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Wind tunnel experiments on how thermal stratification affects flow in and above urban street canyons
Atmospheric Environment 34 (2000) 1553}1562
Wind tunnel experiments on how thermal strati"cation a!ects #ow in and above urban street canyons Kiyoshi Uehara!,*, Shuzo Murakami", Susumu Oikawa#, Shinji Wakamatsu$ !National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-0045, Japan "Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106-8558, Japan #Research Laboratory, Shimizu Construction Co., Ltd., 3-3-17 Etchujima, Koto-ku, Tokyo 135-0044, Japan $National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-0045, Japan Received 21 September 1998; received in revised form 7 September 1999; accepted 7 September 1999
Abstract The e!ects of atmospheric stability on #ow in urban street canyons were studied using a strati"ed wind tunnel. We conducted experiments using a model that represented city streets with simply shaped block forms, while varying atmospheric stability across seven stages from stable (Rb"0.79) to unstable (Rb"!0.21). We used a laser Doppler anemometer (LDA) and a cold wire to measure the #ow "eld and temperature within and above the street canyon. In addition to mean values of wind speed components and temperatures, we measured turbulence intensity, shear stress, and heat #ux distribution. Our results led to the following conclusions: Cavity eddies that arose in the street canyon tended to be weak when the atmosphere was stable and strong when unstable. Stable atmospheric conditions led to a positive feedback e!ect in which the downward #ow into the street canyon weakened due to buoyancy, which facilitated the formation of a more highly stable strati"cation. As a result, when stability exceeded a certain threshold (somewhere in the range of Rb"0.4}0.8), the wind speed in the street canyon dropped nearly to zero. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: Physical modeling; Buildings and streets; Cavity eddy; Air pollution; Atmospheric stability
1. Introduction In addition to heavy vehicular tra$c, a contributing factor to roadside air pollution in large cities is poor street ventilation caused by the high density of urban land use, which increases concentrations of roadside air pollutants. Many wind tunnel experiments have been conducted with the aims of simulating present conditions and understanding the phenomenon of air pollutant dispersion. Dabberdt and Hoydysh performed a series of wind tunnel experiments using a model of simple block shapes. They investigated the e!ects on concentration distribution caused by di!erences in building heights on
* Corresponding author. E-mail addresses: [email protected] (K. Uehara), murakami @iis.u-tokyo.ac.jp (S. Murakami), [email protected] (S. Oikawa), [email protected] (S. Wakamatsu)
both sides of a street canyon and estimated the residence time of air pollutants using soap-bubble trails (Hoydysh and Dabberdt, 1988). Dabberdt and Hoydysh also investigated the e!ects of wind direction, block shape and street width on concentration distribution (Dabberdt and Hoydysh, 1991), and the concentration at intersections (Hoydysh and Dabberdt, 1994). Pasquill (1962) suggested that the in#uence of atmospheric stability on the dispersion of air pollutants is quite strong. However, very few systematic studies have dealt with the in#uences of atmospheric stability on #ow and dispersion in urban areas. Yang and Meroney (1970) studied plume dispersion in the wake of a building under stable strati"ed conditions. Snyder (1994) examined the in#uences of atmospheric stability on pollutant dispersion around a model cube-shaped building with a water tank experiment; they suggested that strong strati"cation will have an in#uence on the dispersion within the wake of a building.
1352-2310/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 4 1 0 - 0
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K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
Nomenclature d g h H ¸ R b Re H Re H Ri t ¹ ¹ ! ¹ & ¹ H ¹ 0 ¹ . ¹ 700 u H ; H ; 3.!9 ; 700 u = u, v, w ;, <, = X, >, Z
zero-plane displacement, mm acceleration due to gravity, m s~2 height of roughness element"50 mm model block height"100 mm width of street canyon"100 mm bulk Richardson number"gH(¹ !¹ )/M(¹#273)(; )2N H 0 H roughness Reynolds number"u Z /l * 0 model building Reynolds number"; H/l H gradient Richardson number"(gL¹/LZ)/M(¹#273)(L;/LZ)2N turbulent component of temperature, 3C mean temperature, 3C ambient temperature, 3C #oor temperature, 3C temperature at the top of the street canyon, 3C temperature at height Z"0, 3C estimated from ¹ pro"les model surface temperature, 3C temperature at height Z"700 mm, 3C friction velocity, m s~1 mean wind speed at the top of the street canyon, m s~1 maximum value of reverse #ow velocity in a street canyon, m s~1 wind speed at height Z"700 mm, m s~1 wind tunnel speed"1.5 m s~1 velocity components in the X, > and Z directions, m s~1 mean velocity components in the X, > and Z directions, m s~1 spatial coordinates in the windward, crosswind, and vertical directions, m} X"0 at the start of the wind tunnel test section, >"0 at the wind tunnel's center line, and Z"0 at the wind tunnel #oor. Z roughness parameter, mm 0 d boundary layer thickness, mm i von Karman constant"0.4 l kinematic viscosity, cm2 s~1 p ,p ,p standard deviations of u, v and w, in m s~1 U V W ( ) quantities at model block height H H ( ) quantities at height Z Z
Dispersion of air pollutants is subject to the e!ects of air#ow, and knowledge about the #ow "eld is indispensable to understanding the dispersion mechanism. However, very few studies have dealt with the #ow "eld within the urban canopy layer. DePaul and Sheih (1985) and Yamartino and Weigand (1986) studied the wind velocity and turbulence intensity within and above urban street canyons. Nakamura and Oke (1988) examined the temperature distribution within an urban street canyon in relation to the outside wind and atmospheric stability. Rotach (1994) studied wind speed and turbulence pro"les within and above the street canyon by long-term observation. More recently, Rafailidis (1997) used a laser Doppler anemometer to measure the #ow above a street canyon and to investigate the e!ects of the street width and the shapes of roofs on buildings around the street canyon. However, the full particulars of the #ow "eld within the urban street canyon are still not clear.
