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1.
Multiple windbreaks: An aeolean ensemble 总被引：1，自引：0，他引：1
Near-neutral measurements of the turbulent wind field within and above a sequence of 15 parallel windbreaks on a flat pastoral site are presented. The windbreak fences each had a porosity of 60% and were equally-spaced at 6 times their height (h = 2 m). The following conclusions seem justified for wind directions within 10 ° of the normal to the array:
- Above the windbreaks (2h), mean windspeeds first decreased and then increased asymptotically to a value in equilibrium with the new surface roughness. At 0.5h, windspeeds exhibited a slow increase down the entire array.
- Reflecting differences in approach flows, the drag on the initial fence was almost twice that on barriers farther downstream. This reduction in momentum extraction per windbreak was associated with an elevation in the zero-plane displacement to a level equal to 0.8h.
- At positions well-removed from the initial fences, mean windspeeds were reduced throughout the entire region below shelter height. In this region, the flow became increasingly dominated by downward moving air with velocities much greater than the local average. The zone of reduced turbulence was small, extending only 2h downstream of a barrier at a height of 0.25h. This corresponded with the region excluded from smoke trails released at the top of windbreaks.
- An approximate TKE budget mid-way between windbreaks 7 and 8 suggests that shear and wake production peak near z = h and that production is balanced by dissipation and vertical transport components. Advective and inertial interaction terms are negligible at this midway position but are likely to be major sources of TKE closer to the windbreak. Local equilibrium is attained above z = 1.5h implying the existence of a constant-stress layer.
2.
The potential for porous windbreaks to enhance wind-turbine power production is studied using linearized theory and wind-tunnel experiments. Results suggest that windbreaks have the potential to substantially increase power production, while lowering mean shear, and leading to negligible changes in turbulence intensity. The fractional increase in turbine power output is found to vary roughly linearly with windbreak height, where a windbreak 10% the height of the turbine hub increases power by around 10%. Wind-tunnel experiments with a windbreak imposed beneath a turbulent boundary layer show the linearized predictions to be in good agreement with particle-image-velocimetry data. Power measurements from a model turbine further corroborate predictions in power increase. Moreover, the wake of the windbreak showed a significant interaction with the turbine wake, which may inform windbreak use in large wind farms. Power measurements from a second turbine downwind of the first with its own windbreak show that the net effect for multiple turbines is dependent on windbreak height. 相似文献
3.
Large-Eddy Simulation of Windbreak Flow 总被引：13，自引：10，他引：3
Edward G. Patton Roger H. Shaw Murray J. Judd Michael R. Raupach 《Boundary-Layer Meteorology》1998,87(2):275-307
A large-eddy simulation has been performed of turbulent flow around multiple windbreaks set within a wheat canopy under neutral stability conditions. The simulation is validated against a wind tunnel data set taken under similar conditions. Velocity profiles and second-order statistics are presented and compared to those found in the wind tunnel. From the numerical simulation, we discuss spatial distributions of instantaneous velocity fields and pressure statistics, which are important and telling features of the flow that are difficult to measure experimentally. We present a discussion of the momentum balance at various locations with respect to the windbreak, and similarly, we introduce the budget of a passive scalar. These discussions show the importance of the terms in each budget equation as they vary upstream and downstream of the windbreak. 相似文献
4.
A Simple Model for Spatially-averaged Wind Profiles Within and Above an Urban Canopy 总被引：2，自引：2，他引：0
This paper deals with the modelling of the flow in the urban canopy layer. It critically reviews a well-known formula for
the spatially-averaged wind profile, originally proposed by Cionco in 1965, and provides a new interpretation for it. This
opens up a number of new applications for modelling mean wind flow over the neighbourhood scale. The model is based on a balance
equation between the obstacle drag force and the local shear stress as proposed by Cionco for a vegetative canopy. The buildings
within the canopy are represented as a canopy element drag formulated in terms of morphological parameters such as λ
f
and λ
p
(the ratios of plan area and frontal area of buildings to the lot area). These parameters can be obtained from the analysis
of urban digital elevation models. The shear stress is parameterised using a mixing length approach. Spatially-averaged velocity
profiles for different values of building packing density corresponding to different flow regimes are obtained and analysed.
The computed solutions are compared with published data from wind-tunnel and water-tunnel experiments over arrays of cubes.
The model is used to estimate the spatially-averaged velocity profile within and above neighbourhood areas of real cities
by using vertical profiles of λ
f
. 相似文献
5.
