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61.
Wind velocity and temperature measurements from a 200 m tower, locatedin a forest near Karlsruhe were used to investigatethe modified profile function of the wind velocity in theroughness sublayer.To avoid determination of the friction velocity we introduced analternative analysis with the expression
instead of
From the observed Fm* profiles we evaluated the profile function m*. The wind profiles observed under neutral conditions were well representedby a modified non-dimensional profile function with physically based boundary values at the top and at the bottom of theroughness sublayer.The results of our analysis can be used to take into consideration themomentum exchange between the atmosphere and a forest in mesoscaleatmospheric models in a refined way. 相似文献
62.
This study details the observed effects ofatmospheric stability on characteristics of thesurface layer in a low wind speed (U = 1.5 m s-1)regime of tropical West Africa. Theaerodynamic roughness length, z0, anddisplacement height, d, obtained from profilewind-speed data at our bush land site (height 2 m)have values of 0.24 ± 0.10 m and 1.54 ± 0.04 mrespectively. In the unstable range (-2.5 < Ri < -0.1; Riis gradient Richardson number), thestandard deviation in wind speed fluctuations, u, increased from 0.57 ± 0.19 m s-1 toa maximum of 0.7 ± 0.2 m s-1 in near-neutralconditions, and in the stable range, the parameterdecreased rapidly to 0.41 ± 0.15 m s-1 at Ri 0.2.In the same stability range, the horizontal winddispersion, , decreased withincreasing stability from 19 ± 8 deg. to 13 ± 5 deg.The surface-layer integral quantity, u/u*, when plottedas a function of stability, is in agreement with theempirical results. The ratio ofsensible heat flux (estimated) to the net radiationranged between 0.1 and 0.2 at nighttime,increasing to about 0.5 during the daytime, and showeda strong dependency on season. 相似文献
63.
影响测风塔代表性的因素很多,为了合理地确定测风塔观测结果能够代表的区域范围,需要通过适当的方法综合考虑这些因素。湍流通量代表性研究中常用的印痕函数是风速、稳定度、下垫面粗糙度等的函数,可以定量地刻画单点观测结果的代表性。利用印痕函数方法对中国风能资源观测网中来自不同下垫面的44个测风塔2010年的观测结果进行了测风塔资料的代表性分析。首先,根据两层(10、70 m)的风速和温度计算稳定度参数Lp并确定中性层结,粗糙度的计算选取了中性层结条件下满足对数风廓线的资料。然后,利用中性层结下70 m高度主导风向和风向频率较大方位的资料分析了两种印痕函数方法(Schuepp和Hsieh方法)的适用性及代表性范围与粗糙度的关系。两种方法得到的印痕函数对应距离(即代表性范围)比较接近。但是,在复杂山地Schuepp方法已不适用。当粗糙度小于10 m时,粗糙度与印痕函数对应距离的关系可用对数-线性函数描述。根据经验推荐了利用印痕函数积分90%对应的距离以及下垫面显著变化位置来确定测风塔代表范围。但是,印痕函数积分阈值仍需要通过示踪实验或数值模拟检验。 相似文献
64.
Momentum and Heat Transfer over Urban-like Surfaces 总被引:2,自引:2,他引:0
Momentum and heat transfer was examined for the urban-like surfaces used within the Comprehensive Outdoor Scale MOdel (COSMO)
experiments. Simultaneous and comparative meteorological measurements were made over a pair of scale models with different
block geometries. These data were analyzed to investigate the influence of height variations, obstacle elongation, and packing
density, λ
p
, of blocks on the aerodynamic properties. In addition, the robustness of theoretical expressions of bulk transfer coefficients
for momentum and heat with respect to geometric parameters was examined. Our analyses showed: (1) the theoretical framework
for the bulk transfer coefficient for momentum, C
m
, and that for heat, C
h
, was applicable for homogeneous building arrays, (2) the sensitivity of C
h
to the surface geometry was smaller than that of C
m
, (3) the transfer coefficients were increased by variations of block heights, but not by elongation of blocks, (4) first-order
approximations of C
m
and C
h
for an array of blocks with two different heights can be made by applying simple theoretical assumptions to include the effects
of height variation, and (5) variations of block heights increased the momentum flux significantly, but caused little change
in the sensible heat flux. This can be explained by the feedback mechanism of aerodynamic– thermal interaction; aerodynamic
mixing decreased both the advective velocity and the vertical temperature gradient. 相似文献
65.
Pietro Salizzoni Lionel Soulhac Patrick Mejean Richard J. Perkins 《Boundary-Layer Meteorology》2008,127(1):97-110
Flow in the urban boundary layer is strongly influenced by the surface roughness, which is composed principally of isolated buildings or groups of buildings. Previous research has shown that the flow regime depends on the characteristic height of these obstacles (H), and the spacing between them (W). In reality, the urban boundary layer contains roughness elements with a wide range of length scales; in many practical situations these can be classified into large-scale roughness—buildings, or groups of buildings—and small-scale roughness, such as street furniture and elements on the façades and roofs. It is important to understand how the small-scale roughness might modify mass and momentum transfer in the urban boundary layer, but relatively little information is available concerning the potential interaction between large- and small-scale roughness elements in the different flow regimes. This problem has been studied using wind-tunnel experiments, by measuring vertical velocity profiles over a two-dimensional obstacle array, adding small-scale roughness elements to the top of larger parallel square bars. The experiments were performed for different cavity aspect ratios: the results show that the small-scale roughness increases the turbulence intensities and the momentum transfer when the large-scale obstacles are closely packed (H/W > 1) but it has very little effect for more widely-spaced obstacles (H/W < 1). 相似文献
66.
