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Abstract This paper presents a grid-independent numerical methodology that couples the strip theory based discrete vortex method (SDVM) with the finite element method (FEM) to simulate the vortex-induced vibration (VIV) of a long flexible vertical riser. Based on the strip theory, a three-dimensional flow filed is approximately simulated by a series of computational 'flow strips'. A Lagrangian discrete vortex method is employed to numerically solve the unsteady vorticity transport equations of each 'flow strip'. The flexible riser is modelled as a tensioned Bernoulli-Euler beam with the dynamical equation solved by the finite element method in time domain. The two-dimensional DVM code is firstly validated for the VIV simulation of a rigid cylinder that was experimentally studied by Khalak and Williamson (1996). Referring to two typical experimental configurations of Lehn (2003) and Chaplin et al. (2005), the VIV of a long flexible riser immersed in a uniform and stepped incoming flow are numerically simulated, respectively. A good agreement was achieved through detailed comparisons between the present numerical prediction and the experimental data, including the structural in-line and cross-flow VIV response modes, root mean square amplitudes and the dominant frequency. The occurrence of standing and travelling wave responses, dual resonance between in-line and cross-flow motions and figure-eight trajectory are reported. The wake patterns corresponding to two response waves are also presented and investigated. Related to structural local vibration amplitude, two principle vortex patterns resembling the '2S' and '2P' modes are identified in the wake. The standing wave component of structural response determines the vortex shedding pattern. The travelling wave component affects the spanwise vortices shedding at different phases. Highlights • A SDVM-FEM coupled numerical methodology is developed to simulate the VIV of long flexible risers. • Simulation has been carried on two typical experimental configurations to validate the numerical model. • The occurrence of standing and travelling wave responses are reported. • The wake patterns corresponding to two response waves are presented and investigated. [ABSTRACT FROM AUTHOR]

Abstract Over the past seven years, the capsizing of considerable bulk carriers with nickel ores in the South China Sea has made shipping industry shocked. Although a number of countermeasures have been taken by ship owners, it doesn't seem to prevent the ship accidents from happening, possibly implying that we may not yet fully understand some fundamental aspects of the accidents. Clay lateritic nickel ore is characterized by liquefaction under the influence of waves or long-time engine vibrations during transportation. However, the existing methods based on the calculation of GM (metacentric height) values fail in explaining the capsizing of the bulk carriers during shipment. Experimental investigation into the accidents shows that the blending liquid (not pure water) after liquefied is highly viscous and can generate free surface. In contrast to the prevailing theories of ship stability, this free surface may create negative influence on ship's stability, and sometimes may result in a severe consequence of capsizing. Therefore, we speculate that the viscosity coefficient may play a critical role in ship's capsizing. Here we take advantage of numerical simulations to make a thorough analysis on the physical mechanisms of capsizing. The conclusion has been drawn that the collapse of the shear force which is mainly determined by the viscosity coefficient has led to a shift of the liquefied cargo when the inertial force of the liquefied cargo is greater than the shear stress, thus causing an abrupt loss of the stability. Finally, the increasing external moments result in capsizing and sinking. This finding may be profound because it reveals a substantial relationship between the capsizing of a ship and the viscosity of the liquefied cargo which has never been described in previous literature. Highlights • Liquefaction of the nickel ores during shipment has led to numerous capsizing and sinking. • The collapse of the shear stress force may play a critical role. • Numerical simulations have demonstrated that the sloshing of the cargo after liquefied increases the heeling moments. • The collapse of the shear stress force is significantly associated with the frequency and amplitude of the ship rolling. [ABSTRACT FROM AUTHOR]

Ji, Q. and Kun, L., 2018. Numerical simulation solutions for wind-induced vibration of ship rearview mirrors caused by airflow noise in shallow seas. In: Ashraf, M.A. and Chowdhury, A.J.K. (eds.), Coastal Ecosystem Responses to Human and Climatic Changes throughout Asia. The fluid–solid coupling method was used for numerical simulation of the problem of wind-induced vibration of an exterior rearview mirror caused by airflow. The finite element model was established. The fluid–solid coupling method was used to consider the process of interaction between fluid and solid. The dynamic characteristics of the rearview mirror under the coupling condition at different speeds were calculated. Experimental results show that when the vortex shedding frequency is close to the natural frequency of the rearview mirror structure, a strong vibration in the rearview mirror can result. This numerical simulation can simulate the actual situation. [ABSTRACT FROM AUTHOR]

