共查询到10条相似文献，搜索用时 109 毫秒
1.
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
2.
Hiroshi Takimoto Ayumu Sato Janet F. Barlow Ryo Moriwaki Atsushi Inagaki Shiho Onomura Manabu Kanda 《Boundary-Layer Meteorology》2011,140(2):295-314
We investigate the spatial characteristics of urban-like canopy flow by applying particle image velocimetry (PIV) to atmospheric
turbulence. The study site was a Comprehensive Outdoor Scale MOdel (COSMO) experiment for urban climate in Japan. The PIV
system captured the two-dimensional flow field within the canopy layer continuously for an hour with a sampling frequency
of 30 Hz, thereby providing reliable outdoor turbulence statistics. PIV measurements in a wind-tunnel facility using similar
roughness geometry, but with a lower sampling frequency of 4 Hz, were also done for comparison. The turbulent momentum flux
from COSMO, and the wind tunnel showed similar values and distributions when scaled using friction velocity. Some different
characteristics between outdoor and indoor flow fields were mainly caused by the larger fluctuations in wind direction for
the atmospheric turbulence. The focus of the analysis is on a variety of instantaneous turbulent flow structures. One remarkable
flow structure is termed ‘flushing’, that is, a large-scale upward motion prevailing across the whole vertical cross-section
of a building gap. This is observed intermittently, whereby tracer particles are flushed vertically out from the canopy layer.
Flushing phenomena are also observed in the wind tunnel where there is neither thermal stratification nor outer-layer turbulence.
It is suggested that flushing phenomena are correlated with the passing of large-scale low-momentum regions above the canopy. 相似文献
3.
Large-Eddy Simulation of Coherent Turbulence Structures Associated with Scalar Ramps Over Plant Canopies 总被引：1，自引：9，他引：1
Tsutomu Watanabe 《Boundary-Layer Meteorology》2004,112(2):307-341
Large-eddy simulations were performed of a neutrally-stratified turbulent flow within and above an ideal, horizontally- and vertically-homogeneous plant canopy. Three simulations were performed for shear-driven flows in small and large computational domains, and a pressure-driven flow in a small domain, to enable the nature of canopy turbulence unaffected by external conditions to be captured. The simulations reproduced quite realistic canopy turbulence characteristics, including typical ramp structures appearing in time traces of the scalar concentration near the canopy top. Then, the spatial structure of the organised turbulence that caused the scalar ramps was examined using conditional sampling of three-dimensional instantaneous fields, triggered by the occurrence of ramp structures. A wavelet transform was used for the detection of ramp structures in the time traces. The ensemble-averaged results illustrate that the scalar ramps are associated with the microfrontal structure in the scalar, the ejection-sweep structure in the streamwise and vertical velocities, a laterally divergent flow just around the ramp-detection point, and a positive, vertically-coherent pressure perturbation. These vertical structures were consistent with previous measurements made in fields or wind tunnels. However, the most striking feature is that the horizontal slice of the same structure revealed a streamwise-elongated region of high-speed streamwise velocity impacting on another elongated region of low-speed velocity. These elongated structures resemble the so-called streak structures that are commonly observed in near-wall shear layers. Since elongated structures of essentially similar spatial scales were observed in all of the runs, these streak structures appear to be inherent in near-canopy turbulence. Presumably, strong wind shear formed just above the canopy is involved in their formation. By synthesis of the ensemble-averaged and instantaneous results, the following processes were inferred for the development of scalar microfronts and their associated flow structures: (1) a distinct scalar microfront develops where a coherent downdraft associated with a high-speed streak penetrates into the region of a low-speed streak; (2) a stagnation in flow between two streaks of different velocities builds up a vertically-coherent high-pressure region there; (3) the pressure gradients around the high-pressure region work to reduce the longitudinal variations in streamwise velocity and to enhance the laterally-divergent flow and lifted updrafts downstream of the microfront; (4) as the coherent mother downdraft impinges on the canopy, canopy-scale eddies are formed near the canopy top in a similar manner as observed in conventional mixing-layer turbulence. 相似文献
4.
Edge Flow and Canopy Structure: A Large-Eddy Simulation Study 总被引：4，自引：4，他引：0
Sharp heterogeneities in forest structure, such as edges, are often responsible for wind damage. In order to better understand
the behaviour of turbulent flow through canopy edges, large-eddy simulations (LES) have been performed at very fine scale
(2 m) within and above heterogeneous vegetation canopies. A modified version of the Advanced Regional Prediction System (ARPS),
previously validated in homogeneous conditions against field and wind-tunnel measurements, has been used for this purpose.
