ABSTRACT In this study, the analysis of 2DOF (2 Degree Of Freedom) motion and added resistance of a ship in regular head waves is carried out using RANS (Reynolds Averaged Navier-Stokes) approach. In order to improve the accuracy for large amplitude motions, the dynamic overset scheme is adopted. SUGGAR++ which is the dynamic overset scheme library is applied to WAVIS2, the in-house RANS code of KRISO (Korea Research Institute of Ships and Ocean Engineering). The grid convergence test is carried out using the present scheme before the analysis. The target hull form is KRISO VLCC tanker (KVLCC2) and 13 wave length conditions are applied. The added resistance coefficients, heave and pitch motion RAO computed by using Suggar++ are compared with those from the experiment. The present scheme shows the improved results comparing with the non-inertial reference frame based solver. It is confirmed that the dynamic overset scheme has the advantage for the large amplitude motion cases. INTRODUCTION With the introduction of the EEDI (Energy Efficiency Design Index) regulation of the IMO (International Maritime Organization) in 2015, there has been a steady increase in interest of the added resistance in waves in the industrial and research field. The added resistance in waves is related to the weather correction factor of EEDI index and is a component that affects the actual operating efficiency of the ship as a resistance component added by the waves when the vessel is operating at sea. The method for predicting the additional resistance in waves of a ship can be divided into an experimental method and a numerical analysis method. As the experimental studies, Storm-Tejsen et al. (1973) introduced an added resistance test for Series 60 hull form and then Journee (1992) carried out the Wigley hull test. In order to see the effect of the devices for reducing the added resistance, Hirota et al. (2004) and Kuroda et al. (2012) tested Ax-bow and Leadge-bow for slow speed ships and STEP for fast ships, respectively. Recently, Valanto and Hong (2015) published the test results for HSVA hull model and Park et al. (2015) introduced the uncertainty analysis for added resistance experiment of KVLCC2 ship using ITTC procedure. Numerical analysis can be divided into two approaches, one is potential theory based method and the other is CFD (Computational Fluid Dynamics) method using Navier-Stokes equations. Traditionally, potential based methods have been widely used to predict the effects of radiation wave. Recently, due to rapid development of computational techniques, there are many attempts to approach for predicting added resistance using CFD. As potential based approaches, Salvesen (1978) and Faltinsen et al. (1980) introduced the near-field method and Maruo (1960) and Gerritsma and Beukelman (1972) suggested the far-field method. Recently, Joncquez et al. (2008) computed the added resistance of Wigley, Series 60, and a bulk carrier using BEM (Boundary Element Method) and Kim and Kim (2011) proposed the appropriate criterion of using a time-domain Rankine panel method for the prediction of added resistance in irregular waves. Seo et al. (2013, 2014) conducted the comparative study of different numerical schemes and formulations for computing added resistance. As CFD approaches, Orihara and Miyata (2003) calculated the added resistance of SR108 container ship and mid speed tanker using WISDAM-X based on RANS approach. Carrica et al. (2007) and Castiglione et al. (2009) carried out 6DOF motion analysis of DTMB5415 and computed added resistance of DELFT 372 catamaran using the overset grid, respectively. Simonsen et al. (2013) introduced motion analysis of KCS in head sea condition and Seo and Park (2017) used OpenFOAM for motion analysis of KCS in regular head waves.

ABSTRACT An improved multi-phase WCSPH (Weakly Compressible Smoothed Particle Hydrodynamics) model has been established to numerically analyzethe2D sloshing phenomenon in a rectangular tank undergoing the forced rolling motion. The multiphase SPH model is applied taking care on combining the advantages of different SPH models. The application of artificial parameters is avoided in order to simulate physics phenomena realistically. The density re-initialization which is suitable for two-phase flow is applied to obtain smooth pressure field. A coupled dynamic solid boundary treatment (SBT) is adopted here to reduce numerical oscillations of pressure close to the boundary and to prevent particles' unphysical penetration into the solid boundary. Besides, in order to quantitatively study the wave elevation, a wave elevation measurement method based on linear interpolation is proposed. INTRODUCTION Sloshing is a typical fluid-structure coupling flow phenomenon with strong nonlinearity and randomness, which is involved in many fields such as marine engineering, aerospace engineering and nuclear engineering, and so on. For instance, in the process of liquefied natural gas and oil transportation, the sloshing of liquid mineral fuels has bad effect on the structure safety of LNG tankers and FPSO vessels. A lot of work has been done for studying the sloshing problem using different methods including the model test (Bass et al., 1985, Liu and Lin, 2009, Pistani and Thiagarajan, 2012), theoretical analysis (Faltinsen, 1978, Faltinsen and Timokha, 2009, Chen et al., 2015) and numerical calculation (Armenio and Rocca, 1996, Wu et al., 1998, Frandsen, 2004). SPH method which is an effective mesh-free method has been widely concerned for its natural advantages in solving sloshing problems. In SPH, physical objects are represented by a set of particles with individual masses. One particle interacts with nearby ones within a finite area called support domain, and the influence of the neighboring particles depends on the weight function or the smoothing function. The density and the acceleration of the particle are obtained by solving the governing equations descretized according to SPH algorithm (Liu et al., 2004). Much work has been done to improve the efficiency and accuracy for solving the sloshing problem with SPH method. Liu et al. (2012) proposed a kernel gradient correction and a coupled dynamic solid boundary treatment (SBT) which show great advantages in single phase sloshing problems. Chen et al. (2013) presented a boundary pressure correction to improve the measurement precision of the pressure field in boundary origin. Chowdhury and Sannasiraj (2014) compared the effects of different diffusive terms in sloshing problems. Bouscasse et al. (2014) studied the sloshing phenomenon in shallow water with experiment and numerical method of SPH. Delormeet al. (2009) studied the pressure field at the wall with SPH method.

