共查询到20条相似文献，搜索用时 781 毫秒
1.
Hrvoje Kozmar 《Theoretical and Applied Climatology》2010,100(1-2):153-162
Precise urban atmospheric boundary layer (ABL) wind tunnel simulations are essential for a wide variety of atmospheric studies in built-up environments including wind loading of structures and air pollutant dispersion. One of key issues in addressing these problems is a proper choice of simulation length scale. In this study, an urban ABL was reproduced in a boundary layer wind tunnel at different scales to study possible scale effects. Two full-depth simulations and one part-depth simulation were carried out using castellated barrier wall, vortex generators, and a fetch of roughness elements. Redesigned “Counihan” vortex generators were employed in the part-depth ABL simulation. A hot-wire anemometry system was used to measure mean velocity and velocity fluctuations. Experimental results are presented as mean velocity, turbulence intensity, Reynolds stress, integral length scale of turbulence, and power spectral density of velocity fluctuations. Results suggest that variations in length-scale factor do not influence the generated ABL models when using similarity criteria applied in this study. Part-depth ABL simulation compares well with two full-depth ABL simulations indicating the truncated vortex generators developed for this study can be successfully employed in urban ABL part-depth simulations. 相似文献
2.
Hrvoje Kozmar 《Theoretical and Applied Climatology》2011,106(1-2):95-104
The boundary layer wind tunnel at the Technische Universit?t München was tested for atmospheric boundary layer (ABL) simulations. The ABLs developing above rural, suburban, and urban terrains were reproduced using the Counihan method, i.e., castellated barrier wall, vortex generators, and a fetch of surface roughness elements. A series of flow-characteristic evaluations was performed to investigate the flow development and uniformity. Experimental results presented as mean velocity, turbulence intensity, integral length scale of turbulence, Reynolds stress, and power spectral density of velocity fluctuations were compared with the ESDU data and/or theoretical models. Generated ABL wind-tunnel simulations compare well with the rural, suburban, and urban ABLs. In the test section area used for experiments on structural models, the ABL simulation is developed and uniform. Results of this study indicate the boundary layer wind tunnel at the Technische Universit?t München can be successfully employed in a broad spectrum of engineering, environmental, and micrometeorological studies, where it is required to accurately reproduce ABL characteristics. 相似文献
3.
4.
The turbulent flow in and above plant canopies is of fundamental importance to the understanding oftransport processes of momentum,heat and mass between plant canopies and atmosphere,and to microme-teorology.The Reynolds stress equation model(RSM)has been applied to calculate the turbulence in cano-pies in this paper.The calculated mean wind velocity profiles,Reynolds stress,turbulent kinetic energy andviscous dissipation rate in a corn canopy and a spruce forest are compared with field observed data and withWilson's and Shaw's model.The velocity profiles and Rynolds stress calculated by both models are in goodagreement,and the length scale of turbulence appears to be similar. 相似文献
5.
Large-Eddy Simulation of Flows over Random Urban-like Obstacles 总被引：2，自引：2，他引：0
Further to our previous large-eddy simulation (LES) of flow over a staggered array of uniform cubes, a simulation of flow
over random urban-like obstacles is presented. To gain a deeper insight into the effects of randomness in the obstacle topology,
the current results, e.g. spatially-averaged mean velocity, Reynolds stresses, turbulence kinetic energy and dispersive stresses,
are compared with our previous LES data and direct numerical simulation data of flow over uniform cubes. Significantly different
features in the turbulence statistics are observed within and immediately above the canopy, although there are some similarities
in the spatially-averaged statistics. It is also found that the relatively high pressures on the tallest buildings generate
contributions to the total surface drag that are far in excess of their proportionate frontal area within the array. Details
of the turbulence characteristics (like the stress anisotropy) are compared with those in regular roughness arrays and attempts
to find some generality in the turbulence statistics within the canopy region are discussed. 相似文献
6.
Mathias W. Rotach 《Boundary-Layer Meteorology》2001,99(3):379-410
7.
