Showing papers by "Rex Britter published in 2007"

Transfer processes in a simulated urban street canyon

Abstract: 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.

Modelling environmental & economic impacts of aviation: Introducing the aviation integrated modelling project

Abstract: The Aviation Integrated Modelling project is developing a policy assessment capability to enable comprehensive analyses of aviation, environment and economic interactions at local and global levels. It contains a set of inter-linked modules of the key elements relevant to this goal. These include models for aircraft/engine technologies, air transport demand, airport activity and airspace operations, all coupled to global climate, local environment and economic impact blocks. A major benefit of the integrated system architecture is the ability to model data flow and feedback between the modules. Policy assessment can be conducted by imposing policy effects on the upstream modules and following implications through the downstream modules to the output metrics, which can then be compared to a baseline case. A case study involving different evolution scenarios of the US air transportation system from 2000 to 2030 is used to show the importance of feedback and to model a sample policy scenario in order to illustrate current capabilities.

Application and validation of FLUENT flow and dispersion modelling within complex geometries

Abstract: Flow patterns around buildings have a strong influence on pollutant dispersion derived from sources placed within the urban area. Computational fluid dynamics (CFD) codes are used to provide solutions to the fundamental fluid dynamics equations at spatial scales smaller than the typical urban ones. In this work, dispersion of pollutant from sources near buildings forming various street canyons is studied by means of the general purpose CFD code FLUENT to investigate the influence of small geometric features on pollutant concentration distributions. Firstly, we study the effects of a complex geometry on the flow near the ground by considering a finite array of rectangular and square-shaped rings of buildings with different aspect ratios. Secondly, we study transport and diffusion of pollutant within a finite array of rectangular buildings. FLUENT concentration results are validated against wind tunnel data (CEDVAL, 2002). Numerical simulations are performed using the Reynolds Averaged Nervier–Stokes (RANS) k–e turbulence model and the advection–diffusion model. The paper documents the potential of a general purpose CFD model for the simulation of pollutant dispersion close to emission sources and within complex building arrangements in an operational context.

Guidelines issued by the Royal Meteorological Society

Abstract: These guidelines were prepared for the Royal Meteorological Society by a working group with the following membership: Dr Rex Britter (Cambridge University), Prof. Chris Collier (Meteorological Office), Prof. Richard Griffiths (UMIST), Dr Paul Mason (Meteorological Office), Dr David Thomson (Meteorological Office), Dr Roger Timmis (Her Majesty's Inspectorate of Pollution) and Dr Brian Underwood (AEA Technology Consultancy Services). The Chairman of the working group was Prof. Griffiths. The guidelines are published in collaboration with the Department of the Environment.

A simple approach for rapid operational air quality modelling at airports

Abstract: INTRODUCTION There is increasing concern about the likelihood of airport vicinity pollution levels violating ambient air quality regulations as airport usage continues to grow. Recent research has shown that, in the EU regulatory context, the annual average NO2 limit value is one of the primary constraints on traffic growth. Well-developed modeling tools such as EDMS and ADMSAirport are able to generate hour-by-hour air quality predictions of considerable accuracy. These approaches are appropriate for detailed investigations, but it may be beneficial to develop an operational screening model for expansion plans and policy options. Such a model will be used in the UK Aviation Integrated Modelling Project, based in Cambridge, which aims to simulate worldwide aviation, environmental and economic interactions to a 2050 timeframe (Reynolds et al, 2007).

Chapter 1.6 Modelling urban heat island in the context of a Mediterranean city

Abstract: Urban heat island (UHI) is one of the most well known forms of localised anthropogenic climate modification. It causes a local alteration of atmospheric stability. According to a top-down methodology, mesoscale meteorological modelling is a commonly used approach to study the impact of UHI on atmospheric stability at the urban scale. Our work is an effort to investigate UHI using a bottom-up approach by looking at the UHI through a computational fluid dynamics (CFD) model applied to the street canyons of a neighbourhood area. The CFD code is set up to model the thermal response (structure surface temperature and ambient air temperature) of an urban system to the outside climate. The determination of the air temperature in an urban unit allows the calculation of the Δ T u−r factor representing the difference between the air temperature in the urban system (u) and the air temperature recorded at the closest meteorological station (r), generally situated in the countryside. This factor, introduced by Oke in Boundary Layer Climate, (1987), enables the analysis of the heat island generated by an urban system. The simulation results obtained from the CFD model allows the estimation of the Δ T u−r factor in relation to physical aspects and geometrical configurations. We apply this technique to study UHI of a Mediterranean city of which some urban temperature measurements and morphometry from a digital elevation model (DEM) are available.

Modelling the flow within and above the Urban Canopy Layer

Abstract: The neighbourhood scale is recognised as being the connection between the street and the city scale. The present challenge in urban air quality context is the prediction, using simple models, of wind velocity profiles which take into account building morphology and layout. In this work a simple model to predict the spatially averaged flow field over real urban neighbourhoods is presented, based on the momentum balance between the inertial and the urban canopy layer. The buildings within the canopy were represented as a canopy element drag formulated in terms of the known morphological parameters λp and λf (the planar and frontal area density of buildings). These parameters were derived from a Digital Elevation Model (DEM). The nature of the model, being based on spatially averaged entities, is such that is suitable for inclusion into operational dispersion models for assessing urban air quality. INTRODUCTION Urban areas encompass a large number of neighbourhoods, which may in turn contain areas with similar surface characteristics. At a typical neighbourhood scale (up to about 5 km) the flow and pollutant dispersion can be modelled using spatially averaged approaches, where the buildings are considered as creating a region of porous resistance to the flow (Britter, R.E. and S. Hanna, 2003). In the present work, a simple model to estimate spatially averaged velocity profiles in real cities was adopted. Variation in height of λf, derived from the analysis of DEMs, was taken into account. The use of DEM methodology provided a powerful tool for a statistical treatment of the urban canopy layer in terms of morphological parameters. Capability of the model in modelling real flow patterns was evaluated using published data from wind and water tunnel experiments over array of cubes. METHODOLOGY The starting point of this work was the model introduced by Cionco (1969) for a vegetative canopy and successively adopted by Macdonald, R.W. (2000) for application to urban-type of roughness, intended as array of cubes. The model was based on the momentum balance between the urban canopy layer and the atmosphere above, expressed in terms of the drag force exerted by the buildings on the wind flow as: