Large eddy simulation of fire-induced buoyancy driven plume dispersion in an urban street canyon under perpendicular wind flow

Investigation of the Wind-Direction Effect on Buoyancy-Driven Fire Smoke Dispersion in an Urban Street Canyon

When a fire occurs in a street canyon, smoke recirculation is the most harmful factor to human beings inside the canyon, while the wind condition is an essential factor determining if the smoke is recirculated. This paper focuses on the wind direction's effect on buoyancy-driven fire smoke dispersion in a street canyon, which is innovative research since the effect of wind direction has not been reported before. In this study, an ideal street canyon model with a height-width ratio of 1 was established, and both the wind velocity and wind direction were changed to search for the critical point at which smoke recirculation occurs. The results show that with an increase in the wind direction angle (the angle of wind towards the direction of the street width), the smoke recirculation could be distinguished into three regimes, i.e., the "fully re-circulation stage", the "semi re-circulation stage", and the "non-recirculation stage". The critical recirculation velocity was increased with the increase in the wind direction angle, and new models regarding the critical wind velocity and the Froude number were proposed for different wind direction conditions.

Numerical Simulation of the Smoke Recirculation Behavior in Street Canyons with Different Aspect Ratios and Cross-Wind Conditions

This study investigated smoke dispersion inside a street canyon in a series of numerical simulations. The building height and street width as well as the cross-wind velocity were changed during the simulation, and the smoke recirculation behavior inside the canyon is presented and discussed. The results show that the smoke recirculation behavior could be distinguished into two different stages, i.e., the “fully recirculation stage” and “semi recirculation stage”, which is strongly determined by the canyon aspect ratio (the building height divided by street width). It was found that the critical wind velocity at which the smoke recirculation would take place was almost constant for an ideal street canyon with an aspect ratio of 1; however, this velocity was decreased with increasing building height or decreasing street width, indicating a much more dangerous circumstance when the aspect ratio is greater. Finally, a new piecewise function is proposed for the critical smoke recirculation velocity for all cases, which can provide some theoretical basis for building designs and emergency rescue for human beings inside the street canyon.

Large eddy simulation of dispersion of hazardous materials released from a fire accident around a cubical building

The turbulent smoke dispersion from a pool fire around a cubical building is studied using large eddy simulation at a high Reynolds number, corresponding to existing experimental measurements both in laboratory and field test scales. Emphasis of this work is on the smoke dispersion due to two different fuel pool fire accident scenarios, initiated behind the building. For the setup of fire in the first case, crude oil was used with a heat release rate of 7.8 MW, and in the second, diesel oil with a heat release rate of 13.5 MW. It is found that in both fire scenarios, the downstream extent of the toxic zone is approximately the same. This is explained in terms of the fact that the smoke concentration and dispersion are influenced mainly by the convective buoyant forces and the strong turbulence mixing processes within the wake zone of the building. It is suggested that wind is the dominating factor in these accident scenarios, which represent the conditions resulting in the highest toxicity levels.

Numerical study of critical re-entrainment velocity of fire smoke within the street canyons with different building height ratios

Traffic accident may bring vehicle fire in the street canyons. With its high temperature and numerous hazardous materials, the smoke produced by the vehicle fire may cause serious damage to the human body and the properties nearby, such as the glass curtain walls of buildings. The influence of the ambient air flow speed and street aspect ratio on the dispersion of fire smoke in street canyon has been analyzed by FDS software and theoretical analysis in this study. The impact of different windward building heights and different ambient air flow speeds u₀ on the fire smoke were investigated. The results show that the fire smoke tilts towards the opposing direction of the ambient air flow within the street canyon, while the ambient air flow is perpendicular to the windward building. The results indicate that the critical re-entrainment velocity decreases at first, and then increases until it attains a constant with the building height ratio H₁/H₂. Finally, a predictive model of the critical re-entrainment velocity was developed under different building height ratios H₁/H₂.

