Showing papers by "Rex Britter published in 2008"

Flow and Pollutant Dispersion in Street Canyons using FLUENT and ADMS-Urban

Abstract: This paper is devoted to the study of flow within a small building arrangement and pollutant dispersion in street canyons starting from the simplest case of dispersion from a simple traffic source. Flow results from the commercial computational fluid dynamics (CFD) code FLUENT are validated against wind tunnel data (CEDVAL). Dispersion results from FLUENT are analysed using the well-validated atmos pheric dispersion model ADMS-Urban. The k − e turbulence model and the advection-diffusion (AD) method are used for the CFD simulations. Sensitivity of dispersion results to wind direction within street canyons of aspect ratio equal to 1 is investigated. The analysis shows that the CFD model well reproduces the wind tunnel flow measurements and compares adequately with ADMS-Urban dispersion predictions for a simple traffic source by using a slightly modified k − e model. It is found that a Schmidt number of 0.4 is the most appropriate number for the simulation of a simple traffic source and in street canyons except for the case when the wind direction is perpendicular to the street canyon axis. For this last case a Schmidt number equal to 0.04 gives the best agreement with ADMS-Urban. Overall the modified k − e turbulence model may be accurate for the simulation of pollutant dispersion in street canyons provided that an appropriate choice for coefficients in the turbulence model and the Schmidt number in the diffusion model are made.

A Simple Model for Spatially-averaged Wind Profiles Within and Above an Urban Canopy

Abstract: This paper deals with the modelling of the flow in the urban canopy layer. It critically reviews a well-known formula for the spatially-averaged wind profile, originally proposed by Cionco in 1965, and provides a new interpretation for it. This opens up a number of new applications for modelling mean wind flow over the neighbourhood scale. The model is based on a balance equation between the obstacle drag force and the local shear stress as proposed by Cionco for a vegetative canopy. The buildings within the canopy are represented as a canopy element drag formulated in terms of morphological parameters such as λ f and λ p (the ratios of plan area and frontal area of buildings to the lot area). These parameters can be obtained from the analysis of urban digital elevation models. The shear stress is parameterised using a mixing length approach. Spatially-averaged velocity profiles for different values of building packing density corresponding to different flow regimes are obtained and analysed. The computed solutions are compared with published data from wind-tunnel and water-tunnel experiments over arrays of cubes. The model is used to estimate the spatially-averaged velocity profile within and above neighbourhood areas of real cities by using vertical profiles of λ f .

Simulation of the evolution of aircraft exhaust plumes including detailed chemistry and segregation

Abstract: [1] The Field Monte Carlo or Stochastic Fields (SF) method for turbulent reacting flows has been applied to the chemical evolution of the early part of a hot jet with bypass flow producing 7kN of thrust, using a 23 species chemical mechanism. This is done to broadly approximate a turbofan engine at idle thrust setting. Much of the chemistry was found to take place inside the core of the jet before mixing occurs, as there is no reactant gradient there, considering segregation makes little difference. Radical concentrations, however, were found to be changed. The reaction between NO and ambient O3, which is slow compared to the fast mixing timescale of the turbulent jet, is unaffected by segregation. The local Damkohler number was calculated based on an estimate of the chemical timescale and the local large-eddy timescale. It was found that only those species which had local Da greater than five were affected by segregation. In this work we have applied the SF method the early part of the plume, however the method developed here could equally be employed to study the plume over a longer distance.

The influence of buoyancy on flow and pollutant dispersion in street canyons

Abstract: In this paper, the effect of buoyancy on flow and pollutant dispersion within street canyons is studied by means of computational fluid dynamics simulations. We consider a neutral boundary layer approaching a 3D street canyon assuming a wind direction perpendicular to the street canyon. The Boussinesq hypothesis for incompressible fluids is chosen for modelling buoyancy. We distinguish three cases: leeward, ground and windward wall heating. Thermal effects on both the flow and dispersion are investigated for several Richardson numbers. The analysis focuses on the influence of street canyon geometry on flow and temperature distribution, by considering different aspect ratios W/H canyon between 0.5 and 2, where W is the width and H the height of the street canyon. Three-dimensional effects are observed, depending on L/H, where L is the length of the canyon. Three dimensional effects become negligible for aspect ratio L/H larger than 20. Results obtained for the case with a large Richardson number show that dispersion patterns in a street canyon differ substantially the isothermal case. In case with windward heating large concentration values are found close to the windward wall. Our findings can be of interest for many urban environment applications in which natural ventilation and thermal comfort are being of concern.