Validation of computer modelling of vehicular exhaust dispersion near a tower block

Building and Environment, Vol. 25, No. 2, pp. 125-131, 1990. Printed in Great Britain.

0360-1323/90 $3.00+0.00 ~! 1990 Pergamon Press pie.

Validation of Computer Modelling of Vehicular Exhaust Dispersion Near a Tower Block Y. QIN* S. C. KOT't The wind flow around a 31-storey tower block with a low-rise block on the other side o f a 50 mwide street was computed by the S I M P L E method. The convection-diffusion equation was then used to compute the average NO~ concentration level near the building. Good agreement was found between the computer model and the .field measurements. The computer model showed that the tower block induced updrafts at the leeward side of the tower to provide relief o[ street level pollution.

In the paper [1] a general survey was given on different levels of sophistication in numerical modelling of the dispersion problem around a building. It was shown that the computers required, ranged from mini-computers to super computers. In this paper only the Navier-Stokes equations with effective viscosity to represent turbulence are used. After the wind field around the building is computed, a convection~liffusion equation is used to calculate the pollutant concentration. The purpose of this simplified model is to enable local planning agencies without access to large computers to make some predictions before tower blocks are built. The other alternative is of course the use of scaled wind-tunnel modelling. This requires a large boundary layer wind-tunnel that is only available in national research centres or a few universities. Cost and waiting time are the major problems of wind-tunnel modelling. The question of accuracy of computer modelling is always at the back of the mind of town planners and pollution control agencies. To check the usefulness of the present computer model, the wind field and concentration near the object tower block were measured. A good agreement between the computed and observed mean values of wind velocities and NOx concentration were found.

INTRODUCTION THE PROBLEMS of urban pollution have received increasing attention in the past decade. The major cities of Asia are expanding at an alarming rate. Lack of space for the teeming millions caused high-rise buildings to appear in droves. The street canyons created noise and air pollution problems. The main source of pollution is the ever-increasing motor traffic. Shenzhen is a new city in South China rising from farmlands only a decade ago. Compared to the nearby Hong Kong where high-rise buildings have been constructed haphazardly for several decades along its narrow winding streets, Shenzhen is more fortunate to be able to learn from this unsatisfactory situation. In order to minimize the air pollution problem on street levels, the Institute of Environmental Science of Zhongshan University undertook a study of wind flow around a tower block and field measurements of NOx and 03 concentrations near the building. NOx are gaseous pollutants emitted by automobiles and 03 is a secondary product of NO, and solar radiation. The city of Shenzhen was chosen because even though high-rise buildings are constructed in rapid succession, they are still far apart, allowing a single tower block bordered by a low-rise block to be studied without the interference of the wakes of other high-rise buildings. Our choice of a single tower block was in deference to the ability of computers to calculate flows around only one or two buildings at present. The streets of Shenzhen near the building of interest are laid out in almost a grid fashion. They run in east-west or north-south directions. The terrain is flat. This is an ideal site for testing the recommendation of the survey paper written by Kot [1].

FIELD STUDY Measurement for the wind field and NO,. concentration near a tower block was performed for 6 days in January 1986. During this time the prevalent wind is from the north. This period was chosen to match the inflow condition of the computer model. The tower under study is the Nanyang Mansion. It is a thirty-one storey building with square horizontal cross-sections built atop a podium three storeys high. The building is located on the north side of a 50 m wide street running from east to west. On the south side of the street a low-rise block

* Institute of Environmental Science, Zhongshan University, Guangzhou, China. t Department of Mechanical Engineering,Universityof Hong Kong, Hong Kong. 125

Y. Qin and S. C. Kot

126

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Fig. 2. Plan view of the buildings (see Fig. 1). called Xiangjang Restaurant is located. A sketch of the tower block and the low-rise building is given in Fig. 1. All principal dimensions of the buildings are shown in the figure, and a plan view is shown in Fig. 2. The ZDQ-01 observation system was used to monitor temperature and wind parameters. The ZDQ-01 meteorological gradient measuring system was developed by the Atmospheric Science Department of Zhongshan University. Five sensors can be controlled by ZDQ-01 to collect the wind speed, wind direction and temperature at five different levels. Small cup anemometers with a threshold speed of 0.6 m s- ~and counterbalanced vanes were used for horizontal wind data collection. Quartz sensors were used for temperature measurements. The accuracy of the system is 0.1°C. The instruments were located at the south face of the tower block. The temperature and wind were recorded at five heights, namely the ground, seventh, fourteenth, twenty-second and the top floors. The heights of the sensors were the same as Table 1. Vertical variation of NOx for Nanyang Mansion on 25-29 January 1986 (units in mg m- 3) Height (m)