The purpose of this research was to study the air#ow within street canyons and examine how it is a!ected by atmospheric stability, using a strati"ed wind tunnel. 2. Experimental method 2.1. Stratixed wind tunnel, model city blocks and experimental conditions Our experiment used the atmospheric di!usion wind tunnel at the (Japanese) National Institute for Environmental Studies (Ogawa et al., 1981). Its test section is 2 m high, 3 m wide, and 24 m long. The wind speed range is 0.2}10 m s~1. Thermal strati"cation was created in the test section by controlling the wind temperature ¹ in the range of ! 10}903C, and the wind tunnel #oor temperature ¹ be& tween 0 and 1103C. The arrangement of roughness
K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
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Fig. 1. Arrangement of roughness elements and model city blocks.
elements and city blocks (Fig. 1) included a 100 mm high tripping fence set at X"2 m. Styrofoam roughness elements were arrayed from X"3.0}11.8 m and spaced 100 mm apart on all sides. All elements were 100]100]50 mm. They created a long, rough upwind fetch to generate a turbulent boundary layer. The approach #ow pro"le varied with #ow temperature ¹ ! and #oor panel temperature ¹ settings (Table 1), but & it seems that Z did not depend on strati"ed conditions 0 (Z +3.3 mm, d"35 mm, Fig. 2). Table 1 also shows the 0 bulk Richardson number, Rb, of the street canyon (described in the next section); the friction velocity, u ; H the roughness Reynolds number (Re "u Z /l) of H H 0 the approach #ow; and the scaled velocity variances, p /u , p /u and p /u of the approach #ow. The U H V H W H values were close to those obtained by Counihan (1975), Jackson (1978), Panofsky and Dutton (1984), Oikawa and Meng (1995), Rotach (1994), and Yumao et al. (1997). Street canyon models made of Styrofoam were located from X"12.0 to 14.8 m. The models formed a network of street canyons that were all perpendicular to other streets with which they intersected. The city blocks were all 100]100]100 mm, and were spaced 100 mm apart in the X direction and 50 mm apart in the > direction; thus the canyon geometry was 1 : 1 (height : width). Measurements were conducted in the street between the "fth and sixth rows of buildings, located 1 m leeward from the city model's leading edge. Wind speed and temperature measurements were taken at the vertical section in the street canyon center (Fig. 3). We also took a vertical pro"le up to a height of 700 mm from the #oor at street center to measure the air#ow above the street canyon. 2.2. Similarity conditions and wind tunnel speed The model's geometric and #ow "eld similarities were taken into consideration. With regard to atmospheric stability, we assumed the similarity condition to be the congruence of Ri number pro"les (Cermak, 1984; U.S. EPA, 1981; Snyder, 1972; Ogawa et al., 1975 etc.). There is very little data regarding the Richardson number in urban areas. Wakamatsu et al. (1986) made
their observations in winter in Sapporo, when meteorological conditions are much more stable than in other seasons. We compared these Ri pro"les with those of our experiment (Fig. 4) by converting the height Z above ground level used in the "eld measurements to the height dimension in the wind tunnel by assuming that the average height of buildings in the real city was equal to the arti"cial building height in the wind tunnel. For Ri in "eld observations, the local Richardson number is Ri" (gL¹/LZ)/M(¹#273)(L;/LZ)2N, but in the wind tunnel experiments there was a wide variation in wind speed and temperature values below the building height (Z(H), so stability was indicated with the bulk Richardson number Rb"gH(¹ !¹ )/M(¹#273) (; )2N. ¹ was estiH O H O mated by extrapolating ¹ pro"les in the vicinity (Z+0.2 mm) of the tunnel #oor. On the other hand, Nakamura et al. (1988) measured the temperature distribution within an actual urban street canyon and obtained the Rb of a street canyon under unstable strati"ed conditions. In their observations, Rb ranged from !0.45 to !0.17 on a clear mid-summer afternoon (; '0.5). We also compare H Nakamura's Rb and our Rb in Fig. 4. As the Richardson numbers in the present study are similar to "eld observation values, it would seem that the simulated strati"cation conditions in this experiment closely reproduced strati"cation conditions in actual urban areas. Castro and Robins (1977) and Snyder (1994) suggested a critical Reynolds number (Re "; H/l) of 4000, H H based on model height and wind speed at building height ; , for a building model immersed within a deep boundH ary layer. On the other hand, Hoydysh et al. (1974) suggested 3400 (Re"; H/l), based on the model height = and free stream velocity ; , for a building within an = array of modeled city blocks quite similar to the present study. We set the wind tunnel speed to ; "1.5 m s~1 = so as to achieve a building Reynolds number greater than 3500 (Re "; H/l) under neutral strati"ed H H conditions. Snyder (1972,1994) suggested that the roughness Reynolds number (Re "u Z /l) of the boundary layer H H 0 is more important than the building Reynolds number,
d"700 d"64 Z "10 0
Street canyon
d"700, d"35, Z "3.3 0
Approach #ow
Inside canyon
Above canyon
p /u U H p /u V H p /u W H
p /u U H p /u V H p /u W H
p /u U H p /u V H p /u W H Rb H u /; H 700
Re H u /; H 700
¹ 3C ! ¹ 3C &
0 0.063 2.2 1.7 1.6 1.3 1.7 1.5
0.11 0.049 2.2 1.6 1.6 1.2 1.4 1.3
0.43 0.040 2.1 1.8 1.6 1.1 1.1 1.1
0.032 2.2 1.9 1.6 1.1 1.1 1.1
0.79
20.5
14.5 0.061 2 1.7 1.4
20 20
38 21
0.049 2 1.6 1.4
0.037 2 1.7 1.5
10.0
7.1 0.029 2.1 1.8 1.6
58 21
78 21
Present study
0.080 1.8 1.6 1.4
26.8
19 59
0.079 2 1.7 1.5
26.0
20 79
1.5 2 1.7
0.071 2.2 1.8 1.5 1.5 2.3 1.7
0.085 2 1.6 1.4 1.8 3.2 2.2
0.077 2.3 2 1.6
!0.12 !0.19 !0.21
0.073 1.9 1.6 1.4
24.4
19 40
2.4 1.9 1.3
Flat terrain
Panofsky (1984)
2.5 1.9 1.3
Flat & rural
Counihan (1975)
Oikawa (1995)
Small city
2 1.8 1.4
Jackson (1978)
City center
2.1 1.7 1.7
Previously reported values of p /u 1 p /u 1 p /u 1 U H V H W H
2.5 2.2 1.2
Urban
2.4 2.1 1.4
Suburban
Yumao (1997)
1.7 1.2 0.8 Canyon center 1.2 1.1 0.9
Above canyon
Rotach (1994)
Table 1 Flow and tunnel #oor temperature readings; determined values of ReH , uH , Z0 , d and Rb; and comparison of previously reported values of pU /uH , pV /uH , pW /uH with those of the present study
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Fig. 2. Approach #ow pro"les under various temperature conditons.
Fig. 3. Measuring points and the model setting.
and if the model #ow conditions are chosen such that Re *2.5, one can be certain that the boundary layers H are turbulent, so that #ow and dispersion around the building will change very little with the Reynolds number. The roughness Reynolds numbers of the approach #ow under various temperature conditions for the present study, which range from 7.1 (¹ "78, ¹ "21) to 26.8 ! & (¹ "19, ¹ "59, Table 1), are su$ciently large in the ! & light of Snyder's criteria.
