Physical Modeling of the Flow Field Around Twin High-Rise Buildings


EPA/600/A-93/292
PHYSICAL MODELING OF THE FLOW FIELD AROUND
TWIN HIGH-RISE BUILDINGS
Robert E. Lawson, Jr.
Atmospheric Sciences Modeling Division
Air Resources Laboratory
National Oceanic and Atmospheric Administration
Research Triangle Park, NC 27711
and
Masaaki Ohba
Tokyo Institute of Polytechnics
Atsugi, Kanagawa Prcf., Japan 243-02
1. INTRODUCTION
In Japan, many high-rise office buildings have
been built in downtown areas in order to more effectively
use the available space in commercial districts. These
buildings are often designed to have two high-rise structures
atop a common, terrace-shaped lower level. This
configuration serves to protect pedestrians on the sidewalk
from the strong winds which occur due to blockage of the
approach flow by the high-rise buildings.
Typically, as many as 5000 people might work in
these buildings each day. As a result, the heat gain/loss
inside these buildings is so great that electrical generating
plants are necessary in order to provide building air
conditioning for maintaining a comfortable working
environment. If these generating plants were installed near
the high-rise buildings, they would generally be installed in
an underground level and any exhaust would be emitted into
the area near the base of the high-rise buildings. Because of
their energy efficiency, co-generation systems are widely
used for producing heat and electricity, but these
co-generation systems emit large amounts of NO, so that
they may contribute significantly to increases in air pollution
around the buildings. Hence, installation of such
co-generation systems near the base of the high-rise
buildings may result in adverse effects on human health.
Many researchers have investigated air pollution
problems around buildings, but these experiments have
concentrated primarily on examination of the flow structure
and dispersion in the vicinity of an isolated building. Since
high-rise buildings are typically located in a complex city
environment, and since the twin high-rise structure
introduces the additional complexity of a nearby building,
the available data is of limited use in evaluating contaminant
levels.
In this wind tunnel experiment, we selected three
basic types of high-rise buildings and investigated the
"On assignment to the Atmospheric Research and Exposure
Assessment laboratory, U.S. Environmental Protection
Agency.
effects of these buildings on both gaseous diffusion and flow
structure. This report describes the flow-field measurements,
the techniques used to measure the flow field and some
conclusions which can be drawn from the measurements. A
companion paper (Ohba and Lawson, 1993) describes the
concentration measurements. The primary purposes of this
portion of the study were :
¦	to examine the centerline mean
streamline patterns and, hence, determine which possible
source locations would be likely to cause adverse
concentrations on the building surface
¦	to determine whether the addition of a
terrace level significantly altered the flow field
•	to determine how the flow field in the
downstream wake of the downwind building changed as a
result of varying the building height and the separation
between the buildings
¦	to obtain flow-field data for comparison
with the results of numerical simulations based on a k-c
model.
2. SIMILARITY CRITERIA
Similarity criteria for modeling flow around a
building immersed in a neutral atmospheric boundary layer
in a wind tunnel require that the Rossby number, Reynolds
number, Peclet number or Reynolds-Schmidt product, plus a
set of non-dimensional boundiiry conditions be matched in
both model and prototype Referring to Snyder (1981), the
Rossby number can be neglected when modeling prototype
flows with a length scale less than about 5 km. Also,
provided the model Reynolds number is sufficiently large, it
is not necessary to match the Reynolds number, Peclet
number or Reynolds-Schmidt product between model and
prototype. The reference velocity in this study was chosen
such that the building Reynolds number was greater than
0

