共查询到20条相似文献，搜索用时 31 毫秒
1.
A conditional sampling technique using a multilevel scheme was applied to the detection of temperature and humidity microfronts and organized ejection/sweep motions under different atmospheric stabilities. Data were obtained with seven triaxial sonic anemometer/thermometers and three Lyman-alpha hygrometers within and above a deciduous forest. Both temperature and humidity microfronts were identified in unstable cases, but only humidity microfronts could be detected under neutral conditions. Inverted temperature ramps occurred under slightly stable conditions. Occasionally, wave-like patterns appeared within the canopy, seemingly coupled with inverse ramps occurring above the forest. The frequency of occurrence of scalar microfronts appears to have no clear dependence on atmospheric stability, and averages 74–84 s per cycle with a mode of about 50 s per cycle. However, the strength of ejections and sweeps, shown by the vertical velocity averaged within structures, was reduced by increasing atmospheric stability. Structures identified under different stabilities show many similarities in their patterns of scalar ramps, and associated velocity and surface pressure. Profiles of short-term averaged longitudinal velocity at different times during the microfront passage show that the air within the canopy was retarded and an intensified shear above the canopy occurred prior to the passage of the microfront. Results from the present conditional analysis strongly suggest an important role of shear instability in the formation of canopy coherent structure. 相似文献
2.
Observation of organized structure in turbulent flow within and above a forest canopy 总被引：12，自引：2，他引：12
Ramp patterns of temperature and humidity occur coherently at several levels within and above a deciduous forest as shown by data gathered with up to seven triaxial sonic anemometer/thermometers and three Lyman-alpha hygrometers at an experimental site in Ontario, Canada. The ramps appear most clearly in the middle and upper portion of the forest. Time/height cross-sections of scalar contours and velocity vectors, developed from both single events and ensemble averages of several events, portray details of the flow structures associated with the scalar ramps. Near the top of the forest they are composed of a weak ejecting motion transporting warm and/or moist air out of the forest followed by strong sweeps of cool and/or dry air penetrating into the canopy. The sweep is separated from the ejecting air by a sharp scalar microfront. At approximately twice the height of the forest, ejections and sweeps are of about equal strength.In the middle and upper parts of the canopy, sweeps conduct a large proportion of the overall transfer between the forest and the lower atmosphere, with a lesser contribution from ejections. Ejections become equally important aloft. During one 30-min run, identified structures were responsible for more than 75% of the total fluxes of heat and momentum at mid-canopy height. Near the canopy top, the transition from ejection of slow moving fluid to sweep bringing fast moving air from above is very rapid but, at both higher and lower levels, brief periods of upward momentum transfer occur at or immediately before the microfront. 相似文献
3.
Wenjun Chen Michael D. Novak T. Andrew Black Xuhui Lee 《Boundary-Layer Meteorology》1997,84(1):99-124
Air temperature time series within and above canopies reveal ramp patternsassociated with coherent eddies that are responsible for most of thevertical transport of sensible heat. Van Atta used a simple step-changeramp model to analyse the coherent part of air temperature structurefunctions. However, his ocean data, and our own measurements for aDouglas-fir forest, straw mulch, and bare soil, reveal that even withoutlinearization his model cannot account for the observed decrease of thecubic structure function for small time lag. We found that a ramp model inwhich the rapid change at the end of the ramp occurs in a finite microfronttime can describe this decrease very well, and predict at least relativemagnitudes of microfront times between different surfaces. Averagerecurrence time for ramps, determined by analysis of the cubic structurefunction with the new ramp model, agreed well with values determined usingthe Mexican Hat wavelet transform, except at lower levels within theforest. Ramp frequency above the forest and mulch scaled very well withwind speed at the canopy top divided by canopy height. Within the forest,ramp frequency did not vary systematically with height. This is inaccordance with the idea that large-scale canopy turbulence is mostlygenerated by instability of the mean canopy wind profile, similar to aplane mixing layer. The straw mulch and bare soil experiments uniquelyextend measurements of temperature structure functions and ramp frequencyto the smallest scales possible in the field. 相似文献
4.
