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A reliable investigation which allows an accurate prediction on seakeeping performance of a monotricat ship in head-seas is obviously required. To achieve this objective, a Computational Fluid Dynamic simulation approach is proposed by looking into attainable outcomes with precision, whilst a hydrodynamic description that is underlying the rationale behind the results is explained. Two conditions of the monotricat ship model, termed as with and without stepped-hull, were employed in the computational simulation. Several wave properties, such as various wavelengths and wave heights on heaving and pitching performances associated with different Froude number (Fr) were taken into account. The results showed that the increase of Fr was proportional with the increase the heaving and pitching motions. These led to the downgrade seakeeping performances presented in the form of high Response Amplitude Operators (RAO). In shorter wavelengths (λ<1), RAO of the heave and pitch motions were insignificant. However, the subsequent increase of wavelength (λ>1) was proportional with the increased RAO of the heave and pitch motions. The further increase in wavelength (>1.75) resulted in less RAO both of the heave and pitch motions. The increase in wave height had affected a proportionate increase in the heave and pitch motions that may possibly lead to degrade her seakeeping quality. It can be concluded that the effects of Frand wavelength on the heave and pitch motions of the monotricat ship had more complex phenomenon as compared to the wave-height ones. These CFD results are useful as preliminary prediction for navigation safety during sailing. [ABSTRACT FROM AUTHOR]

Many commercial engineering software programmes used to design floating tidal turbines neglect the added mass and damping because most of the code considers floating wind turbine designs, which is acceptable since air is significantly less dense than seawater. However, added mass and damping forces are important parameters that need to be included in the design of a floating tidal turbine. The increase in instantaneous time-dependent loading and changes in the natural frequency of a floating tidal turbine make these hydrodynamic forces non-negligible, especially for large turbines. The present study describes the construction of matrices of added mass and damping that can be used as inputs to simpler engineering models. These matrices were constructed by conducting three-dimensional blade-resolved CFD simulations for a floating tidal turbine operating under a prescribed pitch motion (i.e., pendulum-like motion) under various motion amplitudes and frequencies. The added mass and damping were also empirically extracted from the fluctuating thrust force, the empirical model derivation of which is given in this paper. Higher motion amplitude and frequency increase the pressure on rotor blades, which increases the amplitude of loading variations. A floating tidal turbine's mean power and thrust deviate from that of a stationary turbine due to suboptimal rotor operating conditions caused by the change in tip speed ratio (which is due to the apparent velocity variation). Formulations can be constructed (as functions of motion amplitude and frequency) to calculate added mass and damping forces based on the data of hydrodynamic forces provided in this paper, which can be applied to a conventional turbine design model. The added mass and damping forces increase the overall mass and damping of the floating rotor, respectively, which affects the motion and loading of the device. • Added mass and damping forces on a floating tidal turbine undergoing the pendulum motion. • Development of generalized hydrodynamic coefficients of floating tidal turbines oscillating in pendulum motion. • Shifts in the natural frequency of the floating turbine due to hydrodynamic forces. [ABSTRACT FROM AUTHOR]

The flow-field around the offshore floating vertical axis wind turbines (OF-VAWTs) is affected by the six-degrees of freedom (6-DOF) movement of the platforms. Understanding the impact of a certain DOF motion on the aerodynamics is beneficial to the design of wind turbines. In this paper, the aerodynamics and performance of a scale OF-VAWT in pitch motion are investigated. The computational fluid dynamics (CFD) method with the turbulence model of improved delayed detached eddy simulation (IDDES) and the overset mesh technique are employed to analyze the characteristics of pitch motion of wind turbines. The CFD model is verified by the experimental data from available literature when the turbine has no-pitch motion. Then, the aerodynamic forces, power, and wake of an OF-VAWT in periodical pitch motion are analyzed. Because of the varying wave loads, an unsteady aerodynamic analysis considering the pitch motion with different periods and amplitudes is performed. The results show that the pitch motion can improve the power output of the OF-VAWTs, and enlarge the variation ranges of aerodynamic force coefficients. Meanwhile, the graphs of the vortex structure show that the complex flow interaction emerges around the rotor blades. Additionally, the power coefficient and the instantaneous aerodynamic forces coefficients sensitively change with different pitching periods and pitching amplitudes. [ABSTRACT FROM AUTHOR]

An improved meshing method based on Fluent is used to update the computational meshes in solving the Navier-Stokes (N-S) equations for viscous and incompressible free surface flows with the volume of fluid (VOF) method. To maintain the mesh quality when updating meshes for a moving structure, the computational domain is separated into several parts and each part corresponds to a specific type of body motion. The numerical results of the interaction between the floating body and the regular waves agree well with the experimental data. A total of eight typical motion types are simulated separately to understand the correlation between the motion types and the wave transmission as well as the forces acting on the floating body. Numerical experiments show that the wave transmission increases in the case of sway and heave motions and decreases in the case of pitch motion as compared with the stationary case. It is also found that the sway motion reduces the horizontal wave force acting on the floating body, while the heave motion enhances the vertical wave force. [ABSTRACT FROM AUTHOR]

