Physical Modeling of Flow Field inside Urban Street Canyons

Abstract The flow characteristics inside urban street canyons were studied in a laboratory water channel. The approaching flow direction was horizontal and perpendicular to the street axis. The street width was adjusted to form street canyons of aspect ratios 0.5, 1.0, and 2.0. The velocity field and turbulent intensity were measured with a laser Doppler anemometer at various locations within the street canyons, which were used to elucidate the flow pattern inside the street canyons. It was found that the previous numerical modeling results are in good agreement with the current experimental results at most locations. For the street canyon of aspect ratio 0.5, which belongs to the wake interference flow regime, the mean and fluctuating velocity components were more difficult to measure as compared with the other two cases because of its more complicated flow pattern. Some guidelines for numerical modeling were developed based on the measurement results. The data presented in this paper can also be used as a comprehensive database for the validation of numerical models.

Download Full-text

Numerical Simulation of Turbulent Flow and Pollutant Dispersion in Urban Street Canyons

Atmosphere ◽

10.3390/atmos10110683 ◽

2019 ◽

Vol 10 (11) ◽

pp. 683 ◽

Cited By ~ 2

Author(s):

Van Thinh Nguyen ◽

Thanh Chuyen Nguyen ◽

John Nguyen

Keyword(s):

Numerical Model ◽

Turbulent Flow ◽

Flow Pattern ◽

Pollutant Dispersion ◽

Experimental Measurements ◽

Street Canyons ◽

Urban Street ◽

Urban Street Canyons ◽

Aspect Ratios ◽

Turbulent Schmidt Number

In this study, we have developed a numerical model based on an open source Computational Fluid Dynamics (CFD) package OpenFOAM, in order to investigate the flow pattern and pollutant dispersion in urban street canyons with different geometry configurations. In the new model, the pollutant transport driven by airflow is modeled by the scalar transport equation coupling with the momentum equations for airflow, which are deduced from the Reynolds Averaged Navier-Stokes (RANS) equations. The turbulent flow calculation has been calibrated by various two-equation turbulence closure models to select a practical and efficient turbulence model to reasonably capture the flow pattern. Particularly, an appropriate value of the turbulent Schmidt number has been selected for the pollutant dispersion in urban street canyons, based upon previous studies and careful calibrations against experimental measurements. Eventually, the numerical model has been validated against different well-known laboratory experiments in regard to various aspect ratios (a relationship between the building height and the width of the street canyon), and different building roof shapes (flat, shed, gable and round). The comparisons between the numerical simulations and experimental measurements show a good agreement on the flow pattern and pollutant distribution. This indicates the ability of the new numerical model, which can be applied to investigate the wind flow and pollutant dispersion in urban street canyons.

Download Full-text

Flow and Pollutant Transport in Urban Street Canyons of Different Aspect Ratios with Ground Heating: Large-Eddy Simulation

Boundary-Layer Meteorology ◽

10.1007/s10546-011-9670-9 ◽

2011 ◽

Vol 142 (2) ◽

pp. 289-304 ◽

Cited By ~ 56

Author(s):

Xian-Xiang Li ◽

Rex E. Britter ◽

Leslie K. Norford ◽

Tieh-Yong Koh ◽

Dara Entekhabi

Keyword(s):

Large Eddy Simulation ◽

Pollutant Transport ◽

Street Canyons ◽

Urban Street ◽

Eddy Simulation ◽

Urban Street Canyons ◽

Aspect Ratios ◽

Large Eddy

Download Full-text

Linear Error Dynamics for Turbulent Flow in Urban Street Canyons

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-16-0173.1 ◽

2017 ◽

Vol 56 (5) ◽

pp. 1195-1208 ◽

Cited By ~ 2

Author(s):

K. Ngan ◽

K. W. Lo

Keyword(s):

Linear Theory ◽

Free Atmosphere ◽

Street Canyons ◽

Urban Street ◽

Eddy Simulation ◽

Urban Street Canyons ◽

Aspect Ratios ◽

Large Eddy ◽

Error Dynamics ◽

Insight Into

AbstractThe ability to make forecasts depends on atmospheric predictability and the growth of errors. It has recently been shown that the predictability of urban boundary layers differs in important respects from that of the free atmosphere on the mesoscale and larger; in particular, nonlinearity may play a less prominent role in the error evolution. This paper investigates the applicability of linear theory to the error evolution in turbulent street-canyon flow. Using large-eddy simulation, streamwise aspect ratios between 0.15 and 1.50, and identical-twin experiments, it is shown that the growth rate of the error kinetic energy can be estimated from Eulerian averages and that linear theory provides insight into the spatial structure of the error field after saturation. The results should be applicable to cities with deep and closely spaced canyons. Implications for data assimilation and modeling are discussed.