2.3. Measurement of yow and temperature xelds in and above street canyons
To measure the wind speed we attached the LDA probe to the carriage system in the tunnel and "red an
K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
Fig. 4. Ri pro"les within and above the street canyon.
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Fig. 5. ;/; pro"les at the central part of the street canyon. 700
Argon laser. The beam was split into three colors, one for each of the three component directions, u, v and w. Each color beam was then split into two, and the resulting parts were directed to intersect at the measuring point from the appropriate direction to obtain Doppler signals of the individual components. Each signal was processed by Burst Spectrum Analyzers (BSA Dantec Measurement Technology). Magnesium carbonate powder (particle diameter 5 lm) was released into the air using a small audio speaker as a seeding generator. The cold wire was installed parallel to the #ow, 1}1.5 mm leeward of the laser beams' intersection so as not to disturb the #ow "eld. The sampling time of wind speed and temperature was 180 s, and occasionally extended to about 300 s if necessary. See Uehara et al. (1997a,b) for more information on the seeding method and the e!ects of model material heat conductivity.
3. Results
Fig. 6. (¹!¹ )/(¹ !¹ ) pro"les at the central part of the & ! & street canyon.
3.1. Proxles Wind-speed pro"les (Fig. 5) were measured starting at 2 mm above the #oor, and temperature pro"les (Fig. 6) were taken starting from 0.2 mm above the #oor, both at the street canyon center. Wind speed in and above the street canyon, ;/; , fell as the strati"cation 700 strengthened, but rose as instability increased (inside the street canyon reverse #ow was strong because of the instability). In particular, when strati"cation stability was strong, the wind speed in the lower part of the street canyon was very low, dropping to 1 or 2% of the wind tunnel speed.
The p /; , p /; and ;=/;2 pro"les (Figs. U 700 W 700 700 7, 8 and 10, respectively) include results obtained by Raupach et al. (1986) with hot-wire anemometer measurements of intra-layer turbulence in a plant canopy model (H"60 mm) under neutral strati"ed conditions. Di!erences such as the model shape and the distance from roughness elements prevent direct comparison, but our p was smaller than that of Raupach, while our U p and ;= were similar. Turbulence intensity, shear W stress, and other values in and above the street canyon
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remained low under stable strati"ed conditions, and were high at times of unstable strati"ed conditions; energy transport in the vertical direction changed greatly according to atmospheric stability (Figs. 7}10). Wind speed and turbulence pro"les inside and above the street canyon were roughly divided into three parts: (1) street canyon interior (Z(H); (2) internal boundary layer (IBL), the transition area directly a!ected by the surface of the city block model (H(Z(2.5H); and (3) upper boundary layer (UBL), where there is only a small in#uence from the city block model and which holds properties of the approach #ow (Z'2.5H). These parts approximately correspond, respectively, to (1) the urban canopy layer, (2) the roughness layer, and (3) the surface layer in the urban boundary layer classi"cation scheme used by Oke (1988a,b). Inside the street canyon, turbulence intensity and shear stress change with height above the ground level Z, but large di!erences in these variables are caused by atmospheric stability. In the IBL, just as inside the street canyon, large di!erences are due to atmospheric stability, and there is little height-dependent variation. Finally in the UBL, di!erences due to strati"cation gradually become smaller and turbulence intensity and #uxes decrease as height increases. In the UBL, strati"cation-induced di!erences in average wind speed and temperature nearly disappear. Mean wind velocities "t logarithmic pro"les when the zero-plane displacement was chosen to be d"64 mm (very near the value suggested by Jackson (1980) of d+H]0.7). The roughness parameter was found to be Z "10 mm; seemingly not strongly a!ected by 0 strati"ed conditions in the way that the approach #ow was. Turbulent intensity and shear stress appeared almost constant in the IBL and the velocity variances scaled by the friction velocity, p /u , p /u and p /u , U H V H W H also seemed constant. However our values were slightly
Fig. 8. p /; pro"les at the central part of the street canyon. W 700
Fig. 9. p /D¹ !¹ D pro"les at the central part of the street T ! & canyon.
Fig. 7. p /; pro"les at the central part of the street canyon. U 700
higher than those of Rotach (1994), while the distribution in the Z direction of both showed a similar tendency (Table 1, Fig. 11; p /u and p /u pro"les are not V H W H shown). As the scaled variances above the street canyon showed values 5}15% higher than those of the approach #ow, it is assumed that the fetch length of the model city blocks was not su$cient and thus the IBL did not attain rigorous equilibrium. Hoydysh et al. (1974) examined the in#uence of a fetch length consisting of rows of buildings on the #ow and concentration "elds in a downstream
K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
Fig. 10. ;=/;2 pro"les at the central part of the street 700 canyon.
Fig. 11. p /; pro"les at the central part of the street canyon. U H
street canyon, and concluded that an increase in fetch length of more than 20H brought no change in the #ow or concentration "elds within or above the street canyon. By this account, it seems, at least inside the street canyon, that the fetch length of the present study was su$cient, taking into account the change of roughness height at the leading edge of the model city blocks. 3.2. Air yow in the street canyon Velocity vectors were superimposed on composite speed distributions (;2#=2)1@2/; in the cross sec700 tion (Fig. 3) of the street canyon center under stable,
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neutral and unstable strati"ed conditions (Fig. 12). The stronger the strati"cation, the weaker the cavity eddy that formed in the street canyon, while the greater the instability, the stronger the eddy. According to #ow visualizations carried out by Uehara et al. (1998), a large eddy formed in the street canyon in the majority of cases. The intensity, shape and stability of the cavity eddy varied with the aspect ratio of the street canyon (the ratio of building height and road width, ¸/H). A strong, stable eddy formed when the aspect ratio was ¸/H"1}2, and it seemed that the #ow pattern corresponded to the Skimming Flow Regime used in Oke's classi"cation of the air #ow over building arrays (Oke, 1988b). Fig. 15 graphically represents the #ows inside and above the street canyon (¸/H"1) based on the results of the visualizations of Sasaki et al. (1989) and Uehara et al. (1998). A series of small eddies formed at the interface between the main and cavity #ows due to the strong velocity gradient. As these eddies transferred energy to the cavity #ow, they were pushed leeward and ran into the top edge of the leeward building. Some of the eddies descended vertically into the street canyon as they disintegrated, while the others #owed out of the top of the street canyon. One can see from the distribution diagrams of the vertical component =/; (Fig. 13) that the down700 ward #ow heading into the street canyon arose within a very small range just in front of the leeward buildings. The #ow that ran into the downstream building broke into the street canyon, reached ground level, then headed toward the back of the windward building as a #ow compensating that which was pulled along by the free stream at the top of the street canyon. It was probably the repetition of these processes that formed the cavity #ow. In a series of these processes, shear stress above the street canyon had a strong in#uence on the descending #ow speed along the leeward building wall. The distributions of shear stress ;=/;2 (Fig. 14) shows that it was 700 greatest at the top edge of the street canyon on the leeward building side, where energy transfer from the free stream was large and shear stress at the top of the street canyon changed in accordance with atmospheric stability (Figs. 15 and 16). Presumably, these processes induced a change in the cavity eddy intensity with the strati"cation. The reverse #ow speed under neutral stable strati"ed conditions was about half the wind speed above the street canyon, increasing to about 60% at times of strong instability. Conversely, the wind speed in the lower part of the street canyon decreased to nearly zero under highly stable conditions. The cavity eddy speed changed widely depending on atmospheric stability, with the change being small under unstable conditions and very large under stable conditions (Fig. 17). When the atmosphere was stable, wind speed ; over the street canyon H remained low. Additionally, the downward #ow entering the street canyon was hindered by buoyancy, so that the
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K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
Fig. 12. Contour of (;2#=2)1@2/; and the vector "eld within the street canyon: (a) stable case, Rb"0.78; (b) neutral case, 700 Rb"0; (c) unstable case, Rb"!0.21.