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that regarded as the critical value for Reynolds number
independence (Golden, 1961). The Reynolds number based
on the wind speed at the top of the smallest building was
approximately 33,000.
For geometrical similarity, the details of Hie
prototype of size smaller than the roughness length need not
be reproduced in the model. All of the models used in this
study had smooth walls and sharp edges with no artificial
roughening of the building surfaces.
The general setting was assumed to be an
environment typical of the downtown areas of modern cities.
Ideally, the building height, shape and separation between
the buildings should all be varied over the full range of
typical values; however, the total number of combinations
would quickly become excessive. We therefore restricted
the number of parameters to four building heights, five
building shapes and several separation distances between the
twin building models. Only one parameter was varied at a
time while maintaining all other parameters at their
base-case values.
3.	WIND TUNNEL
The experiments were carried out in the wind
tunnel of the U.S. Environmental Protection Agency's Fluid
Modeling Facility (Snyder, 1979). The wind-tunnel is of the
open-return type with a test section 3.7 m wide, 2.1 m high
and 18.3 m long. The air speed through the test section can
be varied from about 1 to 10 m/s. An automated instrument
carriage system is located inside the tunnel test section and
is driven by a microcomputer linked to the data acquisition
system. It provides the capability for positioning a probe
anywhere in the test section, acquiring data, then moving to
the next measurement location and repeating the process,
entirely without intervention. This automated instrument
carriage system enabled the (normally tedious) process of
making pulsed-wire measurements to be carried out
around-the-clock.
A simulated neutral atmospheric boundary layer
was created in the wind tunnel using spires and floor
roughness elements. The spires were patterned after those
designed by Irwin (1981). In this study, the spires were
chosen to produce a boundary layer with depth of 2000mm
and a power law exponent of about 0.3. Block roughness
elements were used downstream of the spires to maintain the
boundary layer in equilibrium.
4.	BUILDING MODELS
The high-rise building models used in this study
were rectangular blocks with heights (I I,,) of 300, 450, 600
and 1200mm, respectively, with building width and length
fixed at 200mm. These correspond to full-scale dimensions
of 75, 112.5, 150 and 300m, respectively, in accordance with
the scale ratio of 250:1. The terrace-shaped building model
was 150mm high, 1000mm wide and 1400mm long,
corresponding to full-scale dimensions of 37.5m x 250m x
350m. The building models were centered on a point
11 37m downwind of the leading edge of the spires.
5. ANKMOMKTRY
5.1 Pulsed-ll'ire Anemometer
The bulk of the measurements were made with a
pulsed wire anemometer (PWA). The principle of operation
of the PWA is straightforward. The probe consists of three
fine wires, two outside wires being parallel to one another
and a central wire being perpendicular to the outer ones.
The central wire is pulsed with a high current for a few
microseconds which raises the temperature of the wire to
several hundred degrees Celsius. This releases a tracer of
heated air into the flow and it is convected away with the
instantaneous velocity of the air stream. The two outside
wires are operated as resistance thermometers and are used
to measure the time-of-arrival of the heated air parcel. The
use of two sensor wires, one on either side of the pulsed
wire, ensures that the flow direction is unambiguously
determined .
The PWA probe can be oriented to measure
velocity components in all three coordinate directions.
Because of finite wire lengths, the probe has a yaw response
up to about to 70°, so that, for reasonable measurements of
traverse components of the flow, the turbulence intensity
must be relatively high, e.g., above 20 to 25%. For
low-intensity flows, the hot-wire anemometer may be
preferable.
PWA calibrations were performed against a
Pitot-static tube mounted in the free-stream of the wind
tunnel with the spires laid down on the wind tunnel floor. A
capacitance manometer was used with the Pitot tube to
determine reference velocities in the range of 0.5 to 5m/s.
An iterative least-squares procedure was used to obtain a
"best-fit" of these calibration points to the equation
U = A/T+B/!*+C/P,
where U is the wind speed indicated by the Pitot-static tube,
T is the time-of-flight, and A, B and C are constants. A
typical calibration curve is shown in Figure 1.
Positive Wire
Negative Wire .
m
> 819C
-0 C02 -C.3015 -0.00* -0.0005 J 3.00C5 C.031 0.C015 0.0C2
I /trm#—cf-f ijr t (' /mic,cs#ct)
Figure 1. Typical PWA calibration curves.

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All of the PWA measurements described in this
report were obtained using a pulsing rate of lOHz and an
averaging time of 120 seconds. The data acquisition
computer converted the 12-bit digital time-of-flight signal
from the PWA to velocity using the appropriate calibration
curve for each sensor, computed statistics, then displayed
and plotted the results in real time.
5.2 Hot-Wire Anemometer
X-array sensors were used with a hot-wire
anemometer (HWA) to measure the mean velocity and
turbulence intensity profiles of the approach flow in the
absence of any buildings. Calibrations were performed over
a range of 0.5 to 5 m/s in the same manner as the pulsed
wire anemometer. The calibration voltages were used to
calculate a set of best-fit parameters to a King's law form of
equation
E2 - A +BU",
where E is the anemometer output voltage, U is the mean
wind speed, and A,B and n are constants that are determined
by a least-squares fitting procedure. The HWA is useful
when the turbulence intensities are relatively low (e.g. 20%
or so), or where the instantaneous velocity vector remains
within a cone with a total angle of about 30°. Significant
errors can occur when the hot-wire anemometer is used in
high-intensity or reversing flows such as that found near
buildings or obstacles.
The analog output signals from the HWA were
digitized at a rate of 1000Hz and linearized and processed on
a microcomputer using a 12-bit analog-to-digital converter.
A 60-second averaging time was used for all mean
measurements. All time-series measurements were obtained
over a period of 300s at a sample rate of 2000Hz.
6. EXPERIMENTAL CONDITIONS
Table 1 contains pertinent experimental
parameters used in this study. Figure 2 shows the reference
geometry. The reference velocity was maintained at 3.5 m/s
at a position of X=0min and Z = 600mm during all
measurements
Wind
L
Figure 2. Coordinate system and building
geometry'.
Vertical and lateral velocity profiles were first
obtained in die absence of any buildings in order to
characterize the simulated boundary layer. For Cases 1
through 3, longitudinal and vertical components of velocity
were measured in the vertical centerplane. Longitudinal and
lateral components were measured in a single horizontal
plane 50mm above the surface.
In Cases 4 through 6, only the longitudinal and
vertical components were measured on the vertical
centerplane downstream of the downwind building. For
Cases 7 through 16, only the longitudinal component was
measured downwind of the downstream building. The
high-rise building models were situated atop a terrace
section only for Cases 3 through 6.
7. RESULTS AND DISCUSSION
7.1 Boundary Layer Characterization
Figures 3 and 4 show profiles of the longitudinal
mean velocity and all three components of turbulence
Table 1. Experimental parameters.
(.adc
III.
Sep.
Case
Ht.
Sop.
No.
(in)

