Wind tunnel study on the pollutant dispersion over an ... · Wind tunnel study on the pollutant dispersion over an urban ... In recent years wind tunnel techniques have been ... 1

Wind tunnel study on the pollutant dispersionover an urban area

M.F, Yassin, S, Kate, R, Ooka T. Takahashi, &T. OhtsuInstitute of Industrial Science, University of Tokyo, Japan

Abstract

This paper presents a wind tunnel study of the pollutant dispersion over an urbanarea with three different types of thermal stability within the atmosphericboundary layer. The diffision fields in the boundary layer were examined inthree flow obstacle cases: i) boundary layer without flow obstacles, ii) boundarylayer over two-dimensional fence, and iii) boundary layer over three-dimensionalcubic building model, The scale of the model experiment is assumed to be at1:500, In the experiment, gaseous pollutant is discharged in the simulatedboundary layer over the flat terrain, Ethylene, CZH4,is used as tracer gas and ahydrocarbon analyzer detector (FID) is used to measure its concentration. Theeffluent velocity of the pollutant is set to be negligible. The density of pollutantgas is the same at the height of the pollutant effluent in the boundary layer. Theseexperiments are performed in the stratified wind tunnel under three atmosphericconditions: stable (RiB=O.118), neutral (RiB=O.O)and unstable (RiB=-0.096),Wind velocity profile of !4 power law is simulated for all three cases.Concentration distributions (vertical and horizontal) were measured at fourpositions in the leeward direction (2, 3 , 6 and 13 H~ from gas source, whereH~ is obstacle height), Laser Doppler Velocirnetry (LDV) is used to measure thevelocity field and the turbulence characteristics are analyzed. The resultsobtained are as the following: a) a thick internal boundary layer is generated inthe case with 2D fence, b) the inner boundary layer is verj thick around the wakeregion due to the turbulence mixing, c) the reattachment length of the separatedflows with the 2D fence is longer than that with the cubic model, d) the verticaland horizontal distributions of concentration with the 2D fence are smaller thanthat with the cubic model. The concentration distributions measured in theexperiment may be used for the evaluation of numerical models and expertestimating of air quality in the urban environment.

© 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved.Web: www.witpress.com Email [email protected] from: Air Pollution X , CA Brebbia & JF Martin-Duque (Editors).ISBN 1-85312-916-X

626 Air pollution X

1 Introduction

In recent years wind tunnel techniques have been used increasingly oflen tostudy the dispersion of pollutants in an urban areas. New experiments in thisfield, and other related experiments considering different subjects, have revealedcertain criteria that must be considered when modeling an urban area. The levelof pollutant concentrations over an urban area resulting from exhaust emissionshas become increasing concern in recent years. Due to the complexities of citytopography and irregular building shapes and patterns, elaborate predictivemodels of pollutant dispersion and diffision to date have not been able to clarifythe effects of local geometry on pollutant concentrations at street level,

Many wind tunnel experiments have been conducted with the aims ofsimulating present conditions and understanding the phenomenon of air pollutiondispersion. Dabberted and Hoydysh [1] performed a series of wind tunnelexperiments using a model of simple block shapes, They investigated the effectsof difference in building heights on the concentration distribution around bothsides of a street canyon and estimated the residence time of air pollution usingsoap-bubble trails. Dabberted and Hoydysh [2] also investigated the effects ofdirection, block shape and street width on concentration distribution andconcentration at intersections [3], Meroney et al. [4] studied the line sourcecharacteristic.s at different street canopy widths, where only buildings of aspectratio H/W=l were used. Most recently, Pavageau and Schstzmann [5] examinedthe pollutant concentration distribution in street canopy in a wind tunnel withurban roughness. Kastner-klein and Plate [6] studied the pollutant concentrationvariations with building dimensions, upstream building configurations and roofgeometries. Pearce and Baker [7] investigated the dispersion of vehicularpollutants in an urban area by an extensive series of wind tunnel tests. Thepollutant contamination of an urban street canyon was also investigated in a windtunnel by Gerdes and Olivari [8].

