Designing the City According to the Wind: Using WAsP to Minimize the Impacts of High Rise Building...

International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163 Volume 1 Issue 5 (June 2014) http://ijirae.com

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Designing the City According to the Wind: Using WAsP to Minimize the Impacts of High Rise Building Complex on

Human Comfort

Ar. Seemi Ahmed Assistant Professor, Department of Architecture and

Planning, M.A. National Institute of Technology, Bhopal India,

Dr. Alka Bharat Professor in Department of Architecture and Planning,

M.A. National Institute of Technology, Bhopal, India

Abstract: -- Buildings should provide shelter for human activities. With increasing urbanization and increasing urban population the cities are forced to grow vertically. This increase in high rise buildings density influences the outdoor climate especially the wind climate. This paper is focussed on the wind flow patterns around high rise building complex. Incorporation of wind in design process in an important issue, therefore, the design of a building should not only focus on the building envelope and on providing good indoor environment, but should also include the effect of the design on the outdoor environment. The outdoor environment of a building, in particular related to wind, has received relatively little attention in the Building Physics community. The present paper addresses architects and planners and focuses on the outdoor wind environment for human comfort first, a literature review on related wind studies is provided. The relation between wind effects, wind comfort, wind danger and wind climate is outlined. Key words: Wind Flow Patterns, WAsP, Urban roughness, Micro climate

I. INTRODUCTION Architects, amongst others, deal with the indoor and outdoor climate and the building envelope. The outdoor climate

has received relatively little attention in the physical planning community. Where it has been addressed, it has mainly been in order to provide boundary conditions for the study of the indoor climate and of the hygro-thermal behaviour and durability of the building envelope, but not for the outdoor climate itself. The construction of a High Rise building inevitably changes the outdoor climate or the microclimate. Wind speed, wind direction, air pollution, driving rain, radiation and daylight are all examples of physical aspects that constitute the micro climate and that are changed by the presence of the building. The change of these quantities depends on the shape, size and orientation of the building and on the interaction of the building with the surrounding buildings and other obstacles such as trees etc. These changes can be either favourable or unfavourable. Unfavourable changes include: increased wind speeds around the building leading to uncomfortable or even dangerous conditions, decreased wind speeds leading to insufficient removal and accumulation of traffic or industrial exhaust gasses, shadowing or reflection of sunlight by the building, visual pollution acoustical changes, etc. Increased wind speed at ground level is one of the problems that are considered most important. The present paper will be confined to this aspect of the outdoor climate around a high rise building complex. Verifying natural ventilation, micro climatic modifications and human comfort conditions through models assists the architectural design and urban planning process. Wind tunnel tests, soft-wares or mathematical models are important tools in the analysis of micro wind climate changes, allowing greater precision in the airflow assessment in internal and external environments, Prata-Shimomura el al [2009]

II. WIND EFFECTS, WIND COMFORT, WIND DANGER AND WIND CLIMATE A distinction is made between the mechanical and the thermal effects of wind. Mechanical effects of wind on people range from the feeling of a light breeze on the skin to being blown over by a strong gale. [Lawson and Penwarden 1976] have provided an extended "Land Beaufort Scale" showing wind effects on people. The tabulated wind speed refers to the value that is measured at pedestrian height (z = 1.75 m) over open terrain with an aerodynamic roughness length z0 of 1.0 m . It is important to note that the measurement values are averaged over periods of 10 minutes or 1 hour (steady wind). The wind effects mentioned however can be caused by both steady wind and wind gusts (turbulence). Steady wind effects have been investigated by Penwarden [1978], Hunt et al. [1976], Penwarden et al. [1978], Murakami et al. [1980] and Murakami [1980]. From an extensive study, Murakami et al. [1980] have found that a steady wind of 5 m/s only causes a minor disturbance of hair and clothes and wind is felt on the face, a steady wind of 10 m/s causes hair to be disturbed and fluttering clothes, while a steady wind of 25-33 m/s will blow people away. Comparing these values with the values in Table 1 provides an indication of the importance of wind gusts in wind effects. The effects of non-uniform winds on people have been studied by Murakami et al. In a wind tunnel, people were asked to walk through a jet of strong side winds and footstep irregularities were monitored. It was found that these irregularities were roughly comparable with wind effects in uniform flow with a speed of 1.5 times the wind speed in the jet. The effects of gusty winds on people have been studied by (in chronological order): [Hunt and Poulton 1983], [Hunt et al. 1976], [Jackson 1978], Murakami et al. [1980, 1986], [Murakami 1982] and [Ansley 1977]. According to calculations by [Simoes 1996], a sudden increase of wind speed to 15 m/s or more can be sufficient to bring people out of balance. Summarizing the results of other researchers, Beufort [1977] states that: a gust of 4 m/s during 5 s causes hair to be disturbed and clothes



