A review on the study of urban wind at the pedestrian level around buildings

Author’s Accepted Manuscript A review on the study of urban wind at the pedestrian level around buildings Hemant Mittal, Ashutosh Sharma, Ajay Gairola

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S2352-7102(17)30758-1 https://doi.org/10.1016/j.jobe.2018.03.006 JOBE428

To appear in: Journal of Building Engineering Received date: 28 November 2017 Revised date: 3 March 2018 Accepted date: 17 March 2018 Cite this article as: Hemant Mittal, Ashutosh Sharma and Ajay Gairola, A review on the study of urban wind at the pedestrian level around buildings, Journal of Building Engineering, https://doi.org/10.1016/j.jobe.2018.03.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A review on the study of urban wind at the pedestrian level around buildings

Hemant Mittal*1, Ashutosh Sharma1a, Ajay Gairola2 1

Centre of Excellence in Disaster Mitigation and Management, Indian Institute of Technology Roorkee, Roorkee-247667, India 2

Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee-247667, India

*

Corresponding author: Hemant Mittal. Centre of Excellence in Disaster Mitigation and Management, Indian institute of Technology Roorkee, Roorkee-247667, India. Tel. +91-9568724532. E-mail: [email protected] Abstract Urbanization is leading towards the change of local wind climate in the vicinity of tall buildings, which influences the pedestrian level wind environment to an uncomfortable or even dangerous level. Therefore nowadays, building design should not be limited only for the consideration of wind load and indoor environment, but outdoor wind environment should also be considered. This study presents a review of the methods for the assessment of pedestrian level wind climate, different wind comfort criterion and various techniques to evaluate the wind speed at the pedestrian level. In later sections, brief review for the influence of different parameters related to building design and configuration on pedestrian level wind is presented. After analyzing previous literature it is suggested that there is a strong need for the homogenization of different wind comfort criterion, as it may lead to different consequences for the architects. Among various wind tunnel measurement techniques, use of Irwin probe is simple and accurate compared to hot-wire anemometry and it can be installed at numerous locations for simultaneous measurement of pedestrian level wind speed. For numerical simulation, Reynolds Averaged Navier Stokes based technique has been used by various researchers, although this technique is not accurate as much as large eddy simulation and detached eddy simulation. But this technique is cost effective and requires less computing resources. Keywords: Urban wind climate, Wind comfort, Building design, Wind tunnel test, CFD 1. Introduction The socio-economic growth of a nation is majorly driven by urbanization, as it accommodates the increased demand for business and residential space. As an evidence of the current scenario, many cities of developed and developing nation like Japan, Hong-Kong, China, Malaysia, and India are moving towards the construction of cohesive skyline structures or mega tall buildings. On contrary to these advancements, mega tall buildings in the urban area affect the surrounding wind flow pattern and pedestrian level wind (PLW) comfort. Presence of tall building in the urban area tends to deflect the upper-level high-speed wind to the ground, which creates conditions that could be unpleasant or even dangerous to pedestrians. There are many such incidents which are reported due to strong winds. But nowadays modern megacities are packed with the high density of high rise buildings, which influences the air movement. The reduced air movement at pedestrian level causes weak natural ventilation and allow the pollutants to be accumulated at ground level which increases air pollution. Such wind conditions are persistent in Hong Kong, Tokyo and New Delhi [1]. Many causalities have been reported due to the accumulation of air-borne SARS virus (Severe Acute Respiratory Syndrome) because of low wind speed zone at a building site in Hong Kong 1

