effects of elevated metro rail tracks on pollutant

International Journal of Applied Environmental Sciences

ISSN 0973-6077 Volume 11, Number 2 (2016), pp. 425-439

© Research India Publications

http://www.ripublication.com

Effects of Elevated Metro Rail Tracks on Pollutant

Dispersion in Urban Street Canyons

Abhishek Pratap Singh

Research Scholar, Department of Mechanical Engineering, The NorthCap University, Gurgaon

Correspondence Author

Pramod Bhatia

Associate Professor, Department of Mechanical Engineering, The NorthCap University, Gurgaon, Haryana, India.

K K Chaudhary

Professor (retired), IIT Delhi, India.

Abstract

Objectives of this study are to analyze the effects of elevated metro rail tracks

(EMRT) on pollutant dispersion phenomena in an urban street canyon. Street

canyon of aspect ratio 1 (regular street canyon) and aspect ratio 0.5 (wide

street canyon) were simulated using two dimensional geometry. Wind flow

has been simulated using k-Ɛ turbulence model. Pollutant dispersion

simulation using Standard, RNG and Realizable k-Ɛ turbulence models were

also compared. CFD technique has been used to simulate the wind flow and

pollutant dispersion modeling. ANSYS FLUENT software were used to

simulate the street canyon cases. Two dimensional geometrical models have

been taken. Transport species model has been used to simulate air-acetylene

mixture dispersion as pollutants. Results of velocity vectors, contours of

pollutant concentration and pollutant concentration plots have been discussed

and compared. Simulation results of different turbulence models were also

compared. Results show that by introduction of EMRT in street canyon of

aspect ratio 1, pollutant concentrations have been increased on both leeward

and windward sides. There were no such significant changes in dispersion

phenomena and pollutant concentrations on street canyon of aspect ratio 0.5

when a cross section of EMRT was introduced. Standard k-Ɛ turbulence

model has predicted similar results to the previous experimental results.

Keywords: Street canyon; pollutant concentration; k-Ɛ turbulence model; air

pollution modeling.

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426 Abhishek Pratap Singh et al

1. Introduction A quality transport system is one of the current requirements of highly populated

cities of the world. Metro trains are one of the solutions to develop an effective and

comfortable transportation system. Elevated Metro Rail Tracks (EMRT) are

constructed to operate metro trains. These tracks mostly constructed along the urban

street canyons. This further changes the geometrical configuration of the urban street

canyon. The geometrical configurations are one of the major parameters that affect

pollutant dispersion in street canyons. Comfortable life style, opportunities in jobs,

and quality of education & medical facilities are the major attractions to shift

population from rural to urban areas. This further increase pressures on development

authorities of the cities to provide residential, commercial, and transport

infrastructures. Population growth is also responsible in increasing the number of

vehicles which leads to high emission of pollutants. Vehicle emission represents the

main group of pollutants in urban areas [1]. High pollution levels have been often

observed in urban street canyons due to the increased traffic emission and reduced

natural ventilations [2]. Research area of investigation in this field is to find out

dispersion phenomena of pollutants in street canyons under different conditions.

There are number of parameters which affect dispersion phenomena i.e. wind

orientation, building geometrical configuration, pollutant emission rate, traffic

volume, and road obstacles. Building geometrical configuration plays major role in

dispersion phenomena. Some of the geometrical parameters are, building aspect ratio

(average building height to road width ratio), building configuration, elevated road,

uneven building layouts, roof designs, etc. The street canyon configuration has

significant influence on the pollutant dispersion [3]. Numbers of studies have reported

the influence of street canyon’s geometrical parameters on pollutant dispersion

phenomena [1, 3-10]. Effects on pollutant dispersion in street canyon due to inside

obstacles such as traffic volume and tree planting have also been discussed in

previous studies [10, 11-14]. The Navier stokes and transport species equations

govern the dispersion mechanism of pollutant in street canyon. Mathematical model

calculate pollutant concentration by solving analytically simplified parametric

equations [2]. CFD technique is the best tool to predict the fluid flow and mixing

behavior of pollutants. The major advantages of CFD techniques are low cost

analysis, time saving, and reliability in accuracy of results. In the present study two

dimensional fluid domains have been analyzed for numerical simulation. Numbers of

studies have reported the analysis of 2 dimensional street canyon fluid domains [3, 10,

15-17]. Geometrical fluid models have been modeled using geometry tool of ANSYS

Workbench. Mesh has been generated using meshing tool of ANSYS Workbench.

