Thermal Wave Effect and Sintering Activation Energy at the Initial Stage of Field Assisted Sintering Process for Non-conductive Al_2O_3 Powders

The effect of thermal wave at the initial stage for non-conductive Al_2 O_3 powders compact in field assisted sintering technique(FAST) was investigated. The Lord and Shulman type generalized thermoselastic theory was introduced to describe the influence of thermal-mechanical interaction, as well as the heat transport and thermal focusing caused by thermal wave propagation. The expression of vacancy concentration difference of the particles was deduced by considering transient thermal stress. Subsequently, the relationship between activation energy and vacancy concentration difference was obtained. The mechanism of surface diffusion, volume diffusion, simultaneous surface and volume diffusion was analyzed. The numerical simulations indicate that low sintering temperature can obtain high local temperature by the superposition effect of thermal wave. Vacancy concentration differences were improved during FAST compared with hot-pressure and pressureless sintering, thereby decreasing the sintering time. By contrast, the activation energy declined with the decrease of vacancy concentration difference in the neck growth process.

Two Energy-Preserving Compact Finite Difference Schemes for the Nonlinear Fourth-Order Wave Equation

In this paper,two fourth-order compact finite difference schemes are derived to solve the nonlinear fourth-order wave equation which can be viewed as a generalized model from the nonlinear beam equation.Differing from the existing compact finite difference schemes which preserve the total energy in a recursive sense,the new schemes are proved to per-fectly preserve the total energy in the discrete sense.By using the standard energy method and the cut-off function technique,the optimal error estimates of the numerical solutions are established,and the convergence rates are of O(h^(4)+τ^(2))with mesh-size h and time-step τ.In order to improve the computational efficiency,an iterative algorithm is proposed as the outer solver and the double sweep method for pentadiagonal linear algebraic equations is introduced as the inner solver to solve the nonlinear difference schemes at each time step.The convergence of the iterative algorithm is also rigorously analyzed.Several numerical results are carried out to test the error estimates and conservative properties.

Applications of EMD to analyses of high-frequency beachface responses to Storm Bebinca in the Qing’an Bay,Guangdong Province,China

On average,five to six storms occur in the Qiongzhou Strait every year,causing significant damage to coastal geomorphology and several property losses.Tropical Storm Bebinca is the most unusual and complex storm event that has occurred in this region over the last 10 years.To detect the high-frequency beachface responses to the storm,a pressure sensor was deployed in the surf zone to record the free sea surface height,and the heights of grid pile points on the beachface were measured manually to determine beach elevation changes during this storm.Empirical Mode Decomposition and related analysis techniques were used to analyze the high-frequency topography and wave data.The results showed that:(1)the beachface response process occurred in three stages.The first stage was the rapid response stage,wherein the spring tide berm began to erode significantly,and the front edge of the beach berm reacted closely.The two beach sections resisted the harmful energy of the main storm.In the second stage,the beach slope increased after a large sediment loss on the beach berm and its front edge.To adapt to the storm energy,the beach at the low tide line began to erode,and the beach slope decreased.In the third stage,after the storm turned,the wave energy was significantly attenuated,and the beach berm eroded to resist the residual wave energy.The beachface began to oscillate and recover.(2)The main wave surface was the superimposed product of a few internal mode functions.Similar results were observed in beachface changes.High-frequency driving factors determine the local characteristics of beach evolution,and low-frequency driving factors determine the beach evolution trend.(3)The response of sediment to the storm was not a single sea-transportation,but a single-or two-way conversion driven by factors such as wave energy,swash flow,and secondary wave breaking.(4)TheΩ-RTR model is not completely applicable to beach states that undergo rapid changes during storms.Therefore,it is necessary to carry out further research on beach state identification during storms.

Short-term variations and influencing factors of suspended sediment concentrations at the Heisha Beach,Guangdong,China

Knowledge of sediment variation processes is essential to understand the evolution mechanism of beach morphology changes.Thus,a field measurement was conducted at the Heisha Beach,located on the west coast of the Zhujiang River(Pearl River)Estuary,to investigate the short-term variation in suspended sediment concentrations(SSCs)and the relationship between the SSC and turbulent kinetic energy,bottom shear stress(BSS),and relative wave height.Based on extreme event analysis results,extreme events have a greater influence on turbulent kinetic energy than SSC.Although a portion of the turbulent kinetic energy dissipates directly into the water column,it plays an important role in suspended sediment motion.Most of the time,the wave-current interaction is strong enough to drive sediment incipience and resuspension.When combined,the wave-current interaction and wave-induced BSSs have a greater influence on suspended sediment transport and SSC variation than current-induced BSS alone.The relative wave height also has a strong correlation with SSC,indicating that the combined effect of water depth and wave height significantly impacts SSC variation.Water depth is mainly controlled by the tide on the beaches;thus,the effects of tides and waves should be conjunctively considered when analyzing the factors influencing SSC.

