Re Britter

THE DYNAMICS OF THE HEAD OF A GRAVITY CURRENT ADVANCING OVER A HORIZONTAL SURFACE

The motion behind the head of a gravity current advancing over a no-slip horizontal surface is a complex three-dimensional flow. There is intense mixing between the current and its surroundings and the foremost part of the head is raised above the surface. Experimental results are obtained from (i) an apparatus in which the head is brought to rest by using an opposing flow and a moving floor and (ii) a modified lock exchange flow. The dimensionless velocity of advance, rate of mixing between the two fluids and the depth of the mixed layer left behind the head and above the following gravity current are determined for an extended range of the dimensionless gravity current depth. The mixing between the two fluids is the result of gravitational and shear instabilities at the gravity current head. A semi-empirical analysis is presented to describe the results. The influence of Reynolds number is discussed and comparison with a documented atmospheric flow is presented.

Experiments on the dynamics of a gravity current head

Some of the dense fluid at the front of an advancing gravity current is observed to be mixed with the ambient fluid. This process continues when the cross-stream non-uniformities at the head of the current are suppressed by advancing the floor beneath the head. In the resulting two-dimensional flow regular billows are visible. This paper considers experimentally and analytically the inviscid gravity current head and specifically includes the observed mixing at the head. Experimental results were obtained with an apparatus in which the head of the gravity current was brought to rest by an opposing uniform flow. The mixing appears to occur through Kelvin-Helmholtz billows generated on the front of the head and controls the dynamics of the head. A momentum balance is used to analyse the flow and the problem is closed by quantitatively introducing the billow structure.

A random walk model for dispersion in inhomogeneous turbulence in a convective boundary layer

It is necessary for a random walk model to satisfy the well-mixed criterion which requires that if particles of a tracer are initially well mixed in the ambient fluid they will remain so. Models applied so far to dispersion in a convective boundary layer where the turbulence is inhomogeneous and skew require a non-Gaussian random forcing and do not satisfy this well-mixed condition. In this work a random walk model is developed based on the approach of Thomson (1987, J. Fluid Mech.180,529–556) which satisfies the well-mixed condition, incorporates skewness in the vertical velocity and has Gaussian random forcing. The skewed probability distribution function (PDF) equation of Baerentsen and Berkowicz (1984, Atmospheric Environment18, 701–712) is used to derive the model equation. The model is applied to diffusion in a convective boundary layer. The validity of the closure assumption that σA = wAand σb = wA, where σA and σB are the updraft and downdraft velocity standard deviations, respectively and wA andwB are the mean updraft and downdraft velocities, respectively, is analyzed quantitatively with the measured values of various statistical parameters involved in the PDF equation. Results reveal that the assumption is quite satisfactory. The new model is general and reduces to the one-dimensional model equations of Wilson et al. (1983, Boundary-Layer Met. 27,163–169) and Thomson (1987, J. Fluid Mech. 180, 529–556) when the turbulence is Gaussian without any mean flow, and to the basic Langevin equation when the turbulence is homogeneous. Predictions are made for the dimensionless crosswind integrated concentrations, mean particle height, and particle spread for three source heights and three step sizes. The comparison of the model results with laboratory measurements of Willis and Deardorff(1976, Q. Jl R. met. Soc.102,427–445; 1978, Atmospheric Environment12,1305–1311; 1981, Atmospheric Environment15,109–117) and the random walk results of de Baas et al. (1986, Q. Jl R. met. Soc.112,165–180) and Sawford and Guest (1987, J. atmos. Sci.44,1152–1165) in which the models require non-Gaussian random forcing, shows that the new model simulates the experimental features quite well. The particle distribution becomes homogeneous after X≈6. The maximum ground level concentrations are better predicted by the new model.

