Turbulence Structure in the Wake Region of a Meteorological Tower   总被引：2，自引：0，他引：2  

Christian Barthlott  Franz Fiedler 《Boundary-Layer Meteorology》2003,108(1):175-190

A meteorological tower significantly modifies the air flow, the mean windspeed and wind direction as well as the turbulencestructure of the air. Suchchanges can be noticed in particular in the wake region of the tower.Measurementson the 200 m tower ofForschungszentrum Karlsruhewere carried outusing Solent sonic anemometers in the lee of the towerand cup anemometers on both sides.In the wake region, spectral energydensity is increased in the high-frequency range. Superposition of this disturbance spectrum on the undisturbedspectrum yields a `knee' in the resulting spectrum. In the case of low turbulence intensity with stable stratification,a plateau with a constant energy content is observed in front of the knee.This effect is caused by the new production of turbulence energy from the mean flow as well as by an energy transfer fromlarger to smaller vortices. Power spectra in strongly stable conditionsshow a more rapid decrease of intensity in the region where the inertialsubrange is expected.The relevant scales of wake turbulence are derived from the maximum of the disturbance spectrum.Locations of the high-frequency peak do not depend on atmospheric stability,but are controlled mainly by mean wind speed.Apart from the reduction of the mean wind speed, the spectra and cospectra exhibit a strong anisotropy for such cases.The results demonstrate the significant influence of a tower on turbulence spectra in the wake region.  相似文献   

应用非线性高阶谐波衰减和尾波干涉监测高温作用后水泥制品的损伤演化     

张尧  马强  肖武军 《震灾防御技术》2018,13(1):52-64

非线性高阶谐波和尾波波速变化均能够反映水泥材料内部微结构的应力变化。利用高阶谐波和尾波干涉实验测量系统,对引入高温作用后的3类不同粒径共6块水泥试样进行单轴加载的损伤演化实验,并与无高温作用的完整试样的实验结果进行对比。结果表明,从初始状态到25%抗压强度的过程中,高温作用后的试样的谐波幅值和尾波波速变化出现明显增强的现象（谐波幅值最大增幅约20%）,而无高温作用的完整试样的谐波幅值和尾波波速变化较为平稳（谐波幅值最大增幅约5%）;在达到65%抗压强度的过程中,高温作用后的试样的谐波幅值和尾波波速变化急剧增大（谐波幅值最大增幅约100%）,且粒径较大的试样的增幅高于粒径较小的试样,而无高温作用的完整试样的谐波幅值和尾波波速变化的增幅较小（谐波幅值最大增幅约10%）;当抗压强度超过75%以后,高温作用后的试样的谐波幅值和尾波波速变化急剧衰减（谐波幅值最大衰减幅度约140%）,而无高温作用的完整试样的谐波幅值和尾波波速的最大衰减幅值在40%以内。基于以上观测结果对高温作用后水泥制品损伤演化的物理机制以及这两类监测方法的适用性进行了讨论。  相似文献   

Elastic properties of sands,Part 1: Micro computed tomography image analysis of grain shapes and their relationship with microstructure     

Zubair Ahmed  Maxim Lebedev 《Geophysical Prospecting》2019,67(4):723-744

Elastic properties of an unconsolidated sand are largely dependent on the elastic properties of its constituent grain and the micro-structure that defines how the grains are arranged within themselves. Coordination number, that is the average number of contacts a grain has with its neighbours, and contact surface area are the two parameters closely related to the microstructure. Moreover, grain shapes and sorting also have substantial influence on these parameters. To calculate these parameters and find any potential relationships with the shape factors, we acquire high-resolution micro computed tomography images of four mechanically compacted unconsolidated dry sand samples that are of different shape factors and sorting indices. After a comprehensive voxel-based data processing, we calculate shape factors such as sphericity and roundness of each grain in all samples. Using own algorithm, we then calculate the coordination number and contact surface area. Results show that samples of well-sorted and higher spherical and rounded grains have higher coordination number and contact surface area than the samples of poorly sorted and lower spherical and rounded grains. Among the poorly sorted samples, coordination number is largely dependent on the fraction of larger grain sizes present in the sample. Inside any given sample, grains of lower sphericity tend to have higher coordination numbers. Moreover, more spherical and rounded grains have greater contact surface area with their neighbours.  相似文献   

