The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Jong-jin Baik - One of the best experts on this subject based on the ideXlab platform.
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relationship between rooftop and on road concentrations of traffic related pollutants in a busy Street Canyon ambient wind effects
Environmental Pollution, 2016Co-Authors: Sanghyun Lee, Kyunghwan Kwak, Jaemyeong Mango Seo, Seungbu Park, Jong-jin BaikAbstract:Rooftop and on-road measurements of O3, NO2, NOx, and CO concentrations were conducted to investigate the relationship between rooftop and on-road concentrations in a busy and shallow Street Canyon with an aspect ratio of ∼0.3 in Seoul, Republic of Korea, from 15 April to 1 May 2014. The median road-to-roof concentration ratios, correlation coefficients between rooftop and on-road concentrations, and temporal variations of rooftop and on-road concentrations are analyzed according to the rooftop wind directions which are two cross-Canyon and two along-Canyon directions. The analysis results indicate that the relationship is strong when the rooftop is situated on the downwind side rather than on the upwind side. Relative to the cross-Canyon wind directions, one of the along-Canyon wind directions can more enhance the relationship. A conceptual framework is proposed to explain the effect of ambient wind direction on the relationship between rooftop and on-road concentrations in a Street Canyon.
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a large eddy simulation study of thermal effects on turbulent flow and dispersion in and above a Street Canyon
Journal of Applied Meteorology and Climatology, 2012Co-Authors: Seungbu Park, Jong-jin Baik, Siegfried Raasch, Marcus Oliver LetzelAbstract:Thermal effects on turbulent flow and dispersion in and above an idealized Street Canyon with a Street aspect ratio of 1 are numerically investigated using the parallelized large-eddy simulation model (“PALM”). Each of upwind building wall, Street bottom, and downwind building wall is heated, and passive scalars are emitted from the Street bottom. When compared with the neutral (no heating) case, the heating of the upwind building wall or Street bottom strengthens a primary vortex in the Street Canyon and the heating of the downwind building wall induces a shrunken primary vortex and a winding flow between the vortex and the downwind building wall. Heating also induces higher turbulent kinetic energy and stronger turbulent fluxes at the rooftop height. In the neutral case, turbulent eddies generated by shear instability dominate mixing at the rooftop height and appear as band-shaped perturbations in the time–space plots of turbulent momentum and scalar fluxes. In all of the heating cases, buoyancy-generated turbulent eddies as well as shear-generated turbulent eddies contribute to turbulent momentum and scalar fluxes and band-shaped or lump-shaped perturbations appear at the rooftop height. A quadrant analysis shows that at the rooftop height, in the neutral case and in the case with upwind building-wall heating, sweep events are less frequent but contribute more to turbulent momentum flux than do ejection events. By contrast, in the case with Street-bottom and downwind building-wall heating, the frequency of sweep events is similar to that of ejection events and the contribution of ejection events to turbulent momentum flux is comparable to that of sweep events.
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a cfd modeling study of the impacts of nox and voc emissions on reactive pollutant dispersion in and above a Street Canyon
Atmospheric Environment, 2012Co-Authors: Kyunghwan Kwak, Jong-jin BaikAbstract:Abstract A computational fluid dynamics (CFD) model that includes the carbon bond mechanism IV (CBM-IV) is developed and used to investigate reactive pollutant dispersion in and above a Street Canyon with an aspect ratio of 1. Fourteen emission scenarios of NOx and volatile organic compounds (VOCs) are considered. Dispersion types are classified into NO-type, NO2-type, and O3-type dispersion that exhibit concentration maxima at the Street bottom, near the center of the Street Canyon, and above the Street Canyon, respectively. For the base emission scenario, the number of reactive species is 9 in the NO-type dispersion, 10 in the NO2-type dispersion, and 15 in the O3-type dispersion. As the NOx emission level decreases or the VOC emission level increases, some species in the O3-type dispersion are shifted to the NO2-type dispersion. The VOC-to-NOx emission ratio is found to be an important factor in determining the transition of dispersion type. In this transition process, OH plays a key role through a radical chain including HO2, RO, and RO2. Because of their high OH reactivities, XYL (xylene) and OLE (olefin carbon bond) among VOCs are largely responsible for the transition of dispersion type. The O3 sensitivity is examined by reducing NOx or VOC emission level by a half. Because the NO titration of O3 is more pronounced than the NO2 photolysis and the radical chain process in the Street Canyon, the O3 concentration therein is negatively correlated with the NOx emission level and weakly correlated with the VOC emission level. As a result, the Street Canyon is a negatively NOx-sensitive regime.