Organised Motion and Radiative Perturbations in the Nocturnal Canopy Sublayer above an Even-Aged Pine Forest 总被引：2，自引：3，他引：2
Using time series measurements of velocity, carbon dioxideand water vapour concentration, and temperature collected justabove a 15 m tall even-aged pine forest, we quantify the roleof organized motion on scalar and momentum transport withinthe nocturnal canopy sublayer (CSL). We propose a frameworkin which the nocturnal CSL has two end-members, bothdominated by organised motion. These end-members representfully developed turbulent flows at near-neutral or slightly stablestratification and no turbulence for very stable stratification.Our analysis suggests that ramps dominate scalar transport fornear-neutral and slightly stable conditions, while linear canopywaves dominate the flow dynamics for very stable conditions.For intermediate stability, the turbulence is highly damped andoften dominated by fine scale motions. Co-spectral analysissuggests that ramps are the most efficient net scalar mass-transportingagent while linear canopy waves contribute little to net scalartransport between the canopy and atmosphere for averagingintervals that include complete wave cycles. However, canopywaves significantly contribute to the spectral properties of thescalar time series. Ramps are the most frequently occurringorganised motion in the nocturnal CSL for this site.Numerous night-time runs, however, resided between thesetwo end-members. Our analysis suggests that whenradiative perturbations are sufficient large (>20 W m-2 innet radiation), the flow can switch from being highly dampedfine-scale turbulence to being organized with ramp-like properties. We also found that when ramps are already the dominant eddymotion in the nocturnal CSL, radiative perturbations have aminor impact on scalar transport. Finally, in agreement withprevious studies, we found that ramps and canopy waves havecomparable length scales of about 30–60 metres. Consequencesto night-time flux averaging are also discussed. 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.
Two-Point Correlation Analysis Of Neutrally Stratified Flow Within And Above A Forest From Large-Eddy Simulation 总被引：5，自引：5，他引：0
Two-point space-time correlations ofvelocities, a passive scalar and static pressure arecalculated using the resolvable flow fields computedby large-eddy simulation (LES) of neutrally stratifiedflow within and above a sparse forest. Zero-time-lagspatial auto-correlation contours in thestreamwise-vertical cross-section for longitudinal andlateral velocities and for a scalar are tilted fromthe vertical in the downstream direction, as istypical in near-wall sheared flow. On the other hand,auto-correlations of vertical velocity and of staticpressure are vertically coherent. Zero-time-lagspatial auto-correlations in the spanwise-verticalcross-section show no distinct tilt, and those forboth longitudinal and vertical velocities demonstratedistinct negative side lobes in the middle forest andabove, while longitudinal velocity in the subcrowntrunk space is laterally in-phase. Static pressureperturbations appear to be spatially coherent in thespanwise direction at all heights, especially insidethe forest. Near the forest floor, longitudinalvelocity is found to be in-phase with static pressureperturbation and to be closely linked to theinstantaneous streamwise pressure gradient, supportinga previous proposal that longitudinal velocity in thisregion is dominantly modulated by the pressurepatterns associated with the coherent sweep/ejectionevents. Near treetop height, a lack of linkage betweenthe pressure gradient and the local time derivative ofthe longitudinal velocity supports the hypothesis ofadvection dominating turbulent flow.The major phase characteristics of the two-pointcorrelations essentially remained the same from fourLES runs with different domain size and/or gridresolution. A larger LES domain yielded betteragreement with field observations in a real forest onboth the magnitudes of the correlations and thesingle-point integral time scales. A finer gridresolution in the LES led to a faster rate of decreaseof correlation with increasing separation in space ortime, as did the higher frequency fluctuations in theturbulent records from field measurements. Convectivevelocities estimated from the lagged two-pointauto-correlations of the calculated flow fields werecompared with similar calculations from wind-tunnelstudies. At the canopy top, estimates from thecorrelation analyses agree with the translationvelocity estimated from instantaneous snapshots of ascalar microfront using both LES and field data. Thistranslation velocity is somewhat higher than the localmean wind speed. Convective velocities estimated fromlagged correlations increase with height above thecanopy. It is suggested that an appropriate filteringprocedure may be necessary to reduce the effects ofsmall-scale random turbulence, as was reported in astudy over an orchard canopy. The mean longitudinalvelocity near the treetops is found to be moreappropriate than the local mean longitudinal velocityat each height to link single-point integral timescales with directly calculated spatial integralstreamwise length scales. 相似文献
7.
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
8.
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
9.
Li Fitzmaurice Roger H. Shaw Kyaw Tha Paw U Edward G. Patton 《Boundary-Layer Meteorology》2004,112(1):107-127
Large-eddy simulation is used to reproduce neutrallystratified airflow inside and immediately above a vegetation canopy. A passive scalaris released from the canopy and the evolution of scalar concentration above the canopyis studied. The most significant characteristic of the scalar concentration is the repeatedformation and dissipation of scalar microfronts, a phenomenon that has been observedin nature. These scalar microfronts consist of downstream-tilted regions of highscalar concentration gradients. Computer visualization tools and a conditional samplingand compositing technique are utilized to analyze these microfronts. Peaks in positivepressure perturbation exceeding an experimental threshold are found to be effectiveindicators of scalar microfronts. Convergence of the streamwise velocity componentand divergence of the cross-stream velocity component are observed in the immediatevicinity of scalar microfronts, which helps explain their relatively longlifetimes. Many of these three-dimensional features have been observedin previous field studies of canopy flow. 相似文献
10.
