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Strategic guidelines for street canyon geometry to achieve sustainable street air quality—part II: multiple canopies and canyons
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Atmospheric Environment 37 (2003) 2761–2772
Strategic guidelines for street canyon geometry to achieve sustainable street air quality—part II: multiple canopies and canyons Andy T. Chan*, William T.W. Au, Ellen S.P. So Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong Received 13 December 2002; received in revised form 6 March 2003; accepted 19 March 2003
Abstract The flow field and pollutant dispersion characteristics in a three-dimensional urban street canyon are investigated for various building array geometries. The street canyon in consideration is located in a multi-canopy building array that is similar to realistic estate situations. The pollutant dispersion characteristics are studied for various canopy aspect ratios, namely: the canyon height to width ratio, canyon length to height ratio, canyon breadth ratio and crossroad locations are studied. A three-dimensional field-size canyon has been analysed through numerical simulations using k e turbulence model. As expected, the wind flow and mode of pollutant dispersion is strongly dependent on the various flow geometric configurations and that the results can be different from that of a single canyon system. For example, it is found that the pollutant retention value is minimum when the canyon height-to-width ratio is approximately 0.8, or that the building height ratio is 0.5. Various rules of thumbs on urban canyon geometry have been established for good pollutant dispersion. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Building arrays; Urban street canyon; k e turbulence model; Pollutant dispersion; Canopy aspect ratios; Urban air quality
1. Introduction Urban canyon climate is a key component in defining the urban environment and has been discussed extensively (Oke, 1988; Hunter et al., 1992; Ca et al., 1995). In spite of this, many metropolises still suffer from poor ventilation and air quality problems due to improper urban planning. Unstructured planning of urban canopies is common in areas of rapid urbanization. Rise in traffic density with the rapid development of urban activities consequently result in obstinate situation of unfavourable ambient air quality. Since canyon flow is dominated by interactions of incoming wind field and surrounding buildings, building geometry effects and rules of thumb in canyon designation are often under *Corresponding author. Fax: +852-2858-5415. E-mail address: [email protected] (A.T. Chan).
investigations (Theurer, 1999; Chan et al., 2002). However, it is not easy to arrive at a general formula that gives a compromising wind field for a particular canyon geometry for application in urban design due to the participation of so many intertwining factors. For instance while strong wind is favourable to pollutant dispersion, it could be an annoyance to pedestrians. The self-circulating eddies are found in the corners of the canyon and accumulate pollutants and debris at the corners of the buildings especially when continuous source of pollutants are generated inside the canyon. Therefore, study in this area is necessary to understand all associated phenomena and to provide up to date reference data for urban plannings. Simulations of flow characteristics in canyon using wind tunnel experiments demonstrate three-dimensional flow phenomena around canyons (Bottema, 1997; Pavageau and Schatzmann, 1999; Kastner-Klein and
1352-2310/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00252-8
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Plate, 1999). These works provide experimental data supporting results done numerically. Xia and Leung (2001) worked on the flow field over buildings in street canyons. Sini et al. (1996) and Huang et al. (2000) have discussed the regimes formation and effect of inclined inflow. On the other hand, many of these works involved only very simple canyon geometries that are not indicative of real-life situation and are not particularly useful in real-life scenarios (Chan et al., 2002). For example, many of the above-mentioned works studied the pollutant dispersion inside a two-dimensional Tshaped canyon model or isolated canyons. Many known phenomena like backward contamination or city roughness effects cannot be captured from these idealised models. Besides it is also known that thermal effects on canyon can be equally important in characterising canyon flow (Kim and Baik, 1999, 2001). Real urban flows are obviously more complicated and it is the motivation of this work to look into these in a more systematic approach. The objective of this work is to find out critical building configuration relationships that would enhances better ventilation and thus better pollutant dispersion in a multi-block urban street canyon. The present work represents as an ongoing study of the previous work (Chan et al., 2001) for a two-canopy system. In particular the pollutant dispersion characteristics are studied for various canopy aspect ratios, namely: the canyon height to width ratio, canyon length to height ratio, canyon breadth ratio and crossroad locations are studied. We would like to develop guidelines for urban planning based on these critical ratios for use in multi-canyon cities. Another purpose of this work
is to apply these relationships on a real case model and to suggest sensible modifications that can be made to exalt the situation. The simulations are done numerically by the commercial code CFX-5. The validations of the settings and the advantages of the CFX-5 package have been discussed previously (Chan et al., 2001 and references therein). Despite the popularity and accuracy of the standard k e model used in the package, it must be emphasised that the k e model is known to have many problems like over-prediction of turbulent kinetic energy near building edges.
2. Simulation model In the present work, the canyon model has been modified in order to investigate the multi-block effects on canyon flow. The system with canyon domain in dimension L H W (200 m 60 m 350 m) and canyon length l; building height h; breadth b and the canyon width w is shown in Fig. 1. Canyon ratios l=h and h=w are selected in reference with the regimes classified by Oke (1988). The standard length l; height h; breadth b and canyon width w are thus 180, 20, 10 and 60 m, giving h=w ¼ 0:33 and l=h ¼ 9:0: This implies that there is a lateral spacing of 20 m between the canopy blocks with the adjacent domain and this effectively constitutes the three-dimensionality of the problem. A total of four buildings are established in the model in which, the region under focus is the area between Buildings 2 and 3. Roughness elements in form of small individual blocks of 3 cm high is located upstream of all buildings as in typical wind tunnel and numerical experiments
Fig 1. Simulation model (boundary conditions on each surface are given in Table 1).
