WAVE ATTENUATION BY BOTTOM FRICTION IN CURRENTS

In nature, wave attenuation occurs with propagation. In some cases, it is significant and cannot be neglected. In this paper, the attenuation of wave spectra in current by bottom friction is studied. To simplify the calculation, a linearized bottom friction stress formula is introduced, which gives an equivalent energy loss due to bottom friction. Model test data indicate that the authors' method for the calculation of wave attenuation in this paper agrees well with the experimental results.

Wave-Current Propagation over a Frictional Topography

In this paper the parabolic approximation model based on mild-slope equation is used to study wave propagation over a slowly varying and frictional topography under wave-current interaction. A governing equation considering the friction effects is derived by the authors for the first time. A simplified form for the rate of wave energy dissipation is presented on the basis of the wave-current action conservation equation and the bottom friction model given by Yoo and O'connor (1987). Examples reveal that the present computational method can be used for the calculation of wave elements for actual engineering projects with large water areas.

Rip Currents Have Least Friction

Surface gravity waves continually come to the beach but rarely go back to sea. They bring excess mass to shore which must be returned offshore. Rip currents do that job because there is less overall friction in the nearshore region than there is in the only other imagined circulation, a two-layer scheme, which has in fact never been seen. An argument is presented to support this proposal. If correct, rip currents join a group of geophysical flow phenomena that persist, probably because there is zero friction associated with them, which include surface gravity waves, tornadoes and hurricanes, individually promoted recently.

A New Multigrid 3D-VAR Optimization Method for Bottom Friction Using HF Radar Current Observation

This paper proposes a new method for data assimilation of the surface radial current observed by High Frequency ground wave radar and optimization of the bottom friction coefficient.In this method,the shallow water wave equation is introduced into the cost function of the multigrid three-dimensional variation data assimilation method as the weak constraint term,the surface current and the bottom friction coefficient are defined as the analytical variables,and the high spatiotemporal resolution surface radial flow observed by the high-frequency ground wave radar is used to optimize the surface current and bottom friction coefficient.This method can effectively consider the spatiotemporal correlation of radar data and extract multiscale information from surface radial flow data from long waves to short waves.Introducing the shallow water wave equation into the cost function as a weak constraint condition can adjust both the momentum and mass fields simultaneously to obtain more reasonable analysis information.The optimized bottom friction coefficient is introduced into the regional ocean numerical model to carry out numerical experiments.The test results show that the bottom friction coefficient obtained by this method can effectively improve the accuracy of the numerical simulation of sea surface height in the offshore area and reduce the simulation error.

中美新局势下的贸易摩擦——动因、影响及我国对策

近年来,美国打着“国家安全”和“公平贸易”的幌子,对中国采取了一系列不公正的强硬措施,标志着两国关系进入新局势。美国发起贸易战,名义是平衡贸易关系,实则意图遏制中国经济发展和技术进步,以维护其全球霸主地位。美国拜登政府沿袭了特朗普政府对华贸易政策,以冷战思维与“零和”博弈为基调,通过关税战、芯片战、汇率战和金融战,使中美贸易摩擦蔓延至经济、科技、文化、社会等多个领域,深层次地影响中美经贸合作的长远利益。因此,在新局势下,中国须保持战略定力,构建高效投资框架和应急响应机制,探索“中国式区域合作”新模式,促进内部变革与区域合作协同发展,推动产业技术升级,完善市场体系,优化投资结构,稳固在新国际格局中的地位。

