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Search Results (733)

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Keywords = low-frequency motion

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15 pages, 2931 KB  
Article
Low Poisson’s Ratio Measurement on Composites Based on DIC and Frequency Analysis on Tensile Tests
by Luis Felipe-Sesé, Andreas Kenf, Sebastian Schmeer, Elías López-Alba and Francisco Alberto Díaz
J. Compos. Sci. 2025, 9(10), 570; https://doi.org/10.3390/jcs9100570 - 16 Oct 2025
Viewed by 191
Abstract
Accurate determination of elastic properties, especially Poisson’s ratio, is crucial for the design and modeling of composite materials. Traditional methods often struggle with low strain measurements and non-uniform strain distributions inherent in these anisotropic materials. This research work introduces a novel methodology that [...] Read more.
Accurate determination of elastic properties, especially Poisson’s ratio, is crucial for the design and modeling of composite materials. Traditional methods often struggle with low strain measurements and non-uniform strain distributions inherent in these anisotropic materials. This research work introduces a novel methodology that integrates Digital Image Correlation (DIC) with frequency analysis techniques to improve the precision of Poisson’s ratio determination during tensile tests, particularly at low strain ranges. The focus is on the evaluation of two distinct frequency-based approaches: Phase-Based Motion Magnification (PBMM) and Lock-in filtering. DIC + PBMM, while promising for motion amplification, encountered specific challenges in this application, particularly at very low strain amplitudes, leading to increased variability and computational demands. In contrast, the DIC + Lock-in filtering method proved highly effective. It provided stable, filtered strain distributions, significantly reducing measurement uncertainty compared to traditional DIC and other conventional methods like strain gauges and Video Extensometers. This study demonstrates the robust potential of Lock-in filtering for characterizing subtle periodic mechanical behaviors leading to a reduction of approximately 70% in the standard deviation of the measurement. This work lays a strong foundation for more precise and reliable material characterization, crucial for advancing composite design and engineering applications. Full article
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12 pages, 2841 KB  
Article
Mesoscopic Liquids Emit Thermal Waves Under Shear Strain or Microflow
by Laurence Noirez, Eni Kume and Patrick Baroni
Liquids 2025, 5(4), 27; https://doi.org/10.3390/liquids5040027 - 9 Oct 2025
Viewed by 157
Abstract
Liquids like water are not expected to produce a thermal change under shear strain or flow (away from extreme conditions). In this study, we reveal experimental conditions for which the conventional athermal hydrodynamic assumption is no longer valid. We highlight the establishment of [...] Read more.
Liquids like water are not expected to produce a thermal change under shear strain or flow (away from extreme conditions). In this study, we reveal experimental conditions for which the conventional athermal hydrodynamic assumption is no longer valid. We highlight the establishment of non-equilibrium hot and cold thermal states occurring when a mesoscopic confined liquid is set in motion. Two stress situations are considered: low-frequency shear stress at large strain amplitude and microfluidic transport (pressure gradient). Two liquids are tested: water and glycerol at room temperature. In confined conditions (submillimeter scale), these liquids exhibit stress-induced thermal waves. We interpret the emergence of non-equilibrium temperatures as a consequence of the solicitation of the mesoscopic liquid elasticity. In analogy with elastic deformation, the mesoscopic volume decreases or increases slightly, which leads to a change in temperature (thermo-mechanical energy conversion). The energy acquired or released is converted to heat or cold, respectively. To account for these non-equilibrium temperatures, the mesoscopic flow is no longer considered as a complete dissipative process but as a way of propagating shear and thus compressive waves. This conclusion is consistent with recent theoretical developments showing that liquids propagate shear elastic waves at small scales. Full article
(This article belongs to the Section Physics of Liquids)
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22 pages, 8129 KB  
Article
A Low-Frequency Component Filtering Method for Heave Acceleration Signal of Marine Ship
by Dejian Sun, Xiong Hu, Chongyang Han and Xinqiang Chen
J. Mar. Sci. Eng. 2025, 13(10), 1919; https://doi.org/10.3390/jmse13101919 - 6 Oct 2025
Viewed by 264
Abstract
The motion of ships in the ocean follows six degrees of freedom, and accurately measuring this motion is crucial for improving marine engineering operations. Among the six degree-of-freedom movement of ships, the change in ship heave freedom has the worst impact on offshore [...] Read more.
