Leeward Wall Research Articles

Data-driven prediction of wind pressure on low-rise buildings in complex heterogeneous terrains

This study presents a data-driven methodology for predicting the pressure coefficient statistics on the windward wall, roof, and leeward wall of low-rise buildings situated downwind of complex heterogeneous terrains. Two types of artificial neural network models were developed: the empirical parameter-based ANN (PANN) and the morphology-based ANN (MANN). Pressure data from wind tunnel tests on the Wind Engineering Research Field Laboratory (WERFL) building model (building height H = 4 m) in complex heterogeneous terrain were used to develop the ANN models. These models were evaluated against a non-linear fitted model to assess their predictive performances. PANN and MANN demonstrated superior performance in capturing the effects of terrain complexity on the mean (Cp,mean) and the root-mean-square (Cp,RMS) wind pressure coefficients for the windward wall, roof, and leeward wall. Optimal prediction was achieved with a terrain patch size of W × L = 4 × 2, equating to a full-scale area of approximately 72 m × 23 m. This suggests that the morphology within approximately 100 m × 50 m (25H× 12.5H) in front of a low-rise building has the greatest correlation with the wind pressure coefficient. Despite lower R2 values for max Cp,RMS on the leeward wall across all models, both PANN and MANN showed promising accuracy for the six outputs studied. Moreover, a global sensitivity analysis confirmed the impact of terrain roughness and complexity on the prediction models particularly on max Cp,RMS, and underscored the dominance of effective roughness length z0,eff and the coefficient of variation of roughness length COVz0 in influencing model outcomes.

Three‐Dimensional Electrostatic Hybrid Particle‐In‐Cell Simulations of the Plasma Mini‐Wake Near a Lunar Polar Crater

AbstractLunar polar region has become the focus of future explorations due to the possible ice reservoir in the permanently shadowed craters. However, the space environment near the polar crater is quite complicated, and a plasma mini‐wake can be caused by the topographic obstruction. So far, three‐dimensional (3D) numerical simulations of the mini‐wake around a crater far larger than the Debye length are still limited. Here we present a 3D electrostatic hybrid particle‐in‐cell model to study the plasma mini‐wake of a polar crater on scale of about 1 km. It is found that the mini‐wake can begin upstream from the crater with a cone angle of about 8.8°. There is a plasma void with extra electrons near the leeward crater wall, where the electric potential can be as low as −60 V. A part of solar wind ions can be diverted into the crater, and the ratio of the diverted flux is about 4% on the crater bottom and about 18% on the windward crater wall, which provide an important source for the surface sputtering. Further studies show that the mini‐wake can change with the solar wind parameters and the crater shapes. Our results are helpful to assess the space environment and the water loss rate of a polar crater, and have general implications in studying the plasma mini‐wake caused by a crater on the other airless bodies.

Assessment of pollution removal mechanisms in steep-asymmetric city-type environments using wind deflectors

Traffic emissions directly impact indoor air quality in near-road buildings. Adjustable wind deflectors on building roofs were previously shown to be effective in mitigating air pollution for ideal city-type environments. This was based on the hypothesis that wind deflectors promoted higher pollutant removal by reducing the dependence on turbulent fluctuations. However, the question of whether such a deflector system would work in a more complex city-type environment such as an asymmetric street canyon remains unanswered. In addition, the fundamental impact a deflector could impart on flow dynamics within street canyons in the context of pollution removal through differing mechanisms also requires further research. The current study seeks to answer both questions by introducing adjustable wind deflectors for step-up and step-down asymmetric canyons with two traffic flow directions. For the step-down canyon, the deflectors promoted CO reduction in building facades by 73.55% and 34.79% from leeward and windward walls under a Cross Road Pollution (CRP) source. A 16.57% reduction was achieved on side walls under a Side Road Pollution (SRP) source. However, apart from the 13.87% CO reduction across windward walls under the CRP source, the wind deflectors predominantly resulted in detrimental results for step-up canyons. The ratio of pollution exchange rate achieved by mean flow-induced fluxes and total pollution exchange rate (θ), Sherwood number is the ratio of convective and diffusive mass transfer (Sh) and average canyon concentration (C‾canyon) were used as indices to investigate pollution removal mechanisms. Although both Shv/sC‾canyon and θv/sC‾canyon relationships exhibited good inverse correlations when the deflector was positioned at different locations, the Shv/sC‾canyon showed superior performance in distinguishing the scenarios where a deflector was involved and when not. This implies that the introduction of wind deflectors impacted more in effecting convective fluxes than fluctuations for pollution removal.

