Form, shape and function: segmented blood flow in the choriocapillaris.

The development of fluid transport systems was a key event in the evolution of animals and plants. While within vertebrates branched geometries predominate, the choriocapillaris, which is the microvascular bed that is responsible for the maintenance of the outer retina, has evolved a planar topology. Here we examine the flow and mass transfer properties associated with this unusual geometry. We show that as a result of the form of the choriocapillaris, the blood flow is decomposed into a tessellation of functional vascular segments of various shapes delineated by separation surfaces across which there is no flow, and in the vicinity of which the transport of passive substances is diffusion-limited. The shape of each functional segment is determined by the distribution of arterioles and venules and their respective relative flow rates. We also show that, remarkably, the mass exchange with the outer retina is a function of the shape of each functional segment. In addition to introducing a novel framework in which the structure and function of the metabolite delivery system to the outer retina may be investigated in health and disease, the present work provides a general characterisation of the flow and transfers in multipole Hele-Shaw configurations.

Environmental policy constraints for acidic exhaust gas scrubber discharges from ships.

Increasingly stringent environmental legislation on sulphur oxide emissions from the combustion of fossil fuels onboard ships (International Maritime Organization (IMO) Regulation 14) can be met by either refining the fuel to reduce sulphur content or by scrubbing the exhaust gases. Commonly used open loop marine scrubbers discharge warm acidic exhaust gas wash water into the sea, depressing its pH. The focus on this paper is on the physics and chemistry behind the disposal of acidic discharges in seawater. The IMO Marine Environment Protection Committee (MEPC 59/24/Add.1 Annex 9) requires the wash water to reach a pH greater than 6.5 at a distance of 4m from the point of discharge. We examine the engineering constraints, specifically size and number of ports, to identify the challenges of meeting regulatory compliance.

Compliance of Royal Naval ships with nitrogen oxide emissions legislation.

Nitrogen oxide (NOx) emissions from marine diesel engines pose a hazard to human health and the environment. From 2021, demanding emissions limits are expected to be applied to sea areas that the Royal Navy (RN) accesses. We analyze how these future constraints affect the choice of NOx abatement systems for RN ships, which are subject to more design constraints than civilian ships. A weighted matrix approach is used to facilitate a quantitative assessment. For most warships to be built soon after 2021 Lean Nitrogen Traps (LNT) in conjunction with Exhaust Gas Recirculation (EGR) represents a relatively achievable option with fewer drawbacks than other system types. Urea-selective catalytic reduction is likely to be most appropriate for ships that are built to civilian standards. The future technologies that are at an early stage of development are discussed.

Observing and quantifying airflows in the infection control of aerosol- and airborne-transmitted diseases: an overview of approaches.

With 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 temporal-spatial 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 human-like mannequins using various tracer gas/particle techniques, real human volunteers with real-time 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.

New developments in understanding interfacial processes in turbulent flows.

Interfaces, across which fluid and flow properties change significantly, are a ubiquitous feature of most turbulent flows and are present within jets, plumes, homogeneous turbulence, oceans and planetary atmospheres. Even when the interfaces occupy a small volume fraction of the entire flow, they largely control processes such as entrainment and dissipation and can act as barriers to transport. This Theme Issue brings together some of the leading recent developments on interfaces in turbulence, drawing in many methodologies, such as experiments, direct number simulations, inverse methods and analytical modelling.

Interfaces and inhomogeneous turbulence.

A Euromech colloquium, on interfacial processes and inhomogeneous turbulence, was held in London on 28-30 June 2010. Papers were presented describing and analysing the influence of interfaces that separate turbulent/non-turbulent regions, between regions of contrasting fluid properties, or at the edge of boundaries. This paper describes a summary of the work presented, giving a snapshot of the current progress in this area, along with discussions about future research directions.

The fluid mechanics of root canal irrigation.

