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A dynamic pitch strategy is usually adopted to improve the aerodynamic performance of the blade of a wind turbine. The dynamic pitch motion will affect the linear vibration characteristics of the blade. However, these influences have not been studied in previous research. In this paper, the influences of the rigid pitch motion on the linear vibration characteristics of a wind turbine blade are studied. The blade is described as a rotating cantilever beam with an inherent coupled rigid-flexible vibration, where the rigid pitch motion introduces a parametrically excited vibration to the beam. Partial differential equations governing the nonlinear coupled pitch-bend vibration are proposed using the generalized Hamiltonian principle. Natural vibration characteristics of the inherent coupled rigid-flexible system are analyzed based on the combination of the assumed modes method and the multi-scales method. Effects of static pitch angle, rotating speed, and characteristics of harmonic pitch motion on flexible natural frequencies and mode shapes are discussed. It shows that the pitch amplitude has a dramatic influence on the natural frequencies of the blade, while the effects of pitch frequency and pith phase on natural frequencies are little. [ABSTRACT FROM AUTHOR]

Natural vibration characteristics of a cantilever beam that is parametrically excited by pitch motion are theoretically investigated in this work. A partial differential equation governing the parametrically excited bending vibration of the beam is established by employing the generalized Hamiltonian principle. Normal modal functions of the bending vibration are obtained based on the method of separation of variables. It reveals that the dynamic pitch motion cannot change the modal shapes of the bending vibration of a uniform beam. Principal vibrations of the beam are studied by taking into account two cases of E I 1 = E I 2 and E I 1 ≠ E I 2 , where E I 1 and E I 2 are the first and second principal bending stiffness, respectively. The bending stiffness is considered as the superposition of a constant stiffness with a small perturbation. With the aid of the Fourier series theory and the method of multiple scales, the principal vibrations of the parametrically excited system at non-resonant and resonant cases are analyzed by introducing a small natural parameter , while natural vibration characteristics of the beam are obtained and the orthogonal properties of modal functions are illustrated. Theoretical methods provided in this work are verified by using the finite element method, assumed modes method and direct numerical integrations to the governing equation based on a rectangular beam model. It reveals that theoretical results coincide with numerical results very well. Finally, the influences of design parameters including the beam height, beam width, beam length, slender ratio, aspect ratio, pitch amplitude, pitch frequency and pitch phase on natural frequencies of a harmonic pitching beam are discussed. It shows that the width, height, length, slender ratio, aspect ratio and pitch amplitude have dramatic influences on the natural frequencies of the pitching beam, while the pitch frequency and pitch phase have little effect on natural frequencies. [ABSTRACT FROM AUTHOR]

A series of experiments were carried out to investigate the performance of a WEC rotor with platform motion. To achieve platform motion, a forced motion device with a rotor was installed in the Ocean Engineering Basin. In this study, the rotor's performance was examined at various phase stages between incoming waves at three wave heights, and compared to a rotor without a PTO system. The phase stage where the rotor exhibited the largest pitch motion was identified, and the efficiencies of the rotor with the PTO system were analyzed during this stage. Without the PTO system, the rotor experienced its largest pitch motion when the rotating center was positioned at the zero-up crossing point of the incoming waves at three different wave heights. Furthermore, it was observed that in the optimal phase, the rotor's efficiency increased with relatively large platform motion. These findings provide fundamental data for rotor design. [ABSTRACT FROM AUTHOR]

Since a floating solar panel is set above a water surface, the ground effect due to the small gap between the panel and the free surface may be induced by the wind load, in turn causing a high lift force on the panel. In this paper, the wind load on a three-dimensional thin flat plate floating on surface waves is studied analytically under linear assumptions. The clearance between the panel and the free surface is set to be smaller than the panel length and the effects of the base of the floating system are ignored. The pitch motion of the plate due to linear surface waves is considered and the effects of the aspect ratio of the plate, wave number, and wave speed of the free surface are discussed. The analytical solution shows that, if the plate is stationary and the free surface is flat (no waves), then the lift coefficient increases with aspect ratio. For a pitching plate above surface waves, lift coefficient increases with aspect ratio for longer wavelengths, but decreases when the aspect ratio increases for shorter wavelengths. Minimum lift and the associated wave number and aspect ratio for different wave speeds are also discussed. The findings of this study are anticipated to enhance the design of floating solar panels, improving their capacity to withstand substantial wind and waves. [ABSTRACT FROM AUTHOR]