to flap, a gust of 7 m/s during 5 s can cause hair to be disarranged, a gust of 15 m/s during 2 s can bring people out of balance and is dangerous for the elderly and the infirm, a gust of 20 m/s can be dangerous, even for young people and a gust of 23 m/s will blow people over. Comparing these values with those given above for steady winds [Murakami et al. [1980] again indicate the importance of wind gusts in wind effects: for the same wind effect, gust wind speeds are significantly less. Recent studies have analysed the role of urban street width and street building heights, wind direction and velocity [e.g. Berkowicz et al., 2006; Di Sabatino et al., 2007a,b; Di Sabatino et al., 2008; Soulhac et al., 2008], building roof geometry [e.g. Huang et al., 2009], tree planting [e.g. Gromke et al., 2008; Buccolieri et al., 2009], building packing density [e.g. Belcher et al., 2003; Cheng et al., 2007; Blocken et al., 2008]. As the wind speed increased, its negative impact on the city became clearer, and the tree falls were directly related to it. Between 12.00 p.m. and 4.00 p.m., the wind speed varied between 13 m/s and 15 m/s and 80% of the trees fell, (28 of the total of 35). This peak in occurrences coincided with the period of maximum gust (27m/s). The distribution of occurrences was analysed in relation to the different land uses and ventilation classes defined for the city of Lisbon [Alcoforado et al, 2005]. It was found that 41% of the fallen trees took place in densely built-up areas (where at least 50% of the area is covered with high-density buildings) in the southern part of the city, and 20% in green areas. Although these districts have greater roughness length (z001 m) and therefore diminish regional wind speed by about 30% comparing with non-occupied areas [Lopes, 2003]. Specifically, the objective of this work is to identify the impacts of tall buildings and constructive densification on human comfort after turbulence studies over the southeast part of Bhopal in India, once this sector of the city is experiencing an urban expansion process closely linked to the main New market. Comparing a maximum Building Density scenario with taller buildings and lower urban roughness allowed by local authority to the present situation and an intermediate proposal with medium height buildings with moderate roughness, this research verifies the airflow conditions in several points of the study-area.

III.COMPUTATIONAL SIMULATION a) The Pilot Study: Platinum Plaza Bhopal

Platinum Plaza is a residential cum commercial High Rise Complex having 10 floors with a overall height of 30 mtrs and is considered highest buildings of Bhopal. Bhopal is the capital of the Indian state of Madhya Pradesh and the administrative headquarters of Bhopal District and Bhopal Division.. Bhopal is known as the City of Lakes for its various natural as well as artificial lakes and is also one of the greenest cities in India. A B-1 class city, Bhopal houses various institutions of national importance. Platinum Plaza is situated in the area developed in 1956 of this city.

b) DESCRIPTION OF THE MONITORING SITE

Survey of India, Topo Sheet 55-E/7 -E/8 (1:50,000 Scale). Has been obtained from the Geological Survey of India in which the contours of the site near Bhopal are shown. This site has been chosen for the study (Fig. 3). The site under consideration covers an area of approximately 56 square kilometres in linear topography, interspersed with thick vegetation roads and urban dwellings. These areas were marked off separately in the vector map giving roughness values as defined by WAsP and given in Table 1.The map of the whole area was digitised.