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
11.
Structure functions are used to study the dissipation and inertial range scales of turbulent energy, to parametrize remote turbulence measurements, and to characterize ramp features in the turbulent field. Ramp features are associated with turbulent coherent structures, which dominate energy and mass fluxes in the atmospheric surface layer. The analysis of structure functions to identify ramp characteristics is used in surface renewal methods for estimating fluxes. It is unclear how commonly observed different scales of ramp-like shapes (i.e., smaller ramps and spikes embedded in larger ramps) influence structure function analysis. Here, we examine the impact of two ramp-like scales on structure function analysis using artificially generated data. The range of time lags in structure function analysis was extended to include time lags typically associated with isotropic turbulence to those larger than the ramp durations. The Van Atta procedure (Arch Mech 29:161–171, 1977) has been expanded here to resolve the characteristics of two-scale ramp models. This new method accurately, and in some cases, exactly determines the amplitude and duration of both ramp scales. Spectral analysis was applied to the structure functions for a broad range of time lags to provide qualitative support for the expanded Van Atta procedure results. The theory reported here forms the foundation for novel methods of analyzing turbulent coherent structures. 相似文献
12.
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
13.
A study of the neutrally-stratified flow within and over an array of three-dimensional buildings (cubes) was undertaken using
simple Reynolds-averaged Navier—Stokes (RANS) flow models. These models consist of a general solution of the ensemble-averaged,
steady-state, three-dimensional Navier—Stokes equations, where the k-ε turbulence model (k is turbulence kinetic energy and ε is viscous dissipation rate) has been used to close the system of equations. Two turbulence
closure models were tested, namely, the standard and Kato—Launder k-ε models. The latter model is a modified k-ε model designed specifically to overcome the stagnation point anomaly in flows past a bluff body where the standard k-ε model overpredicts the production of turbulence kinetic energy near the stagnation point. Results of a detailed comparison
between a wind-tunnel experiment and the RANS flow model predictions are presented. More specifically, vertical profiles of
the predicted mean streamwise velocity, mean vertical velocity, and turbulence kinetic energy at a number of streamwise locations
that extend from the impingement zone upstream of the array, through the array interior, to the exit region downstream of
the array are presented and compared to those measured in the wind-tunnel experiment. Generally, the numerical predictions
show good agreement for the mean flow velocities. The turbulence kinetic energy was underestimated by the two different closure
models. After validation, the results of the high-resolution RANS flow model predictions were used to diagnose the dispersive
stress, within and above the building array. The importance of dispersive stresses, which arise from point-to-point variations
in the mean flow field, relative to the spatially-averaged Reynolds stresses are assessed for the building array. 相似文献
14.
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
15.
In this paper we discuss the development of turbulence back from the transition fromopen moorland to a forest. Data from a field study and a wind-tunnel experiment arepresented. These show that the variance in the streamwise velocity begins to adjust tothe new surface between 2 to 4 tree heights downwind of the transition. This is soonerthan either the vertical velocity variance or the shear stress, both of which begin to adjust in a zone 3 to 5 tree heights downwind of the edge. Key terms in the prognostic equations for streamwise and vertical velocity variance are evaluated in order to explain these differences. The flow distortion caused by the forest edge, which extends to 4 tree heights downwind of the forest edge, is shown to be crucial in the delayed turbulence development. Initially the shear production term, which is the dominant source for the streamwise velocity variance, is counteracted by a sink in the vertical advection term. After the flow levels out the pressure redistribution (return-to-isotropy) term becomes the main sink of streamwisevelocity variance and feeds energy into the vertical velocity component. Therefore, thedevelopment of the vertical velocity variance and shear stress cannot begin until afterdevelopment of an increase in the streamwise velocity variance. Results are comparedwith other experiments, including the flow across shelterbelts, and large-eddy simulations of forest flow. 相似文献
16.
