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Journal of Wind Engineering
and Industrial Aerodynamics 90 (2002) 711732
Pressure distributions on a cube in a
simulated thunderstorm downburstPart A:
stationary downburst observations
M.T. Chay, C.W. Letchford*Department of Civil Engineering Wind Science and Engineering Center, Texas Tech University,
Lubbock, TX 79409-1023, USA
Received 10 December 2001; received in revised form 19 March 2002; accepted 20 March 2002
Abstract
Thunderstorms are responsible for a large amount of wind-induced damage around the
world. It is known that the wind characteristics in thunderstorms, particularly downbursts,
differ significantly from those of synoptic scale boundary layer winds. This paper describes a
study aimed at simulating the flow structure in a downburst and obtaining the pressure field on
a cube immersed in such a flow. Part A presents the data obtained from a stationary wall jet
simulation of a thunderstorm downburst, while Part B presents the data from a moving
downburst simulation. The pressure distributions on the cube are compared with data from
uniform and boundary layer flows. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
An atmospheric boundary layer wind profile currently forms the basis for
calculating wind loads on structures [13], and has been the profile employed in wind
tunnel simulations to obtain wind loading data either explicitly for specific buildings
or implicitly through codified data for generic building shapes. However, thunder-
storms are responsible for design wind speeds in many parts of the world [4,5] and
the wind characteristics of thunderstorms are known to be different from boundary
layer wind profiles. From these differences it may be anticipated that wind loading of
both low- and high-rise buildings will be significantly different from those in
traditional boundary layer flows. In particular, at low level the wind profile is more
*Corresponding author. Tel.: +1-806-742-3476; fax: +1-806-742-3446.
E-mail address: [email protected] (C.W. Letchford).
0167-6105/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 1 6 7 - 6 1 0 5 ( 0 2 ) 0 0 1 5 8 - 7
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uniform with height, potentially leading to increased loads on low-rise structures,
while at high level the reduction in wind velocity with elevation could reduce the
wind loads on high-rise buildings. Full-scale observations [69], and physical
simulations [1012] using wall jets have helped give a better understanding of thesephenomena and Letchford et al. [5] recently reviewed this work, however, there has
been little attempt to assess and quantify the impact of thunderstorm wind profiles
on the wind loading of structures.
This study aimed to establish a viable simulation of a thunderstorm downburst
and obtain surface pressure distributions on a generic building shapea cube
immersed in this flow. Comparisons would then be made with pressure distributions
in other flow simulations, namely uniform and turbulent boundary layer flows to
ascertain the significance of the new impinging flow type. The downburst was
simulated by a stationary wall jet, which has previously been shown [1012] to give a
reasonable representation of the mean velocity profile (mean here applies to the
short time averaged (several minutes) wind velocities measured at full-scale). To
obtain better kinematic similarity of the flow and hence report meaningful unsteady
pressure measurements, a moving wall jet was constructed so that the transient
characteristics of a thunderstorm gust front could be obtained. Part A of this paper
presents the results from the stationary wall jet simulation of the downburst, while
Part B [24] presents results from the moving simulation. In the following section the
basic characteristics of thunderstorm downbursts are reviewed. Section 3 reviews
previous wind pressure measurements on cubes. In Section 4, the wall jet
and velocity characteristics of the stationary downburst simulation are described.Section 5 compares the mean pressure distributions on a cube immersed in the
stationary wall jet with earlier studies in uniform and boundary layer flows. A
discussion of the experimental results is presented in Section 6.
2. Thunderstorm downbursts characteristics
Letchford et al. [5] discuss the general characteristics of thunderstorms.
Fundamentally, convection drives an updraft, which transports warm moist, more
buoyant, air to great elevations. Subsequently, the moisture in this air condenses,cools, and the upward motion is halted. The now colder more dense air begins to
accelerate toward the ground as a downdraft. Downbursts occur when a strong
downdraft collides with the surface of the earth and diverges. Close to the point of
impact the flow resembles that of a wall jet. As the flow spreads out over the ground
it behaves as a gravity or density current. The flow field created by such an event,
particularly near the impact point, varies from an atmospheric boundary layer wind
field in a number of fundamental ways.