Fig. 13. Contour of =/; within the street canyon: (a) stable case, Rb"0.78; (b) neutral case, Rb"0; (c) unstable case, Rb"!0.21. 700
Fig. 14. Contour of (;=/;2 )]103 within the street canyon: (a) stable case, Rb"0.78; (b) neutral case, Rb"0; (c) unstable case, 700 Rb"0.21.
K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
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Fig. 15. Schematic of #ow within the street canyon.
Fig. 17. Change in reverse #ow speed, ; /; at bottom of 3.!9 H the street canyon with atmospheric stability Rb.
4. Summary We used a strati"ed wind tunnel to study the e!ects of atmospheric stability on the #ow "eld in a street canyon, and determined the following:
Fig. 16. Change in shear stress, ;=/;2 at top center of the 700 street canyon with atmospheric stability Rb.
wind speed in the street canyon further decreased. The result was the generation of a positive feedback e!ect in which a more stable strati"ed layer was formed. For this reason, the wind speed fell to nearly zero in the lower part of the street canyon under stable strati"cation, somewhere between Rb"0.4}0.8. Conversely, when the atmosphere was unstable, a downward #ow readily entered the street canyon and the cavity eddy became stronger than that under neutral conditions. This resulted in vertical mixing that reduced the high-low temperature difference, thereby weakening the e!ect of buoyancy. Thus, wind speed di!erences caused by the intensity of instability were smaller than those under stable strati"ed conditions.
f The cavity eddy that formed in the street canyon became weaker when the atmosphere was stable, and stronger when unstable. f Stable atmospheric conditions led to a positive feedback e!ect in which the downward #ow into the street canyon weakened owing to buoyancy, which facilitated the formation of a more highly stable strati"cation. As a result, when stability exceeded a certain threshold (somewhere between Rb"0.4 and 0.8), wind speed in the street canyon dropped to nearly zero. On the other hand, when the atmosphere was unstable there was a great deal of mixing in the street canyon, causing the vertical temperature gradient to grow smaller, and thus the instability to decrease.
Acknowledgements We would like to thank Mr. K. Komaba, Mr. Y. Yamao and Mr. T. Kawata for their help in the execution of the wind tunnel experiments.
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Cermak, J.E., 1984. Physical modeling of #ow and dispersion over complex terrain. Boundary-Layer Meteorology 30, 261}292. Counihan, J., 1975. Adiabatic atmospheric boundary layers: a review and analysis of data from the period 1880-1972. Atmospheric Environment 9, 871}905. Dabberdt, W.F., Hoydysh, W.G., 1991. Street canyon dispersion: sensitivity to block shape and entrainment. Atmospheric Environment 25A, 1143}1153. DePaul, F.T., Sheih, C.M., 1985. A tracer study of dispersion in an urban street canyon. Atmospheric Environment 19, 555}559. Hoydysh, W.G., Dabberdt, W.F., 1988. Kinematics and dispersion characteristics of #ows in asymmetric street canyons. Atmospheric Environment 22, 2677}2689. Hoydysh, W.G., Dabberdt, W.F., 1994. Concentration "elds at urban intersections: #uid modeling studies. Atmospheric Environment 28, 1849}1860. Hoydysh, W.G., Ogawa, Y., Gri$ths, R.A., 1974. A scale model study of dispersion of pollution in street canyons. APCA Paper No. 74}157, 67th Annual Meeting of the Air Pollution Control Association et al. Annual Meeting of the Air Pollution Control Association, Denver, Colorado, June 9}13. Jackson, P.S., 1978. Wind structure near a city center. Boundary-Layer Meteorology 15, 323}340. Jackson, P.S., 1980. On the displacement height in the logarithmic velocity pro"le. Journal of Fluid Mechanics 111, 15}25. Nakamura, Y., Oke, T.R., 1988. Wind, temperature and stability condition in an East-West oriented urban canyon. Atmospheric Environment 22, 2691}2700. Ogawa, Y., Diosey, P.G., Uehara, K., Ueda, H., 1981. A wind tunnel for studying the e!ects of thermal strati"cation in the atmosphere. Atmospheric Environment 15, 807}821. Ogawa, Y., Gri$ths, R., Hoydysh, W.G., 1975. A wind tunnel study of sea breeze e!ects. Boundary-Layer Meteorology 8, 141}161. Oikawa, S., Meng, Y., 1995. Turbulence characteristics and organized motion in a suburban roughness sublayer. Boundary-Layer Meteorology 74, 289}312. Oke, T.R., 1998a. The urban heat balance. Progress in Physics and Geography 19, 471}508. Oke, T.R., 1988b. Street design and urban canopy layer climate. Energy and Buildings 11, 103}113. Panofsky, H.A., Dutton, J.A., 1984. Atmospheric turbulence. Wiley, New York. Pasquill, 1962. Atmospheric Di!usion. D. Van Nostrand Company Ltd, London. Rafailidis, S., 1997. In#uence of building areal density and roof shape on the wind characteristics above a town. BoundaryLayer Meteorology 85, 255}271.
