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1.
Large-eddy Simulation of Turbulent Flow across a Forest Edge. Part II: Momentum and Turbulent Kinetic Energy Budgets 总被引：1，自引：0，他引：1
Bai Yang Andrew P. Morse Roger H. Shaw Kyaw Tha Paw U 《Boundary-Layer Meteorology》2006,121(3):433-457
Momentum and turbulent kinetic energy (TKE) budgets across a forest edge have been investigated using large-eddy simulation (LES). Edge effects are observed in the rapid variation of a number of budget terms across this vegetation transition. The enhanced drag force at the forest edge is largely balanced by the pressure gradient force and by streamwise advection of upstream momentum, while vertical turbulent diffusion is relatively insignificant. For variance and TKE budgets, the most important processes at the forest edge are production due to the convergence (or divergence) of the mean flow, streamwise advection, pressure diffusion and enhanced dissipation by canopy drag. Turbulent diffusion, pressure redistribution and vertical shear production, which are characteristic processes in homogeneous canopy flow, are less important at the forest transition. We demonstrate that, in the equilibrated canopy flow, a substantial amount of TKE produced in the streamwise direction by the vertical shear of the mean flow is redistributed in the vertical direction by pressure fluctuations. This redistribution process occurs in the upper canopy layers. Part of the TKE in the vertical velocity component is transferred by turbulent and pressure diffusion to the lower canopy levels, where pressure redistribution takes place again and feeds TKE back to the streamwise direction. In this TKE cycle, the primary source terms are vertical shear production for streamwise velocity variance and pressure redistribution for vertical velocity variance. The evolution of these primary source terms downwind of the forest edge largely controls the adjustment rates of velocity variances. 相似文献
2.
Large-eddy Simulation of Turbulent Flow Across a Forest Edge. Part I: Flow Statistics 总被引：1，自引：1，他引：1
Bai Yang Michael R. Raupach Roger H. Shaw Kyaw Tha Paw U Andrew P. Morse 《Boundary-Layer Meteorology》2006,120(3):377-412
The statistics of turbulent flow across a forest edge have been examined using large-eddy simulation, and results compared with field and wind-tunnel observations. The moorland-to-forest transition is characterized by flow deceleration in the streamwise direction, upward distortion of the mean flow, formation of a high pressure zone immediately in front of the edge, suppression of the standard deviations and covariance of velocity components, and enhancement of velocity skewnesses. For the selected forest density, it is observed that the maximum distortion angle is about 8 degrees from the horizontal. Instead of approaching a downwind equilibrium state in a monotonic manner, turbulence (standard deviations and covariances of velocity components) and mean streamwise velocity undershoot in the transition zone behind the edge. Evolution of flow statistics clearly reveals the growth of an internal boundary layer, and the establishment of an equilibrium layer downwind of the edge. It is evident that lower-order moments generally adjust more quickly over the new rough surface than do higher-order moments. We also show that the streamwise velocity standard deviation at canopy height starts its recovery over the rough surface sooner than does the vertical velocity standard deviation, but completes full adjustment later than the latter. Despite the limited domain size upstream of the edge, large-eddy simulation has successfully reproduced turbulent statistics in good agreement with field and wind-tunnel measurements. 相似文献
3.
Christopher Poëtte Barry Gardiner Sylvain Dupont Ian Harman Margi Böhm John Finnigan Dale Hughes Yves Brunet 《Boundary-Layer Meteorology》2017,163(3):393-421
Landscape discontinuities such as forest edges play an important role in determining the characteristics of the atmospheric flow by generating increased turbulence and triggering the formation of coherent tree-scale structures. In a fragmented landscape, consisting of surfaces of different heights and roughness, the multiplicity of edges may lead to complex patterns of flow and turbulence that are potentially difficult to predict. Here, we investigate the effects of different levels of forest fragmentation on the airflow. Five gap spacings (of length approximately 5h, 10h, 15h, 20h, 30h, where h is the canopy height) between forest blocks of length 8.7h, as well as a reference case consisting of a continuous forest after a single edge, were investigated in a wind tunnel. The results reveal a consistent pattern downstream from the first edge of each simulated case, with the streamwise velocity component at tree top increasing and turbulent kinetic energy decreasing as gap size increases, but with overshoots in shear stress and turbulent kinetic energy observed at the forest edges. As the gap spacing increases, the flow appears to change monotonically from a flow over a single edge to a flow over isolated forest blocks. The apparent roughness of the different fragmented configurations also decreases with increasing gap size. No overall enhancement of turbulence is observed at any particular level of fragmentation. 相似文献
4.
An experiment was set-up to investigate the adjustment of turbulence over a roughness transition (moorland to forest). Results from this experiment support the development of an internal boundary layer (IBL) at the transition, which propagates upwards by turbulent diffusion as a function of distance downwind from the transition. Spectra and length-scale results uphold the hypothesis that, over a transition to a rough surface, the variance distribution shifts towards smaller wavelengths/length scales. However, results suggest that the adjustment of streamwise velocity variance may be faster than the adjustment of the vertical velocity variance. The concept of an equilibrium layer developing above the new surface is supported. Fetch requirements for equilibrium are, however, found to differ between first order and second order (flux) statistics, with second order statistics requiring a longer fetch. Results indicate that fetch should exceed 25 times the height of the measurement above the zero plane, which is a 2° (±0.5) growth angle, for flux equilibrium. 相似文献
5.
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
6.
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
7.
Effects of Thermal Stability and Incoming Boundary-Layer Flow Characteristics on Wind-Turbine Wakes: A Wind-Tunnel Study 总被引：2，自引：1，他引：1
Wind-tunnel experiments were carried out to study turbulence statistics in the wake of a model wind turbine placed in a boundary-layer
flow under both neutral and stably stratified conditions. High-resolution velocity and temperature measurements, obtained
using a customized triple wire (cross-wire and cold wire) anemometer, were used to characterize the mean velocity, turbulence
intensity, turbulent fluxes, and spectra at different locations in the wake. The effect of the wake on the turbulence statistics
is found to extend as far as 20 rotor diameters downwind of the turbine. The velocity deficit has a nearly axisymmetric shape,
which can be approximated by a Gaussian distribution and a power-law decay with distance. This decay in the near-wake region
is found to be faster in the stable case. Turbulence intensity distribution is clearly non-axisymmetric due to the non-uniform
distribution of the incoming velocity in the boundary layer. In the neutral case, the maximum turbulence intensity is located
above the hub height, around the rotor tip location and at a distance of about 4–5.5 rotor diameters, which are common separations
between wind turbines in wind farms. The enhancement of turbulence intensity is associated with strong shear and turbulent
kinetic energy production in that region. In the stable case, the stronger shear in the incoming flow leads to a slightly
stronger and larger region of enhanced turbulence intensity, which extends between 3 and 6 rotor diameters downwind of the
turbine location. Power spectra of the streamwise and vertical velocities show a strong signature of the turbine blade tip
vortices at the top tip height up to a distance of about 1–2 rotor diameters. This spectral signature is stronger in the vertical
velocity component. At longer downwind distances, tip vortices are not evident and the von Kármán formulation agrees well
with the measured velocity spectra. 相似文献
8.
Water-flume experiments are conducted to study the structure of turbulent flow within and above a sparse model canopy consisting
of two rigid canopies of different heights. This difference in height specifies a two-dimensional step change from a rough
to a rougher surface, as opposed to a smooth-to-rough transition. Despite the fact that the flow is in transition from a rough
to a rougher surface, the thickness of the internal boundary layer scales as x
4/5, consistent with smooth-to-rough boundary layer adjustment studies, where x is the downstream distance from the step change. However, the analogy with smooth-to-rough transitions no longer holds when
the flow inside the canopy and near the canopy top is considered. Results show that the step change in surface roughness significantly
increases turbulence intensities and shear stress. In particular, there is an adjustment of the mean horizontal velocity and
shear stress as the flow passes over the rougher canopy, so that their vertical profiles adjust to give maximum values at
the top of this canopy. We also observe that the magnitude and shape of the inflection in the mean horizontal velocity profile
is significantly affected by the transition. The horizontal and vertical turbulence spectra compare well with Kolmogorov’s
theory, although a small deviation at high frequencies is observed in the horizontal spectrum within the canopy. Here, for
relatively low leaf area index, shear is found to be a more effective mechanism for momentum transfer through the canopy structure
than vortex shedding. 相似文献
9.
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.
10.