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(Meroney et al., 1996). Such arrangements favour flow development and suppress the strength of bolster eddy in the first building as in a real multiple canopy system. Additional upstream and downstream buildings will affect the wind flow pattern, thus making the model closer to a complex real urban area. Despite only one canyon upstream of the canyon studied is used, the flow characteristics upstream for a multiple canyon system are actually similar to that used in the simulations as discussed in Meroney et al. (1996) and Pavageau and Schatzmann (1999). This arrangement is used to save computational resources. Figs. 2a and b show the finite element mesh of the domain. The tetrahedral elements discretised are of maximum edge length ranging from 1.5 to 5.0 m. Line source pollutants are assigned in this simulation work to simulate traffic pollution. The pollutant line source is along the datum between Buildings 2 and 3 and is located midway between the two buildings. The source is 180 m long, 0.5 m wide and it emits pollutants with a velocity of 0.5 ms1 based on experiments. The Reynolds number based on the building height is kept at around 18.5 106, based on the height of the windward building and the free stream velocity. Other fluid properties are set with the reference to the general environment in Hong Kong and are found in Chan et al. (2001).
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3. Simulations and analyses All computations are based on the basic assumptions of lower atmosphere and the standard k e turbulence closure model as in the first part of the study (Chan et al., 2001). To evaluate the extent the dispersion of the pollutant from the canyon, we shall calculate the retention value of the canyon for the pollutant. The retention value is defined as the quantity of pollutant found inside the canyon over the total pollutant released in a specified time. This reflects the dispersion ability of air pollutants from the sources near pedestrian level out to the free stream above the canyon. The cavity under investigation is the volume of the canyon noted as the canyon height multiplied by the canyon width. To have a better understanding of the concentration in region where most social activities take place, the average pollutant concentration at pedestrian level is also noted. The following sections present the simulations of different canyon configurations with the clean free air stream flowing perpendicularly across the canyons. Each aspect of investigations is carried out independently keeping all other parameters constant to arrive at the valid relationships in subsequent section.
Fig 2. Mesh generated for the simulation model: (a) isometric view, (b) side view.
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3.1. Individual canyon height-to-width ratio In studies of canyon flow, the aspect ratio h=w is generally accepted as the key factor in determining the characteristics of wind flow within an urban canyon (Oke, 1988; Sini et al., 1996; Jeong and Andrews, 2002). The flow field, transition of flow regimes and corresponding mode of pollutant dispersion for an isolated canyon have been discussed in the previous paper (Chan et al., 2001). We shall study the effect of presence of other buildings on the pollutant dispersion within the canyon. In this four-canopy system, only the canyon width between Buildings 2 and 3 will vary in this section while other canyon widths remain constant. The variation will demonstrate h=w ratios that cover the three regimes stated by Oke (1988). The results obtained in the simulations show similar phenomenon as in a two-canyon system (Chan et al., 2001; Lee and Park, 1994). Isolated roughness flow regime is developed in wider canyon, with h=w below 0.5 and wake interference flow regime lying between h=w ratio 0.5–0.8 before transition into skimming flow (Figs. 3 and 4). It is noted that the retention value shows distinctly the respective flow regimes. At h=w ¼ 0:75; the canyon gives the lowest average pollutant concentration, indicating it is the most favourable configuration for pollutant dispersion. At low h=w; corresponding to a very wide canyon, it is difficult for the upstream wind to carry all the pollutant outside of the wide canyon due to its sheer width; whereas in a deep canyon, secondary vortex develops giving two vortices along the building height rotating in opposite directions (Fig. 3c), thus promoting trapping of pollutants. Similar trend have
been reported in the two-canopy system published in Chan et al. (2001) but with the installment of the additional upstream and downstream canopies, the maximum concentrations are nearly doubled by the interfering effects caused on the strength of flow and thus the extent of dispersion. 3.2. Global canyon height to width ratio The density of canyon building alignments alters the overall flow profile through changes in surface roughness. This produces an effect to the inmost canyon and raises the amount of pollutants trapped in narrow canyons. In early designs of urban estates, designers tend to maximise the land use and to keep a tidy appearance of the estate, resulting in rows of buildings located in equal spacing. Such geometrical effects on pollutant dispersion or natural ventilation inside the canopies are studied by changing the h=w ratios of each consecutive canopy as shown in Figs. 5 and 6. In general, the trend follows closely that of a single-canyon system. Particular interest is on the upstream interference to the roof wind profile. As the buildings become more closely packed, the separated flow seems past across a large barrier in which the distortion of wind profile over the buildings becomes significant. A generally higher retention value in skimming flow cases provides evidence that arrays of buildings constructed closely unfavour ventilation inside the canopies. An overall increase in the pollutant concentration at pedestrian level also indicates the dispersion of pollutants in lower level is not effective in densely packed buildings (Table 1).
(a)
(b)
(c) Fig 3. Velocity plot with pollutant for (a) h=w ¼ 0:33; (b) h=w ¼ 1:0; (c) h=w ¼ 2:0 (ratio refers to that of Canyon #2).
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0.8
0.6
-3
Concentration (g m ) / Retention value
1
0.4
Max Pollutant Concentration Isolated Roughness Flow
0.2
Wake Interference Flow
Retention value
Skimming Flow
Ave. Pollutant Conc. at Pedestrian Level
0 0
0.5
1
1.5
2
2.5
h/w Fig 4. Maximum pollutant concentration, retention value and average pollutant concentration at pedestrian level against h=w between Buildings 2 and 3.
(a)
(b)
(c) Fig 5. Velocity plot with pollutant for all (a) h=w ¼ 0:33; (b) h=w ¼ 1:0; (c) h=w ¼ 2:0 (ratio refers to that of Canyon #2).
3.3. Canopy breadth ratio Canopy breadth relates to the roughness of the urban system. Increasing the canopy breadth increases the intensity of turbulence and the depth of frictional influence. The canopy breadth of Building 3 over 2 in this case serves to examine the roughness effect imposed by the downstream building of the canyon. Plumes of pollutants are less dispersed with wide roughness length
as shown in Fig. 7. As the relative building breadth increases to a value of 2.0, a critical relative roughness fetch (Chan, 2001) and thus a maximum intensity of the turbulence is attained. This makes the pollutant retention value, maximum pollutant concentration and average pollutant concentration at pedestrian level reaches a minimum when the ratio reaches 2.0 as in Figs. 7 and 8. At this ratio, a transition between wake interference flow and skimming flow takes place and
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-3
Concentration (g m ) / Retention value
1.2
1
0.8
0.6
0.4
Max Pollutant Concentration Retention value
0.2
Ave. Pollutant Conc. at Pedestrian Level 0 0
0.5
1
1.5
2
2.5
h/w Fig 6. Maximum pollutant concentration, retention value and average pollutant concentration at pedestrian level against h=w between all buildings.