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modeling reactive pollutant dispersion in an urban Street Canyon
Atmospheric Environment, 2007Co-Authors: Jong-jin Baik, Yoonso Kang, Jae-jin KimAbstract:Abstract Reactive pollutant dispersion in an urban Street Canyon with a Street aspect ratio of one is numerically investigated using a computational fluid dynamics (CFD) model. The CFD model developed is a Reynolds-averaged Navier–Stokes equations (RANS) model with the renormalization group (RNG) k–e turbulence model and includes transport equations for NO, NO 2 , and O 3 with simple photochemistry. An area emission source of NO and NO 2 is considered in the presence of background O 3 and Street bottom heating (Δ T =5 °C) with an ambient wind perpendicular to the along-Canyon direction. A primary vortex is formed in the Street Canyon and the line connecting the centers of cross-sectional vortices meanders over time and in the Canyon space. The cross-Canyon-averaged temperature and reactive pollutant concentrations oscillate with a period of about 15 min. The averaged temperature is found to be in phase with NO and NO 2 concentrations but out of phase with O 3 concentration. The photostationary state defect is small in the Street Canyon except for near the roof level and the upper downwind region of the Canyon and its local minimum is observed near the center of the primary vortex. The budget analysis of NO (NO 2 ) concentration shows that the magnitude of the advection or turbulent diffusion term is much larger (larger) than that of the chemical reaction term and that the advection term is largely balanced by the turbulent diffusion term. On the other hand, the budget analysis of O 3 concentration shows that the magnitude of the chemical reaction term is comparable to that of the advection or turbulent diffusion term. The inhomogeneous temperature distribution itself affects O 3 concentration to some extent due to the temperature-dependent photolysis rate and reaction rate constant.
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Effects of inflow turbulence intensity on flow and pollutant dispersion in an urban Street Canyon
Journal of Wind Engineering and Industrial Aerodynamics, 2003Co-Authors: Jae-jin Kim, Jong-jin BaikAbstract:The effects of inflow turbulence intensity on flow and pollutant dispersion in an urban Street Canyon with a Street aspect ratio of 1 are examined using a two-dimensional numerical model. As the inflow turbulence intensity increases, turbulent kinetic energy and turbulent diffusivity in the Street Canyon increases. Also, the mean horizontal velocity near the roof level increases and the Street-Canyon vortex strengthens. The analyses of the time series and residue ratio of pollutant concentration show that the inflow turbulence intensity significantly affects pollutant concentration in the Street Canyon. As the inflow turbulence intensity increases, the pollutant concentration in the Street Canyon becomes low and hence more pollutants escape from the Street Canyon.
M Santamouris - One of the best experts on this subject based on the ideXlab platform.
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experimental study of temperature and airflow distribution inside an urban Street Canyon during hot summer weather conditions part i air and surface temperatures
Building and Environment, 2008Co-Authors: K Niachou, I Livada, M SantamourisAbstract:Abstract This paper describes the measurements and analysis of an experimental campaign performed in an urban Street Canyon in Athens, Greece. A number of field and indoor experimental procedures were organized during summer 2002 aiming at the investigation of the impact of urban environment on the potential of natural and hybrid ventilation. The present study is focused on the experimental investigation of thermal characteristics of a typical Street Canyon, oriented in ESE–WNW direction, under hot weather conditions. The temporal and spatial distribution of air and surface temperatures is examined. Emphasis was given on the vertical distribution of air and surface temperatures and the air temperature profile in the centre of Canyon under different weather conditions. The measured surface temperature differences across the Street reached almost 30 °C and this favored the overheating of lower air levels. Buoyancy generated mainly from asphalt-Street heating resulted in the development of the predominant recirculation inside the Street Canyon.
