|
|
|
题名
|
作者
|
年代
|
出处
|
被引量
|
| 1 | Mechanism of chain propagation for the synthesis of polyoxymethylene dimethyl ethers显示文摘Polyoxymethylene dimethyl ethers(PODE)were synthesized from the reaction of paraformaldehyde with dimethoxymethane(DMM)over different acid catalysts at different conditions.Products were found to follow the Schulz-Flory distribution law.The chain propagation proceeds through the insertion of an individual segment of CH2O one by one,while the simultaneous insertion of a few CH2O segments or their assembly is unlikely.Due to the restriction of this law,it is difficult to increase the selectivity to the desired products(e.g.,PODE3 4). | Yupei Zhao Zheng Xu Hui Chen Yuchuan Fu Jianyi Shen | 2013 | Journal of Energy Chemistry2013，22，6： | 36 |
| 2 | Numerical simulations of the bi-level and branched structure of intracloud lightning flashes显示文摘Intracloud (IC) lightning flashes have been simulated in fine resolution (12.5 m) by using a bidirectional stochastic lightning parameterization scheme within 2-dimensional domain. The simu-lated results show that the IC flashes have a bilevel channel structure and the altitudes of the hori-zontal channels are at the same heights of potential wells, which are supported by the previous VHF source observations and balloon soundings of electric field profile in the thundercloud. Further con-clusions are: (1) After an IC flash is initiated near the boundary between positve and nagetive charge zone, the negative (or positive) leader tends to propagate into the positive (or negative) charge zone. Both types of positive and negative IC flashes have been reproduced and their polarity depends on the up and down disposition of the positive and negative charge regions. (2) The extension range of leaders is correlative with the cloud charge distribution. The leader is possible to extend through the inverted charge region all over where it is extending, but keeps away from the isolated charge area of the same polarity. (3) The channel structures also depend on the electric potential distributions in the thundercloud. Before propagating into the central area of potential wells, the leader tends to extend along the direction with the maximum of potential gradient. Once extending away from the center of potential wells, the leader tends to extend along the direction with the slowest potential change. (4) The IC flash channels have the fractal feature with fractal dimension 1.45 before leaders pass through the central area of charge regions. The exponent decreases rapidly once leaders extend into the low-density charge regions. (5) The induced charges of opposite polarity are deposited in the leader channels within preexisting positive and negative charge regions during IC flash discharges. This causes a new and complicated charge distribution in the thundercloud, and the potential extremum drops from 200 to 20 Mv when the IC flash terminates. | TAN Yongbo1, TAO Shanchang1, ZHU Baoyou1, MA Ming2 & LU Weitao2 1. School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China 2. Chinese Academy of Meteorological Sciences, Beijing 100081, China | 2006 | Science China Earth Sciences2006，49，6： | 15 |
| 3 | Understanding hydraulic fracture propagation behavior in tight sandstone–coal interbedded formations: an experimentalinvestigation显示文摘Whether hydraulic fractures could connect multiple gas zones in the vertical plane is the key to fracturing treatment to jointly exploit coalbed methane and tight sandstone gas through integrative hydraulic fracturing in tight sandstone–coal interbedded formations. Laboratory true triaxial hydraulic fracturing experiments were conducted on layered specimens with di erent combination types of natural sandstone and coal to simulate the propagation behavior of hydraulic fractures. The effects of the fracture initiation position, fracturing fluid viscosity and injection rate were discussed. The results showed that di erent fracture morphologies could be found. When initiating from coal seams, three patterns of fracture initiation and propagation were obtained:(1) The main hydraulic fracture initiated and propagated along the natural fractures and then diverged due to the effects of in situ stress and formed secondary fractures.(2) The hydraulic fracture initiated and propagated in the direction of the maximum horizontal stress.