Generally, new and renewable energy systems generate electricity by installing and operating multiple modules simultaneously. In the Republic of Korea, recent studies and developments have focused on asymmetric wave energy converters (hereafter referred to as rotors) suitable for marine environments off the western coast of Jeju. These rotors are arranged on a large floating truss-structure platform and designed to harness electricity from the rotors' pitch motion. However, when multiple rotors operate on a platform, their behavior diverges from that of a single module due to hydrodynamic interactions between them. Moreover, because the rotors are connected to the floating platform, their motion is influenced by the platform's dynamics. In this study, a time-domain multibody motion equation was established to analyze changes in the behavioral characteristics of the rotors, both with and without a floating platform. The hydrostatic and hydrodynamic coefficients were derived in the frequency domain using WAMIT, a commercial code based on linear potential flow theory for three-dimensional diffraction/radiation analyses. The motion equation was then applied under regular and irregular wave conditions using OrcaFlex version 11.3, a marine systems design and analysis program. The resulting behaviors were compared to elucidate the influence of the platform and hydrodynamic interactions on the rotors' performance. [ABSTRACT FROM AUTHOR]

Pitch motion is the key factor affecting the performance characteristics of centrifugal pumps on board ships and exacerbates hydraulic excitation to induce the unsteady vibration of pump units. A hydraulic test platform with swing motion is established to explore the effects of pitch motion on a pump's performance characteristics. An obvious hump zone exists in the head characteristic curve in the low-flow-rate condition due to the pitch motion. The pump head in the shut-off condition has a significant decrease due to the pitch motion, compared to the static state. The head decrease gradually increases as the maximum pitch angle increases or the pitch period shortens. Specifically, the head in the rated flow condition decreases by 6.3 % to reach a minimum at the maximum pitch angle of 20 degrees in a period of 5 s. Based on a multiple-reference coordinate system, a large eddy simulation with a shear-modified eddy viscosity model is employed to simulate inner flow characteristics under the influence of pitch motion. A distinct vortex flow appears near the blade suction surface and becomes increasingly turbulent as the pitch period shortens. The pitch motion intensifies the unsteady stretching and deformation of vortices. The periodic variations in fluid-induced pressure over time present parabolic features, and the amplitude in the frequency domain reaches its maximum value within a pitch period of 5 s. [ABSTRACT FROM AUTHOR]

Pitching mode is more crucial than heaving mode in assessing floating offshore wind turbine (FOWT) motion characteristics, especially the operation of the SPAR substructure. The aim of this paper is to develop an experimental method for improving the SPAR substructure to minimize unnecessary pitch motion. Toward this end, three vertical plate configurations based on the novel SPAR are being developed, known as the 3VP, 4VP, and 5VP models. In consideration of 0⁰, 30⁰, 60⁰, 90⁰-incidence, the pitch response characteristics of the proposed novel SPAR models are comprehensively evaluated in terms of submerged volume ratio, wave-induced motion, non-dimensional damping coefficient, and percentage of motion reduction. The model test results indicate that the 4VP model outperforms the other novel models with respect to dynamic response, particularly the incidence of 0⁰ and 90⁰. This study implies that the novel SPAR development is both feasible and effective in the modification of SPAR-type FOWT substructures.

A series of experiments were carried out to investigate the performance of a WEC rotor with platform motion. To achieve platform motion, a forced motion device with a rotor was installed in the Ocean Engineering Basin. In this study, the rotor’s performance was examined at various phase stages between incoming waves at three wave heights, and compared to a rotor without a PTO system. The phase stage where the rotor exhibited the largest pitch motion was identified, and the efficiencies of the rotor with the PTO system were analyzed during this stage. Without the PTO system, the rotor experienced its largest pitch motion when the rotating center was positioned at the zero-up crossing point of the incoming waves at three different wave heights. Furthermore, it was observed that in the optimal phase, the rotor’s efficiency increased with relatively large platform motion. These findings provide fundamental data for rotor design.

The motion of small fishing vessels is significantly affected by small waves, leading to accidents, such as capsizing or sinking. This paper presents the results of two types of basin tests. The first test analyzed the characteristics of roll and pitch motions among regular waves with the same wave steepness using the drifting state of three (3G/T, 7G/T, 10G/T) small fishing vessels. The second test analyzed the motion characteristics of the 7G/T fishing vessel under different wave steepness. The first test showed that heave and roll motions are significant in the beam sea, while pitch motion is significant in the bow and stern seas. The second test shows that wave steepness has a linear relationship with roll and pitch motions in the bow and stern seas.

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Generally, new and renewable energy systems generate electricity by installing and operating multiple modules simultaneously. In the Republic of Korea, recent studies and developments have focused on asymmetric wave energy converters (hereafter referred to as rotors) suitable for marine environments off the western coast of Jeju. These rotors are arranged on a large floating truss-structure platform and designed to harness electricity from the rotors’ pitch motion. However, when multiple rotors operate on a platform, their behavior diverges from that of a single module due to hydrodynamic interactions between them. Moreover, because the rotors are connected to the floating platform, their motion is influenced by the platform’s dynamics. In this study, a time-domain multibody motion equation was established to analyze changes in the behavioral characteristics of the rotors, both with and without a floating platform. The hydrostatic and hydrodynamic coefficients were derived in the frequency domain using WAMIT, a commercial code based on linear potential flow theory for three-dimensional diffraction/radiation analyses. The motion equation was then applied under regular and irregular wave conditions using OrcaFlex version 11.3, a marine systems design and analysis program. The resulting behaviors were compared to elucidate the influence of the platform and hydrodynamic interactions on the rotors’ performance.