四主缆大跨悬索桥抗震性能分析及减震措施优化

以燕矶长江大桥为工程背景,首先,采用动力非线性时程法分析了燕矶长江大桥抗震性能,并研究了设置电涡流-摩擦组合型阻尼器和黏滞阻尼器对大跨悬索桥抗震性能的影响;然后,对附加电涡流-摩擦组合型阻尼器摩擦力、阻尼系数和阻尼指数开展了参数敏感性分析;最后,从耗能角度分析地震作用下电涡流-摩擦组合型阻尼器减震特点.结果表明:在E2地震作用下,桥塔关键截面抗弯能力均大于弯矩需求;在“纵向+竖向”地震作用下梁端纵向位移较大.设置塔梁处纵桥向阻尼器后,可有效降低主桥梁端纵向位移;增大摩擦力、阻尼系数与降低阻尼指数均可提升梁端纵向地震响应的控制效果,但参数变化对桥塔控制截面的地震响应影响较小.阻尼系数较大时,电涡流阻尼主导了电涡流-摩擦组合型阻尼器耗能,相较于黏滞阻尼器,电涡流-摩擦组合型阻尼器对梁端纵向位移控制效果更好.

基于密度聚类算法和广度优先搜索算法的道岔摩擦电流智能分析系统

[目的]现场的道岔摩擦电流测试与调整存在流程繁琐且风险高、对检修人员专业水平要求高、测定数值的主观性占比大3个弊端,为此需要基于各类智能算法及技术提升道岔的智能运维水平。[方法]分析了道岔摩擦电流测试曲线4个阶段的特征,提出建立道岔摩擦电流的智能分析系统。阐述了该系统的功能及工作原理,设定了该系统的摩擦电流标准值及阈值范围。该系统可基于密度聚类算法和广度优先搜索算法自动获取道岔摩擦电流值。介绍了该系统的调试界面截图,以说明系统在获取道岔摩擦电流值如何为现场检修人员提供操作建议。[结果及结论]该智能系统具有良好的可用性,实现了节约检修时间、降低维护成本和提高检修效率的既定目的。

载流条件下导电润滑脂轴承的摩擦学性能

选用碳纳米管、纳米导电炭黑作为添加剂,制备了两种导电润滑脂,对两种导电润滑脂进行了滴点、热重、抗氧化性能等理化性能测试,并基于自主研发的轴承载流摩擦试验台进行轴承台架试验,观察并研究了轴承的载流摩擦学特性,探索润滑脂性能对轴承抗电蚀性能的影响。结果表明:加入碳纳米管的润滑脂,滴点并未下降,且热分解温度提高,氧化诱导期延长约44%,摩擦系数及磨斑直径变化不大,磨损体积减小约50%;加入纳米导电炭黑的润滑脂,滴点降低约40℃,热分解温度降低约3℃,氧化诱导期延长约4%,摩擦系数及磨斑直径变化不大,磨损体积减小约90%。两种润滑脂均可有效减小轴承的电蚀程度。

不同焊接方法下燃料元件包壳用ODS钢焊接接头界面演化规律

ODS钢具有优异的高温力学性能、抗辐照性能和抗热蠕变性能等,是最有潜力的下一代核反应堆包壳候选材料之一。ODS钢的焊接技术主要包括熔焊、钎焊、压力焊等,根据核燃料元件包壳结构,选择电子束焊接、旋转摩擦焊及脉冲电流辅助扩散焊三种焊接方法进行对比研究,并对ODS钢焊接接头微观形貌演化进行分析,揭示了最优焊接方法及界面形貌演化规律。结果表明,电子束焊接及旋转摩擦焊接工艺下,焊接接头的晶界处均有Y_(2)O_(3)析出,而采用脉冲电流辅助扩散焊无氧化物析出和团聚,对于Fe-Cr系ODS钢有突出优势。因此,脉冲电流辅助扩散焊在抑制ODS钢中纳米氧化物的析出、团聚和减少接头变形等方面具有显著优势,是一种适合ODS钢焊接的高质量焊接方法。

