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1.
A method to determine flow specific first-order closure for the turbulent flux of momentum in the atmospheric boundary layer
(ABL) is presented. This is based on the premise that eddy viscosity is a flow rather than a fluid property, and the physically
more realistic assumption that the transfer of momentum and other scalar quantities in a turbulent flow takes place by a large,
but finite number of length scales, than the often used single length scale, the ‘mixing length’. The resulting eddy viscosity
is flow specific and when applied to the study of the ABL, yields the vertical profiles of shear stress and mean wind velocity
in good agreement with observations. The method may be extended to other types of turbulent flows, however it should be recognized
that each type of flow may yield a different eddy viscosity profile. Using the derived eddy viscosity the paper presents simple
analytical solutions of the ABL equations to determine observationally consistent wind speed and shear stress profiles in
the ABL for a variety of practical applications including air pollution modelling. 相似文献
2.
In this study, a modelling methodology is proposed for RANS simulations of neutral Atmospheric Boundary Layer (ABL) flows on the basis of the standard k-ε model, which allows the adoption of an arbitrary shear stress model. This modelling methodology is first examined in the context of an open flat terrain in an empty domain to ascertain there are no substantial changes in the prescribed profiles. The results show that relatively good homogeneity can be achieved with this modelling methodology for various sets of inflow boundary profiles. In addition, to extend the solutions derived from the standard k-ε model to RNG k-ε model, the RNG k-ε model is in detail assembly and tuned. Finally, the topographic effects on surface wind speeds over a complex terrain are assessed with the combined use of the proposed methodology and the modified RNG model. The numerical results are in good agreement with wind tunnel testing results and long-term field observations. A discussion of the effects of horizontal homogeneity and turbulence models on the simulated wind flows over a complex terrain is also given. 相似文献
3.
Wind Tunnel Experiments and Numerical Simulation of Snow Drifting around an Avalanche Protecting Dam
To learn about wind flow and snow drifting around avalanche dams, experiments were done in the Jules Verne Climatic Wind Tunnel. The paper reports the results from numerical wind flow simulations that were done to support the findings from the wind tunnel. Satisfying the model similitude criteria for the wind tunnel configuration was difficult due to the inevitable small geometric scale of the model, while on the contrary the snow drifting conditions in the facilities were full scale. By comparing numerical wind flow results of full scale and model scale dams with the snow pattern observed in the wind tunnel, it was possible to conclude that albeit poor model similitude, the snowdrifts on the windward side of the wind tunnel model are likely to indicate the full scale natural situation. 相似文献
4.
Eugéne C. Joubert Thomas M. Harms Annethea Muller Martin Hipondoka Joh R. Henschel 《Environmental Fluid Mechanics》2012,12(1):23-44
In the Namib Desert seed distribution is greatly influenced by wind patterns. Existing literature regarding wind patterns
over dunes focuses on two-dimensional simulations of flow over simplified dune structures. The three-dimensional geometries
of the sand dunes suggests far more complex flow features exist, which are not captured by two-dimensional simulations. Computational
fluid dynamics (CFD) was used to reproduce the three-dimensional near surface wind patterns around a dune with the aim to
learn more about seed distribution. Field work included terrain mapping, wind speed, direction and temperature metering. The
CFD results show the expected two-dimensional flow features of high pressure at the dune toe, low pressure at the crest and
flow acceleration up windward slope. Also observed are some three-dimensional flow features such as a spiral vortex near the
crest and transverse flow due to crest-line curvature of the dune. It was also observed how the wall shear stress differs
due to the three-dimensional shape of the dune. The wall shear stress suggests that seed accumulation is more likely to occur
behind trailing (down-wind) crest edges. Particle tracking showed how seeds tend to move over the dune crest and recirculate
towards the crest on the lee-side. The study showed that adding the third dimension makes the simulations more complex, adds
to computational requirements and increases simulation time but also provides vital flow information which is not possible
with two-dimensional simulations. 相似文献
5.