The unsteady aerodynamic characteristics and interference effects of floating offshore wind turbine (FOWT) are mainly affected by the pitch motion of the ocean platform. Based on computational fluid dynamics (CFD) and advanced overset grid technology, the aerodynamic performance and wake characteristics of a fully configured wind turbine with rotating blades, nacelle , and tower are studied in this paper. The effects of the amplitude and frequency of pitch motion on the wind turbine aerodynamic loads and flow field are investigated herein in detail. The power and thrust between numerical simulation and experiment are compared. The results show that the grid and simulation parameters used in this study can accurately capture the aerodynamic characteristics and the flow field around wind turbines. The influence of the pitch amplitude and frequency on the performance of wind turbines is discussed. The complex flow interaction between the tip vortex, tower shedding vortex, and the turbulent wake was observed. The present results indicate that the pitch motion amplitude and frequency have a great influence on the power, thrust, and wake characteristics. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines suffer severe aerodynamics under platform motions. Novel control methods are urgently needed to improve power and load performances. A mechanical cyclic pitch control strategy is introduced in this paper, and the load reduction effect on the NREL 5-MW wind turbine is investigated under dominant pitch motions. The simulation is performed by the standalone Aerodyn_Driver code. Results show that the relative airflow under pitch motions can be equated to a linear sheared flow and is only related to wind velocity and pitch angular velocity. Cyclic pitch control hardly affects power and thrust but significantly changes aerodynamic moments. The resultant moment and moment axis azimuth are defined, and they are controlled respectively by pitch amplitude and phase. The optimal pitch parameter equations to minimize pitching and yawing moments are derived from fitting and derivation, and the standard deviations of aerodynamic moments are reduced by over 70 % under the sinusoidal pitch motion and steady inflow below rated operation. This paper provides a new idea for the individual pitch control structure, and the mechanism study for aerodynamic control is important and instructive to fill the technological gap in mature pitch control strategies for floating offshore wind turbines. • Cyclic pitch control is used to reduce aerodynamic loads under pitch motions. • The linear sheared airflow is only related to wind and pitch angular velocity. • Power and thrust are almost unaffected in the limited pitch angle range. • Load magnitude and direction are adjusted respectively by pitch amplitude and phase. • Standard deviations of moments are reduced by over 70 % with the optimal control. [ABSTRACT FROM AUTHOR]

Earlier studies have shown that the pitch motion of the platform can enhance the power coefficient of offshore vertical axis wind turbine (VAWT), particularly in the realm of scaled model. However, there is a shortage of research employing computational fluid dynamics (CFD) to evaluate how that motion affects the aerodynamic performance of full-scale VAWT. Moreover, the differences in aerodynamic characteristics between full-scale and scaled VAWTs are not well-understood. To fill this knowledge gap, we utilize high-fidelity CFD simulations to investigate the aerodynamic performance of a 5.3-MW full-scale offshore H-type VAWT in the presence of pitch motion. Subsequently, we conduct a comparative analysis of the variations between the full-scale VAWT and its scaled counterpart. Furthermore, we explore the influence of pitch amplitude and period on the overall performance. The findings reveal that, with low solidity (i.e., 0.1) and moderate tip speed ratio (i.e., 3.5), periodic pitch motion results in a 16.42% improvement in the power coefficient for the full-scale VAWT. In contrast, the scaled counterpart experiences a significant 56.71% reduction in the power coefficient. Meanwhile, for the full-scale VAWT, selecting a larger pitch amplitude and shorter pitch period holds the potential to enhance the power coefficient; however, the durability concerns stemming from the extensive fluctuations in blade torque need to be addressed. Additionally, it is observed that pitch motion introduces turbulent structures into the wake, facilitating both expansion and recovery. Due to the scale effect (Reynolds number effect), it's crucial to recognize that conclusions drawn about the aerodynamic performance of full-scale VAWT cannot be directly extrapolated from scaled model. • Aerodynamics of full-scale offshore VAWT under pitch motion is studied. • Aero-differences between full-scale and scaled VAWTs are elucidated. • Impact of pitch motion on C P may be contrary for full-scale and scaled VAWTs. [ABSTRACT FROM AUTHOR]

Tuned sloshing dampers have long been used to stabilize tall and thin buildings and are now being applied to marine structures. In this study, the effectiveness of two new passive control devices at stabilizing floating offshore wind turbine substructures was investigated: the bidirectional tuned liquid damper and bidirectional tuned liquid column damper. Numerical simulations were performed to consider the sloshing effect on the passive control devices and the pitch-induced external moment brought about by wave action. The effects of adding baffles and orifices to the passive control devices were explored to optimize the geometric design. The results showed that the optimal combination of substructure geometry and passive control device type could reduce the pitch motion by 20%–40%. [Display omitted] • A numerical model was developed to simulate a floating offshore wind turbine (FOWT) under a regular wave. • An integrated numerical model was developed for integrating various FOWT substructures with tuned liquid dampers (TLDs). • The optimum geometry and location of the TLDs were determined based on the minimum pitch motion of the FOWT substructure. • The effectiveness of the FOWT-TLD was assessed for regular waves with various wave periods and amplitudes. [ABSTRACT FROM AUTHOR]

