Considering usual computation efforts, single-frame models are used in the ultimate strength analyses of container ships. However, the case of large container ships is more complicated, and further investigations should be made. Generally speaking, the modeling extent, boundary conditions, lateral pressure, and initial deflection are key factors in the ultimate strength analysis. As a case study, we investigated their influences on ultimate strength under both vertical bending and torsion cases for a typical 10,000 twenty-foot equivalent unit (TEU) container ship, and also bending–torsion interactions. The results are representative and can be reference values for container ships of similar sizes.
Container ships are getting larger, and their structural strengths are major concerns for their owners and designers, especially after the 2013 crash of the MOL Comfort, an 8,110 twenty-foot equivalent unit (TEU) container ship. According to technical investigations (ClassNK, 2014), the local deformations of local structures such as double bottom are not negligible, which, however, has not been systematically considered previously. Thus, further investigations of large container ships to better understand their specific structural behaviors are meaningful.
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.
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.
Sinusoidal motion of a cylinder in viscous flow has been extensively studied in the past decades. Distinction of flow patterns exists between cylinders in cross-flow freedom restricted and freely vibrating conditions when experiencing oscillatory flow. In this paper, a series of numerical simulations are carried out by the in-house CFD code naoe- FOAM-SJTU, which is developed basing on the open source code OpenFOAM with overset grid capability. The diameter of the cylinder is 0.02m and the KC numbers varies from 3 to 12 corresponding to the attached vortices regime and the transverse street regime. Results of vortex evolution, flow regimes and hydrodynamic force coefficients are compared.
In actual production, offshore floating structures subject to waves, currents or winds will cause the platform to move periodically in the water. Then relatively oscillatory flow is generated between the riser and the water. In recent decades, researches of the sinusoidal motion of a cylinder in viscous fluid have been extensively studied by Bearman (1984, 1985), Sarpkaya (1986,1995) and Williamson (1985).
Williamson (1985) conducted a series of experiments to investigate development of vortices around a single cylinder in relative oscillatory flow. And several vortex regimes were identified within particular ranges of Keulegan-Carpenter (KC) Numbers: the attached vortices regime (0 Kozakiewicz et al., (1996) conducted experiments of a cylinder exposed to oscillatory flow for two Keulegan– Carpenter numbers, KC=10 and 20. Then numerical simulations of a cylinder freely vibrating in the cross-flow direction were carried out at the same KC numbers. Comparisons showed that the number of vortices generated over one oscillating cycle increased when the cylinder was freely vibrating in the cross-flow direction. The vortex shedding direction changed to the opposite side of the cylinder in the transverse street regime when KC=10.
Kozakiewicz et al., (1996) conducted experiments of a cylinder exposed to oscillatory flow for two Keulegan– Carpenter numbers, KC=10 and 20. Then numerical simulations of a cylinder freely vibrating in the cross-flow direction were carried out at the same KC numbers. Comparisons showed that the number of vortices generated over one oscillating cycle increased when the cylinder was freely vibrating in the cross-flow direction. The vortex shedding direction changed to the opposite side of the cylinder in the transverse street regime when KC=10.
With the development and utilization of ocean resources, maritime transportation is becoming increasingly busy. Stopping ability has great effect on the safety of ship maneuvering for those large ships. In this paper, naoe-FOAM-SJTU solver with 6DOF motion module with a hierarchy of bodies developed by Wan Decheng's research team in Shanghai Jiao Tong University is used to numerically investigate complex ship motion problem in stopping maneuver. The simulation starts from the steady state of self-propulsion and the propeller is controlled to a reverse speed to achieve the ship stopping condition. Detail information of the flow field during stopping maneuver are presented and analyzed to explain the stopping effect. The predicted results for the stopping maneuver in calm water are compared with the corresponding experimental data. The comparison is satisfactory and shows that the naoe-FOAM-SJTU solver is feasible for the direct simulation of stopping maneuvers. And the calculation results can provide suggestions when designing a ship or choosing stopping method.
In recent years, ships tend to become larger for reducing the cost of shipping and improving the transport efficiency. Since the large size worsens the maneuverability, accidents can easily occur in the crowded ports and channels. Therefore, it is significant to study the stopping ability of large ships to prevent the ship from collision and to ensure the safety of the ship sailing near the port.
Generally, reversing the propeller is still the most common operation when a large ship needs to brake to prevent collision. In the procedure of stopping manuever, the bow will turn left or right because of the side forces at the aft caused by reversing propeller. The existence of the transversal force caused by reversing propeller is determined by Chislett and Smitt (1972) through a ship model test. The stopping trajectory is shown in Fig. 1. Good stopping ability means minimum stopping distance, horizontal distance and yaw angle.
