九州大学大学院総合理工学府報告第 37 巻 第 1 号 1-7 頁 平成 27 年 9 月

Engineering Sciences Reports, Kyushu University
Vol. 37, No. 1, pp. 1-7, SEP., 2015

Aerodynamic effect of overhang on a turbulent flow fieldwithin a two-dimensional street canyon
Mohd Faizal MOHAMAD*1,2†, Aya HAGISHIMA*3, Naoki IKEGAYA*3,
Jun TANIMOTO*3, and Abd Rahman OMAR*1
E-mail of corresponding author: faizal3744@salam.uitm.edu.my†(Received August 25, 2015, accepted August 28, 2015)
This paper reports the results of flow field analysis within and above a two-dimensional street
canyon with various overhang lengths using large-eddy simulation (LES). Simulations wereconducted for a constant canyon aspect ratio of W/H = 3, where W is the street width and H is thebuilding height. Three different overhang lengths were simulated with P = 0H, 0.5H, and 1H in orderto derive the mean and instantaneous flow characteristics. The results are compared with windtunnel experiments for validation. The LES results of the mean flow with the P = 0H condition agree
fairly well with the wind tunnel data. However, profiles of the standard deviation for the streamwise
and vertical velocity components show large discrepancies at all measured locations. In addition, an
increase in overhang length on both building façades significantly modifies the in-canyon flow
pattern by limiting the penetration of the bulk flow into the canyon layer.
Key words:

1.

Large-eddy simulation (LES), overhang, street canyon, air exchange rate (ACH)Introduction

The turbulent flow nature of urban street
canyons has for decades been a major area ofinterest within the fields of urban climatology,
wind engineering, and building environmental
engineering because of its significant effects on
the comfort and safety of the urban population
relating to urban air quality, thermal comfort,and damage from strong gusts. Although theflow field of a real urban street is highly
complicated and varies with urban topography,
it has been known that several geometricparameters relating to the building packing
density could describe a key feature, namely,
the transition of the flow regime. For example,
DePaul et al.1) revealed that the flow pattern isstrongly affected by the aspect ratio W/H (ratioof street width to the building height). Oke2)
categorized three characteristic flow regimes
under different aspect ratios, namely,
skimming, wake interference, and isolatedroughness flow. Hunter et al.3) also
investigated the flow structures of uniform
building arrays under different conditions of*1 Universiti Teknologi MARA (UiTM)*2 Department of Energy and Environmental
Engineering, Graduate student*3 Department of Energy and Environmental
Engineering

the street aspect ratio, on the basis of anumerical simulation. In addition, Sini et al. 4)
examined the influence of street canyon aspectratio along with thermal conditions on thein-canyon flow structure and pollutant
transport, and revealed that these two factors
are strongly related to the number of mean
flow vortices, and thus significantly affects thevertical transport process.
In addition to these studies which focused on
generic idealized urban geometry, Rafailidis5)
conducted a wind tunnel experiment on theinfluence of pitched roofs on the in-canyon flow
pattern, which shows that pitched roofs are
more effective for enhancing the ventilation
within the street canyon compared to widebuilding spacing.
Meanwhile, in addition to the building
packing density and roof shape, various
architectural and urban elements with muchsmaller size compared to buildings have beenrecognized as secondary urban roughness
affecting urban canopy flow. Building
overhangs, which we focused on this study, is
one of these elements, and commonly seen intropical climates for protection from direct
solar radiation and rain. Foroushani et al.6)
revealed that an overhang on a building façade

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Aerodynamic effect of overhang on a turbulent flow field within a 2D street canyon

significantly disturbed the flow pattern aroundthe building. Peren et al.7) investigatedventilation performance in buildings withvarious configurations of overhang position
and inclination angle. Both studies were
performed upon an isolated building using
Reynolds-averaged Navier Stokes (RANS)
simulations. Furthermore, Mohamad et al.8)
conducted a series of RANS simulations of theflow over a two-dimensional (2D) street canyon,
and demonstrated that a porch overhanging
from a building wall over a 2D street canyon
drastically changes the well-known canopy
flow regimes such as skimming, wakeinterference, and isolated flow. These studies
on the effect of overhangs on the mean flow
field of an urban street canyon suggest that
secondary urban roughness can significantly
hinder urban ventilation in some cases in spiteof its small volume compared to buildings, andraises the necessity for improvement of thetransport modelling of the urban canopy layer.
Under these circumstances, the authors of
the present study investigated how an
overhang affects the characteristics of the
turbulent flow within an idealized 2D street
canyon in order to deepen the understandingof the relation between the urban ventilationand the urban geometry, on the basis of thecomputational fluid dynamics (CFD) technique.
For reproducing the unsteady flow fluctuationin the canyon, large-eddy simulation (LES)
was adopted for the study.