The motion of ships in the ocean follows six degrees of freedom, and accurately measuring this motion is crucial for improving marine engineering operations. Among the six degree-of-freedom movement of ships, the change in ship heave freedom has the worst impact on offshore lifting operations. At present, the most common method for measuring heave displacement is by integrating heave acceleration twice. The heave motion of ships belongs to low-frequency motion, but the low-frequency band range is often easily overlooked. This paper first analyzes the wave spectrum to determine the dominant frequency range of ship heave motion under typical wind speeds, which is found to be between 0.22 Hz and 0.45 Hz. The accuracy of low-frequency ship heave displacement signals largely depends on the heave acceleration signal, and filtering acceleration signals in the low-frequency range is particularly difficult. To address this challenge, this paper proposes a low-frequency component filtering method for heave acceleration signal of marine ships, which effectively avoids the phase and peak-to-peak errors introduced by traditional filters. This method further improves the filtering performance of acceleration signals in the 0.2 Hz to 0.5 Hz low-frequency range and can provide the crane driver with a motion reference for the heave of the ship when the ship is performing lifting operations. Full article
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22 pages, 3598 KB  
Article
Research on Denoising Methods for Magnetocardiography Signals in a Non-Magnetic Shielding Environment
by Biao Xing, Xie Feng and Binzhen Zhang
Sensors 2025, 25(19), 6096; https://doi.org/10.3390/s25196096 - 3 Oct 2025
Viewed by 445
Abstract
Magnetocardiography (MCG) offers a noninvasive method for early screening and precise localization of cardiovascular diseases by measuring picotesla-level weak magnetic fields induced by cardiac electrical activity. However, in unshielded magnetic environments, geomagnetic disturbances, power-frequency electromagnetic interference, and physiological/motion artifacts can significantly overwhelm effective [...] Read more.
Magnetocardiography (MCG) offers a noninvasive method for early screening and precise localization of cardiovascular diseases by measuring picotesla-level weak magnetic fields induced by cardiac electrical activity. However, in unshielded magnetic environments, geomagnetic disturbances, power-frequency electromagnetic interference, and physiological/motion artifacts can significantly overwhelm effective magnetocardiographic components. To address this challenge, this paper systematically constructs an integrated denoising framework, termed “AOA-VMD-WT”. In this approach, the Arithmetic Optimization Algorithm (AOA) adaptively optimizes the key parameters (decomposition level K and penalty factor α) of Variational Mode Decomposition (VMD). The decomposed components are then regularized based on their modal center frequencies: components with frequencies ≥50 Hz are directly suppressed; those with frequencies <50 Hz undergo wavelet threshold (WT) denoising; and those with frequencies <0.5 Hz undergo baseline correction. The purified signal is subsequently reconstructed. For quantitative evaluation, we designed performance indicators including QRS amplitude retention rate, high/low frequency suppression amount, and spectral entropy. Further comparisons are made with baseline methods such as FIR and wavelet soft/hard thresholds. Experimental results on multiple sets of measured MCG data demonstrate that the proposed method achieves an average improvement of approximately 8–15 dB in high-frequency suppression, 2–8 dB in low-frequency suppression, and a decrease in spectral entropy ranging from 0.1 to 0.6 without compromising QRS amplitude. Additionally, the parameter optimization exhibits high stability. These findings suggest that the proposed framework provides engineerable algorithmic support for stable MCG measurement in ordinary clinic scenarios. Full article
(This article belongs to the Section Biomedical Sensors)
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18 pages, 2583 KB  
Article
A Numerical Study on the Seakeeping Performance and Ride Comfort of a Small MonoHull Vessel With and Without Hydrofoil in Regular Head Seas
by Jungeun Kim, Woojun Oh and Wook Kwon
J. Mar. Sci. Eng. 2025, 13(10), 1895; https://doi.org/10.3390/jmse13101895 - 2 Oct 2025
Viewed by 307
Abstract
This study numerically investigates the effect of hydrofoil installation on the motion responses and ride comfort of a 20 m monohull vessel operating at 10 knots in regular waves. Linear seakeeping analysis (Maxsurf Motions) and nonlinear computational fluid dynamics (CFD) simulations (STAR-CCM+) are [...] Read more.
