Larmor size transient structures with density depletions as large as 99%of ambient solar wind density levels occur commonly upstream of Earth’s collisionless bow shock.These "density holes" have a mean dura...Larmor size transient structures with density depletions as large as 99%of ambient solar wind density levels occur commonly upstream of Earth’s collisionless bow shock.These "density holes" have a mean duration of 17.9±10.4s but holes as short as 4 s have been observed.The average fractional density depletion（δn/n） inside the holes is 0.68±0.14.The density of the upstream edge moving in the sunward direction can be enhanced by five or more times the solar wind density. Particle distributions show the steepened edge can behave like a shock,and measured local field geometries and Mach number support this view.Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles.Current densities as large as 150nA·m-2 are observed at the leading compressed edge.The waves are elliptically polarized and rotating in the sense of ions（left hand） in the plasma frame.The waves appear to grow and steepen as the density holes convect with the solar wind toward the Earth.The transient nature of density holes suggests that the temporal features could represent the different stages of nonlinear evolutionary processes that produce a shock-like structure.The density holes are only observed with upstream particles,suggesting that back-streaming particles interacting with the solar wind are important. The significance of these observations is still being investigated.展开更多
针对风电机组叶片排水孔堵塞或被雷击穿孔等问题,提出一种非接触式的声学检测方法。该方法首先对采集到的信号转化为时频图,利用中值滤波和自适应阈值的方法将时频图二值化,根据二值化时频图中哨音轮廓特点,提取轮廓信号时域和频域等9...针对风电机组叶片排水孔堵塞或被雷击穿孔等问题,提出一种非接触式的声学检测方法。该方法首先对采集到的信号转化为时频图,利用中值滤波和自适应阈值的方法将时频图二值化,根据二值化时频图中哨音轮廓特点,提取轮廓信号时域和频域等9个参数作为特征向量,提出了动态半径的支持向量数据描述异常检测模型(dynamic radius support vector data description,DR-SVDD)。将DR-SVDD和SVDD的异常检测模型用于风机叶片哨声诊断,验证了该方法的有效性。展开更多
基金The research at UC Berkeley is performed under the auspices of a NASA Grant No.NNG04GF23GCluster is a joint project of ESA and NASA and Double Star a joint project of ESA and the Chinese Space Agencysupported by the WCU program through NRF funded by MEST of Korea(R31-10016)
文摘Larmor size transient structures with density depletions as large as 99%of ambient solar wind density levels occur commonly upstream of Earth’s collisionless bow shock.These "density holes" have a mean duration of 17.9±10.4s but holes as short as 4 s have been observed.The average fractional density depletion（δn/n） inside the holes is 0.68±0.14.The density of the upstream edge moving in the sunward direction can be enhanced by five or more times the solar wind density. Particle distributions show the steepened edge can behave like a shock,and measured local field geometries and Mach number support this view.Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles.Current densities as large as 150nA·m-2 are observed at the leading compressed edge.The waves are elliptically polarized and rotating in the sense of ions（left hand） in the plasma frame.The waves appear to grow and steepen as the density holes convect with the solar wind toward the Earth.The transient nature of density holes suggests that the temporal features could represent the different stages of nonlinear evolutionary processes that produce a shock-like structure.The density holes are only observed with upstream particles,suggesting that back-streaming particles interacting with the solar wind are important. The significance of these observations is still being investigated.
文摘针对风电机组叶片排水孔堵塞或被雷击穿孔等问题,提出一种非接触式的声学检测方法。该方法首先对采集到的信号转化为时频图,利用中值滤波和自适应阈值的方法将时频图二值化,根据二值化时频图中哨音轮廓特点,提取轮廓信号时域和频域等9个参数作为特征向量,提出了动态半径的支持向量数据描述异常检测模型(dynamic radius support vector data description,DR-SVDD)。将DR-SVDD和SVDD的异常检测模型用于风机叶片哨声诊断,验证了该方法的有效性。
