J1.8 APPLICATION OF CFD SIMULATIONS FOR SHORT-RANGE ATMOSPHERIC DISPERSION OVER OPEN FIELDS AND WITHIN ARRAYS OF BUILDINGS

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1 AMS th Jont Conference on the Applcatons of Ar Pollton Meteorology wth the A&WMA, Atlanta, GA, Jan - Feb, 6. J.8 APPLICATION OF CFD SIMULATIONS FOR SHORT-RANGE ATMOSPHERIC DISPERSION OVER OPEN FIELDS AND WITHIN ARRAYS OF BUILDINGS We Tang Natonal Research Concl Research Assocate at Natonal Exposre Research Laboratory, US Envronmental Protecton Agency, RTP, NC Alan Hber Atmospherc Scences Modelng Dvson, n partnershp wth US EPA Natonal Exposre Research Laboratory, RTP, NC Bran Bell, and Walter Schwar Flent, Inc., Hanover, NH. INTRODUCTION Comptatonal Fld Dynamcs (CFD) technqes are ncreasngly beng appled to ar qalty modelng of short-range dsperson, especally the flow and dsperson arond bldngs and other geometrcally complex strctres. The proper applcaton and accracy of sch CFD technqes needs to be assessed. Flent, Inc and the US EPA Natonal Exposre Research Laboratory are workng cooperatvely to demonstrate CFD smlaton as a proven and appled tool n spport of envronmental assessment stdes (Hber et al., ). Part of the ongong work has focsed on the development of compter models and the evalaton of the performance of the FLUENT code n smlatng: () the atmospherc bondary layer, () plme dsperson over an open feld, and () dsperson wthn arrays of bldngs. FLUENT Verson 6. was sed for ths stdy. The FLUENT code solves conservaton eqatons for mass, momentm and energy. In addton, FLUENT models the mxng and transport of a chemcal speces by solvng conservaton eqatons descrbng convecton and dffson for the speces. For trblent flows, the Reynoldsaveraged approach s employed to solve the Naver-Stokes eqatons. A nmber of trblence models are provded n FLUENT to acheve closre. In ths stdy, we examned reslts sng the standard k-ε trblence model (Lander and Spaldng, 97). The goal s to develop and demonstrate applcatons for applyng steady-state Reynolds-averaged Naver-Stokes eqatons Correspondng athor address: We Tang, NERL/AMD, Mal drop E-, US EPA, RTP, NC 77; emal: tang.we@epa.gov (RANS) wth k-ε trblence model snce t s practcal for rotne applcatons today. If necessary, ftre developments wll contne on to nsteady soltons sng hgher order trblence models. A fll descrpton of the code defalts and optons s provded n the FLUENT User s Gde (Flent Inc, 5). Case stdes based on the Project Prare Grass feld program (Barad, 958) were sed to develop and evalate CFD smlatons of plme dsperson over an open feld nder thermally netral and nstable condtons. CFD predctons of arc concentratons were compared wth and reslts of the dsperson model (Cmorell, et al., 5). Analyses for the near thermally netral cases have been completed and are smmared heren. Analyses for the thermally nstable cases are ongong therefore only a case stdy s presented. Smlatons of dsperson arond bldngs are beng evalated wth data from the Mock Urban Settng Test (MUST) feld experment (Bltoft, ). Methods for applyng CFD smlatons for these complex flow statons are beng developed. Only the setp for a case s presented heren. Progress s ongong and pdated reslts wll be smmared n the oral presentaton.. SIMULATION OF PROJECT PRAIRIE GRASS Whle the prmary nterest n applcaton of CFD methods s to smlate flow arond bldngs, t s mportant to demonstrate the ablty of the CFD code to correctly model a plme n absence of any bldngs, where the flow s relatvely smple and well-defned. If there are problems wthn the code, they can be more easly dentfed and resolved.