The dependence on atmospheric stability of flow characteristics adjacent to a very rough surface was investigated in a larch
forest in Japan. Micrometeorological measurements of three-dimensional wind velocity and air temperature were taken at two
heights above the forest, namely 1.7 and 1.2 times the mean canopy height h. Under near-neutral and stable conditions, the observed turbulence statistics suggest that the flow was likely to be that
of the atmospheric surface layer (ASL) at 1.7h, and of the roughness sublayer (RSL) at 1.2h. However, in turbulence spectra, canopy-induced large coherent motions appeared clearly at both heights. Even under strongly
stable conditions, the large-scale motions were retained at 1.2h, whereas they were overwhelmed by small-scale motions at 1.7h. This phenomenon was probably due to the enhanced contribution of the ASL turbulence associated with nocturnal decay of the
RSL depth, because the small-scale motions appeared at frequencies close to the peak frequencies of well-known ASL spectra.
This result supports the relatively recent concept that canopy flow is a superimposition of coherent motions and the ASL turbulence.
The large-scale motions were retained in temperature spectra over a wider region of stability compared to streamwise wind
spectra, suggesting that a canopy effect extended higher up for temperature than wind. The streamwise spacing of dominant
eddies according to the plane mixing-layer analogy was only valid in a narrow range at near neutral, and it was stabilised
at nearly half its value under stable conditions. 相似文献
6.
We analyse single-point velocity statistics obtained in a wind tunnel within and above a model of a waving wheat crop, consisting of nylon stalks 47 mm high and 0.25 mm wide in a square array with frontal area index 0.47. The variability of turbulence measurements in the wind tunnel is illustrated by using a set of 71 vertical traverses made in different locations, all in the horizontally-homogeneous (above-canopy) part of the boundary layer. Ensemble-averaged profiles of the statistical moments up to the fourth order and profiles of Eulerian length scales are presented and discussed. They are consistent with other similar experiments and reveal the existence of large-scale turbulent coherent structures in the flow. The drag coefficient in this canopy as well as in other reported experiments is shown to exhibit a characteristic height-dependency, for which we propose an interpretation. The velocity spectra are analysed in detail; within and just above the canopy, a scaling based on fixed length and velocity scales (canopy height and mean horizontal wind speed at canopy top) is proposed. Examination of the turbulent kinetic energy and shear stress budgets confirms the role of turbulent transport in the region around the canopy top, and indicates that pressure transport may be significant in both cases. The results obtained here show that near the top of the canopy, the turbulence properties are more reminiscent of a plane mixing layer than a wall boundary layer. 相似文献
7.
Numerical Investigations of Mean Winds Within Canopies of Regularly Arrayed Cubical Buildings Under Neutral Stability Conditions 总被引：2，自引：2，他引：0
Recently, several attempts have been made to model the wind velocity in an urban canopy in order to accurately predict the
mixing and transport of momentum, heat, and pollutants within and above the canopy on an urban scale. For this purpose, unverified
assumptions made by Macdonald (Boundary-Layer Meteorol 97:25–45, 2000) to develop a model for the profile of the mean wind
velocity within an urban canopy have been used. In the present study, in order to provide foundations for improving the urban
canopy models, the properties of the spatially-averaged mean quantities used to make these assumptions have been investigated
by performing large-eddy simulations (LES) of the airflow around square and staggered arrays of cubical blocks with the following
plan area densities: λ
p
= 0.05, 0.11, 0.16, 0.20, 0.25, and 0.33. The LES results confirm that the discrepancy between the spatial average of wind
velocity and Macdonald’s five-point average of wind velocity can be large in both types of arrays for large λ
p
. It is also confirmed that Prandtl’s mixing length varies significantly with height within the canopy, contrary to Macdonald’s
assumption for both types of arrays and for both small and large λ
p
. On the other hand, in accordance with Macdonald’s assumption, the sectional drag coefficient is found to be almost constant
with height except in the case of staggered arrays with high λ
p
. 相似文献
8.
