Large-eddy Simulation of Plume Dispersion over Hypothetical Urban Areas

2020 ◽  
Author(s):  
Zhangquan Wu ◽  
Chun-Ho Liu
Keyword(s):  
Nitric Oxide ◽  
Air Quality ◽  
Large Eddy Simulation ◽  
Urban Areas ◽  
Turbulent Dispersion ◽  
Anthropogenic Sources ◽  
Plume Dispersion ◽  
Street Canyons ◽  
Eddy Simulation ◽  
Large Eddy

<p>More than 80% of people living in urban areas that exposed to air quality levels that exceed WHO guideline limits both indoors and outdoors. Road transport has been found to be one of major anthropogenic sources of aerosol particles and many gaseous pollutants in urban areas. Dispersion of pollutants emitted from vehicles over urban areas largely affects pedestrian-level air quality. A good understanding of pollutant transport, mixing process and removal mechanism is crucial to effectuate air quality management. In this study, turbulent dispersion of reactive pollutants in the atmospheric boundary layer (ABL) over hypothetical urban area in the form of an array of idealised street canyons is investigated using large-eddy simulation (LES). The irreversible ozone O3 titration oxidizes nitric oxide NO to nitrogen dioxide NO2, representing the typical anthropogenic air pollution chemistry. Nitric oxide (NO) is emitted from the ground level of the first street canyon into the urban ABL doped with ozone (O3). From the LES results, negative vertical NO flux is found at the roof level of the street canyons.  By looking into the different plume behavior and vertical flux between the inert pollutant and chemically reactive pollutant, a fundamental understanding of exchange processes of anthropogenic chemicals between an urban surface and the atmosphere is developed. </p>

scholarly journals Large-eddy simulation of plume dispersion within regular arrays of cubic buildings

2011 ◽  
Vol 6 (1) ◽  
pp. 79-86
Author(s):  
H. Nakayama ◽  
K. Jurcakova ◽  
H. Nagai
Keyword(s):  
Large Eddy Simulation ◽  
Urban Areas ◽  
Health Hazard ◽  
Critical Threshold ◽  
Plume Dispersion ◽  
Toxic Substances ◽  
High Concentration ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Regular Arrays

Abstract. There is a potential problem that hazardous and flammable materials are accidentally or intentionally released within populated urban areas. For the assessment of human health hazard from toxic substances, the existence of high concentration peaks in a plume should be considered. For the safety analysis of flammable gas, certain critical threshold levels should be evaluated. Therefore, in such a situation, not only average levels but also instantaneous magnitudes of concentration should be accurately predicted. In this study, we perform Large-Eddy Simulation (LES) of plume dispersion within regular arrays of cubic buildings with large obstacle densities and investigate the influence of the building arrangement on the characteristics of mean and fluctuating concentrations.

scholarly journals Sensitivity of local air quality to the interplay between small- and large-scale circulations: a large-eddy simulation study

2017 ◽  
Vol 17 (11) ◽  
pp. 7261-7276 ◽  
Author(s):  
Tobias Wolf-Grosse ◽  
Igor Esau ◽  
Joachim Reuder
Keyword(s):  
Air Pollution ◽  
Air Quality ◽  
Large Eddy Simulation ◽  
Air Pollutants ◽  
Large Scale ◽  
Wind Speeds ◽  
Street Level ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Air Pockets

Abstract. Street-level urban air pollution is a challenging concern for modern urban societies. Pollution dispersion models assume that the concentrations decrease monotonically with raising wind speed. This convenient assumption breaks down when applied to flows with local recirculations such as those found in topographically complex coastal areas. This study looks at a practically important and sufficiently common case of air pollution in a coastal valley city. Here, the observed concentrations are determined by the interaction between large-scale topographically forced and local-scale breeze-like recirculations. Analysis of a long observational dataset in Bergen, Norway, revealed that the most extreme cases of recurring wintertime air pollution episodes were accompanied by increased large-scale wind speeds above the valley. Contrary to the theoretical assumption and intuitive expectations, the maximum NO2 concentrations were not found for the lowest 10 m ERA-Interim wind speeds but in situations with wind speeds of 3 m s−1. To explain this phenomenon, we investigated empirical relationships between the large-scale forcing and the local wind and air quality parameters. We conducted 16 large-eddy simulation (LES) experiments with the Parallelised Large-Eddy Simulation Model (PALM) for atmospheric and oceanic flows. The LES accounted for the realistic relief and coastal configuration as well as for the large-scale forcing and local surface condition heterogeneity in Bergen. They revealed that emerging local breeze-like circulations strongly enhance the urban ventilation and dispersion of the air pollutants in situations with weak large-scale winds. Slightly stronger large-scale winds, however, can counteract these local recirculations, leading to enhanced surface air stagnation. Furthermore, this study looks at the concrete impact of the relative configuration of warmer water bodies in the city and the major transport corridor. We found that a relatively small local water body acted as a barrier for the horizontal transport of air pollutants from the largest street in the valley and along the valley bottom, transporting them vertically instead and hence diluting them. We found that the stable stratification accumulates the street-level pollution from the transport corridor in shallow air pockets near the surface. The polluted air pockets are transported by the local recirculations to other less polluted areas with only slow dilution. This combination of relatively long distance and complex transport paths together with weak dispersion is not sufficiently resolved in classical air pollution models. The findings have important implications for the air quality predictions over urban areas. Any prediction not resolving these, or similar local dynamic features, might not be able to correctly simulate the dispersion of pollutants in cities.