TI介质中P-SV波传播的SBP-SAT模拟

基于速度-应力形式的弹性波动方程,采用分部求和和同时逼近项技术建立的SBP-SAT方法,推导了横向各向同性(transversely isotropy,TI)介质的矩阵对称型(symmetric matrix form,SMF)弹性波动方程离散形式,并通过能量法进行了稳定性分析。将该方法应用于倾斜横向各向同性(tilted transverse isotropic,TTI)介质模型、垂直横向各向同性(transverse isotropy with a vertical axis of symmetry,VTI)介质和含裂缝及曲线域的复杂介质模型,对所得的速度幅值和单炮记录分析并总结规律;对不同时间步长、单元网格数的结果进行对比,得出计算效率并验证该方法在求解P-SV波传播问题上的正确性。数值模拟结果表明,该方法模拟精度高,适用性好,在地震数值模拟领域有很好的应用价值和前景。

基于ASTFA和SDEO解调的行星齿轮箱故障诊断方法

针对行星齿轮箱故障信号的调制特点,提出基于自适应最稀疏时频分析(Adaptive and Sparsest TimeFrequency Analysis,ASTFA)和对称差分能量算子(Symmetric Difference Energy Operator,SDEO)相结合的解调方法,用于提取故障信号的瞬时幅值和瞬时频率信息。采用ASTFA方法分解行星齿轮箱故障信号,得到若干个单分量信号,采用SDEO进行解调,得到各单分量信号的瞬时幅值和瞬时频率,并计算得到包络谱。采用该方法分析行星齿轮箱故障仿真信号和故障实际信号,结果表明,该方法能准确地提取故障特征,实现行星齿轮箱故障诊断。

基于STOA-VMD的串补输电线路故障测距

由于串补电容的存在,串补输电线路中发生故障时,故障行波过程十分复杂,行波波头检测困难;故障行波信号中存在较大噪声时,传统的故障特征提取方法无法准确提取故障行波的特征信号。为此在串补线路中故障行波信号存在噪声条件下,提出一种基于乌燕鸥优化算法(sooty tern optimization algorithm,STOA)对变分模态分解(variational mode decomposition,VMD)优化的故障行波特征提取方法。该方法利用最大峭度准则对STOA的目标函数不断优化,从而选取VMD中更合适的模态量和惩罚因子值,避免人为设定模态量和惩罚因子值时,VMD对故障行波信号的分解不足或分解过剩造成的故障定位误差。通过对称差分能量算子(symmetric differential energy operator,SDEO)提取VMD后最佳的模态分量的能量突变点,结合双端行波测距方法,实现故障测距。仿真结果表明,所提方法能够实现串补输电线路的故障测距,测距精度高,且不受故障类型、过渡电阻的影响。

不同管材侧向环形切缝装药爆炸能量传递特性

为了探究不同管材侧向环形切缝装药的爆炸能量传递特性,开展常规柱状装药和4种管材侧向环形切缝装药的爆炸实验,借助高速纹影拍照系统和冲击波超压监测系统捕获了爆炸波的传播历程和监测爆炸压力的分布情况,分析了侧向环形切缝装药爆炸能量传递规律,以及管材对其能量传递特性的影响。结果显示:侧向环形切缝装药爆炸之后,爆轰产物和冲击波均先朝着切缝方向向外传播,非切缝方向爆轰产物和冲击波的传播相对滞后;与常规柱状装药的爆炸压力对称分布相比,侧向环形切缝管会减小非切缝方向爆炸压力和增大切缝方向爆炸压力,爆炸压力的非对称分布证明侧向环形切缝装药在切缝方向诱发产生了聚能效应;不同管材侧向环形切缝装药的聚能效果强弱顺序为不锈钢(SS)>聚氯乙烯(PVC)>纤维强化塑料(FRP)>有机玻璃(PMMA)。

