6.4 PASSIVE TRACER DISPERSION OVER A REGULAR ARRAY OF CUBES USING CFD SIMULATIONS

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1 6.4 PASSIVE RACER DISPERSION OVER A REGULAR ARRAY OF CUBES USING CFD SIMULAIONS Jose Lus Santago *, Alberto Martll and Fernando Martn CIEMA (Center for Research on Energy, Envronment and echnology). Madrd, Span. 1. INRODUCION Nowadays, ar polluton s consdered a great envronmental problem n many ctes. hus, t s mportant to study pollutant dsperson nsde urban canopy, where there are complex patterns of ar flow that determne pollutant concentraton. Durng last decades, a large number of nvestgatons has been carred out by means of feld experments (e.g, Rotach, 1995; Vachon et al., 2001), wnd tunnel models (e.g., Brown et al., 2001; Meroney et al., 1996) and numercal smulatons (e.g., Bak and Km, 2002; Santago and Martn, 2005; Sn et al. 1996) to contrbute our understandng about meteorology and pollutant dsperson nsde urban envronment. In ths contrbuton, CFD smulatons of pollutant dsperson over a regular three dmensonal array of cubes are carred out representng a regular confguraton of streets (canyons) and buldngs. Smulatons are based on Reynolds Averaged Naver-Stokes equatons (RANS) usng standard k-ε turbulent closure. Pollutants are modeled by a passve tracer emtted at ground nsde regular array of cubes. Our study s focused on the locaton of maxmum and mnmum of passve tracer concentraton and the relaton of pollutant concentraton to flow pattern. In addton, several passve tracers are emtted nsde each canyon to study the effect of emssons nsde a street on concentraton nsde the other ones. hus, some nformaton on the contrbuton of neghbor canyon emssons on pollutant concentraton s obtaned. smulate a 3-D array of cubes. he cube edge length (H = 0.15 m) and street aspect ratos are all equals to 1, obtanng a regular array. Nonunform grd system wth 202 cells n the x- drecton, 44 n the y-drecton and 40 n the z- drecton (Fgure 1). For further detals of the numercal set up see presentaton J5.1. n the Sxth Symposum on the Urban Envronment (Santago et al., 2005). Fgure 1. he a) sde vew and b) top vew of the numercal doman and grd system. he regular array can be dvded nto 6 cube canyon unts and a zone wth one more cube. 8 passve tracer are released. All sources are located at ground where tracer 1 s emtted only n the frst cube canyon unt (Fgure 2b), tracer 2 n the second unt and so on. racer 8 s emtted n the whole array regon (Fgure 2a). 2. MODEL DESCRIPION AND SE UP he numercal model used s FLUEN (FLUEN Inc., 2005). Smulatons are based on Reynolds Averaged Naver-Stokes equatons (RANS) wth standard k-ε turbulent scheme. In addton a transport equaton for passve scalar s solved to smulate pollutant dsperson. Numercal doman conssts of a 7 cubes row n streamwse drecton wth symmetry boundary condtons at lmts n spanwse drecton to * Correspondng author address: Jose Lus Santago, CIEMA, Dept. of Envronment, Av. Complutense 22, 28040, Madrd, Span; emal: jl.santago@cemat.es Fgure 2. X-Y vew at Z / H = 0 of a) locaton of tracer source n the case of emssons n the whole array regon (racer 8) and b) locaton of tracer source n the case of the frst unt emttng (racer 1). Sources: shaded regons. Scheme of a Cube Canyon Unt: backward slash and Street Canyon: forward slash.

2 In all cases the emssons are unform and wth a value of S C = 1 Kg 2 m -6 s -1 correspondng to a emsson (S C /ρ) of 1/1.225 Kgm 3 s -1. hs s a reference value snce we are not nterested n absolute value of tracer concentraton. he nterestng pont s focused on locaton of maxmums and mnmums, relatonshp bet ween emsson nsde a canyon and concentraton nduced n other, dstrbuton of pollutant, etc. of the release of 7 tracers. Each tracer s only emtted from one unt (tracer 1 from unt 1, tracer 2 from unt 2, ). 3. NOAION o smplfy the expressons used n the followng a new notaton s ntroduced. C u ) represents total concentraton of tracer 8 ( (emssons n the whole array regon) n ponts whch belongs to unt and C ( u ) ndcates the same but for the tracer j. In addton, SC ndcates street canyon number, n other words, the zone between the cubes number and +1 (see Fgure 2b). Also, we use brackets (<> ) to ndcate an average value over the volume of ar nsde a street canyon 4. RESULS 4.1 Emssons n the whole array regon (racer 8) Fgure 3. otal concentraton (tracer 8) at Z/H=0.25, 0.5, 0.75 and 1. Maxmum values of concentraton, as expected (source close to the ground), are located at lower regon (Z/H=0.25), and decrease wth heght. At Z/H=0.25 the maxmum nsde each unt are outsde of canyons. hs fact s explaned by the flow pattern whch near the street bottom s outward. Near the downwnd cube n each canyon the flow s downward and nward n the upper regon and outward, as commented above, n the lower regon. he downward moton of clean ar creates a zone wth low concentraton close to the downwnd cubes (Fgure 3). For further detals of flow pattern see presentaton J5.1 n the Sxth Symposum on the Urban Envronment. Hghest values of concentraton are located n the last unts due to pollutant advecton from the upwnd regon. However n the sxth unt, ths behavor s slghtly dfferent because the flow pattern s affected by unt locaton (at the end of array). Due to the same reason, the flow pattern and pollutant dsperson s also dfferent n the frst unt (Fgure 3). 4.2 Emssons nsde each cube canyon unt (racer 1-7) In ths secton, the nfluence of source locaton over concentraton s studed by means Fgure 4. Concentraton produced by one unt emssons (tracer 1, tracer 2, tracer 3, tracer 4, tracer 5 and tracer 6) at Z/H=0.25.

