Phase field model for electric-thermal coupled discharge breakdown of polyimide nanocomposites under high frequency electrical stress

In contrast to conventional transformers, power electronic transformers, as an integral component of new energy power system, are often subjected to high-frequency and transient electrical stresses, leading to heightened concerns regarding insulation failures. Meanwhile, the underlying mechanism behind discharge breakdown failure and nanofiller enhancement under high-frequency electrical stress remains unclear. An electric-thermal coupled discharge breakdown phase field model was constructed to study the evolution of the breakdown path in polyimide nanocomposite insulation subjected to high-frequency stress. The investigation focused on analyzing the effect of various factors, including frequency, temperature, and nanofiller shape, on the breakdown path of Polyimide(PI) composites. Additionally, it elucidated the enhancement mechanism of nano-modified composite insulation at the mesoscopic scale. The results indicated that with increasing frequency and temperature, the discharge breakdown path demonstrates accelerated development, accompanied by a gradual dominance of Joule heat energy. This enhancement is attributed to the dispersed electric field distribution and the hindering effect of the nanosheets. The research findings offer a theoretical foundation and methodological framework to inform the optimal design and performance management of new insulating materials utilized in high-frequency power equipment.

Exploration of instantaneous frequency for local control assessment in real-time hybrid simulation

Local control parameters such as instantaneous delay and instantaneous amplitude play an essential role in evaluating the performance and maintaining the stability of real-time hybrid simulation(RTHS).However,existing methods have limitations in obtaining this local assessment in either the time domain or frequency domain.In this study,the instantaneous frequency is introduced to determine local control parameters for actuator tracking assessment in a real-time hybrid simulation.Instantaneous properties,including amplitude,delay,frequency and phase,are then calculated based on analytic signals translated from actuator tracking signals through the Hilbert transform.Potential issues are discussed and solutions are proposed for calculation of local control parameters.Numerical simulations are first conducted for sinusoidal and chirp signals with time varying amplitude error and delay to demonstrate the potential of the proposed method.Laboratory tests also are conducted for a predefined random signal as well as the RTHS of a single degree of freedom structure with a self-centering viscous damper to experimentally verify the effectiveness of the proposed use of the instantaneous frequency.Results from the ensuing analysis clearly demonstrate that the instantaneous frequency provides great potential for local control assessment,and the proposed method enables local tracking parameters with good accuracy.

应用FFB-EWT的OFDM辐射源个体识别

针对在传统辐射源个体识别方法中OFDM辐射源细微指纹特征信息会受到数据信号成分和信道噪声的影响会导致分类识别率低的问题,根据短导码的子载波频谱特点设计了一种基于固定频率边界的经验小波变换(frequency fixed boundary-empirical wavelet transform,FFB-EWT)和深度残差网络的OFDM辐射源个体识别方法.首先,提取OFDM信号的短导码,根据短导码中传输信号子载波的频率间隔划分固定边界条件,将频域边界值应用于FFB-EWT对信号进行分解,去除包含前导序列信息的子载波分量;其次,对相邻帧中包含指纹特征的空子载波分量进行积累,提高指纹特征信号的信噪比;然后,使用双通道的结合了非局部注意力模块和通道注意力模块的ResNet18残差网络,对IQ两路数据输入进行特征提取,通过Softmax函数进行分类;最后,选择Oracle公开数据集验证方法的可行性.实验结果表明利用FFB-EWT方法对6个不同辐射源个体在6 dB和0 dB条件下进行识别,准确率可以达到98.17%和89.33%,证明了该方法在低信噪比条件下的有效性.

