EACWE 5

Florence, Italy19th – 23rd July 2009

Flying Sphere image © Museo Ideale L. Da Vinci

Keywords: Pedestrian-level wind environment, Low wind speed areas, Height and width effects, Effects gap width between buildings.

ABSTRACT

It is well known that high-rise buildings affect the surrounding pedestrian-level wind environment. In recent years, awareness and concern has increased about the creation of low wind speed areas around buildings which may lead to poor out-door air ventilation. Moreover, many modern building developments are not restricted to a single building but may comprise a group of buildings. There are very few systematic studies focused on the low wind speed areas around a group of buildings. In this research, a series of parametric wind tunnel studies was carried out to investigate the effects of building width, height and the gap width between buildings on the pedestrian-level wind environment. Mean wind speeds were used to determine the low wind speed areas where poor air ventilation may exist, and Gust Equivalent Mean (GEM) wind speeds were used to indicate the high wind speed areas for discomfort under strong wind conditions.

1. INTRODUCTION

In many densely populated cities, such as Hong Kong, urban renewal is an important kind of sustainable development for the community in terms of good use of land and infrastructure. Under the renewal projects, modern high-rise buildings have been built inside the closely packed old districts. Due to the significant changes of building forms, awareness and concerns have been raised by the communities on how the newly built structures alter the surrounding wind environments. Residents mostly complain that tall and bulky buildings, closely packed together, form undesirable barriers which obstruct winds from penetrating the downstream urban fabric and result in poor natural air ventilation. However, the residents’ complaints and the corresponding solutions suggested by the designers and engineers are mostly based on personal and professional experience which may be subjective. There is a genuine lack of solid scientific literature to support these arguments.

Contact person: 1st C.W.Tsang, CLP Power Wind/Wave Tunnel Facility, The Hong Kong University of Science &

Technology, Kowloon, Hong Kong, China, TEL: 852-2358 0170 and FAX: 852-2243 0040. E-mail ceroytsang@gmail.com

Effects of building dimensions and building separations on pedestrian-level wind environment

1st C.W. Tsang, 2nd K.C.S. Kwok, 3rd P.A. Hitchcock

1) CLP Power Wind/Wave Tunnel Facility, The Hong Kong University of Science and Technology – ceroytsang@gmail.com – Clear Water Bay, Hong Kong S.A.R., P.R. China

2) School of Engineering, University of Western Sydney – kkwok@ust.hk – Sydney, New South Wales, Australia

3) CLP Power Wind/Wave Tunnel Facility, The Hong Kong University of Science and Technology – wtpete@ust.hk – Clear Water Bay, Hong Kong S.A.R., P.R. China

The wind flow pattern around buildings is very complicated and has been investigated for more than 30 years. A comprehensive review focusing on the flow features and pedestrian-level wind environment around buildings was conducted by Blocken and Carmeliet (2004). Since the 1960s, outdoor human comfort has been given a significant amount of attention. Wind environment studies at pedestrian-level around idealized buildings were carried out with the major focus on the discomfort conditions caused by strong winds near the buildings (Wiren 1975, Stathopoulos and Storms 1986, Uematsu 1992, Jamieson et al. 1992, Stathopoulos and Wu 1995, To and Lam 1995, Blocken et al. 2007, 2008). In those studies, the effects of buildings characteristics, such as dimensions, shape, spacing between buildings, orientations of buildings groups and street canyon, have been investigated, from which knowledge-based expert systems were developed (Stathopoulos et al. 1992, Visser et al. 2000). These systems allow a preliminary and simplified evaluation of high wind speed area around buildings. However, very few systematic studies have focused on the low wind speed areas around a group of buildings (Stathopoulos and Wu 1995, Chan et al. 2001, Kubota et al. 2008).

This research aims to provide a better understanding on the pedestrian-level wind environment around modern building designs by using a wind tunnel physical simulation. The natural air ventilation and pedestrian comfort are both evaluated.

2. EXPERIMENTAL SETUP

The experiments were carried out in the 3 m × 2 m × 29 m long high-speed test section of the CLP Power Wind/Wave Tunnel Facility (WWTF) at The Hong Kong University of Science and Technology. The mean wind speed profile of the approaching turbulent wind flow followed a power law exponent of 0.2, using a series of fences and roughness blocks, to simulate wind flow above a typical suburban terrain. All of the wind tunnel tests were conducted using a reference mean wind speed Ur of approximately 10 m/s at 150 m (in prototype scale) above ground.