Time series of wave-induced ship responses are analysed by the Hilbert-Huang Transform (HHT) method. The results of the analysis of numerically simulated motions and loads on a chemical tanker in head waves are compared with the analysis of experimental data from laboratory model tests. The Empirical Mode Decomposition, which is a first part of HHT method is applied to the sea surface elevations of two sea states (high and low seas) and the wave-induced vertical motions and bending moments and the results are discussed. It is found that the decomposition of the experimental data into intrinsic components compares well with the decomposition of the numerically simulated ones for both high and low seas. The first two or three intrinsic mode functions carry the major part of the energy of both experimental and numerical simulated data. The time-frequency distribution of energy, called Hilbert spectrum, is calculated by the second part of HHT method. The Hilbert spectrum of the sea surface elevation is compared with Hilbert spectrum of wave-induced ship motions and moments in an attempt to study the time-frequency characteristics of ship response. The results of the present work provide confidence in the usage of numerical simulation data for the decomposition into intrinsic components of ship responses, avoiding the need for experimental data from laboratory model tests. • Time series of wave-induced ship responses are analysed by the Hilbert-Huang Transform (HHT) method. • The HHT analysis is applied to the sea surface elevations of high and low seas states and the wave-induced motions and loads. • The decomposition of the experimental data into intrinsic components compares well with the numerically simulated ones. • The first 2 or 3 intrinsic mode functions carry the major part of the energy of both experimental and numerical data. [ABSTRACT FROM AUTHOR]

This paper employs the simplified quadrupedal passive dynamic model to analyze the underlying property of the transverse gallop. First, the simplified sagittal quadrupedal planar model of the transverse gallop, the trot, and searching methods for achieving the periodic motion are introduced. Next, we explore the dynamic performance of the transverse galloping pattern on the simplified quadrupedal passive dynamic model at different forward speeds and different stiffnesses of legs in a range of initial angular velocities at which the periodic motion is achieved, and there exists a particular property that is defined as the pitch angular velocity dynamics (PAVD) property. If the forward speed and the stiffness of legs are adequate in the transverse galloping pattern, the PAVD emerges and the forces (e.g. peak ground reaction force and average vertical force) that the model undergoes during locomotion are sharply reduced. The Froude numbers of the minimum forward speed where PAVD holds at a fixed stiffness of legs for different samples are virtually the same. Moreover, a comparison between the trot and the transverse gallop illustrates that a trot is more favorable in the range of forward speeds where the PAVD property is not achieved, while a transverse gallop is better after the property emerges. [ABSTRACT FROM AUTHOR]

Ocean wave energy is a stable renewable energy source, which has realistic application prospects. Based on the intermittent and irregular nature of ocean-wave, researchers have designed many ocean-wave energy harvesters, with several advantages and disadvantages. The main goal of this study is to present a new ocean wave energy harvester, which works based on the small pitch motion of moving floaters. Unlike almost all the previously presented systems, the presented ocean wave energy harvester can work in moving forward ships and boats like emergency boats. To do so, a ibro-impact system is designed to gather the kinetic energy of small pitch motions and transmit it to the vibratory part of the system via impulsive load. A mechanical rectifier that gets its input energy from the oscillations of the vibratory part, provides continuous rotations needed by the electromagnetic transducer. The governing electromechanical equation of motion for the discussed system is derived to study the effects of changing the ocean-wave properties on the application of the energy harvester. It is shown the gap size of the impact part of the system and the ocean-wave period have important effects on the output electrical energy. • This study presents a new ocean wave energy harvester based on the pitch motion of moving floaters. • A vibro-impact system is designed to gather the kinetic energy of pitch motions via impulsive load. • A mechanical rectifier gets input energy from the oscillations of the vibratory part and provides continuous rotations. • The governing electromechanical equation of motion for the discussed system is derived. • The gap size of impact part of the system and the ocean-wave period have important effects on output electrical energy. [ABSTRACT FROM AUTHOR]

The present study is devoted to the computation of the KVLCC2 tanker in head wave with free heave and pitch motion. A RANS solver using finite-volume discretization and free-surface capturing approach is employed for the computation. Free ship motion is captured with a mesh deformation approach. Three different wave lengths (0.6Lpp, 1.1Lpp and 1.6Lpp) are computed. We focus on numerical uncertainty estimation in this paper. For each test case, three different meshes and at least three different time steps have been used to access both time and spatial discretization error. Additional computations with different setups aimed at identifying different numerical discretization errors will also be performed. It is demonstrated that special attention needs to be paid to time discretization. To keep the same time accuracy, time step needs to be reduced on fine mesh for such kind of unsteady free-surface computation involving important pitch or roll motion. [ABSTRACT FROM AUTHOR]