Firstly, the traditional boundary layer profile, of increasing wind velocity with
height is no longer valid as a region of accelerated flow exists close to the surface
with a decrease in velocity with height. Downdrafts and consequently downburstsexist over a range of scales; Fujita [6] classified them as microbursts if the area of
damaging winds was o4 km in extent and macrobursts if >4 km. Typically
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microbursts produced the higher winds. The full-scale observations of downbursts
during Fujitas NIMROD experiments [6] and the JAWS [8] experiments produced
quantitatively similar results for microbursts. The Hjelmfelt [8] summary of the
JAWS results is reproduced in Fig. 1. On average, the maximum wind velocity
occurred at a height ofE80 m at a distance ofE1.5 km from the point of impact.
This was for an average downburst diameter of 1.8 km. Thus, the strongest winds
were observed within about one downdraft diameter from its point of impact. The
primary data sources for wind velocities were three Doppler radars [8] and, a lowlevel, automated mesonets with an average spacing of 4 km. The Doppler radars
were able to scan approximately every 2.5 min. Wilson et al. [9] present a full
discussion on the full-scale analysis techniques and estimation of errors.
Secondly, downdrafts often retain large amounts of the translational momentum
of the parent storm. Storm velocities can be as much as a third the velocity of the
downdraft [5]. The lateral motion of the downdraft causes an increase in the peak
downburst velocity in front of the storm, and a decrease in the velocity on the
trailing side. Due to the translating motion of the downdraft, stationary objects
experience downburst winds as non-stationary events. The effect of the transient
nature of such phenomena, in particular, the vortex ring at the leading edge of thedowndraft has not been previously investigated and is the subject of the Part B of
this paper.
Fig. 1. Velocity profile of a typical microburst during JAWS (Hjelmfelt [8]).
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Thirdly, current procedures for assessing wind loads on structures assume aconstant ambient atmospheric pressure. However, significant variation of atmo-
spheric pressure can be experienced within the flow field of a downburst. Stagnation
occurs in the central region beneath the downdraft as it approaches the ground,
forming a high-pressure dome known as a mesohigh (Fig. 2). A low-pressure ring
forms as the downburst flow diverges and accelerates to the peak horizontal velocity.
The relative magnitude of the pressure decrease is dependant on the translational
velocity of the storm [7]. Beyond the low-pressure ring, deceleration of the outflow
forms another slightly less intense high-pressure region, outside of which the pressure
returns to the ambient pressure. The varying pressure field of a downburst may have
serious implications with respect to design loads on structures. Fujita [7] speculatedthat these rapid pressure changes could be as high as 23 hPa, which may result in
a significant increase in the load applied to sealed structures in the outflow region. As
the downburst is a transient phenomenon, the pressure variation described by Fujita
is naturally a transient one as well and is due to the accelerations within the flow.
3. Previous investigations of pressures on a cube
Numerous previous studies [1320] provide a comprehensive description of the
pressure distribution over a cube immersed in both uniform (i.e., a wind fieldshowing little or no change in velocity as a function of height) and boundary layer
wind fields (i.e., a wind field showing an increase in velocity as a function of height
Fig. 2. The pressure field of a microburst (Fujita [7]).
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due to the effects of surface roughness). These studies have involved physical and
numerical simulations, and full-scale measurements. The following paragraphs detail
a few of the more notable papers.
Baines [13] undertook early wind tunnel studies on a cube immersed in uniformand boundary layer flows. Unfortunately, no turbulence characteristics of the
boundary layer flow are reported. He reports pressure distributions on each face of
the cube, for the cube positioned with one face normal to the direction of flow.
Castro and Robins [14] also undertook wind tunnel tests on a cube. Pressures were
measured on a 60 mm cube in a uniform flow, and a 200 mm cube in a 2 m high
boundary layer (constituting a scale of between 1:1000 and 1:300 for the boundary
layer case) with a turbulence intensity of 27% at eaves height. Measurements were
made with the cube face normal to the flow and at 451 to the flow. Castro and
Robins also investigated the effects of varying the boundary layer characteristics
acting on the cube. They found that a higher turbulence intensity favored
reattachment of flow over the cube, and hence a reduced suction in the reattached
region of the roof and leeward wall, for a flow normal to one face of the model.
The study reported by H .olscher and Niemann [16], involved a comparative
experiment of pressures over a cube across some 15 wind tunnels in Europe. There is
a surprising amount of scatter in the results much of which was attributed to
differences in turbulence intensities in the various boundary layer simulations.