Weak geostrophic wind driven ventilation in street canyons with trees and green walls: Cooperating or opposing dispersions of airborne pollutants?

Urban high-rise buildings and dense street canyons significantly disrupt synchronised wind flows, potentially exacerbating urban heat island effect. Fortunately, urban heat island effect can be mitigated by the green infrastructure in cities, particularly through the presence of roadside trees and green walls. However, the combined mechanisms of tree resistance, shading effects, and cooling effect, along with sensible thermal buoyancy flows within canyon ventilation and pollution dispersion, remain undisclosed. This study employs refined computational fluid dynamics numerical simulations to analyse airflow and pollutant dispersion within urban street canyons. Air exchange rate and pollutant retention time are employed to evaluate ventilation and pollutant dispersion, respectively, inside the street canyons.In scenarios with leeward green wall layout, higher tree canopy spread and trunk height lead to a shift in high pollutant concentrations from the region near the windward side to that near the leeward side of the typical canyon (H/W = 1). Conversely, in deep canyons (H/W = 5), most pollutants are retained near the bottom leeward side. Moreover, differences in pollutant dispersion between typical and deep canyons increase with with rising tree canopy spread. Additionally, higher tree canopy spread and trunk height values result in longer pollutant retention time, which are unfavourable for pollutant removal, and particularly limiting pollutant dispersion as street canyon depth increases. Similarly, increasing tree canopy spread reduces the difference in air exchange rate between windward and leeward green wall layout patterns. Regarding Richardson number variations —indicating thermal buoyancy effects—lower Richardson number values lead to reduced differences in pollutant retention time between typical and deep canyons. Furthermore, ventilation performance of the windward green pattern surpasses that of the leeward pattern. This research provides valuable insights for implementing green infrastructure to locally mitigate urban air pollution.

Effects of building envelope features on airflow and pollutant dispersion within a symmetric street canyon.

Building envelope features (BEFs) have attracted more and more attention as they have a significant impact on flow structure and pollutant dispersion within street canyons. This paper conducted CFD numerical models validated by wind-tunnel experiments, to explore the effects of the BEFs on characteristics of the airflow and pollutant distribution inside a symmetric street canyon under perpendicular incoming flow. Three different BEFs (balconies, overhangs, and wing walls) and their locations and continuity/discontinuity structures were considered. For each canyon with various BEFs, the air exchange rate (ACH), airflow patterns, and pollutant distributions were evaluated and compared in detail. The results show that compared to the regular canyon, the BEFs will reduce the ACH of the canyon, but increase the disturbances (the proportion of ACH') inside the canyon. The BEFs on the leeward wall have the least influence on the in-canyon airflow and pollutant distributions, followed by that on the windward wall. Then when the BEFs are on both walls, the ventilation capacity of the canyon is weakened greatly, and the pollutant concentration in the ground center is increased significantly, especially near the windward side. Moreover, the discontinuity BEFs will weaken the effect of the continuity BEFs on the in-canyon flow and dispersion, specifically, the discontinuity BEFs reduced the region of high pollutant concentration distributions. These findings can help optimize the BEFs design to enhance ventilation and mitigate traffic pollution.