楮岛沙滩不同时间尺度变化特征与波能流密度相关性探讨

在波浪和水流的作用下,泥沙在不同时间尺度下的运动会引起沙滩的冲淤演变,对海岸资源有重要的影响。因此,了解沙滩季节性演变规律,并采取针对性的防护措施,是近岸沙滩亟须解决的问题。目前,现场观测是研究沙滩剖面冲淤演变的重要方法,通过沉积物组成、岸滩坡度及波浪动力的时空变化,了解沙滩剖面的变化特性,对于沙滩管理和海岸保护具有十分重要的意义。基于2017年9月—2019年11月在荣成楮岛南岸沙滩每个月采集一次的剖面数据,以及波浪动力数据,分别探究了沙滩在不同时间尺度下的变化特征,并对沙滩变化特征与波浪动力因素的相关性进行了探讨。研究发现:楮岛南岸沙滩形态变化具有较强的季节性特征,春季沙滩比较稳定;夏季沙滩受台风影响侵蚀严重,但在风暴过后的短时间内,沙滩泥沙恢复较快;冬季沙滩恢复速度逐渐减缓并趋于稳定。在夏季和冬季期间,波能流密度的向岸分量对楮岛南沙滩的演变产生重要作用,而且波能流密度向岸分量的均值(选取数据采集前15d的波浪条件参与计算)与沙滩体积的相关性最好,并给出了两者的线性拟合公式。

A Stable Finite Difference Method for the Elastic Wave Equation on Complex Geometries with Free Surfaces

A stable and explicit second order accurate finite difference method for the elastic wave equation in curvilinear coordinates is presented.The discretization of the spatial operators in the method is shown to be self-adjoint for free-surface,Dirichlet and periodic boundary conditions.The fully discrete version of the method conserves a discrete energy to machine precision.

INVARIANTS-PRESERVING DU FORT-FRANKEL SCHEMES AND THEIR ANALYSES FOR NONLINEAR SCHRÖDINGER EQUATIONS WITH WAVE OPERATOR

Du Fort-Frankel finite difference method(FDM)was firstly proposed for linear diffusion equations with periodic boundary conditions by Du Fort and Frankel in 1953.It is an explicit and unconditionally von Neumann stable scheme.However,there has been no research work on numerical solutions of nonlinear Schrödinger equations with wave operator by using Du Fort-Frankel-type finite difference methods(FDMs).In this study,a class of invariants-preserving Du Fort-Frankel-type FDMs are firstly proposed for one-dimensional(1D)and two-dimensional(2D)nonlinear Schrödinger equations with wave operator.By using the discrete energy method,it is shown that their solutions possess the discrete energy and mass conservative laws,and conditionally converge to exact solutions with an order of for ofο(T^(2)+h_(x)^(2)+(T/h_(x))^(2))1D problem and an order ofο(T^(2)+h_(x)^(2)+h_(Y)^(2)+(T/h_(X))^(2)+(T/h_(y))^(2))for 2D problem in H1-norm.Here,τdenotes time-step size,while,hx and hy represent spatial meshsizes in x-and y-directions,respectively.Then,by introducing a stabilized term,a type of stabilized invariants-preserving Du Fort-Frankel-type FDMs are devised.They not only preserve the discrete energies and masses,but also own much better stability than original schemes.Finally,numerical results demonstrate the theoretical analyses.

Beach rocks and storm deposits on the beaches of Hong Kong

The formation of beach rocks has a close relationship to storm deposits, denoted by beach and storm processes in association with depositional characteristics of the beach rocks found in Pui O and Lower Cheung Sha bays on the southern coast of Lantau Island, Hong Kong. Although not all beach rocks have an origin of storm deposits, it is certain that some of them with very coarse shells and shell fragments developing on sandy beaches originate from storm deposits. The cementation of beach rocks on a beach was affected directly by the texture and structure of the beach rocks and wave energy varying along the beach.

An Energy Absorbing Far-Field Boundary Condition for the Elastic Wave Equation

We present an energy absorbing non-reflecting boundary condition of Clayton-Engquist type for the elastic wave equation together with a discretization which is stable for any ratio of compressional to shear wave speed.We prove stability for a second-order accurate finite-difference discretization of the elastic wave equation in three space dimensions together with a discretization of the proposed non-reflecting boundary condition.The stability proof is based on a discrete energy estimate and is valid for heterogeneous materials.The proof includes all six boundaries of the computational domain where special discretizations are needed at the edges and corners.The stability proof holds also when a free surface boundary condition is imposed on some sides of the computational domain.