大气边界层湍流运动是地球大气运动最重要的能量输送过程之一.当数值模式分辨率接近活跃含能湍涡长度尺度时,湍流运动被部分解析,被称为"灰色区域",传统的边界层方案不适合此时模式湍流问题的描述.为了提高模式边界层方案在包括"灰色区域"的不同网格尺度上的描述能力,适应不同分辨率模式的需要,在雷诺平均湍流理论基础上,修正Mell... 相似文献
8.
Stable Nocturnal Boundary Layers: A Comparison of One-Dimensional and Large-Eddy Simulation Models 总被引：1，自引：1，他引：0
S. Galmarini 《Boundary-Layer Meteorology》1998,88(2):181-210
9.
A Large-Eddy Simulation and Lagrangian Stochastic Study of Heavy Particle Dispersion in the Convective Boundary Layer 总被引：4，自引：1，他引：4
Large-eddy simulation and Lagrangian stochastic dispersion models were used to study heavy particle dispersion in the convective boundary layer (CBL). The effects of various geostrophic winds, particle diameters, and subgrid-scale (SGS) turbulence were investigated. Results showed an obvious depression in the vertical dispersion of heavy particles in the CBL and major vertical stratification in the distribution of particle concentrations, relative to the passive dispersion. Stronger geostrophic winds tended to increase the dispersion of heavy particles in the lower CBL. The SGS turbulence, particularly near the surface, markedly influenced the dispersion of heavy particles in the CBL. For reference, simulations using passive particles were also conducted; these simulation results agreed well with results from previous convective tank experiments and numerical simulations. 相似文献
10.
We study turbulent flow over two-dimensional hills. The Reynolds stresses are represented by a second-order closure model, where advection, diffusion, production and dissipation processes are all accounted for. We solve a full set of primitive non-hydrostatic dynamic equations for mean flow quantities using a finite-difference numerical method. The model predictions for the mean velocity and Reynolds stresses are compared with the measured data from a wind-tunnel experiment that simulates the atmospheric boundary layer. The agreement is good. The performance of the second-order closure model is also compared withthat of lower level turbulence models, including the eddy-viscositymodel and algebraic Reynolds stress models. It is concluded that thepresent closure is a considerable improvement over the other modelsin representing various physical effects in flow over hills. Thefeasibility of running a finite-difference numerical simulationincorporating a full second-order closure model on an IBM workstationis also demonstrated. 相似文献
11.
Mohamed F. YASSIN 《大气科学进展》2009,26(6):1241-1252
To assist validation of the experimental data of urban pollution dispersion, the effect of
an isolated building on the flow and gaseous diffusion in the wake region have been investigated
numerically in the neutrally stratified rough-walled turbulent boundary layer. Numerical studies were
carried out using Computational Fluid Dynamics (CFD) models. The CFD models used for the simulation
were based on the steady-state Reynolds-Average Navier-Stoke equations (RANS) with κ-ε
turbulence models; standard κ-ε and RNG κ-ε models. Inlet conditions
and boundary conditions were specified numerically to the best information available for each fluid modeling
simulation. A gas pollutant was emitted from a point source within the recirculation cavity behind the
building model. The accuracy of these simulations was examined by comparing the predicted results with wind
tunnel experimental data. It was confirmed that simulation using the model accurately reproduces the velocity
and concentration diffusion fields with a fine-mish resolution in the near wake region. Results indicated that
there is a good agreement between the numerical simulation and the wind tunnel experiment for both wind flow
and concentration diffusion. The results of this work can help to improve the understanding of mechanisms of
and simulation of pollutant transport in an urban environment. 相似文献
12.
To investigate the suitability of computational fluid dynamics (CFD) with regard to windbreak aerodynamics, simulations are
performed with a state-of-the-art numerical scheme (Fluent) and compared against experimental data for two- and three-dimensional
disturbances, namely the case of a long straight porous shelter fence and the case of a shelter fence erected in a square
about an enclosed plot. A thorough sensitivity study quantifies the impact of numerical choices on the simulation (e.g. grid-point
density, domain size, turbulence closure), and leads to guidelines that should ensure objective simulation of windbreak flows.