2 The Project Prare Grass feld stdy (Barad, 958) ncldes 7 experments wth contnos releases of netrally boyant tracer gas over an open agrcltral feld. The stdy provdes a good test bed for developng the CFD applcatons and assessng the code s performance. Ths feld stdy has rotnely been sed for evalatng atmospherc dsperson models. In part, ths database s the bass for development of the wdely appled Gassan straght-lne plme models. By comparng reslts of the CFD smlatons wth the Prare Grass data, we can demonstrate how the CFD model compares wth other types of dsperson models. The goal here s not to demonstrate that CFD smlaton shold be rotnely sed where applcatons of Gassan straght-lne plme models have been fond to work. Instead we are tryng to demonstrate comparablty as the bass for applyng CFD smlatons to more complex thermal envronments and where there are sgnfcant bldng nflences. The CFD smlaton needs to be developed for the many applcatons where the Gassan straght-lne plme models do not perform as well.. Model Methodology Smlaton of the atmospherc bondary layer s crtcal to modelng plme dsperson. Good smlaton of blk transport reqres that the mean flow feld be well modeled, whle predctng the rght level of trblence s essental to captrng the trblent dsperson of a polltant. Usng FLUENT, a two-dmensonal (D) doman (x, ; wnd along x, vertcal drecton ) was sed to smlate a well-developed bondary layer. Vertcal profles were generated for pressre, temperatre, mean velocty (U), trblent knetc energy (TKE), and trblent dsspaton rate (ε). Reqred npts nclde frcton velocty, roghness heght and mass flow rate. The generated bondary layer profles were then sed as the nlet profles for the dsperson smlaton n a three-dmensonal (D) fll doman. Plme dsperson was smlated sng the speces transport modelng capablty n FLUENT. An mportant model parameter s the trblent Schmdt nmber (Sc), snce t characteres the relatve dffson of momentm and polltant mass de to trblence: µ t Sc = ρdt where µ t s the trblent vscosty and D t s the trblent dffsvty. Vales of Sc from.8 to. have been reported based on feld observatons nder dfferent atmospherc stablty and wnd condtons (Flesch, ). Three vales (.7,., and.) of Sc were examned here. In ths work, all smlatons were rn nder steadystate condtons assmng constant wnd speed and wnd drecton. These steady soltons were then smoothed over the tme weghted dstrbton of wnd drecton flctaton to accont for the stochastc varaton of wnd drecton. More detals can be fond n Tang et al. (5). The followng sectons dscss the reslts of smlated bondary layer flows and plme dsperson for the Project Prare Grass condtons.. Near Netral Stablty.. Bondary profle Drng the Project Prare Grass, experments can be classfed as near thermally netral. A smmary of the rns can be fond n Tang et al. (5). To demonstrate that a sngle CFD rn can be appled for a gven class of thermal stablty and ths redce overall comptaton tme, a generc case was set p wth npt data determned by averagng meteorologcal data of the near netral rns. The mass flow rate was set to a vale eqvalent to a wnd speed of 7. m/s at m above the grond (U m ). Average was. m/s, and average was. m, whch are smlar to vales reported by other stdes (Brggs, 98; van Ulden, 978; Venkatram, 98). In Fgre, the vertcal profle of U normaled by U m s presented for both the CFD smlaton and Heght, m.5.5 U/Um rn rn rn rn rn7 rn8 rn rn6 rn55 rn56 rn67 rn5s CFD Fgre. Comparson of modeled and measred vertcal wnd profles

3 the observatons drng the rns. Regardless of the varablty among the rns; the agreement between modeled and measred profles s reasonably good. The modeled vales of U are wthn ±5% of the. The dfferences are small and we do not expect sgnfcantly mproved reslts by settng a CFD rn for each case. The vertcal profle of TKE s presented n Fgre for the generc case and compared wth estmates based on smlarty theory. Accordng to Stll (988), for a near netral bondary layer, TKE can be calclated as follows: ' v' w' = A = Av = TKE = / ( A + / where represents the mxng heght whch here s taken as the depth of the CFD model bondary layer. A and A v are emprcal constants. A = A + A v. A range of vales were examned for A and A v at.5 6. Plotted n Fgre are profles for A = 5 and A = 9, where 9 represents the vale reported from feld stdes by Stll (988). The CFD modeled TKE profle falls between the two crves for the lower part of the bondary layer. The crtcal one for ths stdy s wthn the lower m. For ftre stdes where the crtcal one extends Heght, m TKE, m^/s^ ) CFD modeled Theory, A=5 Theory, A=9 Fgre. Vertcal profle of smlated TKE deeper nto the bondary layer, we can modfy or technqe to reslt n a deeper matchng one... Plme concentratons Ratos of observed centerlne concentratons (C o ) to model predctons (C p ) are plotted n Fgre for 8 ot of the netral rns. The other rns are not nclded becase wde varaton of wnd drecton was observed and ther arc concentratons appear to have more than one peak vale. As shown n Fgre, all ratos are wthn the two horontal thn lnes, demonstratng that the model predctons of centerlne concentratons were well wthn a factor of two of the observatons. The colored lnes represent average vales of C o / C p for the 8 rns sng dfferent Schmdt nmbers. It appears that on average, predctons sng a Sc of. agree best wth the observatons. The scattered ponts, representng ratos for each rn, are based on smlatons wth Sc =.. Ratos for each rn wth Sc =.7 and., not shown here, wold appear smlarly scattered abot ther plotted averages. As demonstrated here, t s dffclt to assgn a sngle Sc nmber to charactere dsperson n varos atmospherc bondary condtons. Flesch et al. () reported smlar observatons from a tracer experment. Nevertheless, we fond that smlatons wth Sc n the range of. to. performed best n matchng the of the Project Prare Grass. Co/Cp.5 5m arc m arc m arc m arc 8m arc Fgre : Rato of measred and modeled centerlne concentraton at 5 arcs In addton to centerlne concentratons, comparsons were condcted for the crosswnd concentraton profles at all arcs and the avalable vertcal concentraton profles at the m arc. Fgre presents example comparsons of the crosswnd profles of normaled concentratons at rn rn rn rn rn8 rn55 rn56 rn5s Sc =.7 Sc =. Sc =.

