INTRODUCTION
Urban areas and inhabitants of cities tend
to increase. Nowadays, about half of the world total
population live in urban settlements. This large
number of dwellers produce variety of activities and
modify urban air quality and pattern. Urban wind
flow is driven by a deep, stratified urban boundary
layer with significant wind fluctuations. Solar heating
effects include shadows from buildings and trees,
aerodynamic drag, heat exchange affected by the
surface property variations and turbulent heat
transport.
Causes of wind field modifications in urban areas
In urban areas, main air flows from
prevailing winds are strongly modified, depending
on constructions’ morphology and urban
microclimate effects.3
Wind field modifications have
become particularly significant because of the
increasing number of high-rise buildings, industrial
and vehicular activities etc. Significant differences
can be found in wind speed frequency distributions
Current World Environment Vol. 7(2), 267-273 (2012)
Wind Field Modifications in Habitable Urban Areas
SEEMI AHMED and ALKA BHARAT
Department of Architecture and Planning, M.A.Natonal Institute of Technology, Bhopal, India.
(Received: July 12, 2012; Accepted: September 17, 2012)
ABSTRACT
This paper discusses different criteria for the assessment of wind field environments in
urban areas and how they relate to field observations. The importance of the inclusion of wind
environment studies in the planning process is also discussed.The increasing influence of the built
environment on wind speed and direction makes any forecast for heights below 50 meter very
hazardous1
.This increase in the areas with more built form where the roughness is extremely high.
It is not always possible to make a quantitative forecast of wind speed and direction in urban
environment. Examples are provided to illustrate how development controls can be designed to
ensure that pedestrian amenity is not compromised by new development while at the same time
not become a burden to innovative design approaches or good design practice due to wind
modification. The paper concludes with a number of case studies that provide examples of how
innovative techniques for mitigation of adverse wind environments can achieve the desired level
of pedestrian amenity without having to compromise with the architectural design intent.
Key words: Wind engineering, pedestrian environment comfort, Urban Heat Island (UHI) effect
at vertical levels in urban areas due to following
elements and phenomenon.
´ Buildings, Vegetation ( Physical and thermal
Obstacles/ Roughness)
´ Air conditioners (thermal)
´ Natural Topography (moisture properties of
the surface, undulations etc)
´ Street canal effects due to Vehicular
movements. (thermal)
´ Tall buildings - Turbulence, Roughness
´ Local Climatic conditions/ Seasonal
Variations
´ Shelter from nearby buildings,
´ Urban Heat Island Effect (UHI)
Wind field modifications due to UHI effect
Urban surfaces act as a giant reservoir of
heat energy. Concrete can hold roughly 2,000 times
as much heat as an equivalent volume of air.
The large daytime surface temperature
within the UHI is easily seen via thermal remote
sensing. At night time an inversion layer is formed
268 AHMED & BHARAT, Curr. World Environ., Vol. 7(2), 267-273 (2012)
This traps urban air near the surface, and keeping
surface air warm from the still-warm urban surfaces,
forming the nighttime warmer air temperatures
within the UHI. Lose of heat at night is blocked by
the buildings in an urban area.
Difficulties and peculiarities of an urban wind
regime
Urban Roughness
Prediction of the wind speed in the built
environment is difficult. One of the reasons is
“surface roughness”. The many obstacles and
different heights of buildings give the built
environment a high roughness coefficient2
,
compared to open, rural locations. The roughness
coefficient is generally used to extrapolate wind
speed at different heights from measurement at only
one or two heights and locations. A high roughness
coefficient means slower acceleration of speed as
height increases and therefore lower energy yields.
Table 1a gives the roughness coefficient
(or length) generally used for a type of surface. It is
worth noting the difference between open
agricultural area (even with some houses and
Table 1(b): Urban Building
Roughness – Flow Regime
Low Density –isolated flow
Buildings and trees are small and widely
spaced, eg. Modern single family housing with large
lots and wide roads, light industrial area or shopping
mall with large paved or open space.
Medium Density- wake interference flow
Two to four storey buildings and mature
trees elements of various heights occupy more than
30% surface area and create semi enclosed
spaces (street canyons and courtyards), closely
spaced and large and semi detached hoses, block
of apartments in open surroundings. Mixed houses
with shops, light industry, churches, and schools
High Density- Skimming flow
Buildings and trees closely packed and of
similar height, narrow street canyons eg. Old town
centres dense row and semidetached housing,
dense factory sites.
High rise- chaotic or mixed flow
Scattered or clustered tall towers of
different heights jutting up from dense urban
surroundings, eg. Modern city core, tall apartment,
major institutions
Fig. 1:The wind speeds at Ground and
higher levels (Grimmond et al., 2007)
Table 1(a): Roughness coefficients
for different surfaces 2
Roughness Landscape Type
length m
0.0002 Water surface
0.0024 Complete open terrain with a
smooth surface eg. Concrete
walkways, airports, mowed grass
etc.