Appropriate boundary types have been defined. Appropriate physics of the analysis

have been applied on ANSYS FLUENT tool for simulation of different cases. Results

of velocity vectors, pollutant contours, and pollutant concentration profile have been

discussed.

Effects of Elevated Metro Rail Tracks on Pollutant Dispersion 427

2. Numerical Setup and Solution 2.1. Numerical Cases

The two dimensional geometrical analysis has been done in the present study. Four

geometrical cases have been considered. First two cases have been considered on the

basis of aspect ratio of street canyon. Two aspect ratios, namely, 1 and 0.5 have been

used. For each aspect ratio, two geometrical conditions have been introduced. First,

street canyon without elevated metro rail track (EMRT) and second, street canyon

with elevated metro rail track. CFD simulations of pollutant dispersion have been

analyzed using k-Ɛ Turbulent Model and species transport model. All three sub

models of k-Ɛ Turbulent Model i.e. Standard, RNG and Realizable have been used in

all geometrical cases. ANSYS 12.1, FLUENT software has been used for simulations.

All the cases have been listed in Table 1 below.

Table 1. List of numerical cases

Two Dimensional

Street Canyon

Street Canyon of

Aspect Ratio-1

Street canyon

without EMRT

Standard K-Ɛ turbulence

model

RNG K-Ɛ turbulence

model

Realizable K-Ɛ turbulence

model

Street canon

with EMRT


model

RNG K-Ɛ turbulence

model


model

Street Canyon of

Aspect Ratio – 0.5

Street canyon

without EMRT


model

RNG K-Ɛ turbulence

model


model

Street canon

with EMRT


model

RNG K-Ɛ turbulence

model


model

2.2. Geometrical modeling

In the present study, two dimensional geometrical fluid domains have been created

using ANSYS Workbench. This has been considered due to symmetrical condition of


the geometry of 3-D fluid domain. Horizontal orientations, left to right of the

geometrical models have been kept along the wind flow. Two dimensional planes

have been considered as fluid domain. Geometrical scaling of 150:1 has been

considered between original street canyon and CFD modeling geometrical

dimensions. This has been considered for experimental validations. Geometrical

models of street canyon without EMRT have been considered for pollutant dispersion

study on CFD tool for experimental validation. Therefore, a cross section of EMRT

has been introduced in all the cases. Length and height of the fluid domain have been

taken as 6 m and 1 m respectively. The lengths of 1.37 m and 4 m have been taken for

leeward and windward side respectively from the street canyon building. Width and

height of the building were equal and kept 0.21 m. Width of the street canyon was

equal to the height of the building for aspect ratio 1 and twice the building height for

aspect ratio 0.5. Line source for pollutant inlets have been created along the street

canyon at the midsection of the bottom. Two inlets of pollutant dispersion have been

created for uniform distribution of emission rate on both side of the street canyon

cross section. The cross section of EMRT has been introduced in the centre of the

street canyon. Figure 1 shows the 3-D fluid domain with inlet and outlet boundaries.

The midsection on z axis of the shown figure 1, has been considered as 2-D fluid

domain case. The cross sections at the street canyon have been shown in figure 2 and

figure 3.

Figure 1: 3-dimenssional geometrical model of building and fluid domain

Figure 2: 2-D geometrical model of street canyons without EMRT


Figure 3: 2-D geometrical model of street canyons with EMRT

2.3. Mesh Generation

Differential volumetric analysis needs grid generation of the fluid domain. In the

present study meshing has been created using meshing tool of ANSYS software. To

keep uniformity of elements the different refinement tools have been used. Mapped

face meshing tool has been used for uniform distribution of grid elements in fluid

domain. Concentration of number of elements has been kept larger at cross section of

street canyon. Edge sizing has also been done. Composite planes of two dimensional

fluid domains have been differentiated in various rectangular shapes. This step has

been considered for uniformity of the different region. Surface area of fluid domain

has been calculated as 5.918 m2. Total elements in case of street canyon of aspect

ratio 1 without EMRT are 15850 with nodes number 48017.