(3) Multiple fractures initiated and propagated at the same time. With the same fracturing fluid viscosity and injection rate, the hydraulic fractures initiating in sandstones had greater chances than those in coal seams to penetrate interfaces and enter neighboring layers. Excessively small or large fracturing fluid viscosity and injection rate would do harm to the vertical extension height of the induced fracture and improvement of the stimulated reservoir volume. Compared with operation parameters(fracturing fluid viscosity and injection rate), the natural weak planes in coals were considered to be the key factor that a ected the fracture propagation path. The experimental results would make some contributions to the development of tight sandstone–coal interbedded reservoirs. | Peng Tan Yan Jin Liang Yuan Zhen-Yu Xiong Bing Hou Mian Chen Li-Ming Wan | 2019 | Petroleum Science2019，16，1： | 14 |
| 4 | Detection of subcritical crack propagation for concrete dams显示文摘Subcritical propagation of cracks is a warning sign of fracture.If such propagation is detected at an early stage,timely maintenance measures can be taken to prevent the failure of structures.To detect the subcritical propagation of a crack,the crack needs to be monitored continuously in a long term, which is not realistic under certain conditions.However,cracks in concrete dams can be monitored continuously by dam monitoring to offer possible detection for subcritical propagation.In this paper, with measured crack openings from dam monitoring,a state equation for characterizing crack development is established based on the grey system theory.The relation between the stability of the equation and the subcritical crack propagation is investigated,then a criterion is proposed for detecting subcritical propagation.An example demonstrates the validity of the criterion and its potential for practical application. | BAO TengFei1 ,2&YU Hong 1,2 1College of Water Conservancy&Hydropower Engineering,Hohai University,Nanjing 210098,China 2National Engineering Research Center of Water Resources Efficient Utilization and Engineering Safety,Nanjing 210098,China | 2009 | Science China(Technological Sciences)2009，52，12： | 14 |
| 5 | Laboratory investigation on mechanisms of stress wave propagations in porous media显示文摘A number of porous models having the similar statistical characteristics of pores and physical properties with natural sandstones have been produced using reactive powder concrete(RPC) and polystyrenes.Spit-Hopkinson-Pressure-Bar tests and CT scans have been carried out on the models with the various porosities to probe the performance of wave propagations and the responses of pores and the matrix during wave propagations.It is shown that porosities significantly influence wave propagations.For an identical impact strain rate,the greater the porosity is,the larger the amplitude of the reflected wave appears,the more the peak in the reflected wave presents,and the smaller the amplitude of the trans-mitted wave turns out.A single peak emerges in the reflected wave when the porosity falls down to 5%.The larger the impact strain rate,the much remarkable the phenomena.The energy-dissipated ratio of porous models,i.e.,WJ /WI,linearly increases with the increment of porosities.The ratio is sensitive to the impact strain rate.Differences in the performance of wave propagations and energy dissipation result from the varied mechanisms that pores response to impacts.For the porosity less than 10%,the mechanism appears to be a process fracturing the matrix to generate new surfaces or pores.Energy has primarily been dissipated in creating new surfaces or pores.No apparent pore deformation takes place.The impact strain rate takes little effect on pore geometry.For the porosity of 15% or more,the mechanism works depending on the impact strain rate.When a low impact strain rate applies,the mechanism still appears to crack the matrix to generate surfaces or pores,but the amount is lower as compared to the case with a low porosity.If a large impact stain rate applies,the mechanism combines both fracturing the matrix and deforming the pores,with the deforming pores predominating.The vast majority of energy has been dissipated to deform pores.Only high porosity and impact strain rate can bring significant deformation to the pores.The proposed eccentricity of pores is capable of character-izing the geometry of pores and its change during wave propagations. | JU Yang YANG YongMing MAO YanZhe LIU HongBin WANG HuiJie | 2009 | Science China(Technological Sciences)2009，52，5： | 13 |
