The study of height variation on outdoor ventilation for Singapore’s high-rise residential housing estate study of height variation on outdoor ventilation

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The study of height variation on outdoorventilation for Singapores high-riseresidential housing estates

Rou Xuan Lee*, Steve Kardinal Jusuf and Nyuk Hien WongDepartment of Building, School of Design and Environment, National University ofSingapore, 4 Architectural Drive, Singapore 117566, Singapore

*Corresponding author:

[email protected]

AbstractThis article is concerned with external ventilation levels within a multi-story Housing and Development

Board (HDB) residential estate, focusing toward a deeper understanding of wind flow with respect todifferent levels of height variation (HV). This study analyzed through parametric study, usingnumerical simulations with the realizable k 1 turbulence model, the various scenarios of HV within a

typical residential HDB estate or precinct. It is found that external wind flow within the precinct forboth the pedestrian and mid-height levels are affected differently by the HV value. Some rules ofthumbs can be established for HVs in the efficient use of outdoor ventilation.

Keywords: precinct level; outdoor ventilation; height variation; Housing and Development Board;

ventilation ratio index

Received 1 November 2012; revised 3 February 2013; accepted 4 March 2013

1 INTRODUCTION

Urbanization is often associated with problems such as higher

air temperatures, high pollution concentration and lower windflow, compared with rural areas which are often due to the

buildings roughness and geometry. Unstructured and im-proper urban morphology (geometry and placement) planningare common in areas of rapid urbanization. One good way to

counteract or reduce outdoor ventilation problems is to go fordesigns that are optimized for ample outdoor ventilation to

dissipate built-up heat within through the process of turbulenttransfer. A throughout literature review done by the authorrevealed that there are basically seven main morphological vari-

ables that determine natural outdoor ventilation efficiencywithin a said precinct area or development. They are orienta-tion [13], geometry ratio [47], building shape [8], gross

building coverage ratio (GBCR) [3, 6, 913], height variation[3, 14, 15], permeability [1, 9] and staggering [3, 13, 16, 17].

These studies by previous researchers postulate that there is anassociation between the different morphological variables andoutdoor ventilation potential within a precinct. How some of

these planning and design parameters affect the ventilation po-tential is a research question that needs careful investigation.

This article discusses a detailed parametric study on the

effects of one of the building morphological variables, build-

ings height variation (HV), on outdoor ventilation within a

typical high-rise Housing Development Board (HDB) public

residential precinct area. Parametric studies are conductedwith computational fluid dynamics (CFD), using the realizablek 1 turbulent model, to study the various scenarios of HV. It

is found that external wind flow within a precinct for both the

pedestrian and mid-height levels are affected differently by theHV value. The objective of the present work is to find out how

the HV values vary with the magnitude of outdoor ventilationwithin a precinct. Some rules of thumbs can be established for

HVs in the efficient use of outdoor ventilation.Many research findings point to the advantage of varying

building blocks height rather than having uniform heightsthroughout, in order to provide better ventilation within anestate. Ngs [3] research concluded that varying the heights of

blocks, with decreasing heights toward the prevailing wind dir-ection, can help to optimize the wind-capturing potential ofthe development. This is especially when height differences are

significant, and it is always better to have varying heights

rather than similar/uniform height for diverting winds to thelower pedestrian levels. Chan et al.s numerical study findingsproposed that uniformity of building height in urban planningshould be avoided as nonuniformly constructed roof heights

provide better ventilation [14]. He proposed a ratio of leewardbuilding height to windward building height of 1.001.25, buthis findings are from a study of a field-sized long isolated

canyon. It focused more on the wind characteristics developed

International Journal of Low-Carbon Technologies 2013, 0, 119# The Author 2013. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]:10.1093/ijlct/ctt013 1 of19

International Journal of Low-Carbon Technologies Advance Access published April 16, 2013


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within two canopies instead of an array of building structures

as per this present research. The results might not be applic-able to the present study. Xie et al. [15] research shows that

different flow patterns are formed within the study canyonwith different variation in heights of ambient buildings.

In this present research, a parametric study approach willbe adopted, whereby the effects of HV within a precinct area

against the average outdoor wind speed at pedestrian level andmid level will require a comprehensive systematic study onmany possible values and configurations (random and strati-fied) of the HV variable. Such systematic studies are much too

difficult to be realized in real streets and relatively costly inwind tunnels; hence, CFD simulations offer an appealing alter-

native at this moment for this article. The objective is to applythe numerical simulation method to wind flow around build-ings within a said precinct area under different HV values and

configurations. The methodology adopted and the resultsobtained will be detailed in the following sections.

2 METHODOLOGY

In this study, an in-depth parametric study approach isadopted for the investigation of HV on the implications ofaverage outdoor wind velocity within a said precinct. The HV

index that will be adopted in this study is simply the standarddeviation of the HV of all the high-rise buildings within the

precinct, which is formulated as

HV ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPHi Have2

N 1

s:

Numerical study is adopted to simulate the conditions of atypical public Housing and Development Board (HDB) high-rise residential housing estate, which is set to be a typical pre-

cinct size of 500 500 m. For comparison purposes, twobase case scenarios are used here, one for the point blocks(each block dimension is 30 30 112 m) and another forslab blocks (each block dimension is 100 20 50 m). Thebase case spacing between the blocks is 20 m apart. All the

blocks are confined within an 500 500 m HDB estate,assumed to be the maximum size we have for high-densityliving in Singapore (Figure 1), given the current regulations

and control. The detailed variation in different scenarios willbe described in the following sections. The average outdoor

velocity magnitude values will be extracted at the pedestrianlevel (2 m above ground level) and the mid-level (following

that of the base case) cut at a constrained horizontal planewithin the precinct area.For the mid-level of the point blocks, it will be considered

at 56 m, which is the average of all the buildings mid-levels in

the precinct. For the mid-level of slab blocks, it will be consid-ered at 25 m above ground, which is the average of all build-

ings mid-levels in the precinct.