Influence of tree planting pattern coupled with wall thermal effect on pollution dispersion within urban street canyon

Among the factors influencing the flow field and dispersion characteristics of pollutants in street canyons, trees not only act as obstacles that impede canyon ventilation but also contribute to non-uniform temperature distributions on its walls through transpiration and shading effects. The coupling effects, however, have not been given adequate attention. This study employed numerical simulations validated by wind-tunnel experiments to investigate the coupling effects of tree planting patterns (crown density Cd, planting density Pd) and wall heating conditions on airflow and pollutant diffusion within urban street canyons. Results show that the variations in Cd or Cd of the tree planting configurations studied have no significant impact on flow structure and pollutant dispersion in the isothermal street canyons. When Pd is 0.25, all three monitoring planes exhibit relatively high net escape velocity (NEV*) and notably reduced overall pollutant concentrations, indicating that this planting configuration is optimal for reducing pollution levels under leeward wall heating. When the windward wall is heated, the pollutant concentration is reduced more significantly under Pd of 0.25 and 0.75 while NEV* on the windward side is about 48. The average NEV* is minimized to approximately 21 at a Cd of 80 % when crown density or planting density is increased for all wall heating scenarios. Moreover, the other configurations of tree planting had little impact on ventilation and dispersion of pollutants in street canyons. The findings can offer technical guidance for the optimal design of urban green facilities to improve local microclimate environment and air quality.

Experimental Study of Wind Pressures on Low-Rise H-Shaped Buildings

Recognizing the urgent need for mitigating global warming, natural ventilation presents a potential strategy to reduce cooling energy demands, enhance thermal comfort, and contribute to indoor air quality. H-shaped buildings are prevalent worldwide, and they constitute the majority of the social housing construction in Brazil. Research suggests that the inadequate design of these buildings can result in poor ventilation; however, investigations about their natural ventilation performance are limited. Thus, the present contribution aims to determine the impact of the geometric characteristics of H-shaped buildings on the pressure distribution through wind tunnel experiments. Three models were tested in the wind tunnel experiments, representing different proportions. Their scales were configured to comply with the 5% obstruction limit allowed for wind tunnel testing, which was performed for 20 wind attack angles. Moreover, a scour test was carried out to allow a better understanding of the wind flow. Python scripting was developed to automate data processing, which is openly available in this paper. The results indicate that the proportion of the model influences the pressure distribution on roofs and leeward walls. Additionally, the depth of the recessed cavity affects its side surfaces and can result in a mirrored behavior on the frontal face of deep cavities (i.e., the wind direction is 45°). The model height influences the windward surfaces in its lower portion, since taller models present a recirculation vortex that modifies the pressure near the ground.

Effects of arcade design on wind and thermal environment inside an idealized 2D urban canyon with realistic solar heating

The arcade design has been widely used in the architecture design since it can provide space for commercial retails and improve local thermal comfort. However, the influences of different arcade design parameters on wind and thermal environment under realistic solar heating conditions are not fully understood, which hinders further application of the arcade design. This paper aims to investigate the effects of different arcade design parameters on wind and thermal environment under different solar heating conditions. Numerical wind flow, solar radiation and heat transfer simulations of a two-dimensional deep street canyon with aspect ratio of three are performed. The steady Reynolds-Averaged Navier-Stokes model with standard k−ε turbulence model is employed in this investigation, while considering solar radiation model. Moreover, the realistic solar heating simulations are achieved by setting different local solar times, i.e., LST0900 (leeward wall heating), LST1200 (street ground heating) and LST1500, (windward wall heating). Under realistic solar heating conditions, the building wall heating and the street ground heating are shown to evidently influence the strength and location of the predominant recirculation inside the street canyon, especially for the windward wall heating. Moreover, analysis results have shown that the arcade width has larger influences on wind and thermal environment than that of the arcade height. Particularly, the wind velocity increases 2.1 to 3.8 times, the air temperature decreases 0.42–2.24 °C and the Physiological Equivalent Temperature value drops between 2.2 °C and 7.0 °C when the arcade width increases to half of building width.