[2]. So it is inevitable to assess wind condition for pedestrian level comfort in the view of low wind speed as well as strong winds near buildings corners. Many urban authorities have made it is essential to study the pedestrian level wind environment for large urban projects during initial design stage[3–6]. Initially, studies related to PLW speed measurement had been conducted with on-site field measurement. As it is not viable to conduct full-scale testing for the initial design of a building project site, so wind tunnel measurements on the scaled model make it feasible to investigate the effect of changes in building design at the initial stage of the urban project. During early days, wind tunnel measurement for PLWs was conducted with hot wire or film anemometry at limited measurement points [7–10]. Later on, Irwin [11] devised a simple omni-direction probe for PLW speed measurement. In which pressure difference between tubes at scaled pedestrian height and surface of the tunnel is calibrated with the corresponding velocity. Recently the use of Irwin probes has been paced up due to the availability of high precision simultaneous pressure measuring sensor. Further, the use of sand erosion technique for such studies is limited as it provides qualitative information over the whole area under investigation [12]. There are other measuring techniques which have been used to evaluate PLW speed such as laser Doppler anemometry (LDA), particle image velocimetry PIV, Infrared thermography and thermistor anemometry. Computational fluid dynamics (CFD) technique is also becoming a viable tool for PLW studies with the advent of high-performance computational resources. Till now steady Reynolds averaged NavierStokes (RANS) modelling approach was used successfully which requires less computing cost and time. But this technique is less accurate for predicting the flow in low wind speed region (deviation up to 5 times) as compared to other high-cost techniques such as LES and DES. The present study comprehensively reviews the urban wind at the pedestrian level around buildings. The content of this paper is organized as follows: the second section presents the method for the assessment of PLW climate with different wind comfort criteria. The third section describes the different techniques to evaluate the pedestrian level winds. The fourth section reviews effect of the various parameter related to building design on PLW for generic building configuration. The last section presents different studies related to the actual urban environment, which comprises the effect of building design parameters and general guidelines for the urban planning in response to pedestrian comfort. 2. Assessment of PLW climate and comfort criterion 2.1 Method for assessment of PLW climate The procedure for the assessment of favourable wind climate to pedestrians is comprised of (1) Statistical meteorological data of nearby weather station; (2) Aerodynamic information of the area and (3) Mechanical wind comfort criteria [13]. The aerodynamic information helps to compute the statistical data at particular building site obtained from the weather station. Then transformed data at this location is compared to wind comfort criterion. This procedure is schematically depicted in Fig. 1.

2

Fig. 1. Flowchart for Wind comfort assessment procedure Meteorological data from weather station consist of hourly mean wind speed ( , measured at 10 m height) and wind direction in open terrain ( ). This wind speed data obtained from weather station is analyzed statistically using Weibull distribution function [14–16] to calculate the probability of exceedance of threshold wind speed as following Eq. (1). (

)

( )

Eq. (1)

) represents the exceedance probability of the wind speed; U is the mean wind Where ( velocity magnitude at building site; c is the dispersion parameter and k is the shape parameter. These constants are obtained by fitting Eq. (1) to the meteorological data. Then statistical information has to be transformed to the area of interest by the means of aerodynamic information using amplification factor R (Eq. (2)). This amplification factor consists of design related contribution and terrain related contribution (Eq. (3)) [16]. The design related contribution comprises of modification of statistical wind climate information due to local building design. These modification can be obtained by either wind tunnel measurement or using CFD simulation. The whole research community in this area is devoted to evaluate the design related modification. The terrain related modification accounts for the differences in terrain roughness between the weather station and area of interest [17] and can be obtained using Eq. (4) and Eq. (5). Eq. (2) Eq. (3) ( (

(

(

)

(

)

)

Eq. (4)

)

)

Eq. (5)

Where is the reference wind speed at certain distance upstream of area of interest or without the presence of building or at the inlet of computational domain; and are the friction velocity at building site and meteorological station respectively; and are the aerodynamic surface roughness length at building site and meteorological station respectively. The dependency of the probability of exceedance and amplification factor R is given by Eq. (6) [16]. (

)

(

)

Eq. (6) 3

2.2 Wind comfort Criteria In wind comfort assessment, besides the wind speed, the frequency of its occurrence also matters. Therefore the criteria for wind comfort involves threshold wind speed above which pedestrian will feel discomfort and its frequency of occurrence. A wide variety of wind comfort criterion, based on threshold mean wind speed and the probability of exceedance has been proposed earlier [14,15,18–23]. Details of different wind comfort criterion are shown in Fig. 2, in which threshold mean wind speed (m/s) for different activities and its corresponding exceedance probability (%) is presented. Most of the criterion is based on the same probability of exceedance and different threshold wind speed for different pedestrian activities. While NEN 8100 [23] considers same threshold mean wind speed and different exceedance probability. The comparison of different wind comfort criterion for different activities is shown in Fig. 2, in which typical wind climate for Indian city (Palam Airport, New Delhi) is represented. It can be identified that, based on comparison with mentioned wind comfort criteria except by Melbourne, (1978) [14], this place is vulnerable to high wind speed, which causes danger to pedestrians. Probability of exceedance Vs Threshold mean wind speed Sitting long Lawson (1978) Sitting short NEN8100 Strolling Melbourne, (1978) Walking fast Soligo et al., (1998) Danger