A physical wave maker system is established to generate the undular bores by superimposing successive solitary waves on the surge wave. The successive solitary waves are generated by the long-stroke piston wavemaker, and the surge wave is generated by a special valve-pump system. The valve-pump system is proved to produce the surge wave precisely and delicately. The superimposition appears that the solitary waves ride on the surge wave. The surge wave acts as the deeper water for the solitary wave to propagate, which affects the amplitude of solitary waves. In the present study, two successive solitary waves can be generated and superimposed on the surge wave, and the experimental data show good repeatability. A profile of the undular bores simulated by numerical computation is well reproduced in the basin.
Tsunami is one of the devastating natural hazards, which may cause thousands of fatalities and millions of property loss. Many numerical studies (Behrens and Dias, 2015) on the tsunami have frequently been reported in recent years, which greatly contribute to the understanding on the propagation mechanism of the tsunami and protection to the coastal structures. However, the computational cost and the complex topographies hinder the numerical simulation especially in the modelling of offshore and onshore processes. The physical model experiment is a good choice to study the mechanism of tsunamis in this region, such as the evolution and propagation with the sediment transport, and the inundation and the impact on the onshore structures.
The first question of the physical experiment for tsunami problem is how to generate the tsunami-like wave in the laboratory. As a simplification, the solitary wave generated by the wavemaker is widely used to mimic tsunami wave. Concerning the generation of solitary wave, Goring (1978) specified the movement of wavemaker plate to generate solitary waves, which was advanced by Malek-Mohammadi and Testik (2010). They took into account the unsteady nature of the solitary wave generation process. Xuan et al. (2013) studied the generation of two successive solitary waves of different amplitude. Although the piston-type wavemaker is doubted to satisfy the wavelengths and periods of the tsunami and the stable trough-led wave shape, many valuable achievements are still obtained by this simplification.
In modern computer aided ship design system, ship hull forms are mostly represented by the Non-Uniform Rational B-Spline surfaces and saved in IGES file format for data exchange between different CAD/CAM systems. Ship hull forms represented by NURBS surfaces have many good properties including visually fair and perfectly smooth compared with hull surfaces represented by discrete meshes. The local and global deformation of NURBS surfaces is simple by relocating the NURBS control points, changing the weight of control points, which possesses strong geometric explanation and is suited for the hydrodynamic optimization of ship hull forms. Therefore, a NURBS- based hull surface modification method is developed and integrated into an in-house solver OPTShip-SJTU for the hydrodynamic optimization of ship hull forms. The developed method is applied to the hydrodynamic optimization of Series 60 model. The optimization results demonstrate that the developed method is efficient and flexible for the deformation of hull surfaces and is suited for the optimization of real life ship.
With the development of computer technology and computational fluid dynamics, the CFD-based hydrodynamic optimization of ship hull forms has become a very powerful tool for the design of hull lines. It can be applied to the optimization of ship hull forms in terms of total resistance and sea-keeping performance in the preliminary stage. With the ship industry paying more and more attention to energy conservation and emission reduction such as energy efficiency design index proposed by IMO, the CFD-based hydrodynamic optimization of ship hull forms will play a more important role in the future. In general, the CFD-based hydrodynamic optimization of ship hull forms consists of four modules (Yang, 2016) including a hull surface representation and modification module, a hydrodynamic evaluation module, a surrogate module, and an optimization module. Among them, the hull surface modification module is used to modify the prototype hull forms to produce new hull forms in terms of hydrodynamic performance and design constraints. It is obvious that the ultimate optimization results are highly determined by the efficiency and quality of the hull surface modification module. The representation of hull model can be classified into two categories (Harries-Abt and Hochkirch, 2004): conventional modeling and parametric modeling. Parametric modeling defines hull model in a high level using form parameters. It can use few design variables to achieve the global deformation of ship hull forms and every form parameter has a specific meaning. But it is hard to achieve the local deformation of hull surfaces such as in the bow and stern. On the other hand, the typical example of conventional hull geometry modeling is ship hull forms represented by NURBS surfaces, which uses points to define curves and uses curves to define surfaces forming hierarchical structure. The NURBS surfaces have a simple and uniform mathematical expression to represent any complex surface as it is a mapping from two dimensional parameter space to surface in threedimensional space defined by B-spline basis function, knot vectors of parameters, NURBS control points, and weights of control points (Piegl and Tiller, 2012). It uses piecewise rational polynomials to avoid using high degree formula to fit complex surfaces and meet geometry constraints, which allows for the local deformation of