7.5 EVIDENCE OF ENHANCED VERTICAL DISPERSION IN THE WAKES OF TALL BUILDINGS
IN WIND TUNNEL SIMULATIONS OF LOWER MANHATTAN
David K. Heist*t1, Steven G. Perryt1, and George E. Bowker2
1 Atmospheric Sciences Modeling Division, Air Resources Laboratory, NOAA
2National Exposure Research Laboratory, US EPA
Research Triangle Park, North Carolina
EPA/600/A-04/082
ABSTRACT
1. INTRODUCTION
Observations of flow and dispersion in urban
areas with tall buildings have revealed a
phenomenon whereby contaminants can be
transported vertically upthe lee sides oftall buildings
due to the vertical flow in the wake of the building.
This phenomenon, which contributes to what is
sometimes called "rapid vertical dispersion", has
important consequences for the dispersion of
pollutants in urban areas and its understanding may
be crucial to improving urban dispersion models.
This venting effect was observed in a wind-tunnel
study of dispersion from the site of the destroyed
World Trade Center (WTC) in New York City, using
a scale model of lower Manhattan, including a scaled
representation of the rubble pile.
Enhanced vertical dispersion was seen on the
downwind side of several tall buildings in the highly
urban area surrounding the WTC site using a smoke
tracer. The flow responsible for this vertical
dispersion was measured with laser Doppler
velocimetry, and its effects on the plume were
demonstrated with concentration measurements of
an ethane tracer released from the rubble pile.
Notably, the World Financial Center buildings, which
stood upwind of the WTC site for westerly winds,
caused an initial vertical dispersion of the plume
before it began to move downwind. This vertical
dispersion was caused by a vertical flow in the wake
of these buildings and resulted in rapid transport of
contaminants to heights above the building tops.
The enhancement of the dispersion of the WTC
plume due to tall building wake effects is analyzed
and compared with Gaussian plume model
predictions.
* Corresponding author address: David K.
Heist, MD-81, USEPA, RTP, NC 27711, USA.
f On assignment to the National Exposure
Research Laboratory, U.S. Environmental
Protection Agency.
To evaluate and enhance our numerical
simulation capabilities for lower Manhattan and other
urban areas and to support ongoing risk assessment
and public health studies of the World Trade Center
disaster, EPA's Office of Research and Development
initiated a wind-tunnel study of flow and pollutant
dispersion in the complex Lower Manhattan area.
The wind-tunnel study was conducted using a scale
model of lower Manhattan, including a scaled
representation of the World Trade Center (WTC)
rubble pile. Neither the initial explosions on
September 11, 2001 nor the collapses of the towers
have been simulated. Instead, dispersion from the
smoldering rubble pile was modeled for the time
period approximately two to six weeks after the
catastrophe.
A prominent characteristic of the flow through
this highly urban area of lower Manhattan is the
tendency for some tall buildings to create wakes that
transport contaminants from street level to the
building rooftops. This phenomenon was observed
in each of the three phases of the study: flow
visualization, velocity measurements, and tracer
concentration measurements.
2. EXPERIMENT
A photograph of the 1:600 scale model of the
lower end of Manhattan Island installed in the EPA's
Meteorological Wnd Tunnel (3.7m wide, 2.3m high,
18m long, more fully described by Snyder, 1979) is
shown in Fig. 1. The model was constructed of rigid
polyurethane foam mounted on a plywood base and
centered 250m east and 115m south (full scale) of
the center of the WTC site. It encompasses all of the
southern tip of Manhattan Island.
A simulated atmospheric boundary layer (abl)
was developed using three Irwin (1981) spires and
roughness blocks (18mm high, 27mm square) with
25% area coverage. These blocks ended at roughly
the western edge of the Hudson River, which is
about 1 km wide at this point. Measurements
showed the full-scale (600:1) equivalent of this abl
would have a depth of 1100m, and a roughness
length of 0.4m full scale at the end of the roughness
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WTC Site
Figure I. View of scale model in wind tunnel for the 270° wind direction case, looking downstream (toward east).
m/s, providing a street-canyon Reynolds number
(Res=U0W/v) of approximately 10,000 for the
smallest street-canyon width (35mm or 21m full
scale). W is the street canyon width and v is the
dynamic viscosity of air. Independent
measurements of flow in idealized two-dimensional
street canyons suggested that Reynolds-number
independence would be achieved if Re, exceeded
4200.
A plan view of the model is shown in Figure 2.
A roughly square array of 9 discrete tubes was used
to release effluent from the smoldering rubble pile to
simulate the distribution of emissions. Neutrally
buoyant ethane was used as a tracer for quantitative
measurements of concentrations with flame
ionization detectors. Flow visualization was
performed using a theatrical smoke generator and a
laser light sheet used to illuminate cross sections of
the plume.