100

10

Pexe (%)

1

0.1

0.01

0.001 0

5

10

Vth (m/s)

15

20

25

Fig.2. Comparison of different pedestrian wind comfort criteria with Indian wind climate Since most of these criterion reports different threshold mean wind speed and exceedance probability, so it is difficult for developers, architects and planners to choose a general guideline on comfort criterion. With regard to this, various comparative assessments of wind comfort criteria have been proposed. Based on discussion with developers and building managers, Soligo et al. [20] converted the different wind comfort criteria on the same scale of frequency of occurrence (20%) or for a particular activity. Janssen et al. [24] also focused on the standardization of different wind comfort criterion.

4

However, all of the above-discussed criteria did not consider pedestrian discomfort due to weak wind conditions for the densely-built urban areas. Du et al. [25] proposed the wind comfort criteria for the urban areas of poor wind conditions such as Hong Kong, where the wind comfort becomes worst in hot and humid season. In this criterion overall mean velocity ratio (OMVR) is used as threshold parameter ⁄ ), (Table 1), which represents the integration of direction values of mean velocity ratio ( Where is wind velocity at pedestrian level and is wind velocity measured at 200 m height. Table. 1 Wind comfort criteria based on overall mean velocity ratio (OMVR) representing weak wind conditions by [25]. Category

Threshold Velocity (Jun-Aug)

Threshold Velocity (Dec-Feb)

Activity

Remark

N/A

No Noticeable Wind

2

Sitting long

Light Breeze

2

Sitting sort

Gentle Breeze

2

Strolling

Moderate breeze

Tolerable

2

Walking fast

Fresh breeze

Intolerable

2

No suitable Activity

Strong breeze

Danger

0.05

Dangerous

Gale

Unfavorable

Acceptable

Exceedance Probability (%) 50

3. Measurement techniques for pedestrian level wind speed To obtain design related contribution for the assessment of pedestrian level wind environment, generally field measurement on the real urban environment, wind tunnel testing on the scaled model of the urban area and CFD simulation are employed. But to make changes in the early design stage, it is not possible to conduct field measurement for urban developmental projects. The following sections describe the details of each method and comparison based on the accuracy of each method. 3.1. Different measurement techniques 3.1.1. Field measurement techniques This technique is regarded as a robust method to evaluate wind speed at limited points and was used successfully by several researchers [26–30]. Generally, a portable three cupanemometer with wind vane system is used for this purpose. This instrument has very low onsetspeed of 0.1 m/s and light in weight [28]. The output of this instrument is in the form of one 5

electrical pulse per revolution of the rotor and this pulse rate is calibrated against the wind speed. A comparison of this technique with others is provided in the later section. 3.1.2.

Wind tunnel techniques

In wind tunnel, the PLW measurements are conducted by using hot-wire/film anemometry (HWA, HFA), Irwin probes, thermistor anemometer, sand erosion, laser Doppler anemometry (LDA), infrared thermography, and particle image velocimetry (PIV). Each of the mentioned technique is based on different working principle and unique experimental setup. Since it is beyond the scope of this paper to discuss the working principle of each technique, only their application and capability is discussed in Table 2. Table. 2 Use of different wind tunnel measurement technique by various authors and their pros. and cons. Measurement Authors Comments Technique [7–10,12,31– Pros: High-frequency response and high spatial resolution. HWA 33] Cons: Intrusive technique, only suitable for moderate turbulence intensity, insensitive to directional changes [33,34]. [7,9,13,26,35 Pros: less susceptible to fouling and fragility, easy to clean, shorter HFA –38] sensing length, The agreement between wind tunnel and full-scale measurement is within 10% [26]. Cons: Intrusive technique, only suitable for moderate turbulence intensity, insensitive to direction changes. LDA

[37,39]

Pros: Non-intrusive point-wise technique, allows measurement of high-turbulence intensity and calibration is not required. [40] Cons: This technique is costlier than HFA and HWA.