2.1 Building shapeThe most commonly adopted shapes for HDB flats are the

point block type and slab block type. The base case dimensionsof each and every block are as follows (L W H):

Point block: 30 30 112 m (can be built higherstructurally)

Slab block: 100 20 50 m (due to structural constraints,a slab block cannot go too high).

Other less common shapes are not studied due to their rarity

and also their results are not easily being correlated to othervariables in a parametric study.

2.2 Orientation of wind flowThe 500 500 m precinct in the middle of the cylindricaldomain will be subjected to wind flow coming from five differ-ent directions (08, 22.58, 458, 67.58 and 908) from North, so asto study the behavior of each and every different HV and con-

figuration type, under different orientation effects on thepedestrian-level and mid-level outdoor ventilation within theprecinct (Figure 2). Figure 3 shows the orientation of the wind

with respect to both the point and slab blocks in the base cases(Figure 3).

Figure 1. Base case for point blocks arrangement (left) and slab blocks arrangement (right) in an 500 500 m estate area.

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2.3 Height variation values and configurationtypesThe effects of HV values on the point and slab blocks (build-

ing shapes), under different wind directions (orientation) andunder different configuration types, will also be discussed.

There are two different types of HV configurations that will bestudied hererandom and stratified (Figures 47).

Random HV variation refers to the buildings, under differentHV values, will have their heights randomly varied from eachother. The spread of the HV will be as random as possible, in

order to ensure an even distribution of different buildings

heights. Figure 4 shows the configuration for point blocks,

and Figure 6 shows the configuration for slab blocks.

Stratified HV variation refers to the buildings with their varied

heights, increasing toward the precincts center part (most in-terior area). Figure 5 shows the configuration for point blocks,

and Figure 7 shows the configuration for slab blocks.

The total volume of the blocks will be the same as the basecase in all the variations, regardless of the amount of change

in HVs. The spacing between the blocks will be the same asthe base case, i.e. 20 m. The focus in this article is to study theHV behavior, all else being equal.

The HV values used in this parametric list are as shown inTable 1 for both the point and slab blocks study.

2.4 Numerical simulationsIn our research here, one of the Reynolds-averaged NavierStokes (RANS) model variantsrealizable k 1 turbulence(RLZ) closure is selected for use in the simulation studies. This

model is a revised k 1

turbulence model and is proposed byShih et al. [18]. The six partial differential equations that needto be solved are as follows:

Continuity equation

@uj

@xj 0: 1

RANS equations (in x-, y- and z-directions)

Figure 3. (a) BASE CASE: Point blocks layout in a 500 500 m HDB estate. Each block is 30 30 112 m in dimension with a spacing of 20 m from eachother. The numbers indicate the blocks height. (b) BASE CASE: Slab blocks layout in a 500 500 m HDB estate. Each block is 10020 50 m indimension with a spacing of 20 m from each other. The numbers indicate the blocks height.

Figure 2. Wind coming from the different orientations into the cylindrical

atmospheric domain.

HV on outdoor ventilation for Singapores high-rise residential housing estates

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@ui@t

uj@ui@xj

1r

@p

@xi m

r

@2ui@xj@xj

@@xj

u0iu0j gi 2

where uj is the j component of velocity (m s21), u0j is the root-

mean-square of the velocity fluctuation j component, P is thepressure in Newton per meter square (N/m2), t is the time (s),xj is the j coordinate (m), r is the air density (kg m

23), m is

the dynamic viscosity (m2s) and gi is the gravitational bodyforce (m s22).

The Reynolds stress is

u0iu0j 1

rmt

@ui@xj

@uj@xi

2

3kdij;

where mt rCmk2=1 is the turbulent viscosity, where Cm is a

Figure 4. HV value for point blocks in random configuration: (a) 21, (b) 34, (c), 52, (d) 63.

Figure 5. HV value for point blocks in stratified configuration: (a) 21, (b) 34, (c), 52, (d) 63.

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Figure 6. HV value for slab blocks in random configuration: (a) 11, (b) 17, (c), 21, (d) 27.

Figure 7. HV value for slab blocks in stratified configuration: (a) 11, (b) 17, (c), 21, (d) 27.




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model constant which is not fixed.

Cm 1A0 ASkU=1 ;

where A0 4.04,

As

ffiffiffi6p

cosw;

w 13

cos1ffiffiffi6

pW

;

W SijSjkSki=~S3;

~S ffiffiffiffiffiffiffiffiffiSijSijp ;

Sij 12@uj=@xi @ui=@xj;

U ~S ffiffiffiffiffiffiffiffiffiSijSijp(where there is no rate of rotation in the stationary referenceframe for this study).