Gravitational convection from instantaneous sources on inclined boundaries

Two-dimensional buoyant clouds moving along inclined boundaries under a gravitational force are investigated theoretically and experimentally. It is found that the ‘thermal theory’ gives a good description of the flow in the slope angle range 5° [lsim ] θ ≤ 90°. In this range the spatial growth rates of the cloud height and length are constant for a given slope angle and show a linear dependence on θ. For a cloud released with zero initial velocity the front velocity U f first increases and then decreases, with the characteristic time of acceleration predicted by theory. In the decelerating state U f /( g 0 ′ Q 0 / x f ) ½ is 2·6 ± 0·2 at θ ≃ 15°, and then reduces uniformly with increasing θ to a value of 1·5 ≃ 0·2 at 90° (where g 0 ′ Q 0 is the released buoyancy and x f is the front position measured from a virtual origin). The shape of the cloud is well approximated by a half-ellipse. The variation of the ratio of the principal axes of the half-ellipse with slope angle is identical with that of the head of an inclined starting plume (Britter & Linden 1980). However, the cloud has a greater growth rate than the head of a starting plume.

The rise of low-cost sensing for managing air pollution in cities.

Ever growing populations in cities are associated with a major increase in road vehicles and air pollution. The overall high levels of urban air pollution have been shown to be of a significant risk to city dwellers. However, the impacts of very high but temporally and spatially restricted pollution, and thus exposure, are still poorly understood. Conventional approaches to air quality monitoring are based on networks of static and sparse measurement stations. However, these are prohibitively expensive to capture tempo-spatial heterogeneity and identify pollution hotspots, which is required for the development of robust real-time strategies for exposure control. Current progress in developing low-cost micro-scale sensing technology is radically changing the conventional approach to allow real-time information in a capillary form. But the question remains whether there is value in the less accurate data they generate. This article illustrates the drivers behind current rises in the use of low-cost sensors for air pollution management in cities, while addressing the major challenges for their effective implementation.

Spatially averaged flow within obstacle arrays

Knowledge of the flow velocity within an array of buildings is important in modelling the dispersion of urban air pollution. In this paper a spatially averaged model is derived for UC, the characteristic in-canopy velocity. The models predictions are compared with experimental results from various sources. The model predicts the in-canopy velocity for a broad range of building packing densities (0.01<λf<0.44). An alternative form of the model is proposed for sparse arrays (λf<0.2). A model is also derived for the exchange velocity UE between the in-canopy and above-canopy flows. This model for UE has the correct variation with packing density and gives predictions of the right order but has yet to be directly validated.

Structure of the temperature field downwind of a line source in grid turbulence

A Lagrangian stochastic model is used in conjunction with detailed wind-tunnel measurements to examine the structure and development of the temperature field behind a line source in grid turbulence. It is shown that on the scale of these experiments molecular diffusion and viscosity have an important influence on temperature fluctuations (particularly on the intensity of these fluctuations) and must be explicitly modelled. The model accounts for a wide range of the measured properties of the temperature field and provides a unified treatment of temperature fluctuations through all stages of the development of the temperature field. This development is discussed in terms of a simple physical picture in which the hot plume is initially smooth and is moved about bodily by the turbulence, but gradually develops increasing internal structure or patchiness as it grows with distance downstream until a self-similar state is reached in which this internal structure maintains the temperature fluctuations.

The distortion of turbulence by a circular cylinder

The flow of grid-generated turbulence past a circular cylinder is investigated using hot-wire anemometry over a Reynolds number range from 4·25 × 103 to 2·74 × 104 and a range of intensities from 0·025 to 0·062. Measurements of the mean velocity distribution, and r.m.s. intensities and spectral energy densities of the turbulent velocity fluctuations are presented for various radial and circumferential positions relative to the cylinder, and for ratios of the cylinder radius a to the scale of the incident turbulence Lx ranging from 0·05 to 1·42. The influence of upstream conditions on the flow in the cylinder wake and its associated induced velocity fluctuations is discussed.For all measurements, detailed comparison is made with the theoretical predictions of Hunt (1973). We conclude the following. The amplification and reduction of the three components of turbulence (which occur in different senses for the different components) can be explained qualitatively in terms of the distortion by the mean flow of the turbulent vorticity and the ‘blocking’ or ‘source’ effect caused by turbulence impinging on the cylinder surface. The relative importance of the first effect over the second increases as a/Lx increases or the distance from the cylinder surface increases.Over certain ranges of the variables involved, the measurements are in quantitative agreement with the predictions of the asymptotic theory when a/Lx [Lt ] 1, a/Lx [Gt ] 1 or |k| a [Gt ] 1 (where k is the wavenumber).The incident turbulence affects the gross properties of the flow in the cylinder wake, but the associated velocity fluctuations are probably statistically independent of those in the incident flow.The dissipation of turbulent energy is greater in the straining flow near the cylinder than in the approach flow. Some estimates for this effect are proposed.

