Waste impoundments are usually not protected by any growing crops, soil roughness, or wind barriers, and dust due to wind erosion of the surface waste material can be a major environmental problem. The tailings dam in...Waste impoundments are usually not protected by any growing crops, soil roughness, or wind barriers, and dust due to wind erosion of the surface waste material can be a major environmental problem. The tailings dam in Aitik is highly susceptible to wind erosion when it is dry and windy. Strong dust storms may occur during dry seasons. These events are short-lived but may transport a huge amount of particulate matter and adversely impact air quality in downwind areas. This study estimated material loss from the Aitik tailings dam using SWEEP model. The simulated total material loss, saltation and creep loss, suspension loss and PM10 loss were 4.4941 kg/m2, 0.042 kg/m2, 4.4559 kg/m2, 0.264057 kg/m2 respectively. To control the dust generation, wind breaks could be suggested to be installed at 800 m downwind the west side edge of the tailings dam to reduce the saltation and creep and it would in turn reduce suspended emission. The measured total suspended material for five days by exposure profiling method was 0.475 kg/m2, which had a big difference with a simulate value of 4.4559 kg/m2. Many reasons contributed to the disagreement of simulated value and measured value in this study. In order to use the model for mining industry, more tests are needed to validate the modeling result, and calibration methods could be useful in adjusting the internal coefficients and empirical equations.展开更多
快反镜作为高精度光学系统的核心执行机构,其控制策略的优劣直接影响系统性能。传统的快反镜控制策略依赖独立的控制盒和线性功放模块,导致快反镜控制系统体积大、热量高、应用受限。而基于金属-氧化物半导体场效应晶体管(metal-oxide-s...快反镜作为高精度光学系统的核心执行机构,其控制策略的优劣直接影响系统性能。传统的快反镜控制策略依赖独立的控制盒和线性功放模块,导致快反镜控制系统体积大、热量高、应用受限。而基于金属-氧化物半导体场效应晶体管(metal-oxide-semiconductor field-effect transistor,MOSFET)开关量驱动的快反镜在控制上难以达到精度要求。通过对快反镜的建模扫频分析,提出了开环零极点配置与比例积分控制(proportional integral derivative,PID)相结合的控制策略,并在快反镜的硬件平台上进行了仿真验证。实验结果表明,该控制策略在不依赖额外控制器和驱动模块的前提下,能够保证快反镜控制的高精度要求。展开更多
The aerodynamic forces and flow structure of a model insect wing is studied by solving the Navier-Stokes equations numerically.After an initial start from rest,the wing is made to execute an azimuthal rotation(sweepin...The aerodynamic forces and flow structure of a model insect wing is studied by solving the Navier-Stokes equations numerically.After an initial start from rest,the wing is made to execute an azimuthal rotation(sweeping)at a large angle of attack and constant angular velocity.The Reynolds number(Re)considered in the present note is 480(Re is based on the mean chord length of the wing and the speed at 60% wing length from the wing root).During the constant-speed sweeping motion,the stall is absent and large and approximately constant lift and drag coefficients can be maintained.The mechanism for the absence of the stall or the maintenance of large aerodynamic force coefficients is as follows.Soon after the initial start,a vortex ring,which consists of the leading-edge vortex(LEV),the starting vortex,and the two wing-tip vortices,is formed in the wake of the wing.During the subsequent motion of the wing,a base-to-tip spanwise flow converts the vorticity in the LEV to the wing tip and the LEV keeps an approximately constant strength.This prevents the LEV from shedding.As a result, the size of the vortex ring increases approximately linearly with time,resulting in an approximately constant time rate of the first moment of vorticity,or approximately constant lift and drag coefficients. The variation of the relative velocity along the wing span causes a pressure gradient along the wing- span.The base-to-tip spanwise flow is mainly maintained by the pressure-gradient force.展开更多
Open channel junctions are encountered in urban water treatment plants, irrigation and drainage canals, and natural river systems. Junctions are very important in municipal sewerage systems and river engineering. Adeq...Open channel junctions are encountered in urban water treatment plants, irrigation and drainage canals, and natural river systems. Junctions are very important in municipal sewerage systems and river engineering. Adequate theoretical description of flow through an open channel junction is difficult because numerous variables are to be considered. Equations of junction models are based on mass and momentum or mass and energy conservation. The objective of this study is to compare two junction models for subcritical flows. In channel branches, we solve numerically the Saint-Venant hyperbolic system by combining Preissmann scheme and double sweep method. We validate our results with HEC-RAS using Nash and Sutcliffe efficiency. In junction models, equality of water stage and complete energy conservation equation from HEC-RAS are compared. Outcome of the research clearly indicates that the complete conservation energy model is more suitable in flow through junction than equality of water stage model in serious situations.展开更多
The influence of the magnetic field sweep rate on the hysteresis loops of exchange bias Ni0.8Fe0.2/Fe0.5Mn0.5 bilayers has been investigated with a vibrating sample magnetometer. It was found that the sweep rate of 13...The influence of the magnetic field sweep rate on the hysteresis loops of exchange bias Ni0.8Fe0.2/Fe0.5Mn0.5 bilayers has been investigated with a vibrating sample magnetometer. It was found that the sweep rate of 13.6 kA/4πms is high enough to bring about obvious changes in the hysteresis loops of the exchange bias bilayer. High sweep rate in the magnetization reversal stage enlarges the coercivity of the sample, while high sweep rate in the saturation state reduces the coercivity. The above phenomena were attributed to magnetic viscosity in the ferromagnetic layer enhanced by the interface exchange interaction and domain magnetization reversals assisted by thermal fluctuation in the antiferromagnetic layer respectively.展开更多
