Andy Chan

Strategic guidelines for street canyon geometry to achieve sustainable street air quality

This paper is concerned with the motion of air within the urban street canyon and is directed towards a deeper understanding of pollutant dispersion with respect to various simple canyon geometries and source positions. Taking into account the present days typical urban configurations, three principal flow regimes “isolated roughness flow”, “skimming flow” and “wake interference flow” (Boundary Layer Climates, 2nd edition, Methuen, London) and their corresponding pollutant dispersion characteristics are studied for various canopies aspect ratios, namely relative height (h2/h1), canyon height to width ratio (h/w) and canyon length to height ratio (l/h). A field-size canyon has been analyzed through numerical simulations using the standard k-e turbulence closure model. It is found that the pollutant transport and diffusion is strongly dependent upon the type of flow regime inside the canyon and exchange between canyon and the above roof air. Some rules of thumbs have been established to get urban canyon geometries for efficient dispersion of pollutants.

Indoor–outdoor relationships of particulate matter and nitrogen oxides under different outdoor meteorological conditions

Abstract Respirable suspended particulate matter and nitrogen oxides concentrations were measured inside and outside a student office in an urban location during a 9-month period. Direct reading tapered-element oscillating microbalance (TEOM) instruments and passive sampling techniques were used to provide indoor and outdoor hourly averages of the two pollutants during the sampling period. The variations and correlations of the pollutant concentrations and the indoor–outdoor (IO) ratio against various outdoor meteorological factors, namely temperature, humidity, pressure, wind speed and solar irradiation were studied. Using a data-mining procedure, suitable sets of data were amassed and grouped together to study the effect of these individual weather parameters on the IO ratio. It is found through statistical regression techniques that temperature, humidity and solar irradiation play a vital role in the variation of the IO ratio. In fact, the IO ratio shows convincing tendency of increase with increase of these three weather parameters. On the other hand, both pressure and wind speed seems to have relatively little effect on the IO ratio.

Strategic guidelines for street canyon geometry to achieve sustainable street air quality—part II: multiple canopies and canyons

Abstract The flow field and pollutant dispersion characteristics in a three-dimensional urban street canyon are investigated for various building array geometries. The street canyon in consideration is located in a multi-canopy building array that is similar to realistic estate situations. The pollutant dispersion characteristics are studied for various canopy aspect ratios, namely: the canyon height to width ratio, canyon length to height ratio, canyon breadth ratio and crossroad locations are studied. A three-dimensional field-size canyon has been analysed through numerical simulations using k − e turbulence model. As expected, the wind flow and mode of pollutant dispersion is strongly dependent on the various flow geometric configurations and that the results can be different from that of a single canyon system. For example, it is found that the pollutant retention value is minimum when the canyon height-to-width ratio is approximately 0.8, or that the building height ratio is 0.5. Various rules of thumbs on urban canyon geometry have been established for good pollutant dispersion.

Indoor–outdoor air quality relationships in vehicle: effect of driving environment and ventilation modes

Abstract Nitrogen oxides and carbon monoxide concentration were measured inside and outside of a light-goods-vehicle at different locations and driving conditions for a 6-month period. To investigate the exposure of the vehicle passenger to the specified outdoor pollutant, the indoor–outdoor air quality (IO) relationships under various driving conditions, namely traffic density, ventilation modes and type of roadway were studied. Four main types of driving environments were selected: highway, countryside, urban street and tunnel. The vehicle was driven under the three main types of ventilation conditions: air-conditioning with air-recirculation, air-conditioning with fresh air intake and natural ventilation. It is found that the IO ratio is not specific only to the mode of ventilation but also depends on the driving environment. The IO value can vary drastically even using the same ventilation mode when the vehicle is travelling in a different environment. It is found that using fresh-air ventilation mode, the IO can change from approximately 0.5–3 as it commutes from a highway to the countryside. The results also indicate that indoor NO level increased as the traffic density increases. The fluctuation of indoor NO level of naturally ventilated vehicle followed the variation of outdoor NO concentration with the IO value varying from 0.5 to 5. The results also show that even in an air-conditioned van, the indoor NO and CO concentration is significantly affected by that outdoor. It suggests the use of different ventilation mode when commuting in different environment.

Commuter exposure and indoor–outdoor relationships of carbon oxides in buses in Hong Kong

Abstract The exposure of bus commuters to carbon monoxide (CO) and carbon dioxide (CO 2 ) in various bus routes in Hong Kong has been studied. The buses under investigation travel in the city of Hong Kong and involve both air-conditioned and older buses without air-conditioning. The in- and out-vehicle concentrations of both pollutants were measured simultaneously using portable samplers. It is found that the exposure level of CO 2 inside the air-conditioned vehicle is strongly dependent on the number of passengers but not on the driving environment. During the measurement, it is found that the CO 2 level can reach up to a dangerous 10 times that out-vehicle when the air-conditioned bus is full. On the other hand, it was found that the CO and CO 2 levels for a non-air-conditioned bus remain low due to better air exchange between in- and out-vehicle.

