Search Results (71)

Offshore wind turbine support structures are in harsh and unsteady marine environments, and their dynamic characteristics could change gradually after long-term service. To better understand the status and improve remaining life estimation, it is essential to conduct in situ measurement and update the numerical models of these support structures. In this paper, the modal properties of a 5.5 MW offshore wind turbine were first identified by a widely used operational modal analysis technique, frequency-domain decomposition, given the acceleration data obtained from eight sensors located at four different heights on the tower. Then, a finite element model was created in MATLAB R2020a and a set of model parameters including scour depth, foundation stiffness, hydrodynamic added mass and damping coefficients was updated in a Bayesian inference frame. It is found that the posterior distributions of most parameters significantly differ from their prior distributions, except for the hydrodynamic added mass coefficient. The predicted natural frequencies and damping ratios with the updated parameters are close to those values identified with errors less than 2%. But relatively large differences are found when comparing some of the predicted and identified mode shape coefficients. Specifically, it is found that different combinations of the scour depth and foundation stiffness coefficient can reach very similar modal property predictions, meaning that model updating results are not unique. This research demonstrates that the Bayesian inference framework is effective in constructing a more accurate model, even when confronting the inherent challenge of non-unique parameter identifiability, as encountered with scour depth and foundation stiffness. Full article

The wheel–rail friction coefficient is a critical factor influencing train traction and braking performance. Low-adhesion conditions not only limit the enhancement of railway transport capacity but are also the primary cause of surface damage such as scratches, delamination, and flat spots. This study employs femtosecond laser technology to fabricate wavy groove textures on U75V rail surfaces, systematically investigating the effects of the wavy angle and texture area ratio on friction enhancement under various medium conditions. Findings indicate that parameter-optimized textured surfaces not only significantly increase the coefficient of friction but also exhibit superior wear resistance, vibration damping, and noise reduction properties. The optimally designed wavy textured surface achieves significant friction enhancement under water conditions. Among the tested configurations, the surface with parameters θ = 150°@η = 30% demonstrated the most pronounced friction enhancement, achieving a coefficient of friction as high as 0.57—a 42.5% increase compared to the non-textured surface (NTS). This enhancement is attributed to the unique hydrophilic and anisotropic characteristics of the textured surface, where droplets tend to spread perpendicular to the sliding direction, thereby hindering the formation of a continuous lubricating film as a third body. Analysis of friction vibration signals reveals that textured surfaces exhibit lower vibration signal amplitudes and richer frequency components. Furthermore, comparison of Stribeck curves under different lubrication regimes for the θ = 150°@η = 30% specimen and NTS indicated an overall upward shift in the curve for the textured sample. The amplitude, energy, and wear extent of the textured surface consistently decreased across boundary lubrication, hydrodynamic lubrication, and mixed lubrication regimes. These findings provide crucial theoretical insights and technical guidance for addressing low-adhesion issues at the wheel–rail interface, offering significant potential to enhance wheel–rail adhesion characteristics in engineering applications. Full article

This work presents an experimental and theoretical study of a pitching point-absorber wave energy converter (WEC) equipped with a nonlinear stiffness mechanism (NSM) based on a pre-compressed spring. The mechanism is designed to reduce the equivalent restoring stiffness and enhance the device response without external control. A 1:13 scale prototype of the Lafkenewen WEC, deployed off Lebu (Chile), was tested in regular waves within a wave tank for two configurations: with and without the NSM. The rotational response amplitude operator (RAO) was obtained from experiments and compared against a linear hydrodynamic model formulated via Newtonian mechanics and frequency domain radiation/excitation coefficients. Dry friction at the hinge was represented as an equivalent viscous damping term identified iteratively. Unlike narrow-resonance WECs, both configurations exhibited a broadband response without a sharp resonance peak in the $0.7$–$1.2$ Hz range, due to significant radiation damping and hinge friction. The NSM produced a moderate amplification of the rotational RAO (up to ∼32%) while preserving the broadband character. Theoretical predictions agreed with the measurements when dry friction was included. These results demonstrate that passive stiffness reduction via an NSM enhances wave–structure energy transfer even in systems dominated by effective damping and provides a consistent basis for future nonlinear time domain modeling and control-oriented studies. Full article

