Flow and dispersion in urban areas
TL;DR: In this article, the authors address flow and dispersion in urban areas at four scales: regional, city, neighborhood, and street, and address the most appropriate framework to study and quantify the result.
Abstract: ▪ Abstract Increasing urbanization and concern about sustainability and quality of life issues have produced considerable interest in flow and dispersion in urban areas. We address this subject at four scales: regional, city, neighborhood, and street. The flow is one over and through a complex array of structures. Most of the local fluid mechanical processes are understood; how these combine and what is the most appropriate framework to study and quantify the result is less clear. Extensive and structured experimental databases have been compiled recently in several laboratories. A number of major field experiments in urban areas have been completed very recently and more are planned. These have aided understanding as well as model development and evaluation.
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TL;DR: A review is given of a set of model evaluation methodologies, including the BOOT and the ASTM evaluation software, Taylor’s nomogram, the figure of merit in space, and the CDF approach.
Abstract: This paper reviews methods to evaluate the performance of air quality models, which are tools that predict the fate of gases and aerosols upon their release into the atmosphere. Because of the large economic, public health, and environmental impacts often associated with the use of air quality model results, it is important that these models be properly evaluated. A comprehensive model evaluation methodology makes use of scientific assessments of the model technical algorithms, statistical evaluations using field or laboratory data, and operational assessments by users in real-world applications. The focus of the current paper is on the statistical evaluation component. It is important that a statistical model evaluation exercise should start with clear definitions of the evaluation objectives and specification of hypotheses to be tested. A review is given of a set of model evaluation methodologies, including the BOOT and the ASTM evaluation software, Taylor’s nomogram, the figure of merit in space, and the CDF approach. Because there is not a single best performance measure or best evaluation methodology, it is recommended that a suite of different performance measures be applied. Suggestions are given concerning the magnitudes of the performance measures expected of “good” models. For example, a good model should have a relative mean bias less than about 30% and a relative scatter less than about a factor of two. In order to demonstrate some of the air quality model evaluation methodologies, two simple baseline urban dispersion models are evaluated using the Salt Lake City Urban 2000 field data. The importance of assumptions concerning details such as minimum concentration and pairing of data are shown. Typical plots and tables are presented, including determinations of whether the difference in the relative mean bias between the two models is statistically significant at the 95% confidence level.
942 citations
TL;DR: In this article, Zhou et al. presented the initial condition dependence of Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) mixing layers, and introduced parameters that are used to evaluate the level of mixedness and mixed mass within the layers.
606 citations
TL;DR: The drivers behind current rises in the use of low-cost sensors for air pollution management in cities are illustrated, while addressing the major challenges for their effective implementation.
591 citations
Cites background from "Flow and dispersion in urban areas"
...local emission sources and atmospheric flow conditions (Britter and Hanna, 2003)....
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...Air quality varies over a relatively small scale since the resulting pollutant concentration in a specific place depends predominantly on local emission sources and atmospheric flow conditions (Britter and Hanna, 2003)....
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TL;DR: In this article, a simulation of turbulent flow over regular arrays of urban-like, cubical obstacles is presented, and results are analyzed in terms of a formal spatial averaging procedure to enable interpretation of the flow within the arrays as a canopy flow, and of flow above as a rough wall boundary layer.
Abstract: Direct numerical simulations of turbulent flow over regular arrays of urban-like, cubical obstacles are reported. Results are analysed in terms of a formal spatial averaging procedure to enable interpretation of the flow within the arrays as a canopy flow, and of the flow above as a rough wall boundary layer. Spatial averages of the mean velocity, turbulent stresses and pressure drag are computed. The statistics compare very well with data from wind-tunnel experiments. Within the arrays the time-averaged flow structure gives rise to significant ‘dispersive stress’ whereas above the Reynolds stress dominates. The mean flow structure and turbulence statistics depend significantly on the layout of the cubes. Unsteady effects are important, especially in the lower canopy layer where turbulent fluctuations dominate over the mean flow.
431 citations
TL;DR: The coupled CFD modelling approach and grid generation procedure presented in this paper can be used similarly for future studies of wind flow and related processes in complex urban environments.