This paper argues that the active turbulence and coherent motions near the top of a vegetation canopy are patterned on a plane mixing layer, because of instabilities associated with the characteristic strong inflection in the mean velocity profile. Mixing-layer turbulence, formed around the inflectional mean velocity profile which develops between two coflowing streams of different velocities, differs in several ways from turbulence in a surface layer. Through these differences, the mixing-layer analogy provides an explanation for many of the observed distinctive features of canopy turbulence. These include: (a) ratios between components of the Reynolds stress tensor; (b) the ratio K
H/K
M of the eddy diffusivities for heat and momentum; (c) the relative roles of ejections and sweeps; (d) the behaviour of the turbulent energy balance, particularly the major role of turbulent transport; and (e) the behaviour of the turbulent length scales of the active coherent motions (the dominant eddies responsible for vertical transfer near the top of the canopy). It is predicted that these length scales are controlled by the shear length scale % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamitamaaBa% aaleaacaWGtbaabeaakiabg2da9iaadwfacaGGOaGaamiAaiaacMca% caGGVaGabmyvayaafaGaaiikaiaadIgacaGGPaaaaa!3FD0!\[L_S = U(h)/U'(h)\] (where h is canopy height, U(z) is mean velocity as a function of height z, and % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmyvayaafa% Gaeyypa0JaaeizaiaadwfacaGGVaGaaeizaiaadQhaaaa!3C32!\[U' = {\rm{d}}U/{\rm{d}}z\]). In particular, the streamwise spacing of the dominant canopy eddies is x = mL
s, with m = 8.1. These predictions are tested against many sets of field and wind-tunnel data. We propose a picture of canopy turbulence in which eddies associated with inflectional instabilities are modulated by larger-scale, inactive turbulence, which is quasi-horizontal on the scale of the canopy. 相似文献
9.
10.
Flow around a windbreak in oblique wind 总被引：2，自引：0，他引：2
Ido Seginer 《Boundary-Layer Meteorology》1975,9(2):133-141
Wind speed was measured near the surface, along a line normal to a single, 50 % porous windbreak. The results for near-neutral atmospheric conditions were strongly dependent on the incidence angle of the wind, a, and slightly dependent on the roughness of the surface. A measure of the shelter effect - the protected distance - was found to decrease faster than cos . The drag coefficient of the windbreak decreased proportionally with cos . The effect of increased roughness was to increase the wind speed near the surface in the lee of the windbreak. 相似文献
11.
12.
A Field Study of the Mean Pressure About a Windbreak 总被引：3，自引：0，他引：3
John D. Wilson 《Boundary-Layer Meteorology》1997,85(3):327-358
To provide additional field data for assessingwindbreak flow models, mean ground-level pressurehas been measured upstream and downstream from along porous fence (height H = 1.25 m, resistancecoefficient k
r
= 2.4). Measurements were madeduring periods of near-neutral stability and near-normallyincident flow, with the fence standing on bare soil(roughness length, z
0
0.8 cm;H/z
0
160), or within a plant canopy. The mean pressure field,measured far from the ends of the fence, was foundto be quite insensitive to mean wind direction(
, zero for perpendicular flow), for|
| less than about 25°.In the absence of a canopy, during each measurementperiod the minimum pressure occurred at the closestsampling location to leeward of the windbreak, thepressure-gradient in most cases beingmaximally-adverse in the immediate lee, and decayingwith increasing downwind distance (x). On one day ofmeasurements, however, the pressure gradient over2 x/H 6 (H = windbreak height) resembled theleeward plateau identified by Wang and Taklein their numerical studies. Perhaps thisoccasional feature was only due to instrumenterror. Nevertheless a plateau of sorts wasindicated in similar measurements by Judd andPrendergast (with H = 1.92 m, z
0
1.2 cm;H/z
0
160, k
r
3). Therefore,existence of a leeward pressure plateau behind athin fence cannot be definitely ruled out.When the windbreak was placed in a canopy, minimumsurface pressure was displaced downwind. Thisagrees with the wind-tunnel study of Judd, Raupach and Finnigan,and is consistent with a simple simulation reported here. 相似文献
13.
