Flow In Street Canyon Research Articles

Modeling green wall interactions with street canyons for building energy simulation in urban context

A hygrothermal model of green walls and a model of mass flows in street canyons have been proposed and implemented in a building simulation program (TRNSYS). The coupled models allow the study of the hygrothermal interaction of green walls at the interface of the detailed building model and the urban microclimate of the street. Its use highlights the effects on both urban microclimate and buildings energy loads, especially in the summer period. While reducing anthropogenic heat release, green walls set up on west or east façades mitigate the street air temperature and reduce building cooling loads depending on streets’ aspect ratio. Some of the canyon model parameters were calibrated through numerical comparison with experimental data on a reduced scale mockup. This mockup has been designed specifically to assess the green walls hygrothermal impact and to evaluate the developed numerical tools. This experimental calibration made simulating the green walls interactions with street canyons of different aspect ratios possible. The experimental and numerical results obtained with green façades underline the advantage of this modeling approach for the design of passive cooling for buildings and mitigation of excessive thermal conditions within street canyons in dense cities in warm climates.

Improvement of a stochastic backscatter model and application to large‐eddy simulation of street canyon flow

A stochastic backscatter (SB) subgrid‐scale (SGS) model is applied, for the first time, to large‐eddy simulation (LES) of street canyon flow. We model a ‘skimming flow’ regime under a neutrally stratified atmosphere, in which the approaching wind is perpendicular to the along‐street axis of a street canyon of unity aspect ratio. Previous LESs of this type have shown an under‐prediction of the intensity of the primary eddy (PE) that forms within the street canyon, indicating a lack of momentum transfer across the roof‐level shear layer. The SB model, however, acts to increase this momentum transfer, bringing the simulated PE intensity significantly closer towards that observed in a corresponding wind‐tunnel experiment. A metric for the PE intensity, ωPE, based on the two‐dimensional vorticity field, is increased from around 70% of the wind‐tunnel ωPE value (with the Smagorinsky SGS model) to as much as 90% (with the SB model). Calculation of the air exchange rate at roof‐level confirms that the rate of entrainment into the street canyon is increased with the inclusion of backscatter.We also outline an improvement to the SB model prior to its application. In its previous version, a constraint on the magnitude of the backscatter acceleration variances ensured a theoretically appropriate level of additional grid‐scale (backscattered) energy. Here, a further constraint on the magnitude of the main covariance term also facilitates a better representation of grid‐scale vertical momentum flux. This new constraint alone can help to increase the simulated ωPE value by as much as 10% of the wind‐tunnel ωPE value, and requires almost no additional computational effort. The effect of varying the magnitude and length‐scale of the imposed backscatter (via the backscatter coefficient and length of the filter used to generate the backscatter acceleration fields, respectively) is also investigated.

CFD simulations of flow and dust dispersion in a realistic urban area

ABSTRACTFluid flow and dust transportation in a realistic urban residential community under dust storm weather conditions are investigated using computational fluid dynamics (CFD) with a grid resolution of several meters. The dust transportation and concentration distribution are obtained through the Lagrangian-formulated discrete particle model by integrating the particle velocity between certain time intervals. The fluid flow is solved by the realizable model. It is found that the dust transportation and distribution are very closely related to the flow field. The flow field in a real residential community is very complicated. When the building axes are perpendicular to the wind direction, the flows resemble the classic street canyon flow. Places with a low wind speed and high vorticity usually have a high dust concentration. As the wind direction changes, the fluid flow and dust distribution differ from case to case, but the general features are kept. In addition, the building shape and particle-wall interaction conditions have additional effects on the dust distribution, which need further study in the future.

Creating drafts in urban settings through coloured façades: Exploring a new climate adaptation measure based on thermal stratification

Climate change will lead to more warm and hot days in the Netherlands. Climate adaptation measures are needed to reduce the vulnerability of urban areas. Moreover, proven adaptation measures, such as increased vegetated areas or water surfaces, are not always possible due to a lack of space or undesired aesthetic effects. An alternative option is to make use of coloured façades to create drafts and accelerate wind speed in a street canyon or on a square to attract fresh air from cooler places. Differences in colour and material types already influence the air flow in street canyons in an uncontrolled manner. If this thermal comfort principle can be implemented, it will potentially have a significant impact on many cities throughout the world. This paper presents the results of a first exploratory research based on measurements on scale models and at full scale. This pilot study indicates success of the intended draft principle, advocating further research.