Abstract: Wind flow in urban environments is an important factor governing the dispersion of heat and pollutants from streets, squares and buildings. This paper presents a coupled CFD modelling approach for urban wind flow and indoor natural ventilation. A specific procedure is used to efficiently and simultaneously generate the geometry and the high-resolution body-fitted grid for both the outdoor and indoor environment. This procedure allows modelling complex geometries with full control over grid quality and grid resolution, contrary to standard semi-automatic unstructured grid generation procedures. It also provides a way to easily implement various changes in the model geometry and grid for parametric studies. As a case study, a parametric analysis of natural ventilation is performed for the geometrically complex Amsterdam ArenA stadium in the Netherlands. The turbulent wind flow and temperature distribution around and inside the stadium are solved with the 3D steady Reynolds-averaged Navier-Stokes equations. Special attention is given to CFD solution verification and validation. It is shown that small geometrical modifications can increase the ventilation rate by up to 43%. The coupled modelling approach and grid generation procedure presented in this paper can be used similarly for future studies of wind flow and related processes in complex urban environments.
371 citations
Cites background from "Flow and dispersion in urban areas"
...Examples are atmospheric transport and dispersion of solid, liquid and gaseous air pollutants (Britter and Hanna 2003, Meroney 2004, Gromke and Ruck 2007, 2009, Blocken et al. 2008, Mensink and Cosemans 2008, Berkowicz et al. 2008, Yang and Shao 2008), wind-driven rain (Choi 1993, Blocken and…...
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References
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4,943 citations
Book•
01 Jan 1978
TL;DR: This modern climatology textbook explains those climates formed near the ground in terms of the cycling of energy and mass through systems.
Abstract: This modern climatology textbook explains those climates formed near the ground in terms of the cycling of energy and mass through systems.
4,767 citations
Book•
06 Jan 1994
TL;DR: In this paper, Spectra and cospectra over flat uniform terrain, Flow over plant canopies and Flow over changing terrain are used to observe the boundary layer of a plant canopy.
Abstract: 1 Flow over flat uniform terrain 2 Spectra and cospectra over flat uniform terrain 3 Flow over plant canopies 4 Flow over changing terrain 5 Flow over hills 6 Sensors and techniques for observing the boundary layer 7 Acquisition and processing of boundary layer data Index
1,791 citations
TL;DR: In this article, the mean velocity profile is inflected, second moments are strongly inhomogeneous with height, skewnesses are large, and second-moment budgets are far from local equilibrium.
Abstract: ▪ Abstract The single-point statistics of turbulence in the ‘roughness sub-layer’ occupied by the plant canopy and the air layer just above it differ significantly from those in the surface layer. The mean velocity profile is inflected, second moments are strongly inhomogeneous with height, skewnesses are large, and second-moment budgets are far from local equilibrium. Velocity moments scale with single length and time scales throughout the layer rather than depending on height. Large coherent structures control turbulence dynamics. Sweeps rather than ejections dominate eddy fluxes and a typical large eddy consists of a pair of counter-rotating streamwise vortices, the downdraft between the vortex pair generating the sweep. Comparison with the statistics and instability modes of the plane mixing layer shows that the latter rather than the boundary layer is the appropriate model for canopy flow and that the dominant large eddies are the result of an inviscid instability of the inflected mean velocity profi...
1,484 citations
TL;DR: In this paper, several methods to determine the aerodynamic characteristics of a site through analysis of its surface form (morphometry) are considered in relation to cities, including zero-plane displacement length (zd), roughness length(z0), depth of the roughness sublayer, and aerodynamic conductance.
Abstract: Several methods to determine the aerodynamic characteristics of a site through analysis of its surface form (morphometry) are considered in relation to cities. The measures discussed include zero-plane displacement length (zd), roughness length (z0), depth of the roughness sublayer, and aerodynamic conductance. A sensitivity analysis is conducted on seven formulas to estimate zd and nine to estimate z0, covering a wide range of probable urban roughness densities. Geographic information systems developed for 11 sites in 7 North American cities are used to characterize their morphometry—the height, shape, three-dimensional area, and spatial distribution of their roughness elements (buildings and trees). Most of the sites are in residential suburbs, but one is industrial and two are near city centers. This descriptive survey of urban geometric form is used, together with the morphometric formulas, to derive the apparent aerodynamic characteristics of the sites. The resulting estimates of zd and z0 a...
1,108 citations