2D Street Canyons Research Articles

Impact of rooftop stack configuration on 2D street canyon air quality

We experimentally investigated the impact of rooftop stack position on the pollutant entrapment within 2D street canyon configurations. Analyses were performed in a water channel loop system by using the image analysis techniques: FTV (Feature Tracking Velocimetry) and LIF (Light Induced Fluorescein) respectively for measuring velocity and concentration. The set-up consists in an array of 20 identical buildings with aspect ratio equal to one (ARB = B/H, where H is the height of the eaves and B is the building width), mimicking an idealized 2D urban canopy. They were equally spaced with a unitary canyon aspect ratio (defined as ARC = W/H; where W is the distance between buildings facades), and varied in shape by using flat and gable roofs with pitch α = 45°, with different chimney positions and heights, for a total number of 9 investigated configurations. Results demonstrate that the presence of gable roof significantly varies the flow in the shear layer, which, in combination with different chimney position and height, lead to not trivial effects on pollutants dispersion.

Impact of indoor-outdoor temperature differences on dispersion of gaseous pollutant and particles in idealized street canyons with and without viaduct settings

Computational fluid dynamics simulations were performed to investigate flow and pollutant dispersion in a 2D street canyon of aspect ratio H/W = 1. Different from other works, the combination of the presence of viaduct and indoor-outdoor temperature differences ΔT is investigated for different approaching wind velocities. For larger wind velocity (2 m/s, Froude number Fr~3.06–12.24) the typical clockwise vortex leads to higher concentrations of both gas and small particles at the leeward side of the street and in the leeward-side rooms; the vortex is enhanced under large ΔT (20 K) improving the dispersion of pollutants. For smaller velocity (0.5 m/s, Fr~0.19–0.77) the appearance of an anti-clockwise vortex leads to a strong accumulation of gas and particles at the windward side and in the windward-side rooms under low ΔT (5 K); increasing the ΔT raises the dispersion of pollutants with consequent lower concentrations in the rooms (up to an average of 67% with respect to the isothermal case for gaseous pollutants), but accumulation close to the ground level at both windward-side and leeward-side rooms. In the presence of viaduct, together with the main vortex above the viaduct which causes concentrations increasing from low-level to high-level leeward-side rooms, two vortices are generated below it. Still ΔT = 20 K improves the dispersion of pollutants, leading up to a maximum of about 30% lower gaseous concentrations in the rooms. In general, lower concentrations of gas and particles are found for larger velocity, indicating that the mechanical turbulence dominates over the buoyancy effects, which become crucial for smaller velocity. This study confirms previous findings that viaducts may improve pollutant dispersion under large velocity if only one viaduct-level pollutant source exists and indoor-outdoor ΔT conditions can mitigate street air pollution.

The impacts of viaduct settings and street aspect ratios on personal intake fraction in three-dimensional urban-like geometries

High vehicular pollutants exposure to residents in near-road buildings raises special concerns in micro-scale urban science, as it causes severe health problems for those residents. This paper integrates a new parameter, i.e. personal intake fraction (IF_p), into computational fluid dynamics (CFD) simulations to investigate the flow and the resulted personal exposure of two-phase pollutants (CO and particular matters) in three-dimensional (3D) urban-like models. The impacts of street aspect ratios (building height/street width H/W = 0.5–1.5), viaduct settings, noise barriers and pollutant source locations were considered.3D downward helical flows exist in the secondary streets perpendicular to the parallel approaching wind, which produces lateral pollutant transport across the interface between the main street and secondary streets. Therefore, the overall average IF_p (<IF_p>) of CO (∼0.23–0.59 ppm) in our current 3D urban models (H/W = 1) is much smaller than that in two-dimensional (2D) street canyon (∼3.25–5.21 ppm) models. Although narrower 2D street canyons usually present greater <IF_p>, our current 3D urban models do not show this monotone decreasing trend due to the complicated flow structures. <IF_p> of fine particles are always smaller than that of CO. Furthermore, if a single pollutant source is placed on the viaduct, <IF_p> become much smaller than that in cases with a single ground-level source. If the source location changes to the upstream secondary street, <IF_p> significantly decreases due to stronger local wind. Finally, <IF_p> of leeward-side cases usually exceeds that of windward-side cases by several times, but with viaduct settings, this leeward-to-windward ratio significantly decreases.