A CFD study on the effectiveness of trees to disperse road traffic emissions at a city scale

This paper focuses on the effectiveness of trees at dispersing road traffic emissions on a city scale. CFD simulations of air-pollutant concentrations were performed using the OpenFOAM software platform using the k-ε model. Results were validated against the CODASC wind tunnel database before being applied to a LIDAR database of buildings and trees representing the City of Leicester (UK). Most other CFD models in the literature typically use idealised buildings to model wind flow and pollution dispersion. However, the methodology used in this study uses real buildings and trees data from LIDAR to reconstruct a 3D representation of Leicester City Centre. It focuses on a 2×2 km area which is on a scale larger than those usually used in other CFD studies. Furthermore, the primary focus of this study is on the interaction of trees with wind flow dynamics. It was found that in effect, trees have a regionally beneficial impact on road traffic emissions by increasing turbulence and reducing ambient concentrations of road traffic emissions by 7% at pedestrian height on average. This was an important result given that previous studies generally concluded that trees trapped pollution by obstructing wind flow in street canyons. Therefore, this study is novel both in its methodology and subsequent results, highlighting the importance of combining local and regional scale models for assessing the impact of trees in urban planning.

Footprint Evaluation for Flux and Concentration Measurements for an Urban-Like Canopy with Coupled Lagrangian Stochastic and Large-Eddy Simulation Models

A footprint algorithm, based on a Lagrangian stochastic (LS) model embedded into a parallelized large-eddy simulation (LES) model, is used for the evaluation of flux and concentration footprints of passive scalars in flow in and above an urban-like canopy layer of a neutrally stratified $$440 \hbox { m}$$ deep boundary layer. The urban-like canopy layer is realized by an aligned array of cuboids whose height H is $$40\hbox { m}$$ . The canopy flow involves strong small-scale inhomogeneities although it is homogeneous at the large scale. The source height is $$1\hbox { m}$$ (0.025H) above the ground in the street canyons, roughly mimicking traffic emissions. Footprints are evaluated for four heights from 0.25H to 2.5H, and for up to eight different horizontal sensor positions per measurement height, comprising sensor positions inside as well as outside of the street canyon that extend perpendicular to the mean wind direction. The LES-LS footprints are compared with footprints estimated by a conventional model (Kormann and Meixner, in Boundary-Layer Meteorol 99:207–224, 2001). It becomes evident that the local heterogeneity of the flow has a considerable impact on flux and concentration footprints. As expected, footprints for measurements within and right above the canopy layer show complex and completely different footprint shapes compared to the ellipsoidal shape obtained from conventional footprint models that assume horizontal homogeneity of the turbulent flow as well as the sources of passive scalars. Our results show the importance of street-canyon flow and turbulence for the vertical mixing of scalar concentration.

Predictability of Turbulent Flow in Street Canyons

Although predictability is a subject of great importance in atmospheric modelling, there has been little research on urban boundary-layer flows. Here the predictability of street-canyon flow is examined numerically via large-eddy simulation of a unit-aspect-ratio canyon and neutrally stratified atmosphere. In spectral space there is indication of cascade-like behaviour away from the canyon at early times, but the error growth is essentially independent of scale inside the canyon; in physical space the error field is rather inhomogeneous and shows clear differences among the canyon, shear layer and inertial sublayer. The error growth is largely driven by the shear layer: errors generated above roof level are advected into the canyon while contributions from intermittent bursting and in situ development within the canyon play a relatively minor role. This work highlights differences between the predictability of urban flows and canonical turbulent flows and should be useful in developing modelling strategies for more realistic time-dependent urban flows.

Characteristics of flow and reactive pollutant dispersion in urban street canyons

In this study, the effects of aspect ratio defined as the ratio of building height to street width on the dispersion of reactive pollutants in street canyons were investigated using a coupled CFD-chemistry model. Flow characteristics for different aspect ratios were analyzed first. For each aspect ratio, six emission scenarios with different VOC–NOX ratios were considered. One vortex was generated when the aspect ratio was less than 1.6 (shallow street canyon). When the aspect ratio was greater than 1.6 (deep street canyon), two vortices were formed in the street canyons. Comparing to previous studies on two-dimensional street canyons, the vortex center is slanted toward the upwind building and reverse and downward flows are dominant in street canyons. Near the street bottom, there is a marked difference in flow pattern between in shallow and deep street canyons. Near the street bottom, reverse and downward flows are dominant in shallow street canyon and flow convergence exists near the center of the deep street canyons, which induces a large difference in the NOX and O3 dispersion patterns in the street canyons. NOX concentrations are high near the street bottom and decreases with height. The O3 concentrations are low at high NO concentrations near the street bottom because of NO titration. At a low VOC–NOX ratio, the NO concentrations are sufficiently high to destroy large amount of O3 by titration, resulting in an O3 concentration in the street canyon much lower than the background concentration. At high VOC–NOX ratios, a small amount of O3 is destroyed by NO titration in the lower layer of the street canyons. However, in the upper layer, O3 is formed through the photolysis of NO2 by VOC degradation reactions. As the aspect ratio increases, NOX (O3) concentrations averaged over the street canyons decrease (increase) in the shallow street canyons. This is because outward flow becomes strong and NOX flux toward the outsides of the street canyons increases, resulting in less NO titration. In the deep street canyons, outward flow becomes weak and outward NOX flux decreases, resulting in an increase (decrease) in NOX (O3) concentration.

