WIND TUNNEL SIMULATION OF AN ATMOSPHERIC BOUNDARY

WIND TUNNEL SIMULATION OF

AN ATMOSPHERIC BOUNDARY LAYER

by

WILLIAM V. BURTON, B.S.M.E.

A THESIS

IN

MECHANICAL ENGINEERING

Submitted to the Graduate Faculty of Texas Tech University in

Partial Fulfillment of the Requirements for

the Degree of

MASTER OF SCIENCE

IN

MECHANICAL ENGINEERING

Approved

Accepted

December, 2001

1"^ ACKNOWLEDGEMENTS 2 0^)

r)to .3 T I would like to express sincere appreciation and gratitude to my graduate advisor,

C .^ Dr. Oler, for his guidance, support, and encouragement throughout my graduate studies

and research process. Also, I would like to acknowledge Dr. Letchford for all of his help

and guidance particularly in the analysis. My gratitude is also extended to Dr. James and

Dr. Para for their comments and remarks in relation to this thesis.

I would especially like to thank my father, Vance P. Burton, for his moral and

spiritual support, guidance, patience, encouragement, and sacrifices over the past years.

In memory of my mother, Linda J. Burton, I am extremely grateful for her love and

support throughout the course of my life.

I further wish to acknowledge the downstairs machinist. Norm, for his help during

the fabrication of the traversing mechanism arm and John Walter for all of his help and

support. A special thanks is extended to the rest of my family, fiiends, and colleagues for

their advice and the amount of stress relief they provided during college. Furthermore, I

greatly appreciate the time and effort of the mechanical engineering staff, Tonette

Rittenberry and Carmen Hemandez. Last but not least, I would like to thank the

Mechanical Engineering Department Chair, Dr. Thomas Burton (no relation), for giving

me the opportunity to further my education at Texas Tech University.

11

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ii

ABSTRACT v

LIST OF TABLES vi

LIST OF FIGURES vii

NOMENCLATURE ix

CHAPTER

1 INTRODUCTION 1

2 LITERATURE SURVEY 3

2.1 StmctureofABL 4

2.2 Characteristics of ABL 6

2.2.1 Mean Velocity Characteristics 7

2.2.2 Turbulent Characteristics 9

2.3 Simulation of the ABL 11

3 EXPERIMENTAL ARRANGEMENT AND PROCEDURE 15

3.1 Texas Tech Wind Tunnel 15

3.2 Physical Modeling 16

3.3 Method of Profile Measurements 22

3.3.1 Data Acquisition 23

3.3.2 Calibration Method 24

3.3.3 Traversing Mechanism 26

4 VELOCITY AND TURBULENCE MEASUREMENTS 29

4.1 Empty Wind Tunnel 29

4.2 Effects of Boundary Layer Augmentation Devices 35

4.3 Summary 45

5 RESULTS AND DISCUSSION 47

5.1 Mean Velocity Profiles 47

5.1.1 Characterization of Mean Velocity Profiles 50

5.2 Turbulence Profiles 54

iii

5.3 hitegral Length Scales 57

5.3.1 Autocorrelation 57

5.3.2 Determination of Eddy Size 60

5.4 Power Spectrum Analysis 61

5.5 Unified Scale Factors 65

6 CONCLUSIONS AND RECOMMENDATIONS 69

6.1 Rural Sunulation at 1:350 Scale Model 69

6.2 Suburban Simulation at 1:300 Scale Model 70

REFERENCES 71

IV

ABSTRACT

This thesis exammes the simulation of a rural and suburban area using the Texas

Tech vyind turmel. The objective was to simulate a rural and suburban type terrain by

matching key parameters to a reasonable unifying scale factor. Passive devices such as

spires, fence barriers, and roughness elements were implemented in the tuimel for the

simulations. ASCE 7-98 standards were used for similarity criteria between the full-scale

ABL flow characteristics and the simulations. Comparisons of the velocity profiles,

turbulence intensities, and power spectrum were performed.