On the influence of viaduct and ground heating on pollutant dispersion in 2D street canyons and toward single-sided ventilated buildings

This paper employs Computational Fluid Dynamic (CFD) simulations to investigate the influence of ground heating intensities and viaduct configurations on gaseous and particle dispersion within two-dimensional idealized street canyons (typical aspect ratio H/W = 1) and their transport from outdoor to leeward and windward rooms of naturally-ventilated buildings. Without viaduct, the ground heating exists at the street ground above which the pollutant source is located, while in the presence of viaduct the ground heating only occurs at the viaduct ground surface and pollutant source is slightly above the viaduct. Results show that viaduct significantly reduces overall spatial mean indoor concentrations of gaseous pollutant (<K>) and indoor particle number (PN) in all rooms being the elevated pollutant source above the viaduct compared to those without the viaduct. Road barriers on the viaduct slow down the flow above it and slightly increase indoor <K>, but they reduce indoor PN due to the enhanced particle deposition onto viaduct surfaces. The uniform heating of street ground or viaduct ground surface strengthens recirculation flows in street canyon, reducing <K> and PN of fine particle (diameter d = 1 μm), but for larger particles indoor PN distribution is complicated by the interaction of gravity, buoyancy and wind-induced recirculation. Although further investigations are still required to propose a practical framework on viaduct design, this paper is one of the first attempts to study the effect of viaduct on street pollutant dispersion.

The effect of using a high-albedo material on the Universal Temperature Climate Index within a street canyon

This study investigates the effect of different high-albedo adaptation strategies on air temperature, mean radiant temperature and the Universal Temperature Climate Index (UTCI) for an idealized 2D street canyon. The used numerical model computes the heat transport in the canyon, and specifically takes into account the effect of multiple scattering of radiation. In general the mean radiant temperature has a much larger impact on the UTCI than the air temperature. Moreover, the mean radiant temperature exhibits strong spatial variations in the canyon due to its sensitivity to shading. The impact of albedo-differences on the UTCI is thus relatively small compared to the large shading effects. The best strategy to minimize the UTCI for the outdoor environment with building height to width ratio H/W=0.5 is found to be a uniform albedo of 0.2. For H/W=1.0, an albedo gradient from high at the bottom part to low at the top of the vertical walls showed the lowest UTCI. Although using high-albedo materials can mitigate the atmospheric urban heat island effect, it is very likely to increase pedestrian heat stress.

Numerical study of the effects of Planetary Boundary Layer structure on the pollutant dispersion within built-up areas

The effects of different Planetary Boundary Layer (PBL) structures on pollutant dispersion processes within two idealized street canyon configurations and a realistic urban area were numerically examined by a Computational Fluid Dynamics (CFD) model. The boundary conditions of different PBL structures/conditions were provided by simulations of the Weather Researching and Forecasting model. The simulated results of the idealized 2D and 3D street canyon experiments showed that the increment of PBL instability favored the downward transport of momentum from the upper flow above the roof to the pedestrian level within the street canyon. As a result, the flow and turbulent fields within the street canyon under the more unstable PBL condition are stronger. Therefore, more pollutants within the street canyon would be removed by the stronger advection and turbulent diffusion processes under the unstable PBL condition. On the contrary, more pollutants would be concentrated in the street canyon under the stable PBL condition. In addition, the simulations of the realistic building cluster experiments showed that the density of buildings was a crucial factor determining the dynamic effects of the PBL structure on the flow patterns. The momentum field within a denser building configuration was mostly transported from the upper flow, and was more sensitive to the PBL structures than that of the sparser building configuration. Finally, it was recommended to use the Mellor–Yamada–Nakanishi–Niino (MYNN) PBL scheme, which can explicitly output the needed turbulent variables, to provide the boundary conditions to the CFD simulation.

Reduced order modelling of an unstructured mesh air pollution model and application in 2D/3D urban street canyons

A novel reduced order model (ROM) based on proper orthogonal decomposition (POD) has been developed for a finite-element (FE) adaptive mesh air pollution model. A quadratic expansion of the non-linear terms is employed to ensure the method remained efficient. This is the first time such an approach has been applied to air pollution LES turbulent simulation through three dimensional landscapes. The novelty of this work also includes POD's application within a FE-LES turbulence model that uses adaptive resolution. The accuracy of the reduced order model is assessed and validated for a range of 2D and 3D urban street canyon flow problems. By comparing the POD solutions against the fine detail solutions obtained from the full FE model it is shown that the accuracy is maintained, where fine details of the air flows are captured, whilst the computational requirements are reduced. In the examples presented below the size of the reduced order models is reduced by factors up to 2400 in comparison to the full FE model while the CPU time is reduced by up to 98% of that required by the full model.

