model parameters, and the total model was formulated as a coupled system of six non-linear, first-order differential equations. Time-domain integration was used to simulate the system in order to test the control strategies and compare with experimental results. [Copyright &y& Elsevier]

Abstract: The scale effect of form factor is investigated via a numerical approach in this paper, where the turbulent ship flow is computed by solving the steady and incompressible Reynolds-averaged Navier–Stokes and continuity equations. A wall function approach is employed to bridge the near-wall and outer turbulent flow region. The numerical scheme based on a finite-volume formulation is applied to discretize the coupled governing equation. For the sake of numerical stability, accuracy and economy, an identical grid is employed to compute ship flow at different Reynolds number, where the grid is optimized for the medium Reynolds number of the investigated range. Four surface ships and two sub-bodies with notably different geometrical characteristics are chosen as the investigated cases, where double-model flow without appendages is computed. The calculated total resistance coefficient shows a decreasing tendency against Reynolds number among all studied hulls. Similar to the calculated total resistance coefficient, the calculated friction resistance coefficient decreases with the Reynolds number and varies relatively little for a given Reynolds number among different hulls. The viscous pressure resistance coefficient is less insensitive to the Reynolds number but apparently depends on hull form. Compared with the form factor calculation based on empirical friction lines, the flat-plate friction prediction based on CFD approach clearly gives smaller Re-dependent form factor, which should more realistically reflect the scale effect of form factor. The form factor exhibits a near linear and increasing dependence on Reynolds number. The numerical results show that the dependence of rP on Reynolds number mainly governs the scale effect of form factor. [Copyright &y& Elsevier]