도시 협곡에서 건물 지붕 냉각이 스칼라 물질 확산에 미치는 영향

In this study, the effects of trees on flow and scalar dispersion in an urban street canyon were investigated using a computational fluid dynamics (CFD) model. For this, we implemented the drag terms of trees to the CFD model, and compared the CFD-simulated results to the wind-tunnel results. For comparison, we considered the same building configuration as the wind-tunnel experiment. The trees were located at the center of street canyon with the aspect ratio (defined as the ratio of the street width to the building height) of 1. First, the flow characteristics were analyzed in the tree-free and high-density tree cases and the results showed that the CFD model reproduced well the flow pattern of the wind-tunnel experiment and reflected the drag effect of trees in the street canyon. Then, the dispersion characteristics of scalar pollutants were investigated for the tree-free, low-density tree and medium-density tree cases. In the tree-free case, the nondimensionalized concentration distribution simulated by the CFD model was quite similar to that in the wind-tunnel experiment in magnitude and pattern. The correlation coefficients between the measured and simulated concentrations are more than 0.9 in all the cases. As the tree density increased, nondimensionalized concentration increased (decreased) near the wall of the upwind (downwind) building, which resulted from the decrease in wind speed case by the drag effect of trees. However, the CFD model underestimated (overestimated) the concentration near the wall of upwind (downwind) building.

This paper investigates the impacts of heating intensity and inflow wind speed on the characteristics of reactive pollutant dispersion in street canyons using the computational fluid dynamic (CFD) model that includes the transportation of NO, NO2, and O3 coupled with NO-NO2-O3 photochemistry. The results indicated that the heat intensity and inflow wind speed have a significant influence on the flow field, temperature field and the characteristics of reactive pollutant dispersion in and above the street canyon. With the street canyon bottom heating intensity increasing, NO, NO2 and O3 concentrations in street canyon are decreased. The O3 concentration reductions are even more than the NO and NO2 concentrations. Improving the inflow wind speed can significantly reduce the NO and NO2 concentrations within street canyons. But the O3 concentrations have a slight rise with wind speed increasing. The results would be useful for understanding the interrelation among reactive vehicle emissions, and provide references for urban planners.

The objective of this study is to investigate numerically the effect of building roof shaps on wind flow and pollutant dispersion in a street canyon with one row of trees of pore volume, <TEX>$P_{vol}=96%$</TEX>. A three-dimensional computational fluid dynamics (CFD) model is used to evaluate air flow and pollutant dispersion within an urban street canyon using Reynolds-averaged Navier-Stokes (RANS) equations and the Explicit Algebraic Reynolds Stress Models (EARSM) based on k-<TEX>${\varepsilon}$</TEX> turbulence model to close the equation system. The numerical model is performed with ANSYS-CFX code. Vehicle emissions were simulated as double line sources along the street. The numerical model was validated by the wind tunnel experiment results. Having established this, the wind flow and pollutant dispersion in urban street canyons (with six roof shapes buildings) are simulated. The numerical simulation results agree reasonably with the wind tunnel data. The results obtained in this work, indicate that the flow in 3D domain is more complicated; this complexity is increased with the presence of trees and variability of the roof shapes. The results also indicated that the largest pollutant concentration level for two walls (leeward and windward wall) is observed with the upwind wedge-shaped roof. But the smallest pollutant concentration level is observed with the dome roof-shaped.

