Yunwei Zhang

Numerical Simulation and In Situ Investigation of Fine Particle Dispersion in an Actual Deep Street Canyon in Hong Kong

This paper reports a computational fluid dynamics simulation of airflow and fine particle (PM2.5) dispersion in the street canyon in Hong Kong, using large eddy simulation. An aspect ratio (AR) of 2.7 and a Reynolds number of 5 × 106 with a one main vortex, were used. This study focused mainly on the vehicle-induced momentum source and PM2.5 concentrations were measured at 10 altitudes near the leeward wall in the street canyon, to provide high resolution measurements for model validation. The simulated PM2.5 concentrations agreed well with the measurements, (R = 0.85). The concentration was higher at the lower part near the leeward wall than the upper part. Higher concentration was found near the roof level. A near-uniform vertical dispersion of PM 2.5 near the windward wall was demonstrated; and the average concentrations were lower than found near the leeward wall. The intermittent escape of the PM2.5 above the canyon at the roof level occurred mainly at the centre and near windward wall areas. The results demonstrated that a reduction in the AR can be conducive to pollutant dispersion in street canyon planning. The findings of this research would inform building designers to formulate effective strategies such as positioning of ventilation air intake, for the control of ingress of PM2.5 into building environments.

Numerical simulation of the cone–jet formation and current generation in electrostatic spray—modeling as regards space charged droplet effect

A physical model of the electric field induced by charged droplets taking account of the effect of space charged droplet emitted from the tip of cone–jet to the external electric field is proposed. Combining this model with the fluid flow equations and charge conservation equation, the evolution of the cone–jet is simulated. The diameter of droplets emitted from the cone–jet tip and current on cone–jet are predicted at various applied voltages and flow rates. The calculated droplet diameter agrees well with experimental measurement. For low conductivity liquid, the droplet diameter decreases with the increment of applied voltage, but decreases with the reduction of flow rate. The simulation result also indicates that the current on the cone–jet increases linearly with the applied voltage. The electric field induced by charged droplets results in the decrease of the cone angle and the presence of space charged droplets has a non-negligible effect on the operation parameters.

Time-Series Numerical Simulation of Wind Flow Within Urban Canopy Layer and Its Integration Effect for Prediction of Pollutant Concentration Variation

Source analysis shows that motor vehicle exhausts are the major pollutant emission sources in urban atmosphere. Street canyons are areas where vehicle exhausts are most frequently discharged into and where urban residents could be subjected to exposure of likely high concentrations of these emissions. The spread of pollutants in urban street canyons would affect the neighbourhood environments and indoor air qualities [1]. The numerical simulation and in situ investigation of fine particle dispersion in an actual deep street canyon in Hong Kong has been reported [2]. Motor vehicle emissions in urban street canyons are thus serious threats to the health of pedestrians [3]. The link between certain respiratory health issues, such as the increase in asthma cases and mortality, and traffic-related pollutants, such as suspended particulate matters (PMs), nitrogen dioxide (NO2) and ozone (O3), have been established by epidemiological and toxicological studies [4,5]. The exposure and risk assessments are of significance for urban residents’ health, especially for elderly and children. Thus, the awareness of diurnal variations of pollutant concentrations are urgently needed for real risk assessments [6], in which the urban air pollution monitoring and modelling are important tools. Nowadays, the numerical modelling method is usually favoured by researchers due to its convenience and speed in forecasting and assessment on dispersion of pollutants and variations. Although the numerical model scales are relatively small, the modelling of atmospheric environment in urban canopy layer is complex and difficult since air flow and pollutant dispersion in street canyons are related to weather conditions, atmospheric turbulence and building layout and building ventilation. With regard to the street planning, the uneven building layout along street and the height difference between the opposite buildings in street can induce the amplifications of wind strength inside urban street canyons and the macro-scale convections of air mass inside/outside street canyons, which sufficiently improve the pollutants dispersion efficiency and reduce the street level pollutant concentrations [7]. The diurnal variations of pollutant concentrations within the urban canopy layer are affected by the urban background pollution concentration and emissions together [8]. The urban background pollutant concentration variations can be attributed to the atmospheric boundary layer movements that are induced by regional climate and local meteorological process. The urban background pollutant concentrations reduce with the daytime air mass expansion in atmospheric boundary layer and increase with the nighttime air mass contraction in atmospheric boundary layer. On the other hand, effects

The nature of a universal subgrid eddy viscosity model in a turbulent channel flow

Large-eddy simulation, which is a widely used method of computational fluid dynamics, can accurately simulate wall-bounded turbulent flows due to near-wall treatment. Through analyzing the similarities of the eddy viscosity formulations for large-eddy simulation and Reynolds-averaged Navier-Stokes, we propose a universal subgrid eddy viscosity model (USM) for large-eddy simulation of turbulent flows. The subgrid eddy viscosity in the model is related not only to the norm of the strain rate tensor of the smallest resolved scales but also to the mixing length associated with the subgrid scale, while the subgrid tensor is associated with the strain rate tensor of large scales like in the classical Smagorinsky model. With the friction-velocity–based Reynolds number, 395, the channel flow simulations by USM show that this subgrid eddy viscosity model can physically illustrate the eddy viscosity in the near-wall region and directly simulate the wall-bounded turbulent flow, compared with the classical Smagorinsky model, the dynamic Smagorinsky model, and Moser et al.s direct numerical simulations (DNS).