This study numerically investigates the effect of hydrofoil installation on the motion responses and ride comfort of a 20 m monohull vessel operating at 10 knots in regular waves. Linear seakeeping analysis (Maxsurf Motions) and nonlinear computational fluid dynamics (CFD) simulations (STAR-CCM+) are performed to compute response-amplitude operators (RAOs); for the bare hull, the two methods agree within 5%, confirming methodological reliability. The CFD results show that hydrofoils reduce heave and pitch amplitudes by approximately 16% on average. Motion Sickness Incidence (MSI) analysis indicates negligible seasickness under Gentle Breeze conditions, even during prolonged exposure; under Moderate conditions, no seasickness is predicted within 30 min across all encounter frequencies. Although linear analysis cannot directly estimate MSI for hydrofoil-fitted cases, the observed reductions in RAOs imply improved ride comfort. Overall, these findings demonstrate that hydrofoils can enhance motion stability and passenger comfort in small, low-speed vessels, providing quantitative evidence to support design applications. Full article
(This article belongs to the Section Ocean Engineering)
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12 pages, 1102 KB  
Article
Self-Motion Misperception Induced by Neck Muscle Fatigue
by Fabio Massimo Botti, Marco Guardabassi, Chiara Occhigrossi, Mario Faralli, Aldo Ferraresi, Francesco Draicchio and Vito Enrico Pettorossi
Audiol. Res. 2025, 15(5), 128; https://doi.org/10.3390/audiolres15050128 - 2 Oct 2025
Viewed by 196
Abstract
Background/Objectives: Previous research has demonstrated that the perception of self-motion, as signaled by cervical proprioception, is significantly altered during neck muscle fatigue, while no similar effects are observed when self-motion is signaled by the vestibular system. Given that in typical natural movements, both [...] Read more.
Background/Objectives: Previous research has demonstrated that the perception of self-motion, as signaled by cervical proprioception, is significantly altered during neck muscle fatigue, while no similar effects are observed when self-motion is signaled by the vestibular system. Given that in typical natural movements, both proprioceptive and vestibular signals are activated simultaneously, this study sought to investigate whether the misperception of motion persists during neck muscle fatigue when both proprioceptive and vestibular stimulation are present. Methods: The study evaluated the gain of the perceptual responses to symmetric yaw sinusoidal head rotations on a stationary trunk during visual target localization tasks across different rotational frequencies. In addition, the final localization error of the visual target was assessed following asymmetric sinusoidal head rotations with differing half-cycle velocities. Results: The findings indicated that even with combined proprioceptive and vestibular stimulation, self-motion perceptual responses under neck muscle fatigue showed a pronounced reduction in the gain at low-frequency stimuli and a notable increase in localization error following asymmetric rotations. Notably, spatial localization error was observed to persist after asymmetric stimulation conditioning in the light. Additionally, even moderate levels of muscle fatigue were found to result in increased self-motion misperception. Conclusions: This study suggests that neck muscle fatigue can disrupt spatial orientation, even when the vestibular system is activated, so that slow movements are inaccurately perceived. This highlights the potential risks associated with neck muscle fatigue in daily activities that demand precise spatial perception. Full article
(This article belongs to the Section Balance)
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25 pages, 5853 KB  
Article
Effects of Dish-Shaped Buoy and Perforated Damping Plate on Power Absorption in Floating Two-Body Wave Energy Converters
by Lilei Li, Changdong Wei, Mingchen Li, Xuening Song, Yanjun Liu and Gang Xue
J. Mar. Sci. Eng. 2025, 13(10), 1881; https://doi.org/10.3390/jmse13101881 - 1 Oct 2025
Viewed by 300
Abstract
Floating two-body wave energy converters (WECs) exhibit advantages, including insensitivity to water depth and tidal range, along with adaptability to multi-level sea states. However, WECs suffer from drawbacks, including unstable power generation and low wave energy capture efficiency. To enhance the hydrodynamic performance [...] Read more.