ABSTRACT In this paper, an integrated numerical model is developed to investigate wave-seabed interactions around multiple submerged permeable breakwaters in an elasto-plastic seabed foundation with Bragg effects. In this model, the wave motion is governed by VARANS equation and the Biot's u-p approximation is used to govern soil-pore fluid interactions in porous medium. The advanced elasto-plastic constitutive model (PZIII) is used to reproduce the plastic soil behaviour in seabed foundation under long-term cyclic wave loading. Numerical results show that the wave motion can be largely changed due to Bragg effects. Under the strongest Bragg effect, the soil permeability and degree of saturation can significantly affect the liquefaction potential in seabed foundation. INTRODUCTION Breakwaters are the most commonly used offshore structures for protecting coast lines by reflecting incident waves back to offshore area. There are many forms of breakwaters, among them, the submerged permeable breakwaters have advantages like eco-friendly and wave energy dissipation efficiently because the breakwaters are under water surface that have less impact on ocean environment and porous medium allows it to reduce wave impact more efficiently. Bragg effect is defined as amplification of reflected waves by multiple breakwaters when the distance between two adjacent breakwaters are about half of incident waves length (Mei et al, 2005). Bragg effect is beneficial for breakwaters to protect the coast line as more waves will be reflected, on the other hand, it will imply more chance for seabed foundation to be liquefied as the wave height become larger in front of breakwaters. As reported, numerous damages of submerged breakwaters are caused by wave-induced seabed instability rather than from the construction deficiencies (Sumer, 2014). Therefore, it is essential to have a better understanding of wave induced seabed response around multiple breakwaters with Bragg effects. In the past few decades, considerable studies have been conducted to investigate the wave induced soil response and wave-structures-seabed interactions (Jeng, 2003). In the early stage, analytical solutions are the most widely used method to study the wave induced soil response around submerged permeable breakwater (Jeng, 1996), in which the breakwaters are simplified as lines without width and weight. Later, numerous numerical models are developed to study the wave-breakwater-seabed interactions. These models normally consist of two sub-models: fluid model for fluid-structure interactions and soil model for dynamic soil response. The fluid-structure interactions have been intensively studied previously (Lin and Liu, 1998; Liu et al ., 1999; Hsu et al ., 2002). By integrating fluid and soil models, the integrated numerical models can simulate the more realistic engineering problems as the e_ect of structure shape, porous properties of structures and the gravity of structures can be included (Ye, 2012).

