课程文献中心 更多>>

关键词： post-Newtonian approximation   gravitational waves   neutron stars   black holes  

摘要： We revisit the problem of flux-balance equations for isolated post-Newtonian matter systems due to the emission of gravitational waves. In particular we show by a local derivation confined to the system, using the expression of radiation-reaction forces up to the 3.5 PN order, that not only the energy, angular momentum and linear momentum of the system, but also the position of its center of mass, obey some (non-trivial) flux-balance equations. The balance equation for the center-of-mass position completes the description of the secular evolution by gravitational waves of relativistic post-Newtonian isolated matter systems. We then confirm this result by a direct computation of the gravitational-wave fluxes at future null infinity, obtaining the full multipole-moment expansion of the flux associated with the center-of-mass position (probably new with this paper), and rederiving as well the known multipole-moment expansions of the fluxes of energy, angular momentum and linear momentum. We also check our analysis by a direct calculation of radiation-reaction effects in the case of compact binary systems.

关键词： anthropogenic influences   classification   cluster analysis   hydrologic regime   random forest   regionalization   ungaged watersheds   watershed hydrology  

摘要： Watershed hydrology has often focused on modelling studies of individual watersheds, which consider each river system as unique. Classification is an alternative approach that instead focuses on the similarities among different watersheds. Although both supervised and unsupervised hydrologic classifications have been developed, few previous studies have used classification to assess the degree of anthropogenic modification of hydrologic regime. Here, we conducted an unsupervised hydrologic classification of 189 U.S. Geological Survey gages, including 41 minimally impacted gages from the Hydro-Climatic Data Network (HCDN), in the five major interstate river basins in the U.S. state of Alabama. For the natural classification, the most significant predictor variables for cluster membership were related to compressive strength of bedrock, bedrock depth, hydraulic conductivity, elevation, temperature, and soil texture, and several land-cover variables were also significant in the anthropogenic classification. We then developed two random-forest models: one based on all 189 gages using both natural and anthropogenic variables from the Stream-Catchment (StreamCat) dataset and one based on the 41 HCDN gages using natural StreamCat variables only. We used the random-forest models to predict natural and anthropogenic normative hydrologic class for over 158,000 National Hydrography Dataset Plus catchments in the study area. Catchments that changed their class between the natural and anthropogenic classifications can be identified as those that have a large amount of anthropogenic influences on their hydrologic regime, including many catchments on the coast, in the north-western Coastal Plain, in the Interior Low Plateaus, and in the Piedmont. Using unsupervised hydrologic classifications is a promising approach for uncovering the physical processes that affect hydrologic regime. There are also potential applications in river management, including predicting the hydrologic beha

关键词： Biohydrogen production   Immobilised cells   Hydrodynamic characteristics   Minimum fluidization velocity   Terminal velocity   Fluidized bed reactor  

摘要： A mathematical model of minimum fluidization velocity (U-mf ) was developed based on the hydrodynamic characteristics of the fluidized bed reactors (FBR) with immobilised cells attached to activated carbon at thermophilic biohydrogen fermentation. The maximum hydrogen productivity rate of 7.8 mmolH(2) /L.h and hydrogen yield of 2.2 molH(2) /mol of sugar consumed was obtained when the HRT was shortened from 48 h to 6 h. The presence of the immobilised cells enriched the biomass composition in the FBR from 4.9 to 7.1 g VSS/L and maximum energy generated was 58.7 KJ H-2/L.d. The FBR had to be operated at a high U(mf)( )of 0.05-0.44 cm/s and a low terminal velocity of 2.11 cm/s to prevent the immobilised cells from washed out from the FBR, hence achieved an adequate fluidization system. A screening of the microbial population by DGGE revealed that the T. thermosaccharolyticum sp. was dominant for all the HRTs, thereby indicating that this bacterium is resilient towards environmental disturbances. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

