Time Domain Analysis Research Articles

Spectral Analysis of a Sagged Cable with an Optimal Viscous Inertial Mass Damper under Random Wind in a Generalized State-Space Formulation

In this study, a spectral analysis method is proposed for a sagged cable with an optimal viscous inertial mass damper (VIMD) under random wind loads based on a generalized state-space formulation. The frequency response functions of the cable responses were evaluated for the wind force vector using complex modes orthogonal to the system matrices, from which the power spectral densities of the cable responses were readily obtained for the spectral matrix of the wind force. The accuracy and efficiency of the proposed method were verified by comparing the root mean square responses with those obtained from the time-domain and covariance analyses. The performance of two VIMDs optimized by the fixed-point method and the maximum damping ratio method was investigated for various cases of cable sag and wind properties. Serviceability failure probability analysis was also conducted using the first-passage problem. The results show that the proposed spectral method is computationally more efficient than the other two analysis methods. The VIMDs optimized for the first original cable mode were found to be highly effective in reducing cable vibration and the probability of serviceability failure.

Ice-Induced Vibration Analysis of Fixed-Bottom Wind Turbine Towers

This research tackles the challenge of ice-induced vibrations in fixed-bottom offshore wind turbines, particularly the risk of frequency lock-in (FLI), a critical loading condition arising from slow ice movement against structures. We introduce an innovative FLI analysis process grounded in the ductile damage–collapse (DDC) mechanism, which offers a more accurate and significantly lower probability of FLI occurrence than conventional methods. Through dynamic evaluations employing a time-domain ice load model and FLI displacement analyses, we demonstrate that FLI can lead to higher structural vibrations than those caused by continuous brittle ice crushing. A case study of a 5 MW monopile wind turbine tower utilising Abaqus confirms the necessity for incorporating FLI considerations into structural design to ensure safety and performance in ice-prone environments. Comparing the DDC mechanism with the ISO method, our study reveals that the DDC approach predicts higher displacement and acceleration values during FLI, nearly an order of magnitude greater than those induced by ice loads with a 50-year return period. The research underscores the importance of robust ice-load integration in design strategies for offshore wind turbines, especially in regions susceptible to ice. It highlights the DDC mechanism as a novel strategy for enhancing structural resilience against ice-induced hazards.

Optimal fractional PI controller-based VSC-HVDC system with LCL filter design

For decades, high voltage direct current (HVDC) has been an essential means in designing smart microgrids. Though, a control approach for Voltage Source Converter (VSC)-based HVDC, like dampening oscillations, must be established for improved operational execution, as these oscillations can cause stability issues. Considering its versatility, the Fractional order Proportional Integral (FPI) controller is used in this study's control approach instead of the PI controller. With frequency domain analysis, comprehensive modelling and design of LCL filters for harmonic reduction to improve power quality are presented. The dynamic characteristics of VSC-HVDC are comprehensively presented based on dq axis synchronous frame of reference. The VSC-HVDC improved control outcome is validated by the result analysis using PI and FPI controller using ITLBO. It is found in both frequency and time-domain simulations and analysis that the AFPI-VSC-HVDC system is capable of effectively dampening oscillations to enhance the small-signal stability.

Deep-Learning-Based detection of recreational vessels in an estuarine soundscape in the May River, South Carolina, USA.

This paper presents a deep-learning-based method to detect recreational vessels. The method takes advantage of existing underwater acoustic measurements from an Estuarine Soundscape Observatory Network based in the estuaries of South Carolina (SC), USA. The detection method is a two-step searching method, called Deep Scanning (DS), which includes a time-domain energy analysis and a frequency-domain spectrum analysis. In the time domain, acoustic signals with higher energy, measured by sound pressure level (SPL), are labeled for the potential existence of moving vessels. In the frequency domain, the labeled acoustic signals are examined against a predefined training dataset using a neural network. This research builds training data using diverse vessel sound features obtained from real measurements, with a duration between 5.0 seconds and 7.5 seconds and a frequency between 800 Hz to 10,000 Hz. The proposed method was then evaluated using all acoustic data in the years 2017, 2018, and 2021, respectively; a total of approximately 171,262 2-minute.wav files at three deployed locations in May River, SC. The DS detections were compared to human-observed detections for each audio file and results showed the method was able to classify the existence of vessels, with an average accuracy of around 99.0%.