Estimating the turbulent energy dissipation rate in an airport environment     

Michael Frech 《Boundary-Layer Meteorology》2007,123(3):385-393

This note reports on the influence of aircraft wake vortices on the estimation of the turbulent energy dissipation rate using sonic anemometer measurements near the runway threshold. The wake vortex traces, which are generated at a height of about 65 m and subsequently evolve in ground effect, are clearly visible in the velocity components and temperature. The observed temperature increase of 1 K appears related to the stably stratified atmospheric surface layer. The dissipation rate is estimated from the longitudinal velocity power spectrum for a sample in a nocturnal boundary layer with and without a wake vortex signal. In both cases an inertial subrange is found. For the analyzed sample the estimated dissipation rate is a factor of ten larger compared to the undisturbed sample. Implications for operational wake avoidance systems are discussed.  相似文献   

Behaviour Of An Ultrasonic Anemometer Under Cloudy Conditions     

Holger Siebert  Ulrich Teichmann 《Boundary-Layer Meteorology》2000,94(1):165-169

The use of ultrasonic anemometers under cloudy weather conditions is often doubted, even rejected or not discussed in the literature. To investigate the influence of liquid water content on the behaviour of sonic anemometers a small intercomparison experiment using an ultrasonic anemometer, a fast response propeller anemometer, and a Particulate Volume Monitor was performed at the German Environmental Research Station Schneefernerhaus near the top of Mt. Zugspitze. The results obtained under different conditions (dry/cloudy) give no significant hint on such an influence.  相似文献   

Flow field visualization of sediment-laden flow using ultrasonic imaging     

Martin Crapper  Tom Bruce  Ccile Gouble 《Dynamics of Atmospheres and Oceans》2000,31(1-4)

This investigation concerns the application of particle image velocimetry (PIV) to ultrasonic images of sediment-laden flow in order to obtain accurate, quantitative, two dimensional vector maps of the flow field within the sediment suspension. Experiments were carried out using a medical ultrasound scanner to obtain ultrasonic images in a kaolin/salt water suspension of mean concentration 16 g/l and to compare results obtained with the ultrasound scanner in clear water with those from established measurement techniques. Measurements of mud were obtained, and the comparison with established methods indicated that the application of PIV to the analysis of ultrasound images yields velocity information accurate to within ±15%. This measurement technique will be of great value in furthering the understanding of the behavior of sediment-laden flows.  相似文献   

岩土超声波测试研究进展及应用综述     

王宇  李晓  胡瑞林  武艳芳  高玮 《工程地质学报》2015,23(2):287-300

超声波在岩土介质中传播时携带了许多与岩土物理力学性质有关的信息,这些信息可综合反映到声学参数如超声波速、衰减系数、波形、频率、频谱及振幅等的变化上,根据这些参数可反演得到岩土体的物理力学指标及细观结构特征,从而解决一系列的岩土工程问题。首先对超声波无损检测可以解决的岩土工程问题进行了概述。其次系统地阐述了土体和岩石(体)超声试验的研究进展:土体强度参数声学特性的试验研究、土体微结构参数的超声探测、岩石强度参数的超声测试研究、超声波速在岩石破裂方面的应用研究、超声波声波衰减在岩石材料中的应用、超声波速度和衰减各向异性的研究及超声波流体流动试验的研究。最后,笔者将超声波测试拓宽到土石混合体研究领域,探讨了不同含石量试样的波速和衰减规律,以及损伤声学参数对试样在单轴压缩作用下裂纹扩展的细观损伤特性的研究,更进一步说明了土石混合材料即不有别于土也不同于岩的特殊性。在以上研究基础上,基于超声波岩土试验国内外研究动态的总结,对其未来的发展趋势进行了展望。  相似文献   