Considering the vibration generated by a propeller-driven UAV or encountering gust, the propeller will perform a very complex follower motion. A pitch and rotating coupled motion is proposed in the present work that can take more complex unsteady performance of follower force than a regular fixed-point rotating motion. In order to evaluate the unsteady follower force and conduct parametric study, an extensive ground test bench was designed for this purpose where the whole test system was driven by a linear servo actuator and the follower force was measured by a 6-component balance. For CFD simulation, coupled motion in particular needs detailed unsteady aerodynamic model; therefore, a high-fidelity CFD-based study integrated with the overset mesh method was complemented to solve the unsteady fluid of varying conditions. The results suggest that a significant influence on unsteady follower force is observed, and the mean value of in-plane force does not equal to zero during the coupled motion process. Compared with the regular fixed-point rotation of propeller, the fluctuation frequency of follower force in present work couples the rotation and pitch motion frequencies. In addition, the oscillation amplitude of out-plane force and torque is positively related with the pitch frequency, pitch amplitude, and relative length from leading edge of wing to the rotation center. For example, the oscillation amplitude of 1-blade’s out-plane force and torque increases by 57.122% and 66.542% for the 5 Hz-5 deg case compared with the 5 Hz-3 deg case, respectively. However, the torque is not sensitive to frequency of pitch motion. The generally excellent agreement evident between the ground test and numerical simulation results is important as guidance for our future investigation on “dynamic” aerodynamic performance of a propeller-driven UAV.

The effect of pitch motion on the power characteristics of a floating horizontal-axis tidal turbine was investigated using model experiments. Based on the water flume's size, a tidal turbine test rig with uniform pitch and simple harmonic motion was designed. The effects of various pitch periods (2.7–4.7 s) and amplitudes (3–9°) on the power characteristics of the tidal turbine under the free surface were analyzed. The excitation of the pitch motion caused the instantaneous value of the tidal turbine power to fluctuate. The degree of power fluctuation was similar for different tip-speed ratios. The fluctuation amplitude increased and decreased approximately linearly with the period and amplitude, respectively. The mean power under pitch motion was slightly higher than that under the fixed state, representing an improvement of approximately 2.75–3.84%. Large-amplitude and high-frequency pitch motions were detrimental to the rotational speed control of tidal turbines and led to severe fluctuations in the power output. The experimental results provide detailed data for designing the motion response of the floating platform, power output control of the tidal turbine, and validation of the numerical model for the multi-degree-of-freedom hydrodynamic analysis of a floating tidal turbine. • Designed a test rig for the pitching motion of horizontal axis tidal turbine. • The power fluctuation increased linearly with the pitch frequency and amplitude. • The mean power of the tidal turbine in pitching motion has increased by 2.75–3.84%. • Severe pitching motion is not conducive to the speed control of the tidal turbine. [ABSTRACT FROM AUTHOR]

Active suspension control strategies are proposed to suppress pitch motion and address the issue of motion sickness among electric vehicle passengers. In this work, we investigate the performance of the linear quadratic regulator (LQR), variable universe fuzzy control, adaptive-network-based fuzzy inference system (ANFIS), and fuzzy PID control under continuous acceleration and deceleration and bumpy road conditions. First, a 14-degrees-of-freedom (DOF) dynamics model of an electric vehicle is developed based on a semirecursive multibody dynamics method, which was originally proposed by Javier García de Jalón. The vehicle dynamics simulation is performed to accurately model the vehicle states and responses. Second, the LQR, variable universe fuzzy control, ANFIS, and Fuzzy PID control algorithms are tailored via vehicle states, including pitch, rate, and acceleration. Their performances are investigated and compared in detail. Finally, the robustness of the proposed control strategies is further discussed based on various speed bump parameters and initial velocities. The results indicate that the proposed control strategies are capable of suppressing the pitch motion of electric vehicles, enhancing the riding comfort under diverse operating conditions, thereby reducing the likelihood of motion sickness among passengers of electric vehicles. • A multibody dynamics model of an electric vehicle with active suspensions is developed. • Different control strategies are tailored for active suspension control. • The robustness of the proposed control strategies is discussed in detail. [ABSTRACT FROM AUTHOR]