基于LWT的可见光OFDM-IM系统研究

针对索引调制正交频分复用(OFDM-IM)技术峰均比过高,对相位噪声和载波频偏敏感,系统复杂度高等问题,提出基于小波提升变换(LWT)的可见光OFDM-IM系统。首先选择正交小波基作为子载波,通过对信号的分裂、预测和更新,将信号分为高频信号和低频信号,再结合可见光信道构成LWT-OFDM-IM系统,最后通过理论分析和蒙特卡罗方法对系统的可靠性、峰均比特性和最佳小波分解层数进行了仿真验证,结果表明:当子载波N=256,子块中子载波数量L=4,激活子载波数量k=2,系统误码率为10~(-4)时,LWT-OFDM-IM比FFT-OFDM-IM改善约8 dB,相较于DWT-OFDM-IM提高约4 dB。当系统互补累计分布函数(CCDF)为10^(-1)数量级时,LWT-OFDM-IM的峰均比较FFT-OFDM-IM改善约2.3 dB。随着小波层数的递增,系统的误码性能越好,当小波层数为3层时,大约在10 dB附近,误码率可到10^(-5)量级。

Frequency Effect of Formulaic Sequences on CAF in Academic Writing:Examples from L2 Master’s Theses

This study was conducted to explore the individual uses of formulaic sequence(FS)frequency and their effects on complexity,accuracy,and fluency(CAF)in academic writing.Data was collected from the conclusion sections of a self-compiled corpus of 30 L2 master’s theses.Statistical analysis revealed several notable conclusions.1)Student writers tend to make repetitive use of particular FSs in single texts.2)FS use has a significant frequency effect on fluency,and the high-frequency group slightly outperforms the low-frequency group.3)FS use has a certain frequency effect on accuracy,and the high-frequency group demonstrates the strongest correlation between FS frequency and accuracy.4)FS use has a significant frequency effect on lexical complexity,and the low-frequency group slightly outperforms the high-frequency group.In the low-frequency group,FS use has a significant frequency effect on syntactic complexity as well.Finally,5)The results support Skehan’s trade-off effect,a competition between CAF,which,to a certain degree,is affected by individual uses of FS frequency.The low-frequency group displays a greater trade-off effect than the high-frequency group.

Calculation,Simulation,and Testing for the Natural Frequency of a Micro Accelerometer

A calculation and test method for the natural frequency of a high-g micro accelerometer with complex structures is presented. A universal formula for natural frequency, which can significantly simplify the structural design process, is deduced and confirmed by experiment. A simplified analytical model is established to describe the accelerometer＇s mechanical behavior and deduce the formula for the natural frequency. Finite element modeling is also conducted to evaluate the natural frequency of the micro-accelerometer and verify the formula. The results obtained from the analytical model and the finite element simulation show good agreement. Finally, a shock comparison method designed for acquiring the high frequency characteristics of the accelerometer is introduced to verify the formula by testing its actual natural frequency.

Traditional Chinese medicine for transformation of gastric precancerous lesions to gastric cancer:A critical review

Gastric cancer(GC)is a common gastrointestinal tumor.Gastric precancerous lesions(GPL)are the last pathological stage before normal gastric mucosa transforms into GC.However,preventing the transformation from GPL to GC remains a challenge.Traditional Chinese medicine(TCM)has been used to treat gastric disease for millennia.A series of TCM formulas and active compounds have shown therapeutic effects in both GC and GPL.This article reviews recent progress on the herbal drugs and pharmacological mechanisms of TCM in preventing the transformation from GPL to GC,especially focusing on antiinflammatory,anti-angiogenesis,proliferation,and apoptosis.This review may provide a meaningful reference for the prevention of the transformation from GPL to GC using TCM.

基于OFDM的太赫兹通感一体化波形设计与性能分析

针对当前正交频分复用(OFDM)通信系统,开展了基于二维快速傅里叶变换(2D-FFT)的一体化波形研究。探究了太赫兹OFDM波形感知特性,通过理论和仿真计算相结合,分析不同频率和带宽下波形参数设计的影响,以指导太赫兹OFDM通信感知一体化(ISAC)系统设计。理论和仿真计算分析表明,太赫兹OFDM通信感知一体化系统具有多目标感知能力,太赫兹大带宽特性使其感知距离分辨力高达厘米级,速度分辨力可达分米每秒级,且低感知信噪比情况下仍能解析出目标的位置与速度信息,证明了OFDM通信感知一体化波形能够支撑太赫兹窄波束移动通信。

Method of Detection Abnormal Features in Ionosphere Critical Frequency Data on the Basis of Wavelet Transformation and Neural Networks Combination

The research is focused on the development of automatic detection method of abnormal features, that occur in recorded time series of ionosphere critical frequency fOF2 during periods of high solar or seismic activity. The method is based on joint application of wavelet-transformation and neural networks. On the basis of wavelet transformation algorithms for the detection of features and estimation of their parameters were developed. Detection and analysis of characteristic components of time series are performed on the basis of joint application of wavelet transformation and neural networks. Method's approbation is performed on fOF2 data obtained at the observatory “Paratunka” (Paratunka settlement, Kamchatskiy Kray).