Aerodynamic analysis of building with zigzag-patterned façade: Insights from large eddy simulation

High-rise buildings have diverse shapes and configurations to harmonize with the architectural, engineering, environmental, economic, and functional considerations, and aspirations. However, the architectural shape and configuration plays a vital role in determining the aerodynamic characteristics of the surrounding flow field and wind forces acting on the building. This paper numerically investigates the aerodynamic characteristics of the zigzag-patterned building by adopting the three-dimensional large eddy simulation (LES). The flow around a square cylinder in high Reynolds number is used to valid the numerical method. The results show that the zigzag pattern has a little effect on the mean streamwise velocity but significantly influences the fluctuation of streamwise and vertical velocities in the wake region. It is observed that the zigzag pattern on the leeward wall results in a narrower wake region, while on the windward wall, leads to a thinner and longer shear layers and shorter recirculation region. As for the pedestrian level wind, the flow velocity within the separation area is noticeably reduced when incorporating a zigzag pattern on the windward wall. Moreover, the fluctuating pressure coefficient values at the sidewalls of a building with a zigzag windward wall is significantly smaller than that of a smooth configuration building due to the façade pattern's impact in diminishing the curvature of the nearby separation at the leading edge.

Influence of obstacles on urban canyon ventilation and air pollutant concentration: An experimental assessment

Air pollution in cities, intensified by vehicular traffic emissions and reduced ventilation, poses a significant health risk. Obstacles, like solid or vegetation barriers, are being considered as strategies to reduce pollution exposure for pedestrians and nearby residents in street canyons. This study utilises wind tunnel experiments, to simulate a typical urban canyon with street intersections on both sides, in a 1:200 scale and an H/W=0.5, positioned perpendicular to the free stream wind flow. A passive scalar representing a vehicular pollutant is released along the length of the street canyon. Concentration measurements inside the canyon are performed to determine the effect of parked cars, boundary walls, hedges and trees on pollutant concentration exposure for pedestrians on the sidewalk. Results show that one circulating vortex is generated within the canyon, driving the pollutant to accumulate along the leeward (upwind) wall. Tightly parked cars, boundary walls and hedges, placed along the sidewalk near the leeward wall, can reduce pedestrian pollutant exposure by 15 %, 23 % and 11 % respectively along this sidewalk. Attributes such as obstacle height, surface roughness and porosity play a key role in their performance. However, trees, when placed in the same area, increase pedestrian pollutant exposure by 51 % and 17 % under dense and sparse tree arrangements, respectively. While a broader analysis that considers the variability of vegetation attributes (e.g., porosity, stand density) is desirable, this study remains crucial for validating numerical simulations and suggesting optimal urban measures to reduce pollution exposure for citizens.

On the anti-rolling performance of a train using a vortex generator array

Vortex generators (VGs) have shown the potential to mitigate the train's operational instability issues caused by strong wind. Numerical simulations are used to predict the flow structures around a train with VGs of different heights. The improved delayed detached eddy simulation (IDDES) hybrid modeling method is adopted to predict the trailing vortices on the leeward field. The numerical method is validated by reproducing wind tunnel test results. The study results reveal that VGs are capable of reducing the rolling moment coefficient around the leeward rail of a train by about 5% ∼ 15% while keeping the drag of the train still lower than its operational drag without crosswind. The control mechanism lies on that the streamwise vortices generated by VGs are attracted to the large-scale trailing vortices, resulting in the pressure on the leeward wall rising. The differences in the domain frequencies between VGs and Baseline cases in POD modes indicate that the VGs changed the periodicity and symmetry of the vorticity fluctuation. This study provides a new method to improve the safety of trains under crosswinds.