This paper aimed to examine the effects of a bidirectional tuned liquid damper on the stability of a barge-type floating offshore wind turbine. This analysis considered the pitch-induced external moment resulting from wave action and the sloshing effect on a bidirectional tuned liquid damper. Bidirectional tuned liquid damper geometries with different baffle heights at the middle of the bidirectional tuned liquid damper were examined, and the pitch motion of the floating offshore wind turbine integrated with the bidirectional tuned liquid damper was considered to determine the optimal bidirectional tuned liquid damper configuration. Based on the wave period, optimizing the bidirectional tuned liquid damper may reduce the pitch motion of a barge-type floating offshore wind turbine by 10%–30%. [Display omitted] • A numerical model was developed to simulate a floating offshore wind turbine (FOWT) under the regular waves. • A bidirectional tuned liquid damper (BTLD) was simulated using a numerical model. • To simulate a barge-type FOWT substructure integrated with a BTLD, a numerical model was developed. • The effectiveness of a BTLD on the stability of a barge-type FOWT were investigated to determine the optimum geometry and location of the BTLD. • The optimum integrated FOWT-BTLD was effective for regular waves with various periods and amplitudes. [ABSTRACT FROM AUTHOR]

A CFD numerical method is established under the wave-current condition, used to analyze the hydrodynamic performance of a horizontal-axis tidal turbine based on floating platform with rotation and pitching motion. The inflow direction load, pitch moment and power coefficients with different depths of the blade tip-immersion, the periods and amplitudes of the pitch, wave heights are obtained. The three coefficients have obvious periodically fluctuated with the pitching and wave frequencies, while the time mean of those have changed little with increasing of the depth of blade tip-immersion, wave height, pitch (wave) period and pitching amplitude. So considering damping and additional mass forces, a load fitting model is established, and the damping and additional mass coefficients are calculated by the least square method. The influence of damping coefficient on the hydrodynamic loads is obvious, while the influence of additional mass coefficient on that can be neglected, and the damping coefficient of inflow direction load coefficient is basically stable. The damping coefficients of the pitching moment and power coefficients fluctuate periodically, but the mean value of damping coefficients keep stable. The research results in this paper are important for studying the coupling motion of the turbine and the floating platform. [ABSTRACT FROM AUTHOR]

Background: Inefficient energy transfer from the pelvis and trunk has been shown to increase compensation at the level of the shoulder. Kinetic chain sequencing of the core segments is underexamined in professional baseball pitchers, especially as it relates to changes in upper extremity kinetics. Purpose: To evaluate elbow and shoulder kinetics in a cohort of professional pitchers differentiated by instances of discordant pelvic to upper torso sequencing during the pitch. Study Design: Descriptive laboratory study. Methods: 285 professional baseball pitchers were evaluated using 3D motion capture (480 Hz). Pitchers were divided into "chronological" and "discordant" groups based on whether maximum pelvic rotation velocity occurred before (chronological) or after (discordant) maximum upper torso rotation velocity during the pitch motion. Pelvic, upper torso, and shoulder kinematic parameters, shoulder distraction force, shoulder internal rotation torque, and pitch efficiency (PE) were compared between groups. Results: Pitchers with discordant torso sequencing (n = 30; 110 pitches) had greater shoulder horizontal adduction at maximum external rotation (mean difference, 3.6°; 95% CI, −5.2° to −2.0°; t = −4.5; P <.001) and greater maximum shoulder external rotation (mean difference, 3.7°; 95% CI, 5.7° to 1.5°; t = −3.5; P <.001) than chronological pitchers (n = 255; 2974 pitches). PE did not differ between groups (P =.856), whereas ball velocity was significantly faster in the discordant group (mean difference, 0.6 m/s; 95% CI, −1.1 to −0.3 m/s; t = −3.3; P =.0012). Chronological pitchers had significantly reduced shoulder distraction force (mean difference, −4.7% body weight (BW); 95% CI, −7.9% to −1.5% BW; t = −2.9; P =.004) with no difference in shoulder internal rotation torque (P =.160). These kinematic and kinetic differences were not observed when accounting for interpitcher variability. Conclusion: Between pitchers, those who had a discordant pelvic to upper torso sequence experienced significantly greater shoulder distraction forces, potentially compensating by increasing maximum shoulder external rotation and horizontal abduction. Achieving maximal pelvic rotation velocity before maximal rotation velocity may be advantageous in preventing compensation at the upper extremity and excessive throwing arm loading. Clinical Relevance: Identifying risk factors for increased upper extremity forces has potential implications in injury prevention. Specifically, mitigating shoulder distraction forces may be beneficial in reducing risk of injury. [ABSTRACT FROM AUTHOR]

