共查询到20条相似文献，搜索用时 562 毫秒
1.
The size of suspended particulate matter in air 总被引：2，自引：0，他引：2
R E Lee 《Science (New York, N.Y.)》1972,178(61):567-575
By means of the NASN cascade impactor the particle sizes of ambient aerosols can be measured by fractionating the particles according to their aerodynamic dimensions, and the fractions can be quantified gravimetrically. Data obtained with this instrument show that particulate matter suspended in urban air in the United States and Great Britain is remarkably uniform in distribution and that the particles are predominantly less than 1 microm in diameter. In Ankara, Turkey, the high proportion of particles larger than 1 microm in diameter are associated with the incomplete combustion of lignite; particulate concentrations in Ankara, however, are considerably higher than those measured elsewhere. The growth of particles in air during temperature inversions occurs in Cincinnati; the greater proportion of large particles found there during inversion periods can account for the reduction in visibility. Aerosols in the vicinity of highways are composed of particles larger than those found some distance away because of the reentrainment of debris by the traffic-induced turbulence. On the other hand, highway configuration and traffic volume have little effect on the size distribution of aerosols in samples collected away from the turbulence area of the roadway. Aerosols measured inside buildings are predominantly smaller in size and exhibit a narrower range of sizes than aerosols outside buildings. 相似文献
2.
利用2003年5月27日、28日在长白山原始森林冠层2个不同高度上观测的湍流资料,采用通量处理软件Eddypro计算和分析了森林冠层上、下的湍流谱结构和局地各向同性特征。结果表明:在惯性副区内,归一化湍流谱遵循-2/3的指数规律;在冠层上、下,大气湍流并不是各向同性的;而在冠层内则是近似各向同性的;基本说明了在森林冠层内,物质和能量是以小涡形式输送的。从冠层下和冠层上的风速分量和温度谱的峰值频率比较可以得出冠层下的湍涡尺度要相对较小的结论。 相似文献
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4.
van der Wiel WG De Franceschi S Fujisawa T Elzerman JM Tarucha S Kouwenhoven LP 《Science (New York, N.Y.)》2000,289(5487):2105-2108
We observe a strong Kondo effect in a semiconductor quantum dot when a small magnetic field is applied. The Coulomb blockade for electron tunneling is overcome completely by the Kondo effect, and the conductance reaches the unitary limit value. We compare the experimental Kondo temperature with the theoretical predictions for the spin- 12 Anderson impurity model. Excellent agreement is found throughout the Kondo regime. Phase coherence is preserved when a Kondo quantum dot is included in one of the arms of an Aharonov-Bohm ring structure, and the phase behavior differs from previous results on a non-Kondo dot. 相似文献
5.
For dairy cattle buildings with slatted floor systems, about 40% of the ammonia emission originates from the slurry pit. In order to find a solution to abate this part of emission, a better understanding of the ammonia transportation from the pit to the room space is crucial. Large eddy simulation (LES) was adopted to investigate the transportation of airflow and ammonia under slatted floor. To tackle the involvement of the slatted floor, two approaches were proposed: modelling slatted floors directly with geometrical details (LESD) and treating them as porous media (LESP). The main purpose of this work was to study the potential of using porous media to model the slatted floor. The LES results were validated by the air velocities measured using a LDA (Laser Doppler Anemometer) in a 1:8 scale pit model placed in a wind tunnel. The results showed that LESP was able to estimate the mean air velocities and turbulence kinetic energy in the core of the pit headspace; but it cannot well predict the mean air velocities and turbulence kinetic energy in the space next to the upwind wall. Clear vertical air motion in the top surface of the slot was observed for LESD results. There was not such trend found for LESP results. Both the air velocity and NH3 fraction fluctuated weaker for LESP results. By spectral analysis, LESP was able to capture the entire power spectrum compared with LESD. A dominant Strouhal number 0.23 was found for LESD results but no dominant Strouhal number was found for LESP results. The emission rate and total mass of NH3 in the pit headspace calculated by LESD was double of those calculated by LESP. Pollutants were confined in the headspace for longer time by means of using LESP than using LESD. For both LESD and LESP, turbulence transportation was the dominant removal mechanism to transport pollutants from the headspace to the free stream. 相似文献
6.
刨花对撞流干燥的理论分析与实验研究 总被引：1，自引：0，他引：1
对撞流干燥技术就是利用两股或两股以上的气流相互对撞所产生的高湍流流场,大大提高物料与气流间的传热传质系数和使物料在对撞面附近往复振荡实现较长停留时间,实现对撞流的高效快速干燥.该文以垂直倾斜半环多级组合对撞流干燥系统为研究对象,通过对木质纤维进行初含水率、气流流量、气流温度和带载率等系统参数的实验研究,探讨木材刨花的对撞流干燥特性.并且用对撞流干燥技术对木材刨花品质的影响从微观和宏观两个角度进行了分析,表明该干燥过程对原料的品质无不良影响. 相似文献
7.