Two turbulence closure equations for realizable k 1 (RLZ):

Turbulent kinetic energy (k) (m2 s22)

@k

@t uj @k

@xj 1r

@

@xjm mt

sk

@k

@xj

Gk

r 1; 3

where Gk is the turbulent kinetic energy production (kg m21 s22).

The dissipation rate of turbulent kinetic energy (1) (m2 s23)

@1@t

uj@1@xj

1r

@@xj

mmts1

@1@xj

C1S1 C2 12

k ffiffiffiffiffiv1p ; 4where S is the scalar measure of deformation tensor or mean

strain rate (m2 s22); n is the molecular kinematic viscosity (mt/

r); sk (1.0) and s1 (1.2) which are the turbulent Prandtl

numbers for kand 1, respectively;

C1 max 0:43; hh 5

;

whereby h Sk=1, where S ffiffiffiffiffiffiffiffiffiffiffi

2SijSijpis the scalar measure of

the deformation tensor. C2 ( 1.9) is also a model constant[18, 19].

All the computational calculations are performed using theStar-CCM1 code from CD-Adapco. Solutions to the problem

here utilized the above RLZ turbulence model, in which theNavierStokes equations are discretized using a finite volumemethod and the SIMPLE algorithm is used to handle pres-

surevelocity coupling. The above set of discretized algebraicequations is solved by the segregated method. Studies showedthat it provides best performance in separated flows and flows

Table 1. Tabulated values of HV for the parametric study.

Case Description Min. height (m) Max. height (m) Average height (m) HV (m)

Point blocks

1 Point blocks: BASE (B-1.5 m, G-24.0 m, G-96.0 m) 112.00 112.00 112.00 0.00

2 Point blocks: HV 30120 WA 112, SD 21 30.00 120.00 112.00 21.00



5 Point blocks: HV

50150 WA

112, SD

32 50.00 150.00 112.00 32.006 Point blocks: HV 80180 WA 112, SD 34 80.00 180.00 112.00 34.00






Slab blocks

1 Slab blocks: BASE (B-1.5 m, G-24.0 m, G-96.0 m) 50.00 50.00 50.00 0.00

2 Slab blocks: HV 3060 WA 50, SD 11 30.00 60.00 50.00 11.00










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with complex secondary flow [20]. This is provided it is prop-erly coupled with a two-layer all y wall treatment nearthe wall boundary condition. Superiority of the realizable

k 1 model (RLZ) has been established for flows includingboundary layers under strong adverse pressure gradients, separ-

ation and recirculation [18, 21] when compared with otherRANS models. This simulation exercise is executed understeady-state and isothermal conditions. In addition, air within

the street canyons is regarded as incompressible turbulent inertflow, according to the valid assumption at low subsonic speeds,the air densities are assumed to be constant under varying

pressures. This reasonably applies for lower atmospheric envir-onment as described by Sini [7].

2.5 Near-wall treatmentA two-layer zonal model treatment is adopted here. This near-

wall flow treatment is suitable for situations that encounter bothstrong body forces and important 3D characteristics of boundarylayer. It separates the computational domain into a viscosity-

affected region (in neighborhood of the wall) and a fully turbu-

lent region. The demarcation between these two regions is basedon a turbulent Reynolds number Rey yk1=2=v, where ystandsfor the normal distance from the wall to the grid cell centers, kstands for the turbulence kinetic energy and v stands for the

kinematic viscosity (Figure 8).The realizable k 1 (RLZ) turbulent model is employed in

the fully turbulent region (Rey. 200) as previously described.

On the contrary, in the viscosity near-wall region (Rey, 200),a one-equation model (only for the turbulent kinetic energy)is employed [22]. In comparison with a classical low Reynolds

number approach, the two-layer zonal model treatment leadsto an improved convergence, requires less mesh elements in

the viscosity sublayer and introduces properly the distribution

of the turbulent length scale near walls [23, 24].

2.6 Computational domainSelection of computational domain size depends mainly on theexpected air flow patterns and wakes around the buildingsdimensions and local terrain under investigation. Researchers

have commonly determined the domain size as a multiple ofbuilding dimensions. Domain size testing should be done tofind a large enough size whereby further size increase does not

affect the computed values significantly. The reason is that un-

necessary size increase not only increases the number of gridnodes, but also demands more CPU resources and stability.

The computational domain adopted here consists of a large

cylindrical atmospheric volume of radius 1800 m and height of800 m (Figure 9). The domain radius follows the rule as sug-

gested by Singapores Building Control Authority (BCA) [25]whereby the computational domain radius is extended from thedevelopment edge, three times of the longest distance length

which is measured across the boundary of the development(include the development of interest, immediate surroundingsand buildings), to the domain edge. The domain height follows

the rule as suggested by COST Action 732 [26] whereby theminimum height of the domain from a building should extendat least six times the tallest buildings height from the highest

building in the whole development. The reason for selecting

both requirements from different agencies is both are the strict-est among those suggested by all previous researchers andguidelines. For example, some of the other suggestions include

five times the height of the tallest building for the inlet anddomain height, or up to 15 times of the maximum buildingheight for the outlet. But for the study case here, it is still not

considered the strictest amongst all suggestions.We used the height of the point blocks (112 m), which are

taller compared with the slab blocks (50 m). The developmentis 500 500 m, of which 500 m is the largest dimensionwithin a development. The derivation of the dimensions is as

follows:

(a) Domain radius:Longest development dimension 500 m.Distance from development to domain edge three times of

longest development dimension 3 500 1500 m.Half of longest development dimension 250 m.Radius of domain 250 m 1500 m 1750 m % 1800 m.