Infrared thermography

[10,32,37]

Pros: Non-intrusive Area technique, RMS, peak and spectrum value can be measured [37] Cons: Due to convection wind flow gets disturbed, sturdy and nonstandard experimental set-up. No perfect correlation is obtained for temperature drop and wind speed [10]

PIV

[41,42]

Pros: Non-intrusive area technique, high spatial resolution and directional sensitivity. Cons: Very expensive, sometimes dangerous, laser light shielding and reflection from buildings, not suitable for the cluster of buildings.

Erosion Technique

[12,34,42–45]

Pros: Area technique, results are comparable to HWA for high wind speed [12]. For high turbulent flow, this technique agrees well with PIV measurements of mean wind speed [42]. Cons: It is non-quantitative technique and difficult to ensure the 6

repeatability. Irwin Probes

[3,46–50]

Pros: Allows measurements at numerous locations. No re-alignment for different wind direction. Simple in design and easy to operate [11]. Cons: Less accurate for high turbulence intensity, it cannot accurately measure wind speed below 1.5m/s [47].

Thermistor Anemometer

[51]

Pros: Small sensor size, susceptible to wind direction. The simple circuitry and low cost of thermistors make it economically feasible to operate the probes in large numbers. Cons: Only suitable for mean velocity measurements, fragile, require alignment for change in wind direction; nonlinear calibration of velocity.

3.1.3.

CFD techniques for PLW speed

To simulate PLW environment, CFD methods are gaining much popularity among researchers and industrialists recently owing to the development of the computer hardware and software. One of the major advantages of CFD over wind tunnel testing is that it gives detailed flow field data of associated parameters over the entire computational domain. In addition, similarity law requirements associated with wind tunnel testing is not a limitation of CFD simulation. Most of the studies, to simulate wind environment are based on the use of different RANS turbulence models e.g. Std. , Realizable and RNG model with default values of model closure coefficients in CFD tools. However, the major issue for the acceptance of CFD results is related to the accuracy, which suffers badly in predicting the flow on leeward side of building. Therefore CFD simulation results require verification and validation [40]. Zahid Iqbal et al. [2] mentioned that the simualtion results for PLW speed obtained by std. are also affeted for different model closure coefficients. The accuracy of CFD simulation largely depends on the selection of turbulence model. The capability of each turbulence model and their accuracy is discussed in Table. 3. Table. 3 Use of different CFD technique by various authors and their pros. and cons. Approximate Authors Comments Forms [2,44,52–58] Pros: Computationally efficient and economically viable. Agreement Std. with wind tunnel measurement of wind speed is within 10% for regions with high wind speed ratio ( ⁄ ) [59] Cons: Underestimates wind speed notably five times or more for low wind speed regions [59]. It cannot reproduce the reverse flow on the roof [60] and overpredicts turbulent kinetic energy in separated flows around windward corners of buildings [55]. 7

Realizable

[6,17,50,61– 64]

Pros: Sensitive to flow separation, reattachment and recirculation. For high wind speed region accuracy further improves as compared to std. model [17]. Cons: Less accurate compared to std. model for low wind speed region due to underestimation of TKE in the wake region [17]

[65–68]

RNG

Pros: For high wind speed region, accuracy improves as compared to std. model. [59] Cons: Less accurate compared to std. model for low wind speed region due to underestimation of TKE in the wake region [59].

[69–72]

LES

Pros: Superiority of this method over steady RANS is clearly reported [60]. It can reproduce turbulence intensity and gustiness. Cons: Computationally very expensive as it requires more time and sensitive to many parameters such as sub-grid scale model, mesh resolution and time step size [73].