For the quantitative flow measurements, a
laser-Doppler anemometer (LDA) was used at a
series of points located within various street canyons
throughout the area. The LDA was aimed through
glass windows in the floor of the wind tunnel so as to
eliminate disturbances from the LDA probe as weii
as to avoid building interferences to the LDA
line-of-sight. This arrangement allowed the
measurement of horizontal components of velocity.
A small mirror placed at 45° to the LDA viewing
direction and supported by small-diameter rods
extending through the window enabled the additional
blocks, which is consistent with the built-up
urban/suburban area on the New Jersey (western)
side of the river. Detailed measurements were made
for winds from the west (270°) and the southwest
(225°), although results shown in this paper only
from the 270° case.
The free-stream velocity (U0) was set at 4.23
World Trade
Center Site
0 m 500 m
Figure 2. Plan view of study area. Red asterisks
indicate selected LDV measurement sites.
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measurement of vertical components as well as
vertical traversing (Snyder & Castro, 1998).
Eight basic locations were selected for LDA
measurements, providing a cross section of the
different types of local building topographies in the
region (e.g., low-rise buildings with narrow streets,
open space surrounded by tall buildings, narrow
canyon surrounded by tall buildings, etc.) and
covering a range of distances and directions from the
WTC site. Fortunately, a number of the selected
measurement locations were on the downstream
side of relatively tall buildings. Therefore, the
velocity measurements provided quantitative
information on the vertical transport of pollutants in
those situations.
Each of these measurement locations included
at least a pair of ports or windows for access by the
LDA and, in some locations, as many as 5 ports. The
separation between the pairs was 100mm and, in
general, pairs were oriented along the street axes.
The 100mm separation was the same as the beam
extension beyond the 45° mirror on the LDA.
Therefore, by using the mirror when the LDA head
was in one port of the pair and removing it when in
the other, vertical profiles of both horizontal and
vertical velocities could be measured along the same
line, generally at the center of the street canyon.
Also, by rotating the LDA probe with mirror attached,
it was possible to measure off-axis profiles, thus
providing cross-sectional information on the flow
structure within the canyon.
A large number (133) of sampling ports were
installed on the model surface to facilitate the
measurement of ground-level concentration
distributions. Sampling "rakes" on a traverse system
allowed lateral and vertical concentration profiles to
be measured at virtually any position in the model
city. Samples acquired in this way were analyzed
Figure 4. Visualization of smoke in a horizontal plane
just above the height of the tallest buildings using a
laser light sheet.
with Hydrocarbon Analyzers equipped with flame
ionization detectors to determine mean
concentrations of the tracer material.
3. FLOW VISUALIZATION
One prominent feature observed from the flow
visualization was entrainment of source material
upwind into the lee of the World Financial Center
(WFC). The subsequent upwash or "pumping" of
smoke up the lee side to the building top and even
above, resulted in what appeared to be a continuous
elevated release. This effect can be seen clearly in
Figure 3 where the plume from the WTC site reaches
the top of the WFC building before it leaves the
immediate area of the rubble pile. Similar "pumping"
action was observed from buildings to the south and
somewhat downwind of the source. These results
may be seen in Figure 4, where a laser-light sheet
illuminated a horizontal plane just above one of the
tallest buildings (the Chase Manhattan Building).
The figure appears to show 3 distinct "plumes". The
one on the right originated from the WFC (213m
high), the middle one from the Liberty Plaza Building
(236m), and the (somewhat more diffuse) one on
the left from the Chase Manhattan Building (274m).
These are the three tallest buildings in the vicinity of
the WTC site.
4. VELOCITY VECTORS
Figure 3. Visualization of smoke in a vertical plane
parallel to the direction of the freestream flow (from
left to right).
The vertical transport of pollutants in the wakes
of tall buildings can be attributed to relatively strong
updrafts which have been quantified for a number of
prominent buildings in the lower Manhattan study
area. Using LDA, the velocity fields were measured
WTC
Rubble Pile
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250 ¦
200 ¦
E
N
100 ¦
(a)
0 50 100
Ysc(m)
250
150
E
N
50
(b)
1
\
i .... i
0 50 100
Ysc (m)
250
200
E_
N
100
(c)
-50 0 50 100
Ysc (m)
Figure 5. Velocity vectors on the downwind side of tall buildings (shown in silhouette on the left side of each
figure, a) Chase Manhattan building (Federal Reserve building on right), b) Woolworth building, c) unknown
building approximately 300m northeast of WTC site.
at many locations across the area, several of which
were on the downstream side of tall buildings
(indicated with asterisks in Figure 2). In general, the
air moves vertically up the lee side of tall buildings
with a velocity that can easily exceed 10% of the
freestream velocity aloft (Figure 5).