Modelling of foundation damping in time-domain analysis of monopile supported offshore wind turbine structures

Foundation damping holds large potential for design optimisations of monopile-supported offshore wind turbine structures. However, the contribution of foundation damping is not well understood, in part due to lack of suitable method for incorporating foundation damping in the time-domain analysis of wind turbine structures. This paper presents a practical approach for this purpose in which a dashpot is attached in parallel to each p-y spring along the monopile. In this way, the distributed soil-pile interaction stiffness and damping are accurately modelled. To demonstrate the validity of the proposed approach, a case study exploring the influence of foundation damping on the dynamic response of an IEA 15 MW reference turbine founded on monopile is carried out. The results demonstrate that the foundation damping has relatively limited impact on the overall dynamic response and fatigue loads in power production states. However, inclusion of foundation damping is shown to significantly reduce the structural response after emergency shutdown and under parked conditions. The findings of the case study show promising potential for wind turbine structure design optimization through consideration of foundation damping.

Traction resistance analysis and cutting state recognition of shearer based on numerical simulation

Accurate identification of the shearer cutting state is essential for the intelligent control of the shearer. In addition to the coal-rock strata conditions, the operational parameters of the shearer also influence the shearer cutting state. To analyze the factors on the shearer cutting state and accurately identify its cutting state, the characteristics of shearer traction resistance are analyzed under various working conditions using discrete element numerical simulation. A method of the shearer cutting state recognition based on traction resistance and traction speed is proposed. Firstly, the mechanical properties of coal and rock are obtained through uniaxial compression experiments. The joint simulation model is developed to analyze the change in traction resistance under different traction speeds. The functional relationship between traction resistance and traction speed is obtained. Then, the influence of gangue thickness and position on traction resistance is analyzed. The time domain and frequency domain analysis of traction resistance is conducted to more accurately describe the characteristics of shearer traction resistance. Finally, simulation and experimental data are used to verify the reliability of the proposed method. The recognition accuracy of simulation and experimentation reaches 100 % and 97.1 %, respectively. This indicates that the proposed method can effectively recognize the shearer cutting state.

Analysis of unsteady internal flow characteristics in axial pump with varying number of blades using computational modelling and vibration techniques

Axial pumps were extensively applied in many varying applications because of their large flow and low head. In this research investigation, the comparative analysis of the findings unveiled that the predominant source of hydraulic-induced vibration in the pump stemmed from pressure pulsations at the impeller inlet. Notably, similar patterns in amplitude were observed as flow rates increased. Furthermore, time-domain analysis confirmed that pressure pulsations and vibrations are highly correlated under high flow rates. Pressure pulsation's Frequency-domain analysis also revealed that it was a multiple of shaft frequency and changed from one multiple to three multiples of vibration. While analyzing the flow rate characteristics pertaining to pressure pulsation and vibration, it was determined that pressure pulsations at the impeller inlet had the potential to generate frequency components across a broad spectrum, encompassing both low and high flow rates, as well as their respective multiples. This phenomenon was particularly pronounced in regions characterized by unstable and high flow rates. Vibration ingredients likely influenced by pressure pulsation at the impeller inlet could be as low as design flow rates and as high as high flow rates. A vibration frequency with a multiple did not seem to be influenced by pulsating pressure at the impeller entrance. The present study focuses on investigation of flow behaviors in an axial pump with varying numbers of blades. It is an important geometric parameter that significantly affects the pump's performance. Therefore, the dynamic flow patterns in a pump, considering changed flow and impeller blade configurations, are investigated by employing the sliding mesh technique in combination with SST (k-ω) turbulence model. The numerical results exhibit a commendable alignment with the existing experimental data, enhancing the predictive accuracy of pump performance. Qualitative analyses encompass static pressure, shear stress, and various velocity components. Concurrently, quantitative investigations delve into pressure fluctuations and average pressure across a spectrum of operating conditions and impeller blade configurations. These comprehensive findings underscore the substantial influence of the impeller blade on pressure, velocity magnitudes radial, axial, tangential shear stress, average pressure within the system.

