Frequency Characteristics Research Articles

A Method for Underwater Acoustic Target Recognition Based on the Delay-Doppler Joint Feature

With the aim of solving the problem of identifying complex underwater acoustic targets using a single signal feature in the Time–Frequency(TF) feature, this paper designs a method that recognizes the underwater targets based on the Delay-Doppler joint feature. First, this method uses symplectic finite Fourier transform (SFFT) to extract the Delay-Doppler features of underwater acoustic signals, analyzes the Time–Frequency features at the same time, and combines the Delay-Doppler (DD) feature and Time–Frequency feature to form a joint feature (TF-DD). This paper uses three types of convolutional neural networks to verify that TF-DD can effectively improve the accuracy of target recognition. Secondly, this paper designs an object recognition model (TF-DD-CNN) based on joint features as input, which simplifies the neural network’s overall structure and improves the model’s training efficiency. This research employs ship-radiated noise to validate the efficacy of TF-DD-CNN for target identification. The results demonstrate that the combined characteristic and the TF-DD-CNN model introduced in this study can proficiently detect ships, and the model notably enhances the precision of detection.

Effect of Plasma Gas Type on the Operation Characteristics of a Three-Phase Plasma Reactor with Gliding Arc Discharge

Three-phase gliding arc discharge reactors are devices in which it is difficult to maintain stable plasma parameters, be it electrically, physically, or chemically. The main cause of plasma instability is the source, which is freely burning arcs in a three-phase system. In addition, these arcs burn at low currents and are intensively cooled, further increasing their instability. These instabilities translate into the electrical characteristics of the plasma reactor. The analysis for the four gases nitrogen, argon, helium, and air shows that the type of plasma-generating gas and its physical parameters have a strong influence on the operational characteristics of the plasma reactor. Current–voltage, power and frequency characteristics of the plasma reactor were plotted experimentally. Characteristics obtained in this way make it possible to determine the areas of effective operation of the plasma reactor, and to estimate the quality of the generated plasma. Based on the characteristics obtained, a method of controlling the plasma parameters can be developed.

Wavelet–pixel domain progressive fusion network for underwater image enhancement

Due to the complexity of the underwater environment, underwater images suffer from low-illumination, color deviation, and blurred details. Conventional underwater image enhancement (UIE) approaches have achieved remarkable success in improving color and illumination in the pixel domain. However, they ignore fine-grained details during the enhancement process. To solve these problems, we propose a novel wavelet–pixel domain progressive fusion network (WPFNet) to improve details while enhancing illumination and reducing color deviation for degraded underwater images. Unlike the UIE method, we propose a wavelet domain module (WDM) to obtain different scale frequency features with fine-grained details. Concretely, we develop a residual attention block (RAB) that optimizes low-frequency sub-images to maintain rich details, and enhance color and illumination information for low-frequency features. Transformer block (T_Block) is proposed to convert three high-frequency sub-images into high-frequency features operated by self-attention mechanism. Frequency features are obtained by adding low- and high-frequency features from the same scale, suffering from insufficient color and illumination information. Thus, the pixel domain module (PDM) is introduced to extract spatial features of different scales with rich color and illumination information in the pixel domain. Reconstruction module (REM) is proposed to fuse frequency and spatial features from small to large scale to restructure clear underwater images. The dual-domain fusion block (DFB) is proposed as a key element of REM, which exploits the constraint signal to fuse superior information of frequency and spatial features. Finally, our WPFNet obtains high-quality underwater images in four benchmark datasets and achieves superior performance compared to state-of-the-art UIE methods. The code is available at: https://github.com/LiuShiBen/WPFNet.