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experimental study of temperature and airflow distribution inside an urban Street Canyon during hot summer weather conditions part ii airflow analysis
Building and Environment, 2008Co-Authors: K Niachou, I Livada, M SantamourisAbstract:This paper presents the results of an urban measurement campaign performed in a Street Canyon in Athens, Greece. A number of field experimental procedures were organized during hot weather conditions, on a 24-h basis for five consecutive days during July 2002. Wind velocity measurements were conducted inside and outside the Street Canyon together with air and surface temperature measurements. Based on the results of air and surface temperature measurements, a further analysis is performed for the investigation of airflow inside the Canyon when the ambient flow is parallel, perpendicular and oblique relative to the long Canyon axis. The observed airflow characteristics are associated with the impact of thermal effects mainly induced from ground heating due to the incident solar radiation. However, the role of the finite length Canyon effects related to wind circulation near Street intersections, on the observed airflow patterns, is also identified.
Andy Chan - One of the best experts on this subject based on the ideXlab platform.
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numerical simulation of atmospheric pollutant dispersion in an urban Street Canyon comparison between rans and les
Journal of Wind Engineering and Industrial Aerodynamics, 2011Co-Authors: Salim Mohamed Salim, Riccardo Buccolieri, Andy Chan, Silvana Di SabatinoAbstract:Prediction accuracy of pollutant dispersion within an urban Street Canyon of width to height ratio W/H=1 is examined using two steady-state Reynolds-averaged Navier–Stokes (RANS) turbulence closure models, the standard k–e and Reynolds Stress Model (RSM), and Large Eddy Simulation (LES) coupled with the advection–diffusion method for species transport. The numerical results, which include the statistical properties of pollutant dispersion, e.g. mean concentration distributions, time-evolution and three-dimensional spreads of the pollutant, are then compared to wind-tunnel (WT) measurements. The accuracy and computational cost of both numerical approaches are evaluated. The time-evolution of the pollutant concentration (for LES only) and the mean (time-averaged) values are presented. It is observed that amongst the two RANS models, RSM performed better than standard k–e except at the centerline of the Canyon walls. However, LES, although computationally more expensive, did better than RANS in predicting the concentration distribution because it was able to capture the unsteady and intermittent fluctuations of the flow field, and hence resolve the transient mixing process within the Street Canyon.
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large eddy simulations of wind flow and pollutant dispersion in a Street Canyon
Atmospheric Environment, 2005Co-Authors: Andy Chan, Anton Y T WongAbstract:Abstract The wind flow and pollutant dispersion phenomena in urban Streets of different aspect ratios (h/w) and relative Canyon height ratios (h2/h1) are studied using large-eddy simulations (LES). The concerned large eddies are computed by the filtered Navier–Stokes equations in LES and the unresolved small eddies are modelled using Smagorinsky subgrid scale model. The domain is discretised into uneven staggered grids using marker and cell (MAC) method. The objective of this work is to demonstrate the various flow regimes and their threshold values in urban Street Canyon using LES for various Canyon geometries and Reynolds numbers. All cases are investigated with Reynolds number 400 primarily to obtain information of the three regimes of Canyon flow and the Reynolds number is then increased incrementally to 2000 to study the consequent flow fields and pollutant dilution patterns. In low Reynolds numbers, results presented agree with the generally obtained threshold values for different flow regimes. Increase in Reynolds number has smeared the flow regimes boundary. The ease of pollutant dispersion is mainly promoted by better mixings inside the Canyon, formation of unstable circulations and higher Reynolds numbers. All results show that the flow regime and pollution pattern demarcations depend on both the varying Canyon geometry and Reynolds number.