To improve the adaptability in variable wave conditions, a novel oscillating water column (OWC) wave energy converter (WEC)-bottom mounted breakwater integrated system is proposed, in which the front-wall constrained by a torsional spring can oscillate in the pitch mode. In this study, an analytical model based on the eigenfunction matching method and technique of variables separation is developed to investigate the hydrodynamic performance (including energy conversion and structural safety) of the proposed integrated system. Taking the air compressibility and linear Power Take-off (PTO) system into account, the effects of the front-wall width, draft and spring stiffness on the performance are discussed, respectively. It shows that the interaction between the pitch motion of the front-wall and the air-pressure oscillation significantly broaden the frequency bandwidth of energy conversion efficiency (i.e., capture width ratio, C w), in comparison with the traditional OWC device with the fixed front-wall. It is also found that those frequency points corresponding to the peaks of C w satisfy the condition that the real part of complex air volume flux which is induced by unit air-pressure oscillation inside the chamber, is equal to zero, after decoupling and eliminating the influence of front-wall motion. • Hydrodynamics of an OWC WEC-breakwater integrated system is theoretically investigated. • Hydrodynamic efficiency is considerably enhanced when front-wall freely pitches. • The smaller the front-wall width, the wider the high-performance frequency bandwidth. • The front-wall draft exist an optimal value for the proposed system with pitching front-wall. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines, as large equipment for deep-sea wind energy utilization, must be carefully studied for their aerodynamic performances under platform motions. In experiments, the model wind turbine designed by the traditional thrust coefficient similarity is usually insufficient for unsteady aerodynamic reproductions of the prototype wind turbine. In this paper, a dynamic aerodynamic similarity criterion based on the mapping of the optimal tip speed ratio is introduced for wind tunnel model tests, and a model wind turbine is designed using the NREL 5-MW prototype wind turbine. In this case, to avoid the over-height model tower aroused in pitch motions, a method of equivalent surge motions instead of pitch motions is proposed and validated for feasibility. Then dynamic aerodynamic similarities under different pitch amplitudes and frequencies are validated by the Unsteady Blade Element Momentum theory. Results indicate that temporal and spatial characteristics and hysteresis effects of thrust and power coefficients all maintain better similarity with those of the prototype wind turbine than those of the model wind turbine designed by the traditional criterion. This paper provides a criterion with equivalent pitch motions for unsteady aerodynamic studies in wind tunnel model tests, especially the high-fidelity Hardware/Software-In-the-Loop test. • A new similarity criterion is proposed for FOWT wind tunnel model tests. • The feasibility of equivalent surge motions instead of pitch motions is verified. • The problem of over-height model tower is avoided by equivalent pitch motions. • Spatiotemporal and hysteresis characteristics of unsteady aerodynamics are analyzed. • Unsteady aerodynamic performances of the prototype can be reflected more accurately. [ABSTRACT FROM AUTHOR]

Improving a ship’s stern part could help to reduce greenhouse gases and costs. However, a ship sailing in actual conditions experiences disturbances that can affect its performance. Ship performance is an important aspect of the design process that guarantees ship safety. The heave and pitch motions of an improved fishing boat were predicted numerically by using a hybrid scheme of Eulerian grid-Lagrangian particle, hereinafter improving its stern part and attaching an additional part. The stern part improvement and additional structure attachment affected an increase on the heave amplitude from the ship’s basic form by 5% to 10%. Moreover, the improvement of the stern part in the bottom area contributed to a better heave amplitude than that of the side area. Finally, the pitch amplitude for all forms was relatively small and affected an increase of 5% to 9%, dependent on the form. The improvement had a greater effect on heave motion than pitch.

Among the various wave power systems, Salter’s duck (rotor) is one of the most effective wave absorbers for extracting wave energy. The rotor shape is designed such that the front part faces the direction of the incident wave, which forces it to bob up and down due to wave-induced water particle motion, whereas the rear part, which is mostly circular in shape, reflects no waves. The asymmetric geometric shape of the duck makes it absorb energy efficiently. In the present study, the rotor was investigated using WAMIT (a program based on the linear potential flow theory in three-dimensional diffraction/radiation analyses) in the frequency domain and verified using OrcaFlex (design and analysis program of marine system) in the time domain. Then, an experimental investigation was conducted to assess the performance of the rotor motion based on the model scale in a two-dimensional (2D) wave tank. Initially, a free decay test (FDT) was carried out to obtain the viscous damping coefficient. The pitch response was extracted from the experimental time series in a periodic regular wave for two different wave heights (1 cm and 3 cm). In addition, the viscous damping coefficient was calculated from the FDT result and fluid forces, obtained from WAMIT, are incorporated into the final response of the rotor. Finally, a comparative study based on experimental and numerical results (WAMIT & OrcaFlex) was performed to confirm the performance reliability of the designed rotor.

Tuned sloshing dampers (TSDs) are applied to dissipate and absorb vibrational energy in structures. They can become an appropriate candidate for damping vibration in rotating offshore structures. In this study, the TSD systems are utilized in a semi-submersible platform in order to suppress its pitch motion response. First, the hydrodynamic behaviors of two different types of vessels are evaluated including a typical GVA4000 semi-submersible rig, and a floating oil storage tank using a finite element analysis. The results are compared with the available data from previous research, which the agreement is good. Subsequently, the semi-submersible platform equipped with four TSDs, which are located inside the bilge of pontoons and filled with 20% water is analyzed. It is analyzed in the frequency domain by considering the effect of internal fluid sloshing of TSDs. The results show that TSDs play a significant role for reducing the pitch motion response of the semi-submersible platform.