Association between obstructive sleep apnea and arrhythmia and heart rate variability among hypertensive patients

BackgroundThe relationship between obstructive sleep apnea (OSA) and the occurrence of arrhythmias and heart rate variability (HRV) in hypertensive patients is not elucidated. Our study investigates the association between OSA, arrhythmias, and HRV in hypertensive patients.MethodsWe conducted a cross-sectional analysis involving hypertensive patients divided based on their apnea-hypopnea index (AHI) into two groups: the AHI ≤ 15 and the AHI > 15. All participants underwent polysomnography (PSG), 24-hour dynamic electrocardiography (DCG), cardiac Doppler ultrasound, and other relevant evaluations.ResultsThe AHI > 15 group showed a significantly higher prevalence of frequent atrial premature beats and atrial tachycardia (P = 0.030 and P = 0.035, respectively) than the AHI ≤ 15 group. Time-domain analysis indicated that the standard deviation of normal-to-normal R-R intervals (SDNN) and the standard deviation of every 5-minute normal-to-normal R-R intervals (SDANN) were significantly higher in the AHI > 15 group (P = 0.020 and P = 0.033, respectively). Frequency domain analysis revealed that the low-frequency (LF), high-frequency (HF) components, and the LF/HF ratio were also significantly elevated in the AHI > 15 group (P < 0.001, P = 0.031, and P = 0.028, respectively). Furthermore, left atrial diameter (LAD) was significantly larger in the AHI > 15 group (P < 0.001). Both univariate and multivariable linear regression analyses confirmed a significant association between PSG-derived independent variables and the dependent HRV parameters SDNN, LF, and LF/HF ratio (F = 8.929, P < 0.001; F = 14.832, P < 0.001; F = 5.917, P = 0.016, respectively).ConclusionsHypertensive patients with AHI > 15 are at an increased risk for atrial arrhythmias and left atrial dilation, with HRV significantly correlating with OSA severity.

Research on the characteristics of electro-hydraulic position servo system of RBF neural network under fuzzy rules

A radial basis function neural network PID controller under fuzzy rules (FUZZY-RBF-PID) was designed for the electro-hydraulic position servo system under the influence of uncertain factors such as load mutation, and load stiffness change. Firstly, the mathematical model of the system is established, and the frequency domain and time domain analysis of the system are carried out. Secondly, based on the analysis results, a radial basis function (RBF) neural network PID controller is designed, and fuzzy rules are innovatively used to adjust the learning rate of PID parameters in the RBF neural network learning algorithm in real time. Thirdly, the simulation results show that under the action of the FUZZY-RBF-PID controller, the unit step response of the system has high steady-state accuracy, fast response speed, and under the condition of large load stiffness, the system can recover to the steady-state value faster after being disturbed. At the same time, when the input signal is the sinusoidal signal of 10 HZ, the system under the action of the FUZZY-RBF-PID controller has no obvious phase lag phenomenon, and the tracking error is minimal. The proposed method can effectively improve the comprehensive performance of the electro-hydraulic position servo system under the influence of uncertain factors.

Brillouin expanded time-domain analysis based on dual optical frequency combs

Brillouin Optical Time-Domain Analysis (BOTDA) is a widely-used distributed optical fiber sensing technology employing pulse-modulated pump waves for local information retrieval of the Brillouin gain or loss spectra. The spatial resolution of BOTDA systems is intrinsically linked to pulse duration, so high-resolution measurements demand high electronic bandwidths inversely proportional to the resolution. This paper introduces Brillouin Expanded Time-Domain Analysis (BETDA) as a modified BOTDA system, simultaneously achieving high spatial resolution and low detection bandwidth. Utilizing two optical frequency combs (OFCs) with different frequency intervals as pump and probe, local Brillouin gain spectra are recorded by their spectral beating traces in an expanded time domain. A 2-cm-long hotspot located in a 230 m single-mode fiber is successfully measured in the time domain with a detection bandwidth of less than 100 kHz using dual OFCs with tailored spectral phase, line spacing, and bandwidth.