Paterson and Apelt [18] performed a numerical simulation using a k e
turbulence model to simulate a boundary layer flow around a cube with one face
normal to the direction of flow. They performed the study under similar conditionsto those Castro and Robins [14] investigated. However, the Paterson and Apelt study
yielded greater mean pressures near the windward edge of the roof than Castro and
Robins. Paterson and Apelt observed that increasing turbulence intensity promoted
reattachment on the roof of the cube, supporting Castro and Robins wind tunnel
observations.
Richards et al. [20] collected full-scale data using the Silsoe Cube, which stands
6 m high and had 16 pressure transducers across its centerline. The cube is situated in
an open field at the Silsoe Research Institute in the United Kingdom. Richards et al.
observed pressures on the cube with winds perpendicular and at 451 to the centerline
pressure tap distribution. Turbulence intensities between 12% and 19% at cubeeaves height occurred during the study.
Fig. 3 shows the mean pressure coefficient distribution along the developed
centerline of the cube, with the windward face being between positions 0 and 1, the
roof between 1 and 2 and the leeward wall between 2 and 3. The flow is normal to
the front face (01 orientation). The pressure coefficients were formed by using the
velocity at the top of cube as the reference velocity for the dynamic pressure. Baines
[13] and Castro and Robins [14] observed similar pressures along the centerline of a
cube immersed in uniform flow for this flow direction. However, significant variation
exists between the pressure distributions for boundary layer flow over a cube at this
orientation. The size of the separated flow region, and consequently the magnitudeof the leeward wall pressure, represents the primary difference between the various
studies and differences in turbulence intensities are the likely cause. Richards et al.
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[20] observed reattachment further downstream than the three boundary layer
simulations, which is consistent with the reduced turbulence that occurred during the
full-scale measurements.
4. Downburst simulation
4.1. The Moving Jet Wind Tunnel
For the current study, an inverted wall jet capable of translational movement, and
termed the Moving Jet Wind Tunnel, produced the downburst simulation. Fig. 4
shows a schematic of the arrangement, which was a further development of that used
by Letchford and Mans [21]. A 5.6 kW centrifugal blower running at 3450 rpm drove
air through extensive flow conditioning in the form of a settling chamber, screens,
two layers of honeycomb and a 4:1 contraction. The 0.51 m diameter jet (D) blew
against an extensive flat test surface positioned 870 mm (1.7D) above its outlet. The
test surface was smooth-painted plywood. The asymmetry caused by the adjacent
wall was countered by leaving a 150 mm gap at the edge of the test surface. Full
details may be found in Chay [22]. As downbursts typically contain much colder,denser, air than that surrounding, the effort here has been aimed at producing a non-
entraining jet. Thus, the nozzle outlet should be close to the surface. As a further
0
1
2
3
-1.5
-1
-0.5
0
0.5
1
1.5
0 1 2
Position
Cp
Castro and Robins, Uniform Flow
Castro and Robins, Boundary Layer Flow
Baines, Uniform Flow
Baines, Boundary Layer Flow
Paterson and Apelt, Boundary Layer Flow
Richards et al., Full Scale Measurements
3
Fig. 3. Comparison of centerline pressures on a cube under various flow regimes with one face normal to
the direction of flow.
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means of reducing entrainment, a 50 mm lip around the nozzle outlet was added to
retard shear layer development between the ambient air and the jet.
The outlet velocity and turbulence intensity profiles at half a diameter above the
jet outlet are shown in Figs. 5 and 6, respectively. Longitudinal refers to thetraversing direction of the moving jet while only half the lateral profile is shown, as
this was the limit of the anemometer traverse mechanism. This location was chosen
Switches
Cube
PitotZ
X
3.9m
5m1.1m
2.4m 0.15m
Fig. 4. The Moving Jet Wind Tunnel.
0
2
4
6
8
10
12
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
Diameters from Center
Velocity(m/s)
Longitudinal Traverse
Lateral Traverse
Fig. 5. Velocity profiles half a diameter above the jet outlet.
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to be well clear of the screen and honeycomb at the jet outlet and away from the
influence of the ground plane. The average velocity here (Vref) was E10 m/s with a
turbulence intensity ofE4%. Fig. 7 shows the velocity spectrum on the jet centerline
half a diameter above the jet outlet and indicates little high frequency content in the
jet and lower magnitude fluctuations than might be anticipated in a boundary layer
flow. Fig. 8 shows the reduction in jet velocity as it approaches the testing surface.
The decay is small above 1 jet diameter and decreases linearly below 0.7 jet diameters
to zero at the surface.
The blower was mounted on rails and could be translated manually atapproximately constant velocity of up to 2 m/s, as timed by a pair of switches
mounted on the track 5 m apart. One switch was positioned directly under the model
location (X 0) to synchronize jet motion and model pressures. The results for the
moving jet are presented in Part B of this paper [24].
A model cube of side length 30 mm was constructed from 2 mm thick Perspex and
38 tappings of 1 mm diameter were located around the cube. Tappings were
distributed along a vertical centerline with six equally spaced pressure taps located
on the windward and leeward walls and seven equally spaced taps across the roof.
An additional six equally spaced taps in a horizontal profile recorded pressures along
one sidewall at mid-height. A dense grid of 13 taps provided a more detailed recordof pressures on one roof corner. The tappings were connected by 200 mm of 1.02 mm
diameter tubing to a Scanivalve ZOC33 64Px pressure measurement system and
0
5
10
15
20
25
30
35
40
45
50
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
Diameters from Center
TurbulenceIntensity(%)
Longatudinal Traverse
Lateral Traverse
Fig. 6. Turbulence intensity profiles half a diameter above the jet outlet.
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0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.0 0.2 0.4 0.6 0.8 1.0 1.2
V/Vref
Z/D
Fig. 8. Non-dimensional velocity decay profile between the jet and testing surface along the jet centerline.
The outlet of the jet is located at Z=D 1:7:
0.00001
0.0001
0.001
0.01
0.1
1
0.1 1 10 100
Frequency (Hz)
Magnitude((m/s)^2/Hz)
Fig. 7. Velocity power spectrum at half a diameter above the center of the jet outlet.
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sampled at 400 Hz for 60 s. Dynamic calibration of the pressure measurement system
revealed that there wasE10% amplitude magnification at 100 Hz. While corrections
to the frequency response for stationary tests could have been undertaken, they
would have been problematical for the transient moving jet tests (detailed in Part B)and for consistency and because the amplification was small, no corrections to
pressure data were made. Furthermore, pressure spectra revealed negligible
amplitude at frequencies above 100 Hz. The reference pressure was background
atmospheric pressure in the laboratory, well away from the jet.
A Cartesian coordinate system with an origin at the center of the base of the
model, located on the centerline of the moving jet, was employed, with Z measured
away from the surface and positive X being upstream of the model. The jet diameter
(D) was used to non-dimensionalize all distance in this study.
4.2. Experimental procedure
Velocity profiles produced by the wall jet were obtained using a TSI IFA300 hot
wire anemometer system and single wire hot film probes sampled at 200 Hz for 20 s
and repeated several times. The hot wire was mounted parallel to the test surface.
Velocities were non-dimensionalized by the centerline jet mean velocity (Vref)
measured at the reference location of half a diameter beyond the jet outlet (at
Z=D 1:2 and X=D 0) before and after each traverse. Vertical velocity profileswere obtained for various stationary positions of the jet away from the origin
(0pX=Dp3) for heights ranging from 3 to 153 mm. At each X=D position of the jet,a mean velocity ratio between the reference location and at the roof or eaves height
of the cube, 30 mm, was also obtained for later pressure coefficient reduction of the
cube pressures.
A miniature pitot-static tube positioned half a diameter above the jet outlet
(Z=D 1:2), was sampled at the beginning and end of each pressure test run andwas used as an initial reference dynamic pressure when non-dimensionalizing cube
pressures. Pressure measurements over the cube were obtained for stationary jet
positions ranging from 0pX=Dp3 and for two cube orientations, 01 and 451 to aface. The jet-induced static pressure over the inverted ground surface was obtained
by measuring the pressure at a flush-mounted tap at the model location (X=D 0)without the model in place. This was undertaken for the jet in positions 0pX=Dp3:
4.3. Stationary jet velocity profiles
Fig. 9 shows the mean velocity profiles over the test surface as a function of the
position of the jet. The velocities have been non-dimensionalized by the outlet
velocity of the jet at the reference location, while heights have been non-
dimensionalized by the jet diameter. These velocities were obtained from the single
film probe and as such represent the velocity magnitude. In regions close tostagnation and well above the test surface, the velocity will have significant vertical
component. The height of the cube is also shown on the figure for reference.