The building models were fabricated at a length scale of 1/200 and represented two building configurations, as shown in Figure 1. Case I comprised 25 building models which represented a single building with varying building width (b) and height (h). The building depth (d), in the along-wind direction, was fixed at 25 m at prototype scale. The building width was changed from 1d to 5d and building height was changed from 2d to 6d, both at increments of 1d, to investigate the effects of h and b on the pedestrian-level wind environment. Case II comprised 30 building models representing a pair of rectangular buildings with varying height (h) and gap width (s) between the buildings. The plan dimensions d and b were fixed at 25 m and 50 m respectively, while h was varied from 2d to 6d at increments of 1d or s was varied from 0 to 1.5d at increments of 0.25d. Only one parameter was adjusted at a time. Case II focused on the combined effects of building height and gap width between buildings.

The distribution of wind speeds at pedestrian level was measured using 175 Irwin Sensors (Irwin 1981) that were installed at a height equivalent to 2 m above ground in prototype scale (10 mm at model scale). The sensors covered an area extending 1.5d upstream, 2.5d laterally and 15d downstream from the buildings.

3. WIND SPEED ANALYSIS

In this research, mean wind speed (U) and Gust Equivalent Mean (GEM) wind speed (UGEM) were analyzed to evaluate the pedestrian-level wind environment. Mean wind speeds were used to determine the low wind speed areas where poor air ventilation may exist, while GEM wind speeds were used to indicate the high wind speed areas corresponding to conditions of potential discomfort. The GEM wind speed was defined, by Lawson (1990), as the maximum of: (i) mean wind speeds and (ii) 3 seconds gust wind speed divided by a factor of 1.85. In the current study, 3 seconds gust wind speeds were determined by the ‘multiple extremes method’, in which the maximum wind speeds from

multiple, i.e. not less than five, one-hour samples were identified and the gust wind speed was calculated by averaging those identified maximum wind speeds (AWES Quality Assurance Manual, 2001).

4. RESULTS AND DISCUSSIONS

Mean wind speed U and GEM wind speed UGEM were normalized by the reference mean wind speed Ur of the approach flow at 150 m in prototype scale. The flow approached perpendicularly to the wide face of the buildings.

At the beginning and at the end of the experiment, baseline studies, with no building installed, were conducted as a reference. The corresponding normalized mean wind speed and normalized GEM wind speed were around 0.5.

4.1 Mean wind speed distribution

Figure 2 shows the top view of the normalized pedestrian-level mean wind speed distribution (U/Ur), ranging from 0.0 to 1.0, on the left half of the buildings, assuming the distribution is symmetrical about the centerline. Areas with U/Ur lower than 0.3 were designated as low wind speed zones, which correspond to mean wind speeds around 1 to 2 m/s for an annual probability of exceedance of 50% in an environment such as Hong Kong.

Mean wind speed distribution around a single building

The general features of the normalized mean wind speed distributions around a single building are

Figure 1: Building model configurations

Figure 2: General features of normalized mean wind speed distribution around (a) single and (b) paired buildings

(b) (a)

shown in Figure 2 (a) and for a pair of buildings in Figure 2 (b). It can be seen that there were four low wind speed zones: i) upstream far-field low wind speed (UFLWS); ii) upstream near-field low wind speed (UNLWS); iii) downstream near-field low wind speed (DNLWS) and iv) downstream far-field low wind speed (DFLWS). In Figure 2 (a), the low wind speed in the UFLWS zone is mainly due to the downwash effect on the windward face of the building. The downwash results in flow reversal at pedestrian level approximately along the centerline upstream of the building. This creates a low wind speed zone where the approaching wind flow and opposing backflow meet. The UNLWS zone was situated at the stagnant area bounded by the ground and the windward face of the building. In the downstream area, the DNLWS zone is due to the shielding effect of the building, while the DFLWS zone is due to the reattachment of the vertical recirculation behind the building and it is also affected by the strength of the horizontal recirculation. It can be seen from Figure 2 (b) that the features of the four low wind speed zones for a pair of rectangular buildings are very similar to the single building configuration.

Figure 3Figure 3 shows the normalized mean wind speed distributions for buildings with a fixed depth (d = 25 m) and width (b = 75 m), and a varying height, from 50 m (2d) to 150 m (6d). It was observed that, as building height increased, the UFLWS zone moved further upstream of the building. This is due to the enhanced downwash at the building’s windward face that results in a stronger backflow in front of the building at pedestrian level. Due to the limited coverage of instrumentation upstream of the building, the other features of the UFLWS zone were not able to be captured. In the UNLWS zone, no significant changes were observed with different building height. This zone was mainly affected by the stagnant area bounded by the building surface and the ground.