Transport processes in and above two-dimensional urban street canyons under different stratification conditions: results from numerical simulation

Thermal stratification (neutral, unstable and stable) plays an important role in determining the transport processes in and above urban street canyons. This paper summarizes the recent findings of the effect of thermal stratification on the transport of momentum, heat, and pollutants in the two-dimensional (2D) urban street canyons in the skimming flow regime. Special attention is paid to the results from large-eddy simulations (LESs), while other experimental and numerical results are referred to when necessary. With increasing Richardson number, $$Ri$$ , the drag coefficient of the 2D street canyon as felt by the overlying atmosphere decreases in a linear manner. Under neutral and stable stratification, a nearly constant drag coefficient of 0.02 is predicted by the LESs. Under unstable stratification, the turbulent pollutant transport is dominated by organized turbulent motions (ejections and sweeps), while under stable stratification, the unorganized turbulent motion (inward interactions) plays a more important role and the sweeps are inhibited. The unstable stratification condition also enhances the ejections of turbulent pollutant flux, especially at the leeward roof-level corner, where the ejections dominate the turbulent pollutant flux, outweighing the sweeps. With increasing $$Ri$$ , both the heat (area active scalar source) and pollutant (line passive scalar source) transfer coefficients decrease towards a state where the transfer coefficients become zero at $$Ri \approx 0.5$$ . It should be noted that, due to the limit of the 2D street canyon configuration discussed in this paper, great caution should be taken when generalising the conclusions drawn here.

Numerical simulations of street canyon ventilation and pollutant dispersion

The relationship among air exchange rate (ACH), pollutant exchange rate (PCH) and friction factor ( f ) is proposed in this study to evaluate the performance of ventilation and pollution removal of hypothetical urban areas. Urban areas were simplified to arrays of idealised, repeated two-dimensional (2D) street canyons of different building-height-to-street-width (aspect) ratios (ARs) and building shapes were examined with the use of computational fluid dynamics (CFD). The Reynolds-averaged Navier-Stokes (RANS) equations with the Renormalization Group (RNG) k - є turbulence model were adopted for the CFD. It was found that the turbulent component of ACH contributes more than 60% to the ventilation performance and is linearly proportional to the square root of friction factor ( ACH ’’ ∝ f 1/2 ) for the building shapes tested in this study. Unlike the well-defined relationship between ACH and f , the relationship of pollutant transport (or heat transfer) against f has not yet been formulated because of the different behaviour between building shapes, aspect ratios and locations of pollutant sources.

On the Mechanism of Air Pollutant Removal in Two-Dimensional Idealized Street Canyons: A Large-Eddy Simulation Approach

Flow resistance, ventilation, and pollutant removal for idealized two-dimensional (2D) street canyons of different building-height to street-width (aspect) ratios $$AR$$ are examined using the friction factor $$f$$ , air exchange rate (ACH), and pollutant exchange rate (PCH), respectively, calculated by large-eddy simulation (LES). The flows are basically classified into three characteristic regimes, namely isolated roughness, wake interference, and skimming flow, as functions of the aspect ratios. The LES results are validated by various experimental and numerical datasets available in the literature. The friction factor increases with decreasing aspect ratio and reaches a peak at $$AR = 0.1$$ in the isolated roughness regime and decreases thereafter. As with the friction factor, the ACH increases with decreasing aspect ratio in the wake interference and skimming flow regimes, signifying the improved aged air removal for a wider street canyon. The PCH exhibits a behaviour different from its ACH counterpart in the range of aspect ratios tested. Pollutants are most effectively removed from the street canyon with $$AR = 0.5$$ . However, a minimum of PCH is found nearby at $$AR = 0.3$$ , at which the pollutant removal is sharply weakened. Besides, the ACH and PCH are partitioned into the mean and turbulent components to compare their relative contributions. In line with our earlier Reynolds-averaged Navier–Stokes calculations (Liu et al., Atmos Environ 45:4763–4769, 2011), the current LES shows that the turbulent components contribute more to both ACH and PCH, consistently demonstrating the importance of atmospheric turbulence in the ventilation and pollutant removal for urban areas.

Large-eddy simulation of reactive pollutant dispersion for the spatial instability of photostationary state over idealised 2D urban street canyons

Dispersion of chemically reactive pollutants in the urban canopy layer (UCL) over twelve idealised two-dimensional (2D) street canyons of unity aspect ratio in isothermal conditions is examined. The reversible NO x -O 3 mechanism is integrated into a large-eddy simulation (LES) model. The ground-level NO emission in the first street canyon and the background O 3 in the prevailing wind initiate the NO 2 production. The chemical equilibrium is measured by the photostationary state (PSS) which is a function of the time scales of turbulent mixing and chemical reactions. PSS of the first street canyon increases with increasing NO emission. For small amount of ground-level NO emission and background O 3 in the prevailing flows, (say NO/O 3 = 1/1), PSS increases gradually from the second to last street canyons. It increases faster if the NO-to-O 3 ratio is raised, say 1,000/30. Further increasing the NO-to-O 3 ratio, say 10,000/1, PSS exhibits a non-linear behaviour in which a trough is observed in the fifth street canyon.

