The epidemics of severe acute respiratory syndrome (SARS) in 2003 highlighted both short- and long-range transmission routes, i.e. between infected patients and healthcare workers, and between distant locations. With other infections such as tuberculosis, measles and chickenpox, the concept of aerosol transmission is so well accepted that isolation of such patients is the norm. With current concerns about a possible approaching influenza pandemic, the control of transmission via infectious air has become more important. Therefore, the aim of this review is to describe the factors involved in: (1) the generation of an infectious aerosol, (2) the transmission of infectious droplets or droplet nuclei from this aerosol, and (3) the potential for inhalation of such droplets or droplet nuclei by a susceptible host. On this basis, recommendations are made to improve the control of aerosol-transmitted infections in hospitals as well as in the design and construction of future isolation facilities.

▪ Abstract  Predicting the motion of bubbles in dispersed flows is a key problem in fluid mechanics that has a bearing on a wide range of applications from oceanography to chemical engineering. In this review we synthesize the recent progress made in describing bubble motion in inhomogeneous flow. A trident approach consisting of experimental, analytical, and numerical work has given a clearer description of the hydrodynamic forces experienced by isolated bubbles moving either in inviscid flows or in slightly viscous laminar flows. A significant part of the paper is devoted to a discussion of drag, added-mass force, and shear-induced lift experienced by spheroidal bubbles moving in inertially dominated, time-dependent, rotational, nonuniform flows. The important influence of surfactants and shape distortion on bubble motion in a quiescent liquid is highlighted. Examples of bubble motion in inhomogeneous flows combining several of the effects mentioned above are discussed.

Recent developments in the physics and modeling of interfacial layers between regions with different turbulent intensities are reviewed. The flow dynamics across these layers governs exchanges of mass, momentum, energy, and scalars (e.g., temperature), which determine the growth, spreading, mixing, and reaction rates in many flows of engineering and natural interest. Results from several analytical and linearized models are reviewed. Particular attention is given to the case of turbulent/nonturbulent interfaces that exist at the edges of jets, wakes, mixing layers, and boundary layers. The geometry, dynamics, and scaling of these interfaces are reviewed, and future lines of research are suggested. The dynamics of passive and active scalars is also discussed, including the effects of stratification, turbulence level, and internal forcing. Finally, the modeling challenges for one-point closures and subgrid-scale models are briefly mentioned.

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This paper describes a study of the local and global effect of an isolated group of cylinders on an incident uniform flow. Using high resolution two-dimensional computations, we analysed the flow through and around a localised circular array of cylinders, where the ratio of array diameter (D G ) to cylinder diameter (D) is 21. The number of cylinders varied from N C = 7 to 133, and they were arranged in a series of concentric rings to allow even distribution within the array with an average void fraction φ = N C (D/D G ) 2 , which varied from 0.016 to 0.30. The characteristic Reynolds number of the array was Re G = 2100. A range of diagnostic tools were applied, including the lift/drag forces on each cylinder (and the whole array), Eulerian and Lagrangian average velocity within the array, and the decay of maximum vorticity with distance downstream. To interpret the flow field, we used vorticity and the dimensionless form of the second invariant of the velocity gradient tensor. A mathematical model, based on representing the bodies as point forces, sources and dipoles, was applied to interpret the results. Three distinct flow regimes were identified. For low void fractions (φ < 0.05), the cylinders have uncoupled individual wake patterns, where the vorticity is rapidly annihilated by wake intermingling downstream and the forces are similar to that of an isolated cylinder. At intermediate void fractions (0.05 < φ < 0.15), a shear layer is generated at the shoulders of the array and the force acting on the cylinders is steady. For high void fractions (φ > 0.15), the array generates a wake in a similar way to a solid body of the same scale. For low void fraction arrays, the mathematical model provides a reasonable assessment of the forces on individual bodies within the array, the Eulerian mean velocity and the upstream velocity field. While it broadly captures the change in the rate of decay of the maximum vorticity magnitude Ω max downstream, the magnitude is underpredicted.

Hospital-acquired infection (HAI) is an important public health issue with unacceptable levels of morbidity and mortality, over the last 5 years. Disease can be transmitted by air (over large distances), by direct/indirect contact or a combination of both routes. While contact transmission of disease forms the majority of HAI cases, transmission through the air is harder to control, but one where the engineering sciences can play an important role in limiting the spread. This forms the focus of this themed volume.In this paper, we describe the current hospital environment and review the contributions from microbiologists, mechanical and civil engineers, and mathematicians to this themed volume on the airborne transmission of infection in hospitals. The review also points out some of the outstanding scientific questions and possible approaches to mitigating transmission.

We examine tracer dispersion by the potential flow through a random array of rigid bodies fixed relative to a mean flow. Both Darcy flow through permeable bodies and inviscid irrotational flow past impermeable bodies are treated within one theoretical framework. The variation of the longitudinal dispersivity with body shape and permeability κ is examined for the case of high Péclet number, Pe. In the absence of diffusive effects, the longitudinal dispersivity D xx (where the mean flow is parallel to the x-axis) is calculated by tracing the evolution of a material surface advected by the mean flow and distorted by the array of bodies. For a random array of identical bodies of volume V and low volume fraction α, D xx = α|D f |UL/V. The drift volume, D f , is defined as the volume between the final and initial position of a material surface distorted by a single body moving in an unbounded flow, and L is the length-scale characterizing the associated longitudinal displacement of the surface. The variation of D xx with permeability is illustrated by considering permeable cylinders and spheres, and the effect of body shape on dispersion is illustrated by considering impermeable spheroids.The longitudinal dispersivity arising from the flow past impermeable bodies is D xx = αC xx UL, where C xx is the added-mass coefficient characterizing the mean flow around the body. This indicates that bluff bodies enhance longitudinal dispersion by promoting the longitudinal stretching of fluid elements. For Darcy flow through bodies of low permeability, the longitudinal dispersivity is D xx = α(C xx + 1)UL. The length-scale L, and thus D xx , is singular as κ → 0, owing to the long retention time of fluid within the bodies. For highly permeable two-dimensional bodies, D xx = α(C yy +1)UL, where C yy is the added-mass coefficient characterizing the flow around an impermeable body moving parallel to the y-axis. Consequently, dispersion by highly permeable bodies is enhanced when the bodies are slender, in contrast to the low-permeability limit.The influence of finite tracer diffusivity on longitudinal dispersion is demonstrated to make a negligible contribution when κ > 0 provided Pe max(1, 1/κ) and for impermeable bodies provided Pe 1. When Pe 1/κ, the longitudinal dispersion is dominated by diffusive effects and D xx = O(αU 2 a 2 /D 1 ).

s u m m a r yWith concerns about the potential for the aerosol and airborne transmission of infectious agents, particularly influenza, more attention is being focused on the effectiveness of infection control procedures to prevent hospital-acquired infections by this route. More recently a number of different techniques have been applied to examine the temporalespatial information about the airflow patterns and the movement of related, suspended material within this air in a hospital setting. Closer collaboration with engineers has allowed clinical microbiologists, virologists and infection control teams to assess the effectiveness of hospital isolation and ventilation facilities. The characteristics of human respiratory activities have also been investigated using some familiar engineering techniques. Such studies aim to enhance the effectiveness of such preventive measures and have included experiments with humanlike mannequins using various tracer gas/particle techniques, real human volunteers with realtime non-invasive Schlieren imaging, numerical modelling using computational fluid dynamics, and small scale physical analogues with water. This article outlines each of these techniques in a non-technical manner, suitable for a clinical readership without specialist airflow or engineering knowledge.