Air pollution modelling for environmental impact assessment

The aims of the lectures are: (i) to explain what concentration fluctuations are; (ii) to illustrate their importance in environmental impact assessment; (iii) to discuss some factors relevant to the quantitative description of concentration fluctuations; (iv) to describe a framework for this description. It will be clear from the lectures, and from others later in the Workshop that there is rapidly increasing awareness of the importance of concentration fluctuations and, consequently, much research activity into their properties. Not surprisingly there are still many unsolved problems, and a by-product of the lectures will be to highlight one or two of the most important

Concentration fluctuations in atmospheric dispersion

This report summarizes work done at Brunel University under Agreement No.2066/62 from 15 July 1986 to 14 July 1989. The title of the project was Concentration Fluctuations in Atmospheric Dispersion. The report has three principal components. These are: (i) theoretical work on the electrostatic effects associated with dispersing charged tracers. (ii) extensive analysis of several datasets taken with the CDE sensor system, particularly one obtained at RAF Cardington on 10 May 1988; (iii) interpretation of the results of the analysis. The conclusions of the report include recommendations for further work to exploit the advantages that the system has over many others

Research on continuous and instantaneous heavy gas clouds

This report describes the contribution of Brunel University to the joint CEC project 'Research on Continuous and Instantaneous Heavy Gas Clouds' under the Major Technological Hazards programme (CEC Contract EV4T.0025.UK(H)). Brunel University's main task in this project was concerned with the analysis of experimental data provided by some of the other project collaborators. Liaison with these collaborators, and with others undertaking other aspects of data analysis, was obviously also important. The experimental data were obtained both from full-scale field trials (Tuv/Risφ) and from wind tunnel experiments (TNO, University of Hamburg, Warren Spring Laboratory). Some of the data sets are very large. The main effort of data analysis has been concentrated on the data from Tuv/Ris sφ and Warren Spring Laboratory. This was mainly because of the timely arrival of substantial quantities of data from these sources, and also to avoid direct duplication of work carried out by other collaborators. Nevertheless, some analyses were made of TNO and University of Hamburg data. The Tuv/Risφ data set had one extremely valuable property, namely that the concentrations were measured by several different methods. Analysis here confirmed the view - hitherto essentially a theoretical speculation with no substantial experimental support - that the instrumentation can itself have a significant effect on the measured concentration. One consequence of the results of Brunel's analysis of the Tuv/Risφ data set is therefore that caution must be exercised in validating practical models of hazard assessment. Interest also attaches to this data set in that, in some of the experiments, obstacles were removed while the experiment was running; some analysis of "before and after" effects has been undertaken. For example, comparisons were made of such effects on levels of concentration and concentration variability, and two different algorithms have been developed to illustrate these features and, indeed, to determine, simply from the time series, when the obstacles were removed. A major and most welcome feature of the Warren Spring Laboratory data set was that it recorded many repetitions of gas releases under identical experimental conditions. Because of this, it was possible to study the variations in the concentration data from one release to another and to build up an initial simple statistical understanding of the situation. In such circumstances, statistical measures such as mean and variance may be estimated as ensemble averages, rather than by considering them as time averages within a single release; this latter approach can be questionable, particularly if the data do not exhibit statistical stationarity. The results of Brunel's analysis of this data set, though not yet complete, amply justify the "repetitions" strategy. The report illustrates this conclusion by presenting typical results that could not otherwise have been obtained, and which have important implications for real-life. The TNO wind tunnel experiments were conducted both for the purpose of comparing results with those from other wind tunnels and to provide a simulation of one of the full-scale Tuv/Risø field trials. The resulting data set is potentially very valuable, but Brunei's analysis has identified a number of points for concern. Thus there are some doubts about the behaviour of the instrumentation, while some of the experimental results are atypical of those obtained by other collaborators and occasionally seem hard to reconcile with physical intuition. Concerning the University of Hamburg data set, Brunel was aware that extensive and detailed analyses had been carried out by the Health and Safety Executive. Brunel did not wish to essentially duplicate this effort. Brunei's work here was, therefore, largely confined to replicating some of the HSE analyses for the purpose of confirming results - an aim that was always achieved. The HSE analyses are discussed formally in HSE's report under this contract, and were presented informally to meetings of the collaborators during the summer. Unavoidable resource constraints have prevented much progress in moving forward from data analysis to the development of models. However, work of this nature is still in progress after the termination of the formal contract. Such work is justified by the quantity and quality of the data, and is expected to form an important input to research under the FLADIS contract