ABSTRACT Fuel consumption of ship has been of great interest due to the reinforced environmental regulations. Estimation of fuel consumption requires calculating the added resistance in actual irregular waves. However, most of relevant researches remain in regular wave since nonlinear interaction is hard to be included in numerical computation. In this paper, the added resistance in irregular head sea has been investigated using Reynolds-averaged Navier-Stokes based Computational Fluid Dynamics. The results were compared to those of the model tests conducted in Samsung Ship Model Basin. In the simulation, the irregular waves were generated by the linear superposition of a number of incoming wave components. Since the computation times are highly increased when a large number of waves are used, total time window was divided into a number of partitions, and irregular waves were continuously generated by overlapping the neighboring windows. Three degrees of freedom were considered for ship's motion: heave, pitch and surge. Motion responses from the computation show fairly good agreement to those from the model test. In addition, the simulation predicts the added resistance in the similar level of accuracy to the experiment. The stepwise analysis is made and key findings are discussed. INTRODUCTION Recently, due to regulations of Energy Efficiency Design Index (EEDI), energy conservation and reduction of CO2 emissions have emerged as a major interest for ship design. In general, a ship is designed for specific speed and draft based on her resistance and propulsion in calm sea. However, when a ship advances in a seaway, ship experiences added resistance defined as an increased resistance due to environmental influence relative to that in calm sea. Since the added resistance result in a speed loss to the sailing ship, the precise prediction of that is required to achieve the economical sailing of the vessel. Traditionally, added resistance in real sea has been treated by the spectral analysis employing the transfer function and sea spectrum, for example in the sea trial analysis (ISO, 2015). In this sense, when the transfer function of added resistance is estimated in regular waves, total amount in real sea is estimated by the moment of response spectra. Most of added resistance research has been focused on calculating transfer function accurately at regular waves. Theoretical and numerical method has been developed as like panel method (Jonquez et al., 2009; Kim and Kim, 2011; Söding et al., 2014; Pan et al., 2016), Reynolds- averaged Navier-Stokes (RANS) based Computational Fluid Dynamics (CFD) (Orihara and Miyata, 2003; Guo et al, 2012; Sadat-Hosseini et al., 2013; Hu et al, 2014; Yang and Kim, 2016) and empirical formula (Fujii and Takahashi, 1975; Faltinsen et al., 1980; Tsujimoto et al., 2008; ISO, 2015). Such methodology has shown their validity especially in moderate sea condition. However, their approach is inherently not complete since the concept of transfer function cannot reflect the wave-body interaction dependent on sea environment. For example, it has been reported from numerical and experimental studies that even the transfer function is dependent on the wave steepness of regular wave (Tsujimoto et al., 2008; Kashiwagi, 2013; Yasukawa et al., 2016; Kim et al., 2017; Lee et al., 2017). In addition, nonlinear interaction in irregular sea is hard to be defined by the linear superposition of responses at each regular wave. Although necessity of direct estimation is obvious, a few studies have been performed due to the limitation of basin facility and computation capacity. Kobayashi (2007) and Kuroda et al. (2016) tried to construct the time histories of added resistance in irregular sea by the approximation using response in regular wave. Despite of the partial satisfactory in mild sea, discrepancy is inevitable in harsh waves. Yasukawa et al. (2016) conducted model test in regular waves at two different wave heights as well as in irregular wave. Their results clearly demonstrate that spectral summation is not complete to estimate the added resistance in actual irregular sea appropriately as well as the transfer function is dependent on wave height. Dalzell (1974), Hirayama and Wang (1993) also showed that transfer function obtained in regular waves does not coincide with that indirectly estimated from irregular and transient waves. Consequently, added resistance needs to be verified in irregular wave directly and thus active study is consistently requested.

ABSTRACT The paper presents several key issues for the development of reliable, accurate and computationally efficient fully-Lagrangian computational methods for reproduction of hydroelastic FSI (Fluid-Structure Interaction) problems. The fully-Lagrangian FSI solvers correspond to projection-based MPS (Moving Particle Semi-implicit) and ISPH (Incompressible Smoothed Particle Hydrodynamics) fluid models coupled with Newtonian MPS/SPH or Hamiltonian MPS/SPH structure models. The mentioned particle-based FSI solvers have been validated through a set of benchmark tests. The key aspects for development of robust and reliable fully-Lagrangian mesh-free FSI solvers are discussed in terms of Newtonian/Hamiltonian structure models, fluid- structure coupling schemes and adaptivity corresponding to multiresolution FSI simulations. INTRODUCTION The interactions between fluids and elastic structures have been of substantial importance in ocean engineering (Liu et al., 2014). In specific, in line with advancement of marine technology for application of lightweight elastic materials, the hydroelasticity effects have become more significant, indicating the need for rigorous related studies. With respect to the advantageous contributions that Lagrangian mesh- free methods or particle methods, e.g. Smoothed Particle Hydrodynamics (SPH; Lucy, 1977; Gingold and Monaghan, 1977), bring about in fluid and structure dynamics (Gotoh and Khayyer, 2018), several studies have been conducted by utilizing these new generation computational methods for hydroelastic Fluid-Structure Interaction (FSI) simulations. In some studies, particle methods have been coupled with other numerical frameworks, e.g. coupled SPH-FEM (e.g. Fourey et al., 2017). On the other hand, integrated fully-Lagrangian mesh-free methods (e.g. Antoci et al., 2007; Oger et al., 2010) provide higher levels of computational flexibility as well as potentially more consistent imposition of fluid-structure interface boundary conditions. With respect to superiorities of semi-implicit projection-based particle methods, e.g. Moving Particle Semi-implicit (MPS; Koshizuka and Oka, 1996) and Incompressible SPH (ISPH; Shao and Lo, 2003) with respect to explicit ones, in providing higher order accuracy in pressure calculation and volume conservation (Gotoh and Khayyer, 2018), coupling between projection-based particle methods with MPS/SPHbased structure models would lead to robust and physically/mathematically consistent frameworks for reproduction of incompressible fluid-elastic structures interactions. In this regard, Hwang et al. (2014) coupled a projection-based MPS fluid model with a Lagrangian MPS structure model. Khayyer et al. (2017c) investigated key issues for development of a reliable FSI solver for simulation of hydroelastic slamming in the context of MPS method. Khayyer et al. (2018a) presented a comparative study on MPS-based and ISPH-based FSI solvers with Newtonian and Hamiltonian structure models. Khayyer et al. (2018b) developed a novel Enhanced ISPH-SPH FSI solver for simulation of incompressible fluid flow-elastic structure interaction. Khayyer et al. (2018c) presented a multi-resolution MPSbased FSI solver for accurate and efficient simulations of interactions between incompressible fluid flows with elastic structures. Falahaty et al. (2018a) incorporated stress point integration in projection-based FSI modelling in the framework of DPD (Dual Particle Dynamics).

