Some Thoughts On Local Isotropy And The 4/3 Lateral To Longitudinal Velocity Spectrum Ratio     

C. A. Biltoft 《Boundary-Layer Meteorology》2001,100(3):393-404

Applications of Kolmogorov's universal equilibrium hypothesis and the Taylor transform to velocity spectra derived from measurements within the low frequency portion of the atmospheric surface layer (ASL) inertial subrange are examined. The measured ratios of lateral to longitudinal velocity component spectra exhibit considerable scatter, but suggest convergence towards 1.0 rather than towards the 4/3 expected from theory. Shear and buoyancy introduce anisotropy to the inertial subrange, thereby contributing to the observed scatter. The apparent discrepancy between the 4/3 velocity component spectrum ratio expected from theory and the measurements could arise as a consequence of the processing used to produce spectra. These processing effects must be considered from the perspective of the propagating eddy. Spectral averaging used with sonic anemometer data is done over time periods that are large with respect to inertial subrange eddy correlation decay times. This averaging causes energy from larger scale eddies to appear as `local convection' that dominates the Taylor transform. Spectrum ratio convergence and cospectra approaching zero are necessary, but not sufficient, conditions for onset of local isotropy. Measurements of spectrum ratios and cospectra over the entire inertial subrange are needed to determine whether or not local isotropy might occur within the ASL.  相似文献   

Evidence of Interaction Between Synoptic and Local Scales in the Surface Layer over the Paris Area     

Laurent Menut  Cyrille Flamant  Jacques Pelon 《Boundary-Layer Meteorology》1999,93(2):269-286

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Experimental investigation of single blade and propeller loads by free running model test. Straight ahead sailing     

《Applied Ocean Research》2019

The understanding and the accurate assessment of propeller loads in realistic operative scenario, both design and off-design conditions, is of paramount importance to design low emission and comfortable ships, fulfilling the requirements of structural integrity of the propulsion system, safety and continuity of operations at sea. To this purpose, a deeper characterization of the propeller functioning, in terms of averaged and fluctuating loads, can be attained by means of the quantification of the single blade loads. In this work, a novel set up that allows to monitor the loads developed by a single blade was implemented on a free running, self propelled maneuvering model with the aim to investigate in details the hydrodynamic interactions between the spatially non-homogeneous and time-variant wake of the hull and the propeller and a complete characterization of its performance. This preliminary work introduces the experimental setup and provides a preliminary overview of the results relative to straight ahead motion.  相似文献   

Analysis and numerical simulation of the meteorological observations at a tropical coastal site Chennai in India     

R. Venkatesan  M. Rajendran  Sikha Dasgupta 《Atmospheric Research》2009,92(4):505-521

Meteorological measurements were carried out at North Chennai semi rural area during pre-monsoon period as a part of an air quality study program. Analysis of the data showed the effects of coastal terrain namely the land-sea breeze circulation, temperature cooling during the sea breeze, difference in onset times at these sites etc. Sea breeze onset was observed with a sharp turning of the wind from westerly to south easterly associated with rise in wind speed. Advection speed of the front was about 2.0 m s^− 1. A simple mesoscale meteorological model (MAM-I) developed at Kalpakkam for coastal atmospheric dispersion estimation was used to simulate the observed characteristics. All the major features observed could be simulated by the model while significant difference was noticed in sea breeze frontal movement. MAM results were also inter-compared with MM5. There were no significant differences in the estimate of mean parameters by both the models. It is concluded that the simple model, which takes less run time in a desktop PC, is adequate enough for practical application of providing wind field for plume dispersion models at coastal sites.  相似文献   

Development of hybrid URANS-LES methods for flow simulation in the ship stern area     

N. Kornev  L. Kleinsorge 《Ocean Engineering》2011,38(16):1831-1838

The paper presents the results of the application of a new hybrid URANS-LES method for the investigations of the ship wake behind the tanker KVLCC2. The switching between URANS and LES models is based on the ratio between the turbulence scale and the cell size of the mesh. Ship resistance, fields of the axial velocity and turbulent kinetic energy in the propeller plane are calculated and compared with measurements. Much attention is paid to the analysis of the unsteady velocities, their PDF distributions and spectra. Numerical analysis shows that the instantaneous velocities deviate substantially from their mean values which are usually used as the estimated velocities in modern engineering methodologies. The thrust variation in the unsteady wake is more than twice as large as that in the time averaged (frozen) wake. The results of the present study point out that the unsteadiness in the wake behind full ships can be very large and should be taken into account when propulsion and unsteady loadings are determined.  相似文献   