The rural exposure was simulated with the installation of triangular-shaped spires,

two fence barriers, and carpet as the roughness fetch. For the suburban exposure

simulation, the installation of the spires, fence barriers, and 50 mm square roughness

blocks were used as the roughness fetch. Various scale factors were investigated until a

reasonable match for all the key parameters were obtained. A reasonable compromise

was found at a model scale of 1:350 for the rural simulation and a 1:300 for the suburban

simulation. Adequate simulations were performed at these scale factors with the

exception of insufficient turbulence intensity in the lower boundary layer.

LIST OF TABLES

3.1. Configuration of Mbcing Devices 21

5.1. Results for w,, Zo, and a 51

5.3. hitegral Length Scales 60

VI

LIST OF FIGURES

2.1. Structure of ABL [Garratt, 1992] 4

2.2. Reynolds stress through the ABL [Cook, 1985] 6

2.3. Energy Spectrum of Turbulence [Garratt, 1992] 10

2.4. Types of Turbulent Generators Investigated [Counihan, 1969] 13

3.1. Wind Tunnel 16

3.2. Wind Tunnel Schematic 16

3.3. Location of Mixing Devices 17

3.4. Downstream View of Spires 18

3.5. Upstream View of Spires 18

3.6. Spire Design (units in mm) 19

3.7. Upstream View of Carpeted Tunnel 20

3.8. Upstream View of Roughness Blocks 20

3.9. Anemometer Circuit [TSI Manual] 22

3.10. X-Film Sensor Orientation 23

3.11. Calibration Setup [TSI Manual] 24

3.12. Calibration Curve 25

3.13. Traversing Mechanism 26

3.14. Slider Arm Setup 1 28

3.15. Slider Arm Setup 2 28

4.1. Traversing Positions Looking Upstream 30

4.2. Profiles of Cross-Stream Traverse 31

4.3. Profile Comparison of High Wind Speeds 33

4.4. Profile Comparisons of Wmd Speeds 34

4.5. Effects of Roughness Elements 36

4.6. Effects of Downstream Fence Barriers 37

4.7- Effects of 95 mm Fence Downstream and Roughness Elements 39

4.8. Effects of 270 mm Fence Downstream and Roughness Elements 40

4.9. Effect of Spires vsdth Roughness Elements 42

vii

4.10. Effects of Spires with Roughness Elements and Downstream Fence Barrier 43

4.11. Effects of Spu-es with Roughness Elements and Upstream Fence Barrier 44

5.1. Mean Velocity Profile of Rural Simulation 48

5.2. Mean Velocity Profile of Suburban Simulation 49

5.3. ASCE 7-98 Velocity Profile Comparison 50

5.4. Logarithmic and Power Law Representations of Rural Simulation 52

5.5. Logarithmic and Power Law Representations of Suburban Simulation 52

5.6. Reynolds Stress Distribution 53

5.7. Turbulence Intensity of Rural Simulation 55

5.8. Turbulence Intensity of Suburban Simulation 55

5.9. ASCE-97 Turbulence Intensity Comparison 56

5.10. Autocorrelation of Rural Simulation @ 100mm Height 58

5.11. Autocorrelation of Suburban Simulation @ 100mm Height 58

5.12. Autocorrelation of Rural Simulation @ 250nim 59

5.13. Autocorrelation of Suburban Simulation® 250mm 59

5.14. Illustration of Eddy Size @ 250nim m the Wind Tunnel 60

5.15. Raw Power Spectrum of Suburban Simulation at 100mm 61

5.16. 100 mm Rural Simulation 63

5.17. 250mm Rural Simulation 63

5.18. lOOmm Suburban Simulation 64

5.19. 250nim Suburban Simulation 64

5.20. Mean Velocity Profile for Rural Simulation at 1:350 Scale 65

5.21. Turbulence Intensity Profile for Rural Sunulation at 1:350 Scale 66

5.22. Power Spectrum of Rural Simulation at 1:350 Scale 66

5.23. Mean Velocity Profile for Suburban Simulation at 1:300 Scale 67

5.24. Turbulence Intensity Profile for Suburban Simulation at 1:300 Scale 67

5.25. Power Spectrum of Suburban Simulation at 1:300 Scale 68

Vlll

NOMENCLATURE

cc Power Law Exponent

^ Te Mean hourly wind speed exponent for terrain category {Jc)\ ASCE 7-98

b Te Mean hourly wmd speed factor; ASCE 7-98

c Te Turbulence intensity factor; ASCE 7-98

^ Te Integral length scale power law exponent; ASCE 7-98

k Von Karman constant

/„ , /^, /^ Turbulence Intensities

/^ Turbulence intensity as a fimction of an equivalent height, z ; ASCE 7-98

L^,L^, L^ Integral length scales

L^ Integral length scale as a function of equivalent height, z ; ASCE 7-98

