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1.
The processes influencing turbulence in a deciduous forest and the relevant length and time scales are investigated with spectral and cross-correlation analysis. Wind velocity power spectra were computed from three-dimensional wind velocity measurements made at six levels inside the plant canopy and at one level above the canopy. Velocity spectra measured within the plant canopy differ from those measured in the surface boundary layer. Noted features associated with the within-canopy turbulence spectra are: (a) power spectra measured in the canopy crown peak at higher wavenumbers than do those measured in the subcanopy trunkspace and above the canopy; (b) peak spectral values collapse to a relatively universal value when scaled according to a non-dimensional frequency comprised of the product of the natural frequency and the Eulerian time scale for vertical velocity; (c) at wavenumbers exceeding the spectral peak, the slopes of the power spectra are more negative than those observed in the surface boundary layer; (d) Eulerian length scales decrease with depth into the canopy crown, then increase with further depth into the canopy; (e) turbulent events below crown closure are more correlated with turbulent events above the canopy than are those occurring in the canopy crown; and (f) Taylor's frozen eddy hypothesis is not valid in a plant canopy. Interactions between plant elements and the mean wind and turbulence alter the processes that produce, transport and remove turbulent kinetic energy and account for the noted observations. 相似文献
2.
Turbulence statistics were measured in a natural black-spruce forest canopy in southeastern Manitoba, Canada. Sonic anemometers were used to measure time series of vertical wind velocity (w), and cup anemometers to measure horizontal wind speed (s), above the canopy and at seven different heights within the canopy. Vertical profiles were measured during 25 runs on eight different days when conditions above the canopy were near-neutral.Profiles of s and of the standard deviation (
w
) of w show relatively little scatter and suggest that, for this canopy and these stability conditions, profiles can be predicted from simple measurements made above the canopy. Within the canopy, a negative skewness and a high kurtosis of the w-frequency distributions indicate asymmetry and the persistence of large, high-velocity eddies. The Eulerian time scale is only a weak function of height within the canopy.Although w-power spectra above the canopy are similar to those in the free atmosphere, we did not observe an extensive inertial subrange in the spectra within the canopy. Also, a second peak is present that is especially prominent near the ground. The lack of the inertial subrange is likely caused by the presence of sources and sinks for turbulent kinetic energy within our canopy. The secondary spectral peak is probably generated by wake turbulence caused by form drag on the wide, horizontal spruce branches. 相似文献
3.
Turbulence Statistics Measurements in a Northern Hardwood Forest 总被引：3，自引：0，他引：3
M. G. Villani H. P. Schmid H.-B. Su J. L. Hutton C. S. Vogel 《Boundary-Layer Meteorology》2003,108(3):343-364
Tower-based turbulence measurements were collected in and over a mixed hardwood forest at the University of Michigan BiologicalStation (UMBS) UMBSflux site in the northern summerof 2000. Velocity and temperature fluctuations were measured at five levels within the canopy (up to the canopy height, H = 21.4 m), using one- and three-dimensional sonic anemometers and fine-wire thermocouples. Six additional thermocouples were distributed over the canopy-layer depth. Three-dimensional velocities and sonic temperatures were also measured above the canopy at 1.6H and at 2.15H on the AmeriFlux tower located at the UMBSflux site. Vertical profiles of buoyancy flux, mean horizontal velocity, Reynolds stress, and standard deviation and skewness of velocity components were calculated. The analysis of these measurements aims at a multi-layer parameterization framework of turbulence statistics forimplementation in Lagrangian stochastic models. Turbulence profiles and power spectra above the canopy were analyzed in the context of Monin-Obukhov similarity theory (MOST) and Kolmogorov theory, as determined by stability at the top level (2.15H), to assess the extent to which surface scaling is valid as the canopy top is approached. Velocity spectra were computed to explore the potential of estimating the viscous dissipation rate, and results show that the high frequency range of the spectra above the canopy exhibits the roll-off predicted by Kolmogorov theory. Similarly, velocity standard deviations above the canopy converge to MOST predicted values toward the top level, and spectral peaks shift with stability, as expected. Within the canopy, both turbulence statistics profiles and spectral distributions follow the general known characteristics inside forests. 相似文献
4.
