George E. Bowker*1, Steven G. Perryt2, and David K. Heistt2
1 National Exposure Research Laboratory, US EPA	PB2004-107206
2Atmospheric Sciences Modeling Division, Air Resources Laboratory, NOAA
Research Triangle Park, North Carolina	EPA/600/A-04/083
Dispersion of pollutants in densely populated
urban areas is a research area of clear importance.
Currently, few numerical tools exist capable of
describing airflow and dispersion patterns in these
complex regions in a time efficient manner. QUIC,
Quick Urban & Industrial Complex, a fast-running
flow and dispersion simulation program has shown
promise in this area. QUIC flow patterns were
compared against two wind tunnel data sets,
namely: one for a simple two-dimensional building
array; another for a complex group of buildings
surrounding the World Trade Center in lower
Manhattan. In both cases, QUIC satisfactorily
simulated the flow patterns depicting channeling
and recirculation patterns within particular street
In the days and weeks following the collapse of
the World Trade Center (WTC) buildings the site
smoldered slowly releasing gases, smoke and fine
particulates (fugitive dust) into the air.
Understanding the movement and dispersion of
these contaminants is important to our
understanding of that particular event and to
facilitate our development of tools and technologies
to help prepare for pollutant releases in other
situations. One such new technology is QUIC
version 3.3, an empirically based mass-consistent
diagnostic wind field model coupled with a
Lagrangian particle based dispersion model. QUIC
has shown promise in aiding our understanding of
local urban flow and dispersion (Pardyjak et al.
2004, Pardyjak and Brown 2001). Although still in
* Corresponding author address: George E.
Bowker, MD-81, USEPA, RTP, NC 27711, USA.
f On assignment to the National Exposure
Research Laboratory, U.S. Environmental
Protection Agency.
development, QUIC is increasingly capable of
simulating flow and dispersion in complex urban
situations (Bagal et al. 2004). This paper
describes a comparison of velocity measurements
from the QUIC model and the US EPA
atmospheric wind tunnel for two cases: a simple
two-dimensional building array; and an extremely
complex group of buildings surrounding the WTC
site in lower Manhattan. These two wind tunnel
data sets provided the opportunity to compare the
performance of the QUIC model for a relatively
simple geometry as well as for an extremely
complex urban area.
The two-dimensional building array and lower
Manhattan physical models were constructed at
the US EPA Fluid Modeling Facility and placed into
the atmospheric wind tunnel (cross section of 3.7m
wide by 2.3m high) with air flowing past at 4.23 m/s
in the freestream (Snyder 1979). Ultimately, the
velocity and turbulence data sets acquired
provided a data base for model comparisons as
well as direct information about flow and dispersion
patterns in urban areas. In both cases, the velocity
data were supplemented by extensive flow
visualization (smoke) and dispersion estimates
based on concentration measurements using
hydrocarbon analyzers of a controlled ethane gas
release (3000 cc/min).
Both building models were placed in a typical
logarithmic boundary layer simulating neutral
atmospheric conditions. The boundary layer was
generated using three triangular Irwin spires
(0.34m base width, 2.3m height) and a dense array
of 19 mm high by 27 mm square roughness blocks
(Irwin 1981). The simulated roughness height (z0)
for the approach flow to lower Manhattan was 0.4
m full scale. The boundary layer for both wind
tunnel models was characterized by a 0.16 power
law exponent with a reference wind speed of 3.0
m/s at a height of 15 cm. In the absence of the
models, the boundary layer did not change

