Simulation of Bio-Agent Release in a Room or Offi ce Space

Transcription

1 Simulaion of Bio-Agen Releae in a Room or Offi ce Space Sephen M. Scorpio, Rober P. Roger, and Alan Brand Indoor oxic agen releae ha become a poibiliy of grea concern. Thi aricle preen a numerical fl ow-field diperion model ued o predic he paial and emporal diribuion of oxin in an encloed area. The approach i baed on aumpion peraining o he enrainmen of mall pariculae (wih properie imilar o hoe of anhrax pore) ino he circulaion airfl ow in a ypical offi ce. The model imulae he enrainmen of pore ino he urbulen boundary layer and ubequen ranpor and diperion wihin and ou of he room. For he illuraive example given here, i wa found ha pore would dipere hrough he room and ino he hallway in approximaely 5 o 10 min. ITRODUCTIO Today, here i an increaed naional awarene of he poibiliy of oxic agen releae in inerior building pace. Of pecific inere, owing o recen even, i he pread of anhrax from conaminaed leer ino he office circulaion airflow. One key apec for aeing he effec of uch releae and for planning miigaion and prevenion raegie i he paern of pread wihin room and building. Thi aricle decribe an APLdeveloped imulaion ool ha predic he propagaion and diperion of pariculae hrough inernal building pace. In addiion, he model poulae a mechanim by which pariculae, iniially depoied on a horizonal urface, are enrained ino he ambien air curren. An analyi of he paial and emporal diribuion of a oxic releae in a ypical room/office ha been performed o illurae he applicabiliy of numerical flowfield diperion calculaion. Thee calculaion provide a bai for udie of agen deecion, miigaion, enor repone and placemen, venilaion raegie, and evacuaion procedure. The analyi wa baed on aumpion concerning he enrainmen of mall pariculae (wih properie imilar o hoe of anhrax pore) ino he circulaion airflow in a ypical office. The udy repored in hi aricle provide he mehodology for uch a imulaion, which i illuraed by an example of a hypoheical bio-agen releae. HYPOTHETICAL SCEARIO The analyi wa baed on he following cenario. Some anhrax pore have been depoied on a ableop. The ma and diribuion of pore on he able i iniially known. The pore are expoed o airflow in he room generaed by he venilaion yem. Depending on he flow condiion, ome of he pore may lif off he ableop and propagae hrough he room. The problem i hen o deermine he rae a which pore 376 JOHS HOPKIS APL TECHICAL DIGEST, VOLUME 24, UMBER 4 (2003)

2 SIMULATIO OF BIO-AGET RELEASE will be lifed from he ableop and propagae hroughou he room. Anhrax pore are roughly pherical and 2 6 m in diameer, 1 wih a ma of 1 pg (peronal communicaion, A. Scorpio, U.S. Army Medical Reearch Iniue of Infeciou Dieae, F. Derick, MD, Jan 2002) a hown in Fig. 1. A an example problem, he following parameer are ued: One gram of pore i evenly diribued on a cm area of he ableop. The average ma of one pore = 1 pg. The average diameer of one pore = 5 m. The volume of air occupied by pore on he able V = 10 cm 10 cm ( cm) = 0.05 cm 3. The number of pore on he able 0 = Ambien airfl ow ouide he boundary layer U = 1 cm/. The pach of pore i 100 cm from he edge of he able (he ar of he boundary layer profi le). MODELIG AD SIMULATIO Two apec of he problem are conidered: he enrainmen of pore from he ableop ino he ambien room airflow, and ranpor of he enrained pore wihin and ou of he room. The former require conideraion of enrainmen ino he air boundary layer along he able, and analyi i baed on a urbulen boundary layer model. Flow wihin he room i modeled uing a commercial hree-dimenional, compuaional fluid dynamic (CFD) numerical model wih a ypical officelike layou and airflow rae. The exac mechanim or mechanim by which mall pariculae would be lifed off and enrained ino he ambien airflow i no known wih cerainy. Three fundamenal mechanim were conidered: urbulen diffuion ino he boundary layer, lifoff due o mean verical velociy in he boundary ublayer, and velociy impared o he paricle by near-wall urbulen flucuaion. Only he hird mechanim proved o be ufficien o enrain paricle of he ize conidered. Oher mechanim relaed o wall roughne, horizonal paricle moion, paricle hape, and elecroaic and oher adheion/ repulion force require furher inveigaion. ear-wall Turbulen Flucuaion The ambien airflow over he able i defi ned a U. The pore are expoed o he urbulen boundary layer flow near he able urface. I i hypoheized ha anhrax pore are lifed off he able a a rae proporional o he verical urbulen flucuaion in he boundary layer. In a aiical ene, he roo mean quare (rm) verical velociy will be direced oward or away from he able and hu will hold down half he pore remaining on he able while he oher half are lifed off he able. The volume flux hrough an area A ju above he ableop i hen Q= v rm A 2 where v rm i he roo mean quare verical velociy flucuaion in he boundary layer. Ma Concenraion of Spore on he Table I i aumed ha he iniial ma of pore m i known and i evenly diribued over he area A. Le 0 be he oal number of pore iniially on he able o ha, on average, he ma of a ingle pore i mˆ = m /. Thu for he condiion pecified above, 0 1 g of pore i equivalen o individual pore. The pore are aced on by he verical urbulen velociy flucuaion. Auming he pore are lifed off he able a a rae proporional o he number of pore remaining on he able, hen, = k. Here, k i a conan ha e he rae a which he number of pore remaining on he able decreae, i he number of pore remaining on he able, and i ime. The rae a which pore are enrained ino he room airflow i = k e 0 k. Figure 1. Anhrax pore a 1000X magnifi caion. (Phoograph courey of A. Scorpio, U.S. Army Medical Reearch Iniue of Infeciou Dieae.) Ma Flux of Spore Enering he Flow The pore are iniially diribued uniformly over a cm area on he ableop. The air immediaely JOHS HOPKIS APL TECHICAL DIGEST, VOLUME 24, UMBER 4 (2003) 377