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Ground 21 42 66 93

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Fig. 3. Measured variation of wind speed with height 2 m from south side of Nanyang Mansion.

the NOx sensors listed in Table 1. They were placed 2 m away from the south face to be outside the boundary layer and 5 m from the centre-line of Nanyang Mansion. The monitoring was carried out on a 24 h basis, under computer control. Variations of the minimum, average and maximum wind speed with height 2 m from the south face of the building are shown in Fig. 3. The wind did not increase with height in a logarithmic fashion as would be expected in an open, flat terrain. Instead, the minimum and the average wind remained almost constant with a slight ground level speed up. This constant wind speed was held up to about 70 m and then it increased rapidly. For the maximum wind speed profile, the ground level wind was large and then decreased up to 70 m. From here the wind again increased. The ground level speed-up was obviously due to the channelling effect of the podium and the low-rise building on the south side of the street. From these wind measurements on the leeward face, the wake effect of the tower block for a northern wind was found to be approximately 70 m in height or seven-tenths of the building's height. The daily variations of wind speed with height were similar to that given in Fig. 3. The wind speed was lower in the afternoons and evenings, and at its highest at midnight. For pollutant measurements atmospheric samples were taken three times a day, at 07:00, 11:00 and 17:00 local time. The air samplers used were DQ-2A manufactured by Laoshan Electronic Instrument Laboratory, Tsingdao, China. They were positioned near the exterior wall of the tower block at five levels, namely ground, 21, 42, 66 and 93 m above ground. 30 dm 3of air were sampled each time for 03 and not less than 5 dm 3 of air for NOx. The concentrations of NOx and 03 were determined by colorimetry. The results for five days of monitoring for the concentration levels of NO~ and 03 at different heights are shown in Table 1 and Fig. 4. As observed, the concentration of NOx near the south face of the tower block decreased with height. The decrease was fastest near the ground and then slowed to a constant concentration above 60 m. This again showed that the wake effect of the tower block in the north wind was greater than 60 m. The pollutants caught inside the wake had more or less a uniform distribution due to vigorous turbulent mixing.

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by means of the incompressible Navier-Stokes equations. Eddy diffusivity was used to approximate turbulence effect. After the wind field had been determined a convection-diffusion equation for dispersion of gaseous pollutant was used to calculate the NOx concentration distribution inside the street canyon. A line source was used to model automobile emission. Since the reaction rates of O3 with No, were not known, for simplicity only a NOx calculation was performed assuming no decay. Since the field of interest was small the Coriolis force was neglected. Assuming incompressible flow, the governing equations were :

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Fig. 4. Measured variation of average concentration of NO, and O~ with height.

momentum The NOv pollutant was caused by the exhaust fumes of automobiles. This was considered to be the sum of two contributions. The first was the contribution of automobiles at street level near the tower block under consideration. The other contribution was due to the automobile emission from the other parts of the city which constituted the background emission. The contribution of the nearby automobiles was greatest at ground level ; the concentration then decreased by advection and diffusion towards the background value at higher levels. O3 is a secondary pollutant generated under solar radiation by the exhaust fumes of automobiles. The hourly change of concentration of 03 was very obvious. The concentration of 03 was highest at noon because of strong solar radiation. In the morning and evening it was lower. Contrary to NO,, the concentration of 03 increased with height up to approximately 35 m. This phenomenon was explained by Rodes and Holland [2]. O3 can react easily with NO to form NO2 and 02. The reaction is completed in a matter of seconds. The reaction rate depends on the concentration of NO and 03. The concentration of NO was higher near the street surface where more 03 reacted with NO. The concentration of O3 then became lower. Since concentration of NO decreased with height, therefore the concentration of 03 increased above 70 m and finally the background value was reached. It is not clear why the measured 03 decreased slightly from 35 to 70 m, the transport rates within the wake and reaction rates of reactants may have influences on this. The number of automobiles passing the tower block was determined by actual count. For calculating mean concentration in the computer model the hourly average value was used. As there is no legislation on the installation of pollution abatement equipment for automobiles in China yet, the emission standards of pre-control days in the U.S.A. were used for modelling purposes in the following section. NUMERICAL MODEL The wind field around the tower block and between the two buildings within the street canyon was calculated