4 the 5 samplng arcs for rn 55. Also presented here are the model reslts of, whch were acqred from the model evalaton databases developed by the US EPA (5). The CFD-based smlaton of plme spread and centerlne concentratons was fond to match the at all 5 arcs. Smlar reslts have been observed for the other 8 near netral rns wth relatvely constant wnd drecton. In general, smlatons wth Sc =. have the best agreement Normaled Concentratons m CFD Sc=.7 CFD Sc=. CFD Sc=. Normaled Concentratons m CFD Sc=.7 CFD Sc=. CFD Sc=. Normaled Concentratons 7 x 6 5 m CFD Sc=.7 CFD Sc=. CFD Sc=. Normaled Concentratons.5 x.5.5 m CFD Sc=.7 CFD Sc=. CFD Sc=. Normaled Concentratons 8 x m CFD Sc=.7 CFD Sc=. CFD Sc=. Fgre. Comparson of crosswnd profle of arc concentratons for Rn 55 Normaled Concentratons m CFD Sc=.7 CFD Sc=. CFD Sc= Normaled Concentratons m CFD Sc=.7 CFD Sc=. CFD Sc= Normaled Concentratons x m CFD Sc=.7 CFD Sc=. CFD Sc= Normaled Concentratons.8 x m CFD Sc=.7 CFD Sc=. CFD Sc=. Normaled Concentratons 6 x 5 8m CFD Sc=.7 CFD Sc=. CFD Sc= Fgre 5. Comparson of crosswnd profle of arc concentratons for Rn 6

5 wth the, whle at the 5m arc, smlatons wth Sc =. perform best for 5 ot of the 8 rns. As expected, for smple dsperson n an open feld, predctons by are well wthn the range of the. Smlar comparsons are present n Fgre 5 for rn 6, whch has wde varaton of wnd drecton resltng n two peak vales across the arc of concentratons. The CFD-based smlaton s shown to captre both peaks cased by the wnd drecton varaton. Fgre 6 presents the vertcal profles at two locatons on the m arc for rn 55. The vertcal profles were well predcted by the model. Smlar reslts have been observed for other near netral rns. Heght, m Fgre 6: Comparson of vertcal profle at m arc (Sc =.) for Rn 55. Measred s at degrees to the left of the centerlne; measred degrees to the rght of the centerlne.. Unstable Condtons Smlar methodology as descrbed n Secton. s beng appled to case stdes of expermental rns nder nstable condtons drng Project Prare Grass. Some modfcatons were made to accont for the enhanced trblence and to smlate ts effect on dsperson. Extensve model evalaton s ongong. Some prelmnary reslts are presented here, and more pdated reslts wll be reported drng the oral presentaton... Bondary profle centerlne off degrees off degrees measred measred Normaled Concentraton For smlatons of thermally nstable bondary layers, the FLUENT code was modfed so that s TKE generaton cased by boyancy s calclated based on potental temperatre gradent. Net heat flx (hf) was added on the grond, and grond temperatre was set to the measred vale. One case stdy based on rn 8 was set p wth =. m/s, =.8 m, L = -8 m, and U m = 5.9 m/s. By adjstng the amont of heat flx, we can generate a temperatre gradent to match the measred data as shown n Fgre 7a. (a) Vertcal profles of temperatre Heght, m hf= hf= hf= measred Temperatre, K (b) Vertcal profles of mean velocty Heght, m. hf= hf= hf= measred U, m/s (c) Vertcal profles of TKE Heght, m TKE, m^/s^ hf= hf= hf= Theory Netral Fgre 7: Vertcal characteratons of smlated nstable bondary layer wth dfferent heat flx (hf) 5