3 above he able will conain pore ha have become airborne. The ma flux of pore enering he airflow i &m k = ˆ mk ˆ e. = m 0 The conan k i pecified uing he urbulen boundary layer equaion 2 yielding Conaminaed dek area m 2 Ceiling inflow ven m m 2 1 m 2 &m mv ˆ rm = 2D v rm 0 2D where D i he diameer of a ingle pore. Thee reul, epecially he ime cale for enrainmen, depend criically on he aumpion ha half he paricle remaining on he able are lifed off he urface a each ucceive ime. I i, however, expeced ha oher facor would end o caue a omewha larger fracion of he paricle o adhere o he urface, increaing he releae ime coniderably. Furher reearch i required o udy he deail of pariculae enrainmen ino he boundary layer. Model for Ambien Airflow and Propagaion of Spore A ypical office-ype room i conidered. A hown in Fig. 2, here are hree dek and a bookcae. The air ener hrough he ven on he ceiling and exi hrough he fully open door. A ypical office airflow rae of 5.8 cf i ued, equivalen o a room fluhing rae of abou 300 ime per hour. Three poiion of inere are idenified. Poiion 1 and 2 repreen people working a heir dek. Poiion 1 i a he dek oppoie he conaminaed dek, poiion 2 i a he dek near he door, and poiion 3 i in he open doorway. Conaminan concenraion are ampled a hee hree poin. The flow field analyi wa performed on an SGI Origin 2000 workaion uing a ingle 400-MHz R12000 CPU and CFD2000, a commercial CFD ofware package. 3 A ypical run ime required 6 day (coninuou) for our example problem. The calculaion erminae afer 12 min of imulaed flow. The ambien airflow paern are preened in Fig. 3. The reak line hown are colored by flow peed, ha i, red = fa and blue = low. Wih he ambien airflow eablihed, maerial i releaed from a cm quare area on one of he dek for 12 min, he imulaed ime for he cae conidered. e, Poiion 1 RESULTS 5.5 m Room Diperion Figure 4 illurae he compued conaminan concenraion line conour afer coninuou releae for he 12-min period. Concenraion conour are hown in a cro plane hrough he releae area and he door, wih an enlargemen over he able releae area. Spore Diribuion Given he pore concenraion x from he CFD compuaion, he pore number deniy i n x = 3 pore/cm. mˆ Poiion 2 Poiion m Figure 2. Room geomery ued in a model for ambien airfl ow and he propagaion of pore. Figure 5 how he number deniy of pore a he hree poiion in he room. Poiion 1 and 2 correpond o in-room locaion a wo of he dek in Fig. 2. A hee locaion here would be 4 6 min Figure 3. Two view of eady-ae room fl ow paern prior o conaminan releae; reak line are colored by velociy magniude: red = fa, blue = low. 378 JOHS HOPKIS APL TECHICAL DIGEST, VOLUME 24, UMBER 4 (2003)