Fig. 3.0 Study Site with Platinum Plaza and 12 samples in four cardinal directions



Table 1. Terrain classification due to Davenport and quoted by Wieringa (1)

class Roughness length: m

Landscape features

no name

1 Sea 0.0002 open water, tidal flat, snow with fetch above 3 km

2 smooth 0.005 featureless land, ice

3 Open 0.03 flat terrain with grass or very low vegetation, airport runway

4 roughly open

0.10 cultivated area, low crops, obstacles of height H separated by at least 20 H

5 rough 0.25 open landscape, scattered shelter belts, obstacles separated by 15 H or so

6 very rough 0.5 landscape with bushes, young dense forest etc separated by 10 H or so

7 closed 1.0 open spaces comparable with H, eg mature forest, low-rise built-up area

8 chaotic over 2.0 irregular distribution of large elements, eg city centre, large forest with clearings

c) HUMAN COMFORT AROUND A HIGH RISE BUILDING COMPLEX

[Murakami et al 1980] carried out several outdoor experiments, mainly walking tests. Wind effects on clothes, hair and walking were observed from instantaneous gust speeds of about 7m/s . Murakami choose to co-relate wind effects on people with a mean wind speed averaged over 10 seconds. This result is in a poor co-relation between wind speeds and wind effects. Still it can be concluded from his results that U10<10m/s the effect of head winds on walking is much larger than the effect of tail winds. For validation extensive field surver was carried out and human comfort results were correlated with the table developed by Murakami 1980 (table.2) TABLE 2: WIND EFFECTS ON PEOPLE AS A FUNCTION OF GUST SPEED UG AND ESTIMATED GUST DURATION TG, DATA ARE FROM M ( MELBOURNE ET AL) MU MURAKAMI ET AL, 1980, JA (JACKSON, 1978) AND B THE EXTENDED BEAUFORT SCALE (PENWARDEN, 1973)

Ug m/s tg (s) author Wind effect 4 5

5 B/JA B/JA

Clothing flaps , Hair is disturbed

7 1-10 5

B/JA

Dust and paper being raised, Hair disarranged

10 3 5 10

MU JA JA

Irregular footsteps, walking difficult to control, eyes felt dry violent flapping of clothes

14 2 10

JA JA

Blown sideways, appreciably slowed into wind

15 2 3

MU People can be brought out of balance by gusts, walking difficult

16 10 10

JA JA

Almost halted into wind, uncontrolled walking down wind

20 3 M Great difficulty in balance with gust 21 2 JA Unbalanced, grabbing at supports 23 3 M People blown over by gusts

d) PREPARATION OF VECTOR MAP [Mortensen et al, 2000] have reported that it is possible to obtain accurate assessment of stable wind flows which are close to the measured values with maps of 8 x 8 sq. km and the influence of contour interval on the accuracy of wind speed prediction. Prediction errors can be reduced with smaller contour intervals with a contour interval of 20 m or less. In the present study area within 500 mtrs around Platinum Plaza has been considered.



The topography map has been obtained from Geological Survey of India and the detailed surface features based on aerial photography conducted by the National Remote Sensing Agency were available as 2 x 2 sq. km tiles in the AutoCAD dwg format. These 6 layer map provide contours at 1 m intervals, trees, buildings, temples, tombs, electric and telegraph poles, waterways, marshy areas, roads, footpaths etc in different layers. Information relevant to WAsP, namely contours, open areas, trees, water bodies and buildings were retained by switching off the unwanted layers. Such tiles were joined together in the AutoCAD software and saved as dxf files (drawing exchange format), which could be imported into the WAsP Map Editor. In this study, contour intervals at 5 m were retained and imported into the WAsP Map Editor for calculating the wind flows throughout the area. The map was transformed to the Universal Transverse Mercator (UTM) projection with the datum of WGS 1984. The area falls in Zone 43 with the central meridian of +75° E.

Fig.4 Raster map of platinum plaza with study area marked as concentric rings

e) SPECIFYING OBSTACLES NEAR MEASURING SITE

For the study 12 buildings structures situated in four cardinal directions around Platinum plaza were selected out of which one in cardinal direction was modelled taking Platinum Plaza as an obstacle . Obstacle present near the measuring site affect the wind data collected and it depends Upon Building porosity and roughness of the area. Obstacles are considered by WAsP as “boxes” with a rectangular cross-section and footprint. Obstacle must be specified by its position relative to the site and its dimensions and must be assigned a porosity value. The position of an obstacle is specified in a local, polar coordinate system. Angles (bearings measured with a compass) are given clockwise from north; distance is the radial length from the site to the corner of the obstacle (measured with a measuring tape or a range finder). As a general rule, the porosity can be set equal to zero for buildings and ~ 0.5 for trees. A row of similar buildings with a separation between them of one third the length of a building will have a porosity of about 0.33. For windbreaks the characteristics defined in WAsP may be applied. The porosity of trees changes with the level of foliation, i.e. the time of year and similar to the roughness length, the porosity should be considered as a climatologically influenced parameter.