论我国海洋能的研究与发展

我国在数千年文明发展中,总体上是一个大陆文明的国家,对海洋的关注不如欧洲文明。这样的情况,一直持续到今天。从目前现有文献看,我国对于海洋的研究,还有很多空白,其中,对于海洋能的深层次的研究就是一个重大的空白。我国拥有漫长的海岸线和广阔的海域,蕴藏巨大的海洋能。但对它的研究、开发和利用,还处于开始前的阶段,落后于西方国家上百年。我国目前已经提出海洋发展战略,为了促进我国海洋能研究与发展,应从以下几个方面加以重视:推进制度安排,加强顶层设计;建立海洋能观测体系,完善科学评估机制;以科技创新为抓手,构建海洋能开发利用的基础平台;基于实际,因地制宜地发展海洋能;科学开发,加强海洋能资源保护。

二维声表面波压电声子晶体在射频段的时域有限差分法

对二维声表面波压电声子晶体在射频段的带隙特性,进行了时域有限差分法（FDTD）理论推导和计算,并提出实验方法对比验证。FDTD计算模型考虑了压电效应,引入周期边界条件以节省计算空间和时间,采用完全匹配层以解决声表面波在截断边界处的虚拟反射问题。实验上分别设计有/无二维压电声子晶体的两种宽频带延迟线结构,测量两种延迟线的传输系数取差值,得到了二维压电声子晶体的带隙;其中通过时域加窗函数保留一次传输信号,进行干扰信号的去除。以铝/128°YX-LiNbO3二维压电声子晶体为例,该FDTD方法、商业有限元软件COMSOL、实验方法均得到了100~500 MHz射频段内的多个带隙,三种带隙对比证明了FDTD计算带隙与实验测量带隙一致,比COMSOL计算的计算带隙精度更高。

“双碳”目标下海洋可再生能源资源开发利用

海洋可再生能源开发利用对促进节能减排、应对气候变化具有重要的潜在作用。我国海洋可再生能源资源总量丰富,仅近海海域的海洋可再生能源资源可开发量就高达数百吉瓦。近年来,我国海洋可再生能源资源开发利用技术取得了快速发展,海洋可再生能源装机规模已进入世界前列。碳达峰、碳中和目标下能源电力绿色转型战略的实施,为我国海洋可再生能源资源开发利用提供了至关重要的发展机遇。在介绍我国海洋可再生能源开发利用开展的工作以及取得的重要进展的基础上,梳理我国海洋可再生能源开发利用中存在的主要问题,分析我国海洋可再生能源减排潜力,对"双碳"背景下我国海洋可再生能源的中长期发展愿景进行展望,并对近期的海洋可再生能源开发利用技术发展以及规模化开发利用重点进行分析。

基于EMD的探地雷达信号直达波去除方法研究

结合探地雷达直达波的特点,提出了基于EMD的探地雷达直达波去除方法的基本思想,在去除直达波方法中引入了差异扩大和能量均衡,并给出了计算流程。通过对数值模拟信号和现场实测信号的处理表明,基于EMD的直达波去除方法可以有效去除直达波信号,使用差异扩大和能量均衡后可以较好地将有效信号显现。

岬间海滩泥沙运动趋势与波能流分布的季节变化性

根据对粤东一岬间海滩冬夏季采集的表层泥沙的粒度分析结果,本文采用沉积物输运趋势概率模式对海滩泥沙沿岸运动方向作了统计分析,进而从波折射数值计算得到的波能流的沿岸分布讨论了对沿滩输沙的季节性效应.

波浪在缓坡前的反射特性分析

本文根据波浪在岸前的能量方程，理论分析了波浪反射与波能损失的关系，并进一步通过试验验证分析了波浪反射系数在岸前的变化趋势。

浅滩地形上波浪破碎对低频波能放大的影响

低频长波对港湾共振、泥沙输运、波浪爬高与越浪等过程有重要影响。采用非静压模型SWASH模拟了不规则波在浅滩上的传播及破碎过程,重点探讨了滩顶短波破碎程度对低频波能演变的影响。模拟结果显示,当滩顶短波处于临界破碎状态时,低频波能流沿浅滩持续增长,浅滩对低频长波的放大效应(滩后与滩前低频波能流之比)显著;当滩顶短波轻微破碎、破波仅发生在外破波区时,低频波能流的沿滩增长率进一步变大;当滩顶短波剧烈破碎、破波延续到内破波区时,低频波能流沿滩先增长而后转为衰减,滩后放大率较临界破碎时明显减小。研究结果表明,浅滩顶部水深改变了短波破碎程度,进而影响低频长波的演化过程,浅滩上长波总体增长率随滩顶水深的减小呈现先增大后减小的规律,在短波轻微破碎时最大。