A new approach for inferring traffic-related air pollution: Use of radar-calibrated crowd-sourced traffic data

BACKGROUND

Crowd-sourced traffic data potentially allow prediction of traffic-related air pollution at high temporal and spatial resolution.

OBJECTIVES

To examine associations (1) of radar-based traffic measurements with congestion colors displayed on crowd-sourced traffic data maps and (2) of black carbon (BC) levels with radar and crowd-sourced traffic data.

METHODS

At an off-ramp of an interstate and a small one-way street in a mixed-use area in New York City, we used radar devices to obtain vehicle speeds and flows (hourly counts) for cars and trucks. At these radar sites and at an additional non-radar equipped site at a 2-way street, we monitored BC levels using aethalometers in the summer and early fall of 2017. At all three sites, free-flow traffic conditions typically did not occur due to the nearby presence of traffic lights and forced turns. We also downloaded real-time traffic maps from a crowd-sourced traffic data provider and assigned an ordinal integer congestion color code CCC to the congestion colors, ranging from 1 (dark red) to 5 (gray).

RESULTS

CCC increased with vehicle speed. Traffic flow was highest for intermediate speeds and intermediate CCC. Regression analyses showed that BC levels increased with either segregated or total vehicle flows. At the off-ramp, time-dependent BC levels can be inferred from time-dependent CCC and radar-derived mean vehicle flow data. A unit decrease in CCC for a mean traffic flow of 100 vehicles/h was associated with a mean (95% CI) increase in BC levels of 0.023 (0.028, 0.018) μg/m³. At the small 1-way and the 2-way street, BC levels were also negatively associated with CCC, though at a >0.05 significance level.

CONCLUSIONS

Use of inexpensive crowd-sourced traffic data holds great promise in air pollution modeling and health studies. Time-dependent traffic-related primary air pollution levels may be inferred from radar-calibrated crowd-sourced traffic data, in our case radar-derived mean traffic flow and widely available CCC data. However, at some locations mean traffic flow data may already be available.

Buoyant wind-driven pollutant dispersion and recirculation behaviour in wedge-shaped roof urban street canyons

The present study investigated the buoyant wind-driven pollutant plume dispersion and recirculation behaviour inside urban street canyons formed by buildings with wedge-shaped roofs. Numerical modelling was performed using a computational fluid dynamics (CFD) large eddy simulation (LES). Street canyon models with a strongly buoyant fire source located on the street and environmental winds perpendicular to the canyon were developed using the fire dynamics simulator (FDS). The complex interaction of buoyancy and wind, as well as their combined effects on the pollutant plume dispersion, was simulated inside the urban street canyon. The results showed that the flow pattern of pollutant plume dispersion inside the street canyon with increasing wind speed for different roof inclination angles could be divided into three regimes, including a recirculation regime, a quasi-recirculation regime and a non-recirculation regime. The pollutant levels in the street canyon, as indexed by carbon monoxide (CO) concentration, increased under the recirculation regime. For the quasi-recirculation regime, however, the leeward buildings primarily suffered from the higher pollutant levels. The critical wind speed needed to trigger recirculation was analysed for various roof inclination angles. A correlation was proposed to predict the critical wind speed of various wedge-shaped roof angles for recirculation regime and quasi-recirculation regimes.

Computational Assessment of the Hazardous Release Dispersion from a Diesel Pool Fire in a Complex Building’s Area

A hazardous release accident taking place within the complex morphology of an urban setting could cause grave damage both to the population’s safety and to the environment. An unpredicted accident constitutes a complicated physical phenomenon with unanticipated outcomes. This is because, in the event of an unforeseen accident, the dispersion of the hazardous materials exhausted in the environment is determined by unstable parameters such as the wind flow and the complex turbulent diffusion around urban blocks of buildings. Our case study focused on a diesel pool fire accident that occured between an array of nine cubical buildings. The accident was studied with a Large eddy Simulation model based on the Fire Dynamics Simulation method. This model was successfully compared against the nine cubes of the Silsoe experiment. The model’s results were used for the determination of the immediately dangerous to life or health smoke zones of the accident. It was found that the urban geometry defined the hazardous gasses dispersion, thus increasing the toxic mass concentration around the buildings.