Intermittency of turbulence within open canopies 总被引：1，自引：0，他引：1
Young-Hee Lee 《Asia-Pacific Journal of Atmospheric Sciences》2011,47(2):137-149
Eddy covariance data have been analyzed to examine intermittency and clustering properties of turbulence within open canopies. Intermittency consists of two aspects: one is related to amplitude variation and the other to clustering. Using the telegraph approximation (TA), the clustering properties have been separated from amplitude effects. Intermittency of canopy turbulence has been explored via clustering exponent, probability density distribution of inter-pulse period of TA, intermittency exponent and structure kurtosis. Intermittency and clustering properties of turbulence within open canopies show similar features to those within dense canopy but some differences are also noted. Unlike within a dense canopy, temperature does not show larger clustering than velocity, which seems to be due to a different thermal structure of the sub-canopy and larger vertical scale of canopy eddy within open canopies. Within the crown region, the inter-pulse probability distribution of TA does not show the ‘double regime’ which was observed within the crown of a dense canopy, indicating less influence of near-field source on canopy turbulence within open canopies. For TA series of the flow variables, intermittency exponent is higher for temperature than for two velocity components within open canopies, which are opposite within a dense canopy. When comparing intermittency for flow variables and their TA series, it is shown that amplitude variation mitigates intermittency for both velocity components and temperature although amplitude variations play a much larger role in velocity intermittency than in temperature counterpart. Kurtosis analysis demonstrates that structure kurtosis is higher at large scales in stable conditions than in unstable conditions, indicating the existence of global intermittency due to stable stratification. The intermittency features of canopy turbulence within open canopies have been discussed in comparison with those within a dense canopy. 相似文献
17.
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.
18.
Linearly Organized Turbulence Structures Observed Over a Suburban Area by Dual-Doppler Lidar 总被引：1，自引：1，他引：0
Dual-Doppler lidar observations are used to investigate the structure and evolution of surface-layer flow over a suburban
area. The observations were made during the Joint Urban 2003 (JU2003) field experiment in Oklahoma City, U.S.A. in the summer
of 2003. This study focuses specifically on a 10-h sequence of scan data beginning shortly after noon local time on 7 July
2003. During this period two coherent Doppler lidars performed overlapping low elevation angle sector scans upwind and south
of Oklahoma City’s central business district. Radial velocity data from the two lidars are processed to reveal the structure
and evolution of the horizontal velocity field in the surface layer throughout the afternoon and during the evening transition
period. The retrieved velocity fields clearly show a tendency for turbulence structures to be elongated in the direction of
the mean flow throughout the entire 10-h study period. In order to quantify the observed anisotropy and its dependence on
stability, integral length scales are estimated directly from the spatially resolved velocity retrievals. As the flow became
more stably stratified the characteristic cross-stream dimension of the linear structures decreased. The streamwise component
was consistently more anisotropic than the cross-stream component, and both velocity components exhibited maximum anisotropy
under neutral conditions. The ratio of the streamwise to cross-stream length scale was estimated to be about eight for the
streamwise component, and four for the cross-stream component under neutral conditions. 相似文献
19.
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
20.
Turbulence Structure Within and Above a Canopy of Bluff Elements 总被引：2，自引：2，他引：0
Margi Böhm John J. Finnigan Michael R. Raupach Dale Hughes 《Boundary-Layer Meteorology》2013,146(3):393-419
Measurements of turbulence structure in a wind-tunnel model canopy of bluff elements show many of the features associated with vegetation canopies and roughness sublayers but also display features more characteristic of the inertial sublayer (ISL). Points of similarity include the existence of an inflexion point in the space-time averaged streamwise velocity at the canopy top, the variation with height of turbulent second moments and the departure of the turbulent kinetic energy budget from local equilibrium in and just above the canopy. Quadrant analysis shows characteristic dominance of sweep over ejection events within the canopy although sweeps are more frequent than usually seen in vegetation canopies. Points of difference are a u′, w′ correlation coefficient that is closer to the ISL value than to most canopy data, and a turbulent Prandtl number midway between canopy and ISL values. Within the canopy there is distinct spatial partitioning into two flow regimes, the wake and non-wake regions. Both time-mean and conditional statistics take different values in these different regions of the canopy flow. We explain many of these features by appealing to a modified version of the mixing-layer hypothesis that links the dominant turbulent eddies to the instability of the inflexion point at canopy top. However, it is evident that these eddies are perturbed by the quasi-coherent wakes of the bluff canopy elements. Based upon an equation for the instantaneous velocity perturbation, we propose a criterion for deciding when the eddies linked to the inflexion point will dominate flow structure and when that structure will be replaced by an array of superimposed element wakes. In particular, we show that the resemblance of some features of the flow to the ISL does not mean that ISL dynamics operate within bluff-body canopies in any sense. 相似文献