(b) Domain height:Highest building height in development 112 m.Distance from tallest building in development is six

Figure 9. The computational domain, the middle estate area of 500 500 mwill be subjected to various morphological variations.

Figure 8. Schematic illustration of the two-layer zonal model [22].




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times 6 112 672 m.Height of tallest building 112 m.Height of domain 112 m 672 m 784 m % 800 m.

The middle portion of this cylindrical atmospheric domain,which consists of an estate area (500 500 m) of HDB blocks,will be subjected to a parametric study. The average outdoor

ventilation of the estate will be studied with variations in differ-ent HV morphological changes. Wind will be coming from the

sides of the cylinder (inlet) at various angles. The cylinder top isa symmetry plane (slip wall) and the cylinder bottom (nonslipwall) is the floor of which the boundary layer of the wind will

slowly develop from the inlet before reaching the estate area.With such a domain, there is no need to set an outlet.

2.7 Boundary conditionsThe mean velocity profile is usually obtained from the loga-

rithmic profile corresponding to the upwind terrain via theroughness length Z0 or from profiles of wind tunnel simula-

tions [26]. Available information from Singapores nearby me-teorological stations is used to determine the wind speed at thereference height.

The inlet is placed at a distance of 1500 m from the edgeof area of interest (building estate cluster) to allow the windflow to develop naturally from a distance away. A logarithmic

wind profile is generated, using the velocities of the four

prevailing wind directions, obtained from the National

Environmental Agency (NEA) for a period of 18 years, aver-aged to a single value of 2.7 m s21 (at reference height of

15.00 m; Table 2), is used for the parametric study here [25].The other input variables are as shown in Table 3. Under low

subsonic speeds in this case, the air densities are assumed con-stant under varying pressure condition and flow can be

regarded as an incompressible inert flow [7]. In the study here,different wind directions are to be simulated with the same cy-lindrical computational domain; hence, the lateral inletboundary becomes the inflow and outflow boundaries with

corresponding boundary conditions.

2.8 Meshing type and sizeUnstructured polyhedral grids are generated for the whole

computational domain and used in all the computations(Figure 10). For the grid-independence study here, one of the

nominal flow conditions, e.g. velocity magnitude, was takenas the parameter to evaluate a few grids densities and deter-mine the influence of the mesh size on the solution. It is

observed how the calculated flow reaches an asymptotic valueas the number of cells increases, in other words, size of themeshes decreases [27, 28]. Meshes of sufficient refinement

(resolution and accuracy) are required in order to resolve localsolution gradients whereby further refines do not affect the

results (grid-independent solution). Figure 10 shows anexample of the pattern for unstructured polyhedral meshing

that is used for this study.Owing to the large number of simulation cases, the base

cases for both the point and slab HDB blocks were used for

this study. The optimal localized approximated mesh size forthe blocks will be used throughout all the simulation studies.In this study, it is carried out by running the exactly identical

case on different grids and checking that results are smallerthan a specific tolerance [29] and a tolerance of 0.01 is used.

The base cases for both the point and slab HDB blocks were

Table 2. Tabulation of prevailing wind direction and speed obtained

from NEA over a period of 18 years [25].

Wind direction Mean speed (m s21)

North 2.0

Northeast 2.9South 2.8

Southeast 3.2

Table 3. Input variables for the inlet boundary conditions.

Parameter Value Input Reference

Power law

exponent (a)

a 0.21 [31] Power law : to approximate the vertical upwind profile flow in medium

density suburban areas

DePaul and

Shieh [32]

Roughness length

(Z0)

Z0 0.5 (Suburban terrains, forest, regular

large obstacles etc.) [33]

Ensuring that the minimum threshold speeds of 2 m s21 for the

development of canyon vortices observed by DePaul and Shieh was

comfortably exceededTurbulence

intensity (Ti)

5% (low speed flows for ventilation)

Turbulent kinetic

energy (k)

At Ti 5%, occurs at H 467 m above

ground of the logarithmic wind profile worked

out.

k 3=2UrTi2, where Ti represents the turbulence intensity, Ur is thereference velocity at the level where Ti 5%.

Wind velocity at this height is Ur 5.56 m s21

[31].

Turbulent

dissipation (1)

1 C3=4m k3=2=lEmpirical constant Cm 0.09 and l 0.07 l, where l is thecharacteristic length and in this case, the longest distance measured across

each estate, i.e. the length of the estate 500 m.

Von Karman

constant

K 0.41 (urban areas)

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used. The average external wind velocity (coming from one

direction, 08 North) readings taken at the horizontal plane

(within 500 500 m precinct) of 2 m above ground

(pedestrian level) and mid-level (of the base case) were

extracted. For the point and slab blocks, mesh independence

occurs when the localized mesh size for the blocks approaches

Figure 10. Unstructured polyhedral meshing for domain: (a) overall view, (b) plan view (2 m above ground) and (c) side view (Section AB from Figure 10b).




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1.5 m. This is shown in the tables and figures below

(Figure 11).