Figure 3: Effects of building height on the normalized mean wind speed distribution around a building

For the downstream area, a higher building resulted in shrinkage of the DNLWS zone. This is

attributed to stronger horse-shoe vortices, created by the stronger downwash, which wrap around the building and enhance the air movement immediately behind the building. Furthermore, as building height increased, the size of vertical recirculation increased, causing the DFLWS zone to shift further downstream

Figure 4 shows the effects of building width on the four low wind speed zones. When building width increased, the UFLWS zone was located further upstream of the building and the UNLWS zone remained unchanged. The reasons are similar to the case of increasing building height. For the downstream near-field area, the lateral extent of the DNLWS zone increased with the building width because larger blockage was created by the building. Lastly, as the width increased, the DFLWS zone was located further downstream and the lateral extents of this zone increased to about the same width as the building.

It can be seen from Figure 3 and Figure 4 that the percentage of the instrumented area designated as a low wind speed zone decreased as building height was increased, and increased as building width was increased.

Figure 4: Effects of building width on the normalized mean wind speed distribution around a building

Mean wind speed distribution around a pair of rectangular buildings

The normalized mean wind speed distributions for a pair of buildings with identical dimensions and varying building height are compared in Figure 5. Building width (b), depth (d) and the gap width (s) were fixed at 50 m, 25 m and 12.5 m respectively. The building height was varied from 50 m (2d) to 150 m (6d). From these results, it was observed that the UNLWS zone was not sensitive to the change in building height. At the downstream area, the DNLWS zone was slightly reduced as the building height increased, and became essentially unchanged as h ≥ 4.0d. The DFLWS zone shifted further downstream and gradually became insensitive to the increase in height as h ≥ 5.0d. The area of the DFLWS zone was not sensitive to building height and the total area of the low wind speed zones slightly decreased with increased building height. Evidently, the benefits of increasing building height for a pair of buildings are not as significant as that for a single building.

Figure 6 shows the normalized mean wind speed distribution for the building configurations with different gap width (s). The building dimensions b, d and h were fixed at 50 m, 25 m and 100 m respectively while s was varied from 0 to 31.25 m (1.25d).

Figure 5: Effects of building height on the normalized mean wind speed distribution around a pair of buildings

Figure 6: Effects of building gap width on the normalized mean wind speed distribution around a pair of buildings

The low pressure zone at the leeward side of the building caused horizontal and vertical

recirculation. The horizontal recirculation enhanced the near-field wind movements while the vertical recirculation created a backward flow and dominated the movements at the far-field area. As the gap width was increased, there was a slight improvement at the DNLWS zone due to additional air movements induced by the flow through the gap. However, the DFLWS zone was worsened. Flow visualizations were conducted to investigate the changes of flow pattern downstream of the building. They showed that the backward flow diminished as s was increased, which is attributed to the weakening of vertical recirculation caused by the flow passing through the gap. Therefore, the wind movement around the DFLWS zone was weakened and the extent of the low wind speed zone increased. There was a steady increase in low wind speed area as the gap width increased. However, it was observed that when s ≥ 1d, the overall changes in the pedestrian-level wind environment became less significant.

4.2 GEM wind speed distribution The general features of the normalized GEM

wind speed distribution (UGEM/Ur) on the left half of single and paired building configurations are shown in Figure 7 to indicate zones of potential discomfort caused by strong winds. For this study, a threshold discomfort wind speed was set at 10 m/s for an annual probability of exceedance of 5%, which corresponds to the maximum allowable GEM wind speed for comfortable business walking in Lawson’s comfort criteria (1975). The corresponding UGEM/Ur for Hong Kong is around 0.8 and areas of UGEM/Ur higher than 0.8, were designated as high wind speed (HWS) zones.

GEM wind speed distribution around a single building

A lateral high wind speed (LHWS) zone can be observed in Figure 7 (a), and the location of maximum GEM wind speed was at the upstream corner of the building. This LHWS zone is caused by the high speed horse-shoe vortices created by the downwash from the windward face of the building and the accelerated flow around the building.

Figure 8 shows the effects of building height on the normalized GEM wind speed distribution at pedestrian level around a building with fixed depth (d) of 25 m and width (b) of 75 m, while the building height was varied from 50 m (2d) to 150 m (6d). The stronger downwash flows, caused by increasing the building height, increased the area of the LHWS zone from less than 1% to around 14% of the total sensor coverage area. The maximum normalized GEM wind speed increased from 0.82 to 1.08, which corresponds to GEM wind speeds of 10.3 m/s and 12.5 m/s for 5% annual probability of exceedance.