With these previous experiences done by va~ious scientists in mind, thepurpose of this research study is to conduct a wind tunnel investigation of thepollutant dispersion in an urban area with three diffenent types of thermalstability (stable, neutral and unstable) within the atmospheric boundary layer.The dispersion fields in the boundary layer were examined in three flow obstaclecases: i) boundary layer without flow obstacles, ii) boundary layer after two-dimensional fence, and iii) boundary layer after three-dimensional cubic buildingmodel. All these experiments were carried out with the following conditions: a)wind velocity profile of ‘Apower law is simulated for all three cases, b) gaseouspollutant is discharged in the simulated boundary layer qver the flat terrain, c)the effluent velocity of the pollutant is set to be negligible, d) the density ofpollutant gas is the same at the height of the pollutant effluent in the boundarylayer, e) the stratified wind tunnel experiments were performed under threeatmospheric conditions: stable (Bulk Richardson number, RiB=O.118), neutral(Ri~=O) and unstable (WB =-0,096). The concentration and wind speedmeasurements were done separately in this experiment. The velocitymeasurements are helpfid in interpreting the concentration results; data sets


Air pollution x 627

containing both velocity and concentration measurements are essential fordevelopment and analysis of diffusion in an urban area.

2 Experimental set-up and instrumentation

2.1 Simulation of the atmospheric boundary layer

The experiment was performed in the boundary layer wind tunnel of the Instituteof Industrial Science (11S), University of Tokyo. The schematic diagram of theexperimental conducted in the present study is illustrated in Fig, 1. The windtunnel has a test section of 2,2m wide, 1.8m high, and 17m long. The wind speedrange is 1-33 mls. The atmospheric boundary layer was simulated usingpositioning spires together with roughness blocks arranged at a distance of 13 mjust after the contact section, The vertical velocity distribution in the region,where the boundary layer is fully developed, can be described by a power law:

U/Uref = (Z /Href )“ (1)

where U is the mean velocity in the leeward direction at elevation Z, U,~fis themean velocity at the reference height H,,f and n is the power exponent. In thisstudy, a turbulent boundary layer for the three atmospheric conditions (stable,neutral and unstable) in an urban area was simulated on a scale of 1:500, Thepower exponent, n, is set at 0.25.

I 12.7 m I

Fig. 1: Experimental set-up in wind tunnel


628 Air pollution X

The three different types of thermal stratification (stable, neutral and unstable)within the atmospheric boundary layer were created in the test section bycontrolling the wind temperature and wind tunnel floor temperature. Heating theair and cooling the wind tunnel floor produced a stable stratified layer. The windtemperature ( .) and wind tunnel floor temperature ( J were set at 27.8 ‘C and21.0 “C respectively, where the Bulk Richardson number (Ri~=gH / ~U2,where = ~- fand H= the inflow temperature) was setatO.118 and Reynoldsnumber (&) was 5500. While, cooling the air and heating the wind tunnel floorproduced an unstable stratified layer, The wind temperature and wind tunnelfloor temperature were 11.9 and 16.0 ‘C respectively, where Ri~ was at –0.096and & was 9800. In neutral stratified layer, RiB was 0.0 and ~ was 4000. Thediffusion field in the boundary layer was examined in three flow obstacle cases:i) boundary layer without flow obstacles, ii) boundary layer after two-dimensional fence, and iii) boundary layer after three-dimensional cubic buildingmodel, The scale of the model was also set to be 1:500. The obstacle two-dimensional fence was 60 mm height and 160 mm width. The obstacle three-dimensional cubic building model was 60 60 60 mm

2.2 Measurement techniques

2.201VelocityWind speed is the most fimdamental and important parameter to measure in windtunnel experiments, Two-Dimensional Laser Doppler Velocirnetry, 2D-LDVwas used to measure the mean velocities and turbulence intensities inlongitudinal and vertical directions. To measure the wind speed, we attached the2D - LDV probe to the traverse system in the tunnel. The 2D-probes connectedto each two BSA (Burst Spectrum Analyzer) measure the velocity components u(horizontal direction) and w (vertical direction).