On a fine grid Fluent’s “realizable k–ε closure” gives results that are in qualitative accord with the observed mean winds. 相似文献
13.
The effectiveness of closure assumptions implemented in turbulent boundary-layer models is rather uncertain over complex terrain. Different closure schemes for Reynolds shear stress based on the mixing-length concept are compared with data from wind tunnel experiments over complex terrain and the results are analysed on the basis of second-order moment equations. A good estimation of the vertical momentum flux velocity scale turns out to be given by the standard deviation of the vertical velocity while the turbulent kinetic energy scaling gives less satisfactory results in regions where turbulence anisotropy is large. Fairly good results are given by closure models implementing a shear-limited mixing-length already proposed for non-logarithmic wind profiles, while large errors characterize traditional mixing-length formulations. 相似文献
14.
Measurements and Computations of Flow in an Urban Street System 总被引：1，自引：1，他引：0
Ian P. Castro Zheng-Tong Xie V. Fuka Alan G. Robins M. Carpentieri P. Hayden D. Hertwig O. Coceal 《Boundary-Layer Meteorology》2017,162(2):207-230
We present results from laboratory and computational experiments on the turbulent flow over an array of rectangular blocks modelling a typical, asymmetric urban canopy at various orientations to the approach flow. The work forms part of a larger study on dispersion within such arrays (project DIPLOS) and concentrates on the nature of the mean flow and turbulence fields within the canopy region, recognising that unless the flow field is adequately represented in computational models there is no reason to expect realistic simulations of the nature of the dispersion of pollutants emitted within the canopy. Comparisons between the experimental data and those obtained from both large-eddy simulation (LES) and direct numerical simulation (DNS) are shown and it is concluded that careful use of LES can produce generally excellent agreement with laboratory and DNS results, lending further confidence in the use of LES for such situations. Various crucial issues are discussed and advice offered to both experimentalists and those seeking to compute canopy flows with turbulence resolving models. 相似文献
15.
We use large-eddy simulation (LES) to study the turbulent pressure field in atmospheric boundary layers with free convection, forced convection, and stable stratification. We use the Poisson equation for pressure to represent the pressure field as the sum of mean-shear, turbulence–turbulence, subfilter-scale, Coriolis, and buoyancy contributions. We isolate these contributions and study them separately. We find that in the energy-containing range in the free-convection case the turbulence–turbulence pressure dominates over the entire boundary layer. That part dominates also up to midlayer in the forced-convection case; above that the mean-shear pressure dominates. In the stable case the mean-shear pressure dominates over the entire boundary layer.We find evidence of an inertial subrange in the pressure spectrum in the free and forced-convection cases; it is dominated by the turbulence–turbulence pressure and has a three-dimensional spectral constant of about 4.0. This agrees well with quasi-Gaussian predictions but is a factor of 2 less than recent results from direct numerical simulations at moderate Reynolds numbers. Measurements of the inertial subrange pressure spectral constant at high Reynolds numbers, which might now be possible, would be most useful. 相似文献
16.
Turbulence modelling over two-dimensional hills using an Algebraic Reynolds Stress Expression 总被引：1，自引：1，他引：0
We carry out model studies of turbulence quantities for flow over two-dimensional hills using a non-hydrostatic version of the Regional Atmospheric Modeling System (RAMS). We test two turbulence closure models: the first one is an explicit Algebraic Reynolds Stress Model (ARSM) and the second one is a combination of the ARSM and a transport equation for the shear stress {ovuw}. Model predictions for the turbulent stresses are compared with data from a wind-tunnel experiment containing isolated two-dimensional hills of varying slope. From the comparison, it is concluded that the first model can only predict the normal stresses adequately while the second model provides satisfactory predictions for the normal stresses as well as giving an improved result for the shear stress {ovuw}. 相似文献
17.