Wind Tunnel And Field Measurements Of Turbulent Flow In Forests. Part I: Uniformly Thinned Stands 总被引：1，自引：1，他引：0
Michael D. Novak Jon S. Warland Alberto L Orchansky Rick Ketler Steven Green 《Boundary-Layer Meteorology》2000,95(3):457-495
Many forest management methods alterstand density uniformly. The effectsof such a change on the wind andturbulence regimes in the forest arecritical to a number of processes governingthe stability of the stand and itsmicroclimate. We measured wind speed andturbulence statistics with a Dantec tri-axialhot-film probe in model forests of variousdensities (31–333 trees m-2), created byremoving whole trees in a regular pattern in awind tunnel, and compared them with similarmeasurements made with propeller anemometers insimilarly thinned plots (156–625 trees ha-1)within a Sitka spruce stand in Scotland. The results agree well, in general, with measurements made inother such studies with diverse canopy types.The systematic variations with density and verticalleaf-area distribution (which differed betweenwind-tunnel and field trees) in our work can explainmuch of the variability shown in scaled profiles ofbasic turbulence statistics reported in theliterature. The wind tunnel and field results are shown to be in good agreement overalldespite the difference in vertical leaf-areadistribution. Within-canopy and isolated-treedrag coefficients in the wind tunnel showthat tree-scale shelter effects increase astree density increases. The measurements indicatethat turbulence in the canopy is dominated bylarge-scale structures with dimensions of the sameorder as the height of the canopy as found inother studies but suggest that inter-tree spacing also modulates the size of these structures. These structures are associated with the sweeps that dominatemomentum exchange in the canopy and it is thisfact that allows the tri-axial probe to operate sowell despite the relatively narrow range of anglesin which the wind vector is correctly measured. Theratio of streamwise periodicity of these structuresto vorticity thickness varies systematically withtree density in the range 2.7–5.1, which spans theexpected range of 3.5–5 found in a laboratorymixing-layer, suggesting that tree spacing imposes another relevant length scale. This test andothers show that the results are in agreement withthe idea that canopy turbulence resembles that of a mixing layer even though they disagree with, and challenge the linear relationship between, streamwise periodicity andshear length scale presented recently in theliterature. The measurements are also in goodoverall agreement with simple drag models presented recently by other researchers. 相似文献
14.
B. Kruijt Y. Malhi J. Lloyd A. D. Norbre A. C. Miranda M. G. P. Pereira A. Culf J. Grace 《Boundary-Layer Meteorology》2000,94(2):297-331
The turbulence structure in two Amazon rain forestswas characterised for a range of above-canopystability conditions, and the results compared withprevious studies in other forest canopies and recenttheory for the generation of turbulent eddies justabove forest canopies. Three-dimensional wind speedand temperature fluctuation data were collectedsimultaneously at up to five levels inside and abovetwo canopies of 30–40 m tall forests, during threeseparate periods. We analysed hourly statistics, jointprobability distributions, length scales, spatialcorrelations and coherence, as well as power spectraof vertical and horizontal wind speed.The daytime results show a sharp attenuation ofturbulence in the top third of the canopies, resultingin very little movement, and almost Gaussianprobability distributions of wind speeds, in the lowercanopy. This contrasts with strongly skewed andkurtotic distributions in the upper canopy. At night,attenuation was even stronger and skewness vanishedeven in the upper canopy. Power spectral peaks in thelower canopy are shifted to lower frequencies relativeto the upper canopy, and spatial correlations andcoherences were low throughout the canopy. Integrallength scales of vertical wind speed at the top of thecanopy were small, about 0.15 h compared to avalue of 0.28 h expected from the shear lengthscale at the canopy top, based on the hypothesis that theupper canopy air behaves as a plane mixing layer. Allthis suggests that, although exchange is not totallyinhibited, tropical rain forest canopies differ from other forests in that rapid, coherentdownward sweeps do not penetrate into the lowercanopy, and that length scales are suppressed. This isassociated with a persistent inversion of stability inthat region compared to above-canopy conditions. Theinversion is likely to be maintained by strong heatabsorption in the leaves concentrated near thecanopy top, with the generally weak turbulence beingunable to destroy the temperature gradients over thelarge canopy depth. 相似文献
15.
S. E. Larsen H. E. Jørgensen L. Landberg J. E. Tillman 《Boundary-Layer Meteorology》2002,105(3):451-470
The structures of mean flow and turbulence in the atmospheric surface boundary layer have been extensively studied on Earth, and to a far less extent on Mars, where only the Viking missions and the Pathfinder mission have delivered in-situ data. Largely the behaviour of surface-layer turbulence and mean flow on Mars is found to obey the same scaling laws as on Earth. The largest micrometeorological differences between the two atmospheres are associated with the low air density of the Martian atmosphere. Together with the virtual absence of water vapour, it reduces the importance of the atmospheric heat flux in the surface energy budget. This increases the temperature variation of the surface forcing the near-surface temperature gradient and thereby the diabatic heat flux to higher values than are typical on the Earth, resulting in turn in a deeper daytime boundary layer. As wind speed is much like that of the Earth, this larger diabatic heat flux is carried mostly by larger maximal values of T*, the surface scale temperature. The higher kinematic viscosity yields a Kolmogorov scale of the order of ten times larger than on Earth, influencing the transition between rough and smooth flow for the same surface features.The scaling laws have been validated analysing the Martian surface-layer data for the relations between the power spectra of wind and temperature turbulence and the corresponding mean values of wind speed and temperature. Usual spectral formulations were used based on the scaling laws ruling the Earth atmospheric surface layer, whereby the Earth's atmosphere is used as a standard for the Martian atmosphere. 相似文献
16.