Raupach, M.R., Coppin, P.A., Legg, B.J., 1986. Experiments on scalar dispersion within a model plant canopy. Part 1: the turbulence structure. Boundary-Layer Meteorology 35, 21}52. Rotach, M.W., 1994. Pro"les of turbulence statistics in and above an urban street canyon. Atmospheric Environment 29, 1473}1486. Sasaki, H., Miyakoshi, H., Ito, H., 1989. Water #ow visualization studies on convective di!usion of layering and leakage in rectangular cavities aside a main #ow way. Papers on The Society of Heating, Air-Conditioning and Sanitary Engineers of Japan 39, 121}131. Snyder, W.H., 1972. Similarity criteria for the application of #uid models to the study of air pollution meteorology. Boundary-Layer Meteorology 3, 113}134. Snyder, W.H., 1994. Some observations of the in#uence of strati"cation on di!usion in building wakes. In: Castro, I.P., Rockli!, N.J. (Eds.), Stably Strati"ed Flows and Dispersion over Topography. Claredon Press, Oxford, England, pp. 301}324. Uehara, K., Murakami, S., Oikawa, S., Wakamatsu, S., 1997a. Wind tunnel test on strati"ed #ow "elds around urban street canyons by LDV. Journal of Architecture. Planning and Environmental Engineering (Transaction of AIJ) 492, 39}46. Uehara, K., Murakami, S., Oikawa, S., Wakamatsu, S., 1997b. Wind tunnel test of concentration "elds around street canyons within the strati"ed urban canopy layer. Journal of Architecture. Planning and Environmental Engineering (Transaction of AIJ) 499, 9}16. Uehara, K., Murakami, S., Oikawa, S., Wakamatsu, S., 1998. Wind tunnel evaluation of #ow "elds within street canyons with thermal strati"cation. Journal of Architecture, Planning and Environmental Engineering (Transaction of AIJ) 510, 37}44. U.S. Environmental Protection Agency, 1981. Guidelines for Fluid Modeling of Atmospheric Di!usion. EPA-600/8-81009, EPA, O$ce of Air Quality Planning and Standards, Research Triangle Park, North Carolina. Wakamatsu, S., Uno, I., Nakamura, A., 1986. Research on dispersion phenomena on an urban scale. In: Matsuso, Y. (Ed.). Ministry of Education special report on `Environmental Sciencesa research, pp. B-296, R15-1, 5-104. Yamartino, R.J., Wiegand, G., 1986. Development and evaluation of simple models for the #ow, turbulence and pollutant concentration "elds within an urban street canyon. Atmospheric Environment 20, 2137}2156. Yang, B.T., Meroney, R.N., 1970. Gaseous dispersion into strati"ed building wakes. AEC Rept. No. C00-2053-3. Yumao, X., Chaofu, Z., Zhongkai, L., 1997. Turbulent structure and local similarity in the tower layer over the Nanjing area. Boundary Layer Meteorology 82, 1}21.
Wind tunnel experiments on how thermal strati"cation a!ects #ow in and above urban street canyons Kiyoshi Uehara!,*, Shuzo Murakami", Susumu Oikawa#, Shinji Wakamatsu$ !National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-0045, Japan "Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106-8558, Japan #Research Laboratory, Shimizu Construction Co., Ltd., 3-3-17 Etchujima, Koto-ku, Tokyo 135-0044, Japan $National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-0045, Japan Received 21 September 1998; received in revised form 7 September 1999; accepted 7 September 1999
Abstract The e!ects of atmospheric stability on #ow in urban street canyons were studied using a strati"ed wind tunnel. We conducted experiments using a model that represented city streets with simply shaped block forms, while varying atmospheric stability across seven stages from stable (Rb"0.79) to unstable (Rb"!0.21). We used a laser Doppler anemometer (LDA) and a cold wire to measure the #ow "eld and temperature within and above the street canyon. In addition to mean values of wind speed components and temperatures, we measured turbulence intensity, shear stress, and heat #ux distribution. Our results led to the following conclusions: Cavity eddies that arose in the street canyon tended to be weak when the atmosphere was stable and strong when unstable. Stable atmospheric conditions led to a positive feedback e!ect in which the downward #ow into the street canyon weakened due to buoyancy, which facilitated the formation of a more highly stable strati"cation. As a result, when stability exceeded a certain threshold (somewhere in the range of Rb"0.4}0.8), the wind speed in the street canyon dropped nearly to zero. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: Physical modeling; Buildings and streets; Cavity eddy; Air pollution; Atmospheric stability
1. Introduction In addition to heavy vehicular tra$c, a contributing factor to roadside air pollution in large cities is poor street ventilation caused by the high density of urban land use, which increases concentrations of roadside air pollutants. Many wind tunnel experiments have been conducted with the aims of simulating present conditions and understanding the phenomenon of air pollutant dispersion. Dabberdt and Hoydysh performed a series of wind tunnel experiments using a model of simple block shapes. They investigated the e!ects on concentration distribution caused by di!erences in building heights on
* Corresponding author. E-mail addresses: [email protected] (K. Uehara), murakami @iis.u-tokyo.ac.jp (S. Murakami), [email protected] (S. Oikawa), [email protected] (S. Wakamatsu)
both sides of a street canyon and estimated the residence time of air pollutants using soap-bubble trails (Hoydysh and Dabberdt, 1988). Dabberdt and Hoydysh also investigated the e!ects of wind direction, block shape and street width on concentration distribution (Dabberdt and Hoydysh, 1991), and the concentration at intersections (Hoydysh and Dabberdt, 1994). Pasquill (1962) suggested that the in#uence of atmospheric stability on the dispersion of air pollutants is quite strong. However, very few systematic studies have dealt with the in#uences of atmospheric stability on #ow and dispersion in urban areas. Yang and Meroney (1970) studied plume dispersion in the wake of a building under stable strati"ed conditions. Snyder (1994) examined the in#uences of atmospheric stability on pollutant dispersion around a model cube-shaped building with a water tank experiment; they suggested that strong strati"cation will have an in#uence on the dispersion within the wake of a building.