Here it is validated in a simple forest-clearing-forest configuration. The model is shown to be able to reproduce accurately
the main features observed in turbulent edge flow, especially the “enhanced gust zone” (EGZ) present around the canopy top
at a few canopy heights downwind from the edge, and the turbulent region that develops further downstream. The EGZ is characterized
by a peak in streamwise velocity skewness, which reflects the presence of intense intermittent wind gusts. A sensitivity study
of the edge flow to the forest morphology shows that with increasing canopy density the flow adjusts faster and turbulent
features such as the EGZ become more marked. When the canopy is characterized by a sparse trunk space the length of the adjustment
region increases significantly due to the formation of a sub-canopy wind jet from the leading edge. It is shown that the position
and magnitude of the EGZ are related to the mean upward motion formed around canopy top behind the leading edge, caused by
the deceleration in the sub-canopy. Indeed, this mean upward motion advects low turbulence levels from the bottom of the canopy;
this emphasises the passage of sudden strong wind gusts from the clearing, thereby increasing the skewness in streamwise velocity
as compared with locations further downstream where ambient turbulence is stronger. 相似文献
5.
Ronald M. Cionco 《Boundary-Layer Meteorology》1978,15(1):81-93
Canopy wind profiles can often be represented by an exponential function such that wind-speeds in these vegetative canopies are a function of height and the attenuation coefficient of this wind profile relationship. To be more precise, canopy flow is a function of canopy density, element flexibility, and height. An index of canopy flow, therefore, can be defined as a conservative measure of the gross flow response to the presence of various types of roughness elements. For this study, windspeed profile data of two quite different canopy density experiments — field and wind tunnel - have been analyzed based on least-square fittings. The results indicate that the two sets of index values of canopy flow behave in a similar manner with maxima occurring for optimum densities of one-third the potential full array of roughness elements. These index values also differ by some 0.2, but are still compatible when one accounts for the respective levels of turbulence within these dissimilar canopies. 相似文献
6.
Impact of Terrain Heterogeneity on Coherent Structure Properties: Numerical Approach 总被引：2，自引：2，他引：0
Clement Fesquet Sylvain Dupont Philippe Drobinski Thomas Dubos Christian Barthlott 《Boundary-Layer Meteorology》2009,133(1):71-92
A three-dimensional large-eddy simulation (LES) model, which includes the effects of plant–atmosphere interactions, is used
to study the effects of surface inhomogeneities on near-surface coherent structures over an open field and behind a forest
canopy. These simulated conditions are representative of two wind sectors of the Site Instrumental de Recherche par Télédétection
Atmosphérique (SIRTA) experimental site at the Institut Pierre Simon Laplace, Palaiseau, France. Coherent structure properties
deduced from wavelet transforms of the simulated near-surface vertical velocity time series are not modified by upstream terrain
heterogeneities, in agreement with site measurements. This feature is related to the nature of structures detected from the
vertical velocity time series. The turbulence close to the surface seems composed of both local coherent structures and large
coherent structures reflecting outer-layer properties, which depend on the overall surface heterogeneity or upstream heterogeneity.
It is argued that the streamwise velocity is representative of these large outer-layer structures that impinge onto the ground
through a top-down mechanism as identified through the space–time correlation of the wind velocity components. In contrast,
the vertical velocity is more representative of small structures resulting from the impingement of the large outer-layer structures.
These small structures represent locally-generated, active turbulence, which adjusts rapidly to local surface conditions,
and consequently they are only weakly dependent on upstream heterogeneities. 相似文献
7.
This is the first of a series of three papers describing experiments on the dispersion of trace heat from elevated line and plane sources within a model plant canopy in a wind tunnel. Here we consider the wind field and turbulence structure. The model canopy consisted of bluff elements 60 mm high and 10 mm wide in a diamond array with frontal area index 0.23; streamwise and vertical velocity components were measured with a special three-hot-wire anemometer designed for optimum performance in flows of high turbulence intensity. We found that:
- The momentum flux due to spatial correlations between time-averaged streamwise and vertical velocity components (the dispersive flux) was negligible, at heights near and above the top of the canopy.
- In the turbulent energy budget, turbulent transport was a major loss (of about one-third of local production) near the top of the canopy, and was the principal gain mechanism lower down. Wake production was greater than shear production throughout the canopy. Pressure transport just above the canopy, inferred by difference, appeared to be a gain in approximate balance with the turbulent transport loss.
- In the shear stress budget, wake production was negligible. The role of turbulent transport was equivalent to that in the turbulent energy budget, though smaller.
- Velocity spectra above and within the canopy showed the dominance of large eddies occupying much of the boundary layer and moving downstream with a height-independent convection velocity. Within the canopy, much of the vertical but relatively little of the streamwise variance occurred at frequencies characteristic of wake turbulence.
