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

Influence of geometry on the flow and turbulence characteristics within urban street canyon - Intercomparison of wind tunnel experiments and numerical simulations

International audienc

The influence of solar-induced wall-heating on the wind flow regime within urban street canyons : wind-tunnel and numerical simulations

International audienc

The influence of thermal effects on flow and dispersion in street canyons

The thermal effects due to the variable heating of the building walls by solar radiation and the heating produced by the vehicles on the airflow within a street canyon were studied by this group. The full-scale and wind tunnel measurements suggest that the overall effect of the heated walls on the street canyon flow dynamics is smaller than in 2-D numerical simulations. Thermal effects may generate a thin thermal convective flow close to the heated wall. As the flow in the wall boundaries carries air from the street level upwards, while normally cleaner air is transported from above, thermal effects may still be important for the air quality at pedestrian level and for the pollution transfers indoor. The heated walls affect the three components of the wind close to the wall and, therefore, this topic must be dealt with 3-D numerical calculations.The heat flux close to the heated wall is an important issue of these studies. In numerical calculations the use of heat flux boundary conditions is certainly more appropriate than the use of temperature boundary conditions. The results of the Nantes\u201999 campaign together with the new experiment Nantes 2000 documenting temperature and wind speed close to the wall are expected to give further input to the refinement of the treatment of wall heating effects and to the formulation of the heat flux boundary condition in CFD codes. Traffic heat-induced turbulence can be comparable in magnitude to that due to traffic mechanically-induced turbulence (TPT), especially for low vehicle speeds, and so needs to be considered for inclusion in models

The modelling of traffic produced turbulence

The chapter summarises the results of the TRAPOS working group “Traffic Produced Turbulence” (TPT). The main goals of these working group have been i) to summarise the TPT results that had already been achieved by the different teams, ii) to exchange the views and knowledge regarding TPT effects iii) to find a consensus concerning the relevance of traffic produced turbulence for dispersion modelling, iv) to improve TPT scaling concepts v) to verify TPT parameterisations for numerical dispersion models and vi) to present concepts for an incorporation of TPT effects in regulatory dispersion models. The studies performed within the TRAPOS TPT working group have shown that TPT is an important aspect for dispersion of traffic emissions. Neglecting TPT parameterisations in dispersion models causes significant over-predictions of pollutant concentrations in urban street canyons that range up to a factor of 4 to 5. Empirical formulas like the so-called VDI method lead to improvements, but operationally significant differences between model calculations and measured concentration values still occur for above-roof wind speeds smaller than approximately 4-5 m/s. Based on the findings of the TRAPOS TPT working group the following recommendations are made for practical applications of dispersion models: The TPT parameterisation implemented in the OSPM model works satisfactorily and dispersion models similar to OSPM should include the OSPM TPT concept. The traditional velocity scaling of concentrations and the empirical VDI method have deficiencies and must be reconsidered. The approach presented in Kastner-Klein et al. (2001) and Ketzel et al. (2001) that is based on a velocity scale which is defined as composition of velocity variances due to the external flow and due to traffic motions is an improvement and can be recommended. In CFD models TPT parameterisations must be implemented and the developed concepts are an improvement compared to model calculations without TPT parameterisations. However, for recommendations of particular modifications in the system of equations further verification studies are necessary

Large-eddy simulation of dispersion from surface sources in arrays of obstacles

Towards meeting the objective of simulating heat transfer processes in urban areas, the study of dispersion from a scalar (ground) surface area source has been addressed as a first step, as dispersion from such a source is in some ways analogous to heat transfer from the surface. Two different urban-like geometries are considered in this study: an array with uniform height cubes and an array with random height cuboids. Some point measurement dispersion experiments in a wind tunnel have previously been carried out in identical arrays using a naphthalene sublimation technique. Large-eddy simulations (LES) of these experiments have been performed as a validation study and the details, presented here, demonstrate the influence of the roughness morphology on the dispersion processes and the power of LES for obtaining physically important scalar turbulent flux information