Particle Counts and Size Distributions in the Roadside Environment

Continuous measurements of number concentrations and size distributions of particles with diameters of 7–225 nm and 100–2,000 nm were conducted, using a TSI scanning mobility particle sizer (SMPS) and an optical particle counter (OPC), respectively, in eight weekdays of 2005 at a roadside environment in Hong Kong. PM2.5 and PM10 mass concentrations, black carbon (BC), particle-bound polycyclic aromatic hydrocarbons (p-PAHs), traffic counts and meteorological parameters were simultaneously monitored every hour. On average, the total particle number concentrations were 50,235 ± 27,076 cm−3 in the SMPS size range and 5,771 ± 1,793 cm−3 in the OPC size range in the winter sampling days, exceeding the summer particle number concentrations by a factor of 1.5 and 2.5, respectively. The ultrafine mode particles accounted for ∼90% of the particles in the SMPS size range. Daily cycle of particle numbers in the SMPS size range were obvious, with high values during daytime and low values during nigh time, which is consistent with the traffic pattern, especially the diesel-fuelled vehicles (r = 0.72). Moreover, these particles were found to have good correlations with BC (r = 0.84) and p-PAHs (r = 0.90). The findings have suggested that diesel-fuelled vehicle exhausts are the dominant source for ultrafine particles at roadside environments of Hong Kong.

Surface charges on aerosol particles – Accelerating particle growth rate and atmospheric pollution

Many Chinese mega-cities frequently experienced severe and persistent haze pollution in recent years. Especially, the so-called ‘explosive growth’ of PM2.5 concentration has often been observed at the beginning of a severe pollution event in winter. As shown in Figure 1(a), the explosive growth of PM2.5 concentration could be increased from below 50 mg/m up to 250 mg/m in only 5–10 h. The explosive growth of PM2.5 concentration in cities can have a strong impact on people’s life and health. In contrast, in Figure 1(b), during 1–5 November 2013 in winter, the PM2.5 mass concentration grew accumulatively in Xi’an, increasing the concentration from 23 to 137 mg/m. Meanwhile, the speed of coagulation of new particles in these PM2.5 concentrations during the explosive growth event is evidently far more than the speed of normal accumulation of these PM2.5 concentrations as shown in Figure 1(b) and with long duration. Although many scholars have put forward ideas to analyse the pollution of high PM2.5 concentration from various directions, the details and theory of particulate explosive growth process are still not clearly defined and elucidated. Secondary reactions may have contributed to the increment growth of particle number concentration, but the high speed of chemical reaction is confusing the possible theory. Actually, at the beginning of a severe haze, the process of explosive growth of PM2.5 concentration could be undergoing a series of physical–chemical processes, in which the charging particles have a significant influence. The charges and ions on particle surfaces could affect the collision and coagulation behaviour of particles, and also the subsequent chemical reactions occurring on the particle porous surfaces. Aerosols in any form are usually electrically charged, no matter in solid, air or liquid (including cloud aerosol, fog aerosol and smog aerosol), either in natural or artificial form. There are many natural or artificial factors that could charge the aerosol particles, such as cosmic rays in space, radiation from radioactive material in air and on earth, atmospheric lightning, electromagnetic radiation, high temperature discharge and static electricity caused by particles collision. On the other hand, because fine particles are porous media with rough surfaces, changeable shapes and uneven energy distribution on surfaces, the distribution of ions and electrical charges on particle surfaces must be uneven, and the energy would gather at the location with small curvature. Thus, the high potential and electrical charges could easily aggregate at the cusp. When a fine particle is separated from a larger particle, a static charge transfer would arise giving two charged particles. Particles with different diameters would become charged after their collision and then separate from each other. When aerosol particles are carrying electrical charges, particle interactions and surface chemical reactions could be affected by these surface charges, thus changing some physical characteristics of particles, such as cohesion, adhesion and its stability in the atmosphere. The electron dynamics of these charged particles could also have an adverse effect on human’s health. The recent research suggested that during the process of explosive growth, the reverse growth of volume concentration of aerosol may be influenced by fluctuation of air moisture under stable atmospheric conditions, which means that in a very short period of time, under the condition of stable or declining number concentration of aerosol particles, adsorbing moisture could increase the diameter of aerosols. In fact, water molecules adsorbed on the surface of particles in atmospheric environment could play a very complicated role in the process of particles collision. If thin water film is present on the surfaces of suspended aerosol particles, a ‘water bridge’ could be formed at the contact interface when particles are moving and colliding in the air, by which hydrogen ions would be