ABSTRACT Wind turbine, an efficient way to sustainably generate electricity, of which the noise problem would affect the living environment adversely. This paper presents the results of the aerodynamic and aero-acoustic calculation of a vertical axis wind turbine. The IDDES technique and FW-H acoustic analogy are adopted to conduct all simulations. The results indicate that the combination of thickness and loading noise are the dominant noise sources at tonal peak frequency, and quadrupole noise has negligible influence. Rotational speed and receiver distance will significantly affect noise level. This work can be exploited to design quieter vertical axis wind turbines. INTRODUCTION In recent years, the demands of renewable energy have attracted more public attention. As a clean and sustainable renewable energy, offshore wind energy has been utilized by wind turbines to generate electricity. However, one offensive problem, noise pollution, would affect the living environment of nearby creatures. Especially in several offshore wind turbine farms, birds and other animals, have left for new habitats. Therefore, it is an important issue to simulate and evaluate wind turbine noise. According to the direction of rotation, wind turbines can be divided into two major categories: horizontal axis wind turbines (HAWT) and vertical axis wind turbines (VAWT) (Borg, Collu and Brennan, 2012). Since the wind turbines require further performance optimization to be competitive with other energy devices, the geometrical design, aerodynamic performance and optimal solutions are continuing to be investigated. Bae et al. (Y.H. Bae, M.H. Kim and H.C. Kim, 2017) studied a floating offshore wind turbine with broken mooring line. The power production and structural fatigue life were checked respectively, and some risk assessments were conducted. Rezaeiha et al. (Rezaeiha, Kalkman and Blocken, 2017) conducted researches on the effects of pitch angle on power performance and aerodynamics of a vertical axis wind turbine. A 6.6% increase in power coefficient could be achieved using a pitch angle of 2 degree at a tip speed ratio of 4 was shown in the results. MirHassani and Yarahmadi (MirHassani and Yarahmadi, 2017) investigated the wind farm layout optimization under uncertainty. A mixed integer quadratic optimization model is developed based on the interaction matrix for multi-turbine wake effects considering different hub height wind turbines. Compared to the conventional HAWTs, the VAWTs show many superiorities, including universal wind exposure, relatively simple blade structure, lower maintenance costs and lesser aerodynamic noise (Tjiu, Marnoto, Mat, Ruslan and Sopian, 2015).Although the noise generated by VAWT is lesser than that caused by HAWT, VAWT's noise is not negligible. Noise generated by operating wind turbines can be divided into mechanical noise and aerodynamic noise. Mechanical noise is generated by different machinery parts. Aerodynamic noise is produced from the moving blades and is mainly associated with the interaction of turbulence with the blade surface (Ghasemian, Ashrafi, and Sedaghat, 2017). Mechanical noise can be decreased by some engineering methods, while the reduction of aerodynamic noise is still a problem.