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strategic guidelines for Street Canyon geometry to achieve sustainable Street air quality part ii multiple canopies and Canyons
Atmospheric Environment, 2003Co-Authors: Andy ChanAbstract:Abstract The flow field and pollutant dispersion characteristics in a three-dimensional urban Street Canyon are investigated for various building array geometries. The Street Canyon in consideration is located in a multi-canopy building array that is similar to realistic estate situations. The pollutant dispersion characteristics are studied for various canopy aspect ratios, namely: the Canyon height to width ratio, Canyon length to height ratio, Canyon breadth ratio and crossroad locations are studied. A three-dimensional field-size Canyon has been analysed through numerical simulations using k − e turbulence model. As expected, the wind flow and mode of pollutant dispersion is strongly dependent on the various flow geometric configurations and that the results can be different from that of a single Canyon system. For example, it is found that the pollutant retention value is minimum when the Canyon height-to-width ratio is approximately 0.8, or that the building height ratio is 0.5. Various rules of thumbs on urban Canyon geometry have been established for good pollutant dispersion.
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strategic guidelines for Street Canyon geometry to achieve sustainable Street air quality
Atmospheric Environment, 2001Co-Authors: Andy Chan, Subash C SamadAbstract:This paper is concerned with the motion of air within the urban Street Canyon and is directed towards a deeper understanding of pollutant dispersion with respect to various simple Canyon geometries and source positions. Taking into account the present days typical urban configurations, three principal flow regimes “isolated roughness flow”, “skimming flow” and “wake interference flow” (Boundary Layer Climates, 2nd edition, Methuen, London) and their corresponding pollutant dispersion characteristics are studied for various canopies aspect ratios, namely relative height (h2/h1), Canyon height to width ratio (h/w) and Canyon length to height ratio (l/h). A field-size Canyon has been analyzed through numerical simulations using the standard k-e turbulence closure model. It is found that the pollutant transport and diffusion is strongly dependent upon the type of flow regime inside the Canyon and exchange between Canyon and the above roof air. Some rules of thumbs have been established to get urban Canyon geometries for efficient dispersion of pollutants.
R E Britter - One of the best experts on this subject based on the ideXlab platform.
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simulations of the dispersion of reactive pollutants in a Street Canyon considering different chemical mechanisms and micromixing
Atmospheric Environment, 2009Co-Authors: Andrew Garmory, R E Britter, I S Kim, E MastorakosAbstract:The Stochastic Fields (SF) or Field Monte Carlo method has been used to model the dispersion of reactive scalars in a Street Canyon, using a simple chemistry and the CBM-IV mechanism. SF is a Probability Density Function (PDF) method which allows both means and variances of the scalars to be calculated as well as considering the effect of segregation on reaction rates. It was found that the variance of reactive scalars such as NO2 was very high in the mixing region at roof-top level with rms values of the order of the mean values. The effect of segregation on major species such as O3 was found to be very small using either mechanism, however, some radical species in CBM-IV showed a significant difference. These were found to be the seven species with the fastest chemical timescales. The calculated photostationary state defect was also found to be in error when segregation is neglected.
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comparative study of measured and modelled number concentrations of nanoparticles in an urban Street Canyon
Atmospheric Environment, 2009Co-Authors: R E Britter, Ruwim Berkowicz, Prashant Kumar, Andrew Garmory, Matthias KetzelAbstract:Abstract This study presents a comparison between measured and modelled particle number concentrations (PNCs) in the 10–300 nm size range at different heights in a Canyon. The PNCs were modelled using a simple modelling approach (modified Box model, including vertical variation), an Operational Street Pollution Model (OSPM) and Computational Fluid Dynamics (CFD) code FLUENT. All models disregarded any particle dynamics. CFD simulations have been carried out in a simplified geometry of the selected Street Canyon. Four different sizes of emission sources have been used in the CFD simulations to assess the effect of source size on mean PNC distributions in the Street Canyon. The measured PNCs were between a factor of two and three of those from the three models, suggesting that if the model inputs are chosen carefully, even a simplified approach can predict the PNCs as well as more complex models. CFD simulations showed that selection of the source size was critical to determine PNC distributions. A source size scaling the vehicle dimensions was found to better represent the measured PNC profiles in the lowest part of the Canyon. The OSPM and Box model produced similar shapes of PNC profile across the entire height of the Canyon, showing a well-mixed region up to first ≈2 m and then decreasing PNCs with increased height. The CFD profiles do correctly reproduce the increase from road level to a height of ≈2 m; however, they do not predict the measured PNC decrease higher in the Canyon. The PNC differences were largest between idealised (CFD and Box) and operational (OSPM) models at upper sampling heights; these were attributed to weaker exchange of air between Street and roof-above in the upper part of the Canyon in the CFD calculations. Possible reasons for these discrepancies are given.