R. H. Shaw K. T. U Paw X. J. Zhang W. Gao G. Den Hartog H. H. Neumann 《Boundary-Layer Meteorology》1990,50(1-4):319-338
The relationship between surface pressure fluctuations and the velocity field associated with turbulent coherent structures is examined for flow within and above a deciduous forest. Measurements were taken with tower-mounted sonic anemometer/thermometers at six heights, Lyman-alpha humidiometers at three heights, and a pressure sensor at the forest floor. We find a strong, near-linear relationship between the mean square turbulent velocity and the standard deviation of the high-pass-filtered pressure fluctuations. Lagged cross-correlations between vertical velocity fluctuations and those of pressure show maximum correlations of ± 0.5 but with a phase offset. Examination of surface pressure during the passage of coherent structures, which are characterized by a transition from ejection to sweep, reveals a period of overpressure about 20 s in duration roughly centered on the time of passage of the scalar microfront at the top of the canopy. Pressure patterns associated with coherent structures appear to be largely responsible for the form of the correlations stated above.Pressure patterns calculated from an integrated Poisson equation, using observed velocity and temperature signals during coherent structures, match the main features of the observed pressure. Retrieval of the pressure fluctuations in this manner reveals that the mean wind shear/turbulence interaction term is dominant, but that important contributions arise from two other terms in the equation. Buoyancy effects are negligible. We show that the surface pressure signal is mainly created by the velocity field near the top of the forest, and present evidence to suggest that features of the sub-crown air movement result directly from this pressure field. 相似文献
11.
Lagged cross-correlation analyses between streamwise velocity at several heights within and above a forest, and between streamwise velocity and surface pressure, provide evidence that turbulence in the sub-crown region of the forest is to a large extent driven by pressure perturbations. The analyses support earlier results based on examination of coherent structures observed in the same forest. The phase of the streamwise velocity signal exhibits an increasing delay with decreasing height, indicative of a downwind tilted structure, until the upper region of the forest is reached, at which point the effect is reversed. It is suggested that positive pressure perturbations ahead of advancing microfronts induce streamwise accelerations in the trunk space. This link between the pressure pattern and the wind field explains why velocity spectra in the trunk space are depleted in the higher frequencies, relative to levels above. 相似文献
12.
Antonio Segalini Jens H. M. Fransson P. Henrik Alfredsson 《Boundary-Layer Meteorology》2013,148(2):269-283
An analysis of velocity statistics and spectra measured above a wind-tunnel forest model is reported. Several measurement stations downstream of the forest edge have been investigated and it is observed that, while the mean velocity profile adjusts quickly to the new canopy boundary condition, the turbulence lags behind and shows a continuous penetration towards the free stream along the canopy model. The statistical profiles illustrate this growth and do not collapse when plotted as a function of the vertical coordinate. However, when the statistics are plotted as function of the local mean velocity (normalized with a characteristic velocity scale), they do collapse, independently of the streamwise position and freestream velocity. A new scaling for the spectra of all three velocity components is proposed based on the velocity variance and integral time scale. This normalization improves the collapse of the spectra compared to existing scalings adopted in atmospheric measurements, and allows the determination of a universal function that provides the velocity spectrum. Furthermore, a comparison of the proposed scaling laws for two different canopy densities is shown, demonstrating that the vertical velocity variance is the most sensible statistical quantity to the characteristics of the canopy roughness. 相似文献
13.
A Wind-Tunnel Investigation of Wind-Turbine Wakes: Boundary-Layer Turbulence Effects 总被引：1，自引：1，他引：0
Wind-tunnel experiments were performed to study turbulence in the wake of a model wind turbine placed in a boundary layer
developed over rough and smooth surfaces. Hot-wire anemometry was used to characterize the cross-sectional distribution of
mean velocity, turbulence intensity and kinematic shear stress at different locations downwind of the turbine for both surface
roughness cases. Special emphasis was placed on the spatial distribution of the velocity deficit and the turbulence intensity,
which are important factors affecting turbine power generation and fatigue loads in wind energy parks. Non-axisymmetric behaviour
of the wake is observed over both roughness types in response to the non-uniform incoming boundary-layer flow and the effect
of the surface. Nonetheless, the velocity deficit with respect to the incoming velocity profile is nearly axisymmetric, except
near the ground in the far wake where the wake interacts with the surface. It is found that the wind turbine induces a large
enhancement of turbulence levels (positive added turbulence intensity) in the upper part of the wake. This is due to the effect
of relatively large velocity fluctuations associated with helicoidal tip vortices near the wake edge, where the mean shear
is strong. In the lower part of the wake, the mean shear and turbulence intensity are reduced with respect to the incoming
flow. The non-axisymmetry of the turbulence intensity distribution of the wake is found to be stronger over the rough surface,
where the incoming flow is less uniform at the turbine level. In the far wake the added turbulent intensity, its positive
and negative contributions and its local maximum decay as a power law of downwind distance (with an exponent ranging from
−0.3 to −0.5 for the rough surface, and with a wider variation for the smooth surface). Nevertheless, the effect of the turbine
on the velocity defect and added turbulence intensity is not negligible even in the very far wake, at a distance of fifteen
times the rotor diameter. 相似文献
14.