2.2.2 Concentrations,In accordance with the purpose of the present investigation, a tracer gas was tobe neutrally buoyant and released from a point source. The tracer gas chosen, inthis case is ethylene, C2Hg, which was emitted from a stack that has an innerdiameter of 4 mm, A hydrocarbon analyzer detector (FID) was used to measurethe C2Hq concentration. The concentration profiles were obtained by collectingsamples through the pipeline attached to the carriage system in the tunnel. Thesamples were routed to a hydrocarbon analyzer detector (FID) which producedoutput voltages linearly r?lated to concentration. The output voltage from theFID was sampled by minicomputer system at a rate of 1 ~z over averaging timeof 120 s, which yield reasonably stable values of mesh concentration. Theconcentration measurements are presented in the ratio of C/CO,where C is themeasured concentration, ppm and C. is the reference concentrations, PPQ(CO=QiU~H~2, where Q is the source volume flow rate, U~ is the free streamvelocity at the height of obstacle, Hm).In the present study, the emission velocityfrom the stack was 10’?’.of the free stream velocity. Therefore, the effluentvelocity of the pollutant is assumed to be negligible. Since a density of C2H4gas


Air pollution x 629

is almost same with that of air, the density of pollutant gas can be thought tohave the same density at the height of the pollutant effluent in the boundarylayer,

3 Results and discussion

3.1 Simulated boundary layer

A simulated atmospheric boundary layer was obtained by using a combination ofspires and roughness elements on the floor of the tunnel as shown the schematicdiagram of Fig. 1, This combination of spires and roughness elements produced asimulated atmospheric boundary layer with a normal depth, , of 1,00 m and a

free stream wind speed, Um of 1,2 ins-’, Fig, 2 shows the simulated turbulent

boundary layer in the stratified wind tunnel under three atmospheric conditions:stable, neutral and unstable at X = -180 mm (X = 0,0 and -30 mm correspondingthe position of the model stack and the flow obstacles), Fig. 3 shows typicaltemperatures profiles in the vertical direction on stable and unstable stratifiedboundary layer at X = -180, 0, 400, 600 and 800 mm. On the stable stratifiedflows show ,almost linear profiles in the vertical direction and uniformtemperature ~rofiles at the stream wise direction.

3.2 Flow characteristics of the boundary layer

The measurements were made at six different spots along the centerline of thewind tunnel: X = -180, 0, 120, 400, 800, and 1600 mm. The flow obstacles werelocated at X =-30 mm. All the velocity data are non-dimensionalized by thereference velocity U1efat the height of 600 mm. The vertical profiles of meanvelocity and turbulence velocity in the longitudinal and vertical directions weremeasured in the turbulent boundary layer starting at 3 mm above the floor of thewind tunnel for all three atmospheric conditions: neutral, stable and unstablecases with three flow obstacle cases: i) boundary layer without flow obstacles, ii)boundary layer over two-dimensional fence, and iii) boundary layer over three-dimensional cubic building model are shown in Figs, 4 to 6, In these figures, athick internal boundary layer can be seen in the case with 2D fence obstacle fordue to increased turbulence velocity in the three atmospheric conditions, while inthe cases with 3D cube obstacle and without obstacle, the internal boundary layergenerated is thin and more or less the same in both cases. The reattachmentlength of the separated flows with 2D fence obstacle is lo~ger than that with thecubic model. The value of turbulence velocity in the 1leeward and verticaldirections with the 2D fence is higher than that without flow obstacles and withthe 3D cube. Fig, 7-9 shows the comparisons of mean and turbulent velocitiesunder the neutral, stable and unstable atmospheric conditions without obstacle,with the 2D fence and the 3D cube. In these figures, the profiles of mean velocityin the leeward direction with the three flow obstacle cases in the neutral andstable boundary layer thickness are approximately the same, but the mean


630 Air pollution X

velocity profiles are increased in the unstable boundary layer due to increase inthe turbulence, which augments momentum transfer fi-omhigher to lower levels.The increase of longitudinal and vertical turbulent velocities is greater for thestable case than for the neutral and unstable cases, Furthermore, all meanvelocity in the vertical direction are smaller for the unstable case than for theneutral and stable cases. Physically in general, the mean velocity in the verticaldirection for unstable case is higher than that of neutral and stable cases due to[9, 10 & 11]: (i) heat flux is positive because of the heat transfer is goingupward, (ii) production of turbulent kinetic energy is increased, (iii) buoyantproduction is positive, (iv) Bulk Richardson number is less than –1 .0. In additionto the phenomenon of the turbulence being smaller for stable case than forneutral and unstable cases, the most important influence observed was on thevertical direction [12 & 13]. However, in this experiment, the mean velocity inthe vertical direction decreases. This reason is considered as follows; themeasuring points are located in downstream region of heating panel end. At thispoint, the floor temperature is lower than the air temperature. This creates strongnegative buoyant effect. Therefore, the mean velocity in vertical directionbecomes small.