Impact of height and shape of building roof on air quality in urban street canyons

Characteristics of flow and reactive pollutant dispersion in urban street canyons

Influence of avenue trees on traffic pollutant dispersion in asymmetric street canyons: Numerical modeling with empirical analysis

The effects of building-roof cooling on flow and air temperature in 3D urban street canyons are numerically investigated using a computational fluid dynamics (CFD) model. The aspect ratios of the building and street canyon considered are unity. For investigating the building-roof cooling effects, the building-roof temperatures are systematically changed. The traditional flow pattern including a portal vortex appears in the spanwise canyon. Compared with the case of the control run, there are minimal differences in flow pattern in the cases in which maximum building-roof cooling is considered. However, as the building roof becomes cooler, the mean kinetic energy increases and the air temperature decreases in the spanwise canyon. Building-roof cooling suppresses the upward and inward motions above the building roof, resultantly increasing the horizontal velocity near the roof level. The increase in wind velocity above the roof level intensifies the secondarily driven vortex circulation as well as the inward (outward) motion into (out of) the spanwise canyon. Finally, building-roof cooling reduces the air temperature in the spanwise canyon, supplying much relatively cool air from the streamwise canyon into the spanwise canyon.

The air flow and pollutant concentration fields in a street canyon affected by trees could affect the comfort and health of residents. At present, the description of the non-uniform/discontinuous distribution of leaves is difficult. In this study, the leaf distribution in the canopy was characterized by establishing non-continuous (uniform/random) algorithm based on a numerical simulation method, and the effects of canopy properties including, height, porosity and uniform/random leaf distribution, on the airflow and pollutant concentration fields in urban street canyons were investigated. The position of the tree canopy was found to directly affect the airflow field form and the air velocity distribution in the street canyon at low inflows. The average air velocity in the street canyon could be reduced significantly when the top of the tree canopy is near the top of the street canyon. The air velocity and pollutant concentration in the street canyon would vary only slightly due to the canopy porosity. Due to the increasing canopy porosity, the air velocity would increase, and the pollutant concentration would be reduced. The leaves are non-continuous and uniformly distributed at constant porosity, which does not significantly change the velocity distribution and pollutant concentration in the street canyon.

To reduce energy demand (both fossil fuel and renewable energy) for cooling the urban heat island environment, some solutions have been studied. Among these methods, the water spray system is considered more flexible due to its dynamic controls. This study investigated the cooling effect of water spray systems in the street canyon under different aspect ratios and high relative humidity environments using a computational fluid dynamics model. This model was validated with water channel and wind tunnel experiments. The results showed that the most effective cooling area was the area just under the spray nozzles. However, in a narrow street canyon, people in the middle of the street may feel the cooling effect because of the dispersion and accumulation of the cooled air. Our simulations demonstrated that air under the nozzles was saturated and this revealed that under drier conditions the water spray systems will have higher cooling performance. We also found that using large water droplets created a wider cooling area in the middle of the street canyon, and this phenomenon was not changed much if the nozzle height was increased from 2.5 m to 3.5 m.

Numerical simulations of the impact of tree planting on airflow and traffic pollutant dispersion in urban street canyons have been performed using the commercial CFD (Computational Fluid Dynamics) code MISKAM. A k-e turbulence model including additional terms for the treatment of vegetation, has been employed to close the Reynolds-averaged-Navier-Stokes (RANS) equations. The numerical results were compared to wind tunnel data. In the case of the investigated wind direction perpendicular to the street axis, the presence of trees lead to increased pollutant concentrations inside the canyon. Concentrations increased strongly on the upstream side of the canyon, while on the downstream side a small concentration decrease could be observed. Lower flow velocities and higher pollutant concentrations were found in the numerical simulations when directly compared to the experimental results. However, the impact of tree planting on airflow and concentration fields when compared to the treeless street canyon as a reference configuration were simulated quite well, meaning that relative changes were similar in the wind tunnel investigations and numerical computations. This feature qualifies MISKAM for use as a tool for assessing the impacts of vegetation on local air quality.

Urban air pollution is one of the unsettled issues in urban development and planning process. Urban street canyons are one of the main focused areas of investigation due to the heavy emission of the pollutants from vehicles. In the present study, the effects of elevated metro rail track (EMRT) on pollutant dispersion phenomena have been studied using computational fluid dynamics (CFD) technique. As a practical application of a CFD model, k-e turbulence model was applied for airflow modelling. Species transport model was applied for pollutant dispersion modelling. Geometrical configuration of urban street canyon of aspect ratio 'one', with elevated metro rail track, has been considered. Numerical simulations were carried out for perpendicular wind flow. Pollutant concentrations, at various locations of the street canyon, were largely affected by the introduction of elevated metro rail track. These results were compared with the street canyon without elevated metro rail track. It has been observed that the pollutant concentration increased at lower region and decreased at the upper region of the street canyon. Variations were not uniformly distributed in street canyon volume. Contours and pollutant concentrations have been compared and discussed for different sampled locations in the street canyon.