Two-point, space-time correlations of streamwise and vertical velocity were obtained from a wind tunnel simulation of an atmospheric surface layer with an underlying model wheat canopy constructed of flexible nylon stalks. Velocity data extend from 1/6 canopy height to several canopy heights, with in excess of 2000 three-dimensional vector separations of the two x-wire probes. Isocorrelation contours over anx, z slice show the streamwise velocity autocorrelation to be roughly circular, such that vertical velocities at the same horizontal position but different heights are closely in phase. Cross-correlations between the two velocity components reflect this difference to some extent. Lateral displacements of the probes revealed side lobes with correlations of reversed sign but we cannot positively link this pattern to particular vorticular structures. Integral length scales obtained directly from the spatial correlations match similar scales deduced from single-point time series with Taylor's hypothesis at 2 to 3 times the canopy height but greatly exceed such scales at lower levels, particularly within the wheat. We conclude that the reversed sign lateral lobes are important components of the correlation field and that an integral length scale for the lateral direction must be defined such that they are included. Convective velocities obtained from the time lag to optimally restore correlation lost by physical separation of the probes change only slowly with height and greatly exceed the mean wind velocity within and immediately above the canopy. Thus, mean wind velocity is not a suitable proxy for convective velocity in the application of Taylor's hypothesis in this situation. The ratio of vertical to longitudinal convective velocity for the streawise velocity signal yields a downwind tilt angle of about 39° which is probably a better estimate of the slope of the dominant fluid motions than the tilt of the major axis of the isocorrellation contours mentioned previously. 相似文献
5.
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
6.
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
7.
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
8.
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
9.
K. H. Papadopoulos N. C. Stefantos U. S. Paulsen E. Morfiadakis 《Boundary-Layer Meteorology》2001,101(1):77-107
Four commercial and one research cup anemometers were comparatively tested in a complex terrain site to quantify the effects of turbulence and flow inclination on the wind speed measurements. The difference of the mean windspeed reading between the anemometers was as much as 2% for wind directions where the mean flow was horizontal. This difference was large enough to be attributed to the well-known overspeeding effect related to the differing distance constant (ranging from 1.7 to 5 m) of the cup anemometers. The application of a theoretical model of the cup-anemometer behaviour in athree-dimensional turbulent wind field proved successful in explaining theobserved differences.Additional measurements were taken with the anemometers tilted at known angles into and out of the incident wind flow. Thus, a field-derived angular response curve is constructed for each anemometer and the deviations from publishedwind-tunnel results are discussed.The uncertainties of, or false assumptions about, the angular response characteristics of the anemometers contribute the largest amount inthe observed errors of mean wind speed even for a horizontal mean flow. The angular response curves are finally used to correct the 10-min mean windspeed. The necessary information for the correction is the turbulent intensity (preferably in the vertical direction) and the mean flow inclination.For demanding applications, the angular response parameters of cup anemometers should be taken into account. The incorporation of the angular response parameters in a correction scheme would be most robustly applied if their variation with inclination and wind speed was smooth. 相似文献
10.
11.
Errors in turbulence measurements with a sonic anemometer 总被引：1，自引：0，他引：1
Errors in surface-layer wind data obtained by sonic anemometers are estimated as a function of relative wind direction using a comparison between two identical anemometers. The maximum errors in mean windspeed and the standard deviations of the horizontal wind components are found to be between 10 and 20%. Agreement between the size of the measured errors and those calculated from wind-tunnel data is fair.In addition, vertical velocity data are analysed to show the presence of flow distortion induced by the asymmetry in the sonic anemometer probe and the mounting of the probe on the top of a mast. 相似文献
12.