A theory of ventilation estimate over hypothetical urban areas

Urban roughness is a major factor governing the flows and scalar transport in the atmospheric boundary layer (ABL) but our understanding is rather limited. The ventilation and pollutant removal of hypothetical urban areas consisting of various types of street canyons are examined using computational fluid dynamics (CFD). The aerodynamic resistance, ventilation efficiency, and pollutant removal are measured by the friction factor f, air exchange rate (ACH), and pollutant exchange rate (PCH), respectively. Two source configurations of passive tracer, ground-level-only (Tracer 0) and all-solid-boundary (Tracer 1) are employed to contrast their transport behavior. It is found that the ventilation and pollutant removal are largely attributed to their turbulent components (over 60%). Moreover, with a consistent support from analytical solution and CFD results, the turbulent ACH is a linear function of the square root of the friction factor (ACH'∝f(1/2)) regardless of building geometry. Tracer 0 and Tracer 1 exhibit diversified removal behavior as functions of friction factor so analytical parameterizations have not yet been developed. In view of the large portion of aged air removal by turbulence, it is proposed that the aerodynamic resistance can serve as an estimate to the minimum ventilation efficiency of urban areas.

Modelling the propagation of smoke from a tanker fire in a built-up area

The paper presents the application of a Lagrangian particle model to problems connected with safety in road transport. Numerical simulations were performed for a hypothetical case of smoke emission from a tanker fire in a built-up area. Propagation of smoke was analysed for three wind directions. A diagnostic model was used to determine the air velocity field, whereas the dispersion of pollutants was analysed by means of a Lagrangian particle model (Brzozowska, 2013). The Idrisi Andes geographic information system was used to provide data on landforms and on their aerodynamic roughness. The presented results of computations and their analysis exemplify a possible application of the Lagrangian particle model: evaluation of mean (averaged over time) concentrations of pollutants and their distribution in the considered area (especially important due to the protection of people, animals and plants) and simulation of the propagation of harmful compounds in time as well as performing computations for cases of the potential effects of road incidents.

Simulation of wind-driven dispersion of fire pollutants in a street canyon using FDS

Air quality in urban areas attracts great attention due to increasing pollutant emissions and their negative effects on human health and environment. Numerous studies, such as those by Mouilleau and Champassith (J Loss Prevent Proc 22(3): 316-323, 2009), Xie et al. (J Hydrodyn 21(1): 108-117, 2009), and Yassin (Environ Sci Pollut Res 20(6): 3975-3988, 2013) focus on the air pollutant dispersion with no buoyancy effect or weak buoyancy effect. A few studies, such as those by Hu et al. (J Hazard Mater 166(1): 394-406, 2009; J Hazard Mater 192(3): 940-948, 2011; J Civ Eng Manag (2013)) focus on the fire-induced dispersion of pollutants with heat buoyancy release rate in the range from 0.5 to 20 MW. However, the air pollution source might very often be concentrated and intensive, as a consequence of the hazardous materials fire. Namely, transportation of fuel through urban areas occurs regularly, because it is often impossible to find alternative supply routes. It is accompanied with the risk of fire accident occurrences. Accident prevention strategies require analysis of the worst scenarios in which fire products jeopardize the exposed population and environment. The aim of this article is to analyze the impact of wind flow on air pollution and human vulnerability to fire products in a street canyon. For simulation of the gasoline tanker truck fire as a result of a multivehicle accident, computational fluid dynamics large eddy simulation method has been used. Numerical results show that the fire products flow vertically upward, without touching the walls of the buildings in the absence of wind. However, when the wind velocity reaches the critical value, the products touch the walls of the buildings on both sides of the street canyon. The concentrations of carbon monoxide and soot decrease, whereas carbon dioxide concentration increases with the rise of height above the street canyon ground level. The longitudinal concentration of the pollutants inside the street increases with the rise of the wind velocity at the roof level of the street canyon.