多元复合地基桩土应力比分析 总被引：2，自引：0，他引：2
从单桩与桩间土的荷载-沉降曲线出发,采用荷载-沉降的双曲线模型,给出了多元复合地基桩土应力比的解析解。分析表明,桩土应力比随荷载的变化形式有3种,具体的变化形式与桩、桩间土的初始切向压缩模量和极限承载强度有关。实例计算表明,所得解析解与实测桩土应力比值基本吻合。 相似文献
8.
林带降低风速最根本的问题是在林带背风面形成湍流,湍流的变化与透风系数紧密相关。所以探讨林带的透风系数与湍流度的变化是农田防护林网气象效应的一个重要方面。本文应用二次回归正交旋转组合设计的方法,通过风洞实验和计算建立了透风系数与湍流度之间的回归模型方程。利用该方程可对林带背风面不同高度的透风系数和湍流度相互进行预测,并揭示了两者之间内在的相关关系。同时利用本文的回归模型方程,用最篷透风系数可预测最适湍流度,并对了解林带背风面不同高度上湍流度的变化有重大意义。 相似文献
9.
An infrared radiometer system has been developed that can alert a pilot of an aircraft 2 to 9 minutes in advance of an encounter with clear air turbulence. The time between the warning and the clear air turbulence event varies with the flight altitude of the aircraft. In turbulence-free areas, the incidence of false alarms is found to be less than one in 3.4 hours of flight time compared to less than one per 10 hours of flight time in areas with turbulence. 相似文献
10.
Enhanced turbulence and energy dissipation at ocean fronts 总被引：6，自引：0，他引：6
The ocean surface boundary layer mediates air-sea exchange. In the classical paradigm and in current climate models, its turbulence is driven by atmospheric forcing. Observations at a 1-kilometer-wide front within the Kuroshio Current indicate that the rate of energy dissipation within the boundary layer is enhanced by one to two orders of magnitude, suggesting that the front, rather than the atmospheric forcing, supplied the energy for the turbulence. The data quantitatively support the hypothesis that winds aligned with the frontal velocity catalyzed a release of energy from the front to the turbulence. The resulting boundary layer is stratified in contrast to the classically well-mixed layer. These effects will be strongest at the intense fronts found in the Kuroshio Current, the Gulf Stream, and the Antarctic Circumpolar Current, all of which are key players in the climate system. 相似文献
11.
环境温度导致的气云密度差和大气湍流变化是LNG泄漏扩散的主要影响因素,研究环境温度变化对LNG扩散规律的影响尤为重要。采用Fluent软件中组分输运和Realizable k-ε湍流模型,建立LNG地面泄漏气云扩散数值模型,探究环境温度对LNG泄漏扩散过程中甲烷体积分数的分布规律、气云密度、大气湍流强度的影响。结果表明:当环境温度较低时,LNG气云中各甲烷体积分数线出现"锯齿状"现象,造成甲烷爆炸下限(Low Flammability Limit,LFL)、1/2 LFL的水平扩散范围均增大;当环境温度较高时,甲烷LFL最远扩散距离较低温环境多115 m,造成甲烷1/2 LFL的水平顺风方向扩散距离增大;甲烷体积分数大于1/2 LFL的区域的大气湍流强度增幅则随温度升高而增加,而甲烷体积分数小于1/2 LFL的区域的大气湍流强度增幅随温度的升高而减小;在泄漏源周围100~200 m内,由于"逆温"所造成的大气湍流强度的增幅达0.79倍。研究结果可为LNG泄漏危害区域预测、安全储运、应急救援提供参考。 相似文献
12.
风荷载是影响温室耐久性的重要指标,而温室的荷载能力会受到温室形状的影响。利用标准k-ε模型对高、低两种尺寸的几种不同外形温室流场和顶部压力系数分布进行了数值模拟。结果表明,半圆形顶部的平均压力系数Cp、湍流脉动动能k值和尾部回流区均最小;同时尺寸较高的温室的顶部与较低尺寸的相比,其压力系数的波动和k值变化更大,应给予更高的结构强度设计。 相似文献
13.
本文应用湍流谱分析的方法研究了林带背风面湍涡运动的周期、频率和振幅等谱特征。结果表明,湍流谱密度在低频区存在较大差异;在高频区,不同树高倍数处的谱密度均服从-5/3的指数定律,满足局地各向同性的理论。结合欧拉自相关分析表明,在林缘附近形成较小尺度的湍涡,其频率较高,周期较小,而远离林带处形成较大尺度的湍涡,其频率较低,周期较大。 相似文献
14.