文摘Waste impoundments are usually not protected by any growing crops, soil roughness, or wind barriers, and dust due to wind erosion of the surface waste material can be a major environmental problem. The tailings dam in Aitik is highly susceptible to wind erosion when it is dry and windy. Strong dust storms may occur during dry seasons. These events are short-lived but may transport a huge amount of particulate matter and adversely impact air quality in downwind areas. This study estimated material loss from the Aitik tailings dam using SWEEP model. The simulated total material loss, saltation and creep loss, suspension loss and PM10 loss were 4.4941 kg/m2, 0.042 kg/m2, 4.4559 kg/m2, 0.264057 kg/m2 respectively. To control the dust generation, wind breaks could be suggested to be installed at 800 m downwind the west side edge of the tailings dam to reduce the saltation and creep and it would in turn reduce suspended emission. The measured total suspended material for five days by exposure profiling method was 0.475 kg/m2, which had a big difference with a simulate value of 4.4559 kg/m2. Many reasons contributed to the disagreement of simulated value and measured value in this study. In order to use the model for mining industry, more tests are needed to validate the modeling result, and calibration methods could be useful in adjusting the internal coefficients and empirical equations.
文摘快反镜作为高精度光学系统的核心执行机构,其控制策略的优劣直接影响系统性能。传统的快反镜控制策略依赖独立的控制盒和线性功放模块,导致快反镜控制系统体积大、热量高、应用受限。而基于金属-氧化物半导体场效应晶体管(metal-oxide-semiconductor field-effect transistor,MOSFET)开关量驱动的快反镜在控制上难以达到精度要求。通过对快反镜的建模扫频分析,提出了开环零极点配置与比例积分控制(proportional integral derivative,PID)相结合的控制策略,并在快反镜的硬件平台上进行了仿真验证。实验结果表明,该控制策略在不依赖额外控制器和驱动模块的前提下,能够保证快反镜控制的高精度要求。
基金The project supported by the National Natural Science Foundation of China(10232010)
文摘The aerodynamic forces and flow structure of a model insect wing is studied by solving the Navier-Stokes equations numerically.After an initial start from rest,the wing is made to execute an azimuthal rotation(sweeping)at a large angle of attack and constant angular velocity.The Reynolds number(Re)considered in the present note is 480(Re is based on the mean chord length of the wing and the speed at 60% wing length from the wing root).During the constant-speed sweeping motion,the stall is absent and large and approximately constant lift and drag coefficients can be maintained.The mechanism for the absence of the stall or the maintenance of large aerodynamic force coefficients is as follows.Soon after the initial start,a vortex ring,which consists of the leading-edge vortex(LEV),the starting vortex,and the two wing-tip vortices,is formed in the wake of the wing.During the subsequent motion of the wing,a base-to-tip spanwise flow converts the vorticity in the LEV to the wing tip and the LEV keeps an approximately constant strength.This prevents the LEV from shedding.As a result, the size of the vortex ring increases approximately linearly with time,resulting in an approximately constant time rate of the first moment of vorticity,or approximately constant lift and drag coefficients. The variation of the relative velocity along the wing span causes a pressure gradient along the wing- span.The base-to-tip spanwise flow is mainly maintained by the pressure-gradient force.
文摘Open channel junctions are encountered in urban water treatment plants, irrigation and drainage canals, and natural river systems. Junctions are very important in municipal sewerage systems and river engineering. Adequate theoretical description of flow through an open channel junction is difficult because numerous variables are to be considered. Equations of junction models are based on mass and momentum or mass and energy conservation. The objective of this study is to compare two junction models for subcritical flows. In channel branches, we solve numerically the Saint-Venant hyperbolic system by combining Preissmann scheme and double sweep method. We validate our results with HEC-RAS using Nash and Sutcliffe efficiency. In junction models, equality of water stage and complete energy conservation equation from HEC-RAS are compared. Outcome of the research clearly indicates that the complete conservation energy model is more suitable in flow through junction than equality of water stage model in serious situations.
文摘The influence of the magnetic field sweep rate on the hysteresis loops of exchange bias Ni0.8Fe0.2/Fe0.5Mn0.5 bilayers has been investigated with a vibrating sample magnetometer. It was found that the sweep rate of 13.6 kA/4πms is high enough to bring about obvious changes in the hysteresis loops of the exchange bias bilayer. High sweep rate in the magnetization reversal stage enlarges the coercivity of the sample, while high sweep rate in the saturation state reduces the coercivity. The above phenomena were attributed to magnetic viscosity in the ferromagnetic layer enhanced by the interface exchange interaction and domain magnetization reversals assisted by thermal fluctuation in the antiferromagnetic layer respectively.