The passive control of air pollution exposure in Dublin, Ireland: a combined measurement and modelling case study

This study investigates the potential real world application of passive control systems to reduce personal pollutant exposure in an urban street canyon in Dublin, Ireland. The implementation of parked cars and/or low boundary walls as a passive control system has been shown to minimise personal exposure to pollutants on footpaths in previous investigations. However, previous research has been limited to generic numerical modelling studies. This study combines real-time traffic data, meteorological conditions and pollution concentrations, in a real world urban street canyon before and after the implementation of a passive control system. Using a combination of field measurements and numerical modelling this study assessed the potential impact of passive controls on personal exposure to nitric oxide (NO) concentrations in the street canyon in winter conditions. A calibrated numerical model of the urban street canyon was developed, taking into account the variability in traffic and meteorological conditions. The modelling system combined the computational fluid dynamic (CFD) simulations and a semi-empirical equation, and demonstrated a good agreement with measured field data collected in the street canyon. The results indicated that lane distribution, fleet composition and vehicular turbulence all affected pollutant dispersion, in addition to the canyon geometry and local meteorological conditions. The introduction of passive controls displayed mixed results for improvements in air quality on the footpaths for different wind and traffic conditions. Parked cars demonstrated the most comprehensive passive control system with average improvements in air quality of up to 15% on the footpaths. This study highlights the potential of passive controls in a real street canyon to increase dispersion and improve air quality at street level.

Large eddy simulation of pollutant gas dispersion with buoyancy ejected from building into an urban street canyon

The dispersion of buoyancy driven smoke soot and carbon monoxide (CO) gas, which was ejected out from side building into an urban street canyon with aspect ratio of 1 was investigated by large eddy simulation (LES) under a perpendicular wind flow. Strong buoyancy effect, which has not been revealed before, on such pollution dispersion in the street canyon was studied. The buoyancy release rate was 5 MW. The wind speed concerned ranged from 1 to 7.5m/s. The characteristics of flow pattern, distribution of smoke soot and temperature, CO concentration were revealed by the LES simulation. Dimensionless Froude number (Fr) was firstly introduced here to characterize the pollutant dispersion with buoyancy effect counteracting the wind. It was found that the flow pattern can be well categorized into three regimes. A regular characteristic large vortex was shown for the CO concentration contour when the wind velocity was higher than the critical re-entrainment value. A new formula was theoretically developed to show quantitatively that the critical re-entrainment wind velocities, u(c), for buoyancy source at different floors, were proportional to -1/3 power of the characteristic height. LES simulation results agreed well with theoretical analysis. The critical Froude number was found to be constant of 0.7.

Simulation and assessment of SO2 toxic environment after ignition of uncontrolled sour gas flow of well blowout in hills

To study the sulfur dioxide (SO(2)) toxic environment after the ignition of uncontrolled sour gas flow of well blowout, we propose an integrated model to simulate the accident scenario and assess the consequences of SO(2) poisoning. The accident simulation is carried out based on computational fluid dynamics (CFD), which is composed of well blowout dynamics, combustion of sour gas, and products dispersion. Furthermore, detailed complex terrains are built and boundary layer flows are simulated according to Pasquill stability classes. Then based on the estimated exposure dose derived from the toxic dose-response relationship, quantitative assessment is carried out by using equivalent emergency response planning guideline (ERPG) concentration. In this case study, the contaminated areas are graded into three levels, and the areas, maximal influence distances, and main trajectories are predicted. We show that wind drives the contamination and its distribution to spread downwind, and terrains change the distribution shape through spatial aggregation and obstacles. As a result, the most dangerous regions are the downwind areas, the foot of the slopes, and depression areas such as valleys. These cause unfavorable influences on emergency response for accident control and public evacuation. In addition, the effectiveness of controlling the number of deaths by employing ignition is verified in theory. Based on the assessment results, we propose some suggestions for risk assessment, emergency response and accident decision making.