pressure distributions on a cube in a simultated thunderstorm downburts part a

7/29/2019 Pressure Distributions on a Cube in a Simultated Thunderstorm Downburts Part A

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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

M.T. Chay, C.W. Letchford / J. Wind Eng. Ind. Aerodyn. 90 (2002) 711732712


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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]).

M.T. Chay, C.W. Letchford / J. Wind Eng. Ind. Aerodyn. 90 (2002) 711732 713


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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



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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pressure distributions on a cube in a simultated thunderstorm downburts part a

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