Testing simple models for street wind conditions in Barcelona

Street wind speed and direction drive models to estimate air quality levels at street scale. In this study, simple models are combined with a mesoscale meteorological model to provide wind conditions at street level. Then, wind speed and direction are evaluated using observations collected during an experimental campaign in April 2013 at street level in Barcelona, Spain. Overall, models considering street geometry give better estimates for both wind speed and direction than those assuming homogeneous terrain. For light winds, models tend to produce a large amount of error estimating wind direction

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

Three scenarios of large-eddy simulation (LES) were performed to examine the characteristic flow and pollutant dispersion in urban street canyons under neutral, unstable and stable thermal stratifications. Street canyons of unity aspect ratio with ground-heating or –cooling are considered. In the LESs of the thermal stabilities tested, a large primary recirculation is developed in the center core and the turbulence production is dominated at the roof level of the street canyon. The current LES results demonstrate that unstable stratification enhances the mean wind, turbulence and pollutant removal of street canyons. On the other hand, in stable stratification, which has been less investigated in the past, the ground-level mean wind and turbulence are substantially suppressed by the large temperature inversion. Whereas, the weakened recirculating wind in the street canyon results in a larger velocity gradient that increases the turbulence production at the roof level. It also slows down the turbulence being carried from the roof down to the lower street canyon. Therefore, a higher level of turbulent kinetic energy (TKE) is retained at the mid-level of the windward side in the stably stratified street canyon.postprintThe 5th International Symposium on Computational Wind Engineering (CWE2010), Chapel Hill, N.C., 23-27 May 2010

Pollutant removal, dispersion, and entrainment over two-dimensional idealized street canyons

Idealized two-dimensional (2D) street canyon models of unity building-height-to-street-width (aspect) ratio are employed to examine the pollutant transport over hypothetical urban areas. The results show that the pollutant removal is mainly governed by atmospheric turbulence when pollutant sources exist in the street canyons. Numerous decelerating, uprising air masses are located at the roof level, implying that the pollutant is removed from the street canyons to the urban boundary layer (UBL) by ejections. For the street canyons without pollutant source, the removal by ejections is limited leading to insignificant turbulent pollutant removal. The roof-level turbulent kinetic energy (TKE) distribution demonstrates that its production is not governed by local wind shear but the descending TKE from the UBL. In the UBL, the pollutant disperses rapidly over the buildings, exhibiting a Gaussian-plume shape. The vertical pollutant profiles illustrate a self-similarity behavior in the downstream region. Future studies will be focused on the characteristic plume shape over 2D idealized street canyons of different aspect ratios.postprintThe 13th International Conference on Wind Engineering (ICWE13), Amsterdam, The Netherlands, 10-15 July 2011

Pedestrian-Level Urban Wind Flow Enhancement with Wind Catchers

Dense urban areas restrict air movement, causing airflow in urban street canyons to be much lower than the flow above buildings. Boosting near-ground wind speed can enhance thermal comfort in warm climates by increasing skin convective heat transfer. We explored the potential of a wind catcher to direct atmospheric wind into urban street canyons. We arranged scaled-down models of buildings with a wind catcher prototype in a water channel to simulate flow across two-dimensional urban street canyons. Velocity profiles were measured with Acoustic Doppler Velocimeters. Experiments showed that a wind catcher enhances pedestrian-level wind speed in the target canyon by 2.5 times. The flow enhancement is local to the target canyon with little effect in other canyons. With reversed flow direction, a “reversed wind catcher” has no effect in the target canyon but reduces the flow in the immediate downstream canyon. The reversed wind catcher exhibits a similar blockage effect of a tall building amid an array of lower buildings. Next, we validated Computational Fluid Dynamics (CFD) simulations of all cases with experiments and extended the study to reveal impacts on three-dimensional ensembles of buildings. A wind catcher with closed sidewalls enhances maximum pedestrian-level wind speed in three-dimensional canyons by four times. Our results encourage better designs of wind catchers to increase wind speed in targeted areas

Large-Eddy Simulation of Flow and Pollutant Transport in Urban Street Canyons with Ground Heating

Our study employed large-eddy simulation (LES) based on a one-equation subgrid-scale model to investigate the flow field and pollutant dispersion characteristics inside urban street canyons. Unstable thermal stratification was produced by heating the ground of the street canyon. Using the Boussinesq approximation, thermal buoyancy forces were taken into account in both the Navier–Stokes equations and the transport equation for subgrid-scale turbulent kinetic energy (TKE). The LESs were validated against experimental data obtained in wind-tunnel studies before the model was applied to study the detailed turbulence, temperature, and pollutant dispersion characteristics in the street canyon of aspect ratio 1. The effects of different Richardson numbers (Ri) were investigated. The ground heating significantly enhanced mean flow, turbulence, and pollutant flux inside the street canyon, but weakened the shear at the roof level. The mean flow was observed to be no longer isolated from the free stream and fresh air could be entrained into the street canyon at the roof-level leeward corner. Weighed against higher temperature, the ground heating facilitated pollutant removal from the street canyon.Singapore-MIT Alliance for Research and Technology. Center for Environmental Sensing and Monitorin