Taro Nakai Akihiro Sumida Kazuho Matsumoto Ken’ichi Daikoku Shin’ichi Iida Hotaek Park Mie Miyahara Yuji Kodama Alexander V. Kononov Trofim C. Maximov Hironori Yabuki Toshihiko Hara Takeshi Ohta 《Boundary-Layer Meteorology》2008,128(3):423-443
We used an aerodynamic method to objectively determine a representative canopy height, using standard meteorological measurements.
The canopy height may change if the tree height is used to represent the actual canopy, but little work to date has focused
on creating a standard for determining the representative canopy height. Here we propose the ‘aerodynamic canopy height’ h
a as the most effective means of resolving the representative canopy height for all forests. We determined h
a by simple linear regression between zero-plane displacement d and roughness length z
0, without the need for stand inventory data. The applicability of h
a was confirmed in five different forests, including a forest with a complex canopy structure. Comparison with stand inventory
data showed that h
a was almost equivalent to the representative height of trees composing the crown surface if the forest had a simple structure,
or to the representative height of taller trees composing the upper canopy in forests with a complex canopy structure. The
linear relationship between d and z
0 was explained by assuming that the logarithmic wind profile above the canopy and the exponential wind profile within the
canopy were continuous and smooth at canopy height. This was supported by observations, which showed that h
a was essentially the same as the height defined by the inflection point of the vertical profile of wind speed. The applicability
of h
a was also verified using data from several previous studies. 相似文献
67.
大规模风电场建成后对风能资源影响的研究 总被引:2,自引:0,他引:2
考虑了千万千瓦级风电基地建成后风电机群对近地面层风速的影响,采用Frandsen研究了大规模风电场内部风速损失时所使用的方法,在内边界层已经充分发展成新边界层的区域内,对轮毂高度65m处风速Uh进行了计算。结果表明,风电场建成后研究区内,风速Uh与未建场时的65m风速U0相比变小,存在风速损失,该风速损失随着U0的增大而减小,与风电机的推力系数CT性质有关;大规模风电场建成后,Uh在3~20m.s-1范围内的平均风功率密度与未建场时U0在此范围内的平均风功率密度相比损失约为58.45%,这与建场地区建场前65m处风速值大小以及各风速值出现的概率有关。 相似文献
68.
69.
Studies on critical Reynolds number indices for wind-tunnel experiments on flow within urban areas 总被引:2,自引:2,他引:0
Reynolds-number dependence of flow fields within a modelled urban area was studied in a wind tunnel. We measured flow around a single model building and around model city blocks at various wind speeds, and studied Reynolds number indices more appropriate than the building Reynolds number. Our results led to the following conclusions. Firstly, the flow around the models in the wind tunnel was roughly divided into three parts according to the intensities of viscous stress and Reynolds stress as follows: (1) the flow in the vicinity of the ground or the surfaces of the model, where viscous stress became dominant under certain conditions; (2) the flow detached from the surfaces of the model, where Reynolds stress was always dominant; and (3) the flow around the separation bubble at the leading edge of the building model, where the influences of both viscous stress near the wall and the Reynolds stress in the separated boundary layer were mixed.Secondly, the critical Reynolds number of the flow in the modelled urban area could be defined by using both the roughness Reynolds number Rez0 (= z0u*/) and the dimensionless height z+ (= zu*/). Reynolds-number independence could be expected for whole flow fields in the modelled urban areas as long as the critical values of Rez0 and z+ were satisfied. 相似文献
70.
A comprehensive study of the turbulent structure of the atmospheric boundary layer in unstable conditions has been carried out using turbulence data obtained from the Gobi desert, grassland, suburban and urban sites based on the same instrumentation, data acquisition and data processing systems. The normalized standard deviations of u and v over the suburban and urban sites are systematically smaller than those over the Gobi desert and grassland sites. However, the normalized standard deviations of w, temperature and humidity q over the suburban and urban sites are quite close to those over the Gobi desert and grassland sites. The normalized humidity standard deviations are quite similar to those of temperature over the grassland and suburban sites. The temperature and humidity spectra are found to be independentof atmospheric stability in all frequency ranges, but spectra obtained from the Gobi desert and grassland sites reveal flatter peaks than those of the suburban and urban sites in the lower frequency range. The normalized spectral curves of temperature and humidity are independent of atmospheric stability both at high and low frequencies. The spectral characteristics of humidity over the grassland and suburban sites aresimilar to those of temperature, but the humidity spectra over the Gobi desert site are quite different from temperature spectra due to evaporation and/or large eddies in the boundary layer. 相似文献