[67]

DES

Pros: Capable of producing similar results as LES with less computing time and lower mesh size.[74] Cons: Sensitive to parameters such as sub-grid scale model, mesh resolution and time step size and sampling time. [74]

3.2. Comparison of Different Techniques 3.2.1. Field measurement and wind tunnel measurement To evaluate PLW environment on existing project site, it is suitable to conduct field measurements. But to assess the effect of changes in the initial design of developmental projects, wind tunnel measurements are preferred. Comparison of these techniques is generally associated with practical difficulties such as variability inherent to the atmospheric phenomena, obstructions due to automobiles etc. Isyumov and Davenport [26] obtained full-scale and wind tunnel measurement for mean wind speed at a site project in Toronto, Canada. They reported the agreement between full-scale and wind tunnel measurement within 10% for windy locations. Dye [28] compared the same in a sheltered urban area and found that wind tunnel test generally underpredicts the mean speed of 20% at most severe locations. While Visser et al. [30] reported the comparison of these two techniques to be dependent on the duration of full-scale data measurement, as the wind speed data of longer period will agree well with the model test. 3.2.2.

Field measurement and CFD simulation

The use of CFD technique provides entire flow field data, whereas field measurement can be performed for few location only. However, it is a challenging task to simulate wind environment using CFD techniques accurately. Blocken et al. [6] compared the results of CFD simulation at a university 8

campus. The simulation was performed using a realizable model with standard wall function and field measurement was performed with 3D ultrasonic anemometers. The authors clearly indicate that the short term measurement for few hours cannot serve as a validation data because the data suffers from intrinsic variability of meteorological conditions. Large deviation of 25% between measured and simulated mean wind speed was reported at the locations where the gradient of wind speed is high. In addition, the comparison of the wind speed in the regions of high gradients can yield large deviation by a small shift in location. 3.2.3.

Wind tunnel measurement and CFD simulation

The CFD technique has attracted wide acceptability and strong support due to the establishment of several best practice guidelines [75]. Wind tunnel testing is often used to benchmark CFD models and simulation results. The use of RANS was found to provide quite accurate results for wind speed ratio, however, it significantly underestimates wind speed in the regions of lower wind speed ratios [6]. A possible reason for high wind speed ratio is characterized by lower turbulence intensities and lower wind direction fluctuation, therefore better modelled by the statistically RANS approach. Richards et al. [44] adopted erosion technique to investigate the PLW environment downtown area of Auckland and compared erosion contour with CFD simulation using std. model. The authors observed the difference at the location of highest wind contours. Using CFD simulation, this location was immediately around the corner of the buildings, whereas the wind tunnel shows that the earliest erosion emanates from the corner and sweeps across the buildings. 4. Parametric studies and their impact on generic building configuration In past decade, various parametric studies related to the evaluation of pedestrian level wind environment around generic building configuration have been conducted, which considered the effect of building height, shape and pattern of a group of buildings. These studies are listed in Table 4. In this section brief discussion about the effect of parameters related to the design of building configuration on pedestrian level flow is presented. 4.1. Height and width variation As the height of building increases, the maximum wind speed ratio increases due to strong downwash effect, as taller building catches the more upper-level wind and directs it to pedestrian level. Hence it poses high wind speed conditions but improves near-field air ventilation conditions as shown in Fig. 3.(a) [35,47,51]. Whereas turbulence intensity is not influenced by height variation of building significantly [35]. Wider buildings increases the sheltering effect to the incoming wind, which enlarges the extent of low wind speed zone in the downstream side of the building.

9

(a)

(b)

Fig. 3. Contours of mean wind speed ratio for (a) height modification [35] (b) Corner modification [35] 4.2. Modification of cross-section Changes in cross-section of the building, such as tapering, rounding, and corner cut improve the wind environment around the building corners, as it reduces the extent of high wind speed zone near building corners due to less deviation of separated flow in comparison to the square building as shown in Fig. 3(b) [8,35]. But root mean square value of wind speed distribution remains unaffected due to corner modification [8]. Circular and polygon shaped cross-section of the building also tends to have better wind climate in terms of reduced high wind speed zone as compared to the square-shaped building due to reduced downwash [35]. 4.3. Modification along the height Till now this modification is investigated in detail by Xu et al. [51] for various building models and it was proposed that the change of building width at one-third height from the ground, majorly influences the PLW environment. Whereas the use of permeable floor at mid-height of the building reduces the extent of high wind speed zone. However, it does not lower the maximum wind speed ratio [39], as this permeable floor provides a way for upper-level wind to pass through the building before reaching it to ground level. 4.4. Lift up building design This design consists elevated main structure of the building from the ground by the central core and has a potential solution to improve wind conditions near the building. This design modification increases area averaged high wind speed ratio but decreases area averaged low wind speed zone on the leeward side of the building because of weak downwash [1,3]. To illustrate the effect of this building design, numerical simulation using CFD software Ansys/Fluent 17.0 was conducted. The governing equations of the flow are 3D steady-state Reynoldsaveraged Navier-Stokes with standard turbulence model. Numerical Simulation has been conducted using best practice guidelines [75] for pedestrian level wind flow. All the details related to CFD simulation is not included as it is beyond the scope of this paper. The result of this simulation is presented 10

