普通物理实验教学改革的几点建议

普通物理实验教学是大学物理教学的重要组成部分。针对普通物理实验教学的现状,就其相关的教学内容、教学方法、师资队伍建设、实验室建设和管理等存在的问题,提出相应的解决思路。     

基于油液原子光谱多维时间序列模型的机械磨损状态监测研究

提出了一种利用润滑油原子光谱分析技术对机械磨损状态进行监测的新方法。对磨合期润滑油原子光谱数据建立多维时间序列模型并视为标准模型,将新数据通过此模型后得到残差并选择残差方差阵元素作为新数据所属磨损状态的特征。然后,利用主成分分析法对高维特征进行降维,提取前三个主成分构成对应磨损状态的特征向量。最后,利用欧式距离度量对测试样本进行分类,达到了对机械磨损状态识别的目的。利用上述方法,通过对某型履带车辆发动机台架实验的光谱数据进行分析,对发动机磨损状态进行了有效识别,从而证明了所提方法的有效性。结果表明,将多维时间序列模型引入油液光谱分析技术,能够实现光谱信息的有效融合,提高机械磨损状态监测的准确性。     

蚜虫胁迫下棉叶光谱特征及其遥感估测

在棉蚜危害棉花的主要生育期测试蚜害后不同严重度叶片的理化参数和光谱,分析并比较了不同时期、不同品种蚜害棉叶理化参数和光谱反射率特征,确定了蚜害棉叶光谱敏感波段,建立了蚜害棉叶严重度估测模型。结果表明:蚜害棉叶的理化参数和光谱特征明显,蚜害棉叶平均厚度,含水量,温度和叶片CHL.b均增加,叶片而平均CHL.a,CHL.a+b和Cars含量减少。不同时期、不同品种蚜害棉叶光谱反射率在可见光均表现先升后降的特征,但不同品种蚜害棉叶在近红外波段反射率不同。434～727和648nm可分别作为蚜害棉叶的光谱敏感波段和最佳波段。基于敏感波段建立的蚜害棉叶遥感估测模型均达到显著相关,其中以(R1589-R648)/(R1589+R648)为自变量建立的估算模型精度最高,相对误差最小(0.128),可作为蚜害棉叶严重度最佳识别模型。该研究为遥感大面积监测棉花蚜虫提供了理论依据及参考。     

Numerical visualization of vortical structures in a lean premixed swirl combustor using LES

Abstract  

Large eddy simulation was performed to visualize the three-dimensional vortical structures interacting with a turbulent premixed in a lean premixed swirl combustor with varied equivalence ratio. It was found that the fluctuation of unsteady heat release due to the deformation of flame surface was significantly decreased as the equivalence ratio increased because of the change in interaction between inner vortical structures and flames. This phenomenon was another evidence of the amplification mechanism in the combustion instabilities due to the strong flame–vortex interactions under lean premixed conditions.     

Performance limits of pico-cell systems using radio-over-fiber techniques with an electroabsorption modulator

We investigate the performance limits of a pico-cell system using the radio- over-fiber technique with an electroabsorption modulator (EAM) in a 2.4 GHz wireless local area network (WLAN) environment. To understand the performance limits of the system using an EAM as the optical transceiver, we vary the optical modulation index (OMI) of the transmitter for different bias voltages to the EAM. Power margins in the links are found to estimate a possible service range, and packet loss rates (PLRs) in the pico-cell system are measured to evaluate the transmission performance. The total service range can be extended up to 3 m by adjusting the OMI of the transmitter for different applied bias voltages to the EAM.     