2.

Smagorinsky’s constant Cs of 0.12. All thesimulations were carried out using theopen-source CFD code OpenFOAM 2.0.
2.2 Simulation domain
The computational domain adopted in thesimulation is shown in Fig. 1. It consists of one
idealized 2D street canyon with a buildingheight H of 25 mm, and a street width W of 75mm, and an aspect ratio W/H of 3 for all threecases. The height of the domain was set to 6 H
as proposed by Franke et al.9) and Tominaga etal.10), and the spanwise domain length was
kept at 2H.

Fig. 1. Schematic of the computational domain adopted inthis study.

Computational settings

2.1 Governing equations
The filtered continuity and Navier–Stokes
equations for unsteady incompressible flow are
written as

ui
0
xi

(1)

u i u i u j 2 ui  ij
p


t
x j
x i
x j x j x j

(2)

where u i and p are filtered velocity andpressure, respectively, and ν is the fluid
kinematic viscosity. Here i, j =1, 2 and 3
indicate the streamwise, spanwise, andvertical directions, respectively.  ij represents
the subgrid-scale (SGS) Reynolds stress.
The SGS stress was modelled by thestandard
Smagorinsky
model
with

Fig. 2. Configurations of simulated urban street; (a) case:P = 0H, (b) case: P = 0.5H, and (c) case: P = 1H.
The three vertical lines included in (a) refer to thepositions of vertical profiles shown in Fig. 3.

Figure 2 shows the three calculation caseswith different overhang lengths ( P = 0H, 0.5H,
and 1H). The overhang thickness in each casewas set to 0.1H. The domain was discretized平成 27 年度

九州大学大学院総合理工学報告

into a uniformly structured mesh with a gridsize of H/16.
The flow was driven by a pressure gradient,which was aligned in the x direction, so as tomaintain the streamwise bulk velocity of 2 m/s.
Periodic boundary conditions were employedin the streamwise and spanwise directions tosimulate infinitely repeated street canyons atregularintervals.
Non-slipboundary
conditions were applied on the lower walls andbuilding surfaces, and free slip boundary
conditions were imposed on the velocity
components at the top boundary, where thevelocity normal to the boundary equals zeroand the gradient of velocity parallel to wallshould be zero.

3.

Results and discussion

3.1

Model validation

The vertical profiles of the mean velocities
and standard deviations at three different
locations (x = −0.8H, 0H, and 0.8H) for caseswith no overhang are shown in Figs. 3 and 4.
The data obtained in the wind-tunnel
experiment based on the particle image
velocimetry (PIV) technique11) are also
included for a comparison. The height and thevelocity are normalised by the building height
H and reference velocity uref at a height of 2H.
The mean velocity profiles derived from thepresent LES show generally good agreementwith the wind tunnel results except for thevertical velocity within the canopy at x =−0.8H and 0.8H. Considering the relatively
small values of the vertical velocity comparedto the streamwise component, it is supposedthat the discrepancy shown in Figs. 3(d) and3(f) is acceptable and the current LES properly
reproduces the mean flow field within andabove the street canyon.
On the other hand, the standard deviations
σu and σw at all the three positions derived
from the LES are much smaller than those of
the wind tunnel experiment, as seen in Fig. 4.
One possible reason for this underestimationof the turbulent intensity in the LES is thesmall computational domain of the present
simulation, which includes only one street
canyon. The periodic boundary condition
applied on the streamwise boundaries canseemingly reproduce the flow over a
horizontally infinite 2D canopy; however, thedevelopment of the instantaneous turbulent
structure is restricted by the streamwise
domain size, as Kanda et al.12) mentioned.

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3

Nevertheless, the shapes of the LES profiles
are qualitatively similar to those obtained by
the wind tunnel.