Field and wind tunnel modeling of an idealized street canyon flow

The present work examines the flow field in a simple street canyon that has been modeled at full-scale and at 1:200 scale in a wind tunnel. It relies on the detailed analysis of statistics of both flows including two-point correlation coefficients, an approach not commonly done for canyon flows. Comparison between the field and wind tunnel study has demonstrated good agreement for the mean velocity and turbulence statistics, which are typically within 20%. However, significant differences in the along-canyon mean and turbulent components have been observed and are shown to be a result of the changing of the ambient wind direction and low frequency motion present in the field. As the wind direction changes over time the result is a channeling of flow along the canyon axis. This phenomenon cannot be accurately reproduced by the wind tunnel model, which produces nominally 2D flow. The turbulence dynamics were investigated through two-point spatial correlation of the streamwise, spanwise and vertical components, which show agreement to within 15–30% between the field and wind tunnel results. From estimation of boundary layer log-law parameters it has been shown that using a single point reference velocity measurement at 10 m height to estimate the boundary layer log-law parameters is unreliable in the present case.

A numerical study of diurnally varying surface temperature on flow patterns and pollutant dispersion in street canyons

The impacts of the diurnal variation of surface temperature on street canyon flow pattern and pollutant dispersion are investigated based on a two-dimensional street canyon model under different thermal stratifications. Uneven distributed street temperature conditions and a user-defined wall function representing the heat transfer between the air and the street canyon are integrated into the current numerical model. The prediction accuracy of this model is successfully validated against a published wind tunnel experiment. Then, a series of numerical simulations representing four time scenarios (Morning, Afternoon, Noon and Night) are performed at different Bulk Richardson number (Rb). The results demonstrate that uneven distributed street temperature conditions significantly alters street canyon flow structure and pollutant dispersion characteristics compared with conventional uniform street temperature assumption, especially for the morning event. Moreover, air flow patterns and pollutant dispersion are greatly influenced by diurnal variation of surface temperature under unstable stratification conditions. Furthermore, the residual pollutant in near-ground-zone decreases as Rb increases in noon, afternoon and night events under all studied stability conditions.

Numerical Validation for Wind Flow Field in a Street Canyon under Unstable Atmospheric Condition

This paper reports on the validation study of a commercial computational fluid dynamics program, ANSYS FLUENT v14. The purpose of the study was to determine the best turbulent model that can reproduce appropriately the wind flow field of turbulent flow in street canyon under different thermal atmospheric condition. Standard k-ε (SKE) and Large Eddy Simulation (LES) techniques were chosen as the turbulent model for the validation study. It was found that LES could well calculate a complex simulation involving isothermal flow and thermal flow with different thermal intensity and different heat location in street canyon.

Model Sensitivity Test of Large Eddy Simulation for Wind Flow and Pollutant Dispersion in a Street Canyon

This paper reports on the model sensitivity analysis of a commercial computational fluid dynamics program, ANSYS FLUENT v14. The purpose of the analysis was to determine the appropriate modeling settings for numerical model of the case study. A full scale of a simplified urban street canyon was modelled and the turbulent flow was calculated using Large Eddy Simulation (LES) techniques. The model sensitivity tests involved are mesh sensitivity, statistically steady state and sampling. Adequate numbers of cells, period time to achieve statistically steady state (SST) and sampling time to simulate wind flow and pollutant dispersion in street canyon were determined through systematic tests.