ABSTRACT The vortex-induced motions (VIM) of semi-submersibles depends on the shape of the submerged structure, small changes in column geometries may have a significant impact on the VIM response of the platform. The study on the effect of column rounded ratios on the VIM of semi-submersible platforms has important guiding significance for the design of platforms. In the present study, numerical simulations of the VIM characteristics of a semi-submersible platform with different column comer radius at 45° current heading have been carried out by naoe-FOAM-SJTU. SST-DDES was used for modeling the massively separated turbulent flows. The displacements and hydrodynamic forces information in three DOFs including sway, surge and yaw motion are obtained. Based on the advantages of CFD method, the mechanism of the influence of the column rounded comer on the VIM of the semi-submersible platform is studied. Vortex shedding patterns and other flow field information are given for further analysis. INTRODUCTION As the increase of the depth of offshore oil and gas exploitation, semi-submersible platforms have been widely used in the field of ocean engineering because of their strong loading capacity. The heave motion performance is improved by increasing the platform draft. However, the Vortex-Induced Motions (VIM) response increases significantly because the increase of the effective excitation length of columns will lead to higher fluctuating pressure caused by the vortex shedding. As a kind of multi-column floating platform, the interaction of vortex shedding between the multiple columns makes the VIMs of the semi-submersible platforms more complex than those of the single-column floating platforms such as Spars. The VIM not only affect the operation of platforms, but also severely reduce the fatigue lives of risers and mooring systems. (Volnei et al , 2009) Experimental works on the VIM of semi-submersible platforms were carried out in recent years. (Waals et al , 2007; Magee et al , 2011; Goncalves et al , 2012; Liu et al , 2016). With the deep development of studies, many researchers transferred focus on the geometric parameters of the platforms effect on the VIM of the semi-submersible platforms. Studies have shown that the VIM response depends on the shape of the submerged structure and the columns have the most significant impact. (Gonçalves et al , 2015) At present, most of the columns of the semisubmersible platforms are square section with corner rounding, but the studies on the effect of the column rounded corner on the response of the platform are still very few. The study on the effect of column rounded corner on the VIMs of semi-submersibles platform has important guiding significance for the design of platforms.

Abstract This paper focuses on an innovative modeling approach of a floating body in a continuous wave current. This approach is Lagrange-based providing finite particles, which represent discrete hydrodynamic properties in contrast to continuous hydrodynamics. It is implemented into a computationally efficient numerical multi-body simulation program, which is flexible in geometry and dynamic load spectrum. It features a particle-based liquid model and provides time accurate hydrodynamic pressure results in three dimensions and 6 Degrees-of- Freedom. Based on the results of dam break tests and sloshing analysis, unsteady impact loads are computed for a floating body in a closed-loop simulation. The wave current is modeled as a continuous flux of particles with velocity-based boundary conditions at the inlet and outlet. Thereby, the floating body geometry is flexible in terms of wall friction, elasticity laws and damping coefficients. A special focus is set on the wave interaction characteristics and the resulting acceleration of the floating body due to the hydrodynamic forces. Introduction Dynamic loading of floating bodies is one of the key issues for an improved efficiency in offshore installations and ship operations. Especially the understanding of the hydrodynamic load distribution over the floating body surface in a wave current and the peak pressures during that interaction are of importance. A scientific approach to the floating body dynamics consists of a step-by-step analysis of the physical phenomena involved. First, the flow pattern around a fixed reference body needs to be understood before a floating body is envisaged. In addition to that, in that early step a submerged body undergoing pure hydrodynamic forces is considered, which does not consider buoyancy effects of the real floating body. For that case Arnold et al. (2015) have proposed an experimental setup to analyze submerged body dynamics with the help of a submerged pendulum.

Abstract Motivated by the development of new offshore platforms, a semi-analytical approach is proposed to solve the scattering of free surface waves around a two-dimensional platform supported by a pile array. The macroscale scattering is predicted based on microscale dynamics between piles. The flow field under platform is modelled by Reynolds equations with locally constant eddy viscosity. The open water region is solved by potential flow theory. Using homogenization theory, we obtain a generalized pressure equations in form of a Laplace equation with complex coefficients. By matching the normal velocity and pressure at the interfaces of open water regions and cylinder region, the macro-scale scattering (wave reflection and transmission) can be predicted based on micro-scale dynamics between piles efficiently. Introduction Very large scale floating structures (VLFSs) supported by small piles are a common form of practical structures in coastal and offshore engineering (Kashiwagi, 2000), e.g. offshore airports, large storage facilities and wave energy converters (Singh and Babarit, 2014). The scattering problems of non-viscous water waves by multiple bodies have been well resolved using different methods based on potential theory. In terms of the diffraction characteristics of single members, a general diffraction problem involving a number of separated bodies have been solved exactly (Kagemoto and Yue, 1986). Using multiple scattering technique, the full interaction phenomena between water waves and a multiple body array has been well solved using linear potential theory (Spring and Monkneyer, 1974; Simon, 1982; Linton and Evans, 1990). As the number of piles dramatically increases and more realistic flow conditions need to be considered, the aforementioned approaches become prohibitive and infeasible. Particularly, viscous effects may significantly affect the wake regions of each member in the pile array and dissipate the wave energy along the floating bodies. Combining the viscous flow model in the pile array region and non-viscous flow model outside seems to be a compromise approach. Even though, three dimensional viscous computational fluid dynamics is not suitable for practical purpose yet, when there are hundreds or even more piles in the array.