/ Te Integral length scale factor; ASCE 7-98

;; Frequency

A ; Reduced frequency; ASCE 7-98

p Ambient density

R^ Wind spectrum; ASCE 7-98

û\u2 Auto-correlation of two signals, separated spatially by distance

R(T) Auto-correlation of a single signal, separated spatigdly by time

S^ Power spectrum, with units of F ^ / 1 Hz

cr^, cr^, cr^ Standard deviations

T Wall shear stress w

T; , T; , T ; titegrated Time scale

u,v,w Longitudinal, lateral, and vertical components

w', v ' , w' Fluctuating longitudinal, lateral, and vertical components

Ti . Reference velocity located at an equivalent 10 m fiill-scale height

M, Friction velocity

ix

Ff Mean hourly wind speed at an equivalent fiill-scale height z; ASCE 7-98

lom.c ^^^ g^st speeds at z=l0 m in Exposure C; ASCE 7-98

Zj Zero-plane displacement

ZQ Aerodynamic roughness

z Height above ground

z^^ Reference height located at an equivalent full-scale height z=l 0 m

CHAPTER 1

INTRODUCTION

The atmospheric boundary layer (ABL) is the region of air which is greatly

influenced by the Earth's surface. For stmctural design purposes, it is important to

understand the atmospheric boundary layer and the turbulent flow characteristics

associated with it. Although computer modeling has come a long way in trying to model

the ABL, there are still many imexplained complexities associated with the flow.

Therefore, wind tunnel simulation remains an important investigative tool. Based on

meteorological data gathered from field sites and by other means, most researchers agree

certain target characteristics should be met. These target characteristics, defined by

Simiu and Scanlan (1996), are: (1) the variation of the mean wind speed wdth height, (2)

the variation of turbulence intensities and integral length scales with height, (3) the

spectra and cross-spectra of turbulence in the along-wind, across wind, and vertical

directions.

Most wind tunnels, typically designed for the study of aeronautics with smooth

laminar flow, lack the turbulence intensity and sufficient boundary layer depths to

simulate the ABL. In the past, several researchers have implemented passive devices to

artificially enhance these flow characteristics. Augmentation devices such £is vortex

generators (spires), fence barriers, and various floor roughness are often used as means to

intensify these characteristics.

For the current investigation, the objectives were to develop a reasonably accurate

simulation of the ABL for rural and suburban terrains. The boundary layer simulation(s)

utilized a combmation of four triangular shaped spires, two different size fence barriers,

and various types of floor roughness. The results for each simulation were compared to

ASCE 7-98 standards.

The background and key characteristics that help define the ABL are discussed in

Chapter 2. In the same chapter, some of the simulation techniques employed in the past

are reviewed and discussed. Chapter 3 gives a description of the measurement techniques

and experimental procedure used for the current investigation. In Chapter 4, the results

of implementing the mixing devices individually and in various combinations are

discussed. Profiles for the velocity and turbulence intensities resulting from the various

combinations are shown. Chapter 5 provides the results of a more thorough evaluation

on the rural and suburban configurations introduced as having the greatest potential for

ABL simulations. The final chapter. Chapter 6, gives the conclusions and

recommendations based on the results produced from the simulations.

CHAPTER 2

LITERATURE SURVEY

This chapter provides a review of studies on aspects of atmospheric boundary

layer flow that are of mterest in stmctural design. A usefiil working definition identifies

the atmospheric boundary layer (ABL) as the layer of air du-ectly above the Earth's

surface m which the effects of the surface fiiction are felt du-ectly. The major mfluence

that causes the boundary layer to form is shear stress at the surface.