| 6 | Radio Propagation and Wireless Coverage of LSAA-Based 5G Millimeter-Wave Mobile Communication Systems显示文摘Millimeter-wave(mm Wave) communications will be used in fifth-generation(5G) mobile communication systems, but they experience severe path loss and have high sensitivity to physical objects, leading to smaller cell radii and complicated network architectures. A coverage extension scheme using large-scale antenna arrays(LSAAs) has been suggested and theoretically proven to be cost-efficient in combination with ultradense small cell networks. To analyze and optimize the LSAA-based network deployments, a comprehensive survey of recent advances in statistical mmWave channel modeling is first presented in terms of channel parameter estimation, large-scale path loss models, and small-scale cluster models. Next, the measurement and modeling results at two 5G candidate mmWave bands(e.g., 28 GHz and 39 GHz) are reviewed and compared in several outdoor scenarios of interest, where the propagation characteristics make crucial contributions to wireless network designs. Finally, the coverage behaviors of systems employing a large number of antenna arrays are discussed, as well as some implications on future mmWave cellular network designs. | Haiming Wang Peize Zhang Jing Li Xiaohu You | 2019 | China Communications2019，16，5： | 12 |
| 7 | Statistics of gravity waves in the lower stratosphere over Beijing based on high vertical resolution radiosonde显示文摘The seasonal variation of inertia gravity-wave activity in the lower stratosphere (17―24 km) over Beijing is studied based on the high vertical resolution radionsonde observa- tions (from December 2001 to February 2003) of Beijing Observatory (116°28′E, 39°48′N). Some of the important gravity-wave parameters, such as intrinsic frequency, and propagation direction, are estimated according to the polarized relation among gravity-wave wind components and temperature. Time series of wave energy show that the largest wave amplitudes occur during the winter and the least during the summer, and the average of the KE:PE ratios is about 2.6. Zonal and meridional wind perturbations have almost the same roots of mean variances, which shows that the wave energy in the lower stratosphere is isotropic horizontally. The motion and tem- perature fields are dominated by waves with vertical wavelengths of 1.5―3 km, which occupy above 80% samples, with a mean value of about 2.3 km. The horizontal wavelengths are mainly distributed between 100―800 km, averaging 445 km. The ratio of the mean horizontal to vertical wavelength is about 200:1, which indicates that the wave propagates nearly horizontally, with a very small vertical angle. The intrinsic frequency is estimated by fitting a polarized ellipse to the wind perturbations after band-filtering waves with a wavelength of 1.5―3 km, and the results show that the intrinsic frequency is dominated by 1f―3.5f, with an averaging value of 2f, corre- sponding to an intrinsic time period of 9 hours. Wave energy is found to propagate mainly upward, and in the horizontal direction, there is clear azimuthal anisotropy, with predominate northwest propagation against the prevailing wind. | BIAN Jianchun CHEN Hongbin LU Daren | 2005 | Science China Earth Sciences2005，48，9： | 12 |
| 8 | A continuous/discontinuous deformation analysis(CDDA)method based on deformable blocks for fracture modeling显示文摘In the framework of finite element meshes,a novel continuous/discontinuous deformation analysis(CDDA)method is proposed in this paper for modeling of crack problems.In the present CDDA,simple polynomial interpolations are defined at the deformable block elements,and a link element is employed to connect the adjacent block elements.The CDDA is particularly suitable for modeling the fracture propagation because the switch from continuous deformation analysis to discontinuous deformation analysis is natural and convenient without additional procedures.The SIFs(stress intensity factors)for various types of cracks,such as kinked cracks or curved cracks,can be easily computed in the CDDA by using the virtual crack extension technique(VCET).Both the formulation and implementation of the VCET in CDDA are simple and straightforward.Numerical examples indicate that the present CDDA can obtain high accuracy in SIF results with simple polynomial interpolations and insensitive to mesh sizes,and can automatically simulate the crack propagation without degrading accuracy. | Yongchang CAI Hehua ZHU Xiaoying ZHUANG | 2013 | Frontiers of Structural and Civil Engineering2013，7，4： | 10 |