Floating two-body wave energy converters (WECs) exhibit advantages, including insensitivity to water depth and tidal range, along with adaptability to multi-level sea states. However, WECs suffer from drawbacks, including unstable power generation and low wave energy capture efficiency. To enhance the hydrodynamic performance and energy capture efficiency, a dish-shaped buoy and perforated damping plate configuration was designed based on conventional two-body WECs. First, four two-body WECs were developed according to these configurations. Second, a numerical model based on potential flow theory and the boundary element method (BEM) was established, with its accuracy validated through sea trials. Finally, the frequency domain response, motion response, mooring tension and power absorption effect of the WECs under wave excitation of grades 3, 4 and 5 were analyzed. The results demonstrate that both the dish-shaped buoy and perforated damping plate significantly improve the device stability and energy capture potential. Regarding the motion response, both configurations reduced the peak response amplitudes in heave and roll, enhancing the device stability. For mooring tension, both configurations reduced the mooring line tension. For power absorption, the perforated damping plate effectively increased the energy capture efficiency, while the dish-shaped buoy also demonstrated superior performance under higher-energy wave conditions. Overall, this study provides a theoretical foundation and design guidance for floating two-body WECs. Full article
(This article belongs to the Section Ocean Engineering)
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16 pages, 1756 KB  
Article
The Effects of Vibrotactile Stimulation of the Upper Extremity on Sensation and Perception: A Study for Enhanced Ergonomic Design
by Abeer Abdel Khaleq, Yash More, Brody Skaufel and Mazen Al Borno
Theor. Appl. Ergon. 2025, 1(2), 8; https://doi.org/10.3390/tae1020008 - 29 Sep 2025
Viewed by 323
Abstract
Vibrotactile stimulation has applications in a variety of fields, including medicine, virtual reality, and human–computer interaction. Eccentric Rotating Mass (ERM) vibrating motors are widely used in wearable haptic devices owing to their small size, low cost, and low-energy features. User experience with vibrotactile [...] Read more.
Vibrotactile stimulation has applications in a variety of fields, including medicine, virtual reality, and human–computer interaction. Eccentric Rotating Mass (ERM) vibrating motors are widely used in wearable haptic devices owing to their small size, low cost, and low-energy features. User experience with vibrotactile stimulation is an important factor in ergonomic design for these applications. The effects of ERM motor vibrations on upper-extremity sensation and perception, which are important in the design of better wearable haptic devices, have not been thoroughly studied previously. Our study focuses on the relationship between user sensation and perception and on different vibration parameters, including frequency, location, and number of motors. We conducted experiments with vibrotactile stimulation on 15 healthy participants while the subjects were both at rest and in motion to capture different use cases of haptic devices. Eight motors were placed on a consistent set of muscles in the subjects’ upper extremities, and one motor was placed on their index fingers. We found a significant correlation between voltage and sensation intensity (r = 0.39). This finding is important in the design and safety of customized haptic devices. However, we did not find a significant aggregate-level correlation with the perceived pleasantness of the simulation. The sensation intensity varied based on the location of the vibration on the upper extremities (with the lowest intensities on the triceps brachii and brachialis) and slightly decreased (5.9 ± 2.9%) when the participants performed reaching movements. When a single motor was vibrating, the participants’ accuracy in identifying the motor without visual feedback increased as the voltage increased, reaching up to 81.4 ± 14.2%. When we stimulated three muscles simultaneously, we found that most participants were able to identify only two out of three vibrating motors (41.7 ± 32.3%). Our findings can help identify stimulation parameters for the ergonomic design of haptic devices. Full article
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24 pages, 4130 KB  
Article
Analysis of Electromechanical Swings of a Turbogenerator Based on a Fractional-Order Circuit Model
by Jan Staszak
Energies 2025, 18(19), 5170; https://doi.org/10.3390/en18195170 - 28 Sep 2025
Viewed by 251
Abstract
This paper addresses the issue of rotor swings in a high-power synchronous generator during stable operation with a stiff power grid. The analysis of electromechanical swings was conducted using a circuit model incorporating fractional-order derivatives. Assuming that variations in the load angle under [...] Read more.