0.03 Open agricultural area without
fences and hedgesrows and very
scattered buildings, only smoothly
rounded hills
0.055 Agricultural land with some houses
and 8 mtrs tall sheltering
hedgerows with a distance of
approximately 1250mtrs.
0.1 Agricultural land with some houses
and 8 mtrs tall sheltering
hedgerows with a distance of
approximately 500mtrs.
0.2 Agricultural land with some houses
and 8 mtrs tall sheltering
hedgerows with a distance of
approximately 250mtrs.
0.4 Villages small towns with many or
tall hedgerows, forests or very
uneven and rough terrain
0.8 Large cities with tall buildings
1.6 Very large cities with tall buildings
and skyscrapers
269AHMED & BHARAT, Curr. World Environ., Vol. 7(2), 267-273 (2012)
hedgerows) at 0.055 to 0.1 roughness, compared
to 0.8 for larger cities with tall buildings – which are
typical of the locations now being considered for
small wind installations.
Due to the high roughness in the built
environment, the wind speed close to the ground
becomes a local parameter (dependent on local
conditions near the ground). It is then not possible
to measure a local parameter (wind speed) on the
basis of some average characteristics of the
roughness of the broader area of the built
environment.
Turbulence
The roughness of the earth’s surface,
which causes drag on the wind, converts some of
the wind’s energy into mechanical turbulence.Since
the turbulence is generated at the surface, the
surface wind speed is much less than the wind
speeds at higher levels fig.1. Turbulence includes
vertical as well as horizontal air movement and
hence the effect of the surface frictional drag is
propagated upwards. The mechanical turbulence
and the effect of frictional drag gradually decrease
with height and at the “gradient” level (around 1000
to 2000 feet) the frictional effect is negligible. The
pressure gradient at this level is balanced by the
Coriolis force (and possibly the centrifugal force),
and the wind blows almost parallel to the isobars
Wind Assessment and Planning Controls
The development of appropriate planning
guidelines is an important step in avoiding adverse
wind environments in urban areas. Currently there
are a diverse range of wind assessment criteria
like placing and spacing of buildings on site. Most
of these are effective in general agreement with
each other and with field observations. Other criteria
tend to be either unnecessarily stringent or slightly
lenient.In the case of former, this is also undesirable
as it will present unnecessary restrictions to the
form and appearance of a building.The use of wind
tunnel testing remains the most reliable technique
to model the wind environment effects around
buildings in urban and suburban environments..
The use of Computation Fluid Dynamics (CFD) or
wind tunnel visualisation techniques such as the
scouring technique may be useful only as form of
initial qualitative assessment and should not be
solely relied upon.
Planning Controls Care should be taken
in the formulation of planning controls such that the
requirements are not overly restrict innovative
design. Features such as aerodynamic tower forms,
adequate podiums, provision of awnings, strategic
planting should be encouraged but not mandatory.
At the same time, controls should be provided with
regards to adequate modelling of the wind effects.
The most critical areas around an exposed
building are usually the areas near the corners at
the base of the building (side-stream effects), at the
base of a wide face of the exposed building
(downwash effect, which is applicable for buildings
more than 12 levels in height) and though arcades
or openings at the base of the building that are
Fig. 2(a):Wind movements in
and around an exposed building
Fig. 2(b): Flow patterns around tall, slab-like
building. areas of increased wind
speeds at pedestrian level
270 AHMED & BHARAT, Curr. World Environ., Vol. 7(2), 267-273 (2012)
open to opposite aspects of the base of the tall
building (gap effect).The extent of each will depend
on the level of exposure, the strength and
directionality of the wind climate and the shielding
or funneling effects from the surrounding buildings.
Other aspects that need to be investigated are the
wind conditions like turbulent wind, funneling effect
of wind in any planned outdoor areas within or
adjacent to the proposed development.
Urban Wind Environment Criteria
If a low building is located in the wind
shadow of a tall building fig. 3b., the increase in
height of a obstructing block will increase the air
flow through the low building in a direction opposite
to that of the wind. The lower wing of a large vortex
would pass through the building.
It is has been established experimentally
that wind comfort is more closely related to the gust
wind speeds rather than the mean wind speeds.1
This is particularly so in the case of extreme winds
which can lead to people losing balance in the wind.