IV. OBSERVATION AND CONCLUSIONS FROM THE STUDY It is seen from the analysis of observed wind Flow Patterns for the six peak wind months for the site that the site is endowed different kind of wind patterns around it. However, it is also seen that the average wind speed calculated for different months owing to uneven wind speed distribution due to a strong influence of the Obstacle Platinum Plaza in the urban area is modifying the wind flow patterns. The wind rose showing the wind direction, duration and speed are shown in table 2 with their associated wind climate chart.



For the sample 1 situated on the East of Platinum Plaza wind flow from modelling shows that there is disturbed wind showing turbulence and sudden fluxes of wind. Fig. 4 sample 1 For sample 2 situated on the North of Platinum Plaza wind flow from modelling shows that there is increase of 90% in the wind speed as compared to the prevailing wind the direction remains the same from west Fig. 4 sample 2 For sample 3 situated on the West of platinum Plaza there was a reverse flow of wind, the wind was blowing from East direction where as the velocity was reduced to 20% of the prevailing wind. Fig. 4 sample 3 For sample 4 situated on the South of Platinum Plaza wind flow from modelling shows that there is increase of 90% in the wind speed as compared to the prevailing wind the direction remains the same from west. Fig. 4 sample 4. The building complex located in Bhopal has been investigated taking into account local wind conditions. The analysis was limited to wind speed distribution around buildings. The highest wind speeds have been noted in passages between windward buildings. Wind speed ratios in these areas vary between 1.8 and 2.7. The open character of the buildings complex as well as their height simplifies ventilation process but also disturbs the wind flow. The situation could be improved by introducing windbreaks. Windbreaks are built using various materials and techniques. They can consist of trees, fences palisades, walls or earth berms . Taking into account the fact that west winds are prevailing and that the area is covered only by grass, the construction of the windbreaks on the west side can provide shelter for the buildings. The analysis dealing with the influence of different windbreaks on wind flow in housing estate, carried out by authors, indicate positive role of shelterbelts. The high porosity of the trees decreases the kinetic energy of the wind and at the same time doesn’t allow for local acceleration near the corners. The maximum difference in V/Vo was about 0.7. Besides, the sheltering role of trees can also have a significant influence on the noise and pollution control as well as microclimate and visual effects. The urban planners didn’t cover the wind comfort requirements during the planning process which exposed the residents to wind nuisance. The modelling results corroborate with the assumption that the verticalization of the city and a lower building density allow better air circulation within the city fabric since this urban form is more permeable to the wind. On the other hand, the elevation of the urban roughness layer and the reduction of the air velocity at the immediately posterior regions are the main effects of this urban form. Although the study of natural ventilation conditions should be considered at specific conditions of each locality, since the conditions of air circulation are easily changed by the construction parameters, this research might help urban design process specifically in parts of the city in constructive densification process, indicating that, despite the increase in height, the higher building densities may cause the stagnation of the airflow even if it is associated with lower building’s height.

V. PLANNING OPTIONS

Narrowing the streets of S/h= 1 instead of 2 yields almost the same result but with 10-20% lower turbulence levels. In these narrow streets flow channelling may become important. Initial conditions, which are generated by the high rise building, may be maintained for several street widths.

Building symmetrical conditions—when the wind is perpendicular to the symmetrical blocks, changes in a symmetrical way may have little influence to wind conditions，however there is still comparability that the long side can influence winds conditions a little more.

Enlarging of spaces could involved increasing of winds speed. The open character of the buildings complex as well as their height simplifies ventilation process but also

disturbs the wind flow. The situation could be improved by introducing windbreaks. Besides, the sheltering role of trees can also have a significant influence on the noise and pollution control as

well as microclimate and visual effects [Vinet, et al].

a) Breezeway/air path The more air ventilation to the streets, the better it will be for these dense urban areas. The overall permeability of the district has to be increased at the ground level. This is to ensure that the prevailing wind travelling along breezeways and major roads can penetrate deep into the district. This can be achieved by proper linking of open spaces, creation of open plazas at road junctions, routes, and widening of the minor roads connecting to major roads. maintaining low-rise structures along prevailing wind direction Also avoid obstructing the sea breeze. Any localized wind problem along the waterfront should be dealt with locally and not affect the overall air ventilation of the city.

