共查询到20条相似文献，搜索用时 31 毫秒
1.
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
2.
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
3.
The nature and origin of turbulence and magnetic fields in the intergalactic space are important problems that are yet to be understood. We propose a scenario in which turbulent-flow motions are induced via the cascade of the vorticity generated at cosmological shocks during the formation of the large-scale structure. The turbulence in turn amplifies weak seed magnetic fields of any origin. Supercomputer simulations show that the turbulence is subsonic inside clusters and groups of galaxies, whereas it is transonic or mildly supersonic in filaments. Based on a turbulence dynamo model, we then estimated that the average magnetic field strength would be a few microgauss (microG) inside clusters and groups, approximately 0.1 muG around clusters and groups, and approximately 10 nanogauss in filaments. Our model presents a physical mechanism that transfers the gravitational energy to the turbulence and magnetic field energies in the large-scale structure of the universe. 相似文献
4.
Babcock HW 《Science (New York, N.Y.)》1990,249(4966):253-257
From the earliest days and nights of telescopic astronomy, atmospheric turbulence has been a serious detriment to optical performance. The new technology of adaptive optics can overcome this problem by compensating for the wavefront distortion that results from turbulence. The result will be large gains in resolving power and limiting magnitude, closely approaching the theoretical limit. In other words, telescopic images will be very significantly sharpened. Rapid and accelerating progress is being made today by several groups. Adaptive optics, together with the closely related technology of active optics, seems certain to be utilized in large astronomical telescopes of the future. This may entail significant changes in telescope design. 相似文献
5.
Shear flows undergo a sudden transition from laminar to turbulent motion as the velocity increases, and the onset of turbulence radically changes transport efficiency and mixing properties. Even for the well-studied case of pipe flow, it has not been possible to determine at what Reynolds number the motion will be either persistently turbulent or ultimately laminar. We show that in pipes, turbulence that is transient at low Reynolds numbers becomes sustained at a distinct critical point. Through extensive experiments and computer simulations, we were able to identify and characterize the processes ultimately responsible for sustaining turbulence. In contrast to the classical Landau-Ruelle-Takens view that turbulence arises from an increase in the temporal complexity of fluid motion, here, spatial proliferation of chaotic domains is the decisive process and intrinsic to the nature of fluid turbulence. 相似文献
6.
After mass spawning events, coral embryos, lacking the protective capsule of other metazoans, are directly exposed to the environment at the ocean surface. Here, we present evidence that modest turbulence disrupts the integrity of these embryos, which fragment into totipotent cells that develop into proportionately smaller functional larvae. The level of turbulence required to fragment coral embryos can be generated from small wind-generated waves, which occur frequently during coral spawning on the Great Barrier Reef. The formation of planktonic coral clones, through natural embryo fragmentation of broadcast spawn, is a previously unknown mode of reproduction in the animal kingdom. 相似文献
7.
通过研究过渡状态下的水流紊动规律，层流、过渡状态、充分发展的紊流可以被认为是一发展、渐变、模糊的过程。基于此，通过引进模糊数学的隶属度，过渡状态的紊动规律可表述为程度的层流与程度的紊流的综合作用，导出了过渡状态下时均流速公式和阻力系数公式。并将其应用于明渠水流，理论结果与试验资料比较符合。 相似文献
8.
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
9.
Direct numerical simulations of wall-bounded flow reveal that turbulence production can be suppressed by a transverse traveling wave. Flow visualizations show that the near-wall streaks are eliminated, in contrast to other turbulence-control techniques, leading to a large shear stress reduction. The traveling wave can be induced by a spanwise force that is confined within the viscous sublayer; it has its maximum at the wall and decays exponentially away from it. We demonstrate the application of this approach in salt water, using arrays of electromagnetic tiles that produce the required traveling wave excitation at a high efficiency. 相似文献
10.
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
11.
Stratospheric wave spectra resembling turbulence 总被引：1，自引：0，他引：1
Dewan EM 《Science (New York, N.Y.)》1979,204(4395):832-835
Pollution effects on ozone raise the question of the significance of turbulence in vertical transport in the stratosphere. The aircraft in situ measurements of velocity fluctuations previously employed to estimate turbulence transport were, it is hypothesized, due to atmospheric waves, despite their classical turbulence spectrum. This new hypothesis implies that previous turbulence estimates are invalid. Experimental tests are suggested. 相似文献
12.
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
13.
Observations of the sun reveal highly complex flows and magnetic structures that must result from turbulent convection in the solar envelope. A remarkable degree of large-scale coherence emerges from the small-scale turbulent dynamics, as seen in the cycles of magnetic activity and in the differential rotation profile of this star. High-performance computing now permits numerical simulations of compressible turbulence and magnetohydrodynamics with sufficient resolution to show that compact structures of vorticity and magnetic fields can coexist with larger scales. Such structured turbulence is yielding transport properties for heat and angular momentum at considerable variance with earlier models. These simulations are elucidating the coupling of turbulent fluid motions with rotation and magnetic fields, which must control the interlinked differential rotation and magnetic dynamo action. 相似文献
14.
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
15.
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
16.
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
17.
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
18.
灌水器流道结构对水力性能影响的数值分析 总被引：3，自引：0，他引：3
采用计算软件Fluent,通过将壁面粗糙元抽象为多孔介质的微尺度效应处理方法进行建模,选用realizable k-ε模型,对3种齿形流道的灌水器内流进行数值研究。利用Micro-PIV试验测量结果验证了该模拟方法的可靠性,在多种流动条件下对3种齿形结构灌水器进行数值计算,并进行抗堵性能和水力性能分析。抗堵性分析表明,3种齿形流道在转角内侧都存在回流区,由于其侧均为高速主流而使颗粒沉积缓慢;尖角齿形流道的尖角部位,大部分区域由于具有一定的紊流强度也不易沉积杂质,只有其齿尖和平角齿形流道的左顶角部位易产生颗粒沉积。在水力性能分析中,通过对压力降和流态指数的比较表明,尖角齿形的灌水器在控制和稳定出流方面具有优势。 相似文献
19.
交联壳聚糖/活性炭复配吸附剂对硝酸盐氮的吸附 总被引：1，自引：0，他引：1
制备了以壳聚糖和粉末活性炭为原料的戊二醛交联壳聚糖/活性炭复配吸附剂,并探讨了复配吸附剂对水中硝酸盐的吸附性能的影响。结果表明,在壳聚糖/粉末活性炭复配比6∶4、质子化时间30 min、吸附剂用量为每25 mL用吸附剂0.2 g、硝酸盐氮起始浓度为10 mg/L时,复配吸附剂对废水中的硝酸盐氮的去除率最高,达到70.6%。吸附过程符合二级动力学方程,其相关系数达到0.9998。这为提高壳聚糖的综合利用价值,解决农业硝酸盐污染提供了一条有效的途径。 相似文献
20.
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