Abstract In an analysis of floating body's motion for ocean engineering, potential theory is known as a conservative method. However, This holds that viscous effects are neglected, which can deliver serious problems when predicting vessel's roll motions as well as complicated configurations such as the structures with moon pool, side-by-side moored vessels, and semi-submersible with deep draft need to be evaluated by more accurate methods other than potential theory in order to overcome inaccurate solutions which can be occurred by viscous effects. In this study, CFD simulation for a single vessel is performed as an initial study for motion analysis of side by side vessels. Introduction In this study, we performed a FVM numerical analysis with respect to a six-degree of freedom motion of a floating body by solving the non-steady state Navier-Stokes equation. For this, we employed the open source codes, OpenFOAM C++ libraries. Taking advantages of viscous damping and vortex shedding, three-dimensional numerical simulations for 6 DOF motions of a single vessel are carried out. Two wave directions, head sea and beam sea, are considered for the simulation with external linear stiffness to prevent drift away of the vessel. The results from developed analysis method were compared with linear potential theory and model test results for validation. Numerical Method Coordinate systems Euler's angle is generally used to describe the dynamic behavior of a solid body. The Euler's angle with a single rotational axis shows a "gimbal lock" where two axes are overlapped to be viewed as one single axis, and thus the rotation of two axes appears as the same rotation. In this study, to prevent gimbal lock and to avoid the effect of the order of the coordinate system rotation, quaternion was used as a technique of expressing the coordinate systems.

Abstract Submarine surfacing in currents is three-dimensional unsteady motion and includes complex coupling between force and movement. This paper uses computational fluid dynamics (CFD) to solve RANS equation coupled with six degrees of freedom solid body motion equations. The CFD code used is an in-house developed code. RANS equations are solved by finite difference method and PISO arithmetic. Level-set method is used to simulate the free surface. Computations were performed for the standard DARPA SUBOFF model. The structured dynamic overset grid is applied to the numerical simulation of submarine surfacing in regular currents and computation cases include surfacing in the static water, and steady, uniform flows with different speed (v=0.2, 0.4, 0.6 m/s) and surfacing with different value between gravity and buoyancy(n=2%, 4%). The asymmetric vortices in the process of submarine surfacing can be captured. It is shown that roll instability is caused by the destabilizing hydrodynamic rolling moment overcoming the static righting moment. Relations among maximum roll angle, surfacing velocity fluctuation and current parameters are concluded by comparison with variation trend of submarine motion attitude and velocity of surfacing in different current conditions. Simulation results confirm that current speed has a significant effect on surfacing velocity fluctuation. Maximum pitch angle decreases with the increase of current speed. Especially the law of pitch angle decreasing with the currents speed presents approximate linear relationship. Maximum pitch angle with current speed at 0.2m/s can reach to 12.89° while 3.07°at 0.6m/s. According to the above conclusions, maneuverability can be guided in the process of submarine surfacing in currents in order to avoid potential safety hazard. Introduction Safety is a very important consideration in the design and operation of submarines. To reduce risk at depth, submarines are usually equipped with emergency systems designed to rapidly blow the ballast tanks rising to free surface quickly. In the case of emergency maneuvers, this requires simulating extreme and unsteady motions. In the process of submarine surfacing, motion, force, moment and free surface have complex coupling with each other. Especially in currents with high speed, submarine maneuvering conditions become more unpredicted and roll angle will be negatively affected while surfacing.