2.9 Wind velocity ratioThe wind velocity ratio (VR) is used as an indicator of goodventilation in this study. It is measured and defined as

VR Vp=V1, where V1 is the wind velocity at the top of anurban boundary layer not affected by the ground roughness,buildings and local site features (typically assumed to be at a

certain height above the roof tops of the city center and is site

dependent; Figure 12). According to the incoming Singaporelogarithmic wind profile as mentioned above, V

1will be fixed

(for all VR calculations) at a certain height above ground. It is

taken as the height level where the change in incoming windvelocity between the selected level (1 m interval between each

level) to the top (assumed to be the top of the cylindricaldomain at 800 m above ground) becomes 1% or less. Vp is thewind velocity at pedestrian level (2 m above ground) or it can

be any other levels, after taking into account the effects of

Figure 11. Mesh independence study for (a) point blocks at 2 m pedestrian level, (b) point blocks at 56 m mid-level, (c) slab blocks at 2 m pedestrian level,

and (d) slab blocks at 25 m mid-level. Values are measured within a constrained plane within the precinct in BASE cases at 08 North wind.

Figure 12. Velocity ratio (VR) explained [30].

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buildings [30]. VR indicates how much of the wind availability

of a location could be experienced and enjoyed by pedestrianson the ground taking into account the surrounding build-ingsa simple indicator used to assess the effects of different

proposals, the higher the value, the lesser are the buildingsimpact on wind availability.

At the top of the domain at 800 m above ground, accordingto logarithmic profile stipulated by the boundary conditions,the wind velocity at this level is 6.22 m s21. At the level of

745 m above ground, the wind velocity obtained is 6.13 m s, ofwhich is 1% difference from it. So, we use V

1as 6.13 m s21

for working out the wind velocity ratio (VR). The average

outdoor velocity magnitude values for Vp will be extracted atthe pedestrian level (2 m above ground level) and the mid-level(56 m for point blocks and 25 m for slab blocks), averaged

within a constrained horizontal plane that is confined within

the precinct area (500 500 m).

3 RESULTS AND DISCUSSION

3.1 Point blocks, pedestrian levelFigure 13 shows the overall results for point blocks under therandom and stratified HV configurations for pedestrian level.

3.1.1 Point blocks, random configuration, pedestrian level

From the results of the parametric study, we observed that asthe HV value increases, the VR readings decreases for both therandom and stratified configurations in all the different orien-

tations. The possible explanation is that when HV is low, thevariation in height is lesser, less turbulence being created andthis translates into a more stable and consistent wind flow at

the pedestrian level as we can see in Figure 14a of which theHV 21 (Figure 14a). But when the HV becomes higher, aswe can see in Figure 14b of which the HV 63, the different

heights of the buildings causes the wind flow to be highly tur-bulent and erratic, resulting in the wind at higher levels unable

to reach the pedestrian levels. When some of the buildings aretoo high, winds that hit the walls are not able to reach the ped-

estrian levels and in many cases, counter-rotating vortices areforms at the lowest levels (Figure 14b). The erratic winds alsospoilt the streamline flow from both direction of intersecting

canyons.The other observation is that point blocks have more or less

similar readings for all the different orientations studied when

compared with the slab blocks. This is because when comparedwith the slab blocks, they have more symmetrical dimensions

(four equal sides) and that translates to a higher number ofurban canyons. Wind that is flowing into the precinct is morepredominantly affected by the height differences of the build-

ings rather than the winds orientations (Figure 15a). Whereas

for slab blocks, the different number of canyons in both inter-secting directions have strong implications to the wind speed

values in different orientations (Figure 15b).

3.1.2 Point blocks, stratified configuration, pedestrian levelThe decreasing VR values with increasing HV values have the

same reasons as for the random configuration as mentioned inSection 3.1.1. For point blocks arranged in a stratified config-

uration, the behavior of the pedestrian level is similar to therandom configuration. Generally, the VR readings of stratifiedconfiguration for pedestrian level are slightly lower than

random configuration, especially for winds coming at an angleof 458. The reason for this is that for the stratified configur-ation, as it is arranged with the tallest buildings in the center

of the precinct, about half of the whole precinct is beingblocked by the first half of the taller buildings (Figure 16). But

as the number of canyons is high in both directions within theprecinct, compared with that of the slab blocks, the effect ofthe stratified arrangement do not lower the readings too much

compared with the random arrangement.For the lower readings of the 458 orientation, the reason is

due to the winds coming equally in both directions of the

Figure 13. Pedestrian level VR against HV for Random (top) and Stratified

(bottom) configuration of POINT BLOCKS.




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canyon and this act as an opposing wind flow from both(Figure 17a). This happens here and not for the random con-

figuration as the reason is due to the turbulent winds produced

by the random configuration which tends to neutralize this op-posing canyon winds and hence, this behavior is not apparent

in the random configuration (Figure 17b). Furthermore, in the

Figure 14. (a) Point blocks: random configuration08 North wind orientation. HV21 (section view). (b) Point blocks: random configuration08 North

wind orientation. HV 63 (section view).

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stratified case, half of the precinct is blocked by the first half

which is facing the wind direction. Those that are behind arenot able to receive the wind flow due to the blockage.

3.2 Point blocks, mid-levelFigure 18 shows the overall results for point blocks under therandom and stratified HV configurations for mid-levels.

3.2.1 Point blocks, random configuration, mid-levelFrom the parametric study results, there is an increasing VRvaluewith increasing HV values. The reason for this behavior is thatthe turbulence created and also the wind that hits on the build-

ings surfaces is being channeled down onto the lower levels.These benefit more especially those of the mid-levels whereby thedistribution of the wind flow is felt the most (Figure 19). In

further levels down, the creation of counter vortices from theabove wind do not benefit those at the pedestrian levels.