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When the jet is placed directly over the measuring point, X=D 0; the jet velocity
decreases linearly to the stagnation point on the test surface. Note for this locationthe reference velocity is located at 1.2 diameters above the surface (i.e., at Z=D 1:2;V=Vref 1). The characteristic nose of a wall jet develops after 0.75 diameters,reaching maximum mean velocity at about 1 diameter and then gradually slowing
and thickening as the test surface induces boundary layer growth. At X=D 1; thelargest velocity in the profile is approximately the same as the reference wind velocity
(V=Vref 1). At this point in the flow, the wall jet is thin and the flow ispredominantly horizontal. The almost constant velocity with height indicates that
the effect of surface roughness has not developed in this region, and that the
boundary layer that must form over the testing surface is lower than the lowest
measurement point (3 mm). Castro and Robins [13] observed for their uniform flowtests a boundary layer thickness of between 2 and 6 mm.
The geometric scale of the current simulation may be estimated based on the
velocity profiles of the stationary jet. Hjelmfelt [8] observed that the parent
downdraft of a typical microburst had a 1.8 km diameter and that the maximum
outflow winds occurred atE1.5 km from the center of the descending column of air.
Based on these observations, the Moving Jet Wind Tunnel simulation has a
geometric scale ofE1:35001:3000. However, as Hjelmfelt observed microbursts
with diameters ranging between 1.2 and 3.1 km, the current simulation may represent
a range of scales based upon these dimensions. A velocity scale is more difficult to
determine as the full-scale velocities in the downdraft ranged from 6 to 22 m/s,compared with the B10 m/s here. It becomes more important to define a velocity
scale when the jet translates and this is the subject of Part B [24].
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 0.2 0.4 0.6 0.8 1 1.2 1.4
V/Vref
Z/D
X/D=0.0
X/D=0.5
X/D=0.75
X/D=1.0
X/D=1.25
X/D=1.5
X/D=2.0
X/D=3.0
Fig. 9. Non-dimensional wind velocity profiles over the testing surface as a function of distance from the
stagnation point X=D 0 for a stationary jet.
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The turbulence intensities (standard deviation/mean velocity) for the wall jet as a
function of distance from jet stagnation point (X=D 0) are presented in Fig. 10.Little turbulence occurred in the flow close to the stagnation point, although as themean velocity tended to zero, the turbulence intensity became very large. When
positioned between X=D 0:5 and 0:75; the jet produced turbulence intensities ofE13% over the height of the cube. At the point of maximum mean wind velocity,
X=D 1; the turbulence intensity was 20% over the height of the cube. At greaterdistances from stagnation the turbulence intensities become very large and possibly
beyond the ability of the probe to accurately respond. This is because the mean
velocity in these regions is tending to zero.
Wood et al. [10] investigated the effect of varying the distance between the jet
outlet and the testing surface (Zj) on the wind velocity profile. Fig. 11 comparesWoods results, with those of the current tests, and those of earlier tests at the
University of Queensland [21] at the location of greatest velocity BX=D 1: Woodused a 300 mm diameter jet with Vref 20 m/s, and Letchford and Mans [21] a
430 mm jet with Vref 7 m/s. It is clear that there is a distinct relationship between
velocity profile and distance of jet from test surface (Zj). The closer the jet outlet to
the test surface, the thicker and faster is the wall jet. This is not surprising as the
simulation is effectively a free jet that will dissipate approximately as the square root
of distance from the nozzle or outlet. In the studies shown in Fig. 11, the maximum
mean velocities remained at distances of E1 diameter from jet stagnation,
irrespective of distance between jet and wall (Zj).Hjelmfelt [8] analyzed the wind velocity profiles of eight full-scale downbursts.
Fig. 12 shows a comparison of Hjelmfelts investigation to the wind velocity profiles
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 20 40 60 80 100 120 140 160
Turbulence Intensity (%)
Z/D
X/D=0.0
X/D=0.5
X/D=0.75
X/D=1.0
X/D=1.25
X/D=1.5
X/D=2.0
X/D=3.0
Eaves Height
Fig. 10. Turbulence intensity profiles over the testing surface as a function of distance from the stagnation
point X=D 0 for a stationary jet.
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produced between 0:75pX=Dp1:5 from the current study. The velocities have been
non-dimensionalized by the maximum velocity in each profile while heights havebeen non-dimensionalized by the elevation at which the maximum velocity occurred.