The horizontal and vertical wind velocity fluctuations were measured using two sonic anemometers at a height of 135 cm above
a snow surface under a transverse snow wave-forming condition. A snow-wave was formed when the wind at a height of 1 m blew
at a speed of more than 7 m s−1 after an approximate accumulation of from 10 to 20 cm of new snow on a snowfield. For example, when a snow-wave had a wavelength
of 10 m and a wave height of 15 to 20 cm, the measured horizontal and vertical velocity components showed that they had a
frequency peak of 0.7 Hz in coherence and co-spectrum corresponding to this wavelength. The results suggest that wind turbulence
and snow-wave formation interact with each other. 相似文献
13.
The horizontal and vertical wind velocity fluctuations were measured using two sonic anemometers at a height of 135 cm above a snow surface under a transverse snow wave-forming condition. A snow-wave was formed when the wind at a height of 1 m blew at a speed of more than 7 m s–1 after an approximate accumulation of from 10 to 20 cm of new snow on a snowfield. For example, when a snow-wave had a wavelength of 10 m and a wave height of 15 to 20 cm, the measured horizontal and vertical velocity components showed that they had a frequency peak of 0.7 Hz in coherence and co-spectrum corresponding to this wavelength. The results suggest that wind turbulence and snow-wave formation interact with each other. 相似文献
14.
The Campbell CSAT3 sonic anemometer is one of the most popular instruments for turbulence measurements in basic micrometeorological research and ecological applications. While measurement uncertainty has been characterized by field experiments and wind-tunnel studies in the past, there are conflicting estimates, which motivated us to conduct a numerical experiment using large-eddy simulation to evaluate the probe-induced flow distortion of the CSAT3 anemometer under controlled conditions, and with exact knowledge of the undisturbed flow. As opposed to wind-tunnel studies, we imposed oscillations in both the vertical and horizontal velocity components at the distinct frequencies and amplitudes found in typical turbulence spectra in the surface layer. The resulting flow-distortion errors for the standard deviations of the vertical velocity component range from 3 to 7%, and from 1 to 3% for the horizontal velocity component, depending on the azimuth angle. The magnitude of these errors is almost independent of the frequency of wind speed fluctuations, provided the amplitude is typical for surface-layer turbulence. A comparison of the corrections for transducer shadowing proposed by both Kaimal et al. (Proc Dyn Flow Conf, 551–565, 1978) and Horst et al. (Boundary-Layer Meteorol 155:371–395, 2015) show that both methods compensate for a larger part of the observed error, but do not sufficiently account for the azimuth dependency. Further numerical simulations could be conducted in the future to characterize the flow distortion induced by other existing types of sonic anemometers for the purposes of optimizing their geometry. 相似文献
15.
A short review of experimental findings is given, followed by a theoretical derivation, based on Taylor's hypothesis, of formulas for lateral coherences. It is assumed that the flow is stationary and homogeneous. Explicit formulas are derived assuming an energy spectrum pertaining to the inertial subrange. Even when the last assumption is not fulfilled, there are only four different types of non-zero velocity coherences. These four coherences correspond to the combinations uu, vv, ww, and uv, where u, v, and w are the longitudinal, the transversal, and the vertical component of the turbulent velocity with respect to the direction of the horizontal mean wind velocity U. In the case of small displacements relative to the scale of turbulence, the coherences are shown to be universal functions of the non-dimensional frequency nD/¦U¦, where n is the frequency and D the lateral displacement. It is shown that these theoretical formulas for spectral coherences are in good agreement with atmospheric data. Finally, the role of the scale of the turbulence is discussed. 相似文献
16.
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
17.