Enhancing ventilation in street canyons using adjustable roof-level wind flow deflectors

Traffic emission impacts the air quality at both the road surface and in near road buildings. Previous research has examined the benefit of using roof-level wind catchers to reduce concentration along building facades in urban environments. Although roof-level interventions are more effective in improving the street canyon air quality in its entirety, localized re-distributions that detrimentally affect specific regions in and around the street canyons are unavoidable. In the current work, a deflector system that can adjust its position and orientation to changing ambient conditions is introduced to ensure that no particular region perpetually experiences compromised air quality. At optimal roof-level positions, the wind deflectors resulted in a local maximum and minimum of pollution removal through mean flow-induced fluxes and overall canyon concentrations respectively. In this study, the potential of the wind deflectors was first demonstrated using 2D Computational Fluid Dynamics (CFD) investigations. A maximum reduction in overall canyon concentration of 2.84 fold was predicted when the deflector was placed 2 m from the leeward walls. Subsequently, the benefit of the dynamic nature of the intervention and the efficacy of the same in a more realistic 3D city-type environment was demonstrated by considering two different pollution source conditions. The wind deflectors performed modestly for the Cross Road Pollution (CRP) source model by reducing 7%, 11% and 13% of CO exposure on the leeward wall, upwind side wall and downwind side wall without affecting the windward wall of the target street canyon. Whereas for the Side Road Pollution (SRP) source model, it reduced 91%, 32% and 34% on the same with a 17% reduction on the windward wall of the target street canyon. Finally, the concept of an adjustable deflector system was demonstrated to mitigate prolonged high exposure for building occupants exposed to changing traffic emission sources via all the surrounding building facades and at the ground.

Thermal Control Investigations on Separation Zone of Ceramic Thermal Protection System

Ceramic thermal protection system (TPS) is bonded to the skin of a hypersonic vehicle through a strain isolation pad, and the separation of the TPS will affect the safety of the vehicle. In this paper, the aeroheating of the separation zone for the ceramic TPS is studied and compared with that of the intact TPS. The intact TPS mainly considers the aeroheating of the gap. The heat flux increases sharply in both the gap and separation zone, especially on the windward wall. The difference of peak heat flux between the windward and leeward walls in the separation zone is much larger than that in the gap, and the heat flux away from the gap and separation zone gradually returns to the normal heat flux of the TPS. The heat flux in the gap only exists in the small zone of the upper part, whereas that in the separation zone is widely distributed on the side walls. The heat flux on the bottom of the gap approaches zero, whereas that of the separation zone is high in the middle and low at ends, and the heat flux in the middle reaches 55% of the normal heat flux of the TPS. In addition, an analysis of influencing factors is conducted.

Possible high COVID-19 airborne infection risk in deep and poorly ventilated 2D street canyons

AbstractDespite the widespread assumption that outdoor environments provide sufficient ventilation and dilution capacity to mitigate the risk of COVID-19 infection, there is little understanding of airborne infection risk in outdoor urban areas with poor ventilation. To address this gap, we propose a modified Wells-Riley model based on the purging flow rate (QPFR), by using computational fluid dynamics (CFD) simulations. The model quantifies the outdoor risk in 2D street canyons with different approaching wind speeds, urban heating patterns and aspect ratios (building height to street width). We show that urban morphology plays a critical role in controlling airborne infectious disease transmission in outdoor environments, especially under calm winds; with deep street canyons (aspect ratio > 3) having a similar infection risk as typical indoor environments. While ground and leeward wall heating could reduce the risk, windward heating (e.g., windward wall ~10 K warmer than the ambient air) can increase the infection risk by up to 75%. Our research highlights the importance of considering outdoor infection risk and the critical role of urban morphology in mitigating airborne infection risk. By identifying and addressing these risks, we can inform measures that may enhance public health and safety, particularly in densely populated urban environments.