3.3 Dispersion characteristics of the boundary layer

To establish dispersion characteristics of the simulated boundary layer,concentration measurements were carried out through the three atmosphericconditions with the three flow obstacle at four leeward distances: X=120, 200,400 and 800 mm. The model stack was located at X=O. Measurements weremade with the model stack’s height of 60 mm and changed to 30 mm for thesame three cases. The vertical concentrations were measured starting at 5 mmabove the floor along the centerline of the wind tunnel, while the horizontalconcentrations were measured at Z=25 mm, The vertical and horizontaldistributions of concentration, C/COwere measured, in the boundary layer at stackheight, H,=60 and 30 mm for all three atmospheric conditions with the three flowobstacle cases are shown in Figs. 10 to 13, In generally, when effluents come outof the vertical stack at low momentum or low mean vertical velocity, and thehorizontal flow around the stack is sufficiently strong, the effluent plume maybedrawn down in the low pressure region in the near wake of the stack, Thisphenomenon is referred to as stack downwash [14] In these figures, the peakconcentrations for the three atmospheric conditions with the three flow obstaclecases are ranging horn 2, to 3,5 at a half stack height, where the effluent isemitted near the separation-reattachment region and create$ the downwashes dueto the emission velocity fi-omthe stack was 10’7’.of the fide stream velocity, Thevertical and horizontal distributions of concentration with the 2D fence are lessthan that with the cubic model due to the increased turbulence velocity with the2D fence. While, at the half stack height, the concentration without flow obstacleis higher than that with the 2D fence and 3D cube model. The value ofconcentration with the 2D fence in the three atmospheric conditions isapproximately the same at H, =30 and 60 mm, this is also found in the case with


Air pollution X 631

the 3D cube model. But, the value of concentration without obstacle at H, =30 ishigher than that H, =60 mm because of the increased stream velocity. Themaximum concentration is found around the wake region of the obstacles.Therefore, the dispersion concentration is high near the stack and getting smalleras it is distanced away flom the stack, In general, the vertical and horizontaldistribution of concentration for the stable is higher than that of the neutral andunstable atmospheric conditions [15], However, in the present study, theconcentration in an unstable case is higher than that in neutral and stable cases inthe case of H, = 60 mm, This reason is considered as follows; the floor windtunnel is not heated after the obstacles, Therefore, the region of the strong stableboundary layer is produced after the obstacle at H,= 60 mm high in the unstableboundary layer case. Thus, turbulent diffision of concentration is decreased inthis case,

4 Conclusions

The results obtained from the study of wind tunnel experiment on pollutantdiffision in an urban area may be summarized as the following: (1) a thickinternal boundary layer is generated in the case with 2D fence, (2) a thin internalboundary layer is generated with 3D cube and without obstacle, (3) the innerboundary layer is very thick around the wake region due to the turbulencemixing, (4) the reattachment length of the separated flows with the 2D fence islonger than that with the cubic model, (5) the value of turbulence velocity in theleeward and vertical directions with the 2D fence is higher than the cases withoutflow obstacle and with 3D cube, (6) the peak concentrations for the threeatmospheric conditions with the three flow obstacle cases are from 2 to 3.5 at thehalf stack height, (7) the vertical and horizontal distributions of concentration forthe 2D fence are smaller than that of the cubic model, (8) at the half stack height,the concentration without flow obstacle is higher than the cases with the 2Dfence and 3D cube model, (9) the value of concen~ation for the 2D fence in thethree cases is approximately the same at H, =30 and 60 mm and also in the 3Dcube model, (10) the value of concentration without obstatle at H$ =30 is higherthan at H, =60 mm, (11) the maximums concentration is around the wake regionof the obstacles, (12) the dispersion concentration is high near the stack andsmall at far the stack, (13) the distributions concentration in unstable case ishigher than that in neutral and stable cases because of the floor wind tunnel is notheated after obstacles..