Turbulence structure in a deciduous forest 总被引：5，自引：2，他引：5
Three-dimensional wind velocity components were measured at two levels above and at six levels within a fully-leafed deciduous forest. Greatest shear occurs in the upper 20% of the canopy, where over 70% of the foliage is concentrated. The turbulence structure inside the canopy is characterized as non-Gaussian, intermittant and highly turbulent. This feature is supported by large turbulence intensities, skewness and kurtosis values and by the large infrequent sweeps and ejections that dominate tangential momentum transfer. Considerable day/night differences were observed in the vertical profiles of the mean streamwise wind velocity and turbulence intensities since the stability of the nocturnal boundary layer dampens turbulence above and within the canopy. 相似文献
15.
In order to study the turbulence structure behind a multiscale tree-like element in a boundary layer, detailed particle image
velocimetry measurements are carried out in the near-wake of a fractal-like tree. The tree is a pre-fractal with five generations,
each consisting of three branches and a scale-reduction factor of 1/2 between consecutive generations. Detailed mean velocity
and turbulence stress profiles are documented, as well as their downstream development. Scatter plots of mean velocity gradient
(transverse shear in the wake) and Reynolds shear stress exhibit a good linear relation at all locations in the flow. Therefore,
in the transverse direction of the wake evolution, the data support the Boussinesq eddy-viscosity concept. The measured mixing
length increases with streamwise distance, in agreement with classic wake expansion rates. Conversely, the measured eddy viscosity
and mixing length in the transverse direction decrease with increasing elevation, which differs from the behaviours measured
in the vertical direction in traditional boundary layers or in canopy flows studied before. In order to find an appropriate
single length scale to describe the wake evolution behind a multiscale object, two models are proposed, based on the notion
of superposition of scales. One approach is based on the radial spectrum of the object while the second is based on its length-scale
distribution evaluated using fractal geometry tools. Both proposed models agree well with the measured mixing length. The
results suggest that information about multiscale clustering of branches must be incorporated into models of the mixing length
for flows through single or sparse canopies of multiscale trees. 相似文献
16.
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
17.
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
18.
The ability of subfilter-scale (SFS) models to reproduce the statistical properties of SFS stresses and energy transfers over
heterogeneous surface roughness is key to improving the accuracy of large-eddy simulations of the atmospheric boundary layer.
In this study, several SFS models are evaluated a priori using experimental data acquired downwind of a rough-to-smooth transition
in a wind tunnel. The SFS models studied include the eddy-viscosity, similarity, non-linear and a mixed model consisting of
a combination of the eddy-viscosity and non-linear models. The dynamic eddy-viscosity model is also evaluated. The experimental
data consist of vertical and horizontal planes of high-spatial-resolution velocity fields measured using particle image velocimetry.
These velocity fields are spatially filtered and used to calculate SFS stresses and SFS transfer rates of resolved kinetic
energy. Coefficients for each SFS model are calculated by matching the measured and modelled SFS energy transfer rates. For
the eddy-viscosity model, the Smagorinsky coefficient is also evaluated using a dynamic procedure. The model coefficients
are found to be scale dependent when the filter scales are larger than the vertical measurement height and fall into the production
subrange of the turbulence where the flow scales are anisotropic. Near the surface, the Smagorinsky coefficient is also found
to decrease with distance downwind from the transition, in response to the increase in mean shear as the flow adjusts to the
smooth surface. In a priori tests, the ability of each model to reproduce statistical properties of the SFS stress is assessed.
While the eddy-viscosity model has low spatial correlation with the measured stress, it predicts mean stresses with the same
accuracy as the other models. However, the deficiency of the eddy-viscosity model is apparent in the underestimation of the
standard deviation of the SFS stresses and the inability to predict transfers of kinetic energy from the subfilter scales
to the resolved scales. Overall, the mixed model is found to have the best performance. 相似文献
19.
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
20.
We carried out measurements to test a simple theory of the effect of probe-induced flow distortion on turbulence measurements. We used two three-component sonic anemometers mounted 1.8m apart at a height of 6.7 m. Behind one was a horizontal circular cylinder of radius 0.15 m and length 1.2 m, chosen to model two-dimensional probe-induced flow distortion in the limit where the scale of the turbulence is very large compared to the scale of the probe. The second sonic anemometer measured the undistorted flow. The measured flow-distortion effects on the Reynolds shearing stress and the variances of streamwise and vertical velocity agree well with the theory. 相似文献