* Corresponding author address: George E.Bowker, MD-81, USEPA, RTP, NC 27711,USA.

† On assignment to the National ExposureResearch Laboratory, U.S. EnvironmentalProtection Agency.

J5.4 A COMPARISON OF AIRFLOW PATTERNS FROM THE QUIC MODEL AND ANATMOSPHERIC WIND TUNNEL FOR A TWO-DIMENSIONAL BUILDING ARRAY AND A MULTI-CITY

BLOCK REGION NEAR THE WORLD TRADE CENTER SITE.

George E. Bowker* 1, Steven G. Perry †2, and David K. Heist†2

1National Exposure Research Laboratory, US EPA2Atmospheric Sciences Modeling Division, Air Resources Laboratory, NOAA

Research Triangle Park, North Carolina

ABSTRACT

Dispersion of pollutants in densely populatedurban areas is a research area of clear importance.Currently, few numerical tools exist capable ofdescribing airflow and dispersion patterns in thesecomplex regions in a time efficient manner. QUIC,Quick Urban & Industrial Complex, a fast-runningflow and dispersion simulation program has shownpromise in this area. QUIC flow patterns werecompared against two wind tunnel data sets,namely: one for a simple two-dimensional buildingarray; another for a complex group of buildingssurrounding the World Trade Center in lowerManhattan. In both cases, QUIC satisfactorilysimulated the flow patterns depicting channelingand recirculation patterns within particular streetcanyons.

1. INTRODUCTION

In the days and weeks following the collapse ofthe World Trade Center (WTC) buildings the sitesmoldered slowly releasing gases, smoke and fineparticulates (fugitive dust) into the air. Understanding the movement and dispersion ofthese contaminants is important to ourunderstanding of that particular event and tofacilitate our development of tools andtechnologies to help prepare for pollutant releasesin other situations. One such new technology isQUIC version 3.3, an empirically based mass-consistent diagnostic wind field model coupled witha Lagrangian particle based dispersion model.QUIC has shown promise in aiding ourunderstanding of local urban flow and dispersion

(Pardyjak et al. 2004, Pardyjak and Brown 2001). Although still in development, QUIC isincreasingly capable of simulating flow anddispersion in complex urban situations (Bagal etal. 2004). This paper describes a comparison ofvelocity measurements from the QUIC model andthe US EPA atmospheric wind tunnel for twocases: a simple two-dimensional building array;and an extremely complex group of buildingssurrounding the WTC site in lower Manhattan. These two wind tunnel data sets provided theopportunity to compare the performance of theQUIC model for a relatively simple geometry aswell as for an extremely complex urban area.

2. DESCRIPTION OF THE MODELS

The two-dimensional building array and lowerManhattan physical models were constructed atthe US EPA Fluid Modeling Facility and placedinto the atmospheric wind tunnel (cross section of3.7m wide by 2.3m high) with air flowing past at4.23 m/s in the freestream (Snyder 1979). Ultimately, the velocity and turbulence data setsacquired provided a data base for modelcomparisons as well as direct information aboutflow and dispersion patterns in urban areas. Inboth cases, the velocity data were supplementedby extensive flow visualization (smoke) anddispersion estimates based on concentrationmeasurements using hydrocarbon analyzers of acontrolled ethane gas release (3000 cc/min).

Both building models were placed in a typicallogarithmic boundary layer simulating neutralatmospheric conditions. The boundary layer wasgenerated using three triangular Irwin spires(0.34m base width, 2.3m height) and a densearray of 19 mm high by 27 mm square roughnessblocks (Irwin 1981). The simulated roughnessheight (z0) for the approach flow to lowerManhattan was 0.4 m full scale. The boundarylayer for both wind tunnel models wascharacterized by a 0.16 power law exponent witha reference wind speed of 3.0 m/s at a height of15 cm. In the absence of the models, theboundary layer did not change appreciably

Figure 2. (a) A side view of the two-dimensionalbuilding array in place within the wind tunnel. Flow is from left to right (shown by the red arrow)and the buildings extend the entire width of thewind tunnel. (b) A side view of the model in QUIC(notice the ends of the blocks are unbounded). InQUIC, the data were collected along thecenterline.

Figure 2. A view of the WTC site from the westshowing the extent of the modeled region.

through the working section of the wind tunnel. ForQUIC analysis, the input boundary layer wasformulated as a power law, with the sameparameters as the wind tunnel.