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z~>100m z < 100m,

p = 1.293 kg m - 3. The orientation of the co-ordinates is a right-hand system with x pointing towards the south and z pointing vertically upwards, thus giving the north wind a positive sign. The equations were discretized by finite volume method following the SIMPLE [5] algorithm (semiimplicit method for pressure linked equation) and ran on a mid-size Fujitsu M340 computer. The only difference with SIMPLE was the fact that the staggered grid was not used in order to save computer internal storage and to simplify programming efforts. The computational threedimensional region was 210x 130× 150 m (length× width x height) to include the tower block and the low-rise building plus some nearby space. Uniform grid size of Ax = Ay = Az = 10m was used. Time-marching and iterative methods for the pressure equation were used to compute the solution until a steady-state was reached. The computing time was in the order of 20 min. No waviness in the solution was observed due to large eddy viscosities and coarse grid. Initial conditions used were a zero velocity and hydrostatic pressure field. The approaching flow from the north had a velocity profile of u = 2.6(z/10) °25 m s- ~. The south boundary condition was a free stream condition. For the sides parallel flow conditions were used. The top boundary was again taken to be a free stream one. For the ground and building surfaces, the no-slip conditions were applied. As there are presently controversies on

Y. Qin and S. C. Kot

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Fig. 8. 100 m height horizontal wind field. Fig. 5. 10 m height horizontal wind field.

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Fig. 10. B B section (see Fig. 2) vertical wind field. computational boundary conditions, the simplest conditions that would provide us with some reasonable results seemed to be the most cost-effective. As the region o f interest was between two solid buildings and at the centre of the computational region, sophisticated boundary conditions were thought to be unnecessary. Some o f the computed horizontal wind fields at 10, 30, 70 and 100 m are shown in Figs 5-8. The vertical wind field at sections A A, B B and C - C are shown in Figs 9 I I. Measured and computed wind speeds near the south face o f the building are shown in Fig. 12. The measured wind speeds used are averages o f observations. A partial vacuum was created at the leeward side o f the tower for the head-on north wind case under consideration here. This caused air flow within the street canyon to converge towards the leeward side o f the tower

Fig. l 1. C C section (see Fig. 2) vertical wind field.

Computer Modellin9 of Vehicular Exhaust Dispersion

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and an updraft was induced. At this leeward side of the tower the wind velocity had a large vertical component. This vertical wind increased from 30 m height then decreased to almost zero near 90 m. This was the meeting point between the updraft from the street level and the downdraft slip-stream induced by the free stream in the presence of the tower obstacle. The comparison between computed and measured wind speed showed reasonable agreement from 30 m upwards. The turning point for wind speed decreasing to rapidly increasing at 90 m had been predicted well by the computer model. The discrepancy below 30 m was attributed to the neglect of the strong easterly wind at street level. This easterly wind was always measured during north wind situations. The physical cause of this easterly wind was due to the presence of other buildings in the city and the channelling effect of the street canyon which ran in the east west direction. In this computation only the tower block and the low-rise block across the street on the south side were taken into account. It is suggested that, if necessary, an experimentally derived easterly correction could be added at ground level to make the model more accurate. At heights for above street level the influence of other buildings in the city was weak. As explained before, where the tower block was not in the wake region of any nearby high-rise buildings, the measured and the calculated wind speed correlated well. With the wind field known and if the source strength and emission rate could be estimated, the concentration of NO., at various points within the computational mesh could be calculated by means of the concentration convection~tiffusion equation : c3C

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where C is the concentration, k~ = (k t, k2, k3) diffusion coefficients for gaseous pollutant and S = source. The chemical decay of NOx when reacting with 03 had been neglected. Again this convection~tiffusionequation was discretized by the finite volume method and solved numerically until a steady-state is reached.

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Fig. 13. Ground NO,-isopleths (units in mg m 3).

The source term was approximated by a line source on the street surface : Sgm's

~) = automobile flow rate (car s ') xemission factor (gcar ~m i).