Jerrold D. McAlpine Darko R. Koracin Douglas P. Boyle John A. Gillies Eric V. McDonald 《Environmental Fluid Mechanics》2010,10(6):691-710
Flights of rotorcraft over the desert floor can result in significant entrainment of particulate matter into the atmospheric
boundary layer. Continuous or widespread operation can lead to local and regional impacts on visibility and air quality. To
account for this pollutant source in air quality models, a parameterization scheme is needed that addresses the complex vertical
distribution of dust ejected from the rotorcraft wake into the atmospheric surface layer. A method to parameterize the wind
and turbulence fields and shear stress at the ground is proposed here utilizing computational fluid dynamics and a parameterized
rotor model. Measurements taken from a full scale experiment of rotorcraft flight near the surface are compared to the simulation
results in a qualitative manner. The simulation is shown to adequately predict the forward detachment length of the induced
ground jet compared to the measured detachment lengths. However, the simulated ground vortex widths and vorticity deviate
substantially from the measured values under a range of flight speeds. Results show that the method may be applicable for
air quality modeling assuming slow airspeeds of the rotorcraft, with advance ratios of 0.005–0.02. 相似文献
6.
The atmospheric boundary layer adjustment at the abrupt transition from a canopy (forest) to a flat surface (land or water) is investigated in a wind tunnel experiment. Detailed measurements examining the effect of canopy turbulence on flow separation, reduced surface shear stress and wake recovery are compared to data for the classical case of a solid backward-facing step. Results provide new insights into the interpretation for flux estimation by eddy-covariance and flux gradient methods and for the assessment of surface boundary conditions in turbulence models of the atmospheric boundary layer in complex landscapes and over water bodies affected by canopy wakes. The wind tunnel results indicate that the wake of a forest canopy strongly affects surface momentum flux within a distance of 35–100 times the step or canopy height, and mean turbulence quantities require distances of at least 100 times the canopy height to adjust to the new surface. The near-surface mixing length in the wake exhibits characteristic length scales of canopy flows at the canopy edge, of the flow separation in the near wake and adjusts to surface layer scaling in the far wake. Components of the momentum budget are examined individually to determine the impact of the canopy wake. The results demonstrate why a constant flux layer does not form until far downwind in the wake. An empirical model for surface shear stress distribution from a forest canopy to a clearing or lake is proposed. 相似文献
7.
Wind erosion processes affect soil surfaces across all land uses worldwide. Understanding the spatial and temporal scales of wind erosion is a challenging undertaking because these processes are diverse and highly variable. Wind tunnels provide a useful tool as they can be used to simulate erosion at small spatial scales. Portable wind tunnels are particularly valued because erosion can be simulated on undisturbed soil surfaces in the field. There has been a long history of use of large portable wind tunnels, with consensus that these wind erosion simulation tools can meet real world aerodynamic criteria. However, one consequence of striving to meet aerodynamic reality is that the size of the tunnels has increased, making them logistically difficult to work with in the field and resulting in a tendency to homogenise naturally complex soil surfaces. This homogenisation is at odds with an increasing awareness of the importance that small scale processes have in wind erosion. To address these logistical and surface homogenisation issues we present here the development and testing of a micro wind tunnel (MWT) designed to simulate wind erosion processes at high spatial resolution. The MWT is a duct-type design—0.05 m tall 0.1 m wide and with a 1.0 m working section. The tunnel uses a centrifugal motor to suck air through a flow‐conditioning section, over the working section and then through a sediment collection trap. Simulated wind velocities range from 5 to 18 m s?1, with high reproducibility. Wind speeds are laterally uniform and values of u * at the tunnel bed (calculated by measuring the pressure gradients within the MWT) are comparable with those of larger tunnels in which logarithmic profiles can be developed. Saltation sediment can be added. The tunnel can be deployed by a single person and operated on slopes ranging from 0 to 10°. Evidence is presented here that the MWT provides new and useful understanding of the erodibility of rangelands, claypans and ore stockpiles. 相似文献
8.
An analysis of concentration time series measured in a boundary-layer wind tunnel at the University of Hamburg is presented.