- Quadrant analysis of the shear stress showed only a slight excess of sweeps over ejections near the top of the canopy, in contrast with previous studies. This is a result of improved measurement techniques; it suggests some reappraisal of inferences previously drawn from quadrant analysis.
8.
Coherent Turbulent Structures Across a Vegetation Discontinuity 总被引：3，自引：2，他引：1
The study of turbulent flow across a vegetation discontinuity is of significant interest as such landscape features are common, and as there is no available theory to describe this regime adequately. We have simulated the three-dimensional dynamics of the airflow across a discontinuity between a forest (with a leaf area index of 4) and a clearing surface using large-eddy simulation. The properties of the bulk flow, as well as the large-scale coherent turbulent structures across the forest-to-clearing transition and the clearing-to-forest transition, are systematically explored. The vertical transport of the bulk flow upstream of the leading edge gives rise to the enhanced gust zone around the canopy top, while the transport downstream of the trailing edge leads to the formation of a recirculation zone above the clearing surface. The large-scale coherent structures across the two transitions exhibit both similarities with and differences from those upstream of the corresponding transition. For example, the ejection motion is dominant over the sweep motion in most of the region 1?<?z/h < 2 (h is the canopy height) immediately downstream of the trailing edge, much as in the forested area upstream. Also, the streamwise vortex pair, which has previously been observed within the canopy sublayer and the atmospheric boundary layer, is consistently found across both transitions. However, the inflection observed both in the mean streamwise velocity, as well as in the vertical profiles of the coherent structures in the forested area, disappears gradually across the forest-to-clearing transition. The coherence of the turbulence, quantified by the percentage of the total turbulence kinetic energy that the coherent structures capture from the flow, decreases sharply immediately downstream of the trailing edge of the forest and increases downstream of the leading edge of the forest. The effects of the ratio of the forest/clearing lengths under a given streamwise periodicity on flow statistics and coherent turbulent structures are presented as well. 相似文献
9.
An observational study of wind-induced waving of plants 总被引：1，自引：0，他引：1
T. Maitani 《Boundary-Layer Meteorology》1979,16(1):49-65
The motions of individual plants and the turbulence statistics of surface winds measured near the top of a canopy are obtained over a wheat field and a rush field. Two typical cases of motions of individual plants are presented. The displacements of the ear of wheat (the plant height is 1.0 m) showed a natural oscillation in wind speeds of 1.6 m s–1 measured at a height of 30 cm over a wheat canopy, while displacements of the stem of a rush plant were closely related to the fluctuations of surface winds in wind speeds of 1.7 m s–1 measured at the top of the rush plant. The power spectra of displacements of a rush plant seem to support the negative seven-third power hypothesis proposed by Inoue. The frequency responses of displacements of plants to fluctuations of the instantaneous momentum flux are also presented. 相似文献
10.
Leonardo P. Chamorro R. E. A. Arndt Fotis Sotiropoulos 《Boundary-Layer Meteorology》2011,141(3):349-367
The fundamental properties of turbulent flow around a perfectly staggered wind farm are investigated in a wind tunnel. The wind farm consisted of a series of 10 rows by 2–3 columns of miniature wind turbines spaced 5 and 4 rotor diameters in the streamwise and spanwise directions respectively. It was placed in a boundary-layer flow developed over a smooth surface under thermally neutral conditions. Cross-wire anemometry was used to obtain high resolution measurements of streamwise and vertical velocity components at various locations within and above the wind farm. The results show that the staggered configuration is more efficient in terms of momentum transfer from the background flow to the turbines compared to the case of an aligned wind turbine array under similar turbine separations in the streamwise and spanwise directions. This leads to improved power output of the overall wind farm. A simplified analysis suggests that the difference in power output between the two configurations is on the order of 10%. The maximum levels of turbulence intensity in the staggered wind farm were found to be very similar to that observed in the wake of a single wind turbine, differing substantially with that observed in an aligned configuration with similar spacing. The dramatic changes in momentum and turbulence characteristics in the two configurations show the importance of turbine layout in engineering design. Lateral homogenization of the turbulence statistics above the wind farm allows for the development of simple parametrizations for the adjustment of flow properties, similar to the case of a surface roughness transition. The development of an internal boundary layer was observed at the upper edge of the wind farm within which the flow statistics are affected by the superposition of the ambient flow and the flow disturbance induced by the wind turbines. The adjustment of the flow in this layer is much slower in the staggered situation (with respect to its aligned counterpart), implying a change in the momentum/power available at turbine locations. Additionally, power spectra of the streamwise and vertical velocity components indicate that the signature of each turbine-tip vortex structure persists to locations deep within the wind farm. 相似文献