Large-eddy simulations on pollutant reduction effects of road-center hedge and solid barriers in an idealized street canyon

This study conducted large-eddy simulations (LES) on the pollutant reduction effects of hedge and solid barriers in a three-dimensional idealized street canyon with an aspect ratio of 0.5. The wind direction was perpendicular and oblique (45°) to the street. The results were validated with data from wind tunnel experiments. LES accurately predicted the concentration distribution in the barrier-free case and reproduced well the barrier-induced concentration reduction. In the barrier-free case, a large recirculation vortex was observed. However, the central barriers forced the recirculated airflow in the middle of the canyon and newly formed vortices near the leeward walls. The two counter-direction vortices in the hedge and solid barrier cases transported pollutants toward the center of the canyon and enhanced the vertical pollutant removal at the top of the street canyon. The hedge barrier (solid barrier) reduced spatially-averaged concentration by about 59% (45%) near the leeward wall, 64% (20%) near the windward wall, and 45% (17%) in the whole street canyon compared to the barrier-free case. The effects of leaf area density (LAD) and barrier width were further investigated under the perpendicular wind direction. Increasing the LAD or the width of the hedge barrier decreased concentration near the leeward walls but increased canyon-averaged concentration. Increasing the width of the solid barrier decreased the concentration near the leeward walls and the canyon-averaged concentration. In an oblique wind direction, the hedge and solid barriers reduced by about 30% and 60% the spatially-averaged concentration near the building walls compared to the barrier-free case.

New insights on precise regulation of pollutant distribution inside a street canyon by different vegetation planting patterns.

Mixed-vegetation planting patterns are commonly seen in urban areas for specific reasons like aesthetic, cooling, and particle deposition effects of the vegetation. However, they may have a negative impact on human health by worsening the air quality inside the street canyon due to the decreased air exchange rate. From the view of precise control of pollutant concentration in the sensitive areas of people's concern in the existed street canyons, thirty-four cases with different vegetation planting patterns and pressure loss coefficients (λ) are studied numerically to investigate the effects of vegetation on airflow and pollutant dispersion inside the canyon. The cases of treeless and 2 rows of tree planting patterns in wind-tunnel measurements were selected for the model validation. The results demonstrate that compared to the treeless case, the greenbelts can greatly change the airflow features and reduce the pollutant concentration at the leeward side, while the only-tree planting patterns have little impact on the flow and deteriorate dispersion within the street canyon. Moreover, rows of greenbelts planted under the corresponding trees can reduce the average pollutant concentrations on the leeward wall and the footpath of the street canyon by up to 22.6% and 33.2%, respectively. Besides, the pattern of 1 row of trees with 1 row of greenbelts planted in the street canyon center should be suggested as the optimal mixed vegetation configuration in this study. That is because compared to the treeless case the pollutant concentration on leeward wall, windward wall, leeward footpath, and windward footpath can be reduced by 14.2%, 10.0%, 24.6%, and 37%, respectively. It is helpful to the city planners to consider whether the disadvantages of planting vegetation inside the street canyon would overwhelm the advantages.

The role of flow structures in the effective removal of NOx pollutants by a TiO2-based coating in a street canyon

Photocatalytic titanium dioxide (TiO2) coatings are known to effectively remove harmful nitrogen oxides (NOx) in so-called laminar flow reactors (LFRs). However, their effectiveness in turbulent flow environments, such as street canyons, is not well understood. In this study, we applied physical modelling principles to simulate NOx photocatalysis on the walls of a street canyon polluted by idealised traffic, and compared the results of this modelling with those of experiments in a standard LFR. The results show that the LFR is partially able to simulate photocatalysis in such a turbulent environment, but with about 18 times higher flow rate than recommended in the standards (3 l min−1). However, the results from the street canyon experiment show that the performance of the photocatalyst is spatially dependent due to the mean flow structures that develop in the canyon. The mean vertical recirculation transports the NOx pollutants from the line source first to the leeward wall and then to the windward wall, making the windward wall more favourable for pollutant removal. The secondary corner vortices that form at the bottom of the canyon increase the reaction time, so that NOx pollutants are removed more effectively (up to 33%) than at other locations in the canyon. In contrast, the lowest removal efficiencies were found at the leeward wall (6%), where pollutants are mainly advected from the traffic and have less contact with the wall. These results provide valuable insights into the effectiveness of photocatalytic coatings in turbulent flow environments and may be useful for the development and implementation of these coatings in realistic urban pollution scenarios.