| 9 | Wind power prediction based on variational mode decomposition multi-frequency combinations显示文摘Because of the uncertainty and randomness of wind speed, wind power has characteristics such as nonlinearity and multiple frequencies. Accurate prediction of wind power is one effective means of improving wind power integration. Because the traditional single model cannot fully characterize the fluctuating characteristics of wind power, scholars have attempted to build other prediction models based on empirical mode decomposition(EMD) or ensemble empirical mode decomposition(EEMD) to tackle this problem. However, the prediction accuracy of these models is affected by modal aliasing and illusive components. Aimed at these defects, this paper proposes a multi-frequency combination prediction model based on variational mode decomposition(VMD). We use a back propagation neural network(BPNN),autoregressive moving average(ARMA)model, and least square support vector machine(LS-SVM) to predict high, intermediate,and low frequency components,respectively. Based on the predicted values of each component, the BPNN is applied to combine them into a final wind power prediction value.Finally,the prediction performance of the single prediction models(ARMA,BPNN and LS-SVM)and the decomposition prediction models(EMD and EEMD) are used to compare with the proposed VMD model according to the evaluation indices such as average absolute error, mean square error,and root mean square error to validate its feasibility and accuracy. The results show that the prediction accuracy of the proposed VMD model is higher. | Gang ZHANG Hongchi LIU Jiangbin ZHANG Ye YAN Lei ZHANG Chen WU Xia HUA Yongqing WANG | 2019 | Journal of Modern Power Systems and Clean Energy2019，7，2： | 10 |
| 10 | Propagations of singularities in a parabolic system with coupling nonlocal sources显示文摘This paper deals with propagations of singularities in solutions to a parabolic system coupled with nonlocal nonlinear sources. The estimates for the four possible blow-up rates as well as the boundary layer profiles are established. The critical exponent of the system is determined also. | ZHANG He KONG LingHua ZHENG SiNing | 2009 | Science China Mathematics2009，52，1： | 10 |
| 11 | Direction synthesis in DOA estimation for monostatic multiple input multiple output (MIMO) radar based on synthetic impulse and aperture radar (SIAR) and its performance analysis显示文摘A new direction synthetic method for monostatic multiple input multiple output (MIMO) radar is presented based on synthetic impulse and aperture radar (SIAR) system. Concerned with the monostatic MIMO radar which simultaneously emits orthogonal signals with multi-carrier-frequency and possesses sparsely distributed transmitting and receiving arrays with respective location, as well as the situation for the presence of multipath propagation in the low flying target’s echo, the method integrates the aperture of the transmitting arrays with the receiving arrays to form the digital beam-forming (DBF) in azimuth and elevation dimensions. And a study has been made of planar general MUSIC algorithm based on decorrelating the multipath signals of multi-carrier-frequency MIMO radar. Through compensat-ing the phase delay of both the transmitting and the receiving arrays and synthe-sizing the transmitting beam in two dimensions at the receiver, the angular resolu-tion and measurement accuracy are improved and the computational complexity is reduced after transforming the three-dimensional (3D) parameter estimation prob-lem into a two-dimensional (2D) one. Finally, the Cramer-Rao Bounds (CRBs) of DOA estimation for azimuth and elevation is put forward with the exsiting multipath propagation. Results of computer simulation demonstrate the validity of the new method. | ZHAO GuangHui CHEN BaiXiao ZHU ShouPing | 2008 | Science China(Technological Sciences)2008，51，6： | 9 |