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pseudo simultaneous measurements for the vertical variation of coarse fine and ultrafine particles in an urban Street Canyon
Atmospheric Environment, 2008Co-Authors: Prashant Kumar, Paul S Fennell, David Langley, R E BritterAbstract:The vertical variation of particle number distributions (PNDs) and concentrations in a Street Canyon is the result of the competing influences of meteorology, traffic and transformation processes overall and for various particle size ranges. A recently developed instrument, the ‘fast-response differential mobility spectrometer DMS500’, measured PNDs in the 5–2738nm range, pseudo-simultaneously, at four different heights (z/H ¼ 0.09, 0.19, 0.40 and 0.64) on the leeward side of an 11.6-m-deep Street Canyon which had a height-to-width ratio of near unity. Measurements were made in Cambridge, UK, between 20 and 21 March 2007. The PNDs were bimodal with the same shape at each height, and with similar values of both the peak and geometric mean particle diameters in each mode. This suggested that transformation processes were not important. Coagulation and condensation time scales were comparable and large, and these processes should have had a negligible effect on the PNDs. The particle number concentrations (PNCs) changed significantly with height from a maximum at z/H ¼ 0.19 and decreasing towards both the lowest (z/H ¼ 0.09) and highest (z/H ¼ 0.64) sampling points. The decrease in PNCs with height in the upper part of the Canyon was attributed to the removal of particles as a result of mass exchange between Street Canyon and the wind above while the reduction in the PNC towards Street level was thought to be due to dilution and dry deposition. Over 99% of the PNCs were found in 10–300nm range whereas the particle mass concentrations were almost equally distributed between the 10–1000nm and 1000–2738nm size range at each height. The PNCs in the 10–30nm and the 30–300nm size range were linearly correlated with the traffic volume but poorly correlated with the rooftop wind speed.
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measurements of particles in the 5 1000 nm range close to road level in an urban Street Canyon
Science of The Total Environment, 2008Co-Authors: Prashant Kumar, Paul S Fennell, R E BritterAbstract:A newly developed instrument, the 'fast response differential mobility spectrometer (DMS500)', was deployed to measure the particles in the 5-1000 nm range in a Cambridge (UK) Street Canyon. Measurements were taken for 7 weekdays (from 09:00 to 19:00 h) between 8 and 21 June 2006 at three heights close to the road level (i.e. 0.20 m, 1.0 m and 2.60 m). The main aims of the measurements were to investigate the dependence of particle number distributions (PNDs) and concentrations (PNCs) and their vertical variations on wind speed, wind direction, traffic volume, and to estimate the particle number flux (PNF) and the particle number emission factors (PNEF) for typical urban Streets and driving conditions. Traffic was the main source of particles at the measurement site. Measured PNCs were inversely proportional to the reference wind speed and directly proportional to the traffic volume. During the periods of cross-Canyon flow the PNCs were larger on the leeward side than the windward side of the Street Canyon showing a possible effect of the vortex circulation. The largest PNCs were unsurprisingly near to road level and the pollution sources. The PNCs measured at 0.20 m and 1.0 m were the same to within 0.5-12.5% indicating a well-mixed region and this was presumably due to the enhanced mixing from traffic produced turbulence. The PNCs at 2.60 m were lower by 10-40% than those at 0.20 m and 1.0 m, suggesting a possible concentration gradient in the upper part of the Canyon. The PNFs were estimated using an idealised and an operational approach; they were directly proportional to the traffic volume confirming the traffic to be the main source of particles. The PNEF were estimated using an inverse modelling technique; the reported values were within a factor of 3 of those published in similar studies.