Fig. 3. Vertical distributions of (a)(b)(c) u and (d)(e)(f) w for
the case with no overhang. The data of (a) and (d)are at x = −0.8H, those of (b) and (e) are 0H, and (c)and (f) are at 0.8H at the lateral centre of thestreet (y = 0H). The solid line refers to the present
simulation result, and the open circles refer to thedata of the wind tunnel experiment11)

Fig. 4. Vertical distributions of (a)(b)(c) σu and (d)(e)(f) σw
for the case with no overhang. The data of (a) and(d) are at x = −0.8H, those of (b) and (e) are 0H,
and (c) and (f) are at 0.8H at the lateral centre of
the street (y = 0H). The solid line refers to thepresent simulation result, and the open circles
refer to the data of the wind tunnel experiment11)

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Aerodynamic effect of overhang on a turbulent flow field within a 2D street canyon

3.2 Spatially averaged profiles of turbulent
statistics
Figure 5 shows the vertical distributions of
the spatially averaged mean streamwise
velocity, standard deviations of streamwise
and vertical velocities, and Reynolds stress.
The streamwise mean velocity for the casewith no overhang (Fig. 5(a)) shows negativevalues at the lower half of the cavity, probably
due to the well-known canopy flow regime ofwake interference flow. In contrast, the mean
velocities for the other two cases withoverhang are closer to zero because of thelimited area of penetration of the above flow
into canyon layer. The velocity gradient
immediately above the building height of all
the three cases is much larger than thatwithin the canopy layer. It is caused by thestrong shear acting on the roof and overhang.
With regard to the standard deviation of
streamwise velocity, a peak can be observed at
the roof height, and the values decrease as theheight increases for all three cases (Fig. 5(b)).
Comparing the three cases reveals that thevalues of σu decrease from the case with nooverhang to the case with longer overhang,
and the peak near the building height becomessteep for the case with longer overhang.
In contrast, the profiles of σw show more
moderate positive peaks at approximately
1.3H compared to the data of σu. Similartendency is shown for σw (Fig. 5(c)), where thevalues decrease with the increase in overhang
length.
The profile of the Reynolds stress of the casewith no overhang shows a well-known dogleg
shape, and a peak appears at the roof height(see Fig. 5(d)). In contrast, the cases withoverhang show a strong reduction of the
Reynolds stress inside the cavity.

research2,4).
In contrast, when P = 0.5H (Fig. 6(b)), thecavity is occupied by three vortices. The
primary vortex is smaller than that for thecase without overhang because it iscompressed in the streamwise direction. The
secondary vortex developed in front of the
upstream building façade extends its size
almost twofold in the vertical direction. At theupper corner of the downstream building, atertiary vortex is observed as the overhang
enforces downward flow, resulting in flow
separation at the edge of the overhang.
The increase in overhang length further
modifies the in-canyon flow pattern as shownin Fig. 6(c). The size of the primary vortex
developed at the middle of the canyon isalmost the same with the building height in
the horizontal and vertical directions. In
addition,
almost
the
same
size
of
anticlockwise-rotating vortices exists underboth overhangs. The modification of thein-canyon flow pattern by the varied overhang
length is consistent with results of Mohamad
et al.8)

3.3 Time-averaged flow fields
Figure 6 shows the time-averaged velocity
structures in the x-z plane at the centre of thelateral position of the domain (y = 0H) for allcases. The result of the case with no overhang(Fig. 6(a)) shows a clockwise-rotating primaryvortex within the canyon, which is driven by
the flow above the building. A secondary
anticlockwise-rotating vortex can be also
observed at the lower corner of the upstreambuilding. This flow structure can be classified
as the wake-interference flow, and is nearlyconsistent with the results of previous

Fig. 5. Vertical distributions of spatially averaged
turbulent statistics: (a) streamwise mean
velocity, (b) standard deviations of streamwise
velocity, (c) standard deviations of verticalvelocity, and (d) Reynolds stress for three caseswith overhang length P =0H, 0.5H, and 1H.