Pollutant dispersion over two-dimensional idealised urban roughness: a large-eddy simulation approach

A series of two-dimensional (2D) street canyon models with a wide range of building-height-to-street-width (aspect) ratios are employed in this study to elucidate the pollutant transport over idealised urban areas. The large-eddy simulation (LES) is used to resolve the turbulent flows and pollutant transport in the urban boundary layer (UBL) over the street canyons. The results show that the pollutant removal is governed by atmospheric turbulence in both skimming flow and wake interference regimes. In the UBL, the vertical pollutant profiles illustrate self-similarity behaviours in the downstream region. The pollutant disperses rapidly over the buildings, exhibiting a Gaussian-plume shape. The maximum vertical pollutant dispersion coefficient is observed at aspect ratio equal to 1/10. A strong correlation between friction factor and dispersion coefficient is revealed, implying that the downstream air quality could be improved by increasing the roughness of urban areas.

Large-eddy simulation of turbulent transports in urban street canyons in different thermal stabilities

Five sets of large-eddy simulations (LES) were performed to examine the characteristics of flows and pollutant dispersion in two-dimensional (2D) urban street canyons of unity building-height-to-street-width ratio in neutral, unstable, and stable thermal stratifications. The characteristic flows fall into the skimming flow regime for all the cases tested. The mean wind speed is increased and decreased, respectively, in unstable and stable conditions. Turbulence is enhanced in unstable conditions. Whereas, in stable conditions, the low-level temperature inversion weakens the recirculating flows forming another layer of stagnant air in the vicinity of the ground level. Unexpectedly, an increase in turbulence is found in the street canyon core in the slightly stable condition (Richardson number Rb=0.18). The turbulence promotion could be caused by the unique geometry of 2D street canyon in which the stable stratification slows down the primary recirculation. The rather stagnant flows in turn sharpen the roof-level vertical velocity gradient and deter the entrainment penetrating down to the ground level, leading to a substantial pollutant accumulation. While the pollutant tends to be well mixed in the street canyons in neutral and unstable conditions, a mildly improved pollutant removal in unstable conditions is observed because of the enhanced roof-level buoyancy-driven turbulence.

Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are evenly aligned in a cross-flow in the streamwise direction x. Theflow falls into the skimming flow regime. A larger computational domain is adopted to accurately resolve the turbulence above roof level and its influence on the flow characteristics in the street canyons. The LES calculated statistics of wind and pollutant transports agree well with other field, laboratory and modelling results available in the literature. The maximum wind velocity standard devi- ations σi in the streamwise (σu), spanwise (σv) and vertical (σw) directions are located near the roof-level windward corners. Moreover, a second σw peak is found at z ≈ 1.5h (h is the building height) over the street canyons. Normalizing σi by the local friction velocity u∗ ,i t is found that σu/u∗ ≈ 1.8, σv/u∗ ≈ 1. 3a ndσw/u∗ ≈ 1.25 exhibiting rather uniform values in the urban roughness sublayer. Quadrant analysis of the vertical momentum flux u �� w �� shows that, while the inward and outward interactions are small, the sweeps and ejections dominate themomentumtransportoverthestreetcanyons.Inthe x direction,thetwo-pointcorrelations of velocity Rv,x and Rw,x drop to zero at a separation larger than h but Ru,x (= 0.2) persists even at a separation of half the domain size. Partitioning the convective transfer coefficient � T of pollutant into its removal and re-entry components, an increasing pollutant re-entrain- ment from 26. 3t o 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons.

On the mechanism of air pollutant re-entrainment in two-dimensional idealized street canyons