关键词： fluidized bed   powder coke   MP-PIC method   emission characteristics  

摘要： The fluidized reactor is widely used in a number of chemical processes due to its high gas-particle contacting efficiency and excellent performance on solid mixing. An improved numerical framework based on the multiphase particle-in-cell (MP-PIC) method has been developed to simulate the processes of gas-solid flow and chemical reactions in a fluidized bed. Experiments have been carried out with a 3-MW circulating fluidized bed with a height of 24.5 m and a cross section of 1 m(2). In order to obtain the relationship between pollutant discharge and operating conditions and to better guide the operation of the power plant, a series of tests and simulations were carried out. The distributions of temperature and gas concentration along the furnace from simulations achieved good accuracy compared with experimental data, indicating that this numerical framework is suitable for solving complex gas-solid flow and reactions in fluidized bed reactors. Through a series of experiments, the factors affecting the concentration of NOx and SOx emissions during the steady-state combustion of the normal temperature of powder coke were obtained, which provided some future guidance for the operation of a power plant burning the same kind of fuel.

关键词： surface electromyography   hand-gesture classification   generative flow model  

摘要： Conventional pattern-recognition algorithms for surface electromyography (sEMG)-based hand-gesture classification have difficulties in capturing the complexity and variability of sEMG. The deep structures of deep learning enable the method to learn high-level features of data to improve both accuracy and robustness of a classification. However, the features learned through deep learning are incomprehensible, and this issue has precluded the use of deep learning in clinical applications where model comprehension is required. In this paper, a generative flow model (GFM), which is a recent flourishing branch of deep learning, is used with a SoftMax classifier for hand-gesture classification. The proposed approach achieves 63.86 +/- 5.12% accuracy in classifying 53 different hand gestures from the NinaPro database 5. The distribution of all 53 hand gestures is modelled by the GFM, and each dimension of the feature learned by the GFM is comprehensible using the reverse flow of the GFM. Moreover, the feature appears to be related to muscle synergy to some extent.

关键词： 脉动流量   文丘里流量计   相位差   脉动幅度  

摘要： 为了深入研究周期脉动流对文丘里流量计的影响,本文利用FLUENT软件对文丘里流量计内流场进行仿真分析。研究中对文丘里流量计在不同脉动频率和幅度下的差压输出特性进行了分析。研究表明差压脉动幅度与流量脉动幅值具有较好的线性关系;而脉动频率对差压与流量之间相位差和差压脉动幅度具有显著影响,在脉动频率为50Hz工况,相位差可达90°,差压脉动幅度可达流量脉动幅度的5倍左右。

关键词： 镇安西部   流体包裹体   钨矿   稳定同位素   成矿流体   棋盘沟矿区   陕西省  

摘要： 镇安西部棋盘沟矿区内的钨矿体主要为石英脉型钨矿,产出于矿区内近SN向断裂中,矿石中金属矿物以白钨矿为主,矿体规模大且品位高,但矿区内未见有岩浆岩体出露,因此明确其成矿流体来源是理解矿床成因的关键。为了探究成矿流体特征,分析了主成矿阶段形成的白钨矿与石英颗粒中主要发育的气液两相包裹体,结果显示包裹体均一温度介于291.4～423.7℃之间,热液盐度[w(NaCl)]为5.1%～7.7%,流体的密度范围为0.52～1.08 g/cm3。这表明矿区成矿流体属于中高温、低盐度流体,与典型热液钨矿流体相似,H、O同位素测定结果显示其主要来源为岩浆水,S同位素落入岩浆岩S同位素组成范围内,反映矿区钨矿来源于岩浆热液。与周边矿区似层状钨矿流体包裹体相比,棋盘沟矿区成矿流体均一温度明显较高,说明近SN向构造是成矿热液运移的主要通道,在成矿流体运移过程中,顺着该区NNE向断裂裂隙上升,形成石英脉型白钨矿。随着成矿热液温度持续降低,矿液沿层间裂隙灌入形成似层状型白钨矿。