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8/2/2019 Wong Wai Lun-005008
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Numerical Studies on Airflow in Urban Street Canyons
WONG Wai Lun, UNIMKL-005008
Computational Fluid Dynamics (MM4CFD)
Department of Mechanical, Materials & Manufacturing Engineering
Supervisor: Prof. Andy Chan
Abstract
The objective of this study is to investigate numerically the effect of the ratio of street width,
W to building height, H on wind flow and predicting the pollutant dispersion in a street
canyon within an urban environment. Three-dimensional numerical models based on
Reynolds-Averaged Navier-Stokes - RNG model and Large Eddy Simulation (LES) werecreated to analyze the air flow development within an urban canyon. The dispersion of thepollutant thus can be predicted based on the air flow simulation using Fluent code. Its learnt
that the geometry and configuration of the building play important roles in determining the
complex flow pattern and then the pollutant concentration within urban canyon streets.
The model generated has strong agreement with the literature data. It is observed that
Large Eddy Simulations LES was able to capture the unsteady and intermittent fluctuations
of the flow. However, it did not give much differences compared to the - RNG model forthe cost of higher computational time and cost. In the light of this, - RNG model is morepreferred in this case. Further improvement in ventilation can be done with alterations of
building height and roof shape.
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Introduction
In the light of continuing urbanization and the increase of vehicles in urban area, the
dispersion of the pollutants that mainly emitted by vehicles exhaust has become subject of
interest to environmental analysts, building occupants and so forth. The air quality is
depending upon the air ventilation within an urban area, which is influenced by the traffic
flow, building geometry, ambient wind speed and direction, street configurations such as
roof shape, building height, width of the street and so on. The air quality has direct impact
on the health of the pedestrians, cyclists as well as people living in the urban area. Without
doubts, poor ventilation will lead to high concentration of pollutant. Several studies
targeting on the ventilation issues had been conducted with field experiments (eg. DePaul
and Sheih, 1986; Nakamura and Oke, 1988; Rotach, 1995; Croxfrod, 1988) and numerical
modeling (eg. Lee and Park,1994; Sini et al., 1996; Hassan and Crowther, 1998) in order to
understand the flow patterns and pollutant removing in urban street canyon. Increasingly
pollutants density has been the momentum for researchers such as Baik and Kim (2003), Xieet al. (2005) to develop numerical studies on pollutant dispersion using Computational Fluid
Dynamics (CFD) models to enrich understanding of pollutant transport and hence the
development of pollutant removal mechanism. Xie et al.(2006) studied the flow field and
pollutant dispersion characteristics in the street canyons with different configurations to
identify the influence of the street geometry on the wind flow and the dispersion of
pollutants in the street canyon. However, most numerical studies have been modeled based
on canyons formed by flat-roof building. The main goal of the present work is to provide the
numerical simulations of airflow patterns within urban street canyon of difference height of
building and width of street combinations to understand the effect of these configurations
on pollutants distribution/characteristic within the canyon. Besides that, the effect of
slanted-shaped roof buildings will also be analyzed in this study.
In this study, the geometries with different width of urban streets are employed with
Reynolds-Averaged Navier-Stokes - RNG and Large Eddy Simulation (LES) model and theperformance of both models is compared. The LES model explicitly solve for the large eddies
and implicitly account for the small eddies, which enable the study of the unsteadiness of a
flow, and provides the detail information on the flow structure including turbulence
statistics. The minus point is that it requires a lot of computational effort, time and cost. LES
has to be run for a sufficiently long flow time to obtain stable statistics for research purpose.
Consequentially, the computation cost involved is higher than RANS model in terms of
memory (RAM) and CPU time.
Further improvement which possibly enhance the air ventilation in the densely build urban
area can be done by finding the critical roof shape and width of the street relative to the
height of the buildings.
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2. Methodology
2.1 Physical model configuration
Three dimensional urban street canyon with different widths of street relative to the
building height were used in this simulation, as shown in Figure 1.
Figure 1: The schematic diagram of the 3-D urban street canyon and the structure of the slanted roof.
The model street canyon consisted of 6 rows of shop lots and 5 streets. All the urban street
buildings have a square cross section of length H=20m with a triangle slanted roof profile as
shown in the Figure 1. The air ventilation in the urban area is depending upon the width of
the street relative to the height of the building. The width of the street, W solely is not able
to justify how well the ventilation would be. Hence, the street width to building height ratio
is employed, where
The width of the street is altered where W=0.5H, H, 2H which corresponding to R=0.5, 1.0
and 2.0. The wind direction is orthogonal to the direction of the street.
The coordinate system has its origin at the bottom, middle of the domain inlet, with x
measured as positive in the downwind direction and y is positive for upward.