1352-2310/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 4 1 0 - 0
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K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
Nomenclature d g h H ¸ R b Re H Re H Ri t ¹ ¹ ! ¹ & ¹ H ¹ 0 ¹ . ¹ 700 u H ; H ; 3.!9 ; 700 u = u, v, w ;, <, = X, >, Z
zero-plane displacement, mm acceleration due to gravity, m s~2 height of roughness element"50 mm model block height"100 mm width of street canyon"100 mm bulk Richardson number"gH(¹ !¹ )/M(¹#273)(; )2N H 0 H roughness Reynolds number"u Z /l * 0 model building Reynolds number"; H/l H gradient Richardson number"(gL¹/LZ)/M(¹#273)(L;/LZ)2N turbulent component of temperature, 3C mean temperature, 3C ambient temperature, 3C #oor temperature, 3C temperature at the top of the street canyon, 3C temperature at height Z"0, 3C estimated from ¹ pro"les model surface temperature, 3C temperature at height Z"700 mm, 3C friction velocity, m s~1 mean wind speed at the top of the street canyon, m s~1 maximum value of reverse #ow velocity in a street canyon, m s~1 wind speed at height Z"700 mm, m s~1 wind tunnel speed"1.5 m s~1 velocity components in the X, > and Z directions, m s~1 mean velocity components in the X, > and Z directions, m s~1 spatial coordinates in the windward, crosswind, and vertical directions, m} X"0 at the start of the wind tunnel test section, >"0 at the wind tunnel's center line, and Z"0 at the wind tunnel #oor. Z roughness parameter, mm 0 d boundary layer thickness, mm i von Karman constant"0.4 l kinematic viscosity, cm2 s~1 p ,p ,p standard deviations of u, v and w, in m s~1 U V W ( ) quantities at model block height H H ( ) quantities at height Z Z
Dispersion of air pollutants is subject to the e!ects of air#ow, and knowledge about the #ow "eld is indispensable to understanding the dispersion mechanism. However, very few studies have dealt with the #ow "eld within the urban canopy layer. DePaul and Sheih (1985) and Yamartino and Weigand (1986) studied the wind velocity and turbulence intensity within and above urban street canyons. Nakamura and Oke (1988) examined the temperature distribution within an urban street canyon in relation to the outside wind and atmospheric stability. Rotach (1994) studied wind speed and turbulence pro"les within and above the street canyon by long-term observation. More recently, Rafailidis (1997) used a laser Doppler anemometer to measure the #ow above a street canyon and to investigate the e!ects of the street width and the shapes of roofs on buildings around the street canyon. However, the full particulars of the #ow "eld within the urban street canyon are still not clear.
The purpose of this research was to study the air#ow within street canyons and examine how it is a!ected by atmospheric stability, using a strati"ed wind tunnel. 2. Experimental method 2.1. Stratixed wind tunnel, model city blocks and experimental conditions Our experiment used the atmospheric di!usion wind tunnel at the (Japanese) National Institute for Environmental Studies (Ogawa et al., 1981). Its test section is 2 m high, 3 m wide, and 24 m long. The wind speed range is 0.2}10 m s~1. Thermal strati"cation was created in the test section by controlling the wind temperature ¹ in the range of ! 10}903C, and the wind tunnel #oor temperature ¹ be& tween 0 and 1103C. The arrangement of roughness
K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
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Fig. 1. Arrangement of roughness elements and model city blocks.
elements and city blocks (Fig. 1) included a 100 mm high tripping fence set at X"2 m. Styrofoam roughness elements were arrayed from X"3.0}11.8 m and spaced 100 mm apart on all sides. All elements were 100]100]50 mm. They created a long, rough upwind fetch to generate a turbulent boundary layer. The approach #ow pro"le varied with #ow temperature ¹ ! and #oor panel temperature ¹ settings (Table 1), but & it seems that Z did not depend on strati"ed conditions 0 (Z +3.3 mm, d"35 mm, Fig. 2). Table 1 also shows the 0 bulk Richardson number, Rb, of the street canyon (described in the next section); the friction velocity, u ; H the roughness Reynolds number (Re "u Z /l) of H H 0 the approach #ow; and the scaled velocity variances, p /u , p /u and p /u of the approach #ow. The U H V H W H values were close to those obtained by Counihan (1975), Jackson (1978), Panofsky and Dutton (1984), Oikawa and Meng (1995), Rotach (1994), and Yumao et al. (1997). Street canyon models made of Styrofoam were located from X"12.0 to 14.8 m. The models formed a network of street canyons that were all perpendicular to other streets with which they intersected. The city blocks were all 100]100]100 mm, and were spaced 100 mm apart in the X direction and 50 mm apart in the > direction; thus the canyon geometry was 1 : 1 (height : width). Measurements were conducted in the street between the "fth and sixth rows of buildings, located 1 m leeward from the city model's leading edge. Wind speed and temperature measurements were taken at the vertical section in the street canyon center (Fig. 3). We also took a vertical pro"le up to a height of 700 mm from the #oor at street center to measure the air#ow above the street canyon. 2.2. Similarity conditions and wind tunnel speed The model's geometric and #ow "eld similarities were taken into consideration. With regard to atmospheric stability, we assumed the similarity condition to be the congruence of Ri number pro"les (Cermak, 1984; U.S. EPA, 1981; Snyder, 1972; Ogawa et al., 1975 etc.). There is very little data regarding the Richardson number in urban areas. Wakamatsu et al. (1986) made
their observations in winter in Sapporo, when meteorological conditions are much more stable than in other seasons. We compared these Ri pro"les with those of our experiment (Fig. 4) by converting the height Z above ground level used in the "eld measurements to the height dimension in the wind tunnel by assuming that the average height of buildings in the real city was equal to the arti"cial building height in the wind tunnel. For Ri in "eld observations, the local Richardson number is Ri" (gL¹/LZ)/M(¹#273)(L;/LZ)2N, but in the wind tunnel experiments there was a wide variation in wind speed and temperature values below the building height (Z(H), so stability was indicated with the bulk Richardson number Rb"gH(¹ !¹ )/M(¹#273) (; )2N. ¹ was estiH O H O mated by extrapolating ¹ pro"les in the vicinity (Z+0.2 mm) of the tunnel #oor. On the other hand, Nakamura et al. (1988) measured the temperature distribution within an actual urban street canyon and obtained the Rb of a street canyon under unstable strati"ed conditions. In their observations, Rb ranged from !0.45 to !0.17 on a clear mid-summer afternoon (; '0.5). We also compare H Nakamura's Rb and our Rb in Fig. 4. As the Richardson numbers in the present study are similar to "eld observation values, it would seem that the simulated strati"cation conditions in this experiment closely reproduced strati"cation conditions in actual urban areas. Castro and Robins (1977) and Snyder (1994) suggested a critical Reynolds number (Re "; H/l) of 4000, H H based on model height and wind speed at building height ; , for a building model immersed within a deep boundH ary layer. On the other hand, Hoydysh et al. (1974) suggested 3400 (Re"; H/l), based on the model height = and free stream velocity ; , for a building within an = array of modeled city blocks quite similar to the present study. We set the wind tunnel speed to ; "1.5 m s~1 = so as to achieve a building Reynolds number greater than 3500 (Re "; H/l) under neutral strati"ed H H conditions. Snyder (1972,1994) suggested that the roughness Reynolds number (Re "u Z /l) of the boundary layer H H 0 is more important than the building Reynolds number,