Utilization and performance comparison of several HESSs with cascade optimal-FOD controller for multi-area multi-source power system under deregulated environment

Electric power system network is evolving rapidly towards a more flexible, robust and secured interconnected network which can provide better power quality within a competitive environment commonly known as deregulated power system (DPS). For safeguarding DPS, energy storage systems (ESSs) and an efficient control method are required to maintain equilibrium between load demand and generated power known as automatic generation control (AGC). This study highlights an attempt of performance comparison of several hybrid ESSs (HESSs) for AGC of multi-area multi-source power system under deregulated scenario. The considered system comprises of thermal, hydro and gas (THG) generating units in each area of a two-area DPS. A cascade optimal controller-fractional order derivative (COC-FOD) is suggested for AGC performance advancement and its performance is compared with optimal controller (OC). A nature inspired salp swarm algorithm (SSA) is utilized to optimise secondary FOD controller gains while primary OC is designed via full state feedback control approach. The time domain analysis and bode plot reveal the eminence of COC-FOD over OC by returning 66 % lower value of cost function, 24 % lower undershoot, 98 % lower overshoot, at least 24 % lesser rise time for area frequency deviation, and 30 % greater stability margins with comparable settling time and lesser number of oscillations. The performances of various HESSs like ultra-capacitor (UC) + redox flow battery (RFB), superconducting magnetic energy storage (SMES) + RFB and flywheel energy storage (FES) + RFB in the attendance of OC and COC-FOD controller are compared and performance of COC-FOD in the presence of SMES+RFB is found superior to other HESS combinations by giving 75 % lesser value of cost function and at least 43 % lesser undershoot for area frequency deviation compared to the scenario where COC-FOD is simulated without ESSs. To present a more realistic scenario, system nonlinearities are considered. Finally, COC-FOD controller proved its resilience and efficacy by providing satisfactory stabilized performance under wide variations in the system parameters.

Chatter and Surface Waviness Analysis in Oerlikon Face Hobbing of Spiral Bevel Gears

A vectorized analytical model for the cutting dynamics in the spiral bevel gear face hobbing process has been developed, which is based on machine tool kinematics and vibration vectorization. The structural modal parameters of the cutter head spindle system are obtained through experimental modal analysis with hammer impact testing. The analytical model is utilized to simulate the generation of simulated vibration acceleration signals during spiral bevel gear hobbing. A wavelet threshold denoising method is applied to process the simulated vibration signals of the spiral bevel gear face hobbing with added white noise. Signal processing methods, including short-time Fourier transform are employed for time-domain analysis, frequency-domain analysis, and time–frequency-domain analysis of measured signals and simulated signals, thereby extracting the corresponding statistical features. In addition to the results of the experimental modal analysis, the causes of chatter in spiral bevel gear hobbing are discussed in detail, revealing that the main factor is cutter head vibration in the Y direction of the Hunan ZDCY CNC EQUIPMENT YKA2260 machine tool used in this research. The error in the time-domain characteristic parameters between simulated signals and measured vibration acceleration signals is within 15%, with a difference of 3.5% in spectral peak values. The predicted tooth surface morphology from simulation matches the actual morphology on the workpiece, comprehensively validating the reliability of the cutting dynamics model for the spiral bevel gear face hobbing process. Another conclusion drawn from numerical simulation experiments is that the amount of tooth surface waviness of the spiral bevel gears is the ratio of tool chatter frequency to cutting fundamental frequency.

A Wear Debris Signal Processing Method Based on Inductive Monitoring for Aero-Engine

In view of the high false alarm rate in the oil debris monitoring results of the triple-coil inductive sensor in the transmission lubrication system of the aero-engine, a new debris signal processing method based on inductive monitoring is proposed. A time domain analysis is carried out first, and the signal energy is the most effective index to distinguish the debris signature from the noise signature. On this basis, signal energy values within a fixed-length sliding window is processed through the histogram. Finally, a threshold is set for the detection of the debris signature, which is based on the distribution of data within the histogram. This method is applied to the experimental data from a test run of an aero-engine, and the results show that all the debris is detected even if part of it appears during a change in the working condition of the aero-engine. Therefore, this method shows satisfactory results in debris detection accuracy and especially the inhibition of false alarms. It is also applicable for real-time monitoring due to the similarity between the movement of the sliding window and real-time data acquisition. In addition, it is applicable for various sensing principles, including but not limited to the inductive sensor signal, since the detection performance is only related to the signal itself.