The wind velocity profiles of the current study show less variation than the full-scale
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
V/Vref
Z/D
Wood et al. Zj = 0.5D
Letchford and Mans Zj = 1.25D
Chay Zj = 1.7D
Wood et al. Zj = 2.0D
Wood et al. Zj = 5.0D
Fig. 11. Comparison of non-dimensional velocity profiles at X=D 1 with various separations of jetoutlet from testing surface (Zj).
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 0.2 0.4 0.6 0.8 1 1.2
V/Vmax
Z/Zmax
full-scale minimum
full-scale mean
full-scale maximum
X/D=0.75
X/D = 1.0
X/D=1.25
X/D=1.5
Fig. 12. Comparison of non-dimensional velocity profiles to full-scale data (Hjelmfelt [8]).
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data, which is rather limited. However, the overall trends are similar, indicating that
a wall jet is a reasonable representation of the mean wind profile.
4.4. Static pressure field
A single pressure tap on the testing surface recorded the static pressure variation
within the simulated downburst. Eq. (1) shows the conversion of the mean static
pressures at each jet location (X=D) to coefficient form with respect to the meandynamic pressure of the jet at the reference location. Ambient pressure in the
laboratory away from the jet (PATMOS) provided the reference pressure for the
transducers
CP PSTATIC PATMOS
12rV2ref
: 1
Fig. 13 shows the variation of static pressure beneath the wall jet as a function of
distance from stagnation. The stationary jet produced a high-pressure region
between 0pX=Dp0:25 approximately equal to the stagnation pressure at the outlet.This region represented the mesohigh of a downburst. The pressure coefficient at
stagnation exceeded 1 as the jet produced a slightly non-uniform velocity profile over
the cross-section of the outlet, increasing slightly in magnitude away from the center
of the outlet, where the reference velocity was measured (Fig. 5). As the jet was
positioned further from X=D 0:5 the static pressure showed a sharp decrease and
quickly approached atmospheric pressure at X=D 1:5: The stationary jet did notcreate a negative static pressure region, as indicated by Fujita [7] in Fig. 2. This is
because in this quasi-steady simulation, there are no transient ring vortices formed
-
0.2
0
0.2
0.4
0.6
0.8
1
1.2
00.511.522.533.5
X/D
Cp
Fig. 13. The mean static pressure field of the stationary jet.
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by the downdraft and which subsequently interact with the ground as suggested by
Fujita [7]. This will be discussed in detail in Part B of this paper, which deals with the
moving jet.
5. Results
5.1. Stationary jet pressure tests
Pressures were measured over the cube for two orientations (01 and 451), with the
jet located in the range 0pX=Dp3: Only 01 results are discussed in this paper.Eq. (2) indicates the conversion of mean surface pressures to coefficient form, with
the ambient pressure in the laboratory away from the jet providing the reference
pressure (PATMOS) and the mean dynamic pressure from the pitot-static tube at the
reference location
CPJ P PATMOS
12rV2ref
: 2
Fig. 14 shows the variation of the mean pressure coefficient along the cube centerline
as a function of jet position for wind perpendicular to one face (01). At the
-1.5
-1
-0.5
0
0.5
1
1.5
0 1 2
Position
Cpj
X/D=0.0 X/D=0.5 X/D=0.75
X/D=1.0 X/D=1.25 X/D=1.5
X/D=2.0 X/D=3.0
0
1
2
3
3
Fig. 14. Mean pressure coefficients observed along the centerline of the cube.
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stagnation position, X=D 0; the jet produced pressure coefficients slightly higherthan 1 at all locations on the model. These are larger than 1 for the same reason that
the surface static pressure was larger than 1 at stagnation, as discussed in Section 4.4
above. Loading under these conditions would be particularly relevant to sealedstructures, where the internal pressure would remain at pre-downburst atmospheric
pressure as represented by PATMOS in Eq. (2).
As the jet is positioned further from the model, the influence of the jet stagnation
pressure wanes while the effect of airflow becomes apparent with negative pressure
coefficients developing in separated flow regions on the roof and lee wall for X=D >0:75: The mean pressure coefficient profile X=D 0:5 reflects the action of wind flowover the model, although the mean pressures on the faces of the cube are still
positive.