b) Podium/site coverage The effect of building layout (especially in terms of building site coverage) has a greater impact than that of building height on the pedestrian wind environment. Stepping building heights in rows would create better wind at higher levels if differences in building heights between rows are significant. The ‘‘podium’’ structures commonly found in Hong Kong are not desirable from the viewpoint of maximizing the wind available to pedestrians. The podia with large site coverage not only block most of the wind to pedestrians (affecting comfort and air quality), but also minimize the ‘‘air volume’’ near the pedestrian level (affecting air quality)

c) Building disposition Proper orientation and layout of the buildings with adequate gaps between buildings are needed. Stagger the arrangement of the blocks such that the blocks behind are able to receive the wind penetrating through the gaps between the blocks in



the front row. In the case of a new town, to avoid obstruction of the sea breeze, the axis of the buildings should be parallel to the prevailing wind. In order to maximize the wind availability to pedestrians, towers should preferably abut the podium edge that faces the main pedestrian area/street so as to enable most of the downwash wind to reach the street level.

d) Building heights Vary the heights of the blocks with decreasing heights towards the direction where the prevailing wind comes from. If not, it is better to have varying heights rather than similar/uniform height. Given the extremely high density of the urban fabric and narrow streets, a probable strategy for improving air ventilation is by varying building heights for diverting winds to the lower levels. Nonetheless, assessment will be required to further quantify the actual performance of such potential strategies in view of the common deep urban canyon situations in Bhopal.

e) Building permeability The provision of permeability/gaps nearer to the pedestrian level is far more important than that at high levels. Create permeability in the housing blocks. Try to create voids at ground level to improve ventilation for pedestrians. This will improve not only the air movement at the ground level (thus improving pedestrian comfort), but also help to remove pollutants and heat generated at ground level. The channelling effect created by the void also helps to improve ventilation performance for those residential units at the lower floors. Creation of openings in the building blocks to increase their permeability may be combined with appropriate wing walls that will contribute to pressure differences across the building façades and thus will permit the air to flow through the openings of the buildings. The wing walls have to be designed according to the known standards. For very deep canyons or very tall building blocks, mid-level permeability may be required to improve the ventilation performance for those occupants situated mid-floor.

f) A need for planning to optimize air ventilation It was in general expertly opined that unlike most cities in the world, wind gust may not be a problem in Hong Kong. On the contrary, wind stagnation and blockage is a major problem. And for the tropical climatic conditions of Hong Kong where wind in the summer is a welcome quality, it was opined unanimously that ‘‘the more the better’’ should be the guiding spirit. That is to say, designs and developments should focus on not blocking the incoming wind, as well as minimizing the stagnant zones at the pedestrian levels.

VI. CONCLUSIONS The urban planners didn’t cover the wind comfort requirements during the planning process which exposed the residents to wind nuisance. The modelling results corroborate with the assumption that the verticalization of the city and a lower building density allow better air circulation within the city fabric since this urban form is more permeable to the wind. On the other hand, the elevation of the urban roughness layer and the reduction of the air velocity at the immediately posterior regions are the main effects of this urban form. Although the study of natural ventilation conditions should be considered at specific conditions of each locality, since the conditions of air circulation are easily changed by the construction parameters, this research might help urban design process specifically in parts of the city in constructive densification process, indicating that, despite the increase in height, the higher building densities may cause the stagnation of the airflow even if it is associated with lower building’s height

REFERENCES 1. Ahmed Siraj Wind Energy Theory and Practice, PHI Publication first edition 2010 2. Boris Jay P. Dust in the Wind: Challenges for Urban Aerodynamics,, Laboratory for Computational Physics and

Fluid Dynamics 2002 3. Campbell Neil et. al “Wind Energy For The Built Environment” Paper published in Procs. European Wind Energy

Conference & Exhibition, Copenhagen, 2-6 July 2001 4. Davenport, A.G. An approach to human comfort criteria for environmental conditions, Colloquium on Building

Climatology, (1972). 5. Rafail, Tony “Developing habitable Built Environment” CUTBH 8th World Congress 2008. 6. Representative designs of energy-efficient buildings in India Published by Tata Energy Research Institute 2001 7. Urban Wind Assessment in UK, An introduction to wind resource assessment in the urban environment, Feb 2007

Designing the City According to the Wind: Using WAsP to Minimize the Impacts of High Rise Building...

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