hull surfaces by moving the control points and changing the weights. There are two hull surface modification method based on NURBS. One is directly moving the NURBS control points of hull surfaces. Kim (2009) selected 31 NURBS control points of Wigley hull as design variables to optimize the total drag coefficient at three given speed. Park and Choi (2013) used the movement of NURBS control points along x and z directions in the ship bow as design variables to optimize Series 60 model in terms of resistance. Wang (2015) used the direct NURBS-based hull surface modification method to generate a bulbous bow and a stern in Wigley hull. Then the hull with initial bulbous bow was optimized using radial basis function interpolation method (RBF). The direct NURBS-based hull surface modification method can achieve large and local deformation of hull surfaces. But it needs to define many design variables, which increases the computation cost. The other one is moving the NURBS control points of hull surfaces combined with other deforming methods including the RBF method, the Free Form Deformation Method and the shifting method. Noble and Clapworthy (1999) used the Free Form Deformation method (FFD) to move the control points of NURBS surfaces, which avoids the limitations of ordinary FFDs. Kim and Yang (2013) applied the shifting method and the RBF method to move the NURBS control points of Model 5279 hull to achieve the global and local deformation of ship hull forms. The optimal hull with a bulbous bow and a stern end bulb was obtained. Yang (2016) applied the NURBS-based modification technique to the optimization of a series of Joint High Speed Sealift with different bow configuration. Cheng et al (2018) employed the radial basis function interpolation method to the optimization of Series 60 model and reached a conclusion that the value of support radius of Wendland ψ3,1 basis function has a great impact on the deformation of hull surfaces. In order to avoid generating saddle-shaped surfaces during the modification of hull surfaces, the support radius should be twice the maximum length of the Delaunay triangulation edges. Pérez et al (2007) used the cubic B-spline curves to construct the body plan of bulbous bow subject to certain form parameters and the B-spline surfaces that fit these curves were constructed. Then the initial hull with bulbous bow generated above is optimized via CFD-based optimization method. Compared with the direct NURBS-based hull surface modification method, these methods have its merits. Only a small number of control points are required as design variables to achieve the flexible deformation of hull surfaces. The large deformation of hull surfaces can be achieved where various geometry constraints can be easily satisfied. The modified region can have a fairing connection with the original hull.
The overtaking interaction of the double solitary waves over a plane slope is studied experimentally. The slope of the plane beach is 1:20. For the cases of the double solitary waves of different amplitude ratios and different relative wave crest distances, the time series of the surface elevation and waterline movement are measured by wave gauges and recorded by high speed cameras respectively. Three categories of overtaking solitary wave interactions are reproduced in the wave flume. It is found that the maximum runup amplification coefficient of the double solitary waves is dependent of the relative distance between two initial peaks of the double solitary waves. Breaking of solitary waves plays an important role in damping the wave energy and then changing the maximum runup of the double solitary waves.
It has been a traditional subject to understand the runup of the long wave propagating over a constant depth region and then climbing up a sloping beach of constant slope because of importance of predicting tsunami runup on beaches. Based on the assumption that solitary waves can be used to model some characteristics of the propagation of tsunamis from offshore to beach, much work on physical and mathematical models of propagation and runup of a single solitary wave on the beach has been done, particularly at the W.M. Keck Lab of Hydraulic and Water Resources, California Institute of Technology. Hammack (1972) implemented experimentally the generation of tsunamis in a flume of uniform depth by an impulsively raised or lowered portion of the bottom. Goring (1978) proposed the solitary wave generation method for a wave flume with a piston type wavemaker and presented the experimental and numerical studies on the solitary wave propagating from a water layer of constant depth to the continental shelf. Synolakis (1987) proposed an analytical solution of the solitary wave propagation and runup to the shallow water equations and presented the characteristics of runup of breaking and non-breaking solitary waves. Li & Raichlen (2001) proposed a nonlinear solution to the nonlinear shallow water equations by using a hodograph transformation and reported an experimental study on runup of nonbreaking and breaking solitary wave. Madsen et al (2008) discussed, considering the effects of geophysical scales on wave propagations, the possibility of solitary wave generation in an ocean and presented the numerical results of disintegration of a long wave into an undular bore. Using the Boussinesq model, Baba et al.(2015) presented numerical simulations of the undular bore generation for 2011 Tohoku tsunami event, which shows that there are several solitary waves riding on a long leading wave front in Sendai shallow coastal region. Zhao et al (2016) studied numerically the generation of undular bores and soliton fission for the long waves propagating on the gentle continental shelves in both the East China Sea and the South China Sea.