Figure 2. (a) A side view of the two-dimensional
building 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 the wind
tunnel, (b) A side view of the model in QUiC
(notice the ends of the blocks are unbounded), in
QUIC, the data were collected along the
appreciably through the working section of the wind
tunnel. For QUIC analysis, the input boundary layer
was formulated as a power law, with the same
parameters as the wind tunnel.
A simple two-dimensional building array was
constructed of plywood using seven rectangular
blocks (0.15m by 0.15m by 3.7m spaced 15 cm
apart) oriented with the long axis of the buildings
perpendicular to flow (Figure 1a). The blocks
spanned the entire width of the wind tunnel.
Vertical velocity profiles were measured using a
pulsed-wire anemometer at three centimeter
increments along a longitudinal section following the
centerline of the wind tunnel. In QUIC the two-
dimensional building array, seven regular rows of
blocks spaced 15 cm apart oriented perpendicular
to flow, was simulated with a mesh node present
every 1.25 cm in the model. In contrast to the wind
tunnel study, where the blocks extended across
the entire width of the tunnel, in the QUIC
simulation the ends of the blocks were not closed
by a boundary wall (Figure 1b). Thus, air was able
to flow laterally along the street canyons and out at
the ends of the blocks.
The 1:600 scale model of lower Manhattan
consisted of a circular region with a radius of about
2 km centered at a point on Broadway close to the
WTC site. The buildings were fabricated using
polyurethane foam based on digital building height
measurements made available through the Vexcel
Figure 2. A view of the WTC site from the west
showing the extent of the modeled region.
Corporation (USA) (Figure 2). Extensive
measurements of velocity and turbulence were
taken using a laser Doppler anemometer (LDA) at
specific port locations which were distributed
throughout the city, including the region
surrounding the WTC (Figure 3a). The LDA
eliminates the usual airflow disturbance prevalent
when using probes. For vertical velocity and
turbulence profiles, the LDA was mounted
vertically below the wind tunnel and aimed through
glass windows inset in the floor of the tunnel (Heist
et ai. 2004). Velocity measurements were made for
two wind directions. The wind coming from the
southwest and west at 225 and 270 degrees,
respectively. These are both common wind
directions for the New York City area in the fall
Due to computer limitations, only a small
region of lower Manhattan could be modeled in
QUIC. This encompassed a several block area
extending from the western edge of the WTC site
east to City Hal! Park, including the Woolworth
building (Figure 3b). At 241 m the Woolworth
building is one of the tallest in lower Manhattan. In

u v
Figure 3. (a) An overhead view of the wind tunnel
model showing the region simulated within QUIC.
North is towards the top of the page. The small
gray circular disks in the floor are locations where
velocity and turbulence profiles were taken. The
small blue circles show locations of surface
concentration ports. The small white building in the
green park is City Hall. The uppermost dark brown
building is the Woolworth building, (b) The area
around the WTC (light gray) modeled in QUIC
including flow streamlines with westerly winds (left
to right) at a height of nine meters above the
ground. The peak of the Woolworth building is dark
QUIC, the resolution of the mesh size was three
meters. The lateral size of the buildings were
rounded to an even mesh increment., while heights
were rounded to the nearest mesh increment. The
building dimensions were derived from the same
database used to build the physical model of lower
Manhattan. QUIC requires that buildings be
orthogonal to the coordinate system. Therefore
the buildings were subdivided for the model into
smaller segments. These dimensions were
imported into QUIC with each of the 86 total
building pieces categorized as an individual
building. Upwind buildings and other roughness
parameters present in the physical model which
undoubtedly affect airflow patterns could not be
simulated in QUIC due to computer limitations.
The flow around the two dimensional building
array was essentially skimming flow, passing over
the tops of the blocks once it had accelerated over
the first block. Regions of recirculation were
present in each of the "canyons" between blocks
(Figure 4a) and an upstream "rotor" was found
along the upwind face of the first block (Figure 4b).
These flow features were all well represented in
the QUIC model. The directions of flow nearly
always coinciding with those measured in the wind
tunnel. The flow patterns around the most upwind
block were particularly well described within QUIC.
Additionally, the centers for the recirculation zones
within the canyons coincided well. QUIC appeared
to consistently underestimate the velocities within
the canyons. This trend was most likely
attributable to the three dimensional nature of the
building array used in QUIC which allowed lateral
movement of air within the canyons to the edges of
the array. This flow feature was not evident in the
wind tunnel simulations.
The flow in the wind tunnel model of lower
Manhattan was complex, with significant areas of
backflow and blockage around regions of the city
(Figures 5a,b). As expected, building location and
geometry obviously influenced flow fields and,
thus, dispersion. Recirculation patterns and
channeling down streets also was prevalent. For
example, for westerly winds, notable channeling
down Church St. (a street near the WTC oriented
at 57 degrees to the wind, Figure 3a) was present
at low heights (1,5m) compared to the heights of
the surrounding buildings (around 60m) (Figure
5a). Above the buildings, and in the absence of
tall buildings directly upwind, the flow vectors
aligned with the freestream wind direction (Figure
5b). Also apparent was flow up the leeward faces