Table 1 Boundary conditions Surface no.
Nature
Input value
Surface 1
Inlet
Surface 2
Outlet
Surface 3
Source inlet
Surface 4, 5, 6, 7
Overall exposed building walls System walls Datum
Logarithmic wind profile Standard temperature and pressure Normal speed at 0.5 ms1 No slip condition
Surface 8, 9, 10 Surface 11
Free slip condition No slip condition, roughness 0.01 m
according to Counihan (1971), the maximum roughness effect corresponds approximately to the transition between wake interference and skimming flow. Beyond this critical value, the increase of canopy breadth serves to reduce the roughness fetch, as the interference between individual wakes smothers their role in producing turbulence. This is indicated by the increase in retention and pollutant concentration values from Fig. 8. 3.4. Canyon height ratio Height difference of buildings is known to be a significant factor in altering the flow field. Essentially difference in canopy height shifts the region experiencing
highest velocity from the upstream shear corner to the area above the higher building (Fig. 9). The pressure built up at the corner of the first building accelerates the wind layer before it comes across the shear corner of the higher building. It has been discussed in Chan et al. (2001) that the shear zone accelerating the top layer of canopy air contributes to the strength of the recirculating eddies. The extent of the eddy is determined upon the height difference of the buildings. The leeward building height is of significant importance in the downstream wind profile. A lower windward building (h3 =ho1) does not promote the acceleration of canyon air above the roof, the weaker vortex inside the canyon leads to a slightly higher amount of pollutants trapped near the pedestrian level. This defect can be compensated by the re-entrainment of fresh air into the canyon cavity from the downstream of Building 3. These observations agree qualitatively with that of Baik et al. (2000) obtained in a water-channel experiment. The fresh air dilutes the concentration and shows a particularly low retention value as plotted in Fig. 10. The maximum retention value occurs at h3 =h ¼ 1:2 because of the stagnation at the upstream of the Building 3. This height ratio obtained is close to the previous study for a two-canopy system. Further increase of leeward building height results in a higher velocity of wind layer nearby, promoting the dispersion of pollutant of the upper layer. The stagnation flow zone extends till the ratio h3 =h ¼ 2:0; where Building 3 almost block the entire plume and dispersion can only occurs at the lateral sides of the canopies. This hindrance effect is known as leeward blockage.
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(a)
(b)
(c) Fig 7. Velocity plot with pollutant for all (a) b3 =w ¼ 0:5; (b) b3 =w ¼ 2:0; (c) b3 =w ¼ 4:0 (ratio refers to that of Canyon #2).
Concentration (g m-3) / Retention value
1.2
1
0.8
0.6
0.4
Max Pollutant Concentration 0.2
Retention value Average Pollutant Concentration at Pedestrian Level
0 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
b3 /b
Fig 8. Maximum pollutant concentration, retention value and average pollutant concentration at pedestrian level against b3 =b:
3.5. Crossroad location Introduction of crossroad between buildings is observed to produce a very distinct wind profile within the canyon. The regimes described by Oke (1988) and Theurer (1999) are no longer valid in commenting the flow inside canyon. The settings in Fig. 11 present the
crossroad position at leeward canopy with d denoting the length of Building 3 or the crossroad shift distance. The crossroad gap distance is fixed at 20 m. With an open slot appeared in the canyon, lateral effect is introduced at the sides of the canopies. The separation of flow at the lateral corners and reattachments at the leeward side of the wall bring in the horizontal element
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(a)
(b)
(c) Fig 9. Velocity plot with pollutant for all (a) h3 =w ¼ 0:25; (b) h3 =w ¼ 0:5; (c) h3 =w ¼ 2:0 (ratio refers to that of Canyon #2).
Max Pollutant Concentration Retention value
-3
Concentration (g m ) / Retention value
1.2
Ave. Pollutant Conc. at Pedestrian Level
1
0.8
0.6
0.4
0.2
Leeward Blockage
Stagnation Flow
Isolated Roughness Flow
0 0
0.5
1
1.5
2
2.5
h3 /h Fig 10. Maximum pollutant concentration, retention value and average pollutant concentration at pedestrian level against h3 =h:
to the flow. Three-dimensional flow is found in the canyon. Canyon air near the crossroad passes through the open slot, and this leakage of canyon flow generates a low-pressure zone around the slot. Therefore, it is observed that the nearby cavity flow is sucked towards the zone. The crossroad introduces a horizontal path for
the pollutants to disperse away, resulting in an overall reduction in retention values as compared with continuous canyon (Fig. 12). It is noted that both the retention value and the maximum pollutant concentration show a minimum at the value d=L ¼ 0:1: This is due to the effective dispersion by the two vortices beside the
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(b)
(a)
Fig 11. Vector plot with pollutants for crossroad distance (a) d ¼ 20 m, (b) d ¼ 80 m.
0.8
0.6
-3
Concentration (g m ) / Retention value
1
0.4
Max Pollutant Concentration Retention value
0.2
Ave. Pollutant Conc. at Pedestrian Level 0 0
0.1
0.2
0.3
0.4
0.5
X/L Fig 12. Maximum pollutant concentration, retention value and average pollutant concentration at pedestrian level against d=L:
crossroad. Similar drop in the retention value is obtained in the case when the crossroad is located at the centreline of the model, as the pair of counterrotating vortices at the crossroad enhances the dispersion.