三维超声风速仪观测中风向角计算方法   总被引：2，自引：0，他引：2  

姜明  景元书  郭建侠  王有恒 《气象科技》2011,39(5):615-619

在对三维超声风速仪观测资料处理中,风向角的计算具有一定的技巧性。根据三维超声风速仪自身观测坐标系统与地理坐标系之间的差异,推导出计算地理坐标系下风向角的一种方法,为超声风观测资料的处理和应用提供借鉴。同时利用锡林浩特国家气候观象台2008年全年通量观测资料对风向角进行了计算,并与台站统计资料进行了对比,结果表明两组数据无论从风向角分布趋势还是从相关性来看,均非常吻合（相关系数为0.97）,从而证明了该方法的正确性及可应用性。  相似文献   

A field study of mean and turbulent flow characteristics upwind,over and downwind of barchan dunes   总被引：1，自引：0，他引：1  

Matthew C. Baddock  Giles F. S. Wiggs  Ian Livingstone 《地球表面变化过程与地形》2011,36(11):1435-1448

Field‐measured patterns of mean velocity and turbulent airflow are reported for isolated barchan dunes. Turbulence was sampled using a high frequency sonic anemometer, deriving near‐surface Reynolds shear and normal stresses. Measurements upwind of and over a crest‐brink separated barchan indicated that shear stress was sustained despite a velocity reduction at the dune toe. The mapped streamline angles and enhanced turbulent intensities suggest the effects of positive streamline curvature are responsible for this maintenance of shear stress. This field evidence supports an existing model for dune morphodynamics based on wind tunnel turbulence measurements. Downwind, the effect of different dune profiles on flow re‐attachment and recovery was apparent. With transverse incident flow, a re‐attachment length between 2·3 and 5·0h (h is dune brink height) existed for a crest‐brink separated dune and 6·5 to 8·6h for a crest‐brink coincident dune. The lee side shear layer produced elevated turbulent stresses immediately downwind of both dunes, and a decrease in turbulence with distance characterized flow recovery. Recovery of mean velocity for the crest‐brink separated dune occurred over a distance 6·5h shorter than the crest‐brink coincident form. As the application of sonic anemometers in aeolian geomorphology is relatively new, there is debate concerning the suitability of processing their data in relation to dune surface and streamline angle. This paper demonstrates the effect on Reynolds stresses of mathematically correcting data to the local streamline over varying dune slope. Where the streamline angle was closely related to the surface (windward slope), time‐averaged shear stress agreed best with previous wind tunnel findings when data were rotated along streamlines. In the close lee, however, the angle of downwardly projected (separated) flow was not aligned with the flat ground surface. Here, shear stress appeared to be underestimated by streamline correction, and corrected shear stress values were less than half of those uncorrected. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Determination of the interstitial velocity field in the swash zone by Ultrasonic Doppler Velocimetry (UDV)     

Mustapha Kamel Mihoubi  Michel Bélorgey  Ahmed Kettab 《Comptes Rendus Geoscience》2012,344(5):312-318

The experimental study of the hydrodynamics of the sediments and the physical analysis of the process of exchange at the water-sediment interface in a zone of swash are essential for the understanding of the sedimentary transport processes in coastal environments, in particular to control and forecast the evolution of the coastline. The objective of this study was to exploit the technique of Ultrasonic Doppler Velocimetry (UDV) to examine the evolution of the velocity field at the water-sediment interface in a zone of swash in various conditions of incidental regular waves in a wave flume. The velocities were measured in different conditions of the swash within the sediment bed, and in the fluid vein at the swash edge. These measurements show: an exponential distribution of velocities inside the sediment bed; a difference in the velocity gradient at the water-sediment interface, between the velocity in the water vein at the swash edge and the interstitial flow in the swash. The measurements obtained in the wave flume made it possible to observe an evolution of the velocity profiles according to the phases of the swash, which are characterized by a phase shift between the free flow at the swash edge and the interstitial flow during the two phases of the swash: uprush and backwash.  相似文献