4 SIMULATIO OF BIO-AGET RELEASE Figure 4. Conaminan concenraion conour afer coninuou releae for 12 min in a plane along he room and a he releae area (kg/m 3 ). before he pore arrived. A he door, Poiion 3, he pore would arrive afer 7 min. Thi analyi indicae he ime elaped prior o expoure and ubequen level of expoure. Such reul would be ueful in planning enor placemen and evacuaion planning. umber deniy (pore/cm 3 ) Poiion 1 Poiion 2 Poiion 3 SUMMARY I ha been hown ha he ue of boundary layer modeling and CFD can be ued o imulae he diperion of pariculae wihin a room. Coupled wih enor cenario, doage experienced by occupan a a funcion of ime can be eimaed. Thi diperion model procedure can be ued a he bai for a oxic agen building imulaion and analyi and for deign of miigaion and evacuaion raegie. Moreover, he approach decribed here can be exended o oher oxic agen (biological and chemical) and o oher room and building configuraion (e.g., a erie of room in a HVAC zone or an audiorium). For he illuraive problem preened here, he expoure level wa on he order of 100 pore/cm 3 of air (or 10 5 pore per lier). A doe of anhrax ha i lehal o 50% of he expoed populaion (LD50) i abou 50,000 pore. For an adul wih a breahing idal volume of 0.5 L, he expoure would be 1 LD50 per breah. The implicaion of hi analyi i ha a ingle gram of pore depoied on a dekop can reul in lehal doe reaching he doorway in a lile a 10 min. REFERECES 1 Cielak, T. J., and Eizen, E. M., Clinical and Epidemiologic Principle of Anhrax, Emerging Infeciou Dieae 5(4) (Jul Aug 1999), available a hp:// (acceed 19 Aug 2003). 2 Schliching, H., Boundary-Layer Theory, Sixh Ed., McGraw-Hill Book Company (1968). 3 CFD2000 Verion 3.0 Uer Guide, Adapive Reearch, a Diviion of Pacific Sierra Reearch Corporaion, Sana Monica, CA (1998) Time (min) Figure 5. Spore deniy number a he hree poiion in Fig. 2. ACKOWLEDGMET: Thi work wa funded under he Chem-Bio buine area inernal reearch and developmen program managed by Harvey Ko. THE AUTHORS STEPHE M. SCORPIO received hi B.S.E., M.S.E., and Ph.D. in naval archiecure a well a an M.S.E. in mechanical engineering, all from he Univeriy of Michigan. Dr. Scorpio reearch inere are in naval hydrodynamic and more generally in numerical and experimenal mehod in fl uid mechanic. Hi addre i ephen.corpio@jhuapl.edu. JOHS HOPKIS APL TECHICAL DIGEST, VOLUME 24, UMBER 4 (2003) 379

5 ROBERT P. ROGER i a member of APL Principal Profeional Saff. He received a B.S. in phyic from Loyola Univeriy of he Souh in 1964 and a Ph.D. in phyic from Louiiana Sae Univeriy (LSU) in For 15 year Dr. Roger wa a profeor of phyic a orhweern Sae Univeriy of Louiiana and LSU. From 1984 o 1997 he worked for Lockheed Miile and Space Company and Teledyne Brown Engineering. He joined APL in 1997 and currenly work in he Mechanical and Aeronauical Engineering Group of he Air Defene Syem Deparmen. Hi pecialy i fl uid mechanic and in paricular compuaional fl uid dynamic echnique for fl ow of all kind. Dr. Roger ha conribued o many refereed journal aricle and unrefereed conference paper and ha made numerou preenaion a profeional meeing. He i an acive member of APS and AIAA. Hi addre i rober.roger@jhuapl.edu. ALA BRADT received a Ph.D. in civil engineering from Carnegie Mellon Univeriy in Dr. Brand i a Principal Profeional Saff cieni a APL, where hi echnical area of inere include fl uid dynamic and phyical oceanography, pecifically urface and uburface hydrodynamic relaed o naval iue. Hi addre i alan.brand@jhuapl.edu. 380 JOHS HOPKIS APL TECHICAL DIGEST, VOLUME 24, UMBER 4 (2003)