Transformation Formulas for the First Kind of Lauricella’s Function of Several Variables

Very recently Atash and Al-Gonah [1] derived two extension formulas for Lauricella’s function of the second kind of several variables and . Now in this research paper we derive two families of transformation formulas for the first kind of Lauricella’s function of several variables and with the help of generalized Dixon’s theorem on the sum of the series obtained earlier by Lavoie et al. [2]. Some new and known results are also deduced as applications of our main formulas.

基于OFDM载波通信技术的变压器油色谱数据传输方法

由于常规的变压器油色谱数据传输方法未进行编码处理导致传输效果不佳,文章设计一种基于正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)载波通信技术的变压器油色谱数据传输方法。利用色谱分析仪获取数据,进行归一化处理和特征提取,以获取数据的关键信息,并对其进行编码。计算数据的覆盖范围,并根据覆盖情况制定合适的传输协议。在OFDM支持下评估信道质量,根据信道状况分配子载波,动态调整分配结果,计算传输频率,实现高效的数据传输。实验结果表明,该方法误码率低,传输效果较好。

BOUNDEDNESS OF THE CALDER´ON COMMUTATOR WITH A ROUGH KERNEL ON TRIEBEL-LIZORKIN SPACES

In this paper,we consider the boundedness on Triebel-Lizorkin spaces for the d-dimensional Calder´on commutator defined by TΩ,af(x)=p.v.∫R_(d)Ω(x−y)/|x−y|^(d+1)(a(x)−a(y))f(y)dy,where Ω is homogeneous of degree zero,integrable on Sd−1 and has a vanishing moment of order one,and a is a function on Rd such that∇a∈L^(∞)(R^(d)).We prove that if 1<p,q<∞andΩ∈L(log L)^(2 q)(S^(d−1))with q=max{1/q,1/q′},then TΩ,a is bounded on Triebel-Lizorkin spaces˙F_(p)^(0)q(R^(d)).

Stock profiling using time–frequency‑varying systematic risk measure

This study proposes a wavelets approach to estimating time–frequency-varying betas in the capital asset pricing model(CAPM)framework.The dynamic of systematic risk across time and frequency is analyzed to investigate stock risk-profile robustness.Furthermore,we emphasize the effect of an investor’s investment horizon on the robustness of portfolio characteristics.We use a daily panel of French stocks from 2012 to 2022.Results show that varying systematic risk varies in time and frequency,and that its short and long-run evolutions differ.We observe differences in short and long dynamics,indicating that a stock’s betas differently fluctuate to early announcements or signs of events.However,short-run and long-run betas exhibit similar dynamics during persistent shocks.Betas are more volatile during times of crisis,resulting in greater or lesser robustness of risk profiles.Significant differences exist in short-run and longrun risk profiles,implying a different asset allocation.We conclude that the standard CAPM assumes short-run investment.Then,investors should consider time–frequency CAPM to perform systematic risk analysis and portfolio allocation.

The Primary Configuration and Formulae of the Intermediate Frequency Bend Machine

In this paper the primary configuration and formulae of design parameters of the intermediate frequency bend machine(  1450)are introduced and the main points of its design are explained.

