突扩两相流动中颗粒相对气体湍流的影响

为了研究颗粒相的存在对气体湍流的影响,用相位Doppler激光测速仪(phase Doppler particle anemometer, PDPA)测量了竖直轴对称突扩通道中两相流动中的气体湍流变动,给出单相流动和气粒两相流动时的平均速度和轴向脉动速度,阐明了颗粒对气体湍流的作用,即湍流变动规律.结果表明,平均粒径150 μm的颗粒在流场的不同位置造成的湍流变动现象有所不同.在回流区内,靠近轴线附近的正流区内颗粒增强了气体湍流; 在逆流区以及下游区域,颗粒削弱了气体湍流.     

Nanosecond radio bursts from strong plasma turbulence in the Crab pulsar

The Crab pulsar was discovered by the occasional exceptionally bright radio pulses it emits, subsequently dubbed 'giant' pulses. Only two other pulsars are known to emit giant pulses. There is no satisfactory explanation for the occurrence of giant pulses, nor is there a complete theory of the pulsar emission mechanism in general. Competing models for the radio emission mechanism can be distinguished by the temporal structure of their coherent emission. Here we report the discovery of isolated, highly polarized, two-nanosecond subpulses within the giant radio pulses from the Crab pulsar. The plasma structures responsible for these emissions must be smaller than one metre in size, making them by far the smallest objects ever detected and resolved outside the Solar System, and the brightest transient radio sources in the sky. Only one of the current models--the collapse of plasma-turbulent wave packets in the pulsar magnetosphere--can account for the nanopulses we observe.     

Calcium gradients in single smooth muscle cells revealed by the digital imaging microscope using Fura-2

Calcium is believed to control a variety of cellular processes, often with a high degree of spatial and temporal precision. For a cell to use Ca2+ in this manner, mechanisms must exist for controlling the ion in a localized fashion. We have now gained insight into such mechanisms from studies which measured Ca2+ in single living cells with high resolution using a digital imaging microscope and the highly fluorescent Ca2+-sensitive dye, Fura-2. Levels of Ca2+ in the cytoplasm, nucleus and sarcoplasmic reticulum (SR) are clearly different. Free [Ca2+] in the nucleus and SR was greater than in the cytoplasm and these gradients were abolished by Ca2+ ionophores. When external Ca2+ was raised above normal in the absence of ionophores, free cytoplasmic Ca2+ increased but nuclear Ca2+ did not. Thus, nuclear [Ca2+] appears to be regulated independently of cytoplasmic [Ca2+] by gating mechanisms in the nuclear envelope. The observed regulation of intranuclear Ca2+ in these contractile cells may thus be seen as a way to prevent fluctuation in Ca2+-linked nuclear processes during the rise in cytoplasmic [Ca2+] which triggers contraction. The approach described here offers the opportunity of following changes in Ca2+ in cellular compartments in response to a wide range of stimuli, allowing new insights into the role of local changes in Ca2+ in the regulation of cell function.     

利用偏振态法测量光纤偏振效应

在庞加莱球的基础上,对原来偏振模色散的测量方法进行了理论分析,研究表明,一阶偏振模色散测量的误差影响二阶偏振模色散的测量.通过推导,提出了利用偏振态测量二阶偏振模色散的方法.此外,基于具有偏振相关损失和双折射的密勒矩阵洛伦兹变化特性,推导出利用偏振态测量偏振相关损失的简化方法,证实了测量偏振相关损失取决于密勒矩阵的第一列元素,该方法对于偏振相关损失的测量只需要2个输人偏振态,因此具有测量速度快和测量误差小的优点.实验的测量结果证实了利用偏振态测量2种光纤偏振效应的可靠性.     