Rofail (2005) proposed a set of criteria that converts
the mean comfort criteria into a set of criteria for
maximum gusts, or peak wind speeds. However,
this set of criteria is based on an assumption of
15% turbulence intensity, which in the vast majority
of cases is very conservative. An alternative set of
peak wind speed criteria has been proposed by
Rofail (2007). The average airflow, the dynamic
fluctuations, and the building scale turbulence are
all closely coupled to the complicated geometry of
the building. An equally valid approach would be
to compare the mean wind speed criteria such as
those by Davenport5
against a Gust-Equivalent
Mean (the maximum of either the mean or the gust
wind speed divided by a gust factor). In addition to
comfort criteria there is a safety limit that is
applicable to all accessible outdoor areas
regardless of type or frequency of use. The safety
limit suggested by Melbourne (1978) of 23m/s for
annual maximum1
gust has been adopted by most
consultants and forms part of most sets of criteria.
Requirements for Reliable Wind Tunnel Tests The
key factors that ensure a reliable set of wind speed
measurements in the wind tunnel are:
´ The scale model of the building and
surrounds,
´ The modelling of the behaviour of the
approach wind and
´ The sampling parameters and type of
instruments that are used to measure the
wind speeds.
Main Factors for wind modelling
These three factors are Comparison of
Various Mean and Gust Wind Environment Criteria,
assuming 15% turbulence and a Gust Factor of 1.5.
Wind tunnel model scales should not be less than
1:500m in scale.
1) The model must include the effect of the
surrounds, including the local land
topography. The study building(s) as well as
the buildings in the immediate vicinity need
to be modelled to a greater accuracy. The
proximity model should extend to a radius of
at least 400m. Care should be taken in
modelling of porous elements such as trees,
louvres or porous screens to ensure the same
aerodynamic properties such as Reynold’s
Number similarity. To achieve this, it may be
necessary to distortion of the model’s
geometry.The modelling of features such as
balustrades in balconies may over-constrict
the flow through these areas in the model
scale and require special treatment. Similarly
the modelling of gaps through a building
may need to be distorted to achieve similarity
in the flow regime between model-scale and
full-scale.4
2) The key parameter in modelling the
behaviour of the approach wind is to ensure
that the flow correctly matches with the fullscale
in terms of the variation of the mean
wind speed as well as the turbulence
intensity with height to within 10 percent.The
other parameter is the modelling of the
integral length scale of turbulence to within
a factor of 3 (AWES, 2002). The reference
wind speeds need to be based on an
analysis of the wind climate data obtained
from an observation station located within a
reasonable distance from the study site. It is
recommended that the climate data used of
wind speeds for a period of at least 10years7
.
The wind climate data should be properly
corrected for the effect of upstream terrain,
shielding effects and the effect of the local
land topography.7
271AHMED & BHARAT, Curr. World Environ., Vol. 7(2), 267-273 (2012)
3) Measurements should be made of the peak
and mean wind speeds. Filtering of the
velocity signal needs to be applied for the
maximum gust to represent a 3-second peak.
Measurements should be taken from at least
8 wind directions, although 16 wind
directions is currently the standard practice
and is recommended.There are two types of
instruments that can be used, hot-wire
anemometry and pressure-based sensors
such as the Irwin probe7
. Note that some
types of pressure based sensors are very
sensitive to wind direction and should be
avoided. Pressure based sensors should be
properly calibrated to ensure that they
provide a reliable wind speed estimate for
the range of wind speeds within which they
operate in the wind tunnel.The results should
be expressed as either gust wind speeds or
both gust and gust-equivalent mean wind
speeds.The gust-equivalent mean is defined
as the maximum between the mean and a
gust-equivalent mean wind speed.
The latter is the gust wind speed divided by
an appropriate gust factor.Initial tests should be carried
out without the effect of vegetation to enable the wind
engineering consultant to properly identify the
prevailing wind flow mechanisms. It is also important
to ensure that any recommendation suggested in the
report shall be adequately modelled and tested in
the wind tunnel.This is important since a solution that
would work for one project may not necessarily be
sufficiently effective for another project even if the wind
flow mechanism is similar.
Case Studies
To ensure a viable Design, the formulation
of strategies needs to be carried out in close
collaboration with the architect or designer
responsible for the project. Below are some examples
of wind effects from projects undertaken by Wind-tech
Consultants and details of solutions that were
recommended after confirmation of their effectiveness
through wind tunnel testing. Note that in some cases,
more than one solution may be presented.
Case Study 1
The tower illustrated in Figure 3 is for a
45-storey tower project in Melbourne.
Fig. 3: Is for a 45-storey tower project in Melbourne
Fig. 4: Redevelopment of property
at Civil Lines, located in Delhi
The tower is located at a corner of a city
block and the wide face fronts a narrow street as
well as the prevailing wind direction for Melbourne
(north). Furthermore, the site is relatively exposed
in that direction. The result is that the tower can
potentially generate a significant downwash and
side-stream effect around the corner of that city
block. Figure 6 shows a side profile of the tower
with the wind incident from the north direction. Two
options were presented. One required the tower to
be set back from the lane. The second treatment
required a small podium with a high wall to capture
the down-washed winds and direct them to a
permeable car parking level.