in Fig. 4, which represent the contours of mean wind speed at pedestrian height for lift-up and conventional square buildings. The qualitative results of the present simulation are similar to the experimental study by Tse et al. [1] which shows high wind speed near the lift up area and further lower values of low wind speed as compared to conventional building design. Which helps in improving ventilation near the building In addition to this lift up area can be used for other purposes as the recreational area, parking, or shelter for pedestrians. Besides this, use of lift up building design is limited due to unacceptable or high wind speed zone near the lift- up area [76]. This unacceptable wind condition arises due to flow through openings of positive and negative pressure faces of the building.

(a)

(b)

Fig. 4. Flow pattern with mean wind velocity (m/s) contours at the pedestrian level for (a) lifted building and (b) conventional building design

4.5. Use of podium structure Podium structure creates a sheltering effect at pedestrian level wind flow near the building and results in undesirable low wind speed zone at pedestrian level [47], as podium structure enhances the spatial extent of low wind speed zone near upstream and downstream of the building. In general podium structure is not recommended where the conditions for natural ventilation are required. Besides this, it is believed that podium structure is used to protect pedestrians from high wind speed. 4.6. Different passages between two buildings The orientation of two buildings such as side by side, parallel, angular (converging, diverging and perpendicular) creates an adverse effect on PLW speed differently as shown in Fig. 5. When wind flow is perpendicular to the row of buildings, the windiest condition occurs in the upstream corners due to flow channelling and suppressed horse-shoe vortices [31]. As the separation between the buildings increases, channelling effect gets minimized and flow tends to be independent of the further increase in separation.

11

(a)

(b)

(C)

Fig. 5 flow pattern with mean wind velocity ratio contour for (a) parallel arrangement [57], (b) Parallel shifted arrangement [17], (c) Angular arrangement [58]

For the converging arrangement, the windiest condition occurs for narrowest passage and decreases further for higher passage width with its location moving further away in the downstream direction as shown in Fig. 6(a). Whereas for diverging arrangement wind speed ratio is often more adverse than converging passage as it does not provide shelter to the wind [38] and its value decreases for higher passage width as shown in Fig. 6(b). Fig. 6 Variation of mean wind velocity ratio along passage centre line for (a) Diverging passage [38] (b) Converging passage [38].

(a)

(b)

4.7. Effect of buildings group pattern

12

In urban design, different arrangements of building govern the microclimate of the urban area differently. But orientation and shape of buildings majorly helps to improve PLW climate. Several studies [2,53,56] propose the configuration with square central space with prevailing wind direction towards the windward opened side face offer better PLW environment. As this configuration can effectively contain the air flow movements. 4.8. Effect of twisted wind flow In the lower part of the hilly regions (below 500 m), wind profile gets twisted with large angles i.e. varying wind direction along the height, therefore wind-structure interaction in these conditions is different from interaction with the conventional profile. To investigate the effect of twisted wind flow on pedestrian level, Tse et al. [77] simulated such kind of twisted wind profile in boundary layer wind tunnel. The authors concluded that the spatial extent of low wind speed zone increases with the scattered type of flow conditions. The extent of low wind speed zone further increases with the increase of wind twist angle. The flow features around two parallel buildings were observed to be asymmetric. Table 4 Various parametric studies to evaluate PLW around different building configuration. Author Parameter Effect of Key parameter Generic Building Configurations Lam, [39] (WT) Permeable floor at mid- The spatial extent of high wind speed reduces by using height of CAARC building the permeable intermediate floor. Uematsu et al. Corner modification [8] (WT) building To & Lam [31] Representation (WT) speed

Zhang et al. Different [65] (WT/CFD) arrangement

Asfour (CFD)

Tsang et [47] (WT)

of

of Reduces the high wind speed zone near building corners.