The measurements were conducted with a detailed aerodynamic model of the Oklahoma City (OKC) central business district (CBD)
at the scale of 1:300 and were part of the Joint Urban 2003 (JU2003) project. Concentration statistics, as well as concentration
probability density (PDF) and exceedance probability (EDF) functions were computed for street- and roof-level sites for three
different wind directions. Taking into account the different length scales and wind speeds in the wind-tunnel (WT) and full-scale
experiments, dimensionless concentrations and a dimensionless time scale are computed for the comparison with data from the
JU2003 full-scale tracer experiments, conducted in OKC in 2003. Using such dimensionless time, the WT time series cover a
~20 times longer time span than the JU2003 full-scale time series, which are analysed in detail in an accompanying, first
part of this paper. The WT time series are thus divided into 20 consecutive blocks of equal length and the statistical significance
of parameters based on relatively short records is assessed by studying the variability of the concentration statistics and
probability functions for the different blocks. In particular at sites closer to the plume edge, the results for the individual
blocks vary significantly and at such sites statistics from short records are not very representative. While the location
of three sampling sites in the WT closely matched the sites during the full-scale experiments, the prevailing wind directions
during the JU2003 releases were not exactly matched. The comparison between full-scale and WT concentration parameters should
thus primarily be interpreted in a qualitative rather than direct quantitative sense. Given the differences in mean wind directions
and concerns about the representativeness of full-scale concentration statistics, the WT and full-scale results compared well.
The 98 percentile concentrations for almost all full-scale releases analyzed are within the scatter of the percentiles observed
in the block analysis of the WT time series. Furthermore, the concentration percentiles appear linearly correlated with the
fluctuation intensities and the linear relationships determined in the wind tunnel agree well with full-scale results. 相似文献
9.
William C. Anderson Sukanta Basu Chris W. Letchford 《Environmental Fluid Mechanics》2007,7(3):195-215
Several non-dynamic, scale-invariant, and scale-dependent dynamic subgrid-scale (SGS) models are utilized in large-eddy simulations
of shear-driven neutral atmospheric boundary layer (ABL) flows. The popular Smagorinsky closure and an alternative closure
based on Kolmogorov’s scaling hypothesis are used as SGS base models. Our results show that, in the context of neutral ABL
regime, the dynamic modeling approach is extremely useful, and reproduces several establised results (e.g., the surface layer
similarity theory) with fidelity. The scale-dependence framework, in general, improves the near-surface statistics from the
Smagorinsky model-based simulations. We also note that the local averaging-based dynamic SGS models perform significantly
better than their planar averaging-based counterparts. Lastly, we find more or less consistent superiority of the Smagorinsky-based
SGS models (over the corresponding Kolmogorov’s scaling hypothesis-based SGS models) for predicting the inertial range scaling
of spectra. 相似文献
10.
T. Kubwimana P. Salizzoni E. Bergamini A. Mos P. Méjean L. Soulhac 《Environmental Fluid Mechanics》2018,18(3):769-786
In order to properly size the mechanical ventilation system of a tunnel, it is essential to estimate the wind-driven pressure difference that might rise between its two portals. In this respect, we explore here the pressure distribution over a tunnel portal under the influence of an incident atmospheric boundary layer and, in particular, its dependency on wind direction and on tunnel geometry. Reduced scale models of generic configurations of a tunnel portal are studied in an atmospheric wind tunnel. Pressure distributions over the front section of different open cavities are measured with surface taps, which allows us to infer the influence of the tunnel aspect ratio and wind direction on a pressure coefficient \(C_{P}\), defined as a spatially and time averaged non-dimensional pressure. Experiments reveal that the magnitude of the coefficient \(C_{P}\), as a function of the wind direction, is significantly influenced by the portal height-to-width ratio and almost insensitive to its length. The experimental data set is completed by hot-wire anemometry measurements providing vertical distribution of velocity statistics. The same configurations are simulated by numerically solving the Reynolds-averaged Navier–Stokes equations, adopting the standard \(k - \varepsilon\) turbulence model. Despite some discrepancies between numerical and experimental estimates of some flow parameters (namely the turbulent kinetic energy field), the numerical estimates of the pressure coefficients \(C_{P}\) show very good agreement with experimental data. The latter is also compared to the predictions of an analytical model, based on the estimate of a spatially averaged velocity within an infinitely long street canyon. The results of the model, which takes into account varying canyon aspect ratios, are in reasonable agreement with experimental data for all cases studied. Notably, its predictions are significantly better than those provided by the simple analytical relations usually adopted as a reference in tunnel ventilation studies. 相似文献
11.