A simple two-dimensional building array wasconstructed of plywood using seven rectangularblocks (0.15m by 0.15m by 3.7m spaced 15 cmapart) oriented with the long axis of the buildingsperpendicular to flow (Figure 1a). The blocksspanned the entire width of the wind tunnel. Vertical velocity profiles were measured using apulsed-wire anemometer at three centimeterincrements along a longitudinal section followingthe centerline of the wind tunnel. In QUIC the two-dimensional building array, seven regular rows ofblocks spaced 15 cm apart oriented perpendicularto flow, was simulated with a mesh node present

every 1.25 cm in the model. In contrast to thewind tunnel study, where the blocks extendedacross the entire width of the tunnel, in the QUICsimulation the ends of the blocks were not closedby a boundary wall (Figure 1b). Thus, air wasable to flow laterally along the street canyons andout at the ends of the blocks.

The 1:600 scale model of lower Manhattanconsisted of a circular region with a radius ofabout 2 km centered at a point on Broadway closeto the WTC site. The buildings were fabricatedusing polyurethane foam based on digital buildingheight measurements made available through the

Vexcel Corporation (USA) (Figure 2). Extensivemeasurements of velocity and turbulence weretaken using a laser Doppler anemometer (LDA) atspecific port locations which were distributedthroughout the city, including the regionsurrounding the WTC (Figure 3a). The LDAeliminates the usual airflow disturbance prevalentwhen using probes. For vertical velocity andturbulence profiles, the LDA was mountedvertically below the wind tunnel and aimedthrough glass windows inset in the floor of thetunnel (Heist et al. 2004). Velocity measurementswere made for two wind directions. The windcoming from the southwest and west at 225 and270 degrees, respectively. These are bothcommon wind directions for the New York Cityarea in the fall months.

Due to computer limitations, only a smallregion of lower Manhattan could be modeled inQUIC. This encompassed a several block areaextending from the western edge of the WTC siteeast to City Hall Park, including the Woolworthbuilding (Figure 3b). At 241m the Woolworthbuilding is one of the tallest in lower Manhattan. In

Figure 3. (a) An overhead view of the wind tunnelmodel showing the region simulated within QUIC. North is towards the top of the page. The smallgray circular disks in the floor are locations wherevelocity and turbulence profiles were taken. Thesmall blue circles show locations of surfaceconcentration ports. The small white building inthe green park is City Hall. The uppermost darkbrown building is the Woolworth building. (b) Thearea around the WTC (light gray) modeled in QUICincluding flow streamlines with westerly winds (leftto right) at a height of nine meters above theground. The peak of the Woolworth building isdark red.

QUIC, the resolution of the mesh size was threemeters. The lateral size of the buildings wererounded to an even mesh increment., whileheights were rounded to the nearest meshincrement. The building dimensions were derivedfrom the same database used to build the physicalmodel of lower Manhattan. QUIC requires thatbuildings be orthogonal to the coordinate system. Therefore the buildings were subdivided for themodel into smaller segments. These dimensionswere imported into QUIC with each of the 86 totalbuilding pieces categorized as an individualbuilding. Upwind buildings and other roughnessparameters present in the physical model whichundoubtedly affect airflow patterns could not besimulated in QUIC due to computer limitations.

3. RESULTS

The flow around the two-dimensional buildingarray was essentially skimming flow, passing overthe tops of the blocks once it had accelerated overthe first block. Regions of recirculation werepresent in each of the "canyons" between blocks(Figure 4a) and an upstream “rotor” was foundalong the upwind face of the first block (Figure4b). These flow features were all well representedin the QUIC model. The directions of flow nearlyalways coinciding with those measured in the windtunnel. The flow patterns around the most upwindblock were particularly well described withinQUIC. Additionally, the centers for therecirculation zones within the canyons coincidedwell. QUIC appeared to consistentlyunderestimate the velocities within the canyons. This trend was most likely attributable to the threedimensional nature of the building array used inQUIC which allowed lateral movement of airwithin the canyons to the edges of the array. Thisflow feature was not evident in the wind tunnelsimulations.

