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The prediction of pollution within street canyons remains challenging due to the complexity of turbulent flows in urban environments. To improve our understanding of these flows and their relation to the ventilation of street-canyons, we studied pollution fluxes through all street-canyon openings and their balance with respect to urban-array complexity using large-eddy simulations (LES). We validated the LES with wind-tunnel measurements using mean velocities and concentrations and the mean total and turbulent pollution fluxes at the top and lateral openings of the street canyons with either uniform or nonuniform heights and pitched roofs. The LES confirm and extend recent findings about the importance of mean horizontal pollution fluxes for pollutant transport within a street network. The mean fluxes have at least the same dominance as the turbulent ones in case of lateral openings. In case of top openings, the mean fluxes clearly dominate if openings at the roof eaves are selected. While pollutant removal and entrainment through lateral openings are negligible in case of uniform canyons, they are notable in case of nonuniform canyons. We demonstrate that urban roof height nonuniformity does not necessarily enhance the street-canyon ventilation because it worsens (by a factor of 1.1) or improves (by a factor of 1.5) the ventilation compared with the urban uniform height.

Although most people live in cities, the pollutant transport in so-called street canyons is still not well understood. The main reason is the complexity of the problem, where the roof shape, roof height and position of the pollutant source within a 3D urban array may play a significant role. This paper tries to improve the understanding and investigates the pollutant transport in six different types of street canyons using large-eddy simulations (LES). All street canyons were part of four different 3D urban arrays and differed in the roof shape and height. The results demonstrate that roof shape, roof-height non-uniformity, and the source position have a crucial impact on the advective and turbulent pollutant transport between the studied canyons and urban arrays. Both the flat roofs and roof-height variability enhance the lateral transport between the street canyons and intersections irrespective of the studied cases. This is crucial for the evaluation of the street canyon pollution if one changes the position of the source within the non-uniform urban array. Although the differences in the pollutant transport between the flat and pitched roofs mitigate in the cases of the variable roof heights, they are still appreciable.

The ventilation processes in three different street canyons of variable roof geometry were investigated in a wind tunnel using a ground-level line source. All three street canyons were part of an urban-type array formed by courtyard-type buildings with pitched roofs. A constant roof height was used in the first case, while a variable roof height along the leeward or windward walls was simulated in the two other cases. All street-canyon models were exposed to a neutrally stratified flow with two approaching wind directions, perpendicular and oblique. The complexity of the flow and dispersion within the canyons of variable roof height was demonstrated for both wind directions. The relative pollutant removals and spatially-averaged concentrations within the canyons revealed that the model with constant roof height has higher re-emissions than models with variable roof heights. The nomenclature for the ventilation processes according to quadrant analysis of the pollutant flux was introduced. The venting of polluted air (positive fluctuations of both concentration and velocity) from the canyon increased when the wind direction changed from perpendicular to oblique, irrespective of the studied canyon model. Strong correlations ( $$>$$ 0.5) between coherent structures and ventilation processes were found at roof level, irrespective of the canyon model and wind direction. This supports the idea that sweep and ejection events of momentum bring clean air in and detrain the polluted air from the street canyon, respectively.

The advanced statistical techniques for qualitative and quantitative validation of Large Eddy Simulation (LES) of turbulent flow within and above a two-dimensional street canyon are presented. Time-resolved data from 3D LES are compared with those obtained from time-resolved 2D Particle Image Velocimetry (PIV) measurements. We have extended a standard validation approach based solely on time-mean statistics by a novel approach based on analyses of the intermittent flow dynamics. While the standard Hit rate validation metric indicates not so good agreement between compared values of both the streamwise and vertical velocity within the canyon canopy, the Fourier, quadrant and Proper Orthogonal Decomposition (POD) analyses demonstrate very good LES prediction of highly energetic and characteristic features in the flow. Using the quadrant analysis, we demonstrated similarity between the model and the experiment with respect to the typical shape of intensive sweep and ejection events and their frequency of appearance. These findings indicate that although the mean values predicted by the LES do not meet the criteria of all the standard validation metrics, the dominant coherent structures are simulated well.