Wave characteristics past a flexible fishnet

The scattering of surface waves by a flexible fishnet is studied analytically. The fishnet is modelled as a porous flexible barrier displaced solely by hydrodynamic force like a catenary. The objective is to investigate how a flexible permeable barrier affects the passing waves in the way they are transmitted and reflected, as observed by the fact that the water inside a fishfarm surrounded by fishnets is significantly calmer than that outside. The boundary value problems are solved by defining the reflection coefficient in terms of velocity potential and then the full solutions are obtained by suitable application of the eigenfunction expansion method and the least squares approximation method. The variations of the reflection coefficient, hydrodynamic pressure, barrier deformation and surface wave elevation are determined with respect to the barrier length, porosity and stiffness. It is observed that as the fishnet gets more flexible, its deformation increases and the reflection coefficient decreases, whereas as the fishnet gets more porous, more water can pass through it and thus the reflection coefficient, barrier deformation and the hydrodynamic force are reduced. The flexibility of the barrier behaves like its porosity by allowing more wave energy to act on it through its deformation and hence reduce the reflection and hydrodynamic force of the incident waves acting on the barrier.

Interaction of laminar far wake with a free surface

Wave disturbances caused by the uniform translatory motion of a submerged body on or beneath the free surface of a viscous fluid are investigated analytically. The submerged body is idealized as an Oseenlet or an Oseen doublet, and exact solutions in closed integral forms are obtained. Based on these exact solutions, asymptotic representations of the wave amplitude for large Reynolds numbers based on the deep‐water wavelength at large distances downstream of the body are derived. The results obtained show explicitly the effect of the laminar wake on the amplitude and the phase of the surface waves thus created.

Effect of vegetation and waterbody on the garden city concept: An evaluation study using a newly developed city, Putrajaya, Malaysia

The garden city concept was adopted in the development of a new tropical city, Putrajaya, aimed at mitigating the effect of urban thermal modification associated with urbanisation, such as urban heat island (UHI). WRF/Noah/UCM coupled system was used to estimate the urban environment over the area and the individual thermal contributions of natural land use classes (vegetation and waterbody). A control experiment including all land use types describing the urban conditions of Putrajaya city agreed well with the observations in the region. A series of experiments was then conducted, in which vegetation and waterbody were successively replaced with an urban land use type, providing the basis for an assessment of their respective effect on urban thermal mitigation. Surface energy components, 2-m air temperature (T2m) and mixing ratio (Q2m), relative humidity (RH) and UHI intensity (UHII) showed variations for each land use class. Overall, an increase in urban surfaces caused a corresponding increase in the thermal conditions of the city. Conversely, waterbody and vegetation induced a daily reduction of 0.14 and 0.39 °C of T2m, respectively. RH, UHI and T2m also showed variations with urban fractions. A thermal reduction effect of vegetation is visible during mornings and nights, while that of water is minimally shown during daytime. However, during nights and mornings, canopy layer thermal conditions above waterbody remain relatively high, with a rather undesirable effect on the surrounding microclimate, because of its high heat capacity and thermal inertia.

N-type thermoelectric recycled carbon fibre sheet with electrochemically deposited Bi2Te3

An N-type thermoelectric recycled carbon fibre sheet with bismuth telluride coating has been successfully synthesised through an electrodeposition technique. The Seebeck coefficient and electrical properties of the combined recycled carbon fibre sheet and bismuth telluride films are reported. Classification of the crystal structure, surface morphology and the elemental composition of the resulting deposits are methodically characterised by XRD, SEM and EDX correspondingly. Cyclic voltammetry is also carried out in nitric acid solutions to investigate the right range of deposition potential. An N-type thermoelectric sheet was successfully synthesised with the highest attainable Seebeck coefficient of −54 μV/k with an electrical resistivity of 8.9 × 10−5 ohm m. The results shows slight differences in morphologies and thermoelectric properties for the films deposited at varying deposition potential. The increase in thermoelectrical properties of the recycled carbon fibre is in line with the development of us...

The unsteady stokeslet and oseenlet

Abstract The unsteady low Reynolds number flow of an incompressible viscous fluid past a singular forcelet is investigated analytically. New fundamental three-dimensional solutions for a concentrated impulsive force are derived for the Stokes and the Oseen equations. These elementary solutions can be used as fundamental Greens functions to obtain solutions for flows over singularities with any time-dependent nature. The fundamental singularities are employed to construct some well-known solutions to demonstrate their validity and usefulness in solving unsteady problems governed by the Stokes and the Oseen equations. A new solution is presented for an unsteady Oseen flow with a constant acceleration.