Two Al2O3/Cu composites containing 0.24 wt.% Al2O3 and 0.60 wt.% Al2O3 separately are prepared by internal oxidation. Effects of sliding speed and pressure on the frictional characteristics of the composites and coppe...Two Al2O3/Cu composites containing 0.24 wt.% Al2O3 and 0.60 wt.% Al2O3 separately are prepared by internal oxidation. Effects of sliding speed and pressure on the frictional characteristics of the composites and copper against brass are investigated and compared. The changes in morphology of the sliding surface and subsurface are examined with scanning electron microscope （SEM） and energy dispersive X-ray spectrum （EDS）. The results show that the wear resistance of the Al2O3/Cu composites is superior to that of copper under the same conditions, Under a given electrical current, the wear rate of Al2O3/Cu composites decreases as the Al2O3-content increases, However, the wear rates of the Al2O3/Cu composites and copper increase as the sliding speed and pressure increase under dry sliding condition. The main wear mechanisms for Al2O3/Cu composites are of abrasion and adhesion; for copper, it is adhesion, although wear by oxidation and electrical erosion can also be observed as the speed and pressure rise.展开更多
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 c...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.展开更多
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 consi...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.展开更多
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 ne...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.展开更多
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 wa...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.展开更多
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 profi...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.展开更多
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 ...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.展开更多
基金National Natural Science Foundation of China (50432020)Henan Innovation Project for University Prominent Re- search Talents (2007KYCX008)+3 种基金Henan Education Department Science and Technology Project (2007430004)Henan Plan Project for College Youth Backbone TeacherHenan University of Science and Technology Major Pre-research Foundation (2005ZD003)Henan University of Science and Technology Personnel Scientific Research Foundation (of023)
文摘Two Al2O3/Cu composites containing 0.24 wt.% Al2O3 and 0.60 wt.% Al2O3 separately are prepared by internal oxidation. Effects of sliding speed and pressure on the frictional characteristics of the composites and copper against brass are investigated and compared. The changes in morphology of the sliding surface and subsurface are examined with scanning electron microscope （SEM） and energy dispersive X-ray spectrum （EDS）. The results show that the wear resistance of the Al2O3/Cu composites is superior to that of copper under the same conditions, Under a given electrical current, the wear rate of Al2O3/Cu composites decreases as the Al2O3-content increases, However, the wear rates of the Al2O3/Cu composites and copper increase as the sliding speed and pressure increase under dry sliding condition. The main wear mechanisms for Al2O3/Cu composites are of abrasion and adhesion; for copper, it is adhesion, although wear by oxidation and electrical erosion can also be observed as the speed and pressure rise.
文摘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.
文摘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.
文摘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.
基金supported by the National Natural Science Foundation of China (Nos. 41506039, 41776004, 41775100 and 41606039)the National Key Research and Development Program of China (No. 2016YFC1401800)+1 种基金the Fundamental Research Funds for the Central Universities (No. 2016B12514)the National Programme on Global Change and Air-Sea Interaction of China (No. GASI-IPO VAI-04)
文摘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.
基金financially supported by the National Natural Science Foundation of China(Grant Nos.51879237 and 11602222)the Research Fund of Zhejiang Ocean University(Grant No.11185010817)+2 种基金Zhejiang Provincial Natural Science Foundation of China(Grant No.LR16E090002)the Fundamental Research Funds for the Central Universities(Grant No.2018QNA4041)the Project of Research on structure properties of framed seawall along the Oujiang River in Lucheng District of Wenzhou City
文摘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.
基金supported by Major Research Program of National Natural Science Foundation of China(Grant No. 91026018)National Natural Science Foundation of China(Grant No. 60979017)Doctoral Fund of Ministry of Education of China(Grant No. 20110111110015)
文摘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.