Flows through forest canopies in complex terrain 总被引:6,自引:0,他引:6
Recent progress on boundary layer flow within and above tall forest canopies in complex terrain is reviewed from the perspective of developing methods to interpret carbon dioxide fluxes from tower measurements in real terrain. Two examples of complex terrain are considered in detail: a forest edge, which exemplifies nonuniform forests, and hilly terrain, which can lead to drainage currents at night. Dynamical arguments show that, when boundary layer winds approach a forest edge, the mean wind adjusts on a length scale of approximately 3L(c), where L(c) is the canopy drag length scale, which depends inversely on the leaf area density of the forest. Over a further distance that also scales on L(c), turbulence in the flow adjusts, and the mixing and transport in the canopy approaches the homogeneous limit. Even low hills change the neutral flow within and above the forest canopy substantially. When the canopy is tall, pressure gradients drive flow up both the upwind and downwind slopes of the hill, leading to an ejection of air out of the top of the canopy just downwind of the crest. This flow at the crest can then advect scalar out of the top of the forest, leading to large variations in the flux of scalar across the hill. At night, when the air near the ground cools and becomes stably stratified, turbulence within the canopy can collapse, even when the flow above the canopy remains turbulent. This leads to a decoupling of the air motions within the canopy from those above. The air above the canopy can then continue to pass up and over the hill, as it does in the neutral case, but at the same time, air within the canopy drains down the hill slopes as drainage currents. These analyses will help us understand when flux towers are reliably measuring the net ecosystem exchange and suggest ways of correcting the flux tower data in more complex situations. 相似文献
12.
Justus Kristine A. Murlis John Jones Chris Cardé Ring T. 《Environmental Fluid Mechanics》2002,2(1-2):115-142
The patterns of stimulus available to moths flying along pheromone plumes in a 3-m-long wind tunnel were characterized using a high frequency photoionization detector in conjunction with an inert tracer gas. Four contrasting flow regimes and source conditions were produced: odor released in pulses from a vertical and horizontal array of four sources, odor released continuously from a point source, and odor released continuously from a point source into an oscillatory wake. Although the four flow regimes produced plumes of intermittent and fluctuating concentration, there were considerable differences in the structure of the signal presented to the sensor. Pulses of tracer gas released at 10 Hz retained most of their longitudinal and lateral separation. The plume growing in the disturbed flow (`oscillatory'), was broader in its lateral extent than the plume growing in an undisturbed flow (`continuous'), and the concentrations in the former were lower at each downstream position. The signal recorded in the disturbed flow had higher intermittency, but the ratio between the peak concentration and the signal mean was lower than in the continuous plume. Time scales were typically longer in the tunnel than in a field setting, but length scales and the main features of intermittency and fluctuation were similar. Moths flying along plumes of pheromone in this and similar wind tunnels typically slow their velocity and narrow the lateral excursions of their flight track as they approach a pheromone source. Which features of the plumes measured in this study account for these behavioral reactions remains to be determined. 相似文献
13.
In large-eddy simulations of atmospheric boundary layer turbulence, the lumped coefficient in the eddy-diffusion subgrid-scale
(SGS) model is known to depend on scale for the case of inert scalars. This scale dependence is predominant near the surface.
In this paper, a scale-dependent dynamic SGS model for the turbulent transport of reacting scalars is implemented in large-eddy
simulations of a neutral boundary layer. Since the model coefficient is computed dynamically from the dynamics of the resolved
scales, the simulations are free from any parameter tuning. A set of chemical cases representative of various turbulent reacting
flow regimes is examined. The reactants are involved in a first-order reaction and are injected in the atmospheric boundary
layer with a constant and uniform surface flux. Emphasis is placed on studying the combined effects of resolution and chemical
regime on the performance of the SGS model. Simulations with the scale-dependent dynamic model yield the expected trends of
the coefficients as function of resolution, position in the flow and chemical regime, leading to resolution-independent turbulent
reactant fluxes. 相似文献
14.