Abstract This paper presents numerical results from three-dimensional Large Eddy Simulations (LES) of steady flow around a pipeline close to an uneven seabed with staggered gaps underneath it. It was found that even small gaps below a pipeline can lead to great changes in the hydrodynamics along the span, as well as the seabed shear stress. It is observed that the total forces can be calculated by independently considering the gap section and the embedded section; however, the spanwise variation in the configuration leads to much energetic vibrations in sectional forces. The instantaneous flow feature across the span, however, appears to be less affected by the gaps, where a coherent turbulent structure is found to develop in the upstream and break down above the pipeline. Introduction The research on pipeline hydrodynamics has been largely driven by the wide use of pipelines for transporting oil and gas products across ocean floors, which has been well documented in the monograph by Sumer & Fredsøe (1997) and papers by Bearman & Zdravkovich (1978), Zdravkovich (1985) and Lei, Cheng & Kavanagh (1999), among others. Most of the research was concerned with pipelines either embedded in or spanned above the seabed uniformly along the pipeline. Due to either uneven seabed or local scour below the pipeline, however pipeline/seabed contacts are unlikely uniform. It is expected that the non-uniformity of pipeline-seabed configuration will affect the hydrodynamic forces on the pipeline and the associated flow features around the pipeline. To date, little research has been reported with regard to this issue. This motivated the present study. Problem Definition As illustrated in Fig. 1, this paper investigates steady flow around a pipeline (represented by a circular cylinder) laid on an uneven seabed, mimicking a situation where the pipeline is partially embedded at certain locations, while free spanned at other locations.

Abstract For many years, fastest swimming speeds reported for dolphins are almost through observation. The principal objective of this study was to estimate the swimming speed of dolphin by simulating the thrust. Therefore, this paper primarily aimed to analyze the thrust characteristics of dolphins kick by numerical simulation, which used the FLUENT software solver and UDF dynamic mesh to simulate the dolphin during it is swimming. Three types of peak-to-peak tail motion amplitudes and frequencies are chosen to simulate. The simulation results of the thrust generated by dolphin fluke motion were compared with available data from the references. In conclusion, the dolphin can reach a very high speed and efficiency. Introduction For a long time, dolphins have always been considered as good swimmers with extremely high thrust efficiency and minimum resistance. There is still debate as to whether special properties of the skin itself contribute to the drag reduction or whether it is simply due to the maintaining of an attached turbulent boundary layer (Fish, 1993). Swimming encompasses the transfer of kinetic energy and momentum from the animal's propulsive movements to the water. High speeds allow increased foraging and active pursuit, but require large energy expenditures because thrust power is directly related to the cube of velocity. Low swimming speeds observed for cetaceans, while foraging and migrating (Lang, 1975; Fish, 1998a; Webb, 1975). A dolphin swimming at constant speed balances forces and moments acting on it by the principle of momentum conservation. The total thrust produced by the action of the caudal flukes balances the total resistance (i.e. drag) that the animal's body encounters moving forward (Fish and Rohr, 1999). Nearly 2000 swimming-speed measurements were obtained from recordings of both captive and free-ranging dolphins. In all cases, some form of motivation was provided for the dolphins to swim fast. In the wild, dolphins swim over a wide range of speeds.

Abstract Extensive studies have been conducted, both experimentally and numerically, in the last decades, however mostly on relatively low Reynolds numbers. The flow field around a sphere in an uniform flow is investigated by the numerical simulations using Large Eddy Simulation (LES) with a dynamic Smagorinsky type subgrid stress (SGS) model at the subcritical regimes (Re = 10 4 and 10 5 ) based on the freestream velocity and sphere diameter in the present paper. The CFD solver adopted in the simulations is Fluent and the fluid is water. Numerical results have verified the grid independence using three different grid quantities at Re=10 4 in the current paper. Compared with existing data, the LES can simulate accurately the flow field around a sphere at Re= 10 4 and 10 5 . The hydrodynamic coefficients and Strouhal numbers are consistent with the experimental results. The structures of wake and the separation angle are also satisfied with experimental results. Introduction The study about the flows over a sphere that have a complex nature depending on the Reynolds number is of great interest to computational fluid dynamics (CFD). The vertical structure around it, depending on the Reynolds number, has been known to show diverse flow characteristics such as the axi-symmetric flow, and irregular rotation of separation points, unsteady flows, periodic vortex shedding, etc. A sphere is considered as an idealized model of three-dimensional axisymmetric bluff bodies. For engineering applications, typical bluff bodies whose shape resembles a sphere are balloons, wings at the high attack, thrusts of ship, bombs, oil-storage tanks, etc. In fact, the study about turbulent flows that exhibit massive separation over a sphere can be essential to investigate these complex flow structures as well as to provide useful information for validating CFD models (e.g. LES model). Because a sphere is a basic body, which has a large potential for various applications, many researchers have studied the flow over a sphere.