PLW Quartile level (*) wind speed descriptor found to be a suitable indicator for evaluating PLW, as it avoids the problem of choosing gust factor.

building The arrangement with higher frontal aspect ratio and plan area density found to be suitable for natural ventilation.

[53] Different grouping pattern The configuration in which buildings are arranged of housing blocks around a central space windward opened side facing the prevailing wind direction offer better ventilation. al. Different building width Taller building improves ventilation near the building, and height for a single and Use of podium and width of building adversely affect row of buildings and use of ventilation. podium

Hang et al. [55] Use of urban canopy layer (CFD)

Urban canopy layer worsens the ventilation than open street due to decrease roof ventilation.

Kuo et al. [48] Different street widths and High wind speed inside canyon for taller podiums. (WT) 13

podium heights Xu et al. [51] Super tall buildings with High-speed up ratio for super tall (400 m) buildings and (WT) unconventional shapes mostly influenced by a change in building width at onethird level from the ground. Fan et al. [54] Building openings at Buildings with permeability of 10% is adequate to (CFD) pedestrian level of street improve PLW canyon Hong & Lin Different layout of building Configuration with square central space offers better [56] (CFD) pattern PLW environment. Tse et al. [78] Wind twist angle, isolated Displaced flow features and increased area of low wind (WT) building, wind incidence speed at downstream of buildings. LWS region gets angle intensified with the increase of wind twist angle. Tse et al. [49] Wind twist angle, building Wind environment is asymmetric near the building, (WT) dimensions and passage wind speed in the passage decreases with wind twist width angle. Passage between Two Buildings Stathopoulos & Passage width and height Buildings with different height create most critical storms [7] of one of the building wind velocity condition. (WT) Blocken et al. Effect of wall function, Horizontal inhomogeneity of wind profile affects the [57] (CFD) Passage width between simulation results. Wind speed within the passages is parallel buildings only pronounced at the pedestrian level. Blocken et al. Passage width between two Wind speed amplification in the diverging passage is [38] (WT) perpendicular buildings and often more than the converging passage. building height Li et al. [58] Converging and diverging Converging passage with 15º for cold and temperate (CFD) passage with 15º interval climate and diverging passage with 150º for the highly dense urban area in sub-tropical climates. Allegrini Lopez (WT)

& Converging and diverging From ground to roof wind speed increases for [41] passage for different angle converging case with increasing angle between 60º, 90º and 120º buildings and decreases for the diverging case.

Lift up Building Design Tse et al. [1] Lift up central core height Increase in the lift up core height mostly influences the (WT) and width. area percentage of acceptable wind speed.

14

Zhang et al. [3] Lift up central core shape Speed-up ratio increases with building height, corner (WT) and building aspect ratio. modification of central core controls the HWS zone. Du et al. [79] Different (CFD) configuration Square).

building Better PLW comfort than non-lift up building for (-, L, U, oblique wind direction.

Zahid Iqbal et Building shape, separation Square-shape arrangement yields higher average ** al. [2] orientation and turbulence acceptable wind speed area, higher and lower (WT/CFD) model closure coefficients values for turbulent Prandtl numbers is suggested for accurate simulation. * Quartile-level Wind speed- it is defined as wind speed that is being exceeded for 10% of the observation time ** model closure coefficient for Std. model

5. Studies on Pedestrian environment around actual urban area This section describes the general flow conditions around the actual complex urban area, the effect of various parameters affecting the PLW speeds. In addition, the methodology to assess the pedestrian comfort is discussed and lastly, the key factors for the design of urban development projects in response to pedestrian comfort are discussed. 5.1. General flow conditions When high rise building is located upstream of the low-rise building, the wind tends to flow over low rise building. The presence of tall building at downstream position intercepts high-speed winds and redirects to ground level [70]. The wind conditions within space between tall buildings are accelerated due to so-called venturi effect, which results in high wind movement especially at pedestrian level [57]. Use of podium structure deflects the high wind to the upper level, thereby protects the pedestrian due to high-speed winds [47]. These are the common flow condition, which arises due to different building configurations in an urban area and can be depicted in Fig.7.

