Do we need high temporal resolution modelling of exposure in urban areas? A test case.

Roadside concentrations of harmful pollutants such as NOx are highly variable in both space and time. This is rarely considered when assessing pedestrian and cyclist exposures. We aim to fully describe the spatio-temporal variability of exposures of pedestrians and cyclists travelling along a road at high resolution. We evaluate the value added of high spatio-temporal resolution compared to high spatial resolution only. We also compare high resolution vehicle emissions modelling to using a constant volume source. We highlight conditions of peak exposures, and discuss implications for health impact assessments. Using the large eddy simulation code Fluidity we simulate NOx concentrations at a resolution of 2 m and 1 s along a 350 m road segment in a complex real-world street geometry including an intersection and bus stops. We then simulate pedestrian and cyclist journeys for different routes and departure times. For the high spatio-temporal method, the standard deviation in 1 s concentration experienced by pedestrians (50.9 µg.m-3) is nearly three times greater than that predicted by the high-spatial only (17.5 µg.m-3) or constant volume source (17.6 µg.m-3) methods. This exposure is characterised by low concentrations punctuated by short duration, peak exposures which elevate the mean exposure and are not captured by the other two methods. We also find that the mean exposure of cyclists on the road (31.8 µg.m-3) is significantly greater than that of cyclists on a roadside path (25.6 µg.m-3) and that of pedestrians on a sidewalk (17.6 µg.m-3). We conclude that ignoring high resolution temporal air pollution variability experienced at the breathing time scale can lead to a mischaracterization of pedestrian and cyclist exposures, and therefore also potentially the harm caused. High resolution methods reveal that peaks, and hence mean exposures, can be meaningfully reduced by avoiding hyper-local hotspots such as bus stops and junctions.

The ventilation of buildings and other mitigating measures for COVID-19: a focus on wintertime.

The year 2020 has seen the emergence of a global pandemic as a result of the disease COVID-19. This report reviews knowledge of the transmission of COVID-19 indoors, examines the evidence for mitigating measures, and considers the implications for wintertime with a focus on ventilation.

Displacement ventilation: a viable ventilation strategy for makeshift hospitals and public buildings to contain COVID-19 and other airborne diseases.

The SARS-CoV-2 virus has so far infected more than 31 million people around the world, and its impact is being felt by all. Patients with diseases such as COVID-19 should ideally be treated in negative pressure isolation rooms. However, due to the overwhelming demand for hospital beds, patients have been treated in general wards, hospital corridors and makeshift hospitals. Adequate building ventilation in hospitals and public spaces is a crucial factor to contain the disease (Escombe et al. 2007 PLoS Med. 4; Escombe et al. 2019 BMC Infect. Dis. 19, 88 (doi:10.1186/s12879-019-3717-9); Morawska & Milton 2020 Clin. Infect. Dis. ciaa939. (doi:10.1093/cid/ciaa939)), to exit lockdown safely, and reduce the chance of subsequent waves of outbreaks. A recently reported air-conditioner-induced COVID-19 outbreak caused by an asymptomatic patient, in a restaurant in Guangzhou, China (Lu et al. 2020 Emerg. Infect. Dis. 26) exposes our vulnerability to future outbreaks linked to ventilation in public spaces. We argue that displacement ventilation (either mechanical or natural ventilation), where air intakes are at low level and extracts are at high level, is a viable alternative to negative pressure isolation rooms, which are often not available on site in hospital wards and makeshift hospitals. Displacement ventilation produces negative pressure at the occupant level, which draws fresh air from outdoors, and positive pressure near the ceiling, which expels the hot and contaminated air out. We acknowledge that, in both developed and developing countries, many modern large structures lack the openings required for natural ventilation. This lack of openings can be supplemented by installing extract fans. We have also discussed and addressed the issue of the 'lock-up effect'. We provide guidelines for such mechanically assisted, naturally ventilated makeshift hospitals.

The transport of liquids in softwood: timber as a model porous medium.