From actual traffic counting on the street the automobile flow rate was determined as approximately 1000 car h ~. NOv was the gaseous pollutant to be studied in this paper. As there was no emission control, the emission parameters used in studies in the U.S.A. in the early sixties would be appropriate. According to Duprey [5] the NO, emission rate for this case is 2.3957 x 10 ~ g car ] m 1 Therefore, the source term used in this computation was : S=0.6655x10

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The other physical parameters needed for the model were the diffusion coefficients. Field studies had been carried out by the Institute of Tropical Ocean Meteorology in the Shenzhen City. Under neutral conditions the appropriate diffusion coefficients with a 15 rain moving average were : kl=k2=6.7m2s

i,

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The ground was assumed to have 5% absorption. The initial concentration field was set to be zero everywhere and the calculation was time-marched forward until a steady state was reached. The actual (total) concentration CT was given by Cr = C+Ca where C, was the background concentration and C was the concentration calculated by the above convection-diffusionequation. The background concentration C, was measured to be 0.020 mgm The computer-calculated total NO, concentration field inside the street canyon on the ground, 10, 20 m and across A-A and C-C sections are shown in Figs 13 17. Finally, the computer predictions of NO,. concentration is compared with the observed concentration in Fig. 18. The observed values were the averaged values under north wind conditions. From this comparison we observe that the computer model is good above 30 m. Below 30 m, due to the discrepancy in the calculated wind speed and the neglect of reactions between 03 and NOt, the comparison is poorer. However, the maximum error is less than 25%.

Y. Qin and S. C. Kot

130

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CONCLUSION

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F r o m the reasonable comparison between computed and observed results it is concluded that the computer model was quite adequate in presenting a quantitative picture of wind field and concentration in the street canyon between a tower block and a low-rise building for the head-on wind case. This north wind direction was an extreme case. The other extreme case is the east-west wind direction along the street canyon. However, for that case the street canyon accumulation of vehicular exhaust is minimal. For other wind directions the building wake effect of the tower block would be less. The case studied here represented the most conservative situation in pollutant dispersion study. For city planning purposes the computed results might be regarded as a worst case estimate. As stated in the previous paper [1], the use of a sophisticated turbulence model in the numerical solution of dispersion near buildings might not be warranted. This study supports that idea. With a simple expression and constants for effective eddy diffusivities, the zero equation model resulted in wind field calculations that correlated well with measurements. Similarly the computed concentration field compared favourably with the observed values. We believe understanding of the chemical reaction rates of various gaseous pollutants is more important than improvements on the turbulence modelling technique. With wind tunnel or field studies, only a small amount of data would be collected at great cost in terms of money and time. The rest of the wind and concentration field would then have to be obtained by interpolation. Using computer modelling the whole quantitative wind and concentration fields are generated, which are more useful to urban planners. The goal set out in the introduction, i.e. to enable users of small computers to predict concentration levels in the street at low cost has been fulfilled, at least for the case of a street bordered by only a few buildings. The necessary input data were those normally on file with local pollution control agencies. The most difficult data to obtain were the diffusion coefficients, but values available in the open literature could be used as a stopgap measure. In fact, several

Computer Modellin 9 o f Vehicular E x h a u s t Dispersion different values of diffusion coefficients were tried in this study. As long as the coefficients were of the same order of magnitude, the discrepancies in the calculated results were not too significant. From the results of field measurements and computer simulation, the wake effect of the tower block for headon wind was shown to be quite prominent. The updraft induced by the presence of a high-rise building is important in the transport of street level pollutant out from the

131

street canyons. One suggestion for reducing ground level pollution was to have high-rise buildings constructed far apart on top of low podiums or no podiums at all, to reduce the channelling effect of street canyons. The wakes of these high-rise buildings should keep clear of each other in any prevalent wind direction. This would allow a rule of thumb distance of separation between high-rise buildings. According to this study this wake height is about 0.7 of the tower's height.

REFERENCES l. S.C. Kot, Numerical modelling of contaminant dispersion around buildings. Bldg Envir. 24, 33-37 (1989). 2. C.E. Rodes and D. M. Holland, Variations of NO, NO2 and 03 concentrations downwind of a Los Angeles freeway. Atmosph. Envir. 15, 243-250 (1981). 3. The measurement of atmospheric diffusion parameters and analysis of diffusion characteristics of Shenzhen special economic zone. Unpublished Report of Institute of Tropical Ocean Meteorology, Guangdong Province (in Chinese) (1986). 4. A.S. Monin and A. M. Obukhov, Basic turbulent mixing laws in the atmospheric surface layer, Trudy Geofiz. Inst. AN S.S.S.R. 24, 163-187 (1954). 5. S.V. Patankar, Numerical Heat Transfer and Fluid Flow. Hemisphere, New York (1980). 6. R.L. Duprey, Compilation of air pollutant emission factors. PHS Publication 999-AP-42, Durham, NC, National Center for Air Pollution Control (1968).