3.6. Distance between two-canyon arrays In real urban cities, canyons crossing one another are common and form a primitive crossed-unit of intricate transport network. The variation of the distance between the two building arrays helps to investigate the effect of street width along wind direction on pollutant dispersion. When the street is wide enough (h=co0:75), sufficient wind flows along it and carries the emitted pollutants down the street (Fig. 13). This lowers the retention values and maximum pollutant concentrations. As the width of the street decreases, pollutants driven by the flow along the street are insignificant and dispersion of pollutants are limited to those over the
rooftop. This is shown by the high retention values for h=c from 0.75.
3.7. Modification of buildings in highly polluted district Amongst districts of severe air pollution problems in Hong Kong, Mongkok is a suitable choice for investigation as canyons in the district is regular and straight but buildings composing the urban configurations are complex. To evaluate the usefulness, feasibility and effectiveness of the guidelines developed in the previous sections, these guidelines are put to test in a hypothetical situation to study its effectiveness in solving air pollution problems. The model of the district with the associated heavy traffic is shown in Fig. 14a. The coloured blocks represent three height levels of the buildings. The yellow ones represent heights of 0–30 m, orange one represents building height from 31 to 60 m and the red colour represents building height over 60 m. Two main line sources are located in the horizontal
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(a)
(b)
Fig 13. Vector plot with pollutants for arrays of (a) h=c ¼ 0:5; (b) h=c ¼ 2:0 (ratio refers to that of Canopy #2).
Mong Kok Road and the vertical Nathan Road. Dispersion patterns developed by the dominant wind along the horizontal street is illustrated in Fig. 14b. Initial simulation results show a large amount of plume accumulated around the area highlighted by the white circle, especially the junction of Tung Choi Street and Fife Street. In most situations, rebuilding the road size is a strategy to achieve better ventilation. However, it involves too much reconstruction and is not beneficial for a single purpose. Therefore altering the size of buildings is considered instead. Modifications of buildings in that zone named 1–4 are done in steps applying some of the relative geometric relationships found from the previous simulations. For the first modification, height ratio relationship is first applied. It is found in Section 3.4 that the retention value reaches its minimum when the ratio is between 0.25 and 0.5. The height ratio of Buildings 3 and 4 is thus adjusted to 0.5, raising Building 3 to 60 m high. It must be emphasised here that we actually raise the building height to achieve better ventilation through redistribution of pollutant. The simulated results displayed on Fig. 14c demonstrate an overall improvement in the site as the plume size decreases. In the second modification, we try to use the relative building breadth relationship to remodel Buildings 1 and 2. Relative building breadth ratio between 1.5 and 2.5 is found to promote better pollutant dispersion, so combining the target buildings to change the ratio to 2.0 is a preferable decision. The plume used to be accumulated near Buildings 2 and 3 dispersed and the air quality in the zone is further improved (Fig. 14d). In the third modification, based on the second one, the improvement of the downstream canopy air of the zone will be the object of the simulation. Height ratio relationship is applied to adjust Building 4 to attain a ratio of 0.5 comparing the height of the building downstream. The simulation result in Fig. 14e shows that almost the whole plume that originally accumulates around the concerning zone has disappeared. Most of the pollutants are dispersed along Mong Kok Road to
the right-hand side. With consecutive modifications, the pollution retention values of Fife Street drops by 40% while that of Tung Choi Street reduces by 30% (Table 2) and the air quality of the zone is much improved.
4. Discussion and summary Investigation on promotion of flow and pollutant dispersion in complex terrains can never result in explicit rules, as numerous considerations are needed. To focus on the geometric impact on urban flow, several rules of thumb are suggested for city planning. Equally spacing of buildings prevents the dispersion of pollutant out from the canyon, unless the canyon is wide or the roof wind velocity is high enough. The critical ratio for effective dispersion is at h=w ¼ 0:7; where there is a change from wake interference to skimming flow. Urban variations in building height and breadth is preferred. Intricate roof level configurations promote ventilation. The critical relative height for minimum retention value lies between height ratio 0.25 and 0.5, while maximum retention values occur in the region between 1.0 and 1.5 where stagnation of flow occurs. When the effect of stagnation of flow diminished, further increase in leeward building height can cause leeward blockage of pollutant. Thin buildings do not necessarily enhance pollutant dispersion. In fact, the critical relative building breadth for maximum retention value lies in the region of either 1.0 or 3.0 and a relative breadth ratio between 1.5 and 2.5 promotes better diffusion. Non-uniform building breadth is recommended. Crossroad introduces the lateral eddies into the canyon. The release of flow through the crossroad creates a relatively low-pressure zone and thus sucks the flow nearby. A general decrease in the retention value is found. Among all cases, crossroad in a location where more vortices generated ahead is more promising in pollutant dispersion.
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Table 2 Numerical outputs of the simulation work of an area in Mongkok Relationships applied
(a)
(b)
(c)
Control simulation 1st Modification 2nd Modification 3rd Modification
h3 =h b3 =b h3 =h
Retention value ( 103) Fife Street
Tung Choi Street
3.34
12.4
2.22 2.17 1.93
9.32 9.18 9.00
path cases, pollutants are forced to disperse from rooftop, which is less effective. It is obvious that the rules themselves do not work independently to promote canyon air quality. Application of the rules on the real case geometries is a complex practice, as lots of elements and tactics in modifications are needed to be well considered. Besides, non-stationary wind flow and non-linear building parameters make the flow even more complicated to demonstrate. Modifications of the heavily polluted district demonstrated in this study show the relationships described here are not merely a paperwork measures. All these are critical theoretically and in real-life situation and will be studied in the future.
Acknowledgements (d)
This research work is sponsored by The University of Hong Kong.
References
(e) Fig 14. Simulation model base on urban configurations of Mongkok: (a) basic model, (b) vector plot and pollutant dispersion in Mongkok before modification, (c) after 1st modification, (d) after 2nd modification, (e) after 3rd modification.