There is plenty of energy contained in the ocean waves, and great efforts have been made over recent decades to make full use of these sustainable energy resources. As a novel device to utilize ocean wave energy, the authors propose in this paper a whipping energy converter, which utilizes a linear generator extracting power from ship’s whipping and springing (hull girder 2-node vibration) responses. The device is designed resonant to the hull girder 2-node vibration frequency. Numerical simulation of the responses of the whipping energy converter was carried out under irregular head sea condition, using time domain ship motion analysis by Rankine source method and subsequent three-dimensional whole ship FE analysis, and the main parameters of the whipping energy converter was studied. A 14,000 TEU large container ship was utilized in this study, because large container ship is prone to constant whipping and springing responses in real seas. As a result, it was found that the whipping energy converter can effectively generate power from the whipping and springing responses, and at the same time, the whipping energy converter functions as a dynamic damper. With the piston weight of 5-10 tons, the whipping vibration is visibly reduced, while if the piston weight is about 100 tons, the spectral peak is reduced to less than one-third. This implies that the whipping energy converter contributes to the prevention of fatigue damage due to whipping and springing, which is an imminent concern in the design of large container ships. It was also found that larger power generation can be expected as the piston weight is heavier, and the bow is the most effective location for this device. [ABSTRACT FROM AUTHOR]

This paper deals with the numerical simulation of low-Reynolds-number flow around a freely vibrating circular cylinder in two-degrees-of-freedom. The governing equations are written in a non-inertial system fixed to the moving cylinder and solved using finite difference method. The natural frequency of the cylinder is chosen to be constant, agreeing with the vortex-shedding frequency for a stationary cylinder at Reynolds number Re 0 . Systematic computations are carried out for Re 0  = 80, 100, 140 and 180 keeping the mass ratio and structural damping coefficient at m*  = 10 and ζ  = 0. The effect of Re 0 on the root-mean-square (rms) values of cylinder displacements and drag coefficients is analyzed. Plotting the data set belonging to different Re 0 values against U* St 0 makes comparison easier. Local extreme values are found in the rms of streamwise displacement and drag coefficient in the range U* St 0  = 0.4–0.65. In the vicinity of U* St 0  = 0.5 the rms of drag approaches zero and the phase angle between the x component of the motion and drag changes abruptly from 0° to 180°. The pressure drag coefficient seems to be responsible for the sudden change. The cylinder follows a distorted figure-eight path in most cases investigated and its orientation changes from clockwise to counterclockwise orbit at around U* St 0  = 0.5. [ABSTRACT FROM AUTHOR]

In the present study, an innovative method for estimating fatigue performance of risers under vortex-induced vibration (VIV) is proposed. Generally, fatigue performance is affected by the surrounding environment such as wind, wave, and current. It is well known that current is the most influential load among all for offshore risers. In structural safety aspect, strength and fatigue are the most important factors for riser design. In addition, fatigue design is affinitive to the VIV phenomenon. For the analysis of fatigue performance of riser, SHEAR7 numerical simulation code which is commonly used in offshore industry is applied. In order to identify the relation between current load and fatigue performance of riser, Fatigue damage versus Current index (F-C) diagram has been proposed. F-C diagram may cover change of current profiles and help predict the fatigue damage under VIV. In case of current profile, a total of sixty cases of current scenarios are considered based on six representative sea-states. The obtained results from this study will be a useful guideline to predict the effect of current on the fatigue performance of riser. [ABSTRACT FROM AUTHOR]

The effects of surface roughness on the vortex-induced vibration (VIV) performances of a circular cylinder were studied numerically. The VIV response amplitude, response frequency, vortex force, vortex phase and vortex shedding flow pattern with different degrees of surface roughness were compared. The numerical results show that the VIV response amplitude decreases with increased surface roughness. For a smooth cylinder and a cylinder with small surface roughness, the VIV response could be divided into three branches: initial branch, upper branch and lower branch. In the initial and upper branches, the vortex shedding flow pattern displays a 2S mode. However, the VIV response produces a 2P mode vortex shedding pattern in the lower branch. For a cylinder with large surface roughness, the VIV response can be divided into only two branches: an initial branch and a lower branch. In the initial branch, the vortex shedding flow pattern displays a 2S mode, but in the lower branch, the pattern changes to the 2P mode. The transition of 2S mode to 2P mode is caused by the abrupt change of the vortex phase between the vortex force and the VIV response. [ABSTRACT FROM AUTHOR]