6 The smlated profles of mean velocty are shown n Fgre 7b, and compared wth the. The best comparson s hf = W/m for temperatre, and hf = W/m for mean velocty. Some fne adjstments on heat flx and mass flow rate are needed to match both measred temperatre and mean velocty wth the same srface bondary condtons. Fgre 7c presents the smlated profles of TKE when dfferent amonts of heat flx are appled at the grond. Also plotted s an estmated profle based on smlarty theory (Stll, 988). The eqatons sed are: ' v' w' w = B = B =.8 TKE = B v /.8 +.8w /.8 where emprcal constants B =, B v =, B = B +B v = 8. The CFD reslts are consstent wth the profle based on smlarty theory, n terms of the heght at whch TKE vale peaks. The profle for hf = W/m matches best wth the theory. There s sgnfcant enhancement of trblence n the thermal bondary layer when compared wth the TKE profle nder near netral condtons... Plme concentratons Fgre 8 presents example comparsons of the crosswnd profles of normaled concentratons at the 5 samplng arcs for rn 8 wth dfferent heat flx added on the grond. Estmates by (US EPA, 5) are also plotted. The CFD reslts are consstent wth reslts, especally when hf = W/m. The degree of agreement vares at the 5 arcs. More case stdes are beng carred ot to mprove the model performance. Schmdt nmber sed for ths example s.7. Other vales need to be examned. Normaled Concentratons m CFD hf= CFD hf= CFD hf= Normaled Concentratons m CFD hf= CFD hf= CFD hf= Normaled Concentratons.5E-.E-.5E-.E- 5.E- m CFD hf= CFD hf= CFD hf=...e Normaled Concentratons 9.E- 8.E- 7.E- 6.E- 5.E-.E-.E-.E-.E- m CFD hf= CFD hf= CFD hf= Normaled Concentratons.E-.5E-.E-.5E-.E- 5.E-5 8m CFD hf= CFD hf= CFD hf=.e+.e Fgre 8. Comparson of crosswnd profle of arc concentratons for Rn 8 6

7 . SIMULATION OF MUST FIELD EXPERIMENT Smlatons of dsperson arond bldngs are beng evalated wth data from the MUST feld experment (Bltoft,, Yee and Bltoft, ). Ths near fll-scale experment was desgned to spport rban dsperson model development and valdaton. The 68 tracer gas releases were condcted n an array of by shppng contaners at the U.S. Army Dgway Provng Grond n Utah. The MUST experment provdes extensve spatal and temporal of dsperson data and meteorologcal data. (a) 6K Mesh (b).m Mesh The same two-step approach as sed n case stdes of Project Prare Grass are beng appled to the MUST experment. Bondary profles are generated sng a D doman and are sed as the nlet bondary profles for smlatons of dsperson wthn arrays of obstacles. A doman of m by m by 5 m was created. The obstacle array, wth overall wdth and length of 9 m and 7 m, s centered on the grond of the doman. Three dfferent volme meshes are beng tested to examne mesh dependency, as shown n Fgre 9. Steady-state solton of the flow feld s frst obtaned sng standard k-ε trblence model. The transent solver s then sed to smlate the release of the tracer gas and the development of the plme. The smlatons are ongong and reslts wll be smmared drng the conference presentaton.. CONCLUSIONS CFD modelng has emerged as a promsng technology for smlatng wnd flow and polltant dsperson, especally n rban mcroenvronments. In order to have confdence n sch models, thorogh evalaton and consderaton of dsperson mechansms are reqred. (c) 6M MESH Fgre 9: Three dfferent meshes for smlaton of MUST Model dsperson n absence of obstacles s a crtcal frst step before applcaton to more complex statons wth flow and dsperson arond bldngs and other geometrcally complex strctres. Case stdes based on the Project Prare Grass feld program were sed to develop and evalate CFD smlatons of plme dsperson over an open feld. Analyss for the near thermally netral cases has been completed. The smlated reslts demonstrated good agreement wth tracer gas release data, n terms of both the whole plme dstrbton and the centerlne concentraton. Work has been extended to thermally nstable wnd 7