Reference

[1] Hoydysh, W.G. & Dabbert, W.F,, Kinematics and dispersion characteristicsof flow in asymmetric street canyons. Atmospheric Environment, Vol. 22,No.12, pp. 2677-2689, 1988.

[2] Dabbert, W.F, & Hoydysh, W,G,, Street canyon dispersion: sensitivity toblock shape and entrainment, Atmospheric Environment, Vol. 25A, pp. 1143-1153, 1991.


632 Air pollution X

[3] Hoydysh, W.G, & Dabbert, W.F., Concentration fields at urban intersections:fluid-modeling studies, Atmospheric Environment, Vol. 28, pp. 1849-1860,1994,

[4] Meroney, R.N., Pavageau, M,, Refailids, S, & Schatzmann, M,, 1996. Studyof line source characteristics for 2-D physical modeling of pollutantdispersion in street canyon. Journal of Wind Eng. & Industrial

Aerodynamics, 62, pp.37-56, 1996.[5] Pavageau, M. & Schstzmann, M., 1999. Wind tunnel measurement of

concentration fluctuations in an urban street canyon. AtmosphericEnvironment, Vol. 33, pp,3961-3971, 1999,

[6] Kastner-klein & P., Plate, E.J., Wind-tunnel study of concentration field instreet canyons. Atmospheric Environment, Vol. 33, pp,3973-3979, 1999.

[7]Pearce, W,& Baker, C,J,, Wind tunnel tests on the dispersion of vehicularpollutants in an urban area. Journal of Wind Eng, & Industrial

Aerodynamics, 80, pp.327-349, 1999.[8] Gerdes, F. & Olivari, D., Analysis of pollutant in an urban street canyon.

Journal of Wind Eng. & Industrial Aerodynamics, 82, pp.105-124, 1999.[9] Synder, W.H, Guide lineforjluid modeling of atmospheric dlj%sion. EPA -

600/8-81-009, 1981.[10] Panofsky, H.A.& Dutton, J.A., Atmospheric turbulence. John Wiley &

Sons, New York, 1984.[11] Tennekes, H.& Lumley, J,L., Ajrst course in turbulence, The Mit Press,

1994,[12] Ogawa, Y., Diosey, P,G,, Uehara, K.& Ueda, H., A wind tunnel for

studying the effects of thermal stratification in the atmosphere. AtmosphericEnvironment, Vol. 15, No.5, pp. 807-821, 1981.

[13] Zegadi, R,, Ayrault, M,& Mejean, P., Effect of a two-dimensional low hillin thermally neutral and stable stratified turbulent boundary layer.Atmospheric Environment, Vol. 28, No.11, pp.1871-1879, 1994.

[14] Arya, S, P., Air pollution meteorology and dispersion. Oxford UniversityPress, Inc. Oxford, 1999.

[15] Ogawa, Y., Griffiths R., Hoydush, W.G., A wind tunqel study of sea breezeeffects. Boundary Layer Meteorolofl, 8, pp. 141-161, 1974.


-c+Umy, Uulcm+ _ %

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1

Fig.2:

al 1 13 Im

(1) neutral

!Y

Iao

1al 1 10 Im

(2) stable

Air pollutionx 633

+ Ek@, Uulm+-%

ml

ml

10

101 1 10 ml

(3) unstable

Vertical profiles of mean velocity and turbulent intensity in the simulatedboundary layer at X = -180 mm

1690

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Fig.3: Vertical profiles of temperature in the simulated boundary layer


.05 05-051.8505 0515~5 05 15 -05 05 15

;~bij /j ‘/-luuref ““’; ~

oJ

-1 -0,5 0 05 1 1.5 2 2,5 3

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I

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Z/Href Cube +,“04

0,2

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.1 -0,5 0 05 i 15 2 2.5 3

X/Href

● without flow obstacle A with fence [, with cube

Fig. 4: Mean and turbulence velocity components in longitudinal and verticaldirections for neutral case (RiB = 0.0)