Scalar transport in turbulent shear flows

Turbulent diffusion can be defined as the study of how a fluid in turbulent motion transports foreign substances that it contains. There are many examples, including smoke, acid rain, and other pollution in the atmosphere, salt in the sea and estuaries, and hot water in factory cooling systems. The foreign substance may have properties (especially its density and overall volume) that affect the motion of the ambient fluid, but it is often the case that the contaminant (as the foreign substance will be called in these lectures) is passive, i.e. the motion of the ambient fluid is the same as it would be in the absence of the contaminant. In practice nearly all cases of atmospheric dispersion fall into this category, as do many industrial applications. On the other hand the presence of dissolved salt has a profound effect on the behaviour of estuaries. These lectures will deal primarily with passive contaminants, although much of what is said applies qualitatively or, sometimes with minor modifications, even quantitatively to non-passive diffusion. Host fluid flows are, in practice, incompressible, i.e. the density of each fluid element is invariant during the motion (or can be regarded as invariant for all practical purposes). Variations of density from fluid element to fluid element are, of course, quite consistent with incompressible behaviour (e.g. in the atmosphere), but they will not be relevant in these lectures. The ambient fluid will therefore be taken as having constant uniform density p. The velocity field at position x and time t in the ambient fluid will be denoted by T(x,t), where (by incompressibility).........

Joint PDF modelling of turbulent flow and dispersion in an urban street canyon

The joint probability density function (PDF) of turbulent velocity and concentration of a passive scalar in an urban street canyon is computed using a newly developed particle-in-cell Monte Carlo method. Compared to moment closures, the PDF methodology provides the full one-point one-time PDF of the underlying fields containing all higher moments and correlations. The small-scale mixing of the scalar released from a concentrated source at the street level is modelled by the interaction by exchange with the conditional mean (IECM) model, with a micro-mixing time scale designed for geometrically complex settings. The boundary layer along no-slip walls (building sides and tops) is fully resolved using an elliptic relaxation technique, which captures the high anisotropy and inhomogeneity of the Reynolds stress tensor in these regions. A less computationally intensive technique based on wall functions to represent boundary layers and its effect on the solution are also explored. The calculated statistics are compared to experimental data and large-eddy simulation. The present work can be considered as the first example of computation of the full joint PDF of velocity and a transported passive scalar in an urban setting. The methodology proves successful in providing high level statistical information on the turbulence and pollutant concentration fields in complex urban scenarios.Comment: Accepted in Boundary-Layer Meteorology, Feb. 19, 200

Quantifying vertical mixing in estuaries

© 2008 The Authors. This is an open-access article distributed under the terms of the Creative Commons Attribution Noncommercial License. The definitive version was published in Environmental Fluid Mechanics 8 (2008): 495-509, doi:10.1007/s10652-008-9107-2.Estuarine turbulence is notable in that both the dissipation rate and the buoyancy frequency extend to much higher values than in other natural environments. The high dissipation rates lead to a distinct inertial subrange in the velocity and scalar spectra, which can be exploited for quantifying the turbulence quantities. However, high buoyancy frequencies lead to small Ozmidov scales, which require high sampling rates and small spatial aperture to resolve the turbulent fluxes. A set of observations in a highly stratified estuary demonstrate the effectiveness of a vessel-mounted turbulence array for resolving turbulent processes, and for relating the turbulence to the forcing by the Reynolds-averaged flow. The observations focus on the ebb, when most of the buoyancy flux occurs. Three stages of mixing are observed: (1) intermittent and localized but intense shear instability during the early ebb; (2) continuous and relatively homogeneous shear-induced mixing during the mid-ebb, and weakly stratified, boundary-layer mixing during the late ebb. The mixing efficiency as quantified by the flux Richardson number Rf was frequently observed to be higher than the canonical value of 0.15 from Osborn (J Phys Oceanogr 10:83–89, 1980). The high efficiency may be linked to the temporal–spatial evolution of shear instabilities.The funding for this research was obtained from ONR Grant N00014-06-1-0292 and NSF Grant OCE-0729547