Effect of upstream flow regime on street canyon flow mean turbulence statistics

The effect on the flow over a street canyon (lateral length/height, L/h $$=$$ 30) of using either 3D (cube) or 2D (rectangular block) upstream roughness arrays, of the same height as the canyon, has been studied for two streamwise canyon width to height aspect ratios (AR $$=$$ W/h) of 1 and 3, in a wind tunnel using Particle Image Velocimetry. The mean streamwise velocity, shear stress, turbulent intensities and length scales, together with shear layer boundaries and mass fluxes across the canyon opening are presented for different combinations of skimming and wake-interference regimes using different upstream roughness and canyon configurations. These results show significant trends with canyon aspect ratio and roughness array plan area packing density $$(\uplambda _{\mathrm{p}})$$ with respect to 2D and 3D configurations. The mean streamwise velocity for configurations of equal $$\uplambda _{\mathrm{p}}$$ is higher in 3D than 2D configurations, while the spatially averaged shear stress is shown to be lower in 3D than 2D configurations. The relative contribution to the total turbulent kinetic energy (TKE) demonstrates that staggered and aligned arrays or 2D and 3D arrays do not produce similar profiles of TKE. Finally, the integral length scale is larger in 2D cases than 3D cases of equal $$\uplambda _{\mathrm{p}}$$ . Urban air quality is a significant concern for human health. By investigating the influence of upstream roughness on canyon flow one can determine which cases or flow regimes in both the upstream roughness and canyon will result in decreased ventilation and negatively effect the air quality of urban areas. From the present work decreased ventilation occurs in the skimming flow regime and is lowest in the case of upstream 2D bar roughness with $$\uplambda _{\mathrm{p}} = 50~\%$$ and canyon AR $$=$$ 1.

Impacts of shape and height of upstream roof on airflow and pollutant dispersion inside an urban street canyon.

A two-dimensional numerical model for simulating flow and pollutant dispersion in an urban street canyon is firstly developed using the FLUENT code and then validated against the wind tunnel results. After this, the flow field and pollutant dispersion inside an urban street canyon with aspect ratio W/H = 1 are examined numerically considering five different shapes (vaulted, trapezoidal, slanted, upward wedged, and downward wedged roofs) as well as three different roof height to building height ratios (Z H /H = 1/6, 1/3, and 1/2) for the upstream building roof. The results obtained reveal that the shape and height of an upstream roof have significant influences on flow pattern and pollutant distribution in an urban canyon. A large single clockwise vortex is generated in the canyon for the vaulted upstream roof at Z H /H = 1/6, 1/3, and 1/2, the trapezoidal and downward wedged roofs at Z H /H = 1/6 and 1/3, and the slanted and upward wedged roofs at Z H /H = 1/6, while a main clockwise vortex and a secondary counterclockwise vortex are established for the trapezoidal and downward wedged roofs at Z H /H = 1/2 and the slanted and upward wedged roofs at Z H /H = 1/3 and 1/2. In the one-vortex flow regime, the clockwise vortex moves upward and grows in size with increasing upstream roof height for the vaulted, trapezoidal, and downward wedged roofs. In the two-vortex flow regime, the size and rotational velocity of both upper clockwise and lower counterclockwise vortices increase with the upstream roof height for the slanted and upward wedged roofs. At Z H /H = 1/6, the pollution levels in the canyon are close among all the upstream roof shapes studied. At Z H /H = 1/3, the pollution levels in the canyon for the upward wedged roof and slanted roof are much higher than those for the vaulted, trapezoidal, and downward wedged roofs. At Z H /H = 1/2, the lowest pollution level appears in the canyon for the vaulted upstream roof, while the highest pollution level occurs in the canyon for the upward wedged roof.

Investigation of Re -independence of turbulent flow and pollutant dispersion in urban street canyon using numerical wind tunnel (NWT) models

Abstract This paper numerically and experimentally studied the Reynolds number independence (Re-independence) of turbulent flow and pollutant dispersion in urban areas. The concept of numerical wind tunnel (NWT) is proposed and validated by prototype wind-tunnel experimental measurements. A new physical quantity: the ratio of relative change, RRC, is proposed to investigate the Re-independence quantitatively. For the given street canyon, numerically predicted variations of RRC vs. building Reynolds number (ReH) from three k–ɛ turbulence models agree well with each other, and the variation trend shows that there exist two flow regimes in the range of ReH studied: flow in low ReH region is strongly affected by ReH, while that in higher ReH region is weakly affected by ReH, giving a strong support to the concept of Re-independence of turbulent flow. A criterion of RRC less than 5% is suggested to determine the value of the critical Reynolds number. For the street canyon studied, such determined critical building Reynolds number (ReH,crit) is 3.3 × 104. Examinations of dimensionless velocity contours, local velocity vectors, and concentration contours demonstrate the feasibility of the suggested critical Reynolds number.