3 Fgure 5. Same as Fgure 4 but for Z/H=0.5. Fgure 6. Same as Fgure 4 but for Z/H=0.75. Followng the notaton above explaned, C ( u ) C ( u ), where N s the = N j= 1 number of unts, s fulflled due to lnearty of transport equaton and the emssons whch are equal n all cases. hs statement has been checked wth the computed results. In all cases the emssons nsde each cube canyon unt only affect the concentraton n downwnd unts (Fgures 4, 5 and 6). he nfluence of emssons n each case can be large nsde the downwnd neghbor unt and nsde further downwnd unts can be non neglgble n certan zones ncreasng ths contrbuton at hgher heghts (Fgures 4, 5 and 6). Concentraton patterns of tracer 1 are very dfferent n comparson wth other cases. he flow nsde the frst unt (hgher wnd velocty and turbulence) affects pollutant dsperson. In ths case relatvely low concentraton appears and at Z/H=0.25 the hghest values are located n two bands from the corners of upwnd cube (Fgure 4). he most smlar patterns are obtaned n the cases of tracer 4 and 5 also ndcatng a smlar flow pattern nsde these unts. 4.3 Average concentraton In ths secton, pollutant concentraton s averaged over the volume of ar nsde street canyons to search for relatonshps between average concentraton nsde a street canyon and emsson locatons. We take the cases of the tracer 4 and 5 as the most representatves due to these flow and concentraton patterns are very smlar between them. he average total concentraton (tracer 8) ncreases wth the number of street canyon (n downwnd drecton). Average concentraton s hgher n a street canyon than n the prevous one, but the dfference of concentraton between them s less n the last street canyons (Fgure 7a) ( SC ) > - ) > < ( ( SC 2 ) > - < ) > ( SC 1 ( SC 1 C ). here are more upwnd emttng unts n the last street canyons. he sxth street canyon s the excepton and t does not follow ths tendency due to ts locaton, at the end of the array, affects tracer dsperson. o study the contrbuton of unt emssons n average concentraton nsde a street canyon, we select the fourth and ffth street canyons and compare average concentraton nsde them produced by the dfferent tracers.

4 RACER ( SC ( SC ) > ) > (wth j from 1 to ) s ftted for the 4 th and 5 th street canyon to a functon as B X + A, where X s ( j). Fttng gves A = 0.1 and B = 1.1 wth a correlaton coeffcent of 0.99 n both cases (Fgure 7b and 7c). he fttng functon gves a smple relatonshp between pollutant source locaton and average concentraton nsde a street canyon,.e. the nfluence of emssons of other areas (or unts) n the selected street canyon. levels they are stuated outsde of street canyons. hese locatons are due to flow patterns whch s outwards n ths regon. In addton, lower concentraton zones are created at downwnd face of street canyons. otal tracer concentraton n the last unts s hgher due to an accumulatve effect, more upwnd emttng unts contrbute to total concentraton. Insde a selected unt, the emssons located just upwnd can notably contrbute to total concentraton whle the downwnd unt emssons are neglgble. A smple relatonshp between average concentraton nsde a street canyon and emsson locatons has been proposed. 6. ACKNOWLEDGEMENS he authors wsh to thanks CIEMA for doctoral fellowshp held by J.L. Santago. REFERENCES Bak, J.-J., and Km, J.-J.: 2002, On the escape of pollutant from urban street canyons, Atmos. Envron. 36, Brown, M.J., Lawson, R.E., DeCrox, D.S., and Lee, R. L.: 2001, Comparson of centrelne velocty measurements obtaned around 2D and 3D buldngs arrays n a wnd tunnel, Report LA- UR , Los Alamos Natonal Laboratory, Los Alamos. Fluent Inc.: 2005, FLUEN 6.2 User s Gude, Volumes 1-3, Fluent Inc., Lebanon. Fgure 7. a) Average total street canyon nsde each street canyon. b) Average concentraton due to ndvdual unt emssons (tracer j) nsde the 4 th street canyon normalsed by average concentraton of tracer 4 nsde the same street canyon ( functon ( B X + A RACER4 for the 5 th street canyon. 5. CONCLUSIONS > ( SC 4 > ( SC 4 ) ) ). Also, fttng ) s plotted. c) Same as b but Pollutant dsperson s determned by flow patterns. Hghest concentraton values appear at lower levels and decrease wth heght. At lower Meroney, R.N., Pavegeau, M., Rafalds, S., and Schatzmann, M.: 1996, Study of lne source characterstcs for 2-D physcal modellng of pollutant dsperson n street canyons, J. Wnd Eng. Indust. Aero. 62, Rotach, M.W.: 1995, Profles of turbulence statstcs n and above an urban street canyon, Atmos. Envron. 29, Santago, J.L., Martll, A., and Martn, F.: 2005, Valdaton of CFD smulaton of turbulent ar flow over a regular array of cubes aganst wnd tunnel data and a 3-D analyss f the flow n the Sxth Symposum on the Urban Envronment, Atlanta, Georga, USA. Santago, J.L., and Martn, F.: 2005, Modellng the ar flow n symmetrc and asymmetrc street canyons, Int. J. Envron. Pollut. 25, Sn, J-F., Anquetn, S. and Mestayer, P.G.: 1996, Pollutant dsperson and thermal effects n

5 urban street canyons, Atmos. Envron. 30, Vachon, G., Louka, P., Rosant, J-M., Mestayer, P., and Sn, J-F.: 2001, Measurements of traffc-nduced turbulence wthn a street canyon durng the Nantes 99 experment n hrd Internatonal Conference on Urban Ar Qualty, Loutrak, Greece.