No.
Iml


HI)
I./HI)

HI.
I./HI)
I
0(H)
0.00
!l
o.:to
0.7!")
•)
Il.tiO
O.fiO
10
o.:i0
1.00

0.15(1
O.'i')
11
0.00
O.'.'fi
•
Il.tiO
0.:.';')
I:,'
O.liO
0.7
r>
0.(50
O.T'i
n
0.00
1.00
i; : n.oo
1.00
11
i :jii
0,'J.',
7 1
O.'.'.'i
If.
_ 1-0,
II.fill
H : i> .'»>
II 50
ii;
!1 0 7.".
1500
1000
z, mm
500
0
15 U. m/s 3
4.5
Figure 3. Approach flow velocity profiles.

-------
A	i/AJ hot-w4re
~	Wflj hot-v*re
O	i/AJ p^Jted-wtre
O	VAJ potewJ-we
~	WrtJ p*4s«6-v*re
1500
1000
z, mm
500 f
intensity
0	.1	.2	.3	.4
Figure 4. Approach flow turbulence intensity.
~A
OSr
ESV
<©
OJrO
Br a
•too
OOA
 OA
10
3' 1
5>
z
01
.001 -
0001
A u" specturi «tz=200mni
~ W spectum *\z=200nrtl
— Katmal i/ specf\*n
Ka*nal V spectum

.0001 .001 .01 .1 1 10 100 1000
Figure 5. Approach flow turbulence spectra.
intensity measured near the center of the test area.
Measurements with the HWA were at X = 0mm while
measurements with the PWA were slightly further upwind
at X=-700mm. The mean velocity profile was found to fit
well to the power law U(Z) = 0.522(Z+10)°JM. A log-law
fit to the mean velocity profile over the range 0< Z < 200mm
yielded a roughness length of 5.9mm (~1.5m full-scale), a
displacement height of 28mm and a friction velocity of 0.273
m/s. These values are consistent with those obtained by
Cook (1973) and the power law exponent falls within the
range of full-scale values from ESDU( 1972). The vertical
component of turbulence intensity measured at the upstream
location is slightly greater than that at X=0 due primarily to
proximity to individual roughness elements.
Lateral profiles of mean velocity and turbulence
intensity were measured at heights of 200, 500 and 1000mm
near the center of the test area. These lateral profiles
indicated peak deviations over the width of the test section
on the order of ± 0.1 m/s (apparently an artifact of the
spires), but deviations were deemed acceptably small near
the center of the test area.
Time-series of digitized velocities were collected
at heights of 200, 500 and 1000mm, and were subsequently
analyzed to obtain turbulence spectra. Figure 5 shows both
the u' and W spectra. The solid and dashed lines represent
the surface layer spectra due to Kaimal et al (1972). In
accordance with the model scale ratio of 250:1, the
measuring height of Z = 200mm corresponds to a full-scale
height of 50m. The wind tunnel spectra at this level compare
favorably with the Kaimal spectra, hence the boundary layer
simulates the prototype atmospheric boundary layer
reasonably well.
7.2 Mean Velocity Vectors and Streamlines
Figures 6a through 8b show the mean velocity
vectors around the building models as measured with the
pulsed wire anemometer and streamlines constnicted from
these measurements.
In the U-W How field for Case 1 (Figure 6a), a
stagnation point was observed on the upwind face of the
building model near Z/Hb = 2/3. Below this level, the
oncoming flow flowed downward along the upwind face and
reached ground level. The flow separated on the upwind
edge of the rooftop and reattached on the rooftop. Reverse
flow was not clearly seen on the rooftop because there were
no data points sufficiently close to the surface. A
recirculating eddy was formed just downstream of and
slightly below the top of the building. Centerplane mean
streamlines constnicted from the measurements clearly
show the salient features of the flow. The streamline pattern
is topologically consistent with the results of Davies et al
(1980) for a square-section building with height six times its
width; however, the center of the recirculating eddy is
located below the top of the building in the present case. The
taller building used by Davies et al showed fully separated
flow on top of the building and this is probably a controlling
factor in determining the height of the downwind eddy. An
elevated "free stagnation point" was observed downstream
near Z/H^sl/2. In the U-V flow field at Z = 50 mm (Figure
6b), the flow separated at the upwind edge of the building
model and reattached on the side. Streamlines constructed
from measurements in the horizontal plane assume that
vertical motion near the surface is restricted sufficiently to
allow two-dimensional streamlines to be representative of
the near-surface flow. It is clear from the streamline
patterns that emissions from sources located near the surface
in the downwind wake of the building will be swept directly
toward the downwind face of the building.
The U-W flow field for Case 2, like that for Case
I, shows the flow separating on the upwind rooftop edge of
the upwind building model and reattaching on the same
rooftop (see Figure 7a). After reattachment, the flow was
directed downwards along the upwind face of the downwind
building model and was directed upstream at the position of
7A\= 1/3. A recirculating flow was clearly created between
the twin building models In the areas near the rooftop of
the downwind building model, the flow was parallel to the