Root canal treatment is a common dental operation aimed at removing the contents of the geometrically complex canal chambers within teeth; its purpose is to remove diseased or infected tissue. The complex chamber is first enlarged and shaped by instruments to a size sufficient to deliver antibacterial fluids. These irrigants help to dissolve dying tissue, disinfect the canal walls and space and flush out debris. The effectiveness of the procedure is limited by access to the canal terminus. Endodontic research is focused on finding the instruments and clinical procedures that might improve success rates by more effectively reaching the apical anatomy. The individual factors affecting treatment outcome have not been unequivocally deciphered, partly because of the difficulty in isolating them and in making the link between simplified, general experimental models and the complex biological objects that are teeth. Explicitly considering the physical processes within the root canal can contribute to the resolution of these problems. The central problem is one of fluid motion in a confined geometry, which makes the dispersion and mixing of irrigant more difficult because of the absence of turbulence over much of the canal volume. The effects of treatments can be understood through the use of scale models, mathematical modelling and numerical computations. A particular concern in treatment is that caustic irrigant may penetrate beyond the root canal, causing chemical damage to the jawbone. In fact, a stagnation plane exists beyond the needle tip, which the irrigant cannot penetrate. The goal is therefore to shift the stagnation plane apically to be coincident with the canal terminus without extending beyond it. Needle design may solve some of the problems but the best design for irrigant penetration conflicts with that for optimal removal of the bacterial biofilm from the canal wall. Both irrigant penetration and biofilm removal may be improved through canal fluid agitation using a closely fitting instrument or by sonic or ultrasonic activation. This review highlights a way forward by understanding the physical processes involved through physical models, mathematical modelling and numerical computations.

Movement of airborne contaminants in a hospital isolation room.

We analyse the characteristics of a force-ventilated isolation room, and the contributions to transport caused by the movement of people and doors opening/closing. The spread of fine droplets and particles can be understood, to leading order, by considering the movement of passive contaminants. A scaled (1:10) model of an isolation room (with water instead of air) was used to analyse the dilution of a passive contaminant (food dye), released either instantaneously or at a constant rate. The high level of turbulence, typical of isolation rooms, ensures that the dye concentration is uniform within the model room and mixing is perfect, and the measured mean concentration can be predicted theoretically. In a second series of experiments, the exchange generated by a door opening/closing is measured for different opening angles. A dipolar vortex is generated at the tip of the door which moves into the centre of the room, with a large coherent structure moving along the wall. The exchange volume is comparable to the swept volume of the door. Larger droplets and particles do not move passively. Their movement within a turbulent flow is studied by combining a Lagrangian model of particle movement with a kinematic simulation of a pseudo turbulent flow. The results show that while the mean fall velocity of particles is largely unchanged, turbulence significantly enhances horizontal and vertical dispersion. The horizontal spread as a function of the level of turbulence and droplet properties is estimated. The conclusions from both studies are brought together and discussed in the context of the airborne spread of contaminants within a general hospital room.

Airborne transmission of disease in hospitals.

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.

New perspectives on dispersed multiphase flows. Introduction.

Dispersed multiphase flows--the study of the individual or collective motion of a discrete (solid/liquid/gas) phase in a continuous (liquid/gas) phase--has broad implications for health physics, processes in the natural environment, new technological developments (microelectromechanical systems) and many industrial problems. Many of these processes are important in our daily activities. In this paper, a general overview of the papers in this Theme Issue is described and some of the common issues are identified.

Disappearing bodies and ghost vortices.

In many dispersed multiphase flows bubbles, droplets, and particles move and disappear due to a phase change. Practical examples include vapour bubbles condensing in subcooled liquids, fuel droplets evaporating in a hot gas and ice crystals melting in water. After these 'bodies' have disappeared, they leave behind a remnant 'ghost' vortex as an expression of momentum conservation. A general framework is developed to analyse why and how a ghost vortex is generated. A study of these processes is incomplete without a detailed discussion of the concept of momentum for unbounded flows. We show how momentum can be defined unambiguously for unbounded flows and show its connection with other expressions, particularly that of Lighthill. We apply our analysis to interpret new observations of condensing vapour bubbles and discuss droplet evaporation. We show that the use of integral invariants, widely applied in turbulence, introduces a new perspective to dispersed multiphase flows.