Case Study 2
The development shown in Figure 4 is
located in Delhi. This development is exposed to
all 3 prevailing wind directions for Delhi.6
The north-easterly winds and westerly
winds were of particular concern for this
development as the two towers are aligned in the
north-south direction. This particular site happens
to be situated near the top of a ridgeline in the land
form that runs north-south.This results in potentially
strong funneling effects between the two tower
272 AHMED & BHARAT, Curr. World Environ., Vol. 7(2), 267-273 (2012)
system. Eco-friendly absorption technology adopted
for air-conditioning. Careful planning of air
distribution system. Air-conditioning standards set
by acceptance level of office staff and not by
international norms. Energy-efficient lighting system
and daylight integration with controls. Optimization
of structure and reduction of embodied energy by
use of less energy-intensive materials6
Case Study 4
Another example is a development in
Waterloo, Sydney fig. 6. The results indicate that
this development will be subject to un-favourable
wind conditions due to the effect of the westerly
winds
It was established that this is due to a
combination of direct ground level winds as well as
a side-stream effect that was accentuated by the
effect of the proposed high colonnade at the northwest
corner of the proposed building. This effect is
further complicated by the fact that an outdoor café
is proposed under the high colonnade at that corner
of the building. The optimum solution involved the
use of strategic tree planting along the western
aspect of the development as well as a freestanding
canopy under the north-western corner of
the colonnade area to act a deflector. With this
treatment the wind conditions were improved from
exceeding the safety limit to being acceptable for
seating and therefore acceptable for use as an
outdoor café area.
Fig.5: Transport Corporation of India Ltd,Gurgaon
buildings that are well in excess of the safety limit.
A number of treatment options were investigated
including large canopies along the entire length of
the two tower buildings and over the gap. This is
possibly due to the significant contribution from the
upwash of the ground level winds over the northern
edge of the podium.This effect seems to have been
accentuated in case by the fact that the site is
situated at the apex of a saddle formation in the
landform, being exposed to the north-east winds
along the wide aspect. The only treatment that
worked effectively was the least expensive.
Case study 3
Transport Corporation of India Ltd,
Gurgaon- Inward-looking compact form, with
controlled exposure.
Two types of windows designed: peep
windows for possible cross-ventilation and view,
the other being for day-lighting. The courts towards
which the building has more transparency have
structural framework to provide support for shading
screens. Landscaping acts as a climate modifier.
The window reveals of the peep window cut out
summer sun and let in winter sun. Adjustable
Venetian blinds in double window sandwich to cut
of insulation and allow daylight. Polyurethane board
insulation on wall and roof. Fountain court with water
columns as environment moderator. Building
systems designed so as to draw upon external
environment to supplement the air-conditioning
Fig. 6: Photographs of the model of theWaterloo,
Sydney development in Windtech Consultants’
wind tunnel showing details of the optimum
treatment for the effect of the westerly winds on
the wind conditions at the north-western corner 1
273AHMED & BHARAT, Curr. World Environ., Vol. 7(2), 267-273 (2012)
Fig. 7:The Abu Dhabi development with the
areas affected by the gap effect indicated
A wind tunnel model study was carried
out and indicated that a significant speed up effect
occurs within these gaps due to the gap effect. For
each gap, the effect was successfully ameliorated
by means of a baffle screen located at each end of
the gap.
One of these screens served as a wall for
a covered gazebo. Other areas that required
treatment included one of the street corners and
the base of one of the towers.The wind effects there
were successfully ameliorated by means of strategic
planting. Fig. 7. The Abu Dhabi development with
the areas affected by the gap effect indicated.
CONCLUSION
This paper demonstrates how it is possible
to plan for habitable wind environments, while still
accommodating the architectural intent. The
average airflow, the dynamic fluctuations, and the
building scale turbulence are all closely related to
the complicated geometry of the building. Features
such as aerodynamic building forms, adequate
podiums, provision of awnings, strategic planting
should be encouraged in the design.
With the proper modeling and simulation
techniques and appropriate wind field study. it is
possible to achieve a favourable outcome for both
the owners and end users. Local authorities can
also have a role in stipulating development controls
without over-specifying the building form, which
runs the risk of stifling innovation.
Case Study 5
A wind environment study was conducted
for the project in Abu Dhabi described in Fig. 7.This
development includes 3 linked residential tower
buildings with two 9-level high undercroft areas
located below the linkages.
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4. Davenport, A.G. An approach to human
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