.22 4212-!)2 12 4Z2 12 422 12 42 12

-1 45 0 0.5 1 15 2 2.5 3

Xrrt’ef0 01 002 01 mzoozolozooloz o 01 02

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.1 -05 0 05 T 1,5 2 2.5 3

X/Hrei

. without flow obstacle A with fence with cube

Fig. 5: Mean and turbulence velocity components in longitudinal and verticaldirections for stable case (RiB=O.118)


08

06.2/Hrel

0,4

02

0-1 -05 0 0.5 1 15 2 2.5 3

Xmrefo 0030 03 003 03 0 03 0 03

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1

08

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04

02

0

11, (w2)0”5/uref

-1 -05 0 05 1 1.5 2 2.5 3

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● without flow obstacle A with fence with cube

Fig. 6: Mean and turbulence velocity components in longitudinal and verticaldirections for unstable case (RiB = -0.096)


o 010< 04 10 1 0 1

1-

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493 umref i0,6- ;

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1

-1 .05 0 05 1 1.5 2 2.5 3

Miref

● neutral case A stable case unstable case

Fig. 7: Mean and turbulence velocity components in longitudinal and verticaldirections without flow obstacle


-92 42 .02 1 .a2

1

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-1 -05 0 05 1 15 2 2.5 3

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02

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-1 -05 0 05 1 1,5 2 2.5 3

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● neutral case A stable case ~unstable case

-.

Fig. 8: Mean and turbulence velocity components in longitudinal and vertical directionswith 2D fence


42 -021 -02 1 4Z 1 42 1 42 1

0 1 2 3

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025

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● neutral case A stable case unstable case

3

Fig. 9: Mean and turbulence velocity components in longitudinal and verticaldirections with 3D cubic model


Cube Fence Stack

WindLine D

o 2 4 6 6 40 12 14

Neutral

!2

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\

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Stable case

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Fig, 10: Vertical distributions of concentration, C/Co for the three atmosphericconditions in the three flow obstacles with stack height, H,= 30 mm & Y=O


Cube Fence Stack

Neutral

00 10 20 30 40.,,-n

Stable

00 10 2.0 3.0 4,0

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● without flow obstacle with 2D fence A with 3D cube model

Fig. 11: Horizontal distributions of concentration, C/Co for the three atmosphericconditions in the three flow obstacles with stack height, H, =30 mm & Z=30


Cube Fence Stack

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1

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6

0,3 .D

o00 TO 20 3,0 40

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Line A Line B Line C Line D

● without flow obstacle with 2D fence A with 3D cube model

Fig. 12: Vertical distributions of concentration, C/Co for the three atmosphericconditions in the three flow obstacles with stack height, H,= 60 mm & Y=O


Cube Fence Stack

Line A LineB

Whd “’80,4

●4.8

+–+.2 4

Neutral

04

~---

.-.

c

; 0.0. . --–J

o

44+ —

,’

‘D.0- ~----

00 ?0 20 30 40

Unstable08

‘)’”

04~

Q

$ 0 -u.>

0,

44 .-= - -~– -.,

48- —--- - ..-. —

003 ~w 2W 303 4m

U*

Line A

1.04

f“ ‘----’--!/

-08A“ L. —-——.

00 10 20 30 40

C/a

00 30 20 30 40

(7Cn

Line B

LineC

8’ 8

Xmll

08 -- --–-——–

I~~04 ——-—-

0: -+,:

-94 _—. + — k

48-- -—-—–———

00 10 20 30 40

Ucn

00 10 20 30 40

m

081 —.–—––r–n

)’,,

04~ .--. +—+

;:

0.4 —.:

0:

04- * .——

08 –-––--—--—

00 10 20 30 40

am

Line C

LineD

— :—.10 12 14

08 -7—-—--–——

1--- ~

04 +

,,

0

44 ‘ —— —~

J28!’

00 10 20 30 40

aca

00 10 20 3.0 40

m

08 -–-–—-—--—––-

1 ‘

04 ——

0 -–-—T—

44 --L ~

.08 ———.—–+-–—

am Iw 2W 3(0 4ca

Line D● without flow obstacle with 2D fence A with 3D cube model

Fig. 13: Horizontal distributions of concentration, C/Co for the three atmosphericconditions in the three flow obstacles with stack height, H,= 60 mm & Z=30


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