This study investigates the hydrodynamic response of a spar-type buoy equipped with a solid, perforated, and novel corrugated plate appendage introduced here for the first time to enhance motion damping. A hybrid approach combining time-domain CFD simulations and frequency-domain potential-flow analysis was employed, providing a framework to incorporate viscous effects that are often omitted in potential-flow models. In the first stage, free-decay simulations were carried out in ANSYS Fluent for a baseline spar and three appendage-equipped configurations. The resulting heave and pitch decay responses were analyzed to determine natural frequencies and viscous damping coefficients. Prior to that, the CFD solver was validated and verified against published experimental data, confirming the reliability of the numerical setup. In the second stage, frequency-domain hydrodynamic diffraction analysis was conducted in ANSYS AQWA, and the CFD-derived viscous damping coefficients were incorporated into the potential-flow model to improve motion predictions near resonance. The comparison between RAOs with and without viscous damping indicated reductions of approximately 55–62% in heave and 41–60% in pitch at resonance, with the perforated plate consistently yielding the highest damping and lowest RAO peaks. This work introduces the first corrugated plate appendage design for spar buoys and establishes a validated CFD–potential-flow hybrid framework that enables more realistic motion predictions and provides practical design guidance for damping-enhanced spar buoys in offshore energy applications. Full article

The paper studies the dynamic characteristics of a multi-layer foil thrust bearing (MLFTB). A modified efficient dynamic characteristic model is established, and the revised Reynolds equation coupled with the thick plate element and the boundary slip model is adopted. During the solving process, the small perturbation method is implemented. The elasto-hydrodynamic effect under geometric and operational parameters is investigated. It reflects that the dynamic characteristics can be visibly influenced by the slip effect when under tiny clearance with low bearing speed, and ought to be considered. Specifically, the maximum deviation of the axial and direct-rotational stiffness coefficients could be up to −4.93% and −5.02%, respectively. The direct-rotational stiffness is increased with the perturbation frequency; however, a turning point may exist in the cross-rotational stiffness. Additionally, both the rotational stiffness and rotational damping can be expanded at a smaller original clearance. It aims to provide prediction methods with high effectiveness and efficiency, and enrich theoretical guidance for the important MLFTB. Full article

This work proposes a novel design configuration for an elasto-poro-hydrodynamic damper (EPHD damper) that consists of an imbibed, soft, elastic, porous material enclosed by a rubber membrane. The core innovation lies in the device’s ability to collect and re-imbibe expelled fluid during decompression, ensuring potential functionality and durability across repetitive loading cycles. Damping is achieved through the synergy of three mechanisms: friction of the membrane and of the piston with solid boundaries, squeeze flow inside the porous layer, and compression of the poro-elastic structure. The EPHD damper’s behavior was evaluated both theoretically and experimentally through quasi-static, low-speed compression tests, with dynamic evaluation being reserved for future work. A numerical model successfully validated stress-deformation behavior against experimental data, with a simplified analytical model providing a good approximation. The study also identifies that the piston–membrane friction coefficient significantly influences the EPHD damper’s performance. These findings provide a valuable framework for optimizing the design and expanding its potential application to repetitive damping systems. Full article

The paper presents a two-dimensional RANS–SST k–ω investigation of vortex-induced vibration of a square cylinder with two degrees of freedom under combined steady and oscillatory flow at the Reynolds number of 5000, Keulegan–Carpenter number of 10, mass ratio of 2.5, and zero structural damping. Flow ratio a (steady-to-total velocity) is varied from 0 to 1.0, and the reduced velocity $U_r$ from 2 to 25 to map lock-in regimes, response amplitudes, frequency content, hydrodynamic loads, trajectories, and wake patterns. At low a ≤ 0.4, in-line vibrations dominate at $U_r$ > 5, with double-frequency transverse lock-in peaking near $U_r$ = 5. As a → 1.0, in-line motion diminishes, and single-frequency transverse oscillation prevails, with the maximum transverse displacement up to 0.54D. The mean drag coefficient increases with increasing flow ratio; the fluctuating drag coefficient decreases with increasing a; while the lift coefficient peaks at a = 1, $U_r$ = 2. Wake topology transitions from a mixed vortex shedding towards a 2S pattern, as a → 1. Full article