Figure 4: Grid generation of the cross section of street canyon

2.4. Boundary Conditions

Fig. 5 shows all boundaries of the fluid domain i.e. air inlet, pollutant inlets, outlet,

symmetry, wall, building, and ground surface. Ambient air has been applied as wind

flow material with 1.67 m/s mean velocity at inlet boundary. User Defined Function

(UDF) has been applied at inlet for velocity profile. Mixture fluid material of air and

acetylene with 5 % proportion of acetylene and rest 95 % of air has been applied at

both pollutant inlets. Pollutant inlet flow velocity was kept 0.011 m/s. Outlet

condition has been kept as pressure outlet. Symmetry boundary condition has been

applied for upper horizontal boundary. All other boundaries have considered as no

slip wall condition.


Figure 5: Boundaries of fluid domain

2.5. Physics Setups

ANSYS FLUENT CFD solver has been used to develop wind flow and pollutant

dispersion in two dimensional street canyons. Standard, RNG, and Realizable K-Ɛ

turbulence models have been applied in all geometrical cases. Transport species

equations model has been applied for pollutant dispersion. Appropriate boundary

conditions were applied as discussed earlier. A convergence criterion of 10-4 has been

applied in the calculations.

3. Results 3.1. Velocity vectors

Velocity vectors have been analyzed and discussed for all the cases of street canyon

and turbulence model. Figure 7 shows the velocity vectors at street canyon of aspect

ratio 1. From the figure, it is clear that the main vortex in the canyon dominates the

dispersion of pollutants. It is also clear that the significant variations in velocity

vectors pattern have been predicted by different k-Ɛ turbulence models. It has been

observed that standard k-Ɛ turbulence model computes velocity vectors similar to

those as in experimental study [18], as shown in figure 6. RNG k-Ɛ turbulence model

and realizable k-Ɛ turbulence model have predicted reverse vortex, which is not

acceptable while comparing to previous experimental and numerical studies. When

comparing velocity vectors of street canyon without EMRT with street canyon with

EMRT, it has been observed that vortex pattern has changed and shifted upward of

the street canyon centre. Velocity magnitudes in the lower corner of leeward and

windward side have decreased, which leads to increase in pollutants concentrations on


both the side. Vortex centre has shifted upward due to introduction of cross section of

EMRT. Lower part of street canyon becomes low velocity region, which leads to

increase in accumulation of pollutant concentrations.

Figure 6: Validation of velocity vectors (B) at street canyon cross section of the

model with simulation study [18] (A) for aspect ratio 1

Figure 7: Velocity vectors at street canyon of aspect ratio 1, (A) Street canyon

without EMRT, (B) Street Canyon with EMRT, (i) Standard k-Ɛ turbulence model (ii)

RNG k-Ɛ turbulence model (iii) Realizable k-Ɛ turbulence model.


Figure 8 shows velocity vectors for street canyon of aspect ratio 0.5 for all the cases.

Standard k-Ɛ turbulence model predicts that, the major vortex is situated little bit to

windward side from centre of the street canyon. A small vortex has also been

observed at lower corner of the leeward side. On the other hand, RNG k-Ɛ turbulence

model and realizable turbulence model predicts a minor vortex on windward side.

While comparison with street canyon having EMRT, it was observed that pollutant

concentration and distribution of the pollutants did not change significantly.

Figure 8: Velocity vectors at street canyon of aspect ratio 0.5, (A) Street canyon

without EMRT, (B) Street Canyon with EMRT, (i) Standard k-Ɛ turbulence model (ii)

RNG k-Ɛ turbulence model (iii) Realizable k-Ɛ turbulence model.

3.2. Pollutant Contours

Figure 9 shows contours of pollutant, acetylene concentration at street canyon of

aspect ratio 1. Standard k-Ɛ turbulence model predicts similar pattern of concentration

contours as that of experiments results in accumulation of pollutants in street canyon.