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transfer processes in a simulated urban Street Canyon
Boundary-Layer Meteorology, 2007Co-Authors: Efisio Solazzo, R E BritterAbstract:The transfer processes within and above a simulated urban Street Canyon were investigated in a generic manner. Computational fluid dynamics (CFD) was used to aid understanding and to produce some simple operational parameterisations. In this study we addressed specifically the commonly met situation where buoyancy effects arising from elevated surface temperatures are not important, i.e. when mechanical forces outweigh buoyancy forces. In a geophysical context this requires that some suitably defined Richardson number is small. From an engineering perspective this is interpreted as the important case when heat transfer within and above urban Street Canyons is by forced convection. Surprisingly, this particular scenario (for which the heat transfer coefficient between buildings and the flow is largest), has been less well studied than the situation where buoyancy effects are important. The CFD technique was compared against wind-tunnel experiments to provide model evaluation. The height-to-width ratio of the Canyon was varied through the range 0.5–5 and the flow was normal to the Canyon axis. By setting the Canyon’s facets to have the same or different temperatures or to have a partial temperature distribution, simulations were carried out to investigate: (a) the influence of geometry on the flow and mixing within the Canyon and (b) the exchange processes within the Canyon and across the Canyon top interface. Results showed that the vortex-type circulation and turbulence developed within the Canyon produced a temperature distribution that was, essentially, spatially uniform (apart from a relatively thin near-wall thermal boundary layer) This allowed the temperatures within the Street Canyon to be specified by just one value Tcan, the Canyon temperature. The variation of Tcan with wind speed, surface temperatures and geometry was extensively studied. Finally, the exchange velocity uE across the interface between the Canyon and the flow above was calculated based on a heat flux balance within the Canyon and between the Canyon and the flow above. Results showed that uE was approximately 1% of a characteristic wind velocity above the Street Canyon. The problem of radiative exchange is not addressed but it can, of course, be introduced analytically, or computationally, when necessary.
Pietro Salizzoni - One of the best experts on this subject based on the ideXlab platform.
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Street Canyon ventilation and atmospheric turbulence
Atmospheric Environment, 2009Co-Authors: Pietro Salizzoni, Lionel Soulhac, P MejeanAbstract:Operational models for pollutant dispersion in urban areas require an estimate of the turbulent transfer between the Street Canyons and the overlying atmospheric flow. To date, the mechanisms that govern this process remain poorly understood. We have studied the mass exchange between a Street Canyon and the atmospheric flow above it by means of wind tunnel experiments. Fluid velocities were measured with a Particle Image Velocimetry system and passive scalar concentrations were measured using a Flame Ionisation Detector. The mass-transfer velocity between the Canyon and the external flow has been estimated by measuring the cavity wash-out time. A two-box model, used to estimate the transfer velocity for varying dynamical conditions of the external flow, has been used to interpret the experimental data. This study sheds new light on the mechanisms which drive the ventilation of a Street Canyon and illustrates the influence of the external turbulence on the transfer process.
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flow in a Street Canyon for any external wind direction
Boundary-Layer Meteorology, 2008Co-Authors: Lionel Soulhac, Pietro Salizzoni, R J PerkinsAbstract:An analytical model has been developed for the flow along a Street Canyon (of height H and width W), generated by an external wind blowing at any angle relative to the axis of the Street. Initially, we consider the special case of a wind blowing parallel to the Street. The interior of the Street is decomposed into three regions, and the flow within each region is assumed to depend only on the external wind and the distance to the closest solid boundary. This decomposition leads to two different flow regimes: one for narrow Streets (H/W > 1/2) and one for wide Streets (H/W < 1/2). The theoretical model agrees well with results obtained from numerical simulations using a Reynolds-Averaged Navier–Stokes model. We then generalize the model to the case of arbitrary wind direction. Numerical solutions show that the streamlines of the mean flow in the Street have a spiral form, and for most angles of incidence, the mass flux along the Street scales on the component of the external wind resolved parallel to the Street. We use this result to generalize the model derived for wind blowing parallel to the Street, and the results from this model agree well with the numerical simulations. The model that has been developed can be evaluated rapidly using only very modest computing power, so it is suitable for use as an operational tool.