Nowadays, the pile-supported structures have been applied in many offshore industries, especially in the wind energy field. As a new type of stationary pile-supported structure, high-rise pile-cap structures are attracting more attention. In the present study, a fully nonlinear solver based on Navier-Stokes equations is established for the investigation into a ten-pile cap structure. The wave loads are obtained by integrating pressure over the surface of each part of the structure. The effects of the interaction between the piles and the cap are investigated. In some cases that the cap is on the top, the wave loads on the piles are 30 percent larger than that without the cap. This is caused by the effect of impact. It means that the wave loads are underestimated significantly by simply using Morison equation. The relationships between the wave loads on piles and the gap between the cap and still, the wave height, and the wavelength are also discussed in detail.
In the recent decades, the pile-supported structures are commonly found in the costal and offshore environment, especially in the wind energy field for Offshore Wind Turbine (OWT). They are generally built by means of a group of piles in different arrangements.The long-term safety of the pile group-supported structures is still a major concern in coastal engineering. As a consequence, the prediction of wave loads on offshore pile group- supported structures is of great importance.
Nowadays, we have already had some typical types of pile-supported structures, such as monopile, suction pile, pile cap and so on (Ryu,et,al.,2012). As a new type of stationary pile group-supported structure, high-rise pile-cap structure is attracting more attention. It has lots of advantages, such as high stiffness, reasonable cost, easy mounting, etc. (Chen & Zhou.et al.,2016). Generally, for pile group-supported structures, the total wave force could be obtained by calculating each single cylinder pile with Morison equation (Morison, J.et al., 1950). However, the effect of different interference parameters is difficult to estimate, especially the pile group effect. Zdravkovich (2003) discussed the effects of the interference parameters on the overall drag forces for pile groups. Through some small-scale and large-scale experiments, Bonakdar et al (2012) and Bonakdar (2014) reviewed that the interference parameters such as the relative spacing between piles and the number of neighbouring piles could noticeably affect the wave loads on every single pile. Anew characteristic geometric scale called effective diameter is introduced by Qu et al (2017), and the total inline wave force on different complex pile-supported structures could be estimated by the unified empirical formulas.Moreover, the interaction between the cap and the piles should also be taken into consideration when the wave hits the cap. Therefore,the Morison Equation is no longer appropriate for estimating the inline wave force of the high-rise pile-cap structure. Meanwhile, the cap is much larger than the other parts of the structure and it will also lead to a larger lift force on the structure.
The impact loads of a disk slamming into both pure and aerated water are investigated experimentally. The impact velocity of the disk (diameter = 140 mm) is 2 m/s. The pressure distribution and evolution of impact loads are recorded with the help of three mini pressure gauges and a force sensor. To minimize the influence of bubbles on the calmness of free surface, the bubble generator is improved with quartz covered on its surface and a flow mass controller is utilized to control the flux of gas introduced into the water precisely. The experimental results show the central region of the disk endures higher impact loads and the decline of impact loads along the radius is not uniform in pure water. For impact tests in aerated water, not only a reduction of impact loads but also a longer duration of impact loading is observed.
The water entry problem has been studied for nearly a century and is still of interest in the fields of hydrodynamics and ocean engineering. The impact load is characterized by its high local pressure and short duration, which will significantly threaten the safety and integrity of a structure. For this reason, the prediction of impact loads is needed in the design of naval and offshore structures, ships even aerospace structures.
Commonly the fluid is assumed to be incompressible and inviscid for convenient to study. The seminal theoretical work of impact loads is conducted by Von Karman (1929) when investigating the landing of a seaplane. Based on the conservation of momentum, Von Karman derived a formula to predict the impact force. Wagner (1932) proposed a refined theory to predict the impact loads later. After that, some researchers improved Wagner's theory to predict the impact loads more accurately. For example, Zhao and Faltinsen (1993), Korobkin (2005) and Oliver (2007).
However, in some cases the compressibility of fluid plays an important role, especially in the initial stage of water entry. It is well known that the sound speed of the water, which is important to the prediction of impact loads, can drop quickly when the water is involved with gas. Moreover, the work of Lamarre and Melville (1991) shows that in the upper layer of the ocean, the void fraction of air bubbles can reach an order of 0.5%. Thus the evaluation of the influence of aeration effect on impact loads is necessary for the design of naval and ship design. Recently, this point is attracting the attention of researchers. Ma et al. (2016) and Elhimer (2017) respectively studied the aeration effect on impact loads of a plate and a cone. The reduction of impact force is verified by experimental results. To have a better understanding of the aeration effect on impact loads, a series of water entry experiments of a disk are conducted in this paper. Except for the relation between maximum impact loads and void fraction in water, special attention is given to the pressure distribution on the bottom surface of a disk.
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.
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.