Figure 15. (a) Point blocks: random configuration08 North wind orientation, HV21 ( plan view, 2 m above ground). (b) Slab blocks: random

configuration 08 North wind orientation, HV21 (plan view, 2 m above ground).

Figure 16. Point blocks: stratified configuration08 North wind orientation. HV 52 (section view).




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Generally, the readings show higher VR readings for all thewind orientations 208, 22.58, 67.58 and 908 compared with

those at pedestrian level, whereas for the 458 wind flow, it ismuch lower compared with the rest at the same level. This isbecause, when wind comes from 458, the inflow from both

directions of the canyon provides an opposing flow that slowsdown the overall outdoor wind flow compared with the others

which are either parallel or oblique to the canyon orientation,which translates to more unobstructed air movement.

3.2.2 Point blocks, stratified configuration, mid-levelFor point blocks arranged in a stratified configuration, thebehavior of the pedestrian level is similar to the random con-

figuration. Generally, the VR readings of stratified configurationfor the pedestrian level are slightly lower than random config-

uration, especially for winds coming at an angle of 458. Thereason for this is that for the stratified configuration, as it isarranged with the tallest buildings in the center of the precinct,

about half of the whole precinct is being blocked by the firsthalf of the taller buildings. But as the number of canyons ishigh in both intersecting directions within the precinct,

compared with that of the slab blocks, the effect of the strati-fied arrangement do not lower the readings too much.

3.3 Slab blocks, pedestrian-levelFigure 20 shows the overall results for slab blocks underthe random and stratified HV configurations for pedestrianlevel.

3.3.1 Slab blocks, random configuration, pedestrian-levelFrom the parametric study results, there is a slight increasing VRvalue with increasing HV values for both the 08 and 22.58 windorientation, whereas for the other three orientations (458, 67.58and 908), there is a slight decreasing relationship. The reason

for the 08 and 22.58 orientation behavior is that there are lesscanyons (three numbers) facing the wind direction and thistranslates to higher wall areas of the buildings facing the wind

Figure 17. (a) Point blocks: stratified configuration458 North wind

orientation. HV21 (plan view, 2 m above ground). (b) Point blocks:

random configuration458 North wind orientation. HV21 ( plan view,

2 m above ground).

Figure 18. Mid-level VR against HV for random (top) and stratified

(bottom) configuration of point blocks.

R. X. Lee et al.



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direction, thus creating higher blockages (Figure 21). The pres-

ence of HV in the precinct morphology helps to channel more

airflow into the lower levels with these blockages and also createmore wind turbulence to improve the overall ventilation within

the precinct (Figure 22). This is unlike the point blocks, whichhave lower wall areas to capture the incoming wind; slab blockscan utilize the benefits of HV more. Furthermore, as slab blocks

are usually shorter than point blocks, the winds are able toreach the pedestrian levels better than point blocks which are

much taller (creating more counter vortices). For winds at 458,67.58 and 908, the readings have a more constant gradient. One

of the reasons given is that there are much more canyons forwinds oblique or parallel to the 908 orientation and also there islesser wall surface facing these directions. In these cases, HV will

not play such an important role in helping to improve ventila-tion in the overall precinct as the situation is more dominatedby the clear canyon pathways found there.

3.3.2 Slab blocks, stratified configuration, pedestrian-levelThe readings for the stratified arrangement are slightly lesser

than that of random configuration. The reasons is similar tothe above-mentioned stratified cases whereby the benefits ofhaving HVs is only felt at the front areas toward wind direction,

whereas those at behind the taller buildings are being more or

less blocked. The VR readings for the 08 and 22.58 orientation

increase with HV values, whereas for 458, 67.58 and 908 orienta-tions, the VR readings decrease with increasing HV values. The

reasons for this behavior are very similar to the case for randomconfiguration, as mentioned in Section 3.3.1.

3.4 Slab blocks, mid-levelFigure 23 shows the overall results for slab blocks under the

random and stratified HV configurations for mid-level.

3.4.1 Slab blocks, random configuration, mid-levelThe readings for the mid-level is generally higher than that ofthe pedestrian level due to the higher airflow at higher levelsand the wind turbulence generated by the HV is able to show

more benefits here. The overall readings are also higher thanthat of the point blocks because of the generally shorter heights

whereby the rotating vortices driven by the ambient winds areable to reach the middle levels more effectively (Figure 22).

As for the different orientations of the wind, the absolute

magnitude of the velocities increases progressively from 08 to908. The reason is due to the increase in the number of unob-structed canyons that are orientated in the 908 direction;

Figure 19. Point blocks: random configuration08 North wind orientation. HV 52 (section view).




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hence, more wind flow is able to reach more areas of the pre-cinct. In terms of the gradient of increase, it is noticed thatfrom 08 to 908 orientation, the increase in VR with increase in

HV decreases in the gradient. The reason is because the benefitof HV is more apparent in situations where the wall areas are

higher (less canyons at 08) compared with another case wherethere are less wall areas (more canyons at 908) to capture thewind flow to bring it to lower levels.

3.4.2 Slab blocks, stratified configuration, mid-levelThe readings for stratified configurations are generally slightly

lower than that of the random configuration for mid-levels.The reasons for the lower readings are similar to the above-mentioned in those of stratified configurations.