星际介质中的纳米尘埃

星际尘埃,作为一种在宇宙中普遍存在的重要成分,在天文学研究中起着重要的作用.星际介质中的尘埃其尺寸分布涵盖从几个埃到几个亚微米的范围.对于有这样大的尺寸差异的星际尘埃,尘埃的热辐射机制差别很大.较大的尘埃颗粒,有比较大的吸收和发射截面,因而其吸收光子的速率和发射的速率也比较大,可以从吸收和发射能量的平衡来得出热平衡温度,并用热平衡温度和黑体辐射来计算其光谱.对于大小为纳米尺度或者更小的尘埃,由于其很小的尺寸,这类尘埃的热容量非常小.当这种尘埃吸收一个与其热容量相当或者更大能量光子的时候,尘埃就会经历一个非常明显的温度涨落:尘埃吸收一个紫外光子瞬间,其温度迅速上升,到达顶点后,由于尚没有外来能量的影响(因吸收截面小,吸收光子几率也非常小),尘埃开始通过热辐射降温,直到吸收另一个光子开始新的循环,这就是单光子加热模型(Single-Photon Heating Model).显然,热平衡和单光子加热是两个明显不同的过程.对于单光子加热,由于尘埃颗粒的温度涨落作用,尘埃不会处于一个稳定的平衡温度状态,因而不能用单一温度来描述其热辐射,必须计算出尘埃在温度涨落过程中的温度分布函数,然后计算其辐射谱.本文主要介绍纳米颗粒在星际空间中的存在证据,单光子加热模型,以及处于温度涨落中纳米颗粒的辐射特征.     

Newly synthesized lithium in the interstellar medium

Astronomical observations of elemental and isotopic abundances provide the means to determine the source of elements and to reveal their evolutionary pathways since the formation of the Galaxy some 15 billion years ago. The abundance of lithium is particularly interesting because, although some of it is thought to be primordial, most results from spallation reactions (in which Galactic cosmic rays break apart larger nuclei in the interstellar medium). Spallation reactions are crucial for the production of other light elements, such as beryllium and boron, so observations of lithium isotopic abundances can be used to test model predictions for light-element synthesis in general. Here we report observations of 7Li and 6Li abundances in several interstellar clouds lying in the direction of the star o Persei. We find the abundance ratio 7Li/6Li to be about 2, which is significantly lower than the average Solar System value of 12.3 (refs 6, 7). An abundance ratio of 2 is clear evidence that the observed lithium must have resulted entirely from spallation, confirming a basic tenet of light-element synthesis. The total lithium abundance, however, is not enhanced as expected.     

Demixing in simple fluids induced by electric field gradients

Phase separation in liquid mixtures is mainly controlled by temperature and pressure, but can also be influenced by gravitational, magnetic or electric fields. However, the weak coupling between such fields and concentration fluctuations limits this effect to extreme conditions. For example, mixing induced by uniform electric fields is detectable only at temperatures that are within a few hundredths of degree or less of the phase transition temperature of the system being studied. Here we predict and demonstrate that electric fields can control the phase separation behaviour of mixtures of simple liquids under more practical conditions, provided that the fields are non-uniform. By applying a voltage of 100 V across unevenly spaced electrodes about 50 micro m apart, we can reversibly induce the demixing of paraffin and silicone oil at 1 K above the phase transition temperature of the mixture; when the field gradients are turned off, the mixture becomes homogeneous again. This direct control over phase separation behaviour depends on field intensity, with the electrode geometry determining the length-scale of the effect. We expect that this phenomenon will find a number of nanotechnological applications, particularly as it benefits from field gradients near small conducting objects.     

天然气中SF_6等多种氟化物气体示踪剂的气相色谱检测

为了监测气驱油藏中的气体示踪剂,利用GC/ECD建立了一种分析天然气中多种氟化物气体示踪剂的方法。该方法选用GDX-104色谱柱,排除了样品中空气峰对检测的干扰,通过正交试验优化最低灵敏度组分的检测条件,并以此作为同时检测天然气或空气中多种气体示踪剂(SF_6、CF_2Br_2和C_4F_8)的条件,其中SF6、CF_2Br_2和C_4F_8的检出限分别为1×10~(-6)、1×10~(-5)和1×10~(-3)μg/L,重复进样7次的相对标准偏差不大于7%。通过对中原油田天然气驱油藏的气体示踪剂的监测,得到了示踪剂的产出曲线,弄清了该区块的非均质性和注采井的连通关系。该检测方法快速、简便,所需样品量少,可实现多种示踪剂同时检测,不仅适用于天然气中微量气体示踪检测,而且适用于大气环流的微量气体示踪检测。     