A passive source ranging method based on the frequency warping transform of the vertical intensity flux in shallow water

The phase of cross-correlation function of two different normal modes contains source range information, which can be extracted by warping transform due to the dispersive characteristics of the shallow water waveguide. The autocorrelation function of the received pressure or particle velocity contains both modal autocorrelation component(MAC) and modal cross-correlation component(MCC), with the former part usually treated as interference for source ranging. Because the real part of the vertical intensity flux(RPVIF) only contains MCC, a passive impulsive source ranging method based on the frequency warping transform of RPVIF with a single vector receiver in shallow water is presented. Using a waveguide-invariant-based frequency warping operator, the cross-correlation components of two different modes in the vertical intensity flux are warped into separable impulsive sequences, the time delays of which are subsequently used for source ranging. The advantages of source ranging based on warping the vertical intensity flux compared with warping the pressure autocorrelation function are pointed out, and the experiment results are also presented.

Analysis of Thermal Behavior of High Frequency Transformers Using Finite Element Method

High frequency transformer is used in many applications among the Switch Mode Power Supply (SMPS), high voltage pulse power and etc can be mentioned. Regarding that the core of these transformers is often the ferrite core;their functions partly depend on this core characteristic. One of the characteristics of the ferrite core is thermal behavior that should be paid attention to because it affects the transformer function and causes heat generation. In this paper, a typical high frequency transformer with ferrite core is designed and simulated in ANSYS software. Temperature rise due to winding current (Joule-heat) is considered as heat generation source for thermal behavior analysis of the transformer. In this simulation, the temperature rise and heat distribution are studied and the effects of parameters such as flux density, winding loss value, using a fan to cool the winding and core and thermal conductivity are investigated.

A WEIGHTED GENERAL DISCRETE FOURIER TRANSFORM FOR THE FREQUENCY-DOMAIN BLIND SOURCE SEPARATION OF CONVOLUTIVE MIXTURES

This letter deals with the frequency domain Blind Source Separation of Convolutive Mixtures (CMBSS). From the frequency representation of the "overlap and save", a Weighted General Discrete Fourier Transform (WGDFT) is derived to replace the traditional Discrete Fourier Transform (DFT). The mixing matrix on each frequency bin could be estimated more precisely from WGDFT coefficients than from DFT coefficients, which improves separation performance. Simulation results verify the validity of WGDFT for frequency domain blind source separation of convolutive mixtures.

Recursion-transform method and potential formulae of the m×n cobweb and fan networks

In this paper, we made a new breakthrough, which proposes a new recursion–transform（RT） method with potential parameters to evaluate the nodal potential in arbitrary resistor networks. For the first time, we found the exact potential formulae of arbitrary m × n cobweb and fan networks by the RT method, and the potential formulae of infinite and semi-infinite networks are derived. As applications, a series of interesting corollaries of potential formulae are given by using the general formula, the equivalent resistance formula is deduced by using the potential formula, and we find a new trigonometric identity by comparing two equivalence results with different forms.

Research on the Seismic Wave Characteristics of Low Frequency Signals before the Alxa Left Banner MS5.8 Earthquake in Inner Mongolia,China

In order to search for the seismic wave characteristics of low frequency signals in the Alxa Left Banner region,Inner Mongolia,the low frequency signals of seismic wave data are extracted from the earthquakes of MS5. 8 in 2015 and MS5. 0 in 2016 in this area. The results show that:① Before the MS5. 8 earthquake,the seismic stations located near the epicenter in Wuhai,Dongshengmiao,and Shizuishan recorded seismic waves that showed the phenomenon of spectrum shift from high to low frequency.② The low frequency signals recorded by different stations have obvious difference.③ According to the data recorded by the station closest to the epicenter,low-frequency signals were recorded about120 hours before the earthquake and had obvious anomalies. This may reflect slow slip before the earthquake.

Power and Cross-Spectra for the Turbulent Atmospheric Motion and Transports in the Domain of Wave Number Frequency Space: Theoretical Aspects

The study of large-scale atmospheric turbulence and transport processes is of vital importance in the general circulation of the atmosphere. The governing equations of the power and cross-spectra for the atmospheric motion and transports in the domain of wave number frequency space have been derived. The contributions of the nonlinear interactions of the atmospheric waves in velocity and temperature fields to the conversion of kinetic and potential energies and to the meridional transports of angular momentum and sensible heat in the atmosphere have been discussed.