Download Full-text

Numerical Modeling of the Flow and Pollutant Dispersion in Street Canyons with Ground Thermal Effect

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.548-549.601 ◽

2014 ◽

Vol 548-549 ◽

pp. 601-606

Author(s):

Ning Bo Zhang ◽

Yan Ming Kang ◽

Ke Zhong ◽

Jia Ping Liu

Keyword(s):

Thermal Effect ◽

Street Canyon ◽

Three Dimensional ◽

Thermal Conditions ◽

Pollutant Dispersion ◽

Ambient Air ◽

Street Canyons ◽

Urban Street ◽

Urban Street Canyons ◽

Aspect Ratios

Thermal stratification affects the flow in and above urban street canyons. Such thermal effect is often not noticed, and can lead to pessimistic or optimistic results of the air quality in urban street canyons under calm conditions and low wind speeds. A three-dimensional CFD model is applied to simulate the flow patterns and particle concentrations in a street canyon under different aspect ratios and ground thermal conditions. The model is validated by the experimental data found in the literature. The simulation results are used to evaluate the flow and pollutant dispersion properties in the canyon. The results show that the ground thermal conditions can significantly affect the ventilation performance of the street canyon, which improves with the increased temperature difference (ΔT) between the ambient air and the ground of the canyon. The increased ΔT enhances the buoyancy induced secondary flow in the street canyon and hence reduce the particle concentrations in the canyon, with this influence more pronounced for small street widths.

Download Full-text

Numerical investigation of the impact of different configurations and aspect ratios on dense gas dispersion in urban street canyons

Tsinghua Science & Technology ◽

10.1016/s1007-0214(07)70051-2 ◽

2007 ◽

Vol 12 (3) ◽

pp. 345-351 ◽

Cited By ~ 5

Author(s):

Rui Yang ◽

Jing Zhang ◽

Shifei Shen ◽

Xiaomeng Li ◽

Jianguo Chen

Keyword(s):

Numerical Investigation ◽

Dense Gas ◽

Street Canyons ◽

Gas Dispersion ◽

Urban Street ◽

Urban Street Canyons ◽

Aspect Ratios ◽

The Impact

Download Full-text

Are Green Walls Better Options than Green Roofs for Mitigating PM10 Pollution? CFD Simulations in Urban Street Canyons

Sustainability ◽

10.3390/su10082833 ◽

2018 ◽

Vol 10 (8) ◽

pp. 2833 ◽

Cited By ~ 9

Author(s):

Hongqiao Qin ◽

Bo Hong ◽

Runsheng Jiang

Keyword(s):

Cfd Simulation ◽

Green Roofs ◽

Navier Stokes ◽

Street Canyons ◽

Coverage Area ◽

Urban Street ◽

Drift Flux Model ◽

Urban Street Canyons ◽

Aspect Ratios ◽

Green Walls

To examine the effect of green roofs (GRs) and green walls (GWs) on coarse particle (PM10) dispersion in urban street canyons, a computational fluid dynamics (CFD) simulation was conducted with a Reynolds-averaged Navier-Stokes (RANS) model and a revised generalized drift flux model. Simulations were performed with different aspect ratios (H/W = 0.5, 1.0, and 2.0), greenery coverage areas (S = 300, 600, and 900 m2), and leaf area densities (LADs = 1.0, 3.5, 6.0 m2/m3). Results indicate that: (1) GRs and GWs all had the reduction ability of PM10 at the pedestrian level; (2) Averaged concentrations of PM10 in GWs and GRs varied little as LAD changed in H/W = 0.5 and 1.0. When H/W = 2.0, the aerodynamic effects of GRs increased since airflow was enhanced within street canyons, resulting in the increasing concentrations in GRs compared with non-greening scenarios; (3) Given equal greenery coverage area and aspect ratio, GWs are more effective in reducing street-canyon PM10, and the averaged concentrations declined with increasing LADs and greenery coverage areas, especially the H/W; (4) At the pedestrian level, the reduction ratio of GRs is greater than that of GWs with the maximum value of 17.1% for H/W = 0.5. However, where H/W = 1.0 and 2.0, the concentrations within GWs are lower than GRs, with maximum reduction ratios of 29.3% and 43.8%, respectively.

Download Full-text