Atmospheric boundary layer flow is generally turbulent and therefore, exhibits

both temporal and spatial variations. To facilitate descriptions and analysis, Reynolds

decomposition is generally applied to divide the flow field mto mean and fluctuating

components,

u{r, t) = u{r, t)i -\- v(r, / ) / + w{r, t)k (2.1)

u{f,t) = u(r) + u'{f,t) (2.2)

where the mean velocity vector is defined as,

1 ^ «(r) = lim -'\u{r,t)dt. (2.3)

At the surface, the velocity is zero due to a no slip condition. With increasing height

above the surface, the velocity increases to a maximum, the freestream velocity, which

marks the edge of the boundary layer. Since the velocity approaches the freestream

velocity asymptotically, the overedl boundary layer thickness is defined as the height at

which the velocity is 99 % of the freestream velocity. In addition to the velocity gradient,

factors such as surface heating and solar heating may cause a thermal gradient to form in

the boundary layer. However, strong wind speeds generally provide enough mixing to

suppress most thermal effects. Therefore, under strong wind conditions (M > \Om/s), the

ABL can be treated as being effectively adiabatic and neutrally stable. This assumption

applies to the vast majority of wind loading applications. With wind speeds that are

considerably lower than the speed of sound, incompressibility may also be assumed

[Simiu and Scanlan, 1996].

2.1 StmctureofABL

Figure 2.1 illustrates the ABL stmcture vyith neutrally stratified conditions and

rough terrain (such as small buildings). The ABL can be divided mto two main regions,

an inner region and an outer region. The inner region is further divided into two layers,

an mterfacial sublayer and an mertial sublayer.

(rs0.1A)"

r»a>.

Outer (Ekman) Uyer

i^ a

Inter (surface) layer

n n n n

Ineitial sublayer

n. Inlexfacial

« ) subUyei t lnlexucu (KUfteVB sublayer

Figure 2.1. Stmcture of ABL [Garratt, 1992].

The interfacial sublayer is the region at the very bottom of the ABL occupied by

the surface roughness features. At the top of the surface feature, the Reynolds stress is at

a maximum value and reduces to zero at the ground surface. This is due to momentum

loss from pressure forces on the uidividual elements of roughness. The thickness of the

interfacial layer is called the 'zero-plane displacement', z^. When the surface roughness

is small, e.g., in open country, the interfacial layer is very shallow (the depth of the

vegetation) and the zero-plane displacement can be assumed zero [Cook, 1985].

The mertial sublayer is the region which is directly affected by the surface

characteristics. The nature of the surface can be categorized into four distinct terrain

types; urban, suburban, rural, and smooth. The ASCE 7-98 standard categorizes each

terrain type by the following exposures:

• Exposure A ; Large city centers with at least 50% of the buildings having a

height in excess of 21.3 m (70 ft.).

• Exposure B; Urban and suburban areas, wooded areas, or other terrain with

numerous closely spaced obstructions having the size of single family dwellings

or larger. This exposure is representative of terrain in the upwind direction for a

distance of at least 460 m (1,500 ft.) or 10 times the height of the building or other

stmcture, whichever is greater.

• Exposure C; Open terrain (rural terrain) with scattered obstmctions having

heights generally less than 9.1 m (30 ft.). This category includes flat open

country, grasslands and shorelines in hurricane prone regions.

• Exposure D; Flat, unobstmcted areas exposed to wind flowing over open water

for a distance of at least 1.61 km (1 mile). Shorelines in this exposure include

inland waterways, the Great Lakes and coastal areas such as California. Exposure

D extends inland from the shoreline a distance of 460 m (1500 ft.) or 10 times the

height of the building or stmcture, whichever is greater.

The outer region, also known as the Ekman layer, is not significantly affected by

the surface roughness. This region is the remainder of the ABL through which the

Reynolds stresses decrease from a maximum in the irmer region to zero at the maximum

boundary layer height (gradient height). This Reynolds stress variation is illustrated in

Figure 2.2.

<V''^

Gradient height

Surface regicin

Ekman layer

7eroplan«di«>lac«fner>i. d

Interfacial layer

Reynolds stress, —pûw

Figure 2.2. Reynolds stress through the ABL [Cook, 1985].

2.2 Characteristics of ABL

In the time frame of 1880-1959, the properties of the ABL were largely quantified