不同表面形貌的接触线对浸金属碳滑板载流摩擦磨损性能的影响

在受电弓-接触网载流摩擦磨损过程中,接触线会产生不同的横截面形状,而接触线形貌的改变可能会影响弓网间的接触关系,进而影响弓网间的载流摩擦磨损性能。为研究不同表面形貌的接触线对浸金属碳滑板载流摩擦磨损性能的影响,利用环-块式高速载流摩擦磨损试验机,研究载流条件下常规形貌、麻点形貌、斜切形貌的接触线与浸金属碳滑板的摩擦磨损性能,比较采用不同形貌接触线时的摩擦因数、电弧能量和浸金属碳滑板的磨损量、表面形貌。试验结果表明:在直流电情况下,常规接触线与浸金属碳滑板组成的摩擦副的摩擦因数最小,滑板磨损量最低;采用斜切形貌接触线时的摩擦因数最大,滑板磨损量最大。通过SEM电镜观测浸金属碳滑板表面的磨损形貌,发现接触线为常规形貌时,滑板以氧化磨损为主,有较多的氧化物产生;接触线为麻点形貌时,滑板以电弧烧蚀和磨粒磨损为主,产生了细小裂纹和烧蚀坑,有较多的磨屑和剥落层出现;接触线为斜切形貌时,滑板以电弧烧蚀为主,有大裂纹和犁沟产生,并且烧蚀区域出现了较多的白色小球。研究表明,当接触线的形貌发生改变时,会导致滑板磨耗增加并加剧接触副电弧放电,从而恶化接触副的状态。因此,当接触线磨损变形严重时,应及时进行更换。

载流滑动摩擦部件表面失效及其修复技术研究进展

滑动电接触部件在电能传递与转换过程中起着举足轻重的作用,其失效和损伤行为带来的安全因素和经济影响重大,因此对其失效机理和修复技术的研究成为当今热点之一.已有众多学者通过试验和计算机模拟仿真等手段研究了电接触部件的表面损伤机制和寿命预测,至今为止,已研究出载流摩擦部件的表面修复技术多达15种以上,修复材料更是有上百种.本文中首先介绍了电接触的基本概念,从有无电流载入2个角度概述了滑动摩擦部件表面失效形式,并从试验研究及仿真模拟研究2个方面对表面损伤机制的研究方法进行了归纳;最后综述了滑动电接触失效部件的表面修复技术和修复材料体系的研究进展,总结了目前滑动电接触部件研究领域存在的问题,提出了利用试验与仿真模拟相结合探究涂层导电耐磨自润滑机理并优化涂层质量的研究思路.

浸石蜡对碳滑板载流摩擦磨损性能影响研究

为研究在载流条件下浸石蜡对碳滑板摩擦磨损性能的影响,在环-块载流摩擦磨损试验台上试验浸石蜡、未浸石蜡2种纯碳滑板与铜银合金接触线在载流条件下的摩擦磨损性能,并对比碳滑板在不同载荷、滑动速度、电流情况下的表面形貌变化。试验结果显示,浸石蜡对碳滑板载流摩擦磨损性能的影响与电弧烧蚀紧密相关。石蜡及其有机产物加剧电弧烧蚀,石蜡的不完全分解会削弱滑板的载流摩擦磨损性能,通过减小法向载荷、加快滑动速度、增大电流均能显著加剧浸石蜡碳滑板在试验中的电弧烧蚀。浸石蜡碳滑板的摩擦系数的变化规律是:随法向载荷、电流的增大呈现减小的趋势,随滑动速度的增加先增大后减小,60km/h时摩擦系数最大;浸石蜡碳滑板的平均电弧能量、磨耗量随法向载荷的增大呈现减少的趋势,随电流的增大而增加,平均电弧能量随滑动速度的增加而增大,磨耗量随滑动速度的增加先增加后减少,60km/h时磨耗量最大;在低速(40km/h)和高速(80km/h)时浸石蜡增强碳滑板的耐磨性能。