The two-dimensional (2D) idealized street canyon, which is the generic unit of a city, is the platform for our fundamental understanding of ventilation and pollutant removal at the neighborhood scale. The building-height-to-street-width aspect ratio h/b is the key geometric parameters affecting the flow structures in a street canyon. In this study, a series of computational fluid dynamics (CFD) sensitivity tests were performed to examine how the air pollutant concentration in a street canyon is related to the aspect ratio. The Reynolds-averaged Navier–Stokes (RANS) equations and the Renormalization Group (RNG) k−ε turbulence model were used in the mathematical model. The spatial behaviors of air pollutant transport from the facades, streets, and roofs to the shear layer were depicted by the local pollutant exchange rate ω. Besides, the bulk quantities, air exchange rate ACH, pollutant exchange rate PCH, and volume average pollutant concentration Θ, were used to elucidate the ventilation and pollutant removal mechanisms of the street canyon. The aspect ratios tested were in the range 0.067 ≤ h/b ≤ 2.5 that covered the isolated roughness, wake interference and skimming flow regimes in 2D street canyons. A local maximum Θ was determined in 0.2 ≤ h/b ≤ 0.5 that is different from the monotonic ACH or PCH. The CFD results showed that the mildly elevated air pollutant concentration is not caused by poor pollutant removal but the pollutant re-entrainment from the shear layer aloft back into the street canyon. It is thus suggested that ACH, PCH, and Θ should be used as complementary indicators.

On the correlation of air and pollutant exchange for street canyons in combined wind-buoyancy-driven flow

The ventilation and pollutant transport in a two-dimensional (2D) street canyon of building-height-to-street-width (aspect) ratio h/ b = 1 under different unstable stratifications were examined. To characterize the combined wind-buoyancy-driven flow and pollutant transport at different Richardson number Ri, a computational fluid dynamics (CFD) model based on the Reynolds-averaged Navier–Stokes (RANS) equations with the Renormalization Group (RNG) k − ε turbulence model was adopted. Unlike the isothermal condition, a secondary recirculation is initiated at the ground-level windward corner of the street canyon once the unstable stratification is switched on (Ri < 0). It traps the ground-level pollutant leading to elevated pollutant concentration there. As Ri further decreases, the enlarging secondary recirculation enables direct pollutant removal from its core to the shear layer that offsets the ground-level pollutant accumulation. The ventilation and pollutant removal performance under different unstable stratifications are compared by the air (ACH) and pollutant (PCH) exchange rates, and pollutant retention time ( τ). Both the mean and turbulent components of ACH are found to increase with decreasing Ri, suggesting that unstable stratification promotes ventilation in street canyons. Moreover, the CFD results agree well with our theoretical model that ACH 2 varies linearly with Ri. Turbulent transport originally dominates the pollutant removal under isothermal condition. However, progressive domination of pollutant removal by mean wind can be observed with decreasing stability (decreasing Ri from 0 to −10.6). The critical value is estimated to be Ri = −8, below which mean wind is the major pollutant removal carrier. Reduction in τ is also observed with decreasing Ri. Hence, in unstable stratification, pollutant resides shorter time in the street canyon compared with its isothermal counterpart, and the ventilation and pollutant removal are more favorable.

Experimental study of the transfer velocity for urban surfaces with a water evaporation method

A major problem in urban climate modelling is determining how the heat fluxes from various canyon surfaces are affected by canyon flow. To address this problem, we developed a water evaporation method involving filter paper to study the distribution of the convective transfer velocity in urban street canyons. In this method, filter paper is pasted onto a building model and the evaporation rate from the paper is measured with an electric balance. The method was tested on 2D (two-dimensional) street canyon models and 3D model arrangements. Moreover, in this technique, it is easy to restrict the flux within an arbitrary surface in question. That is, the evaporation distribution on a surface can be studied by using several small pieces of filter paper. In the 2D case, the wall transfer velocity was strongly dependent on the canyon aspect ratio for perpendicular wind directions and it varied widely with height within both windward and leeward wall surfaces. For 3D cubic arrays, the relation to canyon aspect ratio was largely different from that of the 2D canyon. And, as a case study, the variation of wind direction was investigated for a city-like setting. The area-averaged transfer velocity was insensitive to wind direction but its local deviation was significant. Finally, we measured the transfer velocity for a clustered block array surrounded by relatively wide streets. The effect of spatial heterogeneity on the transfer velocity was significant. Moreover, for a fixed total building volume, the transfer velocity was considerably larger when the building height varied than when it was uniform. Therefore, the water evaporation method with filter paper is expected to be useful for studying the transfer velocity and ventilation rates in urban areas with various canyon shapes.

3D stream and vortexes in the urban canopy layer and transport of motor vehicles exhaust gas

Wind-tunnel experiments were conducted to investigate the transport of motor vehicle exhaust gas in the vicinity of a street. The stream and the vertical and horizontal vortexes in 2D and 3D street canyons were visualised using smoke. They were recorded by video camera and the transport of the exhaust gas by vortexes was then investigated. The vertical vortex plays a significant role in the vertical exchange between in-canyon flow and outer flow, and transports the exhaust gas outside the street canyons. The horizontal vortexes transport air pollutants to the next intersecting street. The relationships between functions of the vortexes and several assumptions in the Osaka Municipal Government (OMG) volume-source model are discussed.