海上风电测风塔的选型     

万文涛 《海洋地质译丛》2011,(1):90-94

风资源的获取与评估是风电前期投资的一项重要工作。作为海上风电,需要动用船舶、浮吊等海上工程机械,施工局限性大,成本高。对目前国内外已经使用的各种测风塔进行了比较,分析了各种测风塔的优缺点,选出适用于海上风电的测风塔塔架和基础形式,为海上风电的开发提供了理论指导。  相似文献   

Effects of Turbulence and Flow Inclination on the Performance of Cup Anemometers in the Field     

K. H. Papadopoulos  N. C. Stefantos  U. S. Paulsen  E. Morfiadakis 《Boundary-Layer Meteorology》2001,101(1):77-107

Four commercial and one research cup anemometers were comparatively tested in a complex terrain site to quantify the effects of turbulence and flow inclination on the wind speed measurements. The difference of the mean windspeed reading between the anemometers was as much as 2% for wind directions where the mean flow was horizontal. This difference was large enough to be attributed to the well-known overspeeding effect related to the differing distance constant (ranging from 1.7 to 5 m) of the cup anemometers. The application of a theoretical model of the cup-anemometer behaviour in athree-dimensional turbulent wind field proved successful in explaining theobserved differences.Additional measurements were taken with the anemometers tilted at known angles into and out of the incident wind flow. Thus, a field-derived angular response curve is constructed for each anemometer and the deviations from publishedwind-tunnel results are discussed.The uncertainties of, or false assumptions about, the angular response characteristics of the anemometers contribute the largest amount inthe observed errors of mean wind speed even for a horizontal mean flow. The angular response curves are finally used to correct the 10-min mean windspeed. The necessary information for the correction is the turbulent intensity (preferably in the vertical direction) and the mean flow inclination.For demanding applications, the angular response parameters of cup anemometers should be taken into account. The incorporation of the angular response parameters in a correction scheme would be most robustly applied if their variation with inclination and wind speed was smooth.  相似文献   

沙尘暴期间的温度脉动特征分析     

谢亮  张静红 《中国沙漠》2011,30(3):649-654

 对沙尘暴期间的近地表0.5 m、1.0 m、8.0 m和16 m的高频风速和温度进行了同步测量,风速和温度的测量频率均为50 Hz。结果表明,4月24日发生在民勤地区的沙尘暴的前2 h为特强沙尘暴,其后为沙尘暴阶段,且在24—27日的沙尘暴期间风速经历了3个增大、平稳和衰减的变化阶段。对第一阶段(24日19时至25日18时)的分析表明,特强沙尘暴期间温度的脉动强度明显增大,其变化原因是温度平流输送引起的,而在其后沙尘暴阶段,温度的均值和脉动强度的变化则基本上是辐射变化导致的。对温度脉动的概率密度计算结果表明,沙尘暴期间温度脉动服从高斯分布,说明它不受沙尘颗粒的影响;而温度脉动的谱分析表明,在低于1 Hz的频率范围内证实了浮力标度区的存在,当频率高于1 Hz时分析表明,夜间和正午的温度谱有明显区别。  相似文献   

A novel maneuverable propeller for improving maneuverability and propulsive performance of underwater vehicles     

《Applied Ocean Research》2019

The existing propulsor that can perform both propulsion and maneuvering along axis of rotation is propeller/rotor for a helicopter. Helicopter propellers when maneuvering increase or decrease their blades’ pitch cyclically to create imbalanced thrust and hence maneuvering force/torque. A “maneuverable propeller” was developed and its performance on both maneuvering and propulsion is assessed. The “maneuverable propeller” is an alternative of the existing helicopter rotors. The novelty of this propulsor is that the imbalanced thrust force/torque is created by cyclically increasing or decreasing the angular speed of their blades relatively to the hubs/shafts, to provide the desired maneuvering torque. This maneuverable propeller is hence defined as the Cyclic Blade Variable Rotational Speed Propeller (CBVRP). One of the best advantages is that the maneuvering torque created by the “maneuverable propeller” is much higher, about 5 times of the shaft torque of the same propeller at thrust only mode. The “maneuverable propeller” has wide applications for both surface ships and underwater vehicles that require high maneuverability for cruising inside the narrow passage.  相似文献   

风杯风速仪过高效应的订正研究     

张宏升  朴淳雄 《应用气象学报》1999,10(3):257-266

该文基于对风杯风速仪动力方程的数值计算和风杯风速仪与超声风速仪平行对比的观测结果,发现专用于近地面层精细风廓线测量的轻型低阈值风杯风速仪测风的过高效应引起的u-error约为1%,因风速平均方法不同（标量平均和矢量平均）引起的DP-error是风杯风速仪测风过高的主要原因,在小风情况下更为明显。DP-error经修正后可获得合理的风速值。  相似文献