高/低湿度条件下弓网载流摩擦磨损性能的试验研究

为探究不同湿度条件下地铁刚性弓网系统摩擦磨损的性能,采用弓网系统常用的浸金属碳滑板与铜银合金接触线为试验材料,利用高速载流摩擦磨损试验机模拟地铁刚性弓网的运行。试验研究了环境相对湿度分别为20%、40%和60%时,地铁常见运营速度下浸金属碳滑板的磨耗率、摩擦系数、平均电弧能量和振动加速度的变化规律。结果表明:低湿度(20%)环境中,滑板与弓网之间的摩擦类似于干摩擦,滑板磨粒会损坏接触线表面形成犁沟。在高湿度(60%)环境中,空气中的水分子会润滑接触表面降低接触副之间的摩擦系数,还可以扩大电流回路的通流面积,降低接触副之间的振动幅度。

纺织服装行业贸易摩擦案件现状及特点研究

纺织服装作为我国主要出口行业,从1993年至今共遭遇了204起贸易摩擦案件,涉案数量居所有行业的第四位。贸易摩擦已成为我国纺织服装出口仅次于汇率和关税的第三大贸易障碍。文中通过对纺织服装贸易摩擦案件的总体情况以及在立案时间、案件类型、终审结论、反制措施、市场分布和案件关联性等方面的特点进行分析,得出纺织服装行业贸易摩擦案件高发且案件“涟漪”效应明显,反倾销案是应对工作的重点与难点,土耳其、印度和印度尼西亚市场需重点关注等结论,旨在为我国纺织服装行业出口和开展贸易摩擦应对工作提供参考。

脉冲电流对6061T6铝合金流动摩擦挤压成形微观组织的影响

通过在流动摩擦挤压成形过程中施加脉冲电流,研究了脉冲电流对6061T6铝合金流动摩擦挤压成形性能的影响,同时,借助EBSD和TEM研究了脉冲电流对变形区材料组织演变的影响规律。结果表明:脉冲电流降低了材料的变形抗力,提高了材料的流动性,在下行速度0.6 mm·min^(-1)、转速950 r·min^(-1)以及下压量3 mm条件下,施加3800 A的脉冲电流可使6061T6铝合金的流动摩擦挤压成形高度从15 mm提高到20 mm。脉冲电流的施加促进了变形过程中再结晶的发生,细化了不同变形区的组织,并提高了变形区大角度晶界的比例;流动摩擦挤压成形后材料内部存在大量的线性位错、网状位错以及由位错攀移形成的位错墙;施加脉冲电流后,材料在搅拌力和挤压力的复合作用下发生大塑性变形,晶粒破碎,形成大量的纳米晶。

不同载荷下镀镍石墨-铜基复合材料的载流摩擦性能

首先采用化学镀的方法在石墨表面镀Ni,随后采用快速热压方法制备10 mass%镀Ni石墨-铜基复合材料。在不同载荷下对复合材料开展了载流摩擦实验,采用扫描电镜观察了复合材料的磨损表面形貌,进而从磨损表面形貌特征分析其损伤机制。结果表明:在石墨表面成功镀覆了致密且均匀的Ni金属层,化学镀镍有效改善了石墨和铜之间的润湿性;镀镍石墨-铜基复合材料的相组成为石墨和铜基体两相,石墨在铜基体中呈弥散分布,未形成连续网状结构;随载荷的增加,复合材料的摩擦系数逐渐降低,磨痕宽度和磨损率逐渐升高,载流效率和载流稳定性逐渐下降;在低载荷下(20和40 N),复合材料的损伤机制以显微切削为主;随着载荷的增加(60和80 N),电侵蚀加剧,成为主要的损伤机制。

废旧锂离子电池涡流分选模型与影响因素

废旧锂离子电池的涡流分选工艺由于影响参数过多,分选效果并不理想。为探明各参数对分选效果的影响,提高涡流分选法的分离效率,开展废旧磷酸铁锂锂离子电池破碎产物的涡流分选研究。利用COMSOL仿真软件模拟磁辊交变磁场分布,建立产物在交变磁场的受力模型与运动模型,研究磁辊长度、磁极对数、磁辊半径、进料速度、磁辊转速、颗粒尺寸和颗粒形状等因素对分选效果的影响,提出将传送带摩擦因数纳入影响参数之中进行模拟,以指导涡流分选模型的建立。上述因素均会影响颗粒的运动轨迹,减小摩擦因数有利于提高分选效率,颗粒尺寸和形状是主要的影响因素。