An Improved Three-Dimensional Simulation of the Diurnally Varying Street-Canyon Flow

The impact of diurnal variations of the heat fluxes from building and ground surfaces on the fluid flow and air temperature distribution in street canyons is numerically investigated using the PArallelized Large-eddy Simulation Model (PALM). Simulations are performed for a 3 by 5 array of buildings with canyon aspect ratio of one for two clear summer days that differ in atmospheric instability. A detailed building energy model with a three-dimensional raster-type geometry—Temperature of Urban Facets Indoor-Outdoor Building Energy Simulator (TUF-IOBES)—provides urban surface heat fluxes as thermal boundary conditions for PALM. In vertical cross-sections at the centre of the spanwise canyon the mechanical forcing and the horizontal streamwise thermal forcing at roof level outweigh the thermal forces from the heated surfaces inside the canyon in defining the general flow pattern throughout the day. This results in a dominant canyon vortex with a persistent speed, centered at a constant height. Compared to neutral simulations, non-uniform heating of the urban canyon surfaces significantly modifies the pressure field and turbulence statistics in street canyons. Strong horizontal pressure gradients were detected in streamwise and spanwise canyons throughout the day, and which motivate larger turbulent velocity fluctuations in the horizontal directions rather than in the vertical direction. Canyon-averaged turbulent kinetic energy in all non-neutral simulations exhibits a diurnal cycle following the insolation on the ground in both spanwise and streamwise canyons, and it is larger when the canopy bottom surface is paved with darker materials and the ground surface temperature is higher as a result. Compared to uniformly distributed thermal forcing on urban surfaces, the present analysis shows that realistic non-uniform thermal forcing can result in complex local airflow patterns, as evident, for example, from the location of the vortices in horizontal planes in the spanwise canyon. This study shows the importance of three-dimensional simulations with detailed thermal boundary conditions to explore the heat and mass transport in an urban area.

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.

Bayesian calibration of the constants of the [formula omitted]–[formula omitted] turbulence model for a CFD model of street canyon flow

In this paper we carry out a Bayesian calibration for uncertainty analysis in Computational Fluid Dynamics modelling of urban flows. Taking the case of airflow in a regular street canyon, and choosing turbulent kinetic energy (TKE) as our quantity of interest, we calibrate 3-D CFD simulations against wind tunnel observations. We focus our calibration on the model constants contained within the standard RANS k–ε turbulence model and the uncertainties relating to these values. Thus we are able to narrow down the space of k–ε model constants which provide the best match with experimental data and quantify the uncertainty relating to both the k–ε model constants in the case of street canyon flow and the TKE outputs of the CFD simulation. Furthermore, we are able to construct a statistical emulator of the CFD model. Finally, we provide predictions of TKE based on the emulator and the estimated bias between model and observations, accompanied with uncertainties in these predictions.

Flow and turbulence characteristics in a suburban street canyon

Flow and turbulence data collected during a yearlong experiment in a street-canyon configuration located in suburban terrain are analyzed. The instrumentation included 13 sonic anemometers deployed on two masts within the street canyon and on three masts on the building roofs. Flow patterns were classified as being in the wake-interference regime. The in-canyon flow and turbulence structure showed a strong dependence on the above-roof wind direction. While channeling along the street dominates for most wind directions, recirculation patterns develop for narrow sectors with above-roof wind directions perpendicular to the street. For these cross-flow scenarios, different scaling velocities were tested and the influence of upwind fetch and stability was investigated in more detail. Similar to previous studies, our findings confirmed that it is difficult to identify a single velocity scale that unifies both mean flow and turbulence properties inside the canyon. Turbulence properties scaled best with the friction velocity at the upwind roof but scaling with mean wind speeds measured at the upwind roof or at an operational meteorological station 5-km away from the study area, resulted in comparable or even better statistics for the mean flow parameters. Turbulence kinetic energy in the shear-layer region at roof layer varied depending on upwind fetch and stability. As turbulence is transported from the shear layer into the canyon region, the in-canyon turbulence characteristics also varied as a function of these two parameters.

On thermally forced flows in urban street canyons

During sunny days with periods of low synoptic wind, buoyancy forces can play a critical role on the air flow, and thus on the dispersion of pollutants in the built urban environments. Earlier studies provide evidence that when a surface inside an urban street canyon is at a higher temperature than that of local ambient air, buoyancy forces can modify the mechanically-induced circulation within the canyons (i.e., gaps between buildings). The aspect ratio of the urban canyon is a critical factor in the manifestation of the buoyancy parameter. In this paper, computational fluid dynamics simulations are performed on urban street canyons with six different aspect ratios, focusing on the special case where the leeward wall is at a greater temperature than local ambient air. A non-dimensional measure of the influence of buoyancy is used to predict demarcations between the flow regimes. Simulations are performed under a range of buoyancy conditions, including beyond those of previous studies. Observations from a field experiment and a wind tunnel experiment are used to validate the results.