Infrared diffuse interstellar bands in the Galactic Centre region

The spectrum of any star viewed through a sufficient quantity of diffuse interstellar material reveals a number of absorption features collectively called 'diffuse interstellar bands' (DIBs). The first DIBs were reported about 90 years ago, and currently well over 500 are known. None of them has been convincingly identified with any specific element or molecule, although recent studies suggest that the DIB carriers are polyatomic molecules containing carbon. Most of the DIBs currently known are at visible and very near-infrared wavelengths, with only two previously known at wavelengths beyond one micrometre (10,000 ?ngstr?ms), the longer of which is at 1.318?micrometres (ref. 6). Here we report 13 diffuse interstellar bands in the 1.5-1.8 micrometre interval on high-extinction sightlines towards stars in the Galactic Centre. We argue that they originate almost entirely in the Galactic Centre region, a considerably warmer and harsher environment than where DIBs have been observed previously. The relative strengths of these DIBs towards the Galactic Centre and the Cygnus OB2 diffuse cloud are consistent with their strengths scaling mainly with the extinction by diffuse material.     

Quadrupole transitions revealed by Borrmann spectroscopy

The Borrmann effect-a dramatic increase in transparency to X-ray beams-is observed when X-rays satisfying Bragg's law diffract through a perfect crystal. The minimization of absorption seen in the Borrmann effect has been explained by noting that the electric field of the X-ray beam approaches zero amplitude at the crystal planes, thus avoiding the atoms. Here we show experimentally that under conditions of absorption suppression, the weaker electric quadrupole absorption transitions are effectively enhanced to such a degree that they can dominate the absorption spectrum. This effect can be exploited as an atomic spectroscopy technique; we show that quadrupole transitions give rise to additional structure at the L(1), L(2) and L(3) absorption edges of gadolinium in gadolinium gallium garnet, which mark the onset of excitations from 2s, 2p(1/2) and 2p(3/2) atomic core levels, respectively. Although the Borrmann effect served to underpin the development of the theory of X-ray diffraction, this is potentially the most important experimental application of the phenomenon since its first observation seven decades ago. Identifying quadrupole features in X-ray absorption spectroscopy is central to the interpretation of 'pre-edge' spectra, which are often taken to be indicators of local symmetry, valence and atomic environment. Quadrupolar absorption isolates states of different symmetries to that of the dominant dipole spectrum, and typically reveals orbitals that dominate the electronic ground-state properties of lanthanides and 3d transition metals, including magnetism. Results from our Borrmann spectroscopy technique feed into contemporary discussions regarding resonant X-ray diffraction and the nature of pre-edge lines identified by inelastic X-ray scattering. Furthermore, because the Borrmann effect has been observed in photonic materials, it seems likely that the quadrupole enhancement reported here will play an important role in modern optics.     

用生命诠释文化 --浅谈老舍小说的文化意蕴

特定的文化内涵和作者自身的人文品格与悲剧意识,使老舍小说文化意蕴深厚。从孕育老舍创作的北京文化与旗人文化人手,探寻老舍文化心理的形成,并从市民阶层这一特定的人群出发诠释老舍小说的文化内涵及其审美理想。     

电磁波在湍流介质中的衰减效应

研究了大气湍流对电磁波的衰减，得出了有关精确表达式，计算结果表明自由空间的平均大气起伏常数为10^-17，应用所得结果导出了闪烁指数理论公式，数值计算与献一致。     

Observations of clear air turbulence by high power radar

Clear air turbulence is a hazard to aviation and is thought to have important effects on atmospheric dynamics. This article describes the structure and evolution of clear air turbulence at high altitudes as revealed by a high power radar and vertical soundings of wind and temperature.