5H
7H
5H
HH
W
H
45
Z
3H
20H
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2.2 Mathematical model/ Numerical Method
In this study, the transient incompressible Reynolds-Averaged Navier-Stokes (RANS)
equation is employed for the mean flow field in street canyon using the Re-Normalisation
Group (RNG) k-epsilon model. The commercial code FLUENT is adopted in the CFD
simulations. The governing equations with the RNG model are expressed as follow:
Mass conservation (Continuity Equation)
= 0
Momentum Equation
+
) = -
+
kand transport equation (rupanya for convection- diffusion) +
=
+
-
+
=
+
-
Where,
= ith mean velocity componentP = the deviation of pressure from its reference value
= air density
= inverse effective Prandtl number for k = inverse effective Prandtl number for = effective turbulent viscosity = scalar measure for the deformation tensor
RNG k- constants: = 1.42 = 4.38 = 0.012
Large Eddy Simulation (LES)
The original Smagorinsky-Lilly model is used due to its algorithmic simplicity and numerical
stability. LES is suitable in complex flow simulation owing to its less approximation but direct
resolving is achieved as opposed to RANS. This would, however, require substantially finer
meshes and needs to be run for sufficiently longer flow-through time in order to obtain
stable statistics of the flow. Therefore, higher memory (RAM) and CPU time are required.
The governing equations are discretized by the finite volume method and the SIMPLE
algorithm is used to handle the pressure-velocity coupling. The second-order unwind
scheme is adopted for the approximation of the convection terms, and the second-order
central difference for the diffusion terms. The scaled residual criteria for all the flow
properties were set at 1x10-5
.
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2.3 Model domain
A three dimensional computational domain is created, which is 100m x 140m x 400m. Biased
mesh is applied due to the large domain in simulating the urban area and the atmosphere.
Mesh cells are biased towards to the buildings. The grid is finer close to the building and
ground in which the grid intervals near the wall of the building is 2m. The number of grid
cells is about 342,640. Structured hexahedron cells are believed to give more even mesh
distributions and accurate result since false diffusion can be prevented in the case where
second order upwind scheme is adopted for the approximation of the convection terms, the
alignment of the flow direction with the grid is rather important.
For both transient models, the number of time steps employed was 5000 with time step size
of 0.07 and maximum 20 iterations per time step.
2.4 Boundary Conditions
Velocity inlet with boundary layer profile is used in the main inlet wind flow. The initial free
stream wind speed is 5m/s, and this inlet velocity profile is developed with turbulence of 1/7
in the power law in the user defined function (UDF) which was then implemented in the CFD
code.
The ground and building surfaces are defined as walls with no-slip boundary condition. In
FLUENTTM
, the surface roughness is expressed in terms of sand grain roughness, Ks in order
to circumvent problem with coarse grid resolution near the ground due to large Ks value. The
sand grain roughness, Ks is set to same as aerodynamic roughness length, z0 which was
found to be z0=0.0033m in wind tunnel experiments. They agreed that setting K s equal to z0
was not correct in a strong sense, but justify the choice from the result obtained, where only
minor difference in terms of velocity profile and turbulence intensity. The top plane and
both sides of the domain are applied with symmetry boundary condition. Zero gradient
boundary condition is set at the outflow.
The time step size
=
=
=0.07
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3. Result
W/H = 0.5
Figure 2: Air velocity vector for W/H=0.5 modelled by k- RNG (left) and LES (right).
Figure 3: The graph of distance from the ground against horizontal velocity in the W/H=0.5 canyon.
Figure 4: The velocity vector in the W/H=0.5 urban area by k- RNG model.
0
20
40
60
80
100
120
140
160
-1 0 1 2 3 4 5 6
Distancefromt
heground(m)
Velocity in x direction (m/s)
X-velocity in the W/H = 0.5 canyon
k-e RNG
LES
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Figure 5 The air velocity vector in the W/H=0.5 urban area by LES model.
W/H = 1.0
Figure 6: Air velocity vector for W/H=1.0 modelled by k- RNG (left) and LES (right).
Figure 7: The graph of distance from the ground against horizontal velocity in the W/H=1.0 canyon.
0
20
40
60
80
100
120140
160
-1 0 1 2 3 4 5 6Distancefromt
heground(
m)
Velocity in x direction (m/s)
X-velocity in the W/H=1.0 canyon
k-e RNG
LES
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Figure 8: The velocity vector in the W/H=1.0 urban area by k- RNG model.
Figure 9: The velocity vector in the W/H=1.0 urban area by LES model.
WH = 2.0
Figure 10: Air velocity vector for W/H=2.0 modelled by k- RNG (left) and LES (right).
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Figure 11: The graph of distance from the ground against horizontal velocity in the W/H=2.0 canyon.
Figure 12: The velocity vector in the W/H=2.0 urban area by k- RNG model.
Figure 13: The velocity vector in the W/H=2.0 urban area by LES model.
0
20
40
60
80
100
120
140
160
-4 -2 0 2 4 6 8
Distancefromt
heground(m)
Velocity in x direction (m/s)
X-velocity in the W/H=2.0 canyon
k-e RNG
LES
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4. Discussion
From the numerical result obtained from the CFD code, it is obvious that the airflow pattern
inside the canyon is strongly influenced by the ratio of the width of the canyon, W to the
height of the surrounding buildings, H. The airflow pattern can be directly related to the
distribution of the pollutants in the canyon.