Abstract: For straightness profile measurements, the traditional standard for hardware datums has been the straightedge, level and optical beam. Currently, the proliferation of computers has brought about development of the software datum. Conventional software datums are error separation techniques that separate a surface profile from parasitic motions by using multiple sensors and/or multiple arrangements (multiple orientations or multiple alignments). The multiple sensor method can separate the surface profile with just one scanning. Therefore, the multiple sensor method is useful, even if the parasitic motion includes non-repeatable components. However, random error propagation of the multiple sensor method has been reported in the case of large sampling numbers. In this paper, a novel two-point method for straightness profile measurement is proposed. The proposed method can reduce the propagation of uncertainty caused by the cross-axis motion, pitch motion and random error of the estimated surface profile. The proposed method consists of a weighted addition and inverse filtering. The suitable weight of the weighted addition that gives the least uncertainty propagation is derived by a simple equation using the standard deviation of the cross-axis motion, pitch motion and random error. [Copyright &y& Elsevier]

The vehicle body is constantly forced to pitch by the load oscillations occurring on the front and rear axles during the vehicle pitch motion, which deteriorates the ride comfort. In order to solve this problem, a criterion which can improve the ride comfort by decreasing effects of the load transfer on the axles is aimed. In this study, the movement of pitch and bounce oscillation centers is considered as a criterion. To determine the properties of criterion which can improve the ride comfort, the positions of the oscillation center have been changed with respect to center of gravity of the vehicle body. The effects of this change on the ride comfort have been analyzed with respect to acceleration amplitudes computed at middle point of the wheelbase and on the axles using magnification function. The simulation of frequency responses of the changes of these acceleration amplitudes was carried out for different vehicle speeds and wheelbases, using MATLAB software. The simulation results show that the ride comfort can be improved by moving the oscillation centers to certain points in reference to the road excitation frequency. As a result, it has been revealed that the oscillation center positions moving at determined directions can be a criterion for selecting suspension properties. [ABSTRACT FROM AUTHOR]

In this study, vibration control, a behavior which subordinates to stall-induced nonlinear vibration and amplitude control of a wind turbine's blade section, based on unified pitch motion driven by slider-linkage mechanism, is investigated by using an iterative learning control (ILC) method. The nonlinear dynamical system is a nonlinear aeroelastic system. The aeroelastic system equations consist of three parts: the nonlinear structural equations derived by using Lagrange's equations, the improved stall-induced nonlinear ONERA (ISNO) aerodynamic equations, and the pitch control equation. The ISNO model is not only suitable for the actual external pitch motion, but also suitable for the solution by using an ILC algorithm due to its fitted nonlinear aerodynamic coefficients. The ILC algorithm used here is an improved iterative learning algorithm (IILC) which considers the large-range, linearized, residual terms, and realizes gain adaptive tuning based on PID controller. On the one hand, it can control the amplitude of an unsteady flutter through trajectory tracking. On the other hand, when the preset value of the amplitude of the ideal trajectory is very small, it can make the system directly tend to convergence and stability of a nonlinear aeroelastic system. To simplify the extremely difficult iterative process, the pitch movement can track the elastic twist displacement in time, thus simplifying the aeroelastic equations and accelerating the IILC iteration process. Therefore, amplitude control for flap-wise/lead-lag displacements is realized by the unified pitch motion and the trajectory tracking controlled by using the IILC algorithm. [ABSTRACT FROM AUTHOR]

The floating vertical axis wind turbine (VAWT) is considered as a competitive device in the utilization of offshore wind energy. However, the platform pitch motion would affect its aerodynamic behavior. In this paper, the aerodynamic performance of a floating φ-type VAWT under pitch motion is investigated by using the Improved Delayed Detached Eddy Simulation SST k − ω turbulence model. After verifying the feasibility of the numerical model, the effects of pitch motion amplitude and period on the aerodynamic characteristics were evaluated, and the impacts of these observations were elucidated. The results showed that the averaged net power coefficient increment of about 1.5 % –15 % could be obtained under platform pitch motions, and the fluctuation of aerodynamic loads was found to increase. Besides, the pitch motion pattern could be regarded as the combination of surge and heave motions, which explained the similarity of their effects on the wind turbine aerodynamics. Furthermore, it was found that the frequency of the peak torque coefficient would change under different periods of pitch motion, which should be noticed in the design of floating wind turbine. Finally, it was concluded that the current study provided additional information about the effect of pitch motion on wind turbine aerodynamics. • The aerodynamic performance of ϕ-type VAWT under pitch motion is investigated. • The similarity of aerodynamic changes under pitch and surge motions is explained. • The aerodynamic changes of ϕ-type and H-type VAWTs under pitch motion are compared. • The influence of pitch motion on aerodynamic spectrum of wind turbine is studied. [ABSTRACT FROM AUTHOR]