The molecular volume of crystals depends on their isotopic masses. This influence originates from the zero-point motion and the resulting small differences in lattice constants. This effect was measured with high precision by using an x-ray standing wave. The standing wave is generated during Bragg reflection and thus is in phase with the planes of the substrate crystal, which is covered with a homoepitaxial film that has a different isotopic composition than the substrate. The positions of the surface planes of the film with respect to the substrate planes are revealed by the photoelectrons excited by the maxima of the standing wave. For germanium-76 on natural germanium(111), a difference in lattice constant of -1.1 x 10(-5) and -2.5 x 10(-5) at 300 and 54 kelvin, respectively, was found. The results are in good agreement with theoretical predictions. 相似文献
15.
在对排沙漏斗的计算区域作了特殊处理后,选用适当的边界条件,采用VOF方法追踪自由水面,结合RNG双方程湍流模型,对排沙漏斗内三维两相流场作了数值模拟,给出了水面线、静压、速度等参数的分布。将数值计算得到的流速分布与试验实测结果进行了对比,两者吻合较好。计算结果对研究排沙漏斗的流场特性和输沙机理以及优化排沙漏斗设计具有一定的指导意义。 相似文献
16.
Gas-exchange processes control the uptake and release of various gases in natural systems such as oceans, rivers, and lakes. Not much is known about the effect of wind speed on gas exchange in such systems. In the experiment described here, sulfur hexafluoride was dissolved in lake water, and the rate of escape of the gas with wind speed (at wind speeds up to 6 meters per second) was determined over a 1-month period. A sharp change in the wind speed dependence of the gas-exchange coefficient was found at wind speeds of about 2.4 meters per second, in agreement with the results of wind-tunnel studies. However, the gas-exchange coefficients at wind speeds above 3 meters per second were smaller than those observed in wind tunnels and are in agreement with earlier lake and ocean results. 相似文献
17.
We report high-angular-resolution measurements of polarized dust emission toward the low-mass protostellar system NGC 1333 IRAS 4A. We show that in this system the observed magnetic field morphology is in agreement with the standard theoretical models of the formation of Sun-like stars in magnetized molecular clouds at scales of a few hundred astronomical units; gravity has overcome magnetic support, and the magnetic field traces a clear hourglass shape. The magnetic field is substantially more important than turbulence in the evolution of the system, and the initial misalignment of the magnetic and spin axes may have been important in the formation of the binary system. 相似文献
18.
为明确小型水体上的大气湍流特征和涡度相关系统的适用性,基于2018年安徽省滁州市全椒县官渡村小型农业养殖塘的通量观测数据,分析该地的大气稳定状态、湍流方差相似性、湍流速度谱和协谱、湍流强度及湍流动能的变化特征。结果表明,该小型农业养殖塘上1 d内约21 h大气处于不稳定状态;Monin-Obukhov相似理论适用于该农业养殖塘;三维风速归一化标准差随大气稳定度的变化符合1/3次方规律,不稳定条件下的拟合效果优于稳定条件下,且以垂直方向上拟合效果最佳,温度和湿度的归一化标准差在大气不稳定时符合-1/3次方规律;三维风速的湍流谱在惯性子区中符合-2/3次方关系,垂直风速与标量的协谱在惯性子区中符合-4/3次方规律,涡度相关系统能够观测该小型农业养殖塘上的感热、潜热和CO2通量;该小型农业养殖塘上的湍流强度随风速衰减的速度快于大型湖泊,风速大于1 m·s-1时湍流强度趋近于常数,且水平方向上的湍流强度大于垂直方向;该小型农业养殖塘上的湍流动能在中性条件下最大（3.0 m2·s-2）,且以风切变贡献为主;湍流动能随风速增大而增大,并呈现昼高夜低的变化特征。上述结果可为明确小型水体上的大气湍流特征及小型农业养殖塘与大气之间能量和物质的交换机制奠定一定的理论基础。 相似文献
19.
Woods JA 《Science (New York, N.Y.)》1972,177(4054):1100-1102
Radiance gradients determined from data gathered by the infrared spectrometers aboard the Nimbus III and Nimbus IV satellites were related to the probabilities of clear air turbulence, as inferred from regular pilot reports. Such radiance gradients represent rather large-scale vertical wind shear. Clear air turbulence is least likely in regions where the radiance gradient is small. The results of this exploratory study show that satellite data can be used to design flight paths so that the probability of encountering clear air turbulence is extremely small. 相似文献
20.
The "excessive functional activity" of some cancer cells first found by Caspersson has been observed in fixed, stained smears of cervical epidermoid carcinomas from four patients. Preliminary results suggest that there may be a characteristic difference between the absorption profiles of some epidermoid cancer cells and other cells found in cytological smears. It is our belief that with an appropriate electronic scanning system such cells can be detected by measurements of their absorptions at two different wavelengths. However, the effect on the absorptions of cells with abnormalities other than cancer, and whether every epidermoid carcinoma will contain such cells, must yet be determined. 相似文献