This paper proposes novel designs of dual-layer flower-shaped heave plates, featuring both aligned and staggered configurations with three, six, and nine petals. Numerical simulations were conducted to study the hydrodynamic effects of these various heave plate designs integrated with the OC4 DeepCwind semisubmersible floating offshore wind turbine platform under prescribed heave oscillations. The overset mesh technique was employed to treat the floating platform’s motions. Comprehensive assessments of vertical force, radiated wave patterns, vorticity fields, added mass, and damping coefficients were conducted. The results revealed that the novel flower-shaped staggered heave plates significantly outperformed conventional circular plates in terms of damping coefficients. Specifically, the damping coefficient of flower-shaped staggered heave plates was greater than that of circular heave plates, while the aligned configuration exhibited a lower damping coefficient. The damping coefficient increased with a reduction in the number of petals for the staggered heave plates. Among the evaluated designs, the dual-layer flower-shaped staggered heave plates with three petals demonstrated the highest effectiveness in attenuating heave motion of the floating platform. The utilization of novel dual-layer flower-shaped staggered heave plates is therefore a promising practice aimed at damping the heave motion of platforms in rough seas. Full article

Fine-mesh netting, with mesh dimensions of the order of a few millimeters, is widely used in sampling nets for the collection of larval and juvenile fishes. The wave force characteristics of fine-mesh netting significantly affect the operational performance of these nets. This study employed both wave tank experiments and numerical simulations to analyze the hydrodynamic performance of fine-mesh netting under varying wave conditions. A series of numerical simulations and particle image velocimetry (PIV) experiments were conducted to investigate the damping effects of fine-mesh netting on wave propagation. The results revealed that horizontal wave forces increased with both the wave period and wave height. When the wave period was held constant, the drag and inertial coefficients of the netting generally decreased as the Reynolds number and the Keulegan–Carpenter (KC) number increased. The wave transmission coefficients of the netting decreased as the wave height increased for the same wave period. However, at a constant wave height, the transmission coefficients initially increased and then decreased with the increasing wave period. The water particle velocity was significantly affected by the netting, with a notable reduction in velocity downstream of the netting at both the wave crest and trough phases. The simulation results and PIV measurements of the water particle velocity field distribution were in good agreement. This study provides important insights for the design and optimization of sampling nets. Full article

This study investigates the use of viscous dampers (VDs) to reduce the vibration of a deepwater offshore platform under joint wind, wave, and earthquake action. A finite element model was established based on the Opensees software (version 3.7.1), incorporating soil–structure interaction simulated by the nonlinear Winkler springs and simulating hydrodynamic loads via the Morison equation. Turbulent wind fields were generated using the von Kármán spectrum, and irregular wave profiles were synthesized from the JONSWAP spectrum. The 1995 Kobe earthquake record served as seismic input. The time-history dynamic response for the deepwater offshore platform was evaluated under two critical scenarios: isolated seismic excitation and the joint action of wind, wave, and seismic loading. The results demonstrate that VDs configured diagonally at each structural level effectively suppress platform vibrations under both isolated seismic and wind–wave–earthquake conditions. Under seismic excitation, the VD system reduced maximum deck acceleration, velocity, displacement, and base shear force by 9.95%, 22.33%, 14%, and 31.08%, respectively. For combined environmental loads, the configuration achieved 15.87%, 21.48%, 13.51%, and 34.31% reductions in peak deck acceleration, velocity, displacement, and base shear force, respectively. Moreover, VD parameter analysis confirms that increased damping coefficients enhance control effectiveness. Full article

Nonlinear vibration phenomena, such as oil whirl and oil whip, are common indicators of oil film instability in hydrodynamic bearings and are key signs of potential faults in rotating machinery. Excessive vibrations caused by oil film instability can accelerate bearing wear and lead to the failure of the rotating system. This paper presents a model for nonlinear dynamic coefficients, aimed at providing a quantitative approach for monitoring and predicting oil film instability. The impact of operational parameters and perturbation values on both linear and nonlinear stiffness and damping coefficients is investigated. Simulation results and experimental rotor vibration signals demonstrate that the nonlinear dynamic coefficient model effectively characterizes oil film instability and accurately predicts rotor trajectory, while traditional linear models are only applicable under low-speed and small-disturbance conditions. Compared to traditional analytical models and numerical solutions, the nonlinear dynamic coefficients have higher accuracy and efficiency and can reliably identify the onset frequency of oil film instability. This study clarifies the relationship between nonlinear dynamic coefficients and rotor dynamic response, laying a theoretical foundation for the monitoring and prediction of oil film instability. Full article