To investigate the suitability of computational fluid dynamics (CFD) with regard to windbreak aerodynamics, simulations are
performed with a state-of-the-art numerical scheme (Fluent) and compared against experimental data for two- and three-dimensional
disturbances, namely the case of a long straight porous shelter fence and the case of a shelter fence erected in a square
about an enclosed plot. A thorough sensitivity study quantifies the impact of numerical choices on the simulation (e.g. grid-point
density, domain size, turbulence closure), and leads to guidelines that should ensure objective simulation of windbreak flows.
On a fine grid Fluent’s “realizable k–ε closure” gives results that are in qualitative accord with the observed mean winds. 相似文献
17.
Turbulence in the nocturnal boundary layer(NBL) is still not well characterized, especially over complex underlying surfaces. Herein, gradient tower data and eddy covariance data collected by the Beijing 325-m tower were used to better understand the differentiating characteristics of turbulence regimes and vertical turbulence structure of urban the NBL. As for heights above the urban canopy layer(UCL), the relationship between turbulence velocity scale(VTKE) and wind speed(V) was con... 相似文献
18.
Abstract Dawn‐to‐dusk evolution of air turbulence, sensible heat and latent heat above a forest during cloud‐free or near‐cloud‐free summer conditions is modelled by way of a system of differential equations. Temperatures in and above the canopy, near canopy‐top wind velocities, early morning leaf moisture (dew) and afternoon canopy ventilation (i.e. heat released from the canopy and from below the canopy) are included in the mathematical treatment. Computed results are compared with field data for atmospheric temperature and wind speed profiles up to 1200 m, within‐canopy temperature, and canopy‐level radiation, turbulent fluxes and wind speeds. Data were collected at a central New Brunswick mixed‐wood forest site dominated by spruce (Picea spp. ) and shade‐tolerant hardwoods for four representative summer days. It was found that the effective canopy temperature was not only affected by insolation, but also by the extent of canopy ventilation and the amount of dew on the foliage. The growth of the mixing layer was affected by canopy ventilation and by above‐canopy wind speeds. Model calculations closely simulated the meteorological observations. 相似文献
19.
We conduct wind-tunnel experiments on three different uniform roughness arrays composed of sparsely distributed rectangular
cylinders for the estimation of surface parameters. Roughness parameters such as the roughness length z
0 and zero-plane displacement d are extracted using a best-fit approximation of the measured wind velocity. We also perform a large-eddy simulation (LES)
to confirm that four sampling points are sufficient to surrogate a space average above the canopy layer of the sparse roughness
arrays. We propose a new morphological model from a systematic analysis of experimental data on the arrays. The friction velocity
predicted by the proposed model agrees well with the peak value of the measured Reynolds shear stress ${(-\left<\overline{u'w'}\right>)^{0.5}}${(-\left<\overline{u'w'}\right>)^{0.5}}. The proposed model is further validated in an additional wind-tunnel experiment conducted on a scaled configuration of a
real urban area exposed to four wind directions. The proposed model is found to perform very well particularly in the estimation
of the friction velocity, readily leading to a better estimation of turbulence, which is essential for an accurate prediction
of pollutant dispersion. 相似文献
20.
Prediction of windthrow risk to individual or groups of retained trees in harvested stands requires an improved understanding
of canopy airflow dynamics. Large-eddy simulations were used to simulate wind-tunnel experiments in two and three dimensions
to compare with observations for model validation and to address parameter space considerations for the design of subsequent
retention pattern experiments. The three-dimensional simulations were similar to the observed wind-tunnel data for the statistical
profiles for but there were greater differences in skewness and kurtosis. These results were obtained using a common leaf-area drag formulation
without either skin friction or speed dependent drag that enables scaling with U
0 (ambient wind speed) and h (height of the canopy). This scaling results in a single non-dimensional parameter h/h
c
where h
c
(x, y, z) is the momentum range resulting from the canopy drag. The validity of the model scaling was tested using two-dimensional
simulations. The irrotational component of the flow (potential flow) was found to be important when defining vertical domain
limitations and has significant implications for time dependent flow (i.e. turbulent conditions) when considering retention
pattern design. The sudden onset of drag associated with the isolated stand presents some unexpected challenges. The horizontal
scales of the shearing instabilities were simulated in two dimensions and found to range between 2h for early times to 7h for later times. The early-time horizontal scales are in the range of logical retention pattern scales and as such need to
be taken into account as part of the parameter space, i.e. a range of retention pattern lengths need consideration. 相似文献