This paper addresses the issue of rotor swings in a high-power synchronous generator during stable operation with a stiff power grid. The analysis of electromechanical swings was conducted using a circuit model incorporating fractional-order derivatives. Assuming that variations in the load angle under small disturbances from a stable equilibrium are minor, a linearized differential equation describing the electrodynamic state of the synchronous machine was derived. Based on this linearized equation of motion and the identified parameters of the equivalent circuit, calculations were performed for a 200 MW turbogenerator. The results indicate that the electromechanical swings are characterized by a constant pulsation and a low damping factor. Calculations were also carried out using a lumped-parameter equivalent circuit model. Based on the obtained results, it can be stated that the fractional-order model provides a more accurate fit of the frequency characteristics compared with the classical model with the same number of rotor equivalent circuits. The relative approximation errors for the fractional-order model are, for the d-axis (one rotor equivalent circuit), relative magnitude error δm = 1.53% and relative phase error δφ = 6.32%, and for the q-axis (two rotor equivalent circuits), δm = 3.2% and δφ = 8.3%. To achieve comparable approximation accuracy for the classical model, the rotor electrical circuit must be replaced with two equivalent circuits in the d-axis and four equivalent circuits in the q-axis, yielding relative errors of δm = 2.85% and δφ = 6.51% for the d-axis, and δm = 1.86% and δφ = 5.49% for the q-axis. Full article
(This article belongs to the Special Issue Electric Machinery and Transformers III)
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28 pages, 6329 KB  
Article
SparsePose–NeRF: Robust Reconstruction Under Limited Observations and Uncalibrated Poses
by Kun Fang, Qinghui Zhang, Chenxia Wan, Pengtao Lv and Cheng Yuan
Photonics 2025, 12(10), 962; https://doi.org/10.3390/photonics12100962 - 28 Sep 2025
Viewed by 408
Abstract
Neural Radiance Fields (NeRF) reconstruction faces significant challenges under non-ideal conditions, such as sparse viewpoints or missing camera pose information. Existing approaches frequently assume accurate camera poses and validate their effectiveness on standard datasets, which restricts their applicability in real-world scenarios. To tackle [...] Read more.
Neural Radiance Fields (NeRF) reconstruction faces significant challenges under non-ideal conditions, such as sparse viewpoints or missing camera pose information. Existing approaches frequently assume accurate camera poses and validate their effectiveness on standard datasets, which restricts their applicability in real-world scenarios. To tackle the challenge of sparse viewpoints and the inability of Structure-from-Motion (SfM) to accurately estimate camera poses, we propose a novel approach. Our method replaces SfM with the MASt3R-SfM algorithm to robustly compute camera poses and generate dense point clouds, which serve as depth–space constraints for NeRF reconstruction, mitigating geometric information loss caused by limited viewpoints. Additionally, we introduce a high-frequency annealing encoding strategy to prevent network overfitting and employ a depth loss function leveraging Pearson correlation coefficients to extract low-frequency information from images. Experimental results demonstrate that our approach achieves high-quality NeRF reconstruction under conditions of sparse viewpoints and missing camera poses while being better suited for real-world applications. Its effectiveness has been validated on the Real Forward-Facing dataset and in real-world scenarios. Full article
(This article belongs to the Special Issue New Perspectives in Micro-Nano Optical Design and Manufacturing)
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23 pages, 5055 KB  
Article
Effect of Ground Motion Duration and Frequency Characteristics on the Probabilistic Risk Assessment of a Concrete Gravity Dam
by Tahmina Tasnim Nahar, Md Motiur Rahman and Dookie Kim
Infrastructures 2025, 10(10), 259; https://doi.org/10.3390/infrastructures10100259 - 27 Sep 2025
Viewed by 356
Abstract
Evaluation of seismic risk by capturing the influences of strong motion duration and frequency contents of ground motion through probabilistic approaches is the main element of this study. Unlike most existing studies that mainly focus on intensity measures such as peak ground acceleration [...] Read more.
Evaluation of seismic risk by capturing the influences of strong motion duration and frequency contents of ground motion through probabilistic approaches is the main element of this study. Unlike most existing studies that mainly focus on intensity measures such as peak ground acceleration or spectral acceleration, this work highlights how duration and frequency characteristics critically influence dam response. To achieve this, a total of 45 ground motion records, categorized by strong motion duration (long, medium, and short) and frequency content (low, medium, and high), were selected from the PEER database. Nonlinear numerical dynamic analysis was performed by scaling each ground motion from 0.05 g to 0.5 g, with the drift ratio at the dam crest used as the Engineering Demand Parameter. It is revealed that long-duration and low-frequency ground motions induced significantly higher drift demands. The fragility analysis was conducted using a lognormal distribution considering extensive damage threshold drift ratio. Finally, the probabilistic seismic risk was carried out by integrating the site-specific hazard curve and fragility curves which yield the height risk for long durations and low frequencies. The outcomes emphasize the importance of ground motion strong duration and frequency in seismic performance and these findings can be utilized in the dam safety evaluation. Full article
(This article belongs to the Special Issue Advances in Dam Engineering of the 21st Century)
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32 pages, 5256 KB  
Article
The Effect of Wave Signature on the Voltage Output of an Oscillating Water Column
by Marcel Ilie
Vibration 2025, 8(3), 54; https://doi.org/10.3390/vibration8030054 - 22 Sep 2025
Viewed by 330
Abstract
The reduction in carbon footprint and scarcity of energy resources have increased the demand for renewable and sustainable energy resources, and thus, significant efforts have been concentrated on harnessing renewable and sustainable energy resources. The oscillating water column (OWC) wave energy converter has [...] Read more.