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. The flow 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 deviations 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*, it is found that σu/u* ≈ 1.8, σv/u* ≈ 1.3 and σw/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 the momentum transport over the street canyons. In the x direction, the two-point correlations 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 ΩT of pollutant into its removal and re-entry components, an increasing pollutant re-entrainment from 26.3 to 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons. © 2011 The Author(s).published_or_final_versionSpringer Open Choice, 21 Feb 201

Observations and numerical simulations of the train-induced air flow in a subway station

This article presents observations and model simulations of wind and temperature in a subway station. The measurements are taken from an experiment with three ultrasonic anemometers at different levels in the underground facility. The available observations indicate a wind regime with a continuous background flow and train-induced ventilation with very effective air exchange between the platform level and the street level. Model simulations with a resolution of less than 1m were performed with running trains in accordance with a regular timetable. The results demonstrate the applicability of the model to the complex underground geometry. Calculated time series of wind and temperature are very comparable to the observations. The findings of a parameter study indicate the necessity to record all details of train movements in order to define appropriate initial and boundary conditions for the model and to explain the observations correctly.DF

Processes controlling atmospheric dispersion through city centres

We develop a process-based model for the dispersion of a passive scalar in the turbulent flow around the buildings of a city centre. The street network model is based on dividing the airspace of the streets and intersections into boxes, within which the turbulence renders the air well mixed. Mean flow advection through the network of street and intersection boxes then mediates further lateral dispersion. At the same time turbulent mixing in the vertical detrains scalar from the streets and intersections into the turbulent boundary layer above the buildings. When the geometry is regular, the street network model has an analytical solution that describes the variation in concentration in a near-field downwind of a single source, where the majority of scalar lies below roof level. The power of the analytical solution is that it demonstrates how the concentration is determined by only three parameters. The plume direction parameter describes the branching of scalar at the street intersections and hence determines the direction of the plume centreline, which may be very different from the above-roof wind direction. The transmission parameter determines the distance travelled before the majority of scalar is detrained into the atmospheric boundary layer above roof level and conventional atmospheric turbulence takes over as the dominant mixing process. Finally, a normalised source strength multiplies this pattern of concentration. This analytical solution converges to a Gaussian plume after a large number of intersections have been traversed, providing theoretical justification for previous studies that have developed empirical fits to Gaussian plume models. The analytical solution is shown to compare well with very high-resolution simulations and with wind tunnel experiments, although re-entrainment of scalar previously detrained into the boundary layer above roofs, which is not accounted for in the analytical solution, is shown to become an important process further downwind from the source

In-street wind direction variability in the vicinity of a busy intersection in central London

We present results from fast-response wind measurements within and above a busy intersection between two street canyons (Marylebone Road and Gloucester Place) in Westminster, London taken as part of the DAPPLE (Dispersion of Air Pollution and Penetration into the Local Environment; www.dapple.org.uk) 2007 field campaign. The data reported here were collected using ultrasonic anemometers on the roof-top of a building adjacent to the intersection and at two heights on a pair of lamp-posts on opposite sides of the intersection. Site characteristics, data analysis and the variation of intersection flow with the above-roof wind direction (θref) are discussed. Evidence of both flow channelling and recirculation was identified within the canyon, only a few metres from the intersection for along-street and across-street roof-top winds respectively. Results also indicate that for oblique rooftop flows, the intersection flow is a complex combination of bifurcated channelled flows, recirculation and corner vortices. Asymmetries in local building geometry around the intersection and small changes in the background wind direction (changes in 15-min mean θref of 5–10 degrees) were also observed to have profound influences on the behaviour of intersection flow patterns. Consequently, short time-scale variability in the background flow direction can lead to highly scattered in-street mean flow angles masking the true multi-modal features of the flow and thus further complicating modelling challenges

Large-eddy simulation for flow and dispersion in urban streets

Large-eddy simulations (LES) with our recently developed inflow approach (Xie &Castro, 2008a) have been used for flow and dispersion within a genuine city area -the DAPPLE site, located at the intersection of Marylebone Rd and Gloucester Plin Central London. Numerical results up to second-order statistics are reported fora computational domain of 1.2km (streamwise) x 0.8km (lateral) x 0.2km (in fullscale), with a resolution down to approximately one meter in space and one secondin time. They are in reasonable agreement with the experimental data. Such a comprehensiveurban geometry is often, as here, composed of staggered, aligned, squarearrays of blocks with non-uniform height and non-uniform base, street canyons andintersections. Both the integrative and local effect of flow and dispersion to thesegeometrical patterns were investigated. For example, it was found that the peaksof spatially averaged urms, vrms, wrms and < u0w0 > occurred neither at the meanheight nor at the maximum height, but at the height of large and tall buildings. Itwas also found that the mean and fluctuating concentrations in the near-source fieldis highly dependent on the source location and the local geometry pattern, whereasin the far field (e.g. >0.1km) they are not. In summary, it is demonstrated thatfull-scale resolution of around one meter is sufficient to yield accurate prediction ofthe flow and mean dispersion characteristics and to provide reasonable estimationof concentration fluctuation