| 12 | Genetic Algorithm Based on New Evaluation Function and Mutation Model for Training of BPNN显示文摘A local minimum is frequently encountered in the training of back propagation neural networks (BPNN), which sharply slows the training process. In this paper, an analysis of the formation of local minima is presented, and an improved genetic algorithm (GA) is introduced to overcome local minima. The Sigmoid function is generally used as the activation function of BPNN nodes. It is the flat characteristic of the Sigmoid function that results in the formation of local minima. In the improved GA, pertinent modifications are made to the evaluation function and the mutation model. The evaluation of the solution is associated with both the training error and gradient. The sensitivity of the error function to network parameters is used to form a self adapting mutation model. An example of industrial application shows the advantage of the improved GA to overcome local minima. | 周祥 何小荣 陈丙珍 | 2002 | Tsinghua Science and Technology2002，7，1： | 8 |
| 13 | Bidirectional propagation of lightning leader显示文摘The characteristics of initial leader of triggered lightning flashes are studied by use of a highspeed digital camera system with the rate of 1000 frame per second and simultaneous measurements of current and electric field change with 0.1 μs time resolution. The results show that the altitude-triggered lightning was initiated by bidirectional propagation leader because of 'current interruption' by air gap,whereas the grounded-triggered one was not. The onset of downward-moving positive leader was prior to that of the upward-going negative leader for 843 ms in an altitude-triggered lightning under the positive ambient electric field. The downward-moving leader, regardless of its polarity, would produce inhomogeneous distribution of luminosity (current) along the channel on the moment of connection to the ground. | Caiwei Wang Muhong Yan Xinsheng Liu Yijun Zhang Wansheng Dong Cuihua Zhang | 1999 | Chinese Science Bulletin1999，44，2： | 8 |
| 14 | DDoS Attack Detection Scheme Based on Entropy and PSO-BP Neural Network in SDN显示文摘SDN (Software Defined Network) has many security problems, and DDoS attack is undoubtedly the most serious harm to SDN architecture network. How to accurately and effectively detect DDoS attacks has always been a difficult point and focus of SDN security research. Based on the characteristics of SDN, a DDoS attack detection method combining generalized entropy and PSOBP neural network is proposed. The traffic is pre-detected by the generalized entropy method deployed on the switch, and the detection result is divided into normal and abnormal. Locate the switch that issued the abnormal alarm. The controller uses the PSO-BP neural network to detect whether a DDoS attack occurs by further extracting the flow features of the abnormal switch. Experiments show that compared with other methods, the detection accurate rate is guaranteed while the CPU load of the controller is reduced, and the detection capability is better. | Zhenpeng Liu Yupeng He Wensheng Wang Bin Zhang | 2019 | China Communications2019，16，7： | 8 |
| 15 | Weakening effects of hydraulic fracture in hard roof under the influence of stress arch显示文摘For the problem of hydraulic fracture propagation when weakening the hard roof in fully mechanized top-coal caving stope of ultra-thick coal seam, based on the stress arch theory and the fracture mechanics, a two-dimensional model for hydraulic fracture of the roof in the stope was established to investigate the propagation laws of hydraulic fracture. The result shows that, after mining, the principal stress direction of overlaying rock deflects to form the stress arch, whose arrow height and arch thickness increase with the increase of the mining width and the side pressure coefficient. Within the influence range of stress arch, the hydraulic fracture in hard roof deflects towards the stope direction in the course of propagation and forms the ‘‘arch' fracture, which cuts off the roof below the fracture in a laminated way. The deflection angle of hydraulic fracture increases with the increase of the mining width, but decreases with the increase of the side pressure coefficient and the fractured horizon. This research can provide theoretical basis for the application of hydraulic fracturing method in the stope roof weakening. | Xia Binwei Zhang Xuan Yu Bin Jia Jinlong | 2018 | International Journal of Mining Science and Technology2018，28，6： | 7 |