A path along wind direction among perpendicular canyons is preferable with sufficient width. It is found that the retention value drops for wide path of h=w below 0.5 as flows pass along wide paths carrying pollutants down to the end of the path. While in narrow
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Atmospheric Environment 37 (2003) 2761–2772
Strategic guidelines for street canyon geometry to achieve sustainable street air quality—part II: multiple canopies and canyons Andy T. Chan*, William T.W. Au, Ellen S.P. So Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong Received 13 December 2002; received in revised form 6 March 2003; accepted 19 March 2003
Abstract The flow field and pollutant dispersion characteristics in a three-dimensional urban street canyon are investigated for various building array geometries. The street canyon in consideration is located in a multi-canopy building array that is similar to realistic estate situations. The pollutant dispersion characteristics are studied for various canopy aspect ratios, namely: the canyon height to width ratio, canyon length to height ratio, canyon breadth ratio and crossroad locations are studied. A three-dimensional field-size canyon has been analysed through numerical simulations using k e turbulence model. As expected, the wind flow and mode of pollutant dispersion is strongly dependent on the various flow geometric configurations and that the results can be different from that of a single canyon system. For example, it is found that the pollutant retention value is minimum when the canyon height-to-width ratio is approximately 0.8, or that the building height ratio is 0.5. Various rules of thumbs on urban canyon geometry have been established for good pollutant dispersion. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Building arrays; Urban street canyon; k e turbulence model; Pollutant dispersion; Canopy aspect ratios; Urban air quality
1. Introduction Urban canyon climate is a key component in defining the urban environment and has been discussed extensively (Oke, 1988; Hunter et al., 1992; Ca et al., 1995). In spite of this, many metropolises still suffer from poor ventilation and air quality problems due to improper urban planning. Unstructured planning of urban canopies is common in areas of rapid urbanization. Rise in traffic density with the rapid development of urban activities consequently result in obstinate situation of unfavourable ambient air quality. Since canyon flow is dominated by interactions of incoming wind field and surrounding buildings, building geometry effects and rules of thumb in canyon designation are often under *Corresponding author. Fax: +852-2858-5415. E-mail address: [email protected] (A.T. Chan).
investigations (Theurer, 1999; Chan et al., 2002). However, it is not easy to arrive at a general formula that gives a compromising wind field for a particular canyon geometry for application in urban design due to the participation of so many intertwining factors. For instance while strong wind is favourable to pollutant dispersion, it could be an annoyance to pedestrians. The self-circulating eddies are found in the corners of the canyon and accumulate pollutants and debris at the corners of the buildings especially when continuous source of pollutants are generated inside the canyon. Therefore, study in this area is necessary to understand all associated phenomena and to provide up to date reference data for urban plannings. Simulations of flow characteristics in canyon using wind tunnel experiments demonstrate three-dimensional flow phenomena around canyons (Bottema, 1997; Pavageau and Schatzmann, 1999; Kastner-Klein and
1352-2310/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00252-8
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Plate, 1999). These works provide experimental data supporting results done numerically. Xia and Leung (2001) worked on the flow field over buildings in street canyons. Sini et al. (1996) and Huang et al. (2000) have discussed the regimes formation and effect of inclined inflow. On the other hand, many of these works involved only very simple canyon geometries that are not indicative of real-life situation and are not particularly useful in real-life scenarios (Chan et al., 2002). For example, many of the above-mentioned works studied the pollutant dispersion inside a two-dimensional Tshaped canyon model or isolated canyons. Many known phenomena like backward contamination or city roughness effects cannot be captured from these idealised models. Besides it is also known that thermal effects on canyon can be equally important in characterising canyon flow (Kim and Baik, 1999, 2001). Real urban flows are obviously more complicated and it is the motivation of this work to look into these in a more systematic approach. The objective of this work is to find out critical building configuration relationships that would enhances better ventilation and thus better pollutant dispersion in a multi-block urban street canyon. The present work represents as an ongoing study of the previous work (Chan et al., 2001) for a two-canopy system. In particular the pollutant dispersion characteristics are studied for various canopy aspect ratios, namely: the canyon height to width ratio, canyon length to height ratio, canyon breadth ratio and crossroad locations are studied. We would like to develop guidelines for urban planning based on these critical ratios for use in multi-canyon cities. Another purpose of this work
is to apply these relationships on a real case model and to suggest sensible modifications that can be made to exalt the situation. The simulations are done numerically by the commercial code CFX-5. The validations of the settings and the advantages of the CFX-5 package have been discussed previously (Chan et al., 2001 and references therein). Despite the popularity and accuracy of the standard k e model used in the package, it must be emphasised that the k e model is known to have many problems like over-prediction of turbulent kinetic energy near building edges.
2. Simulation model In the present work, the canyon model has been modified in order to investigate the multi-block effects on canyon flow. The system with canyon domain in dimension L H W (200 m 60 m 350 m) and canyon length l; building height h; breadth b and the canyon width w is shown in Fig. 1. Canyon ratios l=h and h=w are selected in reference with the regimes classified by Oke (1988). The standard length l; height h; breadth b and canyon width w are thus 180, 20, 10 and 60 m, giving h=w ¼ 0:33 and l=h ¼ 9:0: This implies that there is a lateral spacing of 20 m between the canopy blocks with the adjacent domain and this effectively constitutes the three-dimensionality of the problem. A total of four buildings are established in the model in which, the region under focus is the area between Buildings 2 and 3. Roughness elements in form of small individual blocks of 3 cm high is located upstream of all buildings as in typical wind tunnel and numerical experiments
Fig 1. Simulation model (boundary conditions on each surface are given in Table 1).