The effects of building width on the LHWS zones at pedestrian level are shown in Figure 9. While the building depth (d) and height (h) were fixed, building width was changed from 25 m (1d) to 125 m

Figure 7: General features of normalized GEM wind speed distribution around (a) single and (b) paired buildings

(b) (a)

(5d). Due to the limited sensor coverage on the lateral side of the building, the full extent of the LHWS zone for the wide building configurations, b ≥ 4d, were not measured. Nevertheless, it was still possible to observe the expansion of the area of the LHWS zone from less than 1% to more than 13%. The corresponding GEM maximum wind speed increased from 10.3 m/s to 13.3 m/s.

GEM wind speed distribution around a pair of rectangular buildings

High wind speed zones were observed, in a pair of rectangular buildings as shown in Figure 7 (b), at the lateral sides and at the gap between the buildings, which were designated as the lateral high wind speed (LHWS) zone and Gap high wind speed (GHWS) zone respectively. The recorded maximum GEM wind speeds at the GHWS zone were always greater than those at the LHWS zone.

Figure 10 and Figure 11 show the building height effects on the GEM wind speed distribution around a pair of buildings. In general, the increase in building height enhanced the downwash flow at the windward face of the buildings and redirected more winds to the pedestrian-level, increasing the extents of the HWS zones and the corresponding maximum wind speeds. In the case of a narrow gap width configuration, s < 25 m (1d), as shown in Figure 10, when building height was increased, the rate of expansion of the LHWS zone was greater than that of the GHWS zone. However, the recorded maximum wind speed at the GHWS zone was higher than the LHWS zone. In the case of a wider gap width configuration, s > 25 m (1d), as shown in Figure 11, the expansion rate and the extents of the GHWS zone and LHWS zone were similar. Again, the maximum GEM wind speed at the GHWS zone was higher than the LHWS zone.

Figure 8: Effects of building height on the normalized GEM wind speed distribution around a building

Figure 9: Effects of building width on the normalized GEM wind speed distribution around a building

Figure 10: Effects of building height on the normalized GEM wind speed distribution around a pair of buildings (narrow

gap width configuration)

Figure 11: Effects of building height on the normalized GEM wind speed distribution around a pair of buildings (wide gap

width configuration) The effects of gap width on the GEM wind speed distribution are shown in Figure 12 for two

buildings with dimensions of h = 150 m, b = 100 m and d = 25 m. As gap width was increased, the extent of the LHWS zone diminished whereas the GHWS zone expanded, causing the total area of the LHWS and GHWS zones to decrease. In terms of magnitude, the maximum normalized GEM wind speed for the zero gap width was 1.12, which corresponds to a wind speed of 14 m/s for an annual probability of exceedance of 5%. As the gap width increased, the maximum normalized wind speed was relatively constant at around 1.09 to 1.16, which corresponds to a wind speed of around 13.6 m/s to 14.5 m/s respectively.

5. CONCLUSIONS

In this paper, the pedestrian-level wind environments around a single building and a pair of buildings were investigated. The general features of the low wind speed areas around buildings were investigated experimentally in parametric studies of the effects of building width, height and gap width between buildings. It was found that a wider building created a larger blockage to the approaching winds which enlarged the extent of the two downstream low wind speed zones. Therefore, a wide building design is not recommended in terms of ventilation purposes. Moreover, a taller building redirected more upper level wind to the pedestrian-level. As a result, the near-field ventilations were improved. In the study of gap width effects, it was found that the near-field low speed zones were improved as the gap width was increased. However, the increased gap width had a detrimental effect on the magnitude and extent of the far-field low speed zone.

Figure 12: Effects of building gap width on the normalized GEM wind speed distribution around a pair of buildings Pedestrian comfort under strong winds were also investigated. An increase in building width

and/or building height led to stronger downwash that may adversely affect pedestrian comfort conditions. The effects of gap width between buildings were also investigated. It was found that an increase in gap width led to a shrinkage of the lateral high wind speed zones and an enlargement of the high wind speed zone between buildings. However, the total area of the high wind speed zones was reduced while the maximum 3s GEM wind speed was maintained at similar value. This indicated that a larger gap width between buildings may improve comfort conditions by reducing the area of high wind speed zones. However, the usage of the space at the gap between buildings should be limited unless mitigation measures are implemented.

6. ACKNOWLEDGEMENTS

This research project is funded by the Research Grants Council of Hong Kong Special Administrative Region, China (Project HKUST6301/04E).

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