A simulation tool has been developed to model the wind fields, turbulence fields, and the dispersion of Chemical, Biological,
Radiological and Nuclear (CBRN) substances in urban areas on the building to city blocks scale. A Computational Fluid Dynamics
(CFD) approach has been taken that naturally accounts for critical flow and dispersion processes in urban areas, such as channeling,
lofting, vertical mixing and turbulence, by solving the steady-state, Reynolds-Averaged Navier–Stokes (RANS) equations. Rapid
generation of high quality cityscape volume meshes is attained by a unique voxel-based model generator that directly interfaces
with common Geographic Information Systems (GIS) file formats. The flow and turbulence fields are obtained by solving the
steady-state RANS equations using a collocated, pressure-based approach formulated for unstructured and polyhedral mesh elements.
Turbulence modeling is based upon the Renormalization Group variant of the k–ε model (k–ε RNG). Neutrally buoyant simulations are made by prescribing velocity boundary condition profiles found by a power–law relationship,
while turbulence quantities boundary conditions are defined by a prescribed mixing length in conjunction with the assumption
of turbulence equilibrium. Dispersion fields are computed by solving an unsteady transport equation of a dilute gas, formulated
in a Eulerian framework, using the velocity and turbulence fields found from the steady-state RANS solution. In this paper
the model is explained and detailed comparisons of predicted to experimentally obtained velocity, turbulence and dispersion
fields are made to neutrally stable wind tunnel and hydraulic flume experiments. 相似文献
15.
Castelli S. Trini Morelli S. Anfossi D. Carvalho J. Sajani S. Zauli 《Environmental Fluid Mechanics》2004,4(2):157-196
In this work a model intercomparison between RAMS and ETA models is carried out, with the aim of evaluating the quality and accuracy of these mesoscale models in reproducing the time evolution of the meteorology in real complex terrain. This is of great importance not only for meteorological forecast but also for air quality assessment. Numerical simulations are performed to reproduce the mean variables' fields and to compare them with measurements collected during the field campaign TRACT. The domain covers the Rhine valley and surrounding mountainous region and we consider a time period of two days. Results from simulations are compared to observations relative to ground stations and radiosoundings. A qualitative analysis is joined to a quantitative estimation of some reference statistical indexes. Both RAMS and ETA models performances are satisfactory when compared to the measured data and also their relative agreement is good. The mean variable fields are reproduced with a satisfactory degree of reliability, even if the simulated profiles are not able to describe the largest fluctuations of the variables. At the surface stations, the best agreement between predictions and observations is obtained for the wind velocity, while the quality of the results is lower for temperature and humidity. 相似文献
16.
Air temperature and wind speed profiles measured during one year by means of a SODAR-RASS system located within a large park were examined for the urban boundary layer (UBL) over Rome, Italy. These data, combined with velocity and temperature measurements performed near the ground were used to analyze the vertical structure of the boundary layer and to estimate some turbulence parameters characterizing the surface layer. About 52,000 vertical profiles of wind speed and temperature were used for the analysis, allowing investigation for a large variety of stability conditions. First, friction velocity and Obukhov length were examined, showing clearly their dependence on the time of day and season. Second, the applicability of the Monin–Obukhov (MO) similarity theory—developed over rural terrain—was tested up to 200?m above ground level. For the wind speed profiles, the performance of the MO similarity degrades with both increasing height and stability, with maximum errors that are on the order of 300?% at 200?m for the most stable case. In contrast, for the air temperature the error always remains below 50?%. 相似文献
17.