The reduction in carbon footprint and scarcity of energy resources have increased the demand for renewable and sustainable energy resources, and thus, significant efforts have been concentrated on harnessing renewable and sustainable energy resources. The oscillating water column (OWC) wave energy converter has proven to be the most promising approach for harnessing wave energy. The OWC offers the benefits of a long operating time span and low maintenance, as air serves as the driving fluid. The hydrodynamic efficiency of OWC depends on the wave motion and its interaction with the OWC structure. Therefore, the present research concerns the impact of the incident wave signature on the OWC’s efficiency voltage output, and it is carried out experimentally using a laboratory-scale wave tank. Four different waves, of different amplitudes and frequencies, and their impact on the OWC voltage output are experimentally investigated. This study shows that the four waves exhibit different characteristics, such as crests and troughs of different slopes and amplitudes. However, although the wave crests exhibit relatively similar amplitudes, the wave troughs exhibit significantly different characteristics. This study also reveals that the OWC voltage output exhibits a nonlinear behavior due to the nonlinear nature of the incident waves and compressible air inside the OWC chamber. The maximum voltage output is obtained for a maximum air compressibility factor. However, lower voltage outputs are obtained for both compression and decompression of the air inside the OWC chamber. Full article
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23 pages, 3626 KB  
Article
Experimental and Parametric Study on Mechanical and Motion Responses of a Novel Air-Floating Tripod Bucket Foundation with Taut Mooring
by Xianqing Liu, Yun He, Yu Zhang, Puyang Zhang, Shenghong Hu, Yutao Feng and Nan Lv
J. Mar. Sci. Eng. 2025, 13(9), 1786; https://doi.org/10.3390/jmse13091786 - 16 Sep 2025
Viewed by 317
Abstract
In the present study, a novel air-floating tripod bucket foundation (AFTBF) with taut mooring is proposed. The mechanical and motion response characteristics of this foundation were investigated through model tests. Furthermore, a parametric study was performed on the factors influencing the RAOs of [...] Read more.
In the present study, a novel air-floating tripod bucket foundation (AFTBF) with taut mooring is proposed. The mechanical and motion response characteristics of this foundation were investigated through model tests. Furthermore, a parametric study was performed on the factors influencing the RAOs of mooring tension, air cushion pressure, as well as motion in the surge, heave, and pitch directions. The conclusion of this research is as follows: mooring tension, air cushion pressure, and pitch angle exhibit wave-frequency responses in small periods and low-frequency responses in large periods. Surge response is characterized by dual-peak features, while heave response predominantly demonstrates wave-frequency characteristics. As draft increases, the air cushion pressure inside the buckets exhibits a decreasing trend. Changes in water depth have more pronounced impacts on mooring tension and motion responses than on air cushion pressure. The impacts of changes in mooring distance and water depth on mechanical and motion responses are significantly more pronounced than those induced by changes in draft. These findings provide a critical foundation for the optimal design of this foundation in water depths of 30–50 m. Full article
(This article belongs to the Special Issue Optimized Design of Offshore Wind Turbines)
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25 pages, 5278 KB  
Article
Developing a Quality Flag for SAR Ocean Wave Spectrum Partitioning with Machine Learning
by Amine Benchaabane, Romain Husson, Muriel Pinheiro and Guillaume Hajduch
Remote Sens. 2025, 17(18), 3191; https://doi.org/10.3390/rs17183191 - 15 Sep 2025
Viewed by 412
Abstract
Synthetic Aperture Radar (SAR) is one of the few instruments capable of providing high-resolution global two-dimensional (2D) measurements of ocean waves. Since 2014 and then 2016, the Sentinel-1A/B satellites, whenever operating in a specific wave mode (WV), have been providing ocean swell spectrum [...] Read more.