8 transport and there s ongong work that mst be completed. Updated reslts wll be provded drng the conference oral presentaton. Smlatons of dsperson arond bldngs are beng evalated wth data from the MUST feld experment whch nvolves obstacle arrays. Smlatons of pollton dsperson wthn obstacle arrays over perods of varyng wnd speed and drecton are more challengng than for the same over open felds. Best methods for applyng CFD smlatons for these complex flow statons are beng developed. Updated reslts wll be provded drng the conference presentaton. Ths paper demonstrates that CFD methods can well smlate the atmosphere-lke bondary layer and plme dsperson over an open feld. Whle atmospherc trblence s known to be nonsotropc, applcatons of k-ε trblence model appears to be sffcent for smlatng plme dsperson. We antcpate Gassan plme models are adeqate for many applcatons wthot complcated bldng nflences. The goal of or ongong research s to demonstrate applcatons to more complex statons where the Gassan plme models lkely fal. Broader se of CFD modelng shold be expected once applcatons can be demonstrated sng establshed standard practces. For an experenced CFD modeler setp of the bondary layer smlaton shold take less than a day followed by to 7 days to set p and rn the specfc case stdy for a small area srrondng an ndstral faclty dependng on the nmber of bldngs nvolved. More experence s needed to establsh good estmates. Ths research s leadng to the development of general gdelnes to spport ftre broad applcaton of CFD code for smlatng short-range atmospherc dsperson of pollton where refned soltons are needed. Dsclamer The research presented here was performed n part nder the Memorandm of Understandng between the U.S. Envronmental Protecton Agency (EPA) and the U.S. Department of Commerce's Natonal Oceanc and Atmospherc Admnstraton (NOAA) and nder agreement nmber DW958. Althogh t has been revewed by EPA and NOAA and approved for pblcaton, t does not necessarly reflect ther polces or vews. REFERENCES: Barad, M.L. (Edtor), 958. Project Prare Grass. A feld program n dffson. Geophyscal Research Paper, No. 59, Vols. I and II, Report AFCRC-TR- 58-5, Ar Force Cambrdge Research Center, Bedford, MA. Bltoft, C.A.,. Cstomer Report for Mock Urban Settng Test, DPG Docment No. WDTC- FR--, West Desert Test Center, U.S. Army Dgway Provng Grond, Dgway, Utah. Brggs, G.A., 98. Smlarty forms for grond- srface-layer dffson. Bondary-Layer sorce Meteorology,, Cmorell, A. J.; Perry, S. G.; Venkatram, A.; Wel, J. C.; Pane, R. J.; Wlson, R. B.; Lee, R. F.; and Peters, W. D. 5: : A dsperson model for ndstral sorce applcatons. Part : General model formlaton and bondary layer characteraton. J. Appl. Meteorol.,, Flesch, T.K.; Preger, J.H.; Hatfeld, J.L.. Trblent Schmdt nmber from a tracer experment. Agrcltral and Forest Meteorology. :99-7. Flent Inc, 5. FLUENT 6. User s Gde. Flent Inc, Lebanon, NH. Hber, A.H.; Tang, W.; Flowe, A.; Bell, B. ; Kehlert, K.; and Schwar, W.,. Development and applcatons of CFD smlatons n spport of ar qalty stdes nvolvng bldngs. th Jont Conference on the Applcatons of Ar Pollton Meteorology wth the Ar & Waste Management Assocaton, (CD-ROM) Vancover, Brtsh Colmba, Canada, Agst -7,. Lander, B.E. and Spaldng, D.B. 97. Lectres n Mathematcal Models of Trblence. Academc Press, London, England. Stll, R.B., 988. An Introdcton to Bondary Layer Meteorology. st edton. Klwer Academc Pblshers. Dordrecht. Tang, W.; Hber, A.H.; Bell, B.; Kehlert, K.; and Schwar, W. Example applcaton of CFD smlatons for short-range atmospherc dsperson over the open felds of project prare grass. The Ar & Waste Management Assocaton 98th Annal Conference and Exhbton, (CD-ROM) Mnneapols, Mnnesota, Jne -, 5. US EPA, 5, webste accessed, htm#aermod 8

9 van Ulden, A.P., 978, Smple estmates for vertcal dffson from sorces near the grond, Atmospherc Envronment. :5 9. Venkatram, A., 98, Estmatng the Monn- Bondary-Layer Obkhov length n the stable bondary layer for dsperson calclatons. Meteorology, 9, 8-85 Yee, E. and Bltoft, C.A.,. Concentraton flctaton n a plme dspersng throgh a reglar array of obstacles. Bondary- Layer Meteorology,,