4 CONCLUSION

A detailed parametric study on HV within a precinct areawas carried out by the use of numerical simulation. The HV

index used is simply the standard deviation of the differentheights of all high-rise blocks within the studied precinct. Ityields different consistent trends in the VR that can be seen from

its variation under different orientations, building types and

configurations.The general observations of the study can be summarized

into the following points:

At the pedestrian level for point blocks in random configur-

ation, when HV increases, there is a general decrease in VRfor point blocks. The reason is that when HV becomeshigher, the different building heights causes the wind flow to

be highly turbulent and erratic, resulting in the wind athigher levels unable to reach the pedestrian levels.

Furthermore, there is the formation of counter-vortices at thelowest levels that hamper good ventilation. Next, the higher

number of canyons in both intersection directions for pointblocks leads to them not to be so much affected by the differ-ence in wind orientation compared with slab blocks.

At the pedestrian level for point blocks in stratified config-

uration, the only difference of it from random configura-

tions is the lower VR readings. The reason is that, for

stratified configurations, about half of the whole precinct isbeing blocked by the first half of the taller buildings facing

the wind due to the stratified arrangements. But as thenumber of canyons is high in both directions within theprecinct, compared with that of the slab blocks, the effect of

the stratified arrangement do not lower the readings toomuch compared with the random arrangement.

At the mid-level for point blocks in random configuration,

as HV values increase, VR value increases. The reason is dueto the higher turbulence created at the upper levels whereby

the wind is channeled down to the lower levels. These bene-fits more especially to mid-levels compared with the pedes-trian levels where the creation of counter vortices from the

above air circulation does not benefit the lowest levels.

At the mid-level for point blocks in stratified configur-ation, the behavior of the pedestrian level is similar to the

Figure 20. Pedestrian level VR

against HV for random (top) and Stratified

(bottom) configuration of slab blocks.

Figure 21. Slab blocks: random configuration08 North wind orientation.

HV27 ( plan view, 2 m above ground).

R. X. Lee et al.



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random configuration. Generally, the VR readings of strati-

fied configuration for the pedestrian level are slightly

lower than random configuration. The reason for this isabout half of the whole precinct is being blocked by the

first half of the taller buildings as mentioned. But as thenumber of canyons is high in both intersecting directionswithin the precinct, compared with that of the slab

blocks, the effect of the stratified arrangement do not

lower the readings too much. At the pedestrian level for slab blocks in random configur-

ation, if there are lesser canyons facing the wind, which

translates to higher area of wall facing the wind direction(higher blockages), the effect of HV will be that of an in-crease in VR with an increase in HV and vice versa. HV will

only play an important and beneficial role when the build-

ings blockages are higher facing the wind direction com-pared with one where there are more canyons orientated to

the wind direction. In the later case, HV increase does nothelp in improving ventilation at the pedestrian level.

At the pedestrian level for slab blocks in stratified configur-ation, the readings are slightly lower than for randomconfiguration. The reasons are similar as above-mentioned.

Otherwise, the general behavior can be explained as similarto the random configuration.

At the mid-level for slab blocks in random configuration, as

HV increase, VR increases as well for all the orientations. As

for the different orientations of the wind, the absolute mag-nitude of the velocities increases progressively from 08 to

908. The reason is due to the increase in the number of un-obstructed canyons that are orientated in the 908 direction;hence, more wind flow is able to reach more areas of the

precinct. The gradient of increase is less apparent when

there are more canyons alighted toward the wind direction,and this is due to lesser wall areas capturing the wind flowto bring it to lower levels.

At the mid-level for slab blocks in stratified configuration,the readings are only less than its random configurationcounterpart. The reasons are the same as above-mentioned

for all stratified configurations.

This study sheds some light as to how HV of the buildings canaffect the wind flow at both the pedestrian and mid-levels.

There is a general improvement to wind flow at mid-levelswhen the HV increases in value. The improvement can bequite significant at the upper levels; however, the benefits are

not so apparent at the pedestrian levels. One possible way toimprove the pedestrian levels wind might include having awider canyon width or having higher porosities in the form of

Figure 22. Slab blocks: random configuration08 North wind orientation. HV27 (section view).




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void decks and sky gardens in the building blocks. In thisstudy, it has demonstrated that consistent relationshipsbetween HV and VR can be observed and explained. It gives us

a better understanding of the behavior of morphological varia-tions of all buildings within a precinct; in this case, the HV, for

both the pedestrian and mid-levels. Through a better under-standing of morphological variation effects, high-density

estates can be planned optimally and sustainability.Furthermore, this also supports the high possibility of develop-

ing an overall ventilation potential model using HV and thensubsequently in future, including the rest of the six standar-dized morphological variables (in index forms) as independentvariables for input into this overall model. This helps to facili-

tate the comparison of influence and correlation of each inde-pendent variable to the outdoor wind velocity ratio index (VR)

as the dependent variable, for determination of estate-leveloutdoor ventilation potential, which serves as good guide forfuture urban planning.

REFERENCES

[1] Adolphe L. A simplified model of urban morphology: application to an

analysis of the environmental performance of cities. Environ Plan Plan

Des 2001;28:183200.

[2] Kim JJ, Baik JJ. A numerical study of the effects of ambient wind direction

on flow and dispersion in urban street canyons using the RNG k 1 tur-

bulence model. Atmos Environ 2004;38:303948.