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(Meroney et al., 1996). Such arrangements favour flow development and suppress the strength of bolster eddy in the first building as in a real multiple canopy system. Additional upstream and downstream buildings will affect the wind flow pattern, thus making the model closer to a complex real urban area. Despite only one canyon upstream of the canyon studied is used, the flow characteristics upstream for a multiple canyon system are actually similar to that used in the simulations as discussed in Meroney et al. (1996) and Pavageau and Schatzmann (1999). This arrangement is used to save computational resources. Figs. 2a and b show the finite element mesh of the domain. The tetrahedral elements discretised are of maximum edge length ranging from 1.5 to 5.0 m. Line source pollutants are assigned in this simulation work to simulate traffic pollution. The pollutant line source is along the datum between Buildings 2 and 3 and is located midway between the two buildings. The source is 180 m long, 0.5 m wide and it emits pollutants with a velocity of 0.5 ms1 based on experiments. The Reynolds number based on the building height is kept at around 18.5 106, based on the height of the windward building and the free stream velocity. Other fluid properties are set with the reference to the general environment in Hong Kong and are found in Chan et al. (2001).
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3. Simulations and analyses All computations are based on the basic assumptions of lower atmosphere and the standard k e turbulence closure model as in the first part of the study (Chan et al., 2001). To evaluate the extent the dispersion of the pollutant from the canyon, we shall calculate the retention value of the canyon for the pollutant. The retention value is defined as the quantity of pollutant found inside the canyon over the total pollutant released in a specified time. This reflects the dispersion ability of air pollutants from the sources near pedestrian level out to the free stream above the canyon. The cavity under investigation is the volume of the canyon noted as the canyon height multiplied by the canyon width. To have a better understanding of the concentration in region where most social activities take place, the average pollutant concentration at pedestrian level is also noted. The following sections present the simulations of different canyon configurations with the clean free air stream flowing perpendicularly across the canyons. Each aspect of investigations is carried out independently keeping all other parameters constant to arrive at the valid relationships in subsequent section.
Fig 2. Mesh generated for the simulation model: (a) isometric view, (b) side view.
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3.1. Individual canyon height-to-width ratio In studies of canyon flow, the aspect ratio h=w is generally accepted as the key factor in determining the characteristics of wind flow within an urban canyon (Oke, 1988; Sini et al., 1996; Jeong and Andrews, 2002). The flow field, transition of flow regimes and corresponding mode of pollutant dispersion for an isolated canyon have been discussed in the previous paper (Chan et al., 2001). We shall study the effect of presence of other buildings on the pollutant dispersion within the canyon. In this four-canopy system, only the canyon width between Buildings 2 and 3 will vary in this section while other canyon widths remain constant. The variation will demonstrate h=w ratios that cover the three regimes stated by Oke (1988). The results obtained in the simulations show similar phenomenon as in a two-canyon system (Chan et al., 2001; Lee and Park, 1994). Isolated roughness flow regime is developed in wider canyon, with h=w below 0.5 and wake interference flow regime lying between h=w ratio 0.5–0.8 before transition into skimming flow (Figs. 3 and 4). It is noted that the retention value shows distinctly the respective flow regimes. At h=w ¼ 0:75; the canyon gives the lowest average pollutant concentration, indicating it is the most favourable configuration for pollutant dispersion. At low h=w; corresponding to a very wide canyon, it is difficult for the upstream wind to carry all the pollutant outside of the wide canyon due to its sheer width; whereas in a deep canyon, secondary vortex develops giving two vortices along the building height rotating in opposite directions (Fig. 3c), thus promoting trapping of pollutants. Similar trend have
been reported in the two-canopy system published in Chan et al. (2001) but with the installment of the additional upstream and downstream canopies, the maximum concentrations are nearly doubled by the interfering effects caused on the strength of flow and thus the extent of dispersion. 3.2. Global canyon height to width ratio The density of canyon building alignments alters the overall flow profile through changes in surface roughness. This produces an effect to the inmost canyon and raises the amount of pollutants trapped in narrow canyons. In early designs of urban estates, designers tend to maximise the land use and to keep a tidy appearance of the estate, resulting in rows of buildings located in equal spacing. Such geometrical effects on pollutant dispersion or natural ventilation inside the canopies are studied by changing the h=w ratios of each consecutive canopy as shown in Figs. 5 and 6. In general, the trend follows closely that of a single-canyon system. Particular interest is on the upstream interference to the roof wind profile. As the buildings become more closely packed, the separated flow seems past across a large barrier in which the distortion of wind profile over the buildings becomes significant. A generally higher retention value in skimming flow cases provides evidence that arrays of buildings constructed closely unfavour ventilation inside the canopies. An overall increase in the pollutant concentration at pedestrian level also indicates the dispersion of pollutants in lower level is not effective in densely packed buildings (Table 1).
(a)
(b)
(c) Fig 3. Velocity plot with pollutant for (a) h=w ¼ 0:33; (b) h=w ¼ 1:0; (c) h=w ¼ 2:0 (ratio refers to that of Canyon #2).
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0.8
0.6
-3
Concentration (g m ) / Retention value
1
0.4
Max Pollutant Concentration Isolated Roughness Flow
0.2
Wake Interference Flow
Retention value
Skimming Flow
Ave. Pollutant Conc. at Pedestrian Level
0 0
0.5
1
1.5
2
2.5
h/w Fig 4. Maximum pollutant concentration, retention value and average pollutant concentration at pedestrian level against h=w between Buildings 2 and 3.
(a)
(b)
(c) Fig 5. Velocity plot with pollutant for all (a) h=w ¼ 0:33; (b) h=w ¼ 1:0; (c) h=w ¼ 2:0 (ratio refers to that of Canyon #2).
3.3. Canopy breadth ratio Canopy breadth relates to the roughness of the urban system. Increasing the canopy breadth increases the intensity of turbulence and the depth of frictional influence. The canopy breadth of Building 3 over 2 in this case serves to examine the roughness effect imposed by the downstream building of the canyon. Plumes of pollutants are less dispersed with wide roughness length
as shown in Fig. 7. As the relative building breadth increases to a value of 2.0, a critical relative roughness fetch (Chan, 2001) and thus a maximum intensity of the turbulence is attained. This makes the pollutant retention value, maximum pollutant concentration and average pollutant concentration at pedestrian level reaches a minimum when the ratio reaches 2.0 as in Figs. 7 and 8. At this ratio, a transition between wake interference flow and skimming flow takes place and
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-3
Concentration (g m ) / Retention value
1.2
1
0.8
0.6
0.4
Max Pollutant Concentration Retention value
0.2
Ave. Pollutant Conc. at Pedestrian Level 0 0
0.5
1
1.5
2
2.5
h/w Fig 6. Maximum pollutant concentration, retention value and average pollutant concentration at pedestrian level against h=w between all buildings.