Research on Low Voltage Ride-through Characteristics of 300 MW VSPSH Based on BPA

Abstract With the development of new energy, the power grid scale is constantly expanding, which also brings new challenges to the stability of the grid. When some faults occur in the power grid, it leads to a voltage dip; if most of the devices are removed from the power system, it is easy to cause problems of stability and losses. Therefore, it is necessary to investigate the low voltage ride-through (LVRT) characteristics of power electronic equipment with large capacity before grid connection. Different from constant-speed pumped-storage hydropower plants (CSPSH), variable-speed pumped-storage hydropower (VSPSH) units have broad prospects for development in China due to their characteristics of frequency and voltage regulation, energy saving, and high efficiency. In this paper, the three-machine-nine-node model of 300 MW VSPSH is built based on BPA, and the large voltage dip caused by a three-phase short-circuit fault at the grid side is designed to test the LVRT capability of 300 MW doubly fed VSPSH.

Morphological features of myocardial bridging in patients who underwent CT coronary angiography in the National Heart Centre of Nepal

Abstract Objective Multidetector computed tomography is considered a reliable, non-invasive modality for diagnosing myocardial bridging. The prevalence of myocardial bridges varies greatly based on the technique used to identify the anomaly. We aimed to evaluate the frequency and morphological features of myocardial bridging in patients who underwent ECG-gated 640 slice Multidetector computed tomography coronary angiography at Shahid Gangalal National Heart Centre. Methods A retrospective cross-selectional study was conducted at Shahid Gangalal National Heart Centre. From 1 January 2021 to 30 June 2021, 1068 consecutive patients were enrolled, and the frequency and morphological features of myocardial bridging among those patients were evaluated. Results Among the 1068 patients examined, 141 had one or multiple Myocardial bridging (13.2%). Out of 141 patients, 94.3% of bridging was superficial, while only 5.7% was deep. 97.9% of myocardial bridging was located in the left anterior descending artery, and 86.2% was present in the mid-segment of the left anterior descending artery. Conclusion Multidetector computed tomography is an effective noninvasive method for the diagnosis of myocardial bridging. The frequency of myocardial bridging in our study was 13.2%.

The causal relationship between public debt and economic growth in G7 countries: new evidence from time and frequency domain approaches

Investigators of the public debt-economic growth nexus have yet to fully address the crucial issue of determining the direction of causality. There is an implicit assumption—or perception—that the causal relationship is mostly from public debt to economic growth. Beyond this, causal relationships may vary according to the presence of structural breaks as well as different frequency characteristics. The focus of this study is to address these issues. In this context, we comparatively investigate how structural changes and frequency characteristics affect the public debt-economic growth nexus using historical data covering the period 1870–2020 for G7 countries. Methodologically, we use Fourier Toda-Yamamoto and frequency-domain causality techniques from time and frequency-based approaches, respectively. Consistent with our expectations, we show that in the link between public debt and economic growth, they differ from or in some cases confirm each other based on the time and frequency-domain approaches. According to both approaches, in Italy and Japan, the feedback effect is valid, implying a mutual interaction between public debt and economic growth. Also, we find that this relationship is permanent. Similarly, we conclude that there is no causal relationship for France according to both approaches. For the remaining countries, however, we provide diverse evidence on both the direction of causality and the temporary/permanent nature of the causal relationship. The results on temporary or permanent causality at different frequencies offer policymakers and researchers detailed insights into an obscure aspect of the existing literature.

Non-Destructive Inspection Method Using Elastic Waves for Quality Management in the Bending Process Region of BFRTP