The flow in the wind tunnel model of lowerManhattan was complex, with significant areas ofbackflow and blockage around regions of the city(Figures 5a,b). As expected, building location andgeometry obviously influenced flow fields and,thus, dispersion. Recirculation patterns andchanneling down streets also was prevalent. Forexample, for westerly winds, notable channelingdown Church St. (a street near the WTC orientedat 57 degrees to the wind, Figure 3a) was presentat low heights (1.5m) compared to the heights ofthe surrounding buildings (around 60m) (Figure5a). Above the buildings, and in the absence oftall buildings directly upwind, the flow vectorsaligned with the freestream wind direction (Figure

Figure 4. (a) Velocity vectors within the canyonbetween the second and third blocks (gray). (b)Velocity vectors near the upwind surface of thefirst block (gray) for the wind tunnel (black) andQUIC simulation (light green). The gray areaswithin the figures indicate areas where blocks arepresent 150 mm wide by 150 mm tall. Thecanyon between is 150 mm wide. The red vectorat the top shows the direction of the wind, with thelength equal to 3 m/s.

Figure 5. Velocity vectors at a full scale height of(a) 9 m and (b) 54 meters measured using the LDVfrom the wind tunnel model of lower Manhattan. The WTC site is the group of white footprintsbeneath the red vector. The wind (red vector) isfrom the west (270 degrees). The length of the redvector is 3 m/s. The dark blue footprint shows thelocation of the Woolworth building. The top twovelocity vectors show flow channeling Northeast upChurch street, with the flow going with the mainwind once it reaches the tops of the buildings.

5b). Also apparent was flow up the leeward facesof tall buildings. Rapid vertical transport clearlyinfluenced airflow. Also visible was channeling offlow along the eastern side of City Hall Park (thetriangular green area to the east of the Woolworthbuilding, Figure 3a). Generally, QUIC captured themajor aspects of the flow in the area around theWTC site. It captured the channeling down ChurchSt., and the change in flow direction to the north asit passed by St. Paul's Chapel and entered CityHall Park (Figures 3b, 6a, 6b).

4. DISCUSSION

Understanding the nature of air movement inurban areas is critical to understanding of thedispersion of air pollutants. QUIC, was able tocapture many of the major features of flow foundin our wind tunnel simulations suggesting that itmay be a useful tool for describing the dispersal ofpollutants in urban areas. QUIC adequatelydescribed: abrupt changes in flow direction uponencountering bluff buildings; channeling of flowdown streets; and recirculation patterns within

Figure 6. Velocity vectors (dark blue) created byQUIC for the westerly wind direction (270) at afull scale height of (a) 9 m and (b) 54 meters forthe area of lower Manhattan near the WTC. Filled polygons show building locations. TheWTC site is not shown, but would be to the lowerleft of the figures. The dark red box footprintshows the location of the Woolworth building.Vectors are shown every six meters and arescaled up by 1.5 in length.

street canyons. While flow patterns nearly alwaysappeared to coincide between the wind tunnel andQUIC, the velocity magnitudes often differed.

For the lower Manhattan simulation, it is likelythat buildings upwind of the WTC site that were notincluded in the QUIC simulation, account for someof the differences in flow between QUIC and thewind tunnel. However, other differences may bedue to flow features not being incorporated into theQUIC model. For example, the flow phenomenonof rapid vertical dispersion, where street-levelpollutants are transported vertically up thedownwind faces of some tall buildings, was clearlyevident in the physical model but was less well

characterized by the QUIC model. In the lee ofsome of the tall buildings near the WTC, thevelocities predicted by QUIC were not as strong orwere in different directions from the vertical flowpatterns that were measured in the wind tunnel.

Although not shown in this paper, for both thewind tunnel models and QUIC, smokevisualizations and tracer measurementssuggested that dispersion appeared to stronglydepend on the local wind fields created by thesurrounding buildings. Notable regions of plumechanneling down streets and recirculation withinstreet canyons were evident. Initial dispersionappeared to strongly depend on source placementwith respect to the surrounding buildings, localflow patterns, and the general wind direction.

Based on the results presented here, QUICshows great promise in describing the intricatenature of urban flow. However, the complexitiesof flow and dispersion in urban areas continue torequire diligent effort in model design as well asthe creation of suitable data sets for comparison. Further comparisons with existing EPA data setswill allow greater examination of the flow anddispersion modeling capability of the QUIC model.

DISCLAIMER

This paper has been reviewed in accordancewith the United States Environmental ProtectionAgency's peer and administrative review policies and approved for presentation andpublication.

ACKNOWLEDGMENTS

We gratefully acknowledge the contributionsto this project by Dr. Bruno Pagnani, Mr. RogerThompson, Dr. William Snyder, and Mr. RobertLawson. We also acknowledge with thanks thegenerous support of Messrs. Ashok Patel andJohn Rose.

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