Compared with bottom-fixed wind turbines, the supporting platform of a floating offshore wind turbine has a larger range of motion, so the gyroscopic effects of the system will be more obvious. In this paper, the mathematical analytic expression of the gyroscopic moment of a floating offshore wind turbine is derived firstly. Then, FAST software is utilized to perform a numerical analysis on the model of a spar-type horizontal axis floating offshore wind turbine, OC3-Hywind, so as to verify the correctness of the theoretical analytical formula and take an investigation on the characteristics of gyroscopic effect. It is found that the gyroscopic moment of the horizontal axis floating offshore wind turbine is essentially caused by the vector change of the rotating rotor, which may be due to the pitch or yaw motion of the floating platform or the yawing motion of the nacelle. When the rotor is rotating, the pitch motion of the platform mainly excites the gyroscopic moment in the rotor's yaw direction, and the yaw motion of the platform largely excites the rotor's gyroscopic moment in pitch direction, accordingly. The results show that the gyroscopic moment of the FOWT is roughly linearly related to the rotor's inertia, the rotor speed, and the angular velocity of the platform motion. [ABSTRACT FROM AUTHOR]

The aerodynamic performance of floating offshore wind turbines is extremely complex due to the motions of the floating platform. The surge and pitch motions are the most influential motions among the six degrees-of-freedom motions. In view of this, the aim of this study is to investigate the unsteady aerodynamic characteristics of a floating offshore wind turbine under single (surge or pitch) and combined motions using computational fluid dynamics simulations, In addition, the coupling technique of dynamic mesh and sliding mesh is employed, as well as the unsteady Reynolds averaged Navier-Stokes method. The numerical simulation method in this paper is first validated by comparison to the results of the blade element momentum method and the vortex method. Then, the aerodynamic characteristics of the floating offshore wind turbine under harmonic platform motions with different periods and amplitudes are investigated. The results show that the increase of amplitude and frequency aggravate the fluctuation of the overall aerodynamic performance of the wind turbine. In addition, the combined surge-pitch motion reduces the average power generation indicating that complex platform motions adversely affect the power generation of floating offshore wind turbines. • The effects of wind load were analyzed by comparing the platform motions under different amplitudes and frequencies. • Two representative platform motions and the combined motion were considered for analysis. • Analyze the aerodynamic characteristics of different airfoil sections to reveal the variations of overall performance. • The detailed analysis of the wake effects showed the three-dimensional flow under the platform motions. [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.

In this paper, a new type of ocean wave energy converter designated as the Articulated Tower - Ocean Wave Energy Converter (AT-OWEC) is introduced, especially for operation in Indonesian waters. The working principle is converting the rotational pitching motion of AT as the main structure when induced by wave excitation into translational motion of an arm to propel a double action piston pump (DAPP) which drive the water jet to move a turbine and hence generate electricity. At this stage the work is concentrated in the evaluation of the AT dynamic behavior under regular wave excitation. The formulation of the AT rotational pitch motion in 1-DOF is described, and is followed by the modelling utilizing analytical and numerical models. A case study is conducted on 3 variations of AT size to be operated in three different water depths d, namely 10.0 m, 15.0 m and 20.0 m. The variants are made of steel sized, Variant #1 Dh = 2.0 m, Hh = 3.5 m, t = 10.0 mm, Variant #2 Dh = 3.0 m, Hh = 4.5 m, t = 25.0 mm, and Variant #3 Dh = 4.0 m, Hh = 5.5 m, t = 30.0 mm. The results of the analysis show the largest RAO pitch motion occur in the case of Variant #1 when operated in the water depth d = 10.0 m reaching 49.24 deg/m, while the lowest one occurs in the case of this variant when operated in d = 20.0 m with the intensity of only 16.06 deg/m. Nonetheless, AT of Variant #3 is found to be more potential to harvest the greater wave energy in conjunction with the peculiar characteristic of the Indonesia sea wave frequency range. [ABSTRACT FROM AUTHOR]

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Improving a ship’s stern part could help to reduce greenhouse gases and costs. However, a ship sailing in actual conditions experiences disturbances that can affect its performance. Ship performance is an important aspect of the design process that guarantees ship safety. The heave and pitch motions of an improved fishing boat were predicted numerically by using a hybrid scheme of Eulerian grid-Lagrangian particle, hereinafter improving its stern part and attaching an additional part. The stern part improvement and additional structure attachment affected an increase on the heave amplitude from the ship’s basic form by 5% to 10%. Moreover, the improvement of the stern part in the bottom area contributed to a better heave amplitude than that of the side area. Finally, the pitch amplitude for all forms was relatively small and affected an increase of 5% to 9%, dependent on the form. The improvement had a greater effect on heave motion than pitch. [ABSTRACT FROM AUTHOR]