Table 1 Boundary conditions Surface no.
Nature
Input value
Surface 1
Inlet
Surface 2
Outlet
Surface 3
Source inlet
Surface 4, 5, 6, 7
Overall exposed building walls System walls Datum
Logarithmic wind profile Standard temperature and pressure Normal speed at 0.5 ms1 No slip condition
Surface 8, 9, 10 Surface 11
Free slip condition No slip condition, roughness 0.01 m
according to Counihan (1971), the maximum roughness effect corresponds approximately to the transition between wake interference and skimming flow. Beyond this critical value, the increase of canopy breadth serves to reduce the roughness fetch, as the interference between individual wakes smothers their role in producing turbulence. This is indicated by the increase in retention and pollutant concentration values from Fig. 8. 3.4. Canyon height ratio Height difference of buildings is known to be a significant factor in altering the flow field. Essentially difference in canopy height shifts the region experiencing
highest velocity from the upstream shear corner to the area above the higher building (Fig. 9). The pressure built up at the corner of the first building accelerates the wind layer before it comes across the shear corner of the higher building. It has been discussed in Chan et al. (2001) that the shear zone accelerating the top layer of canopy air contributes to the strength of the recirculating eddies. The extent of the eddy is determined upon the height difference of the buildings. The leeward building height is of significant importance in the downstream wind profile. A lower windward building (h3 =ho1) does not promote the acceleration of canyon air above the roof, the weaker vortex inside the canyon leads to a slightly higher amount of pollutants trapped near the pedestrian level. This defect can be compensated by the re-entrainment of fresh air into the canyon cavity from the downstream of Building 3. These observations agree qualitatively with that of Baik et al. (2000) obtained in a water-channel experiment. The fresh air dilutes the concentration and shows a particularly low retention value as plotted in Fig. 10. The maximum retention value occurs at h3 =h ¼ 1:2 because of the stagnation at the upstream of the Building 3. This height ratio obtained is close to the previous study for a two-canopy system. Further increase of leeward building height results in a higher velocity of wind layer nearby, promoting the dispersion of pollutant of the upper layer. The stagnation flow zone extends till the ratio h3 =h ¼ 2:0; where Building 3 almost block the entire plume and dispersion can only occurs at the lateral sides of the canopies. This hindrance effect is known as leeward blockage.
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(a)
(b)
(c) Fig 7. Velocity plot with pollutant for all (a) b3 =w ¼ 0:5; (b) b3 =w ¼ 2:0; (c) b3 =w ¼ 4:0 (ratio refers to that of Canyon #2).
Concentration (g m-3) / Retention value
1.2
1
0.8
0.6
0.4
Max Pollutant Concentration 0.2
Retention value Average Pollutant Concentration at Pedestrian Level
0 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
b3 /b
Fig 8. Maximum pollutant concentration, retention value and average pollutant concentration at pedestrian level against b3 =b:
3.5. Crossroad location Introduction of crossroad between buildings is observed to produce a very distinct wind profile within the canyon. The regimes described by Oke (1988) and Theurer (1999) are no longer valid in commenting the flow inside canyon. The settings in Fig. 11 present the
crossroad position at leeward canopy with d denoting the length of Building 3 or the crossroad shift distance. The crossroad gap distance is fixed at 20 m. With an open slot appeared in the canyon, lateral effect is introduced at the sides of the canopies. The separation of flow at the lateral corners and reattachments at the leeward side of the wall bring in the horizontal element
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(a)
(b)
(c) Fig 9. Velocity plot with pollutant for all (a) h3 =w ¼ 0:25; (b) h3 =w ¼ 0:5; (c) h3 =w ¼ 2:0 (ratio refers to that of Canyon #2).
Max Pollutant Concentration Retention value
-3
Concentration (g m ) / Retention value
1.2
Ave. Pollutant Conc. at Pedestrian Level
1
0.8
0.6
0.4
0.2
Leeward Blockage
Stagnation Flow
Isolated Roughness Flow
0 0
0.5
1
1.5
2
2.5
h3 /h Fig 10. Maximum pollutant concentration, retention value and average pollutant concentration at pedestrian level against h3 =h:
to the flow. Three-dimensional flow is found in the canyon. Canyon air near the crossroad passes through the open slot, and this leakage of canyon flow generates a low-pressure zone around the slot. Therefore, it is observed that the nearby cavity flow is sucked towards the zone. The crossroad introduces a horizontal path for
the pollutants to disperse away, resulting in an overall reduction in retention values as compared with continuous canyon (Fig. 12). It is noted that both the retention value and the maximum pollutant concentration show a minimum at the value d=L ¼ 0:1: This is due to the effective dispersion by the two vortices beside the
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(b)
(a)
Fig 11. Vector plot with pollutants for crossroad distance (a) d ¼ 20 m, (b) d ¼ 80 m.
0.8
0.6
-3
Concentration (g m ) / Retention value
1
0.4
Max Pollutant Concentration Retention value
0.2
Ave. Pollutant Conc. at Pedestrian Level 0 0
0.1
0.2
0.3
0.4
0.5
X/L Fig 12. Maximum pollutant concentration, retention value and average pollutant concentration at pedestrian level against d=L:
crossroad. Similar drop in the retention value is obtained in the case when the crossroad is located at the centreline of the model, as the pair of counterrotating vortices at the crossroad enhances the dispersion.