Three-dimensional wind velocity components were measured above and within a uniform almond orchard. Turbulent statistics associated with the turbulent flow inside the canopy are examined in detail. Turbulence in an almond orchard is characterized by relatively high turbulent intensities and large skewness and kurtosis values. These results indicate that the frequency distribution of wind velocity components is non-Gaussian. Conditional sampling of the turbulent measurements show that large, infrequent sweeps provide the predominant mechanism for tangential momentum stress in the canopy crown. Deep inside the canopy, a secondary wind maximum and small, but positive, tangential momentum stresses are observed. 相似文献
18.
The capability of SODAR to measure the mean wind field in the lower boundary layer is well known and documented. Therefore, mean wind data are easily obtainable by means of the SODAR-technique, and are used to simulate the transport of pollutants after their release into the atmosphere. But when calculating the diffusion of pollutants, information about atmospheric turbulence is needed, too. In principle, a SODAR can measure turbulence data like the standard deviation of the vertical wind speed or horizontal wind direction. But when measuring turbulence data with a SODAR, one is beset by a host of limitations like volume sampling, spatial and temporal separation of sampling volume, attenuation of the acoustic waves and the slow speed of sound. Therefore, successful turbulence measurements with SODAR are not numerous and little is known about the quality of these data. In this context an intercomparison between a REMTECH-SODAR and a sonic anemometer mounted at the 100 m level of our meteorological tower was performed in summer 1990 at the Kernforschungszentrum Karlsruhe. The intercomparison is in two parts:
- Half hour mean values of the standard deviation of the vertical wind speed are intercompared by scatter plots and by a linear regression and correlation analysis.
- During 7 periods, 2 hours each, and covering atmospheric stabilities from unstable to slightly stable, the instantaneous vertical wind speeds were measured by both instruments and spectra were calculated.
19.
T. Maitani 《Boundary-Layer Meteorology》1989,48(1-2):19-31
Observations of wind velocity and air temperature fluctuations were made in the nocturnal surface inversion layer over a sorghum field. Wave-like fluctuations of temperature and wind velocity with a period of 15–20 min were observed for about 2 hours, 3 to 5 hours before sunrise. Wave-like fluctuations of temperature were observed in the air layer above and within a plant canopy and were most noticeable at the top of the plant canopy. Spectral analysis of temperature and wind velocity fluctuations reveals a separation of energy into wave-like and turbulent fluctuations. Cospectral analysis shows that for both momentum and heat, vertical transports are partitioned almost equally in the frequency ranges characteristic of wave-like and turbulent fluctuations. This suggests wave- turbulence interactions at low frequencies in the air layer near a plant canopy. 相似文献
20.
Shuhua Liu Heping Liu M. Xu M. Y. Leclerc Tingyao Zhu Changjie Jin Zhongxiang Hong Jun Li Huizhi Liu 《Boundary-Layer Meteorology》2001,98(1):83-102
Three velocity componentsand temperature were measured usingthree-dimensional sonic anemometers/thermometers attwo levels, above and within a forest canopy, in theChangbai Mountains of northeast China. Turbulencespectral structure, local isotropy anddissipation rates above and within the forest canopywere calculated using the eddy correlation method.Results show that the normalized turbulent spectralcurves have -2/3 slopes in the inertial subrange.While the shapes of the spectra are in good agreementwith the Kansas flat terrain results, the atmosphericturbulence is anisotropic above the forest canopy. Dueto breaking down of large eddies by the foliage,branches and trunks, the spectral peak frequencies forvelocity and temperature are higher withinthan above the forest canopy. Compared withmeasurements from previous studies over flat terrain,the velocity and temperature spectra above andinside the forest canopy appear to shift toward higherfrequencies. The turbulence is approximately isotropicin the inertial subrange within the forest canopy, and isanisotropic above the forest canopy. The turbulentkinetic energy and heat energy dissipation rates aboveand inside the forest canopy are much larger thanthose obtained by Kaimal and Hogstrom over grasslandand grazing land. The distinct features in the resultsof the present experiment may be attributed to thedynamic forcing caused by the rough surface of the forestcanopy. 相似文献