基于Voronoi图的农村居民点空间分布特征及其影响因素研究——以北京市昌平区为例 总被引:18,自引:0,他引:18
采用GIS查询统计与Voronoi图CV值相结合的分析方法,研究了地形、公路及河流等环境因素对北京市昌平区农村居民点空间分布特征的影响.结果表明:地形是影响农村居民点空间分布的主导因素,平原地区农村居民点用地比例较高,规模较大,密度较高;山区农村居民点用地比例较低,规模相对较小,密度也较低.公路交通是影响农村居民点空间分布的重要因素,91.19%的农村居民点分布在距离主要公路距离小于1 500 m的范围内;河流对农村居民点分布也有一定影响,距离河流越远,农村居民点密度越低. 相似文献
18.
To better understand the dynamics of Kelvin–Helmholtz instabilities in environmental flows, their evolution is investigated using direct numerical simulations (DNS). Two-dimensional DNS is used to examine the large-scale and small-scale structures of the instability at high Reynolds and Prandtl numbers that represent real environmental flows. The semi-analytical model of Corcos and Sherman (J Fluid Mech 73:241–264, 1976) is used to explain the physics of these simulations prior to saturation of the KH billow, and also provide a computationally efficient prediction of the vortex dynamics of the instability. The DNS results show that the large-scale structure of the billow does not depend on the Reynolds number for sufficiently high Reynolds numbers. The billow structure reveals a less straightforward dependence on the Prandtl number. Predictions of the model of Corcos and Sherman (J Fluid Mech 73:241–264, 1976) improve as Reynolds number and Prandtl number increase. The small-scale structure of the vorticity and density fields vary with both Reynolds and Prandtl numbers. Three-dimensional DNS of KH flows and their transition to turbulence are used to study small length scales. Based on the thickness of the braid, a simple method is introduced to estimate the Batchelor scale, which can be used as a guide for the resolution required for the direct numerical simulation of two and three-dimensional Kelvin–Helmholtz flow fields. 相似文献
19.
Stephan F. J. De Wekker D. G. Steyn J. D. Fast M. W. Rotach S. Zhong 《Environmental Fluid Mechanics》2005,5(1-2):35-62
Data from a comprehensive field study in the Riviera Valley of Southern Switzerland are used to investigate convective boundary layer structure in a steep valley and to evaluate wind and temperature fields, convective boundary layer height, and surface sensible heat fluxes as predicted by the mesoscale model RAMS. Current parameterizations of surface and boundary layer processes in RAMS, as well as in other mesoscale models, are based on scaling laws strictly valid only for flat topography and uniform land cover. Model evaluation is required to investigate whether this limits the applicability of RAMS in steep, inhomogeneous terrain. One clear-sky day with light synoptic winds is selected from the field study. Observed temperature structure across and along the valley is nearly homogeneous while wind structure is complex with a wind speed maximum on one side of the valley. Upvalley flows are not purely thermally driven and mechanical effects near the valley entrance also affect the wind structure. RAMS captured many of the observed boundary layer characteristics within the steep valley. The wind field, temperature structure, and convective boundary layer height in the valley are qualitatively simulated by RAMS, but the horizontal temperature structure across and along the valley is less homogeneous in the model than in the observations. The model reproduced the observed net radiation, except around sunset and sunrise when RAMS does not take into account the shadows cast by the surrounding topography. The observed sensible heat fluxes fall within the range of simulated values at grid points surrounding the measurement sites. Some of the scatter between observed and simulated turbulent sensible heat fluxes are due to sub-grid scale effects related to local topography. 相似文献
20.
Evaluation of CFD Simulation using RANS Turbulence Models for Building Effects on Pollutant Dispersion 总被引:5,自引:0,他引:5
CFD evaluations were performed to examine the applicability of the RANS methods in simulating pollutant dispersion near, within
and over three typical building configurations: (1) an isolated building, (2) a building array and (3) an urban intersection.
The CFD results are compared with values obtained from wind tunnel tests. In some situations major differences between the
wind tunnel tests and the CFD results were observed. The main source of difference between the CFD and wind tunnel results
was inadequate modelling of local flow patterns using the RANS turbulence models. Also inappropriate evaluation of high intermittent
turbulent mixing in the RANS approach may lead to either over-prediction or under-prediction of the concentration level, by
up to a factor of 10, depending on the case investigated. 相似文献