Monitoring canopy nitrogen status in winter wheat of growth anaphase with hyperspectral remote sensing

Biomass, leaf area index (LAI) and nitrogen status are important parameters for indicating crop growth potential and photosynthetic productivity in wheat. Nondestructive, quick assessment of leaf dry weight, LAI and nitrogen content is necessary for nitrogen nutrition diagnosis and cultural regulation in wheat production. In order to establish the monitoring model of nitrogen richness in winter wheat of growth anaphase, studying the relationship between the nitrogen richness (NR) containing nitrogen density, LAI and leaf dry weight and the difference of hyperspectral reflectance rates (deltaR), we conducted a comparable experiment with five winter wheat varieties under nitrogen application level of 0, 100, 200 and 400 kg x N x ha(-1). The results indicated the NRs of the different varieties of winter wheat leaves increased with increasing growth stage while in the different nitrogen levels it was sequenced as: NO>N3>N1>N2. Twelve vegetation indices were compared with corresponding NR. The NR had significantly negative correlation to TCARI and VD672 in those vegetation indices, and their correlations (r) arrived at 0.870 and 0.855, respectively. The coefficients of determination (R2) of two models were 0.757 and 0.731 by erecting model with the two indexes and NR Root mean square error (RMSE), relative error (RE) and determination coefficient between measured and estimated NR were employed to test the model reliability and predicting accuracy. Accuracy rates of the models based on TCARI and VD672 achieved 84.56% and 80.13%. The overall results suggested that leaf nitrogen status of growth anaphase in winter wheat has stable relationships with some vegetation indexes, especially index of TCARI and VD672.     

Left–right asymmetry for pion and kaon production in the semi-inclusive deep-inelastic scattering process

We analyze the left–right asymmetry in the semi-inclusive deep-inelastic scattering (SIDIS) process without introducing any weighting functions. With the current theoretical understanding, we find that the Sivers effect plays a key role in our analysis. We use the latest parametrization of the Sivers and fragmentation functions to reanalyze the π ^± production process and find that the results are sensitive to the parametrization. We also extend our calculation on the K ^± production, which can help us know more about the Sivers distribution of the sea quarks and the unfavored fragmentation processes. HERMES kinematics with a proton target, COMPASS kinematics with a proton, deuteron, and neutron target (the information on the neutron target can be effectively extracted from the ³He target), and JLab kinematics (both 6 GeV and 12 GeV) with a proton and neutron target are considered in our paper.     

A method for automatic identification of water and fat images from a symmetrically sampled dual-echo Dixon technique

Sampling water and fat signals symmetrically (i.e., at 0° and 180° relative phase angles) in a dual-echo Dixon technique offers high intrinsic tolerance to phase fluctuations in postprocessing and maximum signal-to-noise performance for the separated water and fat images. However, identification of which image is water and which image is fat after their separation is not possible based on the phase information alone. In this work, we proposed a semiempirical automatic image identification method that is based on the intrinsic asymmetry between the water and fat chemical shift spectra. Specifically, the approximately bimodal feature of the fat spectra and the observation that most in vivo tissues are either predominantly water or predominantly fat are used to construct a spectrum-based algorithm. Additional refinement is accomplished by considering the spatial distribution of the tissues that may have a coexistence of water and fat. The final improved algorithm was tested on a total of 131 three-dimensional patient datasets collected from different scanners and found to yield correct water and fat identification in all datasets.     

Effective potential for highly relativistic neutrinos in a weakly magnetized medium and their oscillation

We have calculated the effective potential experienced by highly relativistic neutrinos in a weakly magnetized electron–positron plasma, where a momentum-dependent finite-width correction to the propagator of W is considered to account for the threshold effect. Magnetars are believed to be sources of TeV–PeV neutrinos which are produced due to photomeson and proton–proton interactions in their atmosphere. We have studied the resonant-oscillation process ν _e ↔ ν _μ,τ of the highly relativistic neutrinos in the atmosphere of SGR 1806-20, which is a magnetar. It is shown that, for high-energy neutrinos propagating within the magnetar atmosphere, the resonance condition can never be satisfied. On the other hand, if GeV neutrinos are produced deep inside the magnetar atmosphere, where the temperature is about 50 keV or more, then these neutrinos can undergo resonant oscillation.