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	>
4m/s
(a)
750
e
JE500
N
250



^ ^





- ¦%. _

-*>

>
















	^
s>-





->
7>
>•





> > > T>-
> > > y
(b)
p250
>
X (mm)
Figure 7. Velocity vectors and streamlines for Case2 in the (a) vertical centerplane and (b) in a horizontal
centerplane at an elevation of 50mm .
under which the wind speed near the ground between the
two buildings would reach a maximum.
7.3 Reattachment Length Behind The Downwind Building
The longitudinal distance from the downwind face
of the downwind building to the point of reattachment of the
flow to the ground surface behind the downwind building
was measured using the pulsed wire anemometer. Figure 9
shows the definition of the reattachment length behind the
downwind building model. Note that the term reattachment
is used in a very broad sense in this context. The flow
downstream of the buildings is highly complex and
three-dimensional; the term reattachment, as seen in the
diagram, really describes the location near the surface or
terrace level where the sign of the longitudinal velocity
component changes from negative (upstream) to positive
(downstream). Figure 10 presents the relationship between
reattachment lengths and separation distances of the twin
building models. From Figure 10, it was found that the
normalized reattachment length increased as the building
height increased, but the separation distances of the twin
building models did not greatly influence the reattachment
length for separation distances in the range of 0.25 < L/H,,
<; 1.0.
8. SUMMARY AND CONCLUSIONS
Velocity vectors around high-rise building models
immersed in a simulated atmospheric boundary layer were
measured with a pulsed wire anemometer. Streamlines were
constructed from these data to illuminate the basic flow
features and to show where exhaust emissions near the base
of the downwind building might impact the building.
Measurements were accomplished with a single building,
two buildings with various heights and separations, and
with the addition of a terrace level A recirculating eddy was
observed in the mean flow field just downstream and near
the top of the high-rise building models. From these wind
tunnel experiments, the following conclusions arc drawn:

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4m/s
750
250
0
1000
500
0
-500
£ 250
0
0
500
1000
-500
X (mm)
Figure 8. Velocity vectors and streamlines for Case3 in the (a) vertical centerplane and (b) in a horizontal
centcrplane at an elevation of 50mm.
(1)	The primary effect of the upwind
building is to retard flow separation on the top and sides of
the downwind building.
(2)	The effects of the upwind building on
flow separation near the top of the downwind building are
diminished when the separation (L/HJ equals or exceeds
1.0.
(3)	Addition of the terrace level did not
substantially affect the flow field downstream of the
downwind building.
(4)	The normalized longitudinal distance to
reattachment behind the downwind building model increased
with the building height, but the separation distance between
the twin building models did not influence the reattachment
Figure 9. Definition of reattachment length
.7




.6
A Hb-0 3

.5 -
O Hb"0 6


.4
A Hb*1.2


~				Q	"~

.2


.1
" " o—¦ o

0
L/H

0 .25 .5 .75 1 1 25
Figure 10. Reattachment length vs. building

separation.