4.1 W/H=0.5
For the first case where the ratio W/H=0.5, it can be seen that two vortices appeared in the
canyon as shown in the Figure 2. The upper one is in clockwise direction, which located in
between the two slanted roof. This vortex is generated due to the ambient wind. The lower
one is driven by the circulation above, therefore it is in counter clockwise direction. As
shown in the Figure 3, the horizontal velocity from ground to approximate 10m above ispositive (counter clockwise circulation) and the velocity slowly reduced as the distance from
the ground increases. It has finally become negative, which indicates a clockwise vortex.
Owing to this lower counter clockwise circulation, pollutants tend to accumulate on the
windward side of the canyon and can hardly escape from the canyon. Comparing the k-RNG and LES model, the LES model present a more realistic result, where the airflow pattern
is not steady and there could have more than two vortices in the canyon. This is illustrated in
Figure 3, where the direction of the horizontal velocity component is fluctuating at the base
of the canyon. Nonetheless, the k- RNG model tends to give higher horizontal velocitycompared to LES model.
4.2 W/H = 1.0
As shown in Figure 6, the upper clockwise vortex has enlarged, and the centre of this vortex
moves downwards. This is because the main stream flow has more space to create a large
circulation in the canyon and generate greater effect on the lower vortex. As a result of it,
the lower counter-clockwise vortex is pressed downwards and smaller in size. Both
numerical models give similar horizontal velocity in the canyon as shown in the Figure 7.
From Figure 7, it can be seen that within the canyon, the changes of horizontal velocity with
respect of distance from ground is similar to first case. The difference is that the positive
horizontal velocity is up to approximately 5m from the ground only, which means the lower
counter clockwise vortex is lowered, due to the enlarged upper clockwise vortex. In the light
of this, some pollutants are carried towards the leeward face by the upper vortex and some
to the windward face by the lower vortex. However, when comparing the k- RNG and LESmodel, k- RNG did not show the effect of the roof shape on the free stream. LES allowsbetter predictions of the transient flow as circulation above the building roofs are observed
in Figure 9. Besides, k- RNG model has higher magnitude of velocity compared to LES modelalthough both model give similar trend in horizontal velocity as shown in the Figure 7.
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4.3 W/H=2.0
When the width of the street is twice the height of the buildings, one strong clockwise
vortex is generated without a driven counter clockwise vortex like previous cases. The wider
street allows better ventilation in the canyon. The main stream flow is able to ventilate the
canyon better without the lid-driven-cavity effect. The graph of horizontal velocity in the
canyon also different compared to the previous cases, where there is only negative (reverse
direction) flow in the base of the canyon which represent that the strong clockwise vortex is
taking place in the canyon. Pollutants tend to accumulate on leeward side of the street due
to the clockwise strong vortex as shown in the Figure 10. Again, the k- RNG model hashigher horizontal velocity of wind in the urban canyon.
Conclusion
In this study, the effects of the street width relative to the buildings height have been
numerically investigated. It can be concluded that different street width will have different
distribution of the pollutants as presented in the Table 1:
Street width to buildings
height ratio, W/H
Number of vortices
generated
Expected pollutant
distribution in the canyon
0.5 2 main vortices, about equal
in size
Windward side of the canyon
1.0 2 main vortices, lower
counter clockwise vortex is
smaller than upper clockwise
vortex.
Leeward side of the canyon
by upper vortex and
windward side by lower
vortex.
2.0 1 main clockwise vortex
driven by the free stream
flow.
Leeward side of the canyon
by the clockwise vortex.
Table 1: Summarized results.
The narrow (W/H=0.5) canyon has poorer air ventilation properties compared to wider
(W/H=2.0) canyon.
Both CFD models (k- RNG and LES model) give good agreements with the literaturenumerical result in Reference [2]. However, the numerical results of k- RNG model aregenerally higher than LES model. LES model is believed to be a model with higher accuracy
compared to k- RNG model, in which the simulated result of transient flow is more realistic.It has also shown the effects of the roof shape to the main stream flow and the circulation
formed. In terms of resources demand and computational cost, the k- RNG took about4hrs while the LES model consumed about 10days for the same geometry. Since the flow
pattern and flow velocity simulated by both models give similar numerical result, the k- RNG model is more suitable to simulate an urban canyon case where computational time
and cost are the constraints.
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References
1. Mohamed F.Yassin (2011). Impact of height and shape of building roof on air qualityin urban street canyons. Atmospheric Environment 45 (2011) 5220-5229.
2. HUANG Yuan-dong, JIN Ming-xia, SUN Ya-nan (2006). Numerical studies on airflowand pollutant dispersion in urban street canyons formed by slanted roof buildings.
Journal of Hydrodynamics (2007) 100-106.
3. HUANG Yuandong, Xiaonan Hu, Ningbin Zeng. Impact of wedge-shaped roofs onairflow and pollutant dispersion inside urban street canyons. Building and
Environment 44 (2009) 2335-2347.
4. Salim Mohemed Salim, Riccardo Buccolieri, Andrew Chan, Silvana Di Sabatino.Numerical simulation of atmospheric pollutant dispersion in an urban street canyon:
Comparison between RANS and LES. Journal of Wind Engineering and Industrial
Aerodynamics (2011)103-113.