Abstract Using numerical method to predict the ships' maneuverability is indispensable for the true understanding and evaluation of ship's performance during its operation and service period. This paper presents the numerical investigation results of modified DTMB5415 model's maneuvering motion. The numerical solver includes 6-DOF model to analysis the motion of the ship, the single phase level set method was applied to capture free surface flow around ship, the overset grid technique to simplify the rudder angle and ship rotating motion under the force of the rudder and a prescribed body-force propeller model is adopted instead of the real propeller. Solving URANS equation with above models, two types of maneuvers are simulated: steady turn and zigzag. Simulation of steady turn at 25°rudder deflection and zigzag 20/20 maneuvers for Fr=0.232 using constant RPM propulsion are compared with experimental time histories of velocity, roll, and trajectories. To validate the numerical simulation results, the maneuvers experiments of full appendages ship model with real propeller was carried out in maneuvering tank. Difference between CFD and experiments are mostly within 20% for both maneuvers. The numerical study matched well with experiment result which means the numerical method used in this paper is effective and feasible in analyzing the navigation attitude of ships in maneuvers motion. Introduction Ship maneuverability is the ability of a ship to keep or change its state of motion under the control action i.e. to keep the straight-ahead course with constant speed, or to change the speed, the course and/or the position of the ship, according to the intention of the helmsman. It relates to security and economy of ship closely. The traditional tools use empiric descriptions of the forces and moments on ship's hull and are generally based on regression analysis of captive maneuvering test data for a preferably wide range of ships. Kuniji Kose and Wojciech Misiag found empiric formulation can provide a convenient and efficient method to predict ship maneuverability. Due to the lack of resolution of hull details or application outside the range of the regression database, the prediction of the maneuverability may be unreliable. The free running model maneuvering test is another common method to predict ship maneuverability by measuring motion trail and relative parameters of ship model in the maneuvering basin. It will produce some inevitable error because of the scale effect.

Abstract Based on the 2-D numerical model averaged in vertical direction which was performed for the tidal current and pollutant diffusion in the Xiyang channel along the southwestern coast of the Yellow Sea, the influence of pollutant diffusion at the effluent discharge sites which are located at water depth 5 m, 7 m and 10 m (under the theory of datum) respectively during spring tide were calculated and analyzed. The results show that the sewage will soon be diluted after discharged into the sea. The envelope areas for both 500 and 200 dilutability contour isoline are small. And obviously, the envelope increment is inversely proportional to the water depth. Based on the model, the envelope areas under different discharge for both 500 and 200 dilutablity contour isoline are calculated, finding the two variables have a positive relationship in series of power.

Abstract In this paper, a modified MPS (Moving Particle Semi-Implicit) method is applied into 3D liquid sloshing to show its capability in modeling 3D sloshing flows. Violent sloshing flow in 3D rectangular is investigated. Furthermore, impact pressure value obtained by 2D and 3D simulation is compared, which indicates that 3D MPS simulation is able to produce smoother pressure field and thus makes better agreement with experiment than 2D MPS simulation. In addition, the snapshots of free surface deformation and velocity field are compared between experiment, 2D and 3D MPS simulation. Results show that the details of free surface deformation, such as the breaking region of the wave and the splash of liquid fluid can be capsized by 3D MPS method effectively.

Abstract A refined grid mathematic model is established to simulate astronomic and typhoon storm of Sea area around Shandong Province of China in this paper. Based on the Advanced Circulation Model (ADCIRC), an astronomical model is established by adding four main constituents to open boundary. The typhoon storm model is established based on typhoon wind field model. Three typical typhoon processes, namely No.8509, No.9216 and No.9711, are hindcasted numerically. The spatial and temporal distributions of storm surges are analyzed. After simulating the series of typhoons from 1960 to 2011 by using the verified ADCIRC model, the 100-year return values around Shandong Peninsula are estimated by a Poisson-Gumbel distribution. The return values will be adopted for storm surge disaster mitigation plan in the nearshore area of Shandong Province.

Abstract The three-dimensional effect of flow past bluff body poses difficult problems for simulation. The work here focuses on using high resolution TVD (total variation diminishing) numerical scheme to simulate the case of flow around a wall-mounted three-dimensional circular cylinder. The well validated TVD code for two-dimensional flow simulation is expanded to solve three-dimensional problems. This work gives a comparison between the results from the newly developed model and the results from previous numerical simulations as well as experiments. It is shown that present model poses reasonable results and predicts better velocity fields than previous model.

Abstract In this paper, a two-phase flow model is presented for simulating breaking wave impact on a rigid wall. The model is based on the Reynolds-averaged Navier-Stokes equations with the k-? turbulence model and developed under the framework of the open source CFD library - OpenFOAM. The governing equations are discretized by using the finite volume method and the air-water interface is captured by employing the volume of fluids (VOF) technique. With suitable predefined initial condition, three different types of wave impact on a vertical wall have been generated successfully. Following that, the pressure distribution and velocity field for air pocket impact are analyzed.