As most of the world’s population lives in cities, it is critical to understand dispersion processes of pollutants in urban areas. This study focuses on so called air exchange rate (ACH) index, which is frequently used by numerical studies to determine ventilation of street canyons without a simulation of a pollution source. These studies applied the ACH on idealised 2D street canyons, where the ventilation acts only through the one opening roof top. There are two pertinent questions: i) is the ACH really capable to predict the ventilation of a street canyon without knowing of a pollutant source; and ii) how much the ACH differs between 2D and 3D street canyons? To answer these questions, we performed large-eddy simulations of pollution of complex 3D street canyons from ground-level line sources. We computed ACHs and spatially-average concentrations for three different street canyons and compared these quantities with those from previous studies. Results clearly demonstrate that these quantities strongly depend not only on street-canyon geometry but also on geometry of surrounding buildings. It is also shown that 2D canyon gives unrealistic result for retention of pollutant within an urban street canyon. The ACH might lead to significant underestimation of the street-canyon ventilation if a source would be outside the canyon.

This paper presents an extension of our previous wind-tunnel study (Nosek et al., 2016) in which we highlighted the need for investigation of the removal mechanisms of traffic pollution from all openings of a 3D street canyon. The extension represents the pollution flux (turbulent and advective) measurements at the lateral openings of three different 3D street canyons for the winds perpendicular and oblique to the along-canyon axis. The pollution was simulated by emitting a passive gas (ethane) from a homogeneous ground-level line source positioned along the centreline of the investigated street canyons. The street canyons were formed by courtyard-type buildings of two different regular urban-array models. The first model has a uniform building roof height, while the second model has a non-uniform roof height along each building's wall. The mean flow and concentration fields at the canyons' lateral openings confirm the findings of other studies that the buildings' roof-height variability at the intersections plays an important role in the dispersion of the traffic pollutants within the canyons. For the perpendicular wind, the non-uniform roof-height canyon appreciably removes or entrains the pollutant through its lateral openings, contrary to the uniform canyon, where the pollutant was removed primarily through the top. The analysis of the turbulent mass transport revealed that the coherent flow structures of the lateral momentum transport correlate with the ventilation processes at the lateral openings of all studied canyons. These flow structures coincide at the same areas and hence simultaneously transport the pollutant in opposite directions.

Owing to the spatial variability of an urban area, even the transport of passive scalar is complex. The examination of the pollutant transport in such turbulent flow as occur within the urban canopy requires the measurements not only the advective but also the turbulent part of this transport. However, recent measurement techniques for the turbulent transport have their limitations. In particular, these measurements are very demanding if one needs to analyze the ventilation processes of the street through the entire streetcanyon openings and which needs to be performed also in the positions where the handling with the measurement instruments might comprise a complex issue or is even impossible. Thus we present a comparison of two methods for the assessment of turbulent and advective pollution fluxes through the lateral openings of two different street canyons with respect to the wind direction and the roof-height nonuniformity to evaluate the importance of the measurement of these fluxes nearby the street-canyon walls. Both of them are based on the simultaneous point measurement of concentration and lateral velocity component and the interpolation between the last measured point and the street-canyon borders, but in the second method the measured area is extended from 77% to 84% of the total area of the lateral openings.

In order to estimate the ventilation of vehicle pollution within street canyons, a wind tunnel experiment and a large eddy simulation (LES) was performed. A model of an idealised urban area with apartment houses arranged to courtyards was designed according to common Central European cities. In the wind tunnel, we assembled a set-up for simultaneous measurement of vertical velocity and tracer gas concentration. Due to the vehicle traffic emissions modelling, a new line source of tracer gas was designed and built into the model. As a computational model, the LES model solving the incompressible Navier-Stokes equations was used. In this paper, we focused on the street canyon with the line source situated perpendicular to an approach flow. Vertical and longitudinal velocity components of the flow with the pollutant concentration were obtained from two horizontal grids placed in different heights above the street canyon. Vertical advective and turbulent pollution fluxes were computed from the measured data as ventilation characteristics. Wind tunnel and LES data were qualitatively compared. A domination of advective pollution transport within the street canyon was determined. However, the turbulent transport with an opposite direction to the advective played a significant role within and above the street canyon.