A hexagonal shape fractal flexible UWB antenna based on jeans material for healthcare applications

This work aims to provide a compact, flexible, lightweight ultra-wideband antenna design with enhanced bandwidth for medical applications. The proposed antenna features fractal geometry and is made by iterating a hexagonal slot within a circular copper construction. The basic radiator’s partial ground plane is defective with a slit at the centre and a down ramp on both sides of the plane. A stub in the feed line improves impedance matching and provides enhanced −10dB bandwidth of 10.6 GHz (1.9 GHz to 12.5 GHz). The antenna is built on a black jeans textile substrate of dielectric constant 1.7 and size of 48 mm × 36 mm × 1.7 mm . The maximum gain of 6.75 dB at the frequency of 2.3 GHz is reported. In the operational band, the developed antenna provides steady radiation properties. Furthermore, the antenna has been verified for use in practical applications by setting up the proposed antenna as a receiver with an NRF24L01 wireless transceiver module and given coverage up to 50 feet in an open environment. The designed prototype is analysed for flexibility with different bending radii. A SAR analysis is also carried out for 1 gram and 10-gram of tissue along with the extensive time domain analysis for wearable UWB applications.

Radial artery pulse wave age-related assessment for diabetic patients based on multiple linear regression time domain analysis method

The global incidence of diabetes increases yearly. About 10 % of adults are affected. This study established a multiple linear regression (MLR) time domain analysis model of radial artery pulse wave characteristic parameters in CUN, GUAN and CHI based on traditional Chinese medicine (TCM) to assess diabetes. Pulse signals of diabetic subjects were collected by a pulse instrument. The locations of CUN, GUAN and CHI were determined by a TCM practitioner, and all pulse signals were normalized. Measurement data from 100 male and 100 female patients were used to establish a new time domain analysis method for extracting pulse wave characteristic parameters called equal pressure pulse transit time (EP-PTT). For these mentioned samples, the EP-PTT is different for male and female diabetic subjects, it was smaller for male diabetic subjects than those of female diabetic subjects. EP-PTT1 and EP-PTT6 are relevant to predicting age based on CUN, GUAN and CHI signals, which showed a linear negative correlation (P < 0.05). The EP-PTTi (i = 2, 3, 4) of female diabetic subjects are linearly correlated with age, and all of these are positively correlated (P < 0.05). These observations were clearly different from observations in healthy controls. The pulse waveform of CUN, GUAN and CHI of diabetic subjects can explore the relationship between the age of diabetic subjects and EP-PTT. The preliminary findings will provide foundational data for type 2 diabetes assessment based on CUN, GUAN and CHI pulse signals of radial artery, and the proposed new analysis method can be used for assessing type 2 diabetes.

Broadband non-reciprocal wave suppression and frequency conversion by active metabeams

Non-reciprocal wave propagation has recently attracted considerable attention as it is potentially beneficial to many applications, ranging from waveguiding, sensing, and communication to vibration control. Here, we propose the design of an active metabeam for broadband non-reciprocal wave suppression, that can strongly suppress the transmitted wave for forward incidence and amplify the transmitted wave for backward incidence. The metabeam consists of piezoelectric sensors and actuators connected by a feedforward control loop. We find that the broadband non-reciprocal wave suppression can be realized by making the second forward transmission dip close to the first one, which can be controlled by the distance between the piezoelectric actuators. The broadband non-reciprocal wave suppression of the metabeam is demonstrated via a set of time domain analyses. We further study the frequency conversion effects of the metabeam based on a time-varying transfer function. In particular, we show that among the frequency converted harmonics, those at high frequencies can be eliminated by destructive interference, resulting in an approximate single-frequency conversion in the transmitted wave. Our study advances the field of non-reciprocal mechanics and offers a reliable platform for designing active broadband elastic wave devices, providing a feasible way for asymmetrical energy control, broadband vibration attenuation, and signal processing.