Large-Eddy Simulation of Reactive Pollutant Dispersion Over Street Canyons of Different Aspect Ratios

2014 ◽  
Author(s):  
T. Z. Du ◽  
Chun-Ho Liu ◽  
Y. B. Zhao
Keyword(s):  
Aspect Ratio ◽  
Large Eddy Simulation ◽  
Chemical Reactions ◽  
Pollutant Dispersion ◽  
Pollutant Removal ◽  
Street Canyons ◽  
Eddy Simulation ◽  
Aspect Ratios ◽  
Large Eddy ◽  
Reactive Pollutant

In urban areas, pollutants are emitted from vehicles then disperse from the ground level to the downstream urban canopy layer (UCL) under the effect of the prevailing wind. For a hypothetical urban area in the form of idealized street canyons, the building-height-to-street-width (aspect) ratio (AR) changes the ground roughness which in turn leads to different turbulent airflow features. Turbulence is considered an important factor for the removal of reactive pollutants by means of dispersion/dilution and chemical reactions. Three values of aspect ratio, covering most flow scenarios of urban street canyons, are employed in this study. The pollutant dispersion and reaction are calculated using large-eddy simulation (LES) with chemical reactions. Turbulence timescale and reaction timescale at every single point of the UCL domain are calculated to examine the pollutant removal. The characteristic mechanism of reactive pollutant dispersion over street canyons will be reported in the conference.

Large eddy simulation of wind field and plume dispersion in building array

2008 ◽  
Vol 42 (6) ◽  
pp. 1083-1097 ◽  
Author(s):  
R.F. Shi ◽  
G.X. Cui ◽  
Z.S. Wang ◽  
C.X. Xu ◽  
Z.S. Zhang
Keyword(s):  
Large Eddy Simulation ◽  
Wind Field ◽  
Plume Dispersion ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Building Array

Test of chemistry boundary conditions large-eddy simulations in urban areas

2020 ◽  
Author(s):  
Renate Forkel ◽  
Basit Khan ◽  
Johannes Werhahn ◽  
Sabine Banzhaf ◽  
Edward C. Chan ◽  
...  
Keyword(s):  
Boundary Conditions ◽  
Urban Areas ◽  
Turbulent Transport ◽  
Pollutant Dispersion ◽  
Time Dependent ◽  
Street Canyons ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Les Model ◽  
Turbulence Generator

<p>Large-Eddy Simulation (LES) allow to simulate pollutant dispersion at a fine-scale turbulence-resolving scale with explicitly resolved turbulent transport around building structures and in street canyons. The microscale urban climate model with atmospheric chemistry PALM-4U (i.e. PALM for Urban applications; Maronga et al., 2019, Met. Z., https://doi.org/10.1127/metz/2019/0909) has been developed within the collaborative project MOSAIK (Model-based city planning and application in climate change). With such a large-eddy simulation (LES) model, pollutant dispersion around buildings and within street canyons can be simulated, with explicitly resolving the turbulent transport in urban environments.</p><p>Cyclic boundaries are frequently applied in LES in order to obtain lateral boundary conditions for the turbulent quantities. In addition to the default cyclic boundary conditions, PALM-4U allows also time-dependent boundary conditions from regional models to account for variable weather conditions and regional scale pollutant transport. Turbulent fluctuations, which are not included in the boundary conditions from the regional simulation but are needed as additional boundary conditions for the LES model are produced by a turbulence generator (Maronga et al, 2019, GMDD, https://doi.org/10.5194/gmd-2019-103).</p><p>PALM-4U simulations with and without time dependent boundary conditions from regional simulations with WRF-Chem are performed for different setups in order to test the impact of the domain configuration. The simulations indicate that cyclic boundary conditions can lead to unrealistic accumulation of pollutants over urban areas with strong sources, which is not the case when time-dependent boundary conditions are applied. However, even though a turbulence generator is applied, explicit setting of time-dependent boundary conditions requires large model domains, in order to obtain fully developed turbulence within the domain of interest, increasing the computational demand of the simulation.</p>

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