A simplified lumped-mass model was established using ordinary differential equations, focusing on the coupled torsional-longitudinal vibrations of a ship's propeller shaft. The numerical simulation based on the presented algorithm was then developed and the dynamic behavior was investigated. A theoretical solution setup with simple model was solved to demonstrate the accuracy of the proposed lumped-mass model. Based on this model, the coupled natural frequencies and the maximum acceleration of each direction were determined. Experimental tests were conducted to validate the applicability of the numerical model, over a range of rotational speeds and loading conditions. It is found that the natural frequencies are unaffected while the maximum acceleration are increased with the rotational speed as well as the loading. Natural frequencies representing other directions are induced by the coupling effect, and enhance the dynamical response. The ultimate response in the direction without excitation is enlarged because of the coupling effect. An appropriate coupling stiffness coefficient value has been proposed based on the discussion on modeling and experimental results. [ABSTRACT FROM AUTHOR]

Wavy cylinders have been proven effective in controlling the flow and reducing the fluid induced forces. However, the hydro-elastic behavior of a flexibly mounted wavy cylinder remains unclear. The current paper endeavours to present a systematic study of the flow around a spring mounted wavy cylinder mainly at a moderate Reynolds number of 5000, the results of which are compared with a normal cylinder with the same fluid and structural attributions. It is discovered that the wavy cylinder, although almost eliminates the Kármán vortices in the fixed configuration, shows only limited efficacy on the mitigation of the flow induced vibration in the case of zero structural damping, and the typical initial-upper-lower type response curve is manifested. The hydrodynamic forces of the wavy cylinder also magnify significantly in the flexibly mounted cases during synchronization. Moreover, the phase lag between the lift coefficient and the displacement displays a clear change from 0° at the initial branch to 180° at the lower branch. The association of the 2 S and 2 P vortex shedding modes to the different branches is also similar to that of the normal cylinder, in which the 2 S mode corresponds to the initial branch and the 2 P to the lower branch. In general, despite the absence of the primary shedding frequency in the fixed configuration, the flexibly mounted wavy cylinder exhibits many features that is also found in the normal cylinder. This implies that the vortex induced vibration may not be initiated by the Kármán vortex shedding, and thus may defy the conventional view on the mechanism of the vortex induced vibrations. Additional simulations are performed with non-zero structural damping. It is disclosed that with sufficiently high structural damping, the vibration of the wavy cylinder could be reduced more efficiently than the normal cylinder. [ABSTRACT FROM AUTHOR]

According to Jauvtis and Williamson's experiment, when the mass ratio is 2.6, the maximum transverse amplitude of the cylinder reaches 1.5 D in the super-upper branch. Many researchers have tried to capture the maximum transverse amplitude in numerical simulation. However, according to the existing references, few of the numerical results can reach such amplitude and the maximum vibration amplitudes are obviously smaller than the experimental values. In order to get more accurate results, a modified SST turbulence model is applied for the numerical simulations based on OpenFOAM. The influence of the magnitude of inflow acceleration in numerical simulation is also investigated. Firstly, the performance of modified SST model is tested by example of flow around a circular cylinder. Secondly, the appropriate inflow acceleration magnitude is determined by analyzing the numerical response under different acceleration magnitudes, finding that the inflow acceleration must be less than 0.017 per normalization time in order to capture the maximum transverse amplitude. Then, the two degrees of freedom vortex induced vibration of a cylinder with a mass ratio of 2.6 and reduced velocity from 2 to 14 is simulated. The numerical results are compared in detail with the experimental data and the maximum transverse amplitude, mutations of phase angle between lift force and displacement, the characteristics and change trends of the trajectory as well as the “2T” vortex model are captured clearly. The results show that the maximum transverse amplitude can be captured more accurately with modified SST turbulence model and appropriate inflow acceleration value. [ABSTRACT FROM AUTHOR]

As one of the most serious “side effects” of blast excavation, blast-induced vibration must be controlled for existing buildings and human beings. This paper proposes a method for blast-induced vibration reduction with water jet assistance according to the cutting characters of low-noised, environment-friendly water jet. The mechanism of vibration-isolation with water jet assistance was analyzed, and the stress wave energy attenuation models were established based on blasting theory and stress wave theory. Influence law on shock wave attenuation by vibration-isolation slot was studied by numerical simulation. Simulation results agree with the theoretical analysis roughly. The results of this study put forward a method for blast-induced vibration near blasting source and provide a certain theoretical basis. [ABSTRACT FROM AUTHOR]