In order to establish a sustainable infrastructure, structures must maintain their minimum required performance for an extended period. Typically, existing concrete structures are reinforced with steel, but the susceptibility of steel reinforcements to corrosion, attributed to factors such as carbonation and chlorides, can lead to performance degradation, rendering the specified requirements unmet. As a remedy, there has been a growing interest in using lightweight and corrosion-resistant fiber-reinforced polymer (FRP) bars as an alternative to traditional steel reinforcement in concrete structures. Particularly noteworthy is the use of FRP bars manufactured with thermoplastic resins (FRTP) due to their ease of processing and convenient applicability on construction sites. This study delves into the mechanical behavior evaluation by examining the propagation characteristics of elastic waves in the curved section of basalt FRTP (BFRTP). Multiple BFRTP bars underwent a 180-degree bending process, subject to various conditions, including heat and twist variations. Subsequent measurements focused on the propagation characteristics of elastic waves in the curved sections, emphasizing wave attenuation, frequency features, and propagation velocity. Additionally, bending tensile tests were conducted on the curved bars, and a comparative analysis was performed between the tensile strength and the aforementioned elastic wave propagation characteristics. The results affirm a consistent correlation between the propagation characteristics of elastic waves (amplitude ratio, centroid frequency, velocity) during the traversal of the curved BFRTP section and its tensile strength. Notably, both centroid frequency and velocity exhibited correlation coefficients exceeding 0.7, suggesting their potential efficacy as reliable indicators for estimating tensile strength.

STAC3 disorder: a common cause of congenital hypotonia in Southern African patients.

STAC3 disorder, or Native American myopathy, is characterised by congenital myopathy, hypotonia, musculoskeletal and palatal anomalies, and susceptibility to malignant hyperthermia. A STAC3 c.851 G > C (p.Trp284Ser) pathogenic variant, common in the Lumbee Native American tribe, has been identified in other populations worldwide, including patients of African ancestry. We report on the frequency of STAC3 c.851 G > C in a cohort of 127 patients presenting with congenital hypotonia that tested negative for spinal muscular atrophy and/or Prader-Willi syndrome. We present a clinical retrospective, descriptive review on 31 Southern African patients homozygous for STAC3 c.851 G > C. The frequencies of various phenotypic characteristics were calculated. In total, 25/127 (20%) laboratory-based samples were homozygous for STAC3 c.851 G > C. A carrier rate of 1/56 and a predicted birth rate of 1/12 500 was estimated from a healthy cohort. A common haplotype spanning STAC3 was identified in four patients. Of the clinical group, 93% had a palatal abnormality, 52% a spinal anomaly, 59% had talipes equinovarus deformity/deformities, 38% had arthrogryposis multiplex congenita, and 22% had a history suggestive of malignant hyperthermia. The novel finding that STAC3 disorder is a common African myopathy has important clinical implications for the diagnosis, treatment and genetic counselling of individuals, with neonatal and/or childhood hypotonia with or without arthrogryposis multiplex congenita, and their families. The spread of this variant worldwide and the allele frequency higher in the African/African-American ancestry than the Admixed Americans, strongly indicates that the STAC3 c.851 G > C variant has an African origin which may be due to anancient mutation with migration and population bottlenecks.

Analysis of impact pressure, rock-breaking effect, and ground vibration induced by the disposable CO2 fracturing tube

The technology of expansion fracturing with liquid CO2 (EFLCO2) has attracted increasing attention due to reduced vibration and damage. The disposable fracturing tube has been developed and is gradually replacing the Cardox tube. However, there is a lack of impact pressure testing of disposable tubes under real working conditions, selection of gas explosion design parameters, and systematic analysis of blasting vibration. This limitation has constrained the widespread application of disposable fracturing tubes in engineering. A joint monitoring of the pressure-time curves in the disposable tubes and boreholes was conducted. The rock-breaking effect of varying hole spacing parameters in the EFLCO2 design was analyzed, and a systematic study was carried out on the vibration peak value, frequency, and energy characteristics. The results show that (1) the pressure distribution characteristics, stress peak value, and duration in the disposable tubes are different from those of Cardox tubes, which show multi-peak distribution, low-pressure peak value, and short duration. The correlation between the pressure in the disposable tube, filling pressure, and liquid CO2 weight is established, and a theoretical calculation method for the borehole wall pressure is proposed; (2) The hole spacing in rocks of different hardness is suggested; and (3) At the same scale distance, the peak particle velocity (PPV) caused by EFLCO2 () is significantly smaller than that caused by blasting (PPVexplosive). The ratio of PPVexplosive to is a power function related to scale distance, and a distance-related zonality exist in this relationship. The frequency composition of the vibration signal caused by EFLCO2 is relatively simple with a narrow frequency band. Its PPV and energy are mainly concentrated in the low-frequency band. This research contributes to the optimization of disposable fracturing tubes, gas explosion design, and vibration hazard control.