Global Mortality Attributable to Aircraft Cruise Emissions

Aircraft emissions impact human health though degradation of air quality. The majority of previous analyses of air quality impacts from aviation have considered only landing and takeoff emissions. We show that aircraft cruise emissions impact human health over a hemispheric scale and provide the first estimate of premature mortalities attributable to aircraft emissions globally. We estimate ∼8000 premature mortalities per year are attributable to aircraft cruise emissions. This represents ∼80% of the total impact of aviation (where the total includes the effects of landing and takeoff emissions), and ∼1% of air quality-related premature mortalities from all sources. However, we note that the impact of landing and takeoff emissions is likely to be under-resolved. Secondary H(2)SO(4)-HNO(3)-NH(3) aerosols are found to dominate mortality impacts. Due to the altitude and region of the atmosphere at which aircraft emissions are deposited, the extent of transboundary air pollution is particularly strong. For example, we describe how strong zonal westerly winds aloft, the mean meridional circulation around 30-60°N, interaction of aircraft-attributable aerosol precursors with background ammonia, and high population densities in combination give rise to an estimated ∼3500 premature mortalities per year in China and India combined, despite their relatively small current share of aircraft emissions. Subsidence of aviation-attributable aerosol and aerosol precursors occurs predominantly around the dry subtropical ridge, which results in reduced wet removal of aviation-attributable aerosol. It is also found that aircraft NO(x) emissions serve to increase oxidation of nonaviation SO(2), thereby further increasing the air quality impacts of aviation. We recommend that cruise emissions be explicitly considered in the development of policies, technologies and operational procedures designed to mitigate the air quality impacts of air transportation.

Effect of wind direction and speed on the dispersion of nucleation and accumulation mode particles in an urban street canyon

There have been many studies concerning dispersion of gaseous pollutants from vehicles within street canyons; fewer address the dispersion of particulate matter, particularly particle number concentrations separated into the nucleation (10-30 nm or N10-30) or accumulation (30-300 nm or N30-300) modes either separately or together (N10-300). This study aimed to determine the effect of wind direction and speed on particle dispersion in the above size ranges. Particle number distributions (PNDs) and concentrations (PNCs) were measured in the 5-2738 nm range continuously (and in real-time) for 17 days between 7th and 23rd March 2007 in a regular (aspect ratio approximately unity) street canyon in Cambridge (UK), using a newly developed fast-response differential mobility spectrometer (sampling frequency 0.5 Hz), at 1.60 m above the road level. The PNCs in each size range, during all wind directions, were better described by a proposed two regime model (traffic-dependent and wind-dependent mixing) than by simply assuming that the PNC was inversely proportional to the wind speed or by fitting the data with a best-fit single power law. The critical cut-off wind speed (Ur,crit) for each size range of particles, distinguishing the boundary between these mixing regimes was also investigated. In the traffic-dependent PNC region (UrUr<<Ur,critUr,crit), concentrations in each size range were approximately constant and independent of wind speed and direction. In the wind speed dependent PNC region (UrUr>>Ur,critUr,crit), concentrations were inversely proportional to Ur irrespective of any particle size range and wind directions. The wind speed demarcating the two regimes (Ur,critUr,crit) was 1.23+/-0.55 m s(-1) for N10-300, (1.47+/-0.72 m s(-1)) for N10-30 but smaller (0.78+/-0.29 m s(-1)) for N30-300.