Numerical study predicts higher pollutant concentration on leeward side. It has been

observed that, with the introduction of EMRT the pollutant concentrations increase at

leeward side and windward side for street canyon having EMRT. On the other side,

RNG k-Ɛ turbulence model and realizable k-Ɛ turbulence model predicts high

pollutant concentration at windward side and increase in pollutant concentration in

street canyon when EMRT is introduced. As RNG and Realizable models do not

validate the model, results may not be accepted. Increase in pollutants may be due to

restrictions in the main flow. Velocity magnitudes reduce in street canyon with

EMRT as compare to the street canyon without EMRT.

Figure 10 shows contours of pollutant concentration at street canyon of aspect ratio

0.5. It is clear from the figure that, introduction of EMRT does not cause any

significant change in accumulation of pollutants. Standard k-Ɛ turbulent model

computes similar pollutant concentration on both the cases.


Figure 9: Contours of pollutant concentration at street canyon of aspect ratio 1, (A)

Street canyon without EMRT, (B) Street Canyon with EMRT, (i) Standard k-Ɛ

turbulence model (ii) RNG k-Ɛ turbulence model (iii) Realizable k-Ɛ turbulence

model.

Figure 10. Contours of pollutant concentration at street canyon of aspect ratio 0.5,

(A) Street canyon without EMRT, (B) Street Canyon with EMRT, (i) Standard k-Ɛ

turbulence model (ii) RNG k-Ɛ turbulence model (iii) Realizable k-Ɛ turbulence

model.


3.3. Pollutant concentration plots

Pollutant concentration on x-y coordinates has been plotted and discussed. Three

vertical sample lines as shown in figure 11, on each side of the street canyon have

been taken to plot pollutant concentrations. Vertical position and pollutant

concentrations have been taken on x-axis and y-axis respectively.

Figure 11: Sample lines taken to plot pollutant concentration on both the side of

street canyon, i.e., leeward (LW) and windward (WW) side.

Figure 12 and figure 13 shows pollutant concentration plot along the sample lines

taken on both leeward and windward side. For aspect ratio 1, it is clear from the figure

of pollutant concentration plot that pollutant concentration on leeward side has been

increased in street canyon having EMRT comparing to street canyon without EMRT.

Pollutant concentration has also been increased on windward side. It has been

observed that increase in pollutant concentration was along the complete line.


Figure 12: Pollutant concentration plot along street height in street canyon of aspect

ratio 1 (Leeward Side), (a) street canyon without EMRT (b) street canyon with EMRT


ratio 1 (Windward Side), (a) street canyon without EMRT (b) street canyon with

EMRT


Figure 14 and figure 15 shows pollutant concentration plot at street canyon of aspect

ratio 0.5. Plot has been taken on three vertical sample lines on the both the side of

street canyon. It has been observed that there is no any significant variation in

pollutant concentration on both leeward and windward side of street canyon.


ratio 0.5 (Leeward Side), (a) street canyon without EMRT (b) street canyon with

EMRT



ratio 0.5 (Windward Side), (a) street canyon without EMRT (b) street canyon with

EMRT

Conclusion Urban street canyon of aspect ratio 1 and 0.5 were analyzed for pollutant dispersion

phenomena. ANSYS FLUENT software were used for CFD simulation of wind flow

and pollutant dispersion at street canyon. Effects of EMRT in street canyon were

compared with street canyon without EMRT for both the cases of aspect ratios.

Standard, RNG, and realizable k-Ɛ turbulence model were used to model wind flow

simulation. Standard k-Ɛ turbulence model had produced similar results when

compared to previous experimental results. Simulation results of the RNG and


realizable k-Ɛ turbulence model were not acceptable when compared to experimental

studies. Pollutant concentrations were increased at different locations of street canyon

having EMRT on both leeward and windward side for aspect ratio 1. On the other side

pollutant concentration were not affected by introduction of EMRT at street canyon of

aspect ratio 0.5. It has been concluded that by introduction of any elevated transport

structure in normal street canyon, pollutant concentration and accumulation of

pollutants increases in street canyon. For two dimensional cases standard k-Ɛ

turbulence model predicts results that are similar to experimental results. Further

study of three dimensional cases will be more predictable model in all complex cases

of street canyon.

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effects of elevated metro rail tracks on pollutant

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