Numerical study of impact of façade ribs on the wind field and wind force of high-rise building under atmospheric boundary layer flow

Façade appurtenances are found to be able to improve the aerodynamic performance of tall building. In present paper, impact of façade ribs on the flow structure and wind force of high-rise building are evaluated through LES approach under atmospheric boundary layer flow. It is found that horizontal ribs can significantly increase the size of wake vortex and suppress the near-wall vertical flow. Vertical ribs obviously affect the flow separation due to the introduction of local recirculation, which induces a narrow shear layer and near wake. Combination of horizontal and vertical rib can together affect the flow structure. All arranged ribs could alleviate the fluctuating wind near the sidewall. The change of wind filed will alter the wind pressure distribution and overall wind force. Horizontal ribs slightly reduce the mean wind pressure on sidewall. Vertical ribs clearly decrease the mean leeward suction. Facade ribs with different arrangement significantly reduce fluctuating wind pressure on the lateral and leeward wall. All models with ribs have smaller wind force than that of reference model, the maximum reduction ratio of drag and lift reach 26% and 47%, respectively.

Numerical Investigation of Flow and Dispersion over Two-Dimensional Semi-Open Street Canyon

A semi-open street canyon is able to protect pedestrians from unpleasant situations such as direct sunlight and rain. However, the protruding elements of the two opposite building facades that form the semi-open configuration can affect the air quality of the urban canopy layer (UCL). Therefore, this paper investigated the influence of the eave structures on the flow and pollutant dispersion over an idealized 2D street canyon with a unity aspect ratio. The length of the eaves was varied into 0.25H and 0.5H (H is the building height) and placed either on the leeward wall, the windward wall, or on both building facades located at the same elevation as the street canyon. Numerical simulations were performed using the steady-state Reynolds-averaged Navier-Stokes (RANS) equations in conjunction with Re-Normalization Group (RNG) k-ε as the turbulence closure model. The pollutant was released from a line source in the center of the bottom of the target canyon with uniform flow rate. Six different eave configurations were simulated in the wind direction perpendicular to the canyon axis, representing the worst condition of canyon ventilation. The evolution of the primary vortex, which occupied the entire canyon with the characteristic of skimming flow, showed less dependence on the length and position of the eave, except for the longest eave on the windward wall. However, the position of the vortex center depicted opposite results. The pollutant concentration is always higher near the leeward wall, but for the eave that protrudes from the windward wall with a length of 0.5H, the pollutant accumulates near the windward region. The ratio of pollutant concentration showed higher concentration in the semi-open configurations compared to the fully open layout as a result of limited penetration of shear flow into the canyon, which leads to deterioration of pollutant removal.

The impact of street level particulate emissions on the energy performance of roof level building ventilation systems

Localised vehicular fine particulate matter (PM2.5) emissions in an urban canyon can influence the energy performance of a building ventilation system at roof level. This paper examines the energy demands of air filtration through an air handling unit (AHU) located in different positions and orientations on a building rooftop. A series of 3D numerical simulations examined the impact of aspiration efficiency (AE) on filter loading rates as the distance from the source increases and AHU orientation relative to ambient wind direction. The ventilation PM2.5 concentration was equal to the ambient when positioned near the windward wall of the target building. A decrease of 33% and 60% in the filter loading rate occurred at a wind speed of 7.5 m/s and 2.5 m/s at the leeward wall. There was no energy savings when the AHU is positioned on the windward side of the target building but a reduction in energy consumption of 9.8% at the centre and 19.4% at the leeward side. Comparing the effect of wind orientation for identical AHU positions on the rooftop centre resulted in a 26–35% reduction in AE when the AHU inlet is not facing into the particle laden wind and incurred energy savings of 25.8%.