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250 300 350
x (mm)
Figure 4. (a) Velocity vectors within the canyon
between the second and third blocks (gray), (b)
Velocity vectors near the upwind surface of the
first block (gray) for the wind tunnel (black) and
QUIC simulation (light green). The gray areas
within the figures indicate areas where blocks are
present 150 mm wide by 150 mm tall. The canyon
between is 150 mm wide. The red vector at the top
shows the direction of the wind, with the length
equal to 3 m/s.
of tall buildings. Rapid vertical transport clearly
influenced airflow. Also visible was channeling of
flow along the eastern side of City Hall Park (the
triangular green area to the east of the Woolworth
building, Figure 3a). Generally, QUIC captured the
major aspects of the flow in the area around the
WTC site. It captured the channeling down Church
St., and the change in flow direction to the north as
it passed by St. Paul's Chapel and entered City Hall
Park (Figures 3b, 6a, 6b).
z = 9 m

heights im>
2 = 54 m

51-1 DO
Figure 5. Velocity vectors at a full scale height of
(a) 9 m and (b) 54 meters measured using the LDV
from the wind tunnel model of lower Manhattan.
The WTC site is the group of white footprints
beneath the red vector. The wind (red vector) is
from the west (270 degrees). The length of the red
vector is 3 m/s. The dark blue footprint shows the
location of the Woolworth building. The top two
velocity vectors show flow channeling Northeast up
Church street, with the flow going with the main
wind once it reaches the tops of the buildings.
Understanding the nature of air movement in
urban areas is critical to understanding of the
dispersion of air pollutants. QUIC, was able to
capture many of the major features of flow found in
our wind tunnel simulations suggesting that it may
be a useful tool for describing the dispersal of
pollutants in urban areas. QUIC adequately
described^abrupt changes in flow direction upon
encountering bluff buildings; channeling of flow
down streets; and recirculation patterns within


.^v? I iKmi	. / / i
Figure 6. Velocity vectors (dark blue) created by
QUIC for the westerly wind direction (270) at a full
scale height of (a) 9 m and (b) 54 meters for the
area of lower Manhattan near the WTC. Filled
polygons show building locations. The WTC site
is not shown, but would be to the lower left of the
figures. The dark red box footprint shows the
location of the Woolworth building. Vectors are
shown every six meters and are scaled up by 1.5
in length.
street canyons. While flow patterns nearly always
appeared to coincide between the wind tunnel and
QUIC, the velocity magnitudes often differed.
For the lower Manhattan simulation, it is likely
that buildings upwind of the WTC site that were not
included in the QUIC simulation, account for some
of the differences in flow between QUIC and the
wind tunnel. However, other differences may be
due to flow features not being incorporated into the
QUIC model. For example, the flow phenomenon
of rapid vertical dispersion, where street-level
pollutants are transported vertically up the
downwind faces of some tall buildings, was clearly
evident in the physical model but was less well
characterized by the QUIC model. In the lee of
some of the tall buildings near the WTC, the
velocities predicted by QUIC were not as strong or
were in different directions from the vertical flow
patterns that were measured in the wind tunnel.
Although not shown in this paper, for both the
wind tunnel models and QUIC, smoke
visualizations and tracer measurements suggested
that dispersion appeared to strongly depend on the
local wind fields created by the surrounding
buildings. Notable regions of plume channeling
down streets and recirculation within street
canyons were evident. Initial dispersion appeared
to strongly depend on source placement with
respect to the surrounding buildings, local flow
patterns, and the general wind direction.
Based on the results presented here, QUIC
shows great promise in describing the intricate
nature of urban flow. However, the complexities of
flow and dispersion in urban areas continue to
require diligent effort in model design as well as
the creation of suitable data sets for comparison.
Further comparisons with existing EPA data sets
will allow greater examination of the flow and
dispersion modeling capability of the QUIC model.
This paper has been reviewed in accordance
with the United States Environmental Protection
Agency's peer and administrative review
policies and approved for presentation and
We gratefully acknowledge the contributions to
this project by Dr. Bruno Pagnani, Mr. Roger
Thompson, Dr. William Snyder, and Mr. Robert
Lawson. We also acknowledge with thanks the
generous support of Messrs. Ashok Patel and
John Rose.
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