The mean pressure coefficient along the cube centerline showed very little
variation over the windward face for most positions of the jet, while there was a
gradual decrease in magnitude across the roof. The jet produced the greatest
magnitude mean suction pressures when positioned in the range 1pX=Dp1:25; withthe dominant component due to wind flow as the static pressure becomes very small.
The magnitude of the mean pressures decreased considerably as the jet moved
beyond X=D 1:5:
5.2. Comparison of mean pressure coefficients with earlier studies
To facilitate comparison with earlier traditional wind tunnel studies, a rooftop oreaves height pressure coefficient was defined by Eq. (3). Here, use is made of the ratio
of mean velocities between the reference velocity location and that at eaves height at
X=D 1 to convert the coefficients defined earlier by Eq. (2). The X=D 1 locationwas chosen because it is where the velocities and pressures were largest, and therefore
constitutes a design case
CPE P PATMOS
12rV2
EAVES;X=D1
: 3
Fig. 15 presents a comparison between these pressure coefficients and Castro and
Robins study [14] in uniform and turbulent boundary layer flows. The present
centerline pressure coefficients bear closer resemblance to the uniform flow results,
although they show slightly greater variation across the roof. On the front face the
wall jet produces significantly higher positive pressures than measured by Castro and
Robins for both their flow types. This apparent increase is due to the eaves height
velocity being only some 90% of velocities at lower heights over the cube, as
indicated in Fig. 9.
Castro and Robins [14] and H .olscher and Niemann [16] identify turbulence
intensity differences as a significant factor with respect to the pressure distributionover the roof of a cube and this could also have contributed to the variations
observed here. At X=D 1; the stationary jet produced an approximately uniform
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velocity profile with a 20% turbulence intensity at eaves height, compared too0.5%
in the uniform flow of Castro and Robins. Contrastingly, the eaves height turbulence
intensity for Castro and Robins boundary layer flow wasB27%, indicating that the
mean flow type plays a very significant role in the surface pressure distribution.
5.3. Effect of the static pressure field
The effect of the static pressure field produced by the wall jet on building pressures
was examined by defining new pressure coefficients as indicated in Eq. (4), in which
the raised static pressure of the wall jet (Fig. 13) is removed from the building
pressures. Once again the mean eaves height dynamic pressure at X=D 1 was used
CPS P PSTATIC;X=D12rV2
EAVES;X=D1
: 4
Permeable structures, in which the internal pressure can quickly equilibrate to
changes in atmospheric pressure, would likely experience the pressures defined by
this coefficient.
Fig. 16 shows these new pressure coefficients and it is seen that a significantreduction in the pressures generated on the cube at locations close to the center of the
jet occurs. However, removal of the raised static pressure of the wall jet caused little
-1.5
-1
-0.5
0
0.5
1
1.5
0 1 2
Position
Cpe
X/D=0.75
X/D=1.0
X/D=1.25
Castro and Robins, Uniform Flow
Castro and Robins, Boundary Layer Flow
0
1
2
3
3
Fig. 15. Comparison of centerline pressure coefficients of the stationary jet with earlier studies.
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change for pressure coefficients in the range 0:875pX=Dp1:25; which correspondsto the largest pressures/suctions under the action of the simulated downburst.
Cassar [23], using a trial wall jet at CSIRO [9], undertook the only other study of
pressures on a building model immersed in a wall jet known to the authors. This jet
had an octagonal outlet ofE1.5 m 0.85 m, with an effective diameter (D) of
1.05 m. Flow from the jet impinged on a smooth particleboard located 1.4 m away
from the outlet (Zj=D 1:33). Fig. 17 shows a comparison of the mean wind velocity
profiles at X=DB1 created by the jet used in the current study and the CSIRO jet.The velocities have been non-dimensionalized by the maximum velocity in the profile
(12.7 m/s for CSIRO) while the heights have been non-dimensionalized by cube
height, which was 100 mm for the CSIRO study. Clearly, the CSIRO model was
relatively larger with the top of that cube extending into the shear layer region above
the wall jet.