Impact of trees on pollutant dispersion in street canyons: A numerical study of the annual average effects in Antwerp, Belgium

The microenvironment, which involves pollutant dispersion of the urban street canyon, is critical to the health of pedestrians and residents. The objectives of this work are twofold: (i) to effectively assess the pollutant dispersion process based on a theory and (ii) to adopt an appropriate stratigy, i.e., wind catcher, to alleviate the pollution in the street canyons. Pollutant dispersion in street canyons is essentially a convective mass transfer process. Because the convective heat transfer process and the mass transfer process are physically similar and the applicability of field synergy theory to turbulence has been verified in the literature, we apply the field synergy theory to the study of pollutant dispersion in street canyons. In this paper, a computational fluid dynamics (CFD) simulation is conducted to investigate the effects of wind catcher, wind speed and the geometry of the street canyons on pollutant dispersion. According to the field synergy theory, Sherwood number and field synergy number are used to quantitatively evaluate the wind catcher and wind speed on the diffusion of pollutants in asymmetric street canyons. The results show that adding wind catchers can significantly improve the air quality of the step-down street canyon and reduce the average pollutant concentrations in the street canyon by 75%. Higher wind speed enhances diffusion of pollutants differently in different geometric street canyons.

A wind tunnel study of organised and turbulent air motions in urban street canyons

The trend of urbanization is increasing due to socioeconomic factors, leading to a decline in air quality and heightened exposure for individuals. Modeling dispersion of emissions within street canyons is crucial for understanding&amp;#160; local-scale air pollution exposure&amp;#160; indicators and devising effective strategies to enhance urban air quality. However, microscale urban air quality simulations are complex and involve a large number of interacting physical processes. Phenomena taken into consideration in simulations of urban pollution dispersion largely vary from one study to another. In this context, we aim at identifying the most influential parameters governing this phenomenon.&amp;#160;&amp;#160;First, we present a comprehensive database of pollution dispersion in street canyon by perturbing the most important parameters. In this study, these parameters include wind speed, background concentration, emission rates, background turbulence intensity, temperature disparity between building surfaces and the atmosphere, as well as model-specific parameters. We employ COMSOL Multiphysics for the computational fluid dynamic simulations. To keep feasible computational times and create a large database, turbulence is modeled by means of the Reynolds Averaged Navier-Stokes (RANS) k-&amp;#949; technique and the domain is considered two-dimensional.&amp;#160;&amp;#160;This simulation bank is used to develop a surrogate model based on Gradient Boosting Machine and the Principal Component Analysis to reduce system complexity. This surrogate model offers a solution to carry out a multivariate global sensitivity analysis. To this end, we compute the Sobol Indexes, providing insights into the global contributions of input factors to the resulting pollution concentrations at each point of the physical domain.&amp;#160;&amp;#160;Results show that, as expected, background concentration dominates the pollution field far from the emission point. The temperature of building surfaces plays a secondary role, acting mainly on the interface between the street canyon cavity and the flow above the buildings. Emission plays a fundamental role, especially in the vertical concentration profile. Other&amp;#160; model-specific parameters have a minor role when compared to the other physical variables. Wind speed plays a key role, along with temperature, in determining how pollution exits the cavity.&amp;#160;By integrating advanced simulation techniques with rigorous sensitivity analyses, this study aims to provide valuable insights into the factors influencing air quality in urban street canyons. Such insights are crucial to perform model calibration, and to understand the implications of typical approximations such as non-buoyancy flows.