Synthetic Aperture Radar (SAR) is one of the few instruments capable of providing high-resolution global two-dimensional (2D) measurements of ocean waves. Since 2014 and then 2016, the Sentinel-1A/B satellites, whenever operating in a specific wave mode (WV), have been providing ocean swell spectrum data as Level-2 (L2) OCeaN products (OCN), derived through a quasi-linear inversion process. This WV acquires small SAR images of 20 × 20 km footprints alternating between two sub-beams, WV1 and WV2, with incidence angles of approximately 23° and 36°, respectively, to capture ocean surface dynamics. The SAR imaging process is influenced by various modulations, including hydrodynamic, tilt, and velocity bunching. While hydrodynamic and tilt modulations can be approximated as linear processes, velocity bunching introduces significant distortion due to the satellite’s relative motion with respect to the ocean surface and leads to constructive but also destructive effects on the wave imaging process. Due to the associated azimuth cut-off, the quasi-linear inversion primarily detects ocean swells with, on average, wavelengths longer than 200 m in the SAR azimuth direction, limiting the resolution of smaller-scale wave features in azimuth but reaching 10 m resolution along range. The 2D spectral partitioning technique used in the Sentinel-1 WV OCN product separates different swell systems, known as partitions, based on their frequency, directional, and spectral characteristics. The accuracy of these partitions can be affected by several factors, including non-linear effects, large-scale surface features, and the relative direction of the swell peak to the satellite’s flight path. To address these challenges, this study proposes a novel quality control framework using a machine learning (ML) approach to develop a quality flag (QF) parameter associated with each swell partition provided in the OCN products. By pairing collocated data from Sentinel-1 (S1) and WaveWatch III (WW3) partitions, the QF parameter assigns each SAR-derived swell partition one of five quality levels: “very good,” “good,” “medium,” “low,” or “poor”. This ML-based method enhances the accuracy of wave partitions, especially in cases where non-linear effects or large-scale oceanic features distort the data. The proposed algorithm provides a robust tool for filtering out problematic partitions, improving the overall quality of ocean wave measurements obtained from SAR. Moreover, the variability in the accuracy of swell partitions, depending on the swell direction relative to the satellite’s flight heading, is effectively addressed, enabling more reliable data for oceanographic studies. This work contributes to a better understanding of ocean swell dynamics derived from SAR observations and supports the numerical swell modeling community by aiding in the refinement of models and their integration into operational systems, thereby advancing both theoretical and practical aspects of ocean wave forecasting. Full article
(This article belongs to the Special Issue Calibration and Validation of SAR Data and Derived Products)
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22 pages, 8319 KB  
Article
An Analytical Model of Motion Artifacts in a Measured Arterial Pulse Signal—Part I: Accelerometers and PPG Sensors
by Md Mahfuzur Rahman, Subodh Toraskar, Mamun Hasan and Zhili Hao
Sensors 2025, 25(18), 5710; https://doi.org/10.3390/s25185710 - 12 Sep 2025
Viewed by 574
Abstract
This paper, the first of two parts, presents an analytical model of motion artifacts (MAs) in measured pulse signals by accelerometers and photoplethysmography (PPG) sensors. As the transmission path from the true pulse signal in an artery to the sensor output (measured pulse [...] Read more.
This paper, the first of two parts, presents an analytical model of motion artifacts (MAs) in measured pulse signals by accelerometers and photoplethysmography (PPG) sensors. As the transmission path from the true pulse signal in an artery to the sensor output (measured pulse signal), the tissue–contact–sensor (TCS) stack is modeled as a 1DOF (degree-of-freedom) system. MAs cause baseline drift of the mass and simultaneously time-varying system parameters (TVSPs) of the TCS stack. With arterial wall displacement and pulsatile pressure serving separately as the true pulse signal, an analytical model is developed to mathematically relate baseline drift and TVSP to a measured pulse signal. With assumed values of baseline drift and TVSPs, the numerical calculation is conducted in MATLAB. While baseline drift is low-frequency additive noise and can greatly swing a measured pulse signal, TVSP generates relatively small, abrupt distortion (e.g., 1% variation in heart rate and <5% change in pulse amplitude) but rides on each harmonic of the true pulse signal. By taking into account the full involvement of the transmission path in pulse measurement, this analytical model serves as a fundamental framework for quantifying baseline drift and TVSPs from a measured pulse signal in the future. Full article
(This article belongs to the Special Issue Biosignal Sensing Analysis (EEG, EMG, ECG, PPG) (2nd Edition))
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