The Chase Manhattan building (located
approximately 500m southeast of WTC site) is one
of the tallest buildings in the Wall Street area with a
rectangular block-like shape that projects above the
neighboring buildings. The flow on the lee side of
that building rises dramatically (Figure 5a)
Figure 5b shows the flow behind the Woolworth
building located approximately 300m east-northeast
of the WTC site. This building, although quite tall
(241 m), tapers toward to the top, thereby reducing its
influence on the flow around it. Nevertheless, the
flow on the lee side of the building exhibits the same
tendency to transport material from ground level to
the top of the building that was seen at Chase
Manhattan building.
Figure 5c shows a much shorter building,
located approximately 300m northeast of the WTC
site on the edge of an area where most of the
buildings are low-rise. Although the building on the
left in Figure 5c is not tall compared to the
Woolworth Building or the Chase Manhattan
building, it does stand taller than the buildings in its
immediate vicinity. The flow vectors in the figure
indicate that even behind this building, the flow is
drawn up the lee side to the top of the building
before being swept downstream by the flow aloft.
5. TRACER MEASUREMENTS
The effect of the tall building upwash can be
seen in the tracer measurements of the simulated
WTC plume. Cross sections of concentration were
measured at 300, 600 and 1200m downwind of the
WTC site. These are shown in Figure 6, where
isoconcentration contours against a background of
the city skyline viewed from downstream provide
some indications about the plume size and behavior.
(The values of the isoconcentrations are
nondimensional, 100CUoH2/Q, where C is the
measured concentration, H is a reference length
scale (150mm) indicative of an average building
height, and Q is the volumetric source flow rate.)
At the 300m distance (Fig. 7a), the most notable
feature of the cross section is the two lobes of higher
concentrations on the sides, with a valley of lower
concentrations in the middle. This is rather clear
evidence of the "pumping" of effluent up the lee
sides of the World Financial Center and the Liberty
Plaza Building. The influence of the Chase
Manhattan Building was not a factor at this
downwind distance, since it is located very near this
plane. The plume was strongly asymmetrical, with
the north lobe being much wider and higher in
elevation than the south lobe. Also at this downwind
distance, the plume appears to be shifted slightly to
the north side of a line aligned with the free-stream
wind and through the center of the WTC site
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Liberty Plaza
World
Financial
Center
t ~ ~
400
Chase Manhattan
-600 -400 -200 0 200 400 600
Cross Wind Distance (m)
Figure 6. Plume cross sections at downwind distances of: a) 300m, b) 600m, and c) 1200m. View looking upstream
against skyline of city. Shorter buildings indicated in lighter colors.
(henceforth referred to as the centerline). The At 600m downwind, two lobes were still evident,
largest concentration, indicated by the circled star, but were more diffuse. A notable feature was the
however, is on the centerline. lateral shift of the plume from a northward bias to a
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southward bias; the maximum measured
concentration was located over 100m south of the
centerline. The maximum concentration is shown
elevated above ground level at this distance, but it
may have been lower; no measurements were made
at lower elevations at this downstream location,
because buildings were in the way.
At 1200m downwind, only the north lobe was
pronounced; a roughly horizontal concentration
contour on the south side suggests that the middle
and south plumes observed in Fig. 3 have essentially
merged at this downwind distance. Also noticeable
here was the strong spread of the plume to the south
side, with the "10"-contour reaching only 170m to the
north of the centerline, but 440m south of the
centerline. The maximum concentration was located
about 240m south of the centerline, indicating an
angular shift of the plume by approximately 11 ° from
a line directly downwind of the source. We believe
this is an indication of a recirculation caused by the
dense cluster of tall buildings in the vicinity of Wall
Street (centered at approximately 500m south of the
centerline through the source). At this distance, the
maximum concentration was elevated approximately
80m above ground level. The dots forming vertical
0
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(a)
400
300
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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 publication.
ACKNOWLEDGMENTS
We gratefully acknowledge the contributions to this
project by Mr. RogerThompson, Dr. William Snyder,
and Mr. Robert Lawson and Mr. Alan Cimorelli. We
also acknowledge with thanks the generous support
of Dr. Bruno Pagnani and Messrs. Ashok Patel and
John Rose.
REFERENCES
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Simulation. J. Wind Engr. Indus. Aerodyn., 7,
361-366.
Snyder, W.H., 1979: The EPA Meteorological Wnd
Tunnel: Its Design, Construction, and Operating
Characteristics. Rpt. No. EPA-600/4-79-051, Envir.
Prot. Agcy., Res. Tri. Pk., NC, 78p.
Snyder, W.H. and Castro, I.P. 1998: The
Yaw-Response of Hot-Wre Probes at Ultra-Low
Wnd Speeds. Meas. Sci. & Tech., 9, 1531-1536.
Cimorelli, A. J., S. G. Perry, A. Venkatram, J. C.
Weil, R. J. Paine, R. B. Wlson, R. F. Lee, and W. D.
Peters, 2003: AERMOD Description of Model
Formulation. EPA 454/R-03-002d, U.S.
Environmental Protection Agency, Research Triangle
Park, NC, 85pp.
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