Understanding stiffness degradation of composite helical springs with multi-braided layers under impact

Understanding stiffness degradation and developing suitable damage detection method for composite helical springs (CHSs) are important for their application and further development. In this study, a coupled plasticity-damage model for capturing stiffness degradation of CHSs with multi-braided layers (MBLs-CHS) is developed. Experimental results show that there is minor damage that only happens in the resin component of MBLs-CHS during impact. The element removal fraction in the simulation result is used to evaluate the damage severity, which is suggested to increase with impact energy (Ei) and decrease sequentially for CHSs with single, double, and triple braided layers (i.e., SCHS, DCHS, and TCHS). Specifically, damage severity of TCHS decreases by 51.3 % under 60 J impaction compared to that of SCHS. Finally, the time domain analysis method is introduced to monitor damage in real time. The amplitude intensity profiles under various Ei of CHSs have been fitted to predict the global stiffness degradation of CHSs in real time.

Design and analysis of grid mooring system for gravity net cage with a comprehensive optimization method

The design of gravity net cage mooring systems is a complex endeavor, which is critical for preventing major accidents caused by mooring system failures. A comprehensive optimization method is proposed to enhance the reliability and performance of the net cage mooring system by integrating static and dynamic phases. The proposed method initiates static simulations based on the catenary theory to refine the initial range of parameters. This preparatory step ensures the subsequent dynamic analysis is carried out based on feasible initial conditions with the lumped mass method. The dynamic simulation captures the complex interactions and behavior of the mooring systems under various environmental conditions. The multi-objective optimization provides feasible solutions with simultaneous consideration of both processes. Three different scenarios are optimized with the forces and motions compared after the time domain analysis, resulting in a satisfactory set of solutions. The results show that variations in the angle of the mooring line of the net cage primarily impact the net cage's movement and volume in different environments.

Probabilistic method for risk analysis of unintentional islanding of distributed generators

Unintentional islanding can be a harmful event, requiring the fast actuation of the anti-islanding protection to disconnect the distributed generators. Although passive techniques are mostly used, they have non-detection zones and fail to detect islanding under specific conditions. Therefore, knowing in advance such operative conditions and the risks associated with the islanding occurrence can be a strategic information to support grid operators in the decision-making process regarding the effectiveness of distributed generators interconnection protection. In this context, this paper proposes a probabilistic method to assess the risk of energized island formation by combining the probability of breaker (or recloser) opening with the probability of anti-islanding protection failure. The method employs probabilistic short circuit associated with overcurrent protection actuation by using Monte Carlo simulation and time-domain analysis to obtain the cases of unintentional islanding and to calculate the risk of distributed generator islanded operation. The results drove more realistic and less conservative conclusions than previous literature. Additionally, it is shown that faults as the cause of breaker opening do play a relevant role in the performance of the anti-islanding protection.

Bandgap tunability and impact mitigation enhancement of hybrid graded origami-inspired metamaterials with multiple resonators

Origami structure has become an important design source of metamaterials because of its extremely rich form and infinite design space. In order to improve the wave attenuation and impact mitigation ability of origami structure, a hybrid graded origami-inspired metamaterials with multiple resonators is proposed. The bandgap characteristics, transmission spectrum and impact resistance of the proposed metamaterials are analyzed in detail by numerical simulation. The results show that multiple local resonance mechanisms can open two new bandgaps compared to the single local resonance case, and the bandgap boundary is very sensitive to the dimension of resonance units. In particular, the resonance peaks of the origami metamaterials are substantially weakened due to the presence of dimensional gradients, which allows multiple bandgaps to merge into an ultra-wide bandgap (bandwidth up to 8.9 kHz), about five times than the original bandwidth. According to the time domain and frequency domain analysis results, it can be found that the amplitude of reaction force is significantly reduced in the bandgap range, and the ultra-wide frequency domain (2.9–10 kHz) with sustainable-low peak appears. Moreover, the asymmetry and graded design of unit-cell also affects the ability of dissipate and attenuate wave energy and delay the wave transmission time. In addition, the local-resonant origami metamaterials are fabricated by 3D printing technology, and the experimental results are in good agreement with the numerical simulation results. This study is expected to provide valuable ideas in crashworthiness design and impact wave protection of important equipments.