The effects of low frequency electromagnetic field on the macro-physical fields in the semi-continuous casting process of aluminum alloys and the microstructure and crack in the billets were studied and analyzed by th...The effects of low frequency electromagnetic field on the macro-physical fields in the semi-continuous casting process of aluminum alloys and the microstructure and crack in the billets were studied and analyzed by the numerical and experimental methods.Comparison of the results for the macro-physical fields in the low frequency electromagnetic casting（LFEC） process with the conventional DC casting process indicates the following characters due to the application of electromagnetic field：an entirely changed direction and remarkably increased velocity of melt flow;a uniform distribution and a decreased gradient of temperature;elevated isothermal lines;a reduced sump depth;decreased stress and plastic deformation.Further,the microstructure of the billets is refined remarkably and the crack in the billets is eliminated in LFEC process because of modification of the macro-physical fields induced by the application of low frequency electromagnetic field.展开更多
Fires play an essential part in the maintenance of the environment, but amplified fire activity often leads to adverse effects in the environment such as destruction of property and loss of life. Botswana has experien...Fires play an essential part in the maintenance of the environment, but amplified fire activity often leads to adverse effects in the environment such as destruction of property and loss of life. Botswana has experienced wildfires that are caused by humans intentionally and unintentionally. Some of these wildfires grow into mega</span><span style="font-family:""> </span><span style="font-family:Verdana;">fires such as the 2008 wildfires. Data of wildfires reported in Botswana from the Department of Forestry and Range Resources and the frequency of occurrence was studied and analyzed. It shows the period of 2006-2017, Ghanzi and Ngamiland districts were mostly affected by wildfires. These districts have protected land such as the wildlife parks which may lead to </span><span style="font-family:Verdana;">the </span><span style="font-family:Verdana;">outbreak of natural wildfires that burns unnoticed due to minimal movements of people. The wildfires reported in each district show an increase over the years and this may affect the smooth running of operations.展开更多
Through analyzing the near-field hydrophone records of the airgun experiment in the Jiemian reservoir,Fujian,we study the time-frequency characteristic of airgun source wavelet and the influence of gun depth and firin...Through analyzing the near-field hydrophone records of the airgun experiment in the Jiemian reservoir,Fujian,we study the time-frequency characteristic of airgun source wavelet and the influence of gun depth and firing pressure,and explain the process of bubble oscillation based on the Johnson( 1994) bubble model. The data analysis shows that:( 1) Airgun wavelet is composed of primary pulse and bubble pulse. The primary pulse,which is of large amplitude,short duration and wide frequency band,is usually used in shallow exploration. The bubble pulse,which is concentrated in the low-frequency range,is usually used in deep exploration with deep vertical penetration and far horizontal propagation.( 2) The variation of primary pulse amplitude with gun depth is very small,bubble pulse amplitude and the dominant frequency increase,and peak-bubble ratio and bubble period decrease. When the gun depth is 10 m,primary pulse amplitude and peakbubble ratio are maximum,which is suitable for shallow exploration; when gun depth is25 m,bubble pulse amplitude is large, and peak-bubble ratio is minimum, which is suitable for deep exploration.( 3) The primary pulse amplitude,bubble pulse amplitude,peak-bubble ratio,and bubble period increase and the dominant frequency decreases with increased firing pressure.展开更多
提出一种改进FIRE(Fuzzy Inference Ruled by Else-action)滤波和改进Prony法相结合的低频振荡模式辨识方法.该方法用改进后具有检测层和调整层的FIRE滤波技术对数据进行快速预处理,再用改进Prony法对滤波后的信号进行分析得到电力系统...提出一种改进FIRE(Fuzzy Inference Ruled by Else-action)滤波和改进Prony法相结合的低频振荡模式辨识方法.该方法用改进后具有检测层和调整层的FIRE滤波技术对数据进行快速预处理,再用改进Prony法对滤波后的信号进行分析得到电力系统的低频振荡模式.将该方法分别用于分析试验信号和IEEE 4机系统振荡信号,并同带有传统模糊滤波器的Prony分析结果比较可以看出,改进后的方法可将低频振荡的主导模式在一定的噪声环境下比较准确和快速的辨识出来,且辨识的阶数更接近于实际阶数,验证了其可靠性和实用性.展开更多
Forest fire history can be reconstructed over past centuries across a wide variety of forest types. Fire scars on living tress, and age classes of forest stands, are the two sources of information for these reconstruc...Forest fire history can be reconstructed over past centuries across a wide variety of forest types. Fire scars on living tress, and age classes of forest stands, are the two sources of information for these reconstructions. Point and area frequencies are used to reconstruct fire history. Point frequencies are useful in forest types that burn with frequent, low intensity fire so that many fire-scarred residual trees exist. A true point is a single tree, but more often point estimates are made by combining fire scar records from several adjacent trees. Area frequences are applied where fires are infrequent but of moderate to high intensity, so that stand ages are used across wide areas to estimate fire return interals. Proper selection and application of fire history methods are essential to deriving useful ecological implications from fire history studies.This review evaluates the common methods of determining fire history :what the techniques are, where they are best applied, and how to interpret them in an ecological context. Emphasis is placed on fire freqency and predictability, but choosing a proper technique may also be a function of fire intensity.展开更多