[3] Ng E. Policies and technical guidelines for urban planning of high-density

cities: air ventilation assessment (AVA) of Hong Kong. Build Environ

2009;44:147888.

[4] Hunter LJ, Watson ID, Johnson GT. Modeling air flow regimes in urban

canyons. Energy Build 1990/1991;1516:31524.

[5] Hussain M, Lee BE. An investigation of wind forces on three-dimensional

roughness elements in a simulated atmospheric boundary layer flow: Part

II. Flow over large arrays of identical roughness elements and the effect of

frontal and side aspect ratio variations. Report no. BS 56. Department of

Building Sciences, University of Sheffield 1980.

[6] Oke TR. Street design and urban canopy layer climate. Energy Build 1988;

11:10313.

[7] Sini JF, Anquetin S, Mestayer PG. Pollutant dispersion and thermal effects

in urban street canyons. Atmos Environ 1996;30:265977.[8] Roberson JA, Crowe CT. Engineering Fluid Mechanics. Houghton Mifflin

Company, 1988.

[9] Brown GZ, Dekay M. Sun, Wind and Light, Architectural Design Strategies,

2nd edn. Wiley, 2001.

[10] Givoni B. Climate Considerations in Building and Urban Design. Wiley,

1998.

[11] Golany GS. Urban design morphology and thermal performance. Atmos

Environ 1996;30:45565.

[12] Kubota T, Miura M, Tominaga Y, et al. Wind tunnel tests on the relation-

ship between building density and pedestrian-level wind velocity: develop-

ment of guidelines for realizing acceptable wind environment in

residential neighborhoods. Build Environ 2008;43:1699708.

[13] Zhang A, Gao CL, Zhang L. Numerical simulation of the wind field

around different building arrangements. J Wind Eng Ind Aerodyn2005;93:891904.

[14] Chan AT, So ESP, Samad SC . Strategic guidelines for street canyon geom-

etry to achieve sustainable street air quality. Atmos Environ

2001;35:568191.

[15] Xie X, Huang Z, Wang JS. Impact of building configuration on air quality

in street canyon. Atmos Environ 2005c;36:360113.

[16] MacDonald RW, Griffiths RF, Hall DJ. An improved method for the esti-

mation of surface roughness of obstacle arrays. Atmos Environ

1998;32:185764.

[17] Raupach MR. Drag and drag partition on rough surfaces. Bound-Layer

Meteorol 1992;60:37595.

[18] Shih T, Liou WW, Shabbir A, et al. A new k1 eddy viscosity model for

high Reynolds number turbulent flows. Comput Fluids 1995;24:22738.

[19] Chan TL, Dong G, Leung CW, et al. Validation of a two-dimensional pol-lutant dispersion model in an isolated street canyon. Atmos Environ

2002;36:86172.

[20] Han HW. A study of entrainment in 2-phase upward concurrent annular

flow in a vertical tube. Ph.D. Thesis. University of Saskatchewan 2005.

[21] Teodosiu C, Rusaouen G. Modelisation des corps de chauffe a laide des

codes de champ. In: Proceedings of the Fourth Fluent Users Meetings, 2000.

[22] Wolfstein M. The velocity and temperature distribution of one-

dimensional flow with turbulence augmentation and pressure gradient. Int

J Heat Mass Transf1969;12:30118.

Figure 23. Mid-level VR against HV for random (top) and stratified(bottom) configuration of slab blocks.

R. X. Lee et al.



19/19

[23] Mohammadi B. Etude du modele k-1 de la turbulence pour les ecoule-

ments compressibles. Ph.D. Thesis. Universite Paris VI 1991.

[24] Xu W, Chen Q, Nieuwstadt FTM. A new turbulence model for

near-wall natural convection. Int J Heat Mass Transf 1998;41:

316176.

[25] Building and Construction Authority (BCA). Code for Environmental

Sustainability of Buildings, 2nd edn. BCA, 2010.

[26] COST Action 732 (Best Practice Guidelines for the CFD Simulation of

Flows in the Urban Environment) 2007.[27] Ferziger JH, Peric M. Numerical modelization of the flow in centrifugal

pump: volute influence in velocity and pressure fields. Int J Rotat Mach

2005;3:24455.

[28] Hooser JD, Wei M, Newton BE, et al. An approach to understanding flow

friction ignition: a computational fluid dynamics (CFD) study on

temperature development of high-pressure oxygen flow inside

micron-scale seal cracks. J ASTM Int 2009;6:115.

[29] Chang WR. Effect of porous hedge on cross ventilation of a residential

building. Build Environ 2006;41:54956.

[30] CUHK. Urban climatic map and standards for wind environment: feasi-

bility study. Working Paper 1A: Draft Urban Climatic Analysis Map, 2008.

[31] Cochran BC. The influence of atmospheric turbulence on the kinetic

energy available during small wind turbine power performance testing. In:

IEA Expert Meeting on: Power Performance of Small Wind Turbines not con-nected to the Grid. , 2002.

[32] De Paul FT, Shieh CM. Measurements of wind velocities in a street

canyon. Atmos Environ 1986;20:45559.

[33] Wieringa J. Updating the Davenport roughness classification. J Wind Eng

Ind Aerodyn 1992;41:35768.


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The study of height variation on outdoor ventilation for Singapore’s high-rise residential housing estate study of height variation on outdoor ventilation