3.4 Spatial distributions of turbulent statistics
Figure 7 shows the contour maps of the平成 27 年度

九州大学大学院総合理工学報告

Reynolds stress in the same x-z plane of Fig. 6.
For the case of P = 0H (Fig. 7(a)), a
horizontally elongated region with large
Reynolds stress is observed at the buildingheight. This feature indicates strong shear
between within and above the canyon layer.
For P = 0.5H (Fig. 7(b)), the protruding
overhangs in both façades restrict thepenetration of the upper air into the street
canyon, resulting in a smaller region withlarge Reynolds stress. In the longest overhangcase (P = 1H) (see Fig. 7(c)), the Reynolds
stress of the entire domain is much smaller
than the others, and only a weak peak area
can be observed at the edge of the downstream
overhang. This suggests that the horizontal
porches projected out at the upper edge of thebuilding walls work as a part of the secondary
earth surface, and make the flow above thebuilding canopy behave like that over asmooth surface where drag force is muchsmaller than that of ordinary block arrays.
The spatial distributions of the standard
deviations σu and σw for the three cases areshown in Fig. 8. In the case with no overhang,a peak region appears above the street at thebuilding height, similar to the distribution
pattern of the Reynolds stress, and thisrelatively high σu area extends downward at
the downwind-half portion of the cavity. Thisis consistent with the vortex generated by the
downward flow shown in Fig. 6(a). The wakeregion near the wall of the upwind building
exhibits a small σu. In contrast, the data of thecase with medium overhang length clearly
exhibits the obstruction of the flow penetration
into the cavity, and the σu values over almost
the entire cavity become small. In the case of
the longer overhang, the overhang morestrongly works to separate the flow within andabove the canopy layer. The standard
deviation of the vertical velocity σw exhibits asimilar tendency to the effect of overhang.
Figure 9 shows the distributions of theskewness of streamwise ( Su) and vertical (Sw)velocities. For P = 0H (Fig. 9(a)), Su is positivein most of the canyon with a peak observed at
the lower corner of the upstream building. In
contrast, negative Su is observed in the wakeregion behind the upstream building. Thissuggests that the relatively high-speed motionof the vortex penetrates the canyon along withthe mean flow pattern at low frequency. The
values of Su and Sw at the building height are
in general positive and negative, respectively.
This tendency implies that the sweep event

第 37 巻第1号

5with high-speed downward motion is dominant,as shown in past research.
Regarding the data for the case with shorter
overhangs, the positive peak of Su becomes
more evident and narrow. It is supposed thatthe relatively high-speed upper air often
penetrates at the building height between thetwo overhangs, similar to the case with nooverhang, but this flow motion toward thecavity is restricted by the overhangs, thus
resulting in the narrower regions of both
positive Su and negative Sw.

Fig. 6. Normalised velocity contours and streamlines ofmean flow in the x-z plane at the middle of thestreet canyon (y = 0H) for (a) P = 0H, (b) P = 0.5H,
and (c) P = 1H. The prevailing wind is to the right.

The distributions of skewness for the case of P
= 1H are much more complex compared to theaforementioned two cases; thus, it is difficult
to simply describe the features. However, theregions of positive Su and negative Sw
elongating from between the gap of overhangs
obliquely downward can still be observed.
They imply the sweep motion dominates theventilation of the cavity beneath theoverhangs.

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Aerodynamic effect of overhang on a turbulent flow field within a 2D street canyon

Fig. 9. Contour of spatial distribution of skewness for
streamwise and vertical velocity components inthe vertical x-z plane through the middle of thestreet canyon (y = 0H) for (a)(b) P = 0H, (c)(d) P =0.5H, and (e)(f) P = 1H. The prevailing wind is to
the right.

Fig.7. Normalised Reynolds stress in the x-z plane at the
middle of the street canyon (y = 0H) for (a) P = 0H,(b) P = 0.5H, and (c) P = 1H. The prevailing wind isto the right.

Fig. 8. Contour of normalised standard deviation of
streamwise and vertical velocity in the x-z planethrough the middle of the street canyon (y = 0H)
for (a)(b) P = 0H, (c)(d) P = 0.5H, and (e)(f) P = 1H.
The prevailing wind is to the right.

4.

Conclusion

The LESs of the flow over the 2D street
canyon with an aspect ratio of 3 wereconducted to investigate the effect of overhang
on the turbulent flow field.
The calculation results of the mean flow
structure for the case with no overhang
indicatethatthewell-known
wakeinterference flow regime consists of twoco-rotating vortices. In contrast, the mean flow
for the two cases with overhangs exhibit aremarkable modification of the flow regime
with an increased number vortices and analteration of the vortex size, because
protruding overhangs on the building façades
restrict the penetration of the upper air into
the canyon layer, resulting in a strong
reduction of the velocity within the canopy.
It is shown that the Reynolds stress within
and near the canopy layer is strongly reducedby the overhangs. In other words, overhangs
extended from a building roof act as aprojected secondary street surface, whichmakes the flow above the building behave likethat over a smooth wall with small surface
drag.
Acknowledgments
This work was supported
KAKENHI Grant No. 25289196.

by

JSPS

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九州大学大学院総合理工学報告第 37 巻第1号

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