Abstract Pitch and surge motions of platforms are two main movements of offshore floating vertical axis wind turbines when they are faced with wind and wave loads. These two movements can not only affect the aerodynamic loads, but also may influence the wake characteristics of the wind turbines. In the present study, the computational fluid dynamics approach with the improved delayed detached eddy simulation (IDDES) and the overset mesh are employed to investigate the wake characteristics of an H-rotor floating vertical axis wind turbine under the platform's pitch and surge motions. The wake profiles and structures between pitch and non-pitch conditions, and surge and non-surge conditions are compared. Then, the wake characteristics under different pitching amplitudes and periods and different surging amplitudes and periods are investigated. It is shown that pitch motion can enlarge the peak values of wake deficits and shift the positions of the mainstream of the wake in the vertical direction. The amplitudes and periods of the 'wake wave' under different pitching amplitudes and periods show regular variations. A 'Spindle' -type of wake structure is formed in the wake region under a platform's surge motion, and the strength and core zone of the 'Spindle' also regularly change with different surging amplitudes and periods. These changes cause wake profile's diversity at certain motion stages for the same downwind distance under different periods and amplitudes of surge and pitch. Highlights • Wake characteristics in pitch and surge motions are investigated by the CFD method. • Pitch and surge motions can distinctly change the wake profiles and structures. • The periods and amplitudes of the 'wake wave' change against the pitching amplitudes and periods. • A 'Spindle' -type wake structure appears and changes with the surging amplitudes and periods. [ABSTRACT FROM AUTHOR]

Pitch motion is the boat's rotation around its horizontal axis and is represented in a cyclical lowering and raising of bow and stern. Pitch amplitude depends on boat trim and rowing technique and yields to increased intra-cyclical fluctuations in boat speed. The study examines different foot-stretcher heights and stroke rates (SR) to investigate their effects on rowing technique and pitch motion using a new measuring system. It was hypothesized that the pitch motion differs depending on foot-stretcher height, and SR. Field testing included 12 male rowers performing a SR step test and 500-m-distances with different foot-stretcher heights in the single scull. Rowing angle, seat position and pitch motion data was variance-analytical evaluated. With raised foot-stretcher height, stroke length and distance travelled by the sliding seat decreased. Reduction of pitch amplitude was greater than the SEM. Higher pitch amplitudes were found at SR 20 compared to 24 and 28. A significant positive correlation between pitch amplitude and body mass revealed at SR 28 and 32. Results verify the expected influence of foot-stretcher height and SR on pitch motion. Changes in pitch motion were observed with practical relevance. An optimal foot-stretcher height should be found for the athletes. [ABSTRACT FROM AUTHOR]

Floating horizontal-axis wind turbines (FHAWTs) experience six degrees of freedom motion in space with their floating platforms. Such dynamic process may induce complex dynamic flow and variable local load on the blades, however, still insufficiently understood from a perspective of unsteady aerodynamics. In this study, the local aerodynamic load in the dynamic process induced by the platforms' pitch motion is investigated by computational fluid dynamics. Our results indicate that the axial force of each blade section fluctuates significantly over time, with varying levels of fluctuation among the blade sections. The pressure distributions around the blade sections also vary significantly. Additionally, we observe the light dynamic stall phenomenon that cause stronger lift in upstroke and weaker lift in downstroke, and the flow separation and reattachment process during the dynamic stall demonstrates a delayed response relative to instantaneous changes in the angle of attack. The load variations due to the dynamic process and dynamic stall can lead to severe fatigue and extreme loads on the surface and structure of the blades. These findings suggest that quasi-steady analysis, which neglects the dynamic process, is not sufficient for accurately predicting the blade loads under pitch motion. Furthermore, we find that the fatigue-load properties vary with location on a blade section, with the leading and trailing edges being more susceptible to fatigue problems. These results highlight the importance of considering the potential harmful effects of pitch motion in the design of FHAWTs. • The local loads of the blades experience dynamic process due to FHAWTs' pitch motion. • CFD is employed to explore local loads of the blade sections of FHAWTs. • The local pressure distributions fluctuate significantly over time during the dynamic process. • Dynamic stall phenomenon occurs during the pitch motion of FHAWTs. • The dynamic process and dynamic stall may cause severe fatigue and extreme loads. The leading and trailing edges are more prone to the fatigue problem during the dynamic process. [ABSTRACT FROM AUTHOR]

A novel type of control law was adopted to reduce the vertical acceleration of a fast ferry as well as the motion sickness incidence suffered by the passengers onboard by means of a submerged T-foil. Considering the system changing characteristics under high disturbances, a model-free approach was adopted. In addition, an upgraded proportional-derivative (PD) controller with correction terms resulting from a fast-online estimation of the system dynamics was designed. The overall controller, known as intelligent PD (i-PD) controller, was tested, and the obtained results were compared with those of a classic PD controller. The controllers were also tested in a changing environment and at different operating velocities. The results confirmed the effectiveness of the i-PD controller to smooth the motions with low computational cost control schemes. Furthermore, thanks to ability of the i-PD controller to continually update the estimated dynamics of the system, it showed a better reduction in both vertical motions and the seasickness level of the passengers with the needed robustness under external disturbances and system changing parameters. [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]