d"700 d"64 Z "10 0
Street canyon
d"700, d"35, Z "3.3 0
Approach #ow
Inside canyon
Above canyon
p /u U H p /u V H p /u W H
p /u U H p /u V H p /u W H
p /u U H p /u V H p /u W H Rb H u /; H 700
Re H u /; H 700
¹ 3C ! ¹ 3C &
0 0.063 2.2 1.7 1.6 1.3 1.7 1.5
0.11 0.049 2.2 1.6 1.6 1.2 1.4 1.3
0.43 0.040 2.1 1.8 1.6 1.1 1.1 1.1
0.032 2.2 1.9 1.6 1.1 1.1 1.1
0.79
20.5
14.5 0.061 2 1.7 1.4
20 20
38 21
0.049 2 1.6 1.4
0.037 2 1.7 1.5
10.0
7.1 0.029 2.1 1.8 1.6
58 21
78 21
Present study
0.080 1.8 1.6 1.4
26.8
19 59
0.079 2 1.7 1.5
26.0
20 79
1.5 2 1.7
0.071 2.2 1.8 1.5 1.5 2.3 1.7
0.085 2 1.6 1.4 1.8 3.2 2.2
0.077 2.3 2 1.6
!0.12 !0.19 !0.21
0.073 1.9 1.6 1.4
24.4
19 40
2.4 1.9 1.3
Flat terrain
Panofsky (1984)
2.5 1.9 1.3
Flat & rural
Counihan (1975)
Oikawa (1995)
Small city
2 1.8 1.4
Jackson (1978)
City center
2.1 1.7 1.7
Previously reported values of p /u 1 p /u 1 p /u 1 U H V H W H
2.5 2.2 1.2
Urban
2.4 2.1 1.4
Suburban
Yumao (1997)
1.7 1.2 0.8 Canyon center 1.2 1.1 0.9
Above canyon
Rotach (1994)
Table 1 Flow and tunnel #oor temperature readings; determined values of ReH , uH , Z0 , d and Rb; and comparison of previously reported values of pU /uH , pV /uH , pW /uH with those of the present study
1556 K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
Fig. 2. Approach #ow pro"les under various temperature conditons.
Fig. 3. Measuring points and the model setting.
and if the model #ow conditions are chosen such that Re *2.5, one can be certain that the boundary layers H are turbulent, so that #ow and dispersion around the building will change very little with the Reynolds number. The roughness Reynolds numbers of the approach #ow under various temperature conditions for the present study, which range from 7.1 (¹ "78, ¹ "21) to 26.8 ! & (¹ "19, ¹ "59, Table 1), are su$ciently large in the ! & light of Snyder's criteria.
2.3. Measurement of yow and temperature xelds in and above street canyons
To measure the wind speed we attached the LDA probe to the carriage system in the tunnel and "red an
K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
Fig. 4. Ri pro"les within and above the street canyon.
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Fig. 5. ;/; pro"les at the central part of the street canyon. 700
Argon laser. The beam was split into three colors, one for each of the three component directions, u, v and w. Each color beam was then split into two, and the resulting parts were directed to intersect at the measuring point from the appropriate direction to obtain Doppler signals of the individual components. Each signal was processed by Burst Spectrum Analyzers (BSA Dantec Measurement Technology). Magnesium carbonate powder (particle diameter 5 lm) was released into the air using a small audio speaker as a seeding generator. The cold wire was installed parallel to the #ow, 1}1.5 mm leeward of the laser beams' intersection so as not to disturb the #ow "eld. The sampling time of wind speed and temperature was 180 s, and occasionally extended to about 300 s if necessary. See Uehara et al. (1997a,b) for more information on the seeding method and the e!ects of model material heat conductivity.
3. Results
Fig. 6. (¹!¹ )/(¹ !¹ ) pro"les at the central part of the & ! & street canyon.
3.1. Proxles Wind-speed pro"les (Fig. 5) were measured starting at 2 mm above the #oor, and temperature pro"les (Fig. 6) were taken starting from 0.2 mm above the #oor, both at the street canyon center. Wind speed in and above the street canyon, ;/; , fell as the strati"cation 700 strengthened, but rose as instability increased (inside the street canyon reverse #ow was strong because of the instability). In particular, when strati"cation stability was strong, the wind speed in the lower part of the street canyon was very low, dropping to 1 or 2% of the wind tunnel speed.
The p /; , p /; and ;=/;2 pro"les (Figs. U 700 W 700 700 7, 8 and 10, respectively) include results obtained by Raupach et al. (1986) with hot-wire anemometer measurements of intra-layer turbulence in a plant canopy model (H"60 mm) under neutral strati"ed conditions. Di!erences such as the model shape and the distance from roughness elements prevent direct comparison, but our p was smaller than that of Raupach, while our U p and ;= were similar. Turbulence intensity, shear W stress, and other values in and above the street canyon
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K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
remained low under stable strati"ed conditions, and were high at times of unstable strati"ed conditions; energy transport in the vertical direction changed greatly according to atmospheric stability (Figs. 7}10). Wind speed and turbulence pro"les inside and above the street canyon were roughly divided into three parts: (1) street canyon interior (Z(H); (2) internal boundary layer (IBL), the transition area directly a!ected by the surface of the city block model (H(Z(2.5H); and (3) upper boundary layer (UBL), where there is only a small in#uence from the city block model and which holds properties of the approach #ow (Z'2.5H). These parts approximately correspond, respectively, to (1) the urban canopy layer, (2) the roughness layer, and (3) the surface layer in the urban boundary layer classi"cation scheme used by Oke (1988a,b). Inside the street canyon, turbulence intensity and shear stress change with height above the ground level Z, but large di!erences in these variables are caused by atmospheric stability. In the IBL, just as inside the street canyon, large di!erences are due to atmospheric stability, and there is little height-dependent variation. Finally in the UBL, di!erences due to strati"cation gradually become smaller and turbulence intensity and #uxes decrease as height increases. In the UBL, strati"cation-induced di!erences in average wind speed and temperature nearly disappear. Mean wind velocities "t logarithmic pro"les when the zero-plane displacement was chosen to be d"64 mm (very near the value suggested by Jackson (1980) of d+H]0.7). The roughness parameter was found to be Z "10 mm; seemingly not strongly a!ected by 0 strati"ed conditions in the way that the approach #ow was. Turbulent intensity and shear stress appeared almost constant in the IBL and the velocity variances scaled by the friction velocity, p /u , p /u and p /u , U H V H W H also seemed constant. However our values were slightly
Fig. 8. p /; pro"les at the central part of the street canyon. W 700
Fig. 9. p /D¹ !¹ D pro"les at the central part of the street T ! & canyon.