Fig. 18 compares the centerline pressure coefficients over the cube for these two
studies. The coefficients are defined by Eq. (4), excepting that for the present studys
X=D 0:75 position, the eaves height dynamic pressure at X=D 0:75 has beenused to ensure compatibility with Cassars pressure coefficient definition. The shape
of the mean pressure profiles is similar, with good agreement on the front face for theX=D 0:75 case, however, Cassars suction pressures are all much lower inmagnitude. It is likely that the differences in velocity profile in relation to the cube
-1.5
-1
-0.5
0
0.5
1
1.5
0 1 2
Position
Cps
X/D=0.0 X/D=0.5 X/D=0.75
X/D=1.0 X/D=1.25 X/D=1.5
X/D=2.0 X/D=3.0
0
1
2
3
3
Fig. 16. Centerline pressures on the cube for 01 orientation, referenced against the static pressure of the
diverging flow and expressed as a ratio of the eaves height dynamic pressure at X=D 1:
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-1.5
-1
-0.5
0
0.5
1
1.5
0 1 2
Position
Cps
Cassar (CSIRO Jet), X/D = 1.0
Chay, X/D = 1.0
Chay, X/D = 0.75
0
1
2
3
3
Fig. 18. Comparison of centerline pressure coefficient profiles created by the TTU Moving Jet Wind
Tunnel and the CSIRO wall jet on a model cube.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 0.2 0.4 0.6 0.8 1 1.2
V/Vmax
Z/H
Chay
Cassar (CSIRO Jet)
Fig. 17. Comparison of mean velocity profiles at X=D 1 for the TTU Moving Jet Wind Tunnel andCSIRO wall jet (Cube height=H).
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height (Fig. 17), i.e., a cube three times larger in a flow nominally only twice as big,
are responsible. In addition, the ambiguity in defining a representative static
reference pressure for this type of flow could also explain the observed differences.
6. Discussion
This paper reports a study of mean pressures generated over a cube in a simulated
thunderstorm downburst flow. A stationary wall jet was employed for this quasi-
steady downburst simulation. Wall jets have been shown to have similar mean
velocity characteristics of reported full-scale downbursts, in particular, the variation
of velocity with height above the ground which leads to a velocity maximum at some
50100 m at distances about 1 jet diameter from the stagnation point of the jet. Thegeometric scale of this simulation was estimated to be 1:3000.
The velocity profiles produced by this simulation have been shown to relate to
earlier studies and it has been shown that wall jet velocities close to stagnation are a
function of separation distance between the jet outlet and the wall. The maximum
mean velocity occurred at a distance of 1 jet diameter from the stagnation point and
produced a nearly constant velocity distribution over the height of the cube.
Turbulence intensities at this location were E20%.
As expected the static surface pressure distribution over the wall produced by the
jet varied significantly from maximum at stagnation to background atmospheric at a
radius of about 1.5 jet diameters. Pressures generated over the cube could be dividedinto three regions based on location from jet stagnation (X=D 0):
* Directly beneath the jet when almost all pressure is due to the static pressure field.* A transition region where pressures are due to both the static pressure field and
diverging wall jet flow X=DB0:5:* A region in which almost all pressure experienced by the cube is due to the
diverging flow of the wall jet X=D > 0:75
Comparing the pressure distributions over the cube with conventional wind tunnel
studies in both uniform and boundary layer flows indicated a greater likeness withuniform flow tests in the region of highest magnitude pressure around X=D 1: Thiswas attributed to the similarity of mean velocity profiles over the height of the cube
in this region. However, the significantly greater turbulence in the wall jet lead to
greater variation in separated flow regimes as might be expected given the
importance of turbulence in separating shear layers and their subsequent
reattachment. In addition, the windward pressures were significantly greater in the
wall jet, due to the inverted velocity profile that decreased slightly with height.
Further away from jet stagnation, X=D > 1:5; boundary layer development of thediverging wall jet flow occurs and consequently the pressures become more like those
of conventional boundary layer tests. However, these regions experience much lowervelocities and consequently pressures and are thus not considered significant from a
wind load design perspective.
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This study has shown that mean pressure distributions on objects immersed in wall
jets differ from those in conventional wind tunnel studies, however, as a realistic
representation of wind pressures generated by thunderstorm downbursts, there
remains considerable debate, not the least because the full-scale storms havesignificantly different kinematic structure compared to the quasi-steady wall jet
simulation. Part B of this paper attempts to address these issues by describing results
from the moving jet simulation, which successfully captured many of the
characteristics of full-scale downbursts, including the transient gust front.
Acknowledgements
The authors would like to acknowledge support from the Wind Science and
Engineering Research Center and a Seed Grant for Multidisciplinary Research from
Texas Tech University.
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