高层建筑附着升降脚手架风振系数研究

附着升降脚手架是适应高层建筑,特别是超高层建筑施工需要的新型脚手架,近年来,在我国应用较为广泛。风荷载是该类脚手架的主导设计荷载,特别是当爬升至150m高度以上时,风荷载对脚手架的安全使用起着决定性作用。风振系数是脚手架风荷载计算中的重要参数之一。本文通过对脉动风作用下附着升降脚手架与建筑结构共同工作的随机振动分析,得到了附着升降脚手架在爬升阶段和下降阶段的风振系数表达式。对本文大量电算结果与现行国家标准和地方规程的有关规定进行了比较分析后认为,脚手架在爬升阶段和下降阶段的风振系数与所附着建筑结构的风振系数有着不同的数值及变化规律。现行国家标准和地方规程对于爬升高度超过100m时将某一位置处脚手架的风振系数用建筑结构在该点的风振系数来近似代替,或根本不计入风振系数的做法,会使得附着升降脚手架的风荷载计算在爬升阶段以及下降阶段产生不同性质或不同程度的误差。本文建议不论附着升降脚手架的使用高度是否已超过100m均应计算脚手架自身的风振系数。     

考虑各向异性的层土-盾构隧道地震反应数值模拟

在层状各向异性土体-盾构隧道地震反应分析中,引入了横观各向同性弹塑性模型理论,建立了适合于横观各向同性介质的双渐近-多向透射边界条件。针对地铁区间盾构隧道抗震设计的特点,基于横观各向同性弹塑性模型,研制了考虑层状土体各向异性和施工开挖效应,适合于盾构隧道动力计算的各向异性弹塑性动力有限元程序。在程序中对于不同的材料采用了不同的本构关系和不同的单元形式,并采用了关联流动法则和多种屈服准则,可同时进行各向异性土体与地下结构的二维平面应力、平面应变和轴对称问题的静力和动力数值分析。最后利用所研制的程序进行了上海地铁二号线石门一路站附近区间隧道在不同超越概率地震动输入下的隧道反应计算。结果表明,在层状土体-地铁区间隧道的抗震分析中考虑土体各向异性的影响是必要的,所提出的计算模型是可行的。     

春季强降水过程的多普勒天气雷达图像特征剖析

对2004年4月25日出现在华北的一次大范围较强降雨过程进行了分析,介绍了多普勒天气雷达径向速度PPI图像和垂直风廓线(VWP)产品的应用技术,根据这两种产品可以判断冷、暖平流,以及各个高度层风的辐合、辐散,综合这些特征进一步判断气流上升或下沉,从而可以推断降水的生消。通过对此次降水个例的分析,探讨了春季强降雨过程的雷达图像特征,发现在降水发生、发展和消亡的不同时段垂直风廓线产品和径向速度图像都有非常明显地体现,为今后预报春季大面积降水提供一定的参考和依据。     

Sensitivity study of the seasonal mean circulation in the northern South China Sea

A study of the circulation in the northern South China Sea （SCS） is carried out with the aid of a three-dimensional, high-resolution regional ocean model. One control and two sensitivity experiments are performed to qualitatively investigate the effects of surface wind forcing, Kuroshio intrusion, and bottom topographic influence on the circulation in the northern SCS. The model results show that a branch of the Kuroshio in the upper layer can intrude into the SCS and have direct influence on the circulation over the continental shelf break in the northern SCS. There are strong southward pressure gradients along a zonal belt largely seaward of the continental slope. The pressure gradients are opposite in the southern and northern parts of the Luzon Strait, indicating inflow and outflow through the strait, respectively. The sensitivity experiments suggest that the Kuroshio intrusion is responsible for generating the imposed pressure head along the shelf break and has no obvious seasonal variations. The lateral forcing through the Luzon Strait and Taiwan Strait can induce the southwestward slope current and the northeastward SCS Warm Current in the northern SCS. Without the lateral forcing, there is the continental slope. The wind forcing mainly causes the The wind-induced water pile-up results in the southward no high-pressure-gradient zonal belt seaward of seasonal variation of the circulation in the SCS. high pressure gradient along the northwestern boundary of the basin. Without the blocking of the plateau around Dongsha Islands, the intruded Kuroshio tends to extend northwest and the SCS branch of the Kuroshio becomes wider and stronger. The analyses presented here are qualitative in nature but should lead to a better understanding of the oceanic responses in the northern SCS to these external influence factors.     

背景场对变分方法海面风速反演结果的影响

利用前期提出的合成孔径雷达反演海面风场变分方法新解法,开展背景场对合成孔径雷达反演海面风场变分方法风速反演结果影响的模拟试验分析.模拟试验表明,变分方法有效解决了直接反演方法的两大缺陷:一是当背景场风速较大的时候,合成孔径雷达后向散射系数对风速不敏感,直接导致较大的风速反演误差;二是当背景场风向存在误差时,将导致后向散...     