Fig. 7. p /; pro"les at the central part of the street canyon. U 700
higher than those of Rotach (1994), while the distribution in the Z direction of both showed a similar tendency (Table 1, Fig. 11; p /u and p /u pro"les are not V H W H shown). As the scaled variances above the street canyon showed values 5}15% higher than those of the approach #ow, it is assumed that the fetch length of the model city blocks was not su$cient and thus the IBL did not attain rigorous equilibrium. Hoydysh et al. (1974) examined the in#uence of a fetch length consisting of rows of buildings on the #ow and concentration "elds in a downstream
K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
Fig. 10. ;=/;2 pro"les at the central part of the street 700 canyon.
Fig. 11. p /; pro"les at the central part of the street canyon. U H
street canyon, and concluded that an increase in fetch length of more than 20H brought no change in the #ow or concentration "elds within or above the street canyon. By this account, it seems, at least inside the street canyon, that the fetch length of the present study was su$cient, taking into account the change of roughness height at the leading edge of the model city blocks. 3.2. Air yow in the street canyon Velocity vectors were superimposed on composite speed distributions (;2#=2)1@2/; in the cross sec700 tion (Fig. 3) of the street canyon center under stable,
1559
neutral and unstable strati"ed conditions (Fig. 12). The stronger the strati"cation, the weaker the cavity eddy that formed in the street canyon, while the greater the instability, the stronger the eddy. According to #ow visualizations carried out by Uehara et al. (1998), a large eddy formed in the street canyon in the majority of cases. The intensity, shape and stability of the cavity eddy varied with the aspect ratio of the street canyon (the ratio of building height and road width, ¸/H). A strong, stable eddy formed when the aspect ratio was ¸/H"1}2, and it seemed that the #ow pattern corresponded to the Skimming Flow Regime used in Oke's classi"cation of the air #ow over building arrays (Oke, 1988b). Fig. 15 graphically represents the #ows inside and above the street canyon (¸/H"1) based on the results of the visualizations of Sasaki et al. (1989) and Uehara et al. (1998). A series of small eddies formed at the interface between the main and cavity #ows due to the strong velocity gradient. As these eddies transferred energy to the cavity #ow, they were pushed leeward and ran into the top edge of the leeward building. Some of the eddies descended vertically into the street canyon as they disintegrated, while the others #owed out of the top of the street canyon. One can see from the distribution diagrams of the vertical component =/; (Fig. 13) that the down700 ward #ow heading into the street canyon arose within a very small range just in front of the leeward buildings. The #ow that ran into the downstream building broke into the street canyon, reached ground level, then headed toward the back of the windward building as a #ow compensating that which was pulled along by the free stream at the top of the street canyon. It was probably the repetition of these processes that formed the cavity #ow. In a series of these processes, shear stress above the street canyon had a strong in#uence on the descending #ow speed along the leeward building wall. The distributions of shear stress ;=/;2 (Fig. 14) shows that it was 700 greatest at the top edge of the street canyon on the leeward building side, where energy transfer from the free stream was large and shear stress at the top of the street canyon changed in accordance with atmospheric stability (Figs. 15 and 16). Presumably, these processes induced a change in the cavity eddy intensity with the strati"cation. The reverse #ow speed under neutral stable strati"ed conditions was about half the wind speed above the street canyon, increasing to about 60% at times of strong instability. Conversely, the wind speed in the lower part of the street canyon decreased to nearly zero under highly stable conditions. The cavity eddy speed changed widely depending on atmospheric stability, with the change being small under unstable conditions and very large under stable conditions (Fig. 17). When the atmosphere was stable, wind speed ; over the street canyon H remained low. Additionally, the downward #ow entering the street canyon was hindered by buoyancy, so that the
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K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
Fig. 12. Contour of (;2#=2)1@2/; and the vector "eld within the street canyon: (a) stable case, Rb"0.78; (b) neutral case, 700 Rb"0; (c) unstable case, Rb"!0.21.
Fig. 13. Contour of =/; within the street canyon: (a) stable case, Rb"0.78; (b) neutral case, Rb"0; (c) unstable case, Rb"!0.21. 700
Fig. 14. Contour of (;=/;2 )]103 within the street canyon: (a) stable case, Rb"0.78; (b) neutral case, Rb"0; (c) unstable case, 700 Rb"0.21.
K. Uehara et al. / Atmospheric Environment 34 (2000) 1553}1562
1561
Fig. 15. Schematic of #ow within the street canyon.
Fig. 17. Change in reverse #ow speed, ; /; at bottom of 3.!9 H the street canyon with atmospheric stability Rb.
4. Summary We used a strati"ed wind tunnel to study the e!ects of atmospheric stability on the #ow "eld in a street canyon, and determined the following:
Fig. 16. Change in shear stress, ;=/;2 at top center of the 700 street canyon with atmospheric stability Rb.
wind speed in the street canyon further decreased. The result was the generation of a positive feedback e!ect in which a more stable strati"ed layer was formed. For this reason, the wind speed fell to nearly zero in the lower part of the street canyon under stable strati"cation, somewhere between Rb"0.4}0.8. Conversely, when the atmosphere was unstable, a downward #ow readily entered the street canyon and the cavity eddy became stronger than that under neutral conditions. This resulted in vertical mixing that reduced the high-low temperature difference, thereby weakening the e!ect of buoyancy. Thus, wind speed di!erences caused by the intensity of instability were smaller than those under stable strati"ed conditions.
f The cavity eddy that formed in the street canyon became weaker when the atmosphere was stable, and stronger when unstable. f Stable atmospheric conditions led to a positive feedback e!ect in which the downward #ow into the street canyon weakened owing to buoyancy, which facilitated the formation of a more highly stable strati"cation. As a result, when stability exceeded a certain threshold (somewhere between Rb"0.4 and 0.8), wind speed in the street canyon dropped to nearly zero. On the other hand, when the atmosphere was unstable there was a great deal of mixing in the street canyon, causing the vertical temperature gradient to grow smaller, and thus the instability to decrease.
Acknowledgements We would like to thank Mr. K. Komaba, Mr. Y. Yamao and Mr. T. Kawata for their help in the execution of the wind tunnel experiments.
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