Global Intermittency and Collapsing Turbulence in the Stratified Planetary Boundary Layer 总被引：1，自引：1，他引：0
Direct numerical simulation of the turbulent Ekman layer over a smooth wall is used to investigate bulk properties of a planetary boundary layer under stable stratification. Our simplified configuration depends on two non-dimensional parameters: a Richardson number characterizing the stratification and a Reynolds number characterizing the turbulence scale separation. This simplified configuration is sufficient to reproduce global intermittency, a turbulence collapse, and the decoupling of the surface from the outer region of the boundary layer. Global intermittency appears even in the absence of local perturbations at the surface; the only requirement is that large-scale structures several times wider than the boundary-layer height have enough space to develop. Analysis of the mean velocity, turbulence kinetic energy, and external intermittency is used to investigate the large-scale structures and corresponding differences between stably stratified Ekman flow and channel flow. Both configurations show a similar transition to the turbulence collapse, overshoot of turbulence kinetic energy, and spectral properties. Differences in the outer region resulting from the rotation of the system lead, however, to the generation of enstrophy in the non-turbulent patches of the Ekman flow. The coefficient of the stability correction function from Monin–Obukhov similarity theory is estimated as \(\beta \approx 5.7\) in agreement with atmospheric observations, theoretical considerations, and results from stably stratified channel flows. Our results demonstrate the applicability of this set-up to atmospheric problems despite the intermediate Reynolds number achieved in our simulations. 相似文献
18.
While turbulent bursts are considered critical for blowing-snow transport and initiation, the interaction of the airflow with the snow surface is not fully understood. To better characterize the coupling of turbulent structures and blowing-snow transport, observations collected in natural environments at the necessary high-resolution time scales are needed. To address this, high-frequency measurements of turbulence, blowing-snow density and particle velocity were made in the Canadian Rockies. During blowing-snow storms, modified variable-interval time averaging enabled identification of periods of near-surface blowing-snow coupling with shear-stress-producing motions in the lowest 2 m of the atmospheric surface layer. The identification of those turbulent motions responsible for blowing snow yields a better understanding of the event-driven mechanics of initiation and sustained transport. The type of coherent structures generating the Reynolds stress are just as important as the magnitude of the Reynolds stress in initiating and sustaining near-surface blowing snow. Our results suggest that blowing-snow models driven by merely the time-averaged shear stress lack physical realism in the near-surface region. The next phase of the development of blowing-snow models should incorporate parametrizations of coherent turbulent structures. 相似文献
19.
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
20.
Single-Column Model Intercomparison for a Stably Stratified Atmospheric Boundary Layer 总被引：2，自引：15，他引：2
J. Cuxart A. A. M. Holtslag R. J. Beare E. Bazile A. Beljaars A. Cheng L. Conangla M. Ek F. Freedman R. Hamdi A. Kerstein H. Kitagawa G. Lenderink D. Lewellen J. Mailhot T. Mauritsen V. Perov G. Schayes G-J. Steeneveld G. Svensson P. Taylor W. Weng S. Wunsch K-M. Xu 《Boundary-Layer Meteorology》2006,118(2):273-303
The parameterization of the stably stratified atmospheric boundary layer is a difficult issue, having a significant impact on medium-range weather forecasts and climate integrations. To pursue this further, a moderately stratified Arctic case is simulated by nineteen single-column turbulence schemes. Statistics from a large-eddy simulation intercomparison made for the same case by eleven different models are used as a guiding reference. The single-column parameterizations include research and operational schemes from major forecast and climate research centres. Results from first-order schemes, a large number of turbulence kinetic energy closures, and other models were used. There is a large spread in the results; in general, the operational schemes mix over a deeper layer than the research schemes, and the turbulence kinetic energy and other higher-order closures give results closer to the statistics obtained from the large-eddy simulations. The sensitivities of the schemes to the parameters of their turbulence closures are partially explored. 相似文献