Research and analysis on fault diagnosis of multistage centrifugal pump for mining

Abstract The vibration data collected by the fault experiment are analyzed in the time domain and frequency domain respectively, and the conclusion shows that the frequency characteristics of some vibration data cannot be fully characterized due to noise interference. In order to solve the problem of noise interference, this paper uses the VMD algorithm based on the kurtosis peak to denoise the collected data, constructs wavelet time-frequency map samples through the wavelet transform method to provide input sources for the subsequent convolutional neural network, and uses the image samples for fault diagnosis and identification. [1] Through the convolutional neural network built to diagnose and identify the different states of the multistage centrifugal pump, it can be seen that the recognition accuracy, training time, and recognition time of a single sample of various image samples in the three cases of no noise reduction, noise reduction and a small number of samples can be found through the method of comparative analysis, and compared with the traditional SVM method, the superiority of the convolutional neural network using image recognition can be found.

Energy dissipation and time–frequency analysis of characteristics induced by vortex breakdown in an axial flow pump

Vortex breakdown in a pump sump is a complex and negative factor for the pump. Different from my previous study that focused mainly on the development process of vortex and its damage to the pump, this paper is from a new perspective that studies the energy dissipation and time–frequency characteristics induced by vortex breakdown. The tested data of pressure and velocity in the process of vortex breakdown were obtained by the model. Considering the gas–liquid two-phase flow of the vortices, a new numerical simulation approach is conducted and verified. The results show that the development rules of vortex breakdown reveal that the breakdown is initiated near the blade. The residual disturbance in the flow field continues to propagate after vortex breakdown, inducing unstable flow inside the runner and causing additional energy dissipation. The time–frequency characteristics induced by vortex breakdown indicated that the runner rotation speed has a significant effect on the vortex breakdown. The frequency of vortex breakdown is relatively small under high-speed rotation. Through discussion, it can be concluded that in order to reduce the harm of vortex breakdown, it can take measures such as controlling the impeller rotation speed, stalling anti-vortex measures, and adjusting operating conditions.

Research on the induced vibration force characteristics of lateral propulsion propeller

Abstract This article conducts unsteady numerical calculations on lateral thrusters under arrival conditions, revealing the induced unsteady force characteristics under the interference between lateral thrusters and pod packages. The calculation results show that the lateral force and torque of the lateral propeller point towards the bow of the ship, and the main characteristic frequencies of pressure pulsation are blade frequency and its harmonics; The thrust and torque pulsation of the side thrusters on the bow side of the ship have maximum values; The bottom of the lateral thrust channel exhibits stronger pressure pulsation compared to other positions. The calculation results of this article determine the position and characterization form of the excitation force of the lateral thruster, providing a basis for the design of low-noise lateral thrusters.

Electromagnetic noise characteristics analysis and noise traceability in induction motor

Abstract To investigate the electromagnetic vibration and noise issues in induction motors, this study employs a combined methodology of experimental, theoretical, and simulation approaches to analyze the characteristics of electromagnetic noise. Initially, a theoretical analysis of the radial electromagnetic force of the motor is conducted, followed by a simulation of the motor’s electromagnetic vibration and noise at a working condition of 3000 rpm, incorporating a Multiphysics coupled analysis of electromagnetic-vibration-noise. Vibration testing on a specific compressor motor reveals that the primary vibration noise characteristic frequency is centered around 2000 Hz. A comparison between simulation and experimental results shows a high degree of congruence, indicating the reliability of the simulation method. Subsequently, an acoustic camera kit, Brüel & Kjær PULSE Reflex, based on a beamforming method, is employed for noise source identification and acoustic holography testing at 3000 rpm, which successfully localizes the noise sources. The analysis and experiments provided herein can serve as a reference for the development of electromagnetic vibration noise suppression techniques in induction motors.