基金Project(2005CB623707)supported by National Basic Research Project of China
文摘The effects of low frequency electromagnetic field on the macro-physical fields in the semi-continuous casting process of aluminum alloys and the microstructure and crack in the billets were studied and analyzed by the numerical and experimental methods.Comparison of the results for the macro-physical fields in the low frequency electromagnetic casting（LFEC） process with the conventional DC casting process indicates the following characters due to the application of electromagnetic field：an entirely changed direction and remarkably increased velocity of melt flow;a uniform distribution and a decreased gradient of temperature;elevated isothermal lines;a reduced sump depth;decreased stress and plastic deformation.Further,the microstructure of the billets is refined remarkably and the crack in the billets is eliminated in LFEC process because of modification of the macro-physical fields induced by the application of low frequency electromagnetic field.
文摘Fires play an essential part in the maintenance of the environment, but amplified fire activity often leads to adverse effects in the environment such as destruction of property and loss of life. Botswana has experienced wildfires that are caused by humans intentionally and unintentionally. Some of these wildfires grow into mega</span><span style="font-family:""> </span><span style="font-family:Verdana;">fires such as the 2008 wildfires. Data of wildfires reported in Botswana from the Department of Forestry and Range Resources and the frequency of occurrence was studied and analyzed. It shows the period of 2006-2017, Ghanzi and Ngamiland districts were mostly affected by wildfires. These districts have protected land such as the wildlife parks which may lead to </span><span style="font-family:Verdana;">the </span><span style="font-family:Verdana;">outbreak of natural wildfires that burns unnoticed due to minimal movements of people. The wildfires reported in each district show an increase over the years and this may affect the smooth running of operations.
基金jointly sponsored the Special Fund for Earthquake Scientific Research of China Earthquake Administration(2015419015)the National Natural Sciences Foundation of China(41474071)
文摘Through analyzing the near-field hydrophone records of the airgun experiment in the Jiemian reservoir,Fujian,we study the time-frequency characteristic of airgun source wavelet and the influence of gun depth and firing pressure,and explain the process of bubble oscillation based on the Johnson( 1994) bubble model. The data analysis shows that:( 1) Airgun wavelet is composed of primary pulse and bubble pulse. The primary pulse,which is of large amplitude,short duration and wide frequency band,is usually used in shallow exploration. The bubble pulse,which is concentrated in the low-frequency range,is usually used in deep exploration with deep vertical penetration and far horizontal propagation.( 2) The variation of primary pulse amplitude with gun depth is very small,bubble pulse amplitude and the dominant frequency increase,and peak-bubble ratio and bubble period decrease. When the gun depth is 10 m,primary pulse amplitude and peakbubble ratio are maximum,which is suitable for shallow exploration; when gun depth is25 m,bubble pulse amplitude is large, and peak-bubble ratio is minimum, which is suitable for deep exploration.( 3) The primary pulse amplitude,bubble pulse amplitude,peak-bubble ratio,and bubble period increase and the dominant frequency decreases with increased firing pressure.
文摘Forest fire history can be reconstructed over past centuries across a wide variety of forest types. Fire scars on living tress, and age classes of forest stands, are the two sources of information for these reconstructions. Point and area frequencies are used to reconstruct fire history. Point frequencies are useful in forest types that burn with frequent, low intensity fire so that many fire-scarred residual trees exist. A true point is a single tree, but more often point estimates are made by combining fire scar records from several adjacent trees. Area frequences are applied where fires are infrequent but of moderate to high intensity, so that stand ages are used across wide areas to estimate fire return interals. Proper selection and application of fire history methods are essential to deriving useful ecological implications from fire history studies.This review evaluates the common methods of determining fire history :what the techniques are, where they are best applied, and how to interpret them in an ecological context. Emphasis is placed on fire freqency and predictability, but choosing a proper technique may also be a function of fire intensity.