3.6. Distance between two-canyon arrays In real urban cities, canyons crossing one another are common and form a primitive crossed-unit of intricate transport network. The variation of the distance between the two building arrays helps to investigate the effect of street width along wind direction on pollutant dispersion. When the street is wide enough (h=co0:75), sufficient wind flows along it and carries the emitted pollutants down the street (Fig. 13). This lowers the retention values and maximum pollutant concentrations. As the width of the street decreases, pollutants driven by the flow along the street are insignificant and dispersion of pollutants are limited to those over the
rooftop. This is shown by the high retention values for h=c from 0.75.
3.7. Modification of buildings in highly polluted district Amongst districts of severe air pollution problems in Hong Kong, Mongkok is a suitable choice for investigation as canyons in the district is regular and straight but buildings composing the urban configurations are complex. To evaluate the usefulness, feasibility and effectiveness of the guidelines developed in the previous sections, these guidelines are put to test in a hypothetical situation to study its effectiveness in solving air pollution problems. The model of the district with the associated heavy traffic is shown in Fig. 14a. The coloured blocks represent three height levels of the buildings. The yellow ones represent heights of 0–30 m, orange one represents building height from 31 to 60 m and the red colour represents building height over 60 m. Two main line sources are located in the horizontal
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(a)
(b)
Fig 13. Vector plot with pollutants for arrays of (a) h=c ¼ 0:5; (b) h=c ¼ 2:0 (ratio refers to that of Canopy #2).
Mong Kok Road and the vertical Nathan Road. Dispersion patterns developed by the dominant wind along the horizontal street is illustrated in Fig. 14b. Initial simulation results show a large amount of plume accumulated around the area highlighted by the white circle, especially the junction of Tung Choi Street and Fife Street. In most situations, rebuilding the road size is a strategy to achieve better ventilation. However, it involves too much reconstruction and is not beneficial for a single purpose. Therefore altering the size of buildings is considered instead. Modifications of buildings in that zone named 1–4 are done in steps applying some of the relative geometric relationships found from the previous simulations. For the first modification, height ratio relationship is first applied. It is found in Section 3.4 that the retention value reaches its minimum when the ratio is between 0.25 and 0.5. The height ratio of Buildings 3 and 4 is thus adjusted to 0.5, raising Building 3 to 60 m high. It must be emphasised here that we actually raise the building height to achieve better ventilation through redistribution of pollutant. The simulated results displayed on Fig. 14c demonstrate an overall improvement in the site as the plume size decreases. In the second modification, we try to use the relative building breadth relationship to remodel Buildings 1 and 2. Relative building breadth ratio between 1.5 and 2.5 is found to promote better pollutant dispersion, so combining the target buildings to change the ratio to 2.0 is a preferable decision. The plume used to be accumulated near Buildings 2 and 3 dispersed and the air quality in the zone is further improved (Fig. 14d). In the third modification, based on the second one, the improvement of the downstream canopy air of the zone will be the object of the simulation. Height ratio relationship is applied to adjust Building 4 to attain a ratio of 0.5 comparing the height of the building downstream. The simulation result in Fig. 14e shows that almost the whole plume that originally accumulates around the concerning zone has disappeared. Most of the pollutants are dispersed along Mong Kok Road to
the right-hand side. With consecutive modifications, the pollution retention values of Fife Street drops by 40% while that of Tung Choi Street reduces by 30% (Table 2) and the air quality of the zone is much improved.
4. Discussion and summary Investigation on promotion of flow and pollutant dispersion in complex terrains can never result in explicit rules, as numerous considerations are needed. To focus on the geometric impact on urban flow, several rules of thumb are suggested for city planning. Equally spacing of buildings prevents the dispersion of pollutant out from the canyon, unless the canyon is wide or the roof wind velocity is high enough. The critical ratio for effective dispersion is at h=w ¼ 0:7; where there is a change from wake interference to skimming flow. Urban variations in building height and breadth is preferred. Intricate roof level configurations promote ventilation. The critical relative height for minimum retention value lies between height ratio 0.25 and 0.5, while maximum retention values occur in the region between 1.0 and 1.5 where stagnation of flow occurs. When the effect of stagnation of flow diminished, further increase in leeward building height can cause leeward blockage of pollutant. Thin buildings do not necessarily enhance pollutant dispersion. In fact, the critical relative building breadth for maximum retention value lies in the region of either 1.0 or 3.0 and a relative breadth ratio between 1.5 and 2.5 promotes better diffusion. Non-uniform building breadth is recommended. Crossroad introduces the lateral eddies into the canyon. The release of flow through the crossroad creates a relatively low-pressure zone and thus sucks the flow nearby. A general decrease in the retention value is found. Among all cases, crossroad in a location where more vortices generated ahead is more promising in pollutant dispersion.
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Table 2 Numerical outputs of the simulation work of an area in Mongkok Relationships applied
(a)
(b)
(c)
Control simulation 1st Modification 2nd Modification 3rd Modification
h3 =h b3 =b h3 =h
Retention value ( 103) Fife Street
Tung Choi Street
3.34
12.4
2.22 2.17 1.93
9.32 9.18 9.00
path cases, pollutants are forced to disperse from rooftop, which is less effective. It is obvious that the rules themselves do not work independently to promote canyon air quality. Application of the rules on the real case geometries is a complex practice, as lots of elements and tactics in modifications are needed to be well considered. Besides, non-stationary wind flow and non-linear building parameters make the flow even more complicated to demonstrate. Modifications of the heavily polluted district demonstrated in this study show the relationships described here are not merely a paperwork measures. All these are critical theoretically and in real-life situation and will be studied in the future.
Acknowledgements (d)
This research work is sponsored by The University of Hong Kong.
References
(e) Fig 14. Simulation model base on urban configurations of Mongkok: (a) basic model, (b) vector plot and pollutant dispersion in Mongkok before modification, (c) after 1st modification, (d) after 2nd modification, (e) after 3rd modification.
A path along wind direction among perpendicular canyons is preferable with sufficient width. It is found that the retention value drops for wide path of h=w below 0.5 as flows pass along wide paths carrying pollutants down to the end of the path. While in narrow
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