Using the Rabinowitsch fluid model, the effects of dilatant fluids (a non-Newtonian factor less than 0) and pseudo-plastic fluids (a non-Newtonian factor greater than 0) on the non-linear stability of hydrodynamic journal bearings is discussed. The modified Reynolds equation is solved by the small perturbation method, and the stiffness and damping coefficients and threshold speed of hydrodynamic bearings are obtained. Through the fourth-order Runge–Kutta method, the trajectory of the journal center is traced and the non-linear stability boundary of hydrodynamic bearings with different non-Newtonian lubricants could be confirmed. The results show that the dilatant fluids could increase the threshold speed and enlarge the non-linear stability region of the hydrodynamic bearing, while the influence of pseudo-plastic fluids is positive. And for the lubricants with a larger non-Newtonian factor, the effect is more obvious. It could be confirmed that the stability of hydrodynamic bearings lubricated with dilatant fluids is better than that of bearings with Newtonian and pseudo-plastic lubricants. The results illustrate that selecting a more appropriate lubricant can enhance the stability of hydrodynamic bearings. Full article

Due to the limited availability of land resources, offshore wind turbines have become a crucial technology for the development of deep-water renewable energy. The multi-floating body platform, characterized by its shallow draft and main body located near the sea surface, is prone to significant motion in marine environments. The proper chamfering of the heave plate can effectively enhance its resistance during wave action, thereby improving the stability of the floating platform. The optimal chamfer angle is 35°. Considering the complexity of the floating body’s motion response, this study focuses on the damping characteristics of the heave plate with 35° chamfered perforations. Using the NREL 5 MW three-column semi-submersible floating wind turbine platform as the research model, the hydrodynamic characteristics of the floating body with a perforated heave plate are systematically studied through theoretical analysis, numerical simulation, and physical tests. The amplitude of vertical force under various working conditions is measured. Through theoretical analysis, the additional mass coefficient and additional damping coefficient for different working conditions and models are determined. The study confirms that the heave plate with 35° chamfered perforations significantly reduces heave in the multi-floating body. Full article

Aiming at the problem of abnormal wear caused by the poor dynamic characteristics of aeroengine segmented annular seals, according to the hydrodynamic lubrication theory, based on the conventional structure featuring the Rayleigh step profile rectangular pocket (RP), novel structures with the circumferential linear convergent pocket (CLCP) and the circumferential parabolic convergent pocket (CPCP) were proposed. A model was developed to analyze both the static and dynamic characteristics of three types of segmented annular seals, utilizing the local differential quadrature (LDQ) method. Once the accuracy of the solution model was confirmed, the effects of working conditions and design features on both static and dynamic characteristics were analyzed. Results indicate that the circumferential wedge convergent pockets can effectively improve the dynamic characteristics of the seal system. Under different rotational speeds, compared with the RP seal, the CLCP seal’s stiffness coefficient and damping coefficient increases by an average of 60.76% and 65.27%, respectively. As the rotational speed increases, the RP seal damping ratio decreases, and the seal system transitions from an overdamped state to an underdamped state, resulting in reduced stability. Nevertheless, under different rotational speeds and pressure ratios, the CLCP and the CPCP seals are both in an overdamped state. Taking into account the static and dynamic characteristics, the CLCP seal is the optimal structure in this study. Full article

Predicting the roll damping coefficient of a ship is a crucial factor in determining the dynamic stability of the vessel. However, a nonlinear analysis that considers the viscosity of the fluid is required to accurately estimate the roll damping coefficient. This study numerically analyzed the hydrodynamic coefficients related to the roll motion of ships, focusing on the eddy-making damping coefficient. A series of forced vibration tests were conducted on a two-dimensional triangular cylinder floating on the water surface. The overset method and the volume-of-fluid method were applied, and the governing equations were solved using the open-source software OpenFOAM v2106. Uncertainties in the grid size and time intervals were identified through the International Towing Tank Conference (ITTC) procedure, and the obtained hydrodynamic coefficients were compared with available experimental data and potential flow results. Additionally, eddy-making damping was extracted from the shed vortex for various excitation frequencies and amplitudes. The study found that the uncertainty in the roll damping coefficient was less than 8%, with eddy-making damping being the dominant factor influencing the results. Numerical results showed a good agreement with experimental data, with an average deviation of 4.4%, highlighting the importance of considering nonlinear effects at higher excitation amplitudes. Comparison with experimental data and empirical formulas revealed that the nonlinearity due to the excitation amplitude must be considered in empirical formulations. Full article