Timber is the only widely used construction material we can grow. The wood from which it comes has evolved to provide structural support for the tree and to act as a conduit for fluid flow. These flow paths are crucial for engineers to exploit the full potential of timber, by allowing impregnation with liquids that modify the properties or resilience of this natural material. Accurately predicting the transport of these liquids enables more efficient industrial timber treatment processes to be developed, thereby extending the scope to use this sustainable construction material; moreover, it is of fundamental scientific value - as a fluid flow within a natural porous medium. Both structural and transport properties of wood depend on its micro-structure but, while a substantial body of research relates the structural performance of wood to its detailed architecture, no such knowledge exists for the transport properties. We present a model, based on increasingly refined geometric parameters, that accurately predicts the time-dependent ingress of liquids within softwood timber, thereby addressing this long-standing scientific challenge. Moreover, we show that for the minimalistic parameterisation the model predicts ingress with a square-root-of-time behaviour. However, experimental data show a potentially significant departure from this [Formula: see text] behaviour - a departure which is successfully predicted by our more advanced parametrisation. Our parameterisation of the timber microstructure was informed by computed tomographic measurements; model predictions were validated by comparison with experimental data. We show that accurate predictions require statistical representation of the variability in the timber pore space. The collapse of our dimensionless experimental data demonstrates clear potential for our results to be up-scaled to industrial treatment processes.

Symmetric coalescence of two hydraulic fractures.

The formation of a fracture network is a key process for many geophysical and industrial practices from energy resource recovery to induced seismic management. We focus on the initial stage of a fracture network formation using experiments on the symmetric coalescence of two equal coplanar, fluid-driven, penny-shaped fractures in a brittle elastic medium. Initially, the fractures propagate independently of each other. The fractures then begin to interact and coalesce, forming a bridge between them. Within an intermediate period after the initial contact, most of the fracture growth is localized along this bridge, perpendicular to the line connecting the injection sources. Using light attenuation and particle image velocimetry to measure both the fracture aperture and velocity field, we characterize the growth of this bridge. We model this behavior using a geometric volume conservation argument dependent on the symmetry of the interaction, with a 2D approximation for the bridge. We also verify experimentally the scaling for the bridge growth and the shape of the thickness profile along the bridge. The influence of elasticity and toughness of the solid, injection rate of the fluid, and initial location of the fractures are captured by our scaling.

Cell geometry across the ring structure of Sitka spruce.

For wood to be used to its full potential as an engineering material, it is necessary to quantify links between its cell geometry and the properties it exhibits at bulk scale. Doing so will make it possible to predict timber properties crucial to engineering, such as mechanical strength and stiffness, and the resistance to fluid flow, and to inform strategies to improve those properties as required, as well as to measure the effects of interventions such as genetic manipulation and chemical modification. Strength, stiffness and permeability of timber all derive from the geometry of its cells, and yet current practice is to predict them based on properties, such as bulk density, that do not directly describe the cell structure. This work explores links between micro-computed tomography data for structural-size pieces of wood, which show the variation of porosity across the wood's ring structure, and high-resolution tomography showing the geometry of the cells, from which we measure cell length, lumen area, porosity, cell wall thickness and the number density of cells. High-resolution scans, while informative, are time-consuming and expensive to run on a large number of samples at the scale of building components. By scanning the same volume of timber at both low and high resolutions (high-resolution scans over a near-continuous volume of timber of approx. 20 mm3 at 15 µm3 per voxel), we are able to demonstrate correlations between the measurements at the two different resolutions, reveal the physical basis for these correlations, and demonstrate that the data from the low-resolution scan can be used to estimate the variation in (small-scale) cell geometry throughout a structural-size piece of wood.

Detrainment of plumes from vertically distributed sources.

We present experimental results demonstrating that, for the turbulent plume from a buoyancy source that is vertically distributed over the full area of a wall, detrainment qualitatively changes the shape of the ambient buoyancy profile that develops in a sealed space. Theoretical models with one-way-entrainment predict stratifications that are qualitatively different from the stratifications measured in experiments. A peeling plume model, where density and vertical velocity vary linearly across the width of the plume, so that plume fluid "peels" off into the ambient at intermediate heights, more accurately captures the shape of the ambient buoyancy profiles measured in experiments than a conventional one-way-entrainment model does.

Pedestrians' perception of environmental stimuli through field surveys: focus on particulate pollution.