| 16 | Modeling hot deformation behavior of low-cost Ti-2Al-9.2Mo-2Fe beta titanium alloy using a deep neural network显示文摘Ti-2 Al-9.2 Mo-2 Fe is a low-cost β titanium alloy with well-balanced strength and ductility, but hot working of this alloy is complex and unfamiliar. Understanding the nonlinear relationships among the strain,strain rate, temperature, and flow stress of this alloy is essential to optimize the hot working process.In this study, a deep neural network(DNN) model was developed to correlate flow stress with a wide range of strains(0.025–0.6), strain rates(0.01–10 s^-1) and temperatures(750–1000℃). The model, which was tested with 96 unseen datasets, showed better performance than existing models, with a correlation coefficient of 0.999. The processing map constructed using the DNN model was effective in predicting the microstructural evolution of the alloy. Moreover, it led to the optimization of hot-working conditions to avoid the formation of brittle precipitates(temperatures of 820–1000℃ and strain rates of 0.01-0.1 s^-1). | Cheng-Lin Li P.L.Narayana N.S.Reddy Seong-Woo Choi Jong-Taek Yeom Jae-Keun Hong Chan Hee Park | 2019 | Journal of Materials Science & Technology2019，35，5： | 7 |
| 17 | Zonal propagation of kinetic energy and convection in the South China Sea and Indian monsoon regions in boreal summer显示文摘Zonal propagation of kinetic energy (KE) and convection in the South China Sea (SCS) and Indian summer monsoon areas are examined in present study. Results suggest that the SCS and Indian summer monsoon prevailed regions (5 —15°N) are dominated by the southwesterly wind, however, the disturbances of KE at 850 hPa and convection are observed mainly coming from the western Pacific Ocean (140—150°E), after passing through the SCS, and westward propagated into the Bay of Bengal (90—100°E). In the Indian summer monsoon domain, where the disturbances of KE are found mainly coming from the Arabian Sea (AS) and eastward propagated into the Bay of Bengal. Therefore, the SCS and the Indian summer mon- soon are quite different in zonal propagation of KE and convection. The SCS summer monsoon is mainly affected by the KE and convection coming from the tropical western Pacific. The Indian summer monsoon, however, can be partly influenced by the AS and the SCS summer monsoon. The analysis also suggests that the interaction region between the SCS and the Indian summer monsoon is around 90—95°E, rather than 105°E as proposed by earlier studies. | CHEN Longxun 1 ,GAO Hui 2 ,HE Jinhai 2 ,TAO Shiyan 3 & JIN Zuhui 3 1.Chinese Academy of Meteorological Sciences,Beijing 100081,China 2.Nanjing Institute of Meteorology,Nanjing 210044,China 3.Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing 100029,China | 2004 | Science China Earth Sciences2004，47，12： | 7 |
| 18 | Water hammer in pipelines with hyperconcentrated slurry flows carrying solid particles显示文摘Water hammers in pseudo-homogeneous flows and heterogeneous flows have been investigated, and equations for the wave propagation speed and the additional pressure due to water hammer in both flows have been developed. These equations have been tested against available experimental data. | 韩文亮 董曾南 柴宏恩 | 1998 | Science China(Technological Sciences)1998，41，4： | 7 |
| 19 | Attenuation of rock blasting induced ground vibration in rock-soil interface显示文摘Blasting has been widely used in mining and construction industries for rock breaking.This paper presents the results of a series of field tests conducted to investigate the ground wave propagation through mixed geological media.The tests were conducted at a site in the northwestern part of Singapore composed of residual soil and granitic rock.The field test aims to provide measurement data to better understand the stress wave propagation in soil/rock and along their interface.Triaxial accelerometers were used for the free field vibration monitoring.The measured results are presented and discussed,and empirical formulae for predicting peak particle velocity (PPV) attenuation along the ground surface and in soil/rock were derived from the measured data.Also,the ground vibration attenuation across the soil-rock interface was carefully examined,and it was found that the PPV of ground vibration was decreased by 37.2% when it travels from rock to soil in the vertical direction. | Bhagya Jayasinghe Zhiye Zhao Anthony Goh Teck Chee Hongyuan Zhou Yilin Gui | 2019 | Journal of Rock Mechanics and Geotechnical Engineering2019，11，4： | 7 |
| 20 | A Method for the Environmental Quality Assessment of Surface Water Based on Artificial Neural Networks | YANG Guo\|dong\+1,\ PAN Da\|feng\+2,\ FAN Ying\|fang\+3 1.Dep. of Environmental Science, Shanxi Uni., Taiyuan 030006, China 2.Shanxi Academy of Agriculture Science, Taiyuan 030031, China 3.Institute of Molecular Science, Shanxi Uni., Taiyuan 030006, C | 2000 | Systems Science and Systems Engineering2000，10，1： | 7 |
您所在的机构暂未开通此部分的功能，请联系管理员