Viscoelastic characterization of coating layers by ultrasonic interference and critical attenuation

This study proposes a characterization technique for a viscoelastic coating layer on a metal substrate immersed in water based on ultrasonic interference and critical attenuation phenomena. Theoretical analysis shows that the reflection spectrum for the normal incidence wave has local minima at the resonance frequencies of the coating layer, which provide its elastic property if the layer thickness is given a priori. Furthermore, it is revealed that the lower envelope of the reflection spectrum takes a minimum, whose frequency can be related to the critical attenuation condition. This feature implies a possibility that the viscous property of the coating material can be estimated by similarly measuring a characteristic frequency to the estimation of the elastic property. Two bonded specimens consisting of polycarbonate (PC) sheets and metal substrates were prepared to validate the proposed method. The reflection spectra measured for the normal wave incidence showed multiple local minima and critical attenuation, which are predicted to appear by theoretical predictions. The wave velocities and loss factors of PC estimated from the two specimens are in fair agreement with the properties shown in the previous paper. This result demonstrates that the ultrasonic interference and critical attenuation behavior can be applied to the characterization of the viscoelastic coating layers.

Research on the excitation force and vortex dynamics characteristics of pump-jet propulsor induced by shafting whirling vibration: Non-uniform blade tip clearance

The propulsion shafting whirling vibration causes non-uniform dynamic changes in the rotor tip clearance, which directly have a significant influence on the excitation force and vortex dynamic characteristics of the pump-jet propulsor. In the current study, based on improved delay detached eddy simulation, the influence of non-uniform blade tip clearance on the excitation force and vortex dynamics characteristics of the pump-jet propulsor is studied under design conditions. The results show that the application of propulsion shafting whirling vibration induces significant changes in the excitation force of the pump-jet propulsor. The rotor blades modulate the excitation forces of the stator blades and duct. The transverse and vertical excitation forces are more significant than the longitudinal excitation force. The magnitude change in the circular orbit shows a linear relationship with the excitation force magnitude. The characteristic frequency of the transverse and vertical excitation forces of each component is the shaft rotation frequency. In contrast, the characteristic frequency of the longitudinal excitation force is twice the shaft rotation frequency. In the elliptical orbit, the excitation force of each component is compressed or stretched in the time domain, and the dominant frequency is shifted in the frequency domain; there is no longer a linear relationship between the vibration magnitude change and the excitation force magnitude. Furthermore, an energy generation mechanism in the wake field of the pump-jet propulsor induces vortex frequency due to the whirling vibration of the propulsion shafting system.

Feature-based maximum entropy for geophysical properties of the seabeda).

The coherent recombination of a direct and seabed reflected path is sensitive to the geophysical properties of the seabed. The concept of feature-based inversion is used in the analysis of acoustic data collected on a vertical line array (VLA) on the New England continental shelf break in about 200 m of water. The analysis approach for the measurements is based on a ray approach in which a direct and bottom reflected path is recombined, resulting in constructive and destructive interference of the acoustic amplitudes with frequency. The acoustic features have the form of prominent nulls of the measured received levels as a function of frequency as a broadband (500-4500 Hz) source passes the closest point of approach to the VLA. The viscous grain shearing (VGS) model is employed to parameterize a two-layer seabed model. The most likely seabed is a sand sediment with a porosity of about 0.42. There is a possibility of a thin (less than 0.5 m) surface layer having a slightly higher porosity between 0.45 and 0.50. Using the estimates for the VGS parameters inferred from the short-range frequency features, a normal mode model is used to predict the received acoustic levels over larger range scales.