This paper presents a computational fluid dynamics (CFD) study of vortex-induced vibration (VIV) for different aspect ratio (L/D) cylinder. Of particular interest was to measure hydrodynamic forces and numerically investigate the wake behaviour of VIV while varying the aspect ratio. The simulation models represented the actual experimental conditions with idealised free-surface boundary condition to capture the responses from fluid-structure interaction phenomenon. The simulations were performed in the subcritical flow region (7.4×103< Re < 2×105), corresponding to a range of reduced velocity (Ur) from 2 to 14. The results of the cases studied were discussed and compared with the experimental data to verify the accuracy and validity of the present simulation. The comparisons have shown a similar curved-shape drag coefficient plot, and however underestimated the value of the drag coefficients over the reduced velocity. Additionally, the simulations seemed to capture a higher lift force response compared with the experimental data for a low aspect ratio. The correlation length was observed to be longer for larger aspect ratio and proportionally decreases as the aspect ratio decreases. [ABSTRACT FROM AUTHOR]

The pile-driving method produces considerable noise and vibrations. Hence, an auger-drilled pile method was developed as a low-noise and -vibration substitute. However, this method does not guarantee the bearing capacity of the pile unless some amount of pile-driving is performed. Therefore, the noise and vibration problems cannot be completely solved. In this study, a prebored screw pile method is proposed to solve these problems. In this method, piles are constructed by the rotary penetration of a screw pile into a prebored hole filled with some cement milk and whose diameter is smaller than that of the screw pile. To determine the shape of the screw pile, laboratory tests with model screw piles were conducted. Also, field load tests were conducted on an actual screw pile fabricated based on the laboratory test result and on a smooth-surfaced pile. In addition, the behavior of the screw pile was estimated by using three-dimensional finite element analysis. The results of the field load test and the numerical simulation showed that the ultimate bearing capacity and the unit skin friction of the screw pile are very superior to those of the smooth-surfaced pile and the cement milk is an important factor in the prebored screw pile method. [ABSTRACT FROM PUBLISHER]

Abstract: A new reduced-order model capable of analyzing the vortex-induced vibration of catenary riser in the ocean current has been developed. This semi analytical–numerical approach is versatile and allows for a significant reduction in computational effort for the analysis of fluid–riser interactions. The incoming current flow is assumed to be steady, uniform, unidirectional and perpendicular to the riser plane of initial equilibrium curvatures. The equations of riser 3-D motion are based on a pinned–pinned, tensioned-beam or flexural cable, modelling which accounts for overall effects of riser bending, extensibility, sag, inclination and structural nonlinearities. The unsteady hydrodynamic forces associated with cross-flow and in-line vibrations are modelled as distributed van der Pol wake oscillators. This hydrodynamic model has been modified in order to capture the effect of varying initial curvatures of the inclined flexible cylinder and to describe the space-time fluctuation of lift and drag forces. Depending on the vortex-excited in-plane/out-of-plane modes and system fluid–structure parameters, the parametric studies are carried out to determine the maximum response amplitudes of catenary risers, along with the occurrence of uni-modal lock-in phenomenon. The obtained results highlight the effect of initial curvatures and geometric nonlinearities on the nonlinear dynamics of riser undergoing vortex-induced vibration. [Copyright &y& Elsevier]

The objective is to propose a quick and accurate vortex-induced vibration (VIV) model to predict the fatigue life of marine drilling risers. The nonlinear wake oscillator model was applied to represent the lift force, which is described by a fourth-order partial differential equation set. Next, we adopted the generalised integral transform technique (GITT) to transform it into second-order ordinary differential equation system, which was solved by the Runge-Kutta method. Validation of the numerical model was implemented and a good agreement was achieved. Next, anS-Ncurve was adopted to conduct fatigue prediction. The results demonstrate that under the conditions set forth in this paper, an increase in the current and outer diameter could reduce fatigue life, except when the outer diameter reaches a certain value, at which point the tendency is reversed. Moreover, when the top tension coefficient reaches a certain value, the fatigue life tendency is also reversed. [ABSTRACT FROM PUBLISHER]