Lofted charged dust distribution above the Moon surface

We developed kinetic theory for the charging processes of small dust grains near the lunar surface due to interaction with the anisotropic solar wind plasma. Once charged, these dust grains, which are exposed to the electric field in the sheath region near the lunar surface, could loft and distribute around such heights off the surface where they reach equilibrium with the local gravitational force. Analytical solutions were derived for the charging time, grain floating potential, and grain charge, characterizing the charging processes of small dust grains in a two-component and in a multi-component solar wind plasma, and further highlighting the unique features presented by the high streaming plasma velocity. We have also formulated a novel kinetic theory of sheath formation around an absorbing planar surface immersed in the anisotropic solar wind plasma in the case of a negligible photoelectric effect and presented solutions for the sheath structure. In this study we combined the results from these analyses and provided estimates for the size distribution function of dust that is expected to be lofted in regions dominated by the solar wind plasma, such as near the terminator and in nearby shadowed craters. Corresponding to the two dominant streaming velocity peaks of 300 and 800 km/s, mean dust diameters of 500 and 350 nm, respectively, are expected to be found at equilibrium at heights of relevance to exploration operations, e.g., around 1.5 m height off the lunar surface. In shadowed craters near the terminator region, where isotropic plasma should be dominating, we estimate mean lofted dust diameter of 800 nm around the same 1.5 m height off the lunar surface. The generally applicable solutions could be used to readily calculate the expected lofted size distribution near the lunar surface as a function of plasma parameters, dust grain composition, and other parameters of interest.     

Simulating OH/H2O formation by solar wind at the lunar surface

We simulate the OH/H₂O production from the action of keV protons on the lunar regolith using a vacuum chamber and a mass analyzer to examine the molecular products released from olivine and SiO₂ powders during their irradiation by deuterium ions. The measured mass spectra, showing the OD/D₂O signature, confirm the possibility of OH/H₂O formation on the lunar surface by solar-wind hydrogen.     

Excitation of ion waves by charged dust beams in ionospheric aerosol release experiments

Ion waves excited by charged dust beams streaming across or along the geomagnetic field in the ionosphere may be generated by plasma instabilities during aerosol release experiments. The injection speed of the dust and gas is comparable to or larger than the ion thermal speed in the background plasma. The dust grains can get charged by plasma collection from the ambient ionosphere, and can thus act as a charged beam that excites instabilities in the background plasma. The theory is applied to relatively early time scales of the order of in the dust-gas cloud expansion, with wave frequencies that are larger than the ion gyrofrequency, and collisions with neutrals are included.     

Deep Space 1 at comet 19P/Borrelly: Magnetic field and plasma observations

On September 22, 2001 the Deep Space 1 spacecraft performed a flyby at comet 19P/Borrelly at a solar distance of 1.36 AU leading the Earth by 74^° in longitude. The spacecraft-comet distance at closest approach was 2171 km. The bow shock had a magnetic compression ratio of 2.5 at a distance of 147 100 km from the nucleus. Deep Space 1 first entered the sheath region essentially from the north polar region. Fluctuations from the cometary ion pickup were present throughout the sheath region and even well upstream of the shock, as expected. The magnetic field pileup region had a peak field strength of 83 nT and was shown to be consistent with a pressure equal to the solar wind ram pressure. The peak field location was offset from the time of closest approach. It is uncertain whether this is a spatial or temporal variation. Draping of magnetic fields around the nucleus was sought, but evidence for this was not apparent in the data. A possible explanation is that the interplanetary solar wind was composed of turbulent short-scale fields, and thus the fields were not symmetric about the point of closest approach. During the flyby phase there were in general few intervals of ACE data where there were large scale Parker spiral fields. With the addition of plasma data, the shock properties are investigated. The characteristics of magnetic draping, pileup and fluctuations are explored. These comet 19P/Borrelly results are contrasted with other cometary flyby results.     

High kinetic energy density jets in the Earth’s magnetosheath: A case study

We study several high kinetic energy density jets observed during a traversal of the dayside magnetosheath by the Cluster spacecraft on March 17, 2001, at various distances from the magnetopause, generally characterised by anomalously high values of the local magnetosonic Mach number. We concentrate on two jets observed just outside the magnetopause, the first almost parallel to the GSM x axis and the second directed northward-tailward along the nominal magnetopause surface. We present evidence that none of them can be ascribed to magnetic reconnection at the magnetopause and show that the magnetopause is severely deformed by the jets, so that its local normal forms an angle of 97° with the quiet time magnetopause normal. On these grounds, we suggest that the indentation of the magnetopause is caused by an anti-sunward jet ramming into the magnetopause slightly equatorward of the northern cusp and that the northward-tailward jet is the result of its reflection at the deformed magnetopause. Finally, we briefly discuss our results by comparing them with past studies of events which in some way recall the one analysed herein.