Nonlinear vibrations and critical angular velocity information of the high-speed rotating eccentric disk made of Gori-metamaterials: Introducing data-driven solution for solving the nonlinear problems

Eccentric discs in rotational motion are commonly utilized in various technical fields, including gas turbine engines, flywheels, gears, and brakes. So, improving its critical angular velocity and frequency characteristics is a challenging issue for engineers. So, in this work for the first time, nonlinear vibrations and buckling analysis of the high-speed rotating eccentric disks using mathematical simulation and data-driven solutions are presented. One of the suggestions for improving its mechanical properties is considering metamaterials in the construction of the eccentric disk. Metamaterial is a novel synthetic material that has distinctive physical and mechanical capabilities that are unattainable in natural materials due to its well-designed structure. The properties of the eccentric disks are controlled by the amount of graphene and the degree of folding of graphene origami (GOri) across the thickness of the eccentric disks. These properties, such as Poisson's ratio, vary depending on the position and can be estimated using micromechanical models assisted by genetic programming (GP). Using von-Karman nonlinearity, transformed differential quadrature method (TDQM), and Newton's method the nonlinear governing equations are obtained and solved, respectively. The results show that when the radius ratio of the rotating eccentric disk increases by 8%, the critical point is decreased from 420 HZ to 370 HZ, a reduction of around 12 %. Another suggestion for improving its mechanical properties is controlling the geometrics and physics of the presented structure according to the designer's purposes.

Quantum chemical computational analysis, electronic transitions, interaction mechanisms analysis by spectroscopic, molecular docking, and molecular dynamic simulation of retinol

Quantum computational simulations based on density functional theory are employed to investigate the molecular structure of Retinol. Both geometrical parameters and electronic transitions for gas and green solvents are calculated. Characteristic frequencies were identified and band assignments were achieved through normal coordinate analysis. The calculated spectra and a comparative study of the vibrational spectra, which included a variety of vibration modes, were compared with the experimental spectra. A compound’s strong reactivity may be indicated by the bandgap, which also indicates the possibility of further charge exchange through the molecule. Determination of relative electrophilicity/nucleophilicity indices of retinol was undertaken through the prediction of condensed Fukui functions, complemented by the generation of molecular electrostatic potential surface maps. This exploration was validated through meticulous correlation with reduced density gradient and isodensity surface plots. The docking analysis of retinol with different proteins was performed to confirm anti-inflammatory activity. To gain comprehensive insights into macromolecule pliability concerning protein–ligand interactions, an effective molecular dynamics simulation was conducted.

Subject review: Human activity recognition based on depth images

Human activity recognition (HAR) has becomes a well-known area of study for researchers due to its multiple possible applications. HAR systems are applied in many areas including supervisory nursing care, medical supervision, surveillance, human- machine interaction, gaming and entertainment, etc. However, developing a robust HAR system that incorporates a comprehensive and scalable set of feature spaces is a challenging task. Much research has been done in this area to develop efficient HAR systems. In this article, a review of the developments in Human Activity Recognition (HAR) will be presented. Human activity recognition works on two basic principles; feature extraction from sensor data and classification of features into action classes. The review presented in this article is hence divided into two parts. The first part deals with feature representation in HAR systems such as Space-time Features, Frequency Features, Local Descriptors, Optical Flow Based, and Skeleton Joints. The second part deals with the classification approaches used in the literature such as Bayesian, and HMM.

Early postoperative complications in open heart surgery patients: A review

Over the past few decades, open heart surgery has undergone significant progress in terms of technologies used, operative techniques and resuscitation care. This has led to a significant increase in patient survival and quality of life after discharge. However, there are still a number of challenges to be addressed, one of which is reducing the incidence of early postoperative complications. They are directly and primarily determined by patients’ demographic and clinical characteristics, as well as by the characteristics of the surgical intervention itself and are a major factor in the outcome of the surgery performed. This review presents the most important and frequent early complications, their epidemiological and clinical features. Their good knowledge is the key to achieving an optimal treatment approach and seeking effective opportunities for their prediction and incidence reduction.