Revisit of Wind Flows between Wind Tunnels and Real Canyons – The Viewpoint of Reynolds Dynamic Similarity

Wind tunnel test and numerical simulation are two powerful methods to study air flows and pollutant dispersions around urban buildings environment. As commonly known, the development of a successful numerical model should be firstly validated by experimental results, usually by wind tunnel data [1,2], a numerical model is ultimately needed to simulate the air flows and pollutant dispersions in real street canyons [3]. It is obvious that wind tunnel models and real canyons are of different scales. The scaling effects for wind tunnel tests have already been investigated on some simple models, such as single building and normal street canyon with the aspect ratio near 1.0 (AR, denoted by building height, H, over road width, W) [4,5]. But the scaling effects on the flows around complex buildings or in real street canyons still need to be examined. In wind tunnel test, air is often used as the experimental fluid. The air velocities in the wind tunnels are several metres per second, rarely more than 20m s 1 [5,6], which are similar to the velocities in the real canyon environment. But wood blocks with height of several centimetres, subject to the experimental conditions, are adopted for the street canyon models, following that the dimensions of wood blocks are about two orders of magnitudes smaller than the real buildings. Thus, the wind tunnel experiments only satisfy the geometric similarity, but miss the Reynolds dynamic similarity in nature. On the other hand, the numerical simulations of turbulent flows with high Reynolds numbers often demand large computing resources, so that the downscaling model is always used to simulate the air flows and pollutant dispersions in urban street canyons. In such cases, building models are set in several centimetres and incoming wind velocities are given similar to the real background wind velocities; this treatment is the same as the wind tunnel test [7,8]. This means that the downscaling simulations would just satisfy the geometric similarity, but miss the Reynolds dynamic similarity. The downscaling modelling was based on the Townsend’s ‘‘Reynolds number similarity’’ hypothesis [9] and determined by the critical Reynolds numbers [1,10]. The Townsend’s hypothesis is that, in the absence of thermal and Coriolis effects and for a specified flow system whose boundary conditions are expressed non-dimensionally in terms of a characteristic length, L, and velocity, UR, the flow structure is similar with all sufficiently high Reynolds numbers. Based on the ‘‘Reynolds number similarity’’ hypothesis, the critical Reynolds numbers are only determined by the representation of minor changes in flow structures or wind profiles, in which actually the wind

Advances in the Fine Scale Simulation of Urban Wind Environment

This work gives a review of studies on the air flow and pollutant dispersion in the fine scale environment of the urban street canyon. The affecting factors on the air flow and pollutant dispersions in urban street canyons are illustrated by analyzing the street canyon physical model and boundary conditions. The building layouts along the street and the variations in the ambient wind velocities and directions could contribute an impact on the basic air flow within urban street canyons, while the atmospheric instabilities inside the street canyons, the vehicle induced turbulence, and tree plantings could affect the turbulence and pollutant distributions inside the street canyons, especially under weak-wind conditions. In the case of the street canyons with high aspect ratio (building height H over street width W), Reynolds number would be related to the vortex structure. The current empirical or semi-empirical models are lacking of careful description of the street canyon’s real geometries and atmospheric conditions, which had often resulted in large errors in the simulated or forecasted results. Based on the computational fluid dynamic (CFD) method, further development of numerical models should focus on the integrated model, taking account of canyon’s real geometries and the real atmospheric conditions.

Study on the Progress of Ecological Fragility Assessment in China

The basic elements of human survival are based on the ecological environment. The development of social economic and the security of the ecological environment are closely linked and interact with each other. The fragility of the environment directly affects the stability of the regional ecosystem and the sustainable development of the ecological environment. As part of the division of the national ecological security, the assessment of ecological fragility has become a hot and difficult issue in environmental research, and researchers at home and abroad have systematically studied the causes and states of ecological fragility. The assessment of regional ecological fragility is a qualitative and quantitative analysis of the unbalanced distribution of ecological environment factors caused by human socio-economic activities or changes in ecosystems. At present, researches on ecological fragility has not formed a complete and unified index assessment system, and the unity of the assessment model has a direct impact on the accuracy of the index weights. Therefore, the discussion on selection of ecological fragility indexes and the improvement of ecological fragility assessment model is necessary, which is good for the improvement of ecological fragility assessment system in China.

Review of Land Use and Land Cover Change research progress

Land Use and Land Cover Change (LUCC) can reflect the pattern of human land use in a region, and plays an important role in space soil and water conservation. The study on the change of land use patterns in the world is of great significance to cope with global climate change and sustainable development. This paper reviews the main research progress of LUCC at home and abroad, and suggests that land use change has been shifted from land use planning and management to land use change impact and driving factors. The development of remote sensing technology provides the basis and data for LUCC with dynamic monitoring and quantitative analysis. However, there is no uniform standard for land use classification at present, which brings a lot of inconvenience to the collection and analysis of land cover data. Globeland30 is an important milestone contribution to the study of international LUCC system. More attention should be paid to the accuracy and results contrasting test of land use classification obtained by remote sensing technology.