Search Results (5)

Improving urban walkability in the face of climate change is a critical challenge for urban designers. Street design strategies can mitigate heat stress and enhance pedestrian livability. Most previous studies conducted in hot climates recommend adopting deep canyons to improve summer conditions, overlooking the potential improvement of wide streets as essential structural elements of the urban fabric. This study was conducted in Biskra city, southern Algeria, where several mitigation strategies were applied to ‘Emir Abdelkader Boulevard’, as the main structural street inside the city, to create an optimal street model for arid climates. Five scenarios were developed based on three criteria: (Sc1) asymmetric profile northeast side (NES) > southwest side (SWS); (Sc2) asymmetric profile SWS > NES; (Sc3) cool paving; (Sc4) horizontal shading; and(Sc5) shading with a linear tree arrangement. ENVI-met software version 5.1.1 and the RayMan model were used to estimate the local climate conditions and outdoor thermal comfort levels based on the physiological equivalent temperature (PET). All scenarios reduced PET values across the street, with optimal reductions of −2.0 °C, −3.1 °C, −1.3 °C, −1.7 °C, and −1.2 °C in Sc1, Sc2, Sc3, Sc4, and Sc5, respectively. Concerning pedestrian areas, the optimal results durations were at the southwest side below the arcades’ sidewalks during peak hours: Sc2, Sc3, Sc4, Sc5 (2.2 °C–3 H, 2.3 °C–3 H, 2.4 °C–3 H, 2.5 °C–2 H). Sc1 performed best during daytime hours on the northeast side. The utilizing of these results can strongly help urban planners and landscape architects in creating climate-responsive streets that enhance citizens’ quality of life. Full article

Due to the rapid advancement of urbanization, traffic–related pollutants in street canyons have emerged as the primary source of PM_2.5, adversely impacting residents’ health. Therefore, it is necessary to reduce PM_2.5 concentrations. In this study, a three–dimensional steady–state simulation was conducted using Computational Fluid Dynamics (CFD). Three representative wind directions (θ = 0°, 45°, and 90°, corresponding to parallel, oblique, and perpendicular winds) and five different building height ratios (BHR = 0.25, 0.5, 1, 2, and 4) were used to explore the effect of building height variations on PM_2.5 dispersion within street canyons. The results indicated that wind direction significantly influenced PM_2.5 dispersion (p < 0.001). As θ increased (θ = 0°, 45°, and 90°), PM_2.5 concentration in the canyon increased, reaching the most severe pollution under perpendicular wind. Building height variations had a minor impact compared to wind direction, but differences in PM_2.5 concentration were still observed among various BHRs. Specifically, under parallel wind, the influence of BHR on PM_2.5 dispersion was relatively small as compared to oblique and perpendicular winds. For oblique wind, PM_2.5 concentrations varied based on BHR. Street canyons composed of low–rise or multi–story buildings (BHR = 0.25 or 4) slightly increased PM_2.5 concentrations within the canyon, while the lowest PM_2.5 concentration was observed at a BHR of 0.5. Under perpendicular wind, symmetrical (BHR = 1) and step–down canyons (BHR = 2 and 4) exhibited comparable peak concentrations of PM_2.5, whereas step–up canyons (BHR = 0.25 and 0.5) showed relatively lower concentrations. Full article

Mean wind profiles within a unit-aspect-ratio street canyon have been estimated by solving the three-dimensional Poisson equation for a set of discrete vortex sheets. The validity of this approach, which assumes inviscid vortex dynamics away from boundaries and a small nonlinear contribution to the growth of turbulent fluctuations, is tested for a series of idealised and realistic flows. In this paper, the effects of urban geometry on accuracy are examined with neutral flow over shallow, deep, asymmetric and realistic canyons, while thermal effects are investigated for a single street canyon and both bottom cooling and heating. The estimated mean profiles of the streamwise and spanwise velocity components show good agreement with reference profiles obtained from the large-eddy simulation: the canyon-averaged errors (e.g., normalised absolute errors around 1%) are of the same order of magnitude as those for the unit-aspect-ratio street canyon. It is argued that the approach generalises to more realistic flows because strong spatial localisation of the vorticity field is preserved. This work may be applied to high-resolution modelling of winds and pollutants, for which mean wind profiles are required, and fast statistical modelling, for which physically-based estimates can serve as initial guesses or substitutes for analytical models. Full article

This study was conducted to examine the effect on airflow of the shape of an urban road green belt in an asymmetrical street canyon. In this paper, the airflow field at pedestrian height in an asymmetrical street with different building height ratios (ASF) on both sides of the street is modeled and simulated using computational fluid dynamics (CFD) software, ANSYS FLUENT, and the flow rate characteristic distribution index and the average airflow intensity index are used to evaluate and analyze the airflow at the pedestrian level. The study shows that: (1) in an empty street scheme with different building ratios, the static wind area is located on the roof of the downstream building; the closer to the ground in a street with an ASF = 1/3, the lower the airflow rate. However, the situation is the opposite of that in other streets (2/3, 3/1, and 3/2). (2) The position of the green belt makes the windward side flow rate in the step-up street higher than that of the leeward side, and the flow rate of the leeward side in the step-down street is higher than that of the windward side. (3) Compared with other green belt forms, the use of two plates and three belts in the incremental street can increase the circumferential sinking at the roofs of the windward side of the street, thereby improving the wind environment in the entire street. The use of one plate, two-belt and three-plate, four-belt scenarios in the step-down street allows the two ends of the corner vortex to carry more airflow into the interior of the street and reduces both the “wind shadow effect” area in the middle of the street and the “air outlet effect” at both ends. Full article

A three-dimensional geometrical model was established based on a section of street canyons in the 2nd Ring Road of Wuhan, China, and a mathematical model describing the fluid flow and pollutant dispersion characteristics in the street canyon was developed. The effect of traffic tidal flow was investigated based on the measurement results of the passing vehicles as the pollution source of the CFD method and on the spatial distribution of pollutants under various ambient crosswinds. Numerical investigation results indicated that: (i) in this three-dimensional asymmetrical shallow street canyon, if the pollution source followed a non-uniform distribution due to the traffic tidal flow and the wind flow was perpendicular to the street, a leeward side source intensity stronger than the windward side intensity would cause an expansion of the pollution space even if the total source in the street is equal. When the ambient wind speed is 3 m/s, the pollutant source intensity near the leeward side that is stronger than that near the windward side (R = 2, R = 3, and R = 5) leads to an increased average concentration of CO at pedestrian breathing height by 26%, 37%, and 41%, respectively. (R is the ratio parameter of the left side pollution source and the right side pollution source); (ii) However, this feature will become less significant with increasing wind speeds and changes of wind direction; (iii) the pollution source intensity exerted a decisive influence on the pollutant level in the street canyon. With the decrease of the pollution source intensity, the pollutant concentration decreased proportionally. Full article