15

Fig.7. Common flow features around the cluster of buildings obtained by sand erosion technique [45]. 5.2. Effect of related parameters The wind flow over the ground surface is mostly affected by roughness element and building configuration, e.g. width, height, the arrangement of buildings and its density. It is considered that an increase in building density reduces the wind velocity in an urban area due to increased friction near the ground [80]. Stathopoulos & Wu [46] investigated the effect of spatial density of street blocks and the relative height of buildings. The authors concluded that the wind speed along streets decreases with blockage ratio ( describe the percentage of oncoming air flow captured by building block) and proposed a simplified relation for wind speed up ratio as given in Eq. (7). They also concluded that, the maximum wind speed ratio increases with the height difference between reference and surrounding buildings. In addition to this Wu & Kriksic [4] reported that, the narrow streets and uniform building height results in low wind speed. (7) 5.3. Assessment method for pedestrian level wind environment Most of the studies related to PLW environment tackle the problem based on discrete point analysis [17,24,81]. In this method, wind speed is evaluated at discrete locations. Then this wind speed is compared to the suitable wind comfort criteria. But obviously, pedestrian wind environment varies spatially in an urban area. So this type of discrete analysis is not efficient to address the real needs of urban planner in design [66]. In order to improve assessment method, Shi et al. [66] proposed that the practical pedestrian wind comfort for urban planning design must be space-based. The authors defined two parameters to analyze the pedestrian wind, the spatially averaged wind (Eq. (8)) and the maximum wind speed in a plane. 16

∑

(

∑

)

(8)

In above equation is the proportion of i-th wind direction; is the wind speed in j-th mesh grid; n is the total number of mesh grid in the plane. The above calculated wind speed should be within the specified limit defined in comfort criteria. 5.4. Key factors for urban design in response to pedestrian comfort To achieve better outdoor pedestrian comfort, there are following considerations which needs to be incorporated while designing urban area. (1) For hot and humid climate, the streets should be aligned parallel to the prevailing wind direction to remove accumulated pollution and improved air movements [4]. (2) In low wind speed and high-temperature areas, use of lift up building design [3] and a cluster of buildings with central square space [2,56] promote the ground level natural ventilation. (3) For cold climate regions, corner modified buildings [8,35] and a large stepped podium [47] around the building are convenient to reduce the high wind speeds at the pedestrian level. (4) Modification in building width at one-third level from the ground influences the PLW environment mostly [51]. (5) The positioning of taller buildings should be on the downside of a street to catch more winds at the pedestrian level [4]. Moreover, the addition of recreation spaces and wide streets, removing the obstacles to the flow throughout the urban area improves the pedestrian comfort level. (6) For decision making in response to pedestrian comfort, suitable wind comfort criterion, improved terrain mapping and accurate design related modifications are required.

6. Conclusion The construction of high rise building in the vicinity of urban area alters the wind climate significantly. Eventually, it becomes necessary to evaluate the wind environment at the initial design phase of urban projects so that suitable modification could be suggested to improve wind climate for pedestrians. While designing urban area, either it is considered to minimize high wind speed zone to reduce mechanical forces on pedestrians or to improve natural outdoor air ventilation. The present study reviews the different wind comfort criterion for pedestrians and measurement techniques to evaluate these wind speeds. Further, the effect of various parameters related to building design is analyzed and at last, evaluation of urban wind at pedestrian level and urban design consideration are discussed. The findings from this review study are summarized as follows. (1) Wind climate statistics obtained from nearby weather station are obligatory for selecting the building shape, orientation and alignment of streets in urban planning and these wind statistics are compared with suitable wind comfort criterion. (2) The probability of exceedance of threshold wind velocity at the particular site requires terrain and design related modifications. To obtain terrain related modification accurately, improved roughness mapping is required. Otherwise, it may lead to wrong decisions in urban planning. 17

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Highlights     

Wind comfort due to high as well as low wind speeds should be considered. Irwin probes provide relatively accurate measurements of wind speed. Reynolds Averaged Navier Stokes based method is economic to simulate urban wind flow. Corner modified and lift-up buildings and use of podiums are positive design approach. Building width at 1/3rd height from ground affects the pedestrian wind mostly.

23