-------
V
length for separation distances in the range of 0.25 <
iyn,< 10.
(4) Mean flow streamlines show that
location of emission sources near the downwind base of any
of the buildings will lead to a potential for contaminating the
downwind building face.
Efforts are underway to model both the flow field
and dispersion characteristics using a k-e numerical model.
These data will provide comparative measurements against
which the model will be compared and evaluated.
DISCIjUMER: Iliis paper has been reviewed in accordance
with the U.S. Environmental Protection Agency's peer and
administrative review policies and approved for presentation
and publication. Mention of trade names or commercial
products does not constitute endorsement or recommendation
for use.
REFERENCES
Britter, R E. & Hunt, J.C.R., 1979: Velocity Measurements
and Order of Magnitude Estimates of the Flow Between Two
Buildings in a Simulated Atmospheric Boundary Layer. J.
Indus. Aerodyn., 4, 165-82.
Cook, N.J., 1973: On Simulating the Lower Third of the
Urban Adiabatic Boundary Layer in a Wind Tunnel. Atmos.
Envir., 7, 691-706.
Davies, M.E., Quincey, V.G. & Tindall, S.J., 1980: The
Near-Wake of a Tall Building Block in Uniform and
Turbulent Flows. Proc. 5th Int. Conf. Wind Engr., Fort
Collins, CO, July, 1979 (J.E. Cermak, ed ), v. 1, p 289-98.
Pergamon Press, NY, NY.
ESDU, 1972: Characteristics of Wind Speed in the Lower
Layers of the Atmosphere near the Ground: Strong Winds
(Neutral Atmosphere). Item No. 72026, Engineering
Sciences Data Unit, London, UK.
Golden, J., 1961: Scale Model Techniques. M.S. Thesis,
College of Engr.,New York Univ., NY, NY, 48p.
Irwin, H.P.A.H., 1981: The Design of Spires for Wind
Simulation. J. H'indEngr. Indus. Aerodyn., 7, 361-66.
Kaimal, J.C., Wyngaard, J.C., Izumi, Y. & Cote, O.R., 1972:
Spectral Characteristics of Surface Layer Turbulence.
Quart. J. Roy. Meteorol. Soc., 98, 563-89.
Ohba, M. and Lawson, R.E., Jr., 1993: Physical Modeling
of Concentration Distributions Around Twin High-Rise
Buildings With a District Heating Plant. AMS Eighth Joint
Conf. On Applications of Air Pollution Meteorology, Jan
23-28, Nashville, TN.
Snyder, W.H., 1981: Guideline for Fluid Modeling of
Atmospheric Diffusion. Rpt. No. EPA-600/8-81-009,Envir.
I'rot. Agcy., Res. Tri. Pk., NC, 200p.
Wise, A.F.E., 1971: Effects Due to Groups of Buildings.
Phil. Trans. Roy. Soc. Lond., A, 469-85.
«

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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completi
1. REPORT NO. 2.
EPA/600/A-93/292
3.
4. TITLE AND SUBTITLE
Physical Modeling of the Flow Field Around
Twin High-Rise Buildings
5. REPORT DATE ggg
C. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Robert E. Lawson, Jr.1 and Masaaki Ohba2
8, PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Atmospheric Research & Exposure Assessment Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 2771 1
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
1Z SPONSORING AOENCY NAME AND ADDRESS
Atmospheric Research & Exposure Assessment Laboratory-RTP, NC
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
1X TYPE OF REPORT AND PERIOD COVERED
Presentation
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
Presentation: AMS Eighth Joint Conf. on Applications of Air Pollution Meteor., Jan., 1994, Nashville, TN.
' On assignment from the National Oceanic and Atmos. Admin., US Dept. of Commerce
2 Tokvo Institute of Polytechnics, Atsugi, Kanagawa Pref.. Japan 243-02
IS. ABSTRACT
A wind tunnel study was conducted to investigate the flow characteristics near three configurations of high-rise
buildings - an isolated high-rise building, two high-rise buildings separated in the streamwise direction, and4wo
high-rise buildings separated in the streamwise direction, but situated atop a terrace-shaped lower leveJ. A
pulsed-wire anemometer was used with an automated traversing system to make detailed velocity
measurements In the vertical centerplane and in a horizontal plane just above the surface. For each of the
three basic configurations, measurements were taken while systematically varying the building height or, for
the twin buildings, varying both the height and separation between the buildings. The measured mean velocity >>
components were used to construct plots showing velocity vectors and streamline patterns and, hence, the
height and downwind extent of areas of recirculating flow. The mean flow streamline plots were used to identify
emission source locations that might result in adverse concentrations on the downwind building face.
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