Compared with the conventional floating offshore platforms, the FOWTs (floating offshore wind turbines) are subjected to larger wind heeling moments to cause significant pitch motions which bring challenges to the power generation efficiency and structure safety, due to the existence of wind turbines. In addition, the dynamic cable connected with FOWT is subject to a load that is greatly affected by horizontal motions of FOWT. Therefore, the inhibitions of horizontal and pitch motions are vital to FOWT. However, conventional mooring systems mainly inhibit the horizontal motions of floating structures. In this paper, an innovative type of mooring system is proposed to inhibit both the horizontal and pitch motions of FOWT. The analytical results based on the innovative mooring system are compared with those of conventional mooring system. In addition, the mechanism of restoring forces and moments of mooring systems are analyzed and discussed. • An innovative type of mooring system is proposed to inhibit the horizontal and pitch motions of FOWT. • The horizontal and pitch motions of FOWT with the innovative mooring system are significantly reduced. • The restoring stiffnesses of innovative mooring system in surge and pitch DOFs are significantly enlarged. • The damping contributions of innovative mooring system in surge and pitch DOFs are significantly enlarged. • The pitch motions will improve the horizontal motion inhibition performance of innovative mooring system. [ABSTRACT FROM AUTHOR]

The aerodynamic performance of wind turbines is essential to evaluate their electricity-generating capability. Compared with the wind turbines with traditional fixed structures, the aerodynamic performance of floating offshore wind turbines (FOWTs) is affected by the additional platform motion, especially for the pitch motion. In view of this, the computational fluid dynamics (CFD) method with the improved delayed detached eddy simulation (IDDES) is applied. Before the numerical simulation, the CFD model of the redesigned 1:50 scale rotor from NREL 5 MW wind turbine is verified with the available experimental data and numerical comparison. Then, the aerodynamics of the FOWT under the harmonic pitch motion with different periods and amplitudes is investigated. It is shown that the aerodynamic performance of the FOWT is sensitive to these parameters of the pitch motion. First, amplitudes of the rotor thrust and torque decrease with the increment of the pitch period. In particular, the wake interference phenomenon is the most evident when the pitch period is short and the pitch amplitude is high, which may compensate some energy to the rotor. In addition, amplitudes of the rotor thrust and torque increase with the increment of the pitch amplitude. Also, the stall phenomenon happens when the pitch amplitude is high, which may impact FOWTs' aerodynamic performance adversely. Finally, the rotor power increases under the periodic pitch motion, especially by decreasing the period or increasing the amplitude. It is concluded that the pitch motion of the platform will change the aerodynamic performance of a wind turbine and should be taken into consideration during design procedure. • IDDES model is employed to analyze the aerodynamics of a FOWT under pitch motion. • Effects of pitch periods are studied, and wake interference phenomenon is observed. • Effects of pitch amplitudes are investigated, and stall phenomenon is revealed. • The rotor power under periodic pitch motion is investigated. [ABSTRACT FROM AUTHOR]

The evaluation of speed loss is the most important studies in the optimal ship routing. However, the mode of speed loss during rough sea voyages is complicated because of the involvement of the human factor. A speed loss analysis is conducted to improve the accuracy of evaluation of measured data extracted from actual marine conditions. First, the conventional criteria to judge the deliberate speed reduction is summarized with the conventional ship maneuvering techniques. Second, a nationwide questionnaire for marine engineers implemented for engine safety during rough sea voyages is detailed. Third, these conventional criteria are validated by analyzing the collated data on accelerations, ship motions, navigation, and engine parameters. The limiting value of vertical acceleration agrees with that obtained via practical measurements. On the contrary, the limiting values of lateral acceleration, roll motions, probabilities of deck wetness, and slamming do not agree with that obtained via measurements. The probabilities of deck wetness, slamming, and propeller racing start to increase at approximately the same instant where the deliberate speed reduction occurs. Some relations between the deck and engine parameters detailed in this paper were found to be highly correlated. Finally, the results of the analysis were used to model simpler estimations. • This study makes clear the detailed situations on the speed loss in rough sea voyages. • Related parameters are validated with the measured database for 28,000-DWT bulk carrier. • Deliberate speed reduction occurs at the starting point of deck wetness or propeller racing. • Speed loss has a boundary region at Δ V = 3–4 knots, and are correlated with pitch motion. [ABSTRACT FROM AUTHOR]