Continuous flushing of contaminants from ballast water tanks.

The transport of non-indigenous species (NIS) with ship ballast water is a major environmental problem. The International Maritime Organisation (IMO) have recommended that ballast tanks are flushed through with sea water to remove NIS contaminants. The flushing efficiency is studied using mathematical models and a scaled experimental model of a ballast tank. The density contrast between the ballast water and water used for flushing is important when the Froude number Fr(w)=U(w)/sqr rt|g(')|H << 1 (defined in terms of average horizontal flow U(w), reduced buoyancy g', and H the vertical dimension in the tank). When denser water is used to flush a ballast tank, from below, it efficiently displaces lighter ballast water; but flushing through with light water creates a buoyant gravity current which effectively short circuits part of the tank. When Fr(w)>>1, the density contrast between the ballast water and water used for flushing is not important and flushing is controlled by a bulk Péclet number, Pe(w). For Pe(w)<<1 perfect mixing occurs, while for Pe(w)>>1 displacement flushing occurs. Laboratory experiments of flushing were performed using a model two-dimensional ballast tank employing dye attenuation to measure the whole concentration field and these experiments confirm the essential features of the mathematical models. The results of this study are discussed in the context of current IMO flushing protocols.

Factors involved in the aerosol transmission of infection and control of ventilation in healthcare premises.

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.

Door-opening motion can potentially lead to a transient breakdown in negative-pressure isolation conditions: the importance of vorticity and buoyancy airflows.

A patient with severe chickenpox was admitted to a negative-pressure isolation room. He remained sedated, intubated and mechanically ventilated throughout his admission. He was managed only by nurses immune to chickenpox. A non-immune male nurse occasionally handed equipment through the doorway, without entering the room. Ten days later, he also developed chickenpox. Sequencing of viruses from the patient and nurse showed the same rare genotype, indicating nosocomial transmission. An experimental model demonstrated that, despite negative pressure, opening the door could have resulted in transport of infectious air out of the isolation room, leading to a breakdown in isolation conditions.

The concept of drift and its application to multiphase and multibody problems.

The concept of drift is built around understanding how a rigid body moving in a straight line distorts a material sheet in an unbounded perfect fluid. As the body moves from infinity through a material sheet, which is initially perpendicular to the direction of translation of the body, the sheet is permanently distorted. Darwin showed that the 'drift' volume, D(f), formed between the distorted and undistorted sheet is equal to C(m)V, where the added-mass coefficient, C(m), characterizes the shape of the body whose volume is V. Darwin's result is important for two reasons: first, it provides a means of quantifying how dyed fluid is transported from one place to another and dispersed; second, it provides a fundamental Lagrangian coordinate system to study inhomogeneous inviscid problems. The aim of this article is to review Darwin's contribution to fluid mechanics. By drawing on recent experimental measurements of drift and the drift volume, we aim to demonstrate how Darwin's drift concept has developed and to describe its broader significance for multiphase and multibody problems.

Experimental and theoretical study of the spread of fluid from a point source on an inclined incontinence bed-pad.

The spread of fluid from a localized source on to a flat fibrous sheet is studied. The sheet is inclined at an angle, alpha, to the horizontal, and the areal flux of the fluid released is Qa. A new experimental study is described where the dimensions of the wetted region are measured as a function of time t, Qa and alpha (>0). The down-slope length, Y, grows according to Y approximately (Qa t)(2/3) (sin alpha)(1/3); for high discharge rates and low angles of inclination, the cross-slope width, X, grows as approximately (Qa t)(1/2), while for low discharge rates or high angles of inclination, the cross-slope transport is dominated by infiltration and X approximately 2(2Ks psi* t)(1/2), where Ks is the saturated permeability and psi* is the characteristic value of capillary pressure. A scaling analysis of the underlying non-linear advection diffusion equation describing the infiltration process confirms many of the salient features of the flow observed. Good agreement is observed between the collapse of the numerical solutions and experimental results. The broader implications of these results for incontinence bed-pad research are briefly discussed.