The between perception of individual exposure to different environmental stimuli; microclimate, noise and especially particulate matter (PM) was examined. Microclimate, noise and PM were monitored during field surveys with 260 questionnaire-guided interviews at a road construction site and a traffic site on the UC San Diego campus. The overall comfort was determined primarily by the thermal environment. The air quality was considered to be poor by 42% of the interviewees at the construction site, which was burdened with higher PM counts and sound levels. Overall, higher PM concentrations were correlated with perception of poor air quality. Similarity between the overall air quality and how dusty it feels suggests that visual clues of PM, such as dust, affect the perception of air quality and pollution. The effect of medical or smoking history on the perceived air quality was also examined. People with a medical history of hay fever voted more frequently for poor air quality conditions than those without, whereas current smokers were the least sensitive to ambient air quality conditions. Through the exposure-response relationships between the various perception votes and PM, it was possible to predict perceived air cleanliness using the PM count. Understanding the human assessment of environmental stimuli could inform the design and development of urban spaces, in relation to the allocation of uses and activities, along with air quality management schemes.

Particle transport in low-energy ventilation systems. Part 2: Transients and experiments.

UNLABELLED: Providing adequate indoor air quality while reducing energy consumption is a must for efficient ventilation system design. In this work, we study the transport of particulate contaminants in a displacement-ventilated space, using the idealized 'emptying filling box' model (P.F. Linden, G.F. Lane-serff and D.A. Smeed (1990) Emptying filling boxes: the fluid mechanics of natural ventilation, J. fluid Mech., 212, 309-335.). In this paper, we focused on transient contaminant transport by modeling three transient contamination scenarios, namely the so called 'step-up', 'step-down', and point source cases. Using analytical integral models and numerical models we studied the transient behavior of each of these three cases. We found that, on average, traditional and low-energy systems can be similar in overall pollutant removal efficiency, although quite different vertical gradients can exist. This plays an important role in estimating occupant exposure to contaminant. A series of laboratory experiments were conducted to validate the developed models. PRACTICAL IMPLICATIONS: The results presented here illustrate that the source location plays a very important role in the distribution of contaminant concentration for spaces ventilated by low energy displacement-ventilation systems. With these results and the knowledge of typical contaminant sources for a given type of space practitioners can design or select more effective systems for the purpose at hand.

Particle transport in low-energy ventilation systems. Part 1: theory of steady states.

UNLABELLED: Many modern low-energy ventilation schemes, such as displacement or natural ventilation, take advantage of temperature stratification in a space, extracting the warmest air from the top of the room. The adoption of these energy-efficient ventilation systems still requires the provision of acceptable indoor air quality. In this work we study the steady state transport of particulate contaminants in a displacement-ventilated space. Representing heat sources as ideal sources of buoyancy, analytical models are developed that allow us to compare the average efficiency of contaminant removal between traditional and modern low-energy systems. We found that on average traditional and low-energy systems are similar in overall pollutant removal efficiency, although quite different vertical distributions of contaminant can exist, thus affecting individual exposure. While the main focus of this work is on particles where the dominant mode of deposition is by gravitational settling, we also discuss additional deposition mechanisms and show that the qualitative observations we make carry over to cases where such mechanisms must be included. PRACTICAL IMPLICATIONS: We illustrate that while average concentration of particles for traditional mixing systems and low energy displacement systems are similar, local concentrations can vary significantly with displacement systems. Depending on the source of the particles this can be better or worse in terms of occupant exposure and engineers should take due diligence accordingly when designing ventilation systems.

'Optimal' vortex rings and aquatic propulsion mechanisms.

Fishes swim by flapping their tail and other fins. Other sea creatures, such as squid and salps, eject fluid intermittently as a jet. We discuss the fluid mechanics behind these propulsion mechanisms and show that these animals produce optimal vortex rings, which give the maximum thrust for a given energy input. We show that fishes optimize both their steady swimming efficiency and their ability to accelerate and turn by producing an individual optimal ring with each flap of the tail or fin. Salps produce vortex rings directly by ejecting a volume of fluid through a rear orifice, and these are also optimal. An important implication of this paper is that the repetition of vortex production is not necessary for an individual vortex to have the 'optimal' characteristics.

Modelling the global efficiency of dissolved air flotation.

The purpose of this paper is to examine how the efficiency of dissolved air flotation is affected by the size of bubbles and particles. The rise speed of bubble/particle agglomerates is modelled as a function of bubble and particle size, while the kinematics of the bubble attachment process is modelled using the population balance approach adopted by Matsui, Fukushi and Tambo. It is found that flotation, in general, is enhanced by the use of larger particles and larger bubbles. In particular, it is concluded that for the ultra-high surface loading rates of 25 m/hr or more planned for future flotation tanks, bubble size will have to be increased by a factor of two over the size currently employed in many facilities during dissolved air flotation.