Linear Analysis of Longshore Currents Instability over Mild Slopes

Longshore current instability is important to nearshore hydrodynamic and sediment transport. This paper investigates the longshore current instability growth model based experimental data with different velocity profiles of slopes1:100 and 1:40 by adopting a linear shear instability model with the bottom friction effects. The results show that:(1)Only backshear mode exists in the instability of longshore current for slope 1:40 and frontshear and backshear modes may exist slope 1:100.(2) The peaks of linear instability growth mode for slope 1:100 correspond to three cases: the dominant peak is formed by the joint action of both frontshear and backshear, or by backshear alone without the existence of the smaller peak or formed by either the frontshear or backshear.(3) Bottom friction can decrease the corresponding unstable growth rate but it cannot change the unstable fluctuation period. The results of fluctuation period, wavelength and spatial variation obtained by the analysis of linear shear instability are in good agreement with experimental results.

Effect of Electrical Current on the Tribological Behavior of the Cu-WS_2-G Composites in Air and Vacuum

As the traditional graphite-based composites cannot meet the requirement of rapid developing modern industry, novel sliding electrical contact materials with high self-lubricating performance in multiple environments are eagerly required. Herein a copper-based composite with WS2 and graphite as solid lubricant are fabricated by powder metallurgy hot-pressed method. The friction and wear behaviors of the composites with and without current are investigated under the condition with sliding velocity of 10 m/s and normal load of 2.5N/cm 2 in both air and vacuum. Morphologies of the worn surfaces are observed by optical microscope and compositions of the lubricating films are analyzed by XPS. Surface profile curves and roughness of the worn surfaces are obtained by 2205 surface profiler. The results of wear tests show that the friction coefficient and wear volume loss of the composites with current are greater than that without current in both air and vacuum due to the adverse effects of electrical current which damaged the lubricating film partially and roughed the worn surfaces. XPS results demonstrate that the lubricating film formed in air is composed of oxides of Cu, WS2 , elemental S and graphite, while the lubricating film formed in vacuum is composed of Cu, WS2 and graphite. Because of the synergetic lubricating action of oxides of Cu, WS2 and graphite, the composites show low friction coefficient and wear volume loss in air condition. Owing to the fact that graphite loses its lubricity which makes WS2 become the only lubricant, severe adhesive and abrasive wear occur and result in a high value of wear rate in vacuum condition. The formation of the lubricating film on the contact interface between the brush and ring is one of the factors which can greatly affect the wear performance of the brushes. The low contact voltage drop of the composites in vacuum condition is attributed to the high content of Cu in the surface film. This study fabricated a kind of new sliding electrical contact self-lubricating composite with dual-lubricant which can work well in both air and vacuum environments and provides a comprehensive analysis on the lubrication mechanisms of the composite.

界面氧化对碳–铜接触副电接触性能的影响

弓网系统滑动电接触是列车能量传递的唯一途径。且界面氧化程度对C/Cu接触副载流摩擦磨损影响显著。文中设计直线往复式载流摩擦试验台,提出一种通过定量控制碳滑板表面氧化膜厚度来模拟不同氧化状态的方法,研究不同氧化程度下C/Cu接触副的电接触特性,探究界面氧化对载流摩擦副电接触性能的影响机理。实验结果表明:适当的氧化膜厚度有助于获得更低的接触电阻,较初值降低了17.9%,提升C/Cu接触副的电接触性能,有利于能量传输;但过厚的氧化膜会增大接触电阻,导致接触界面放电频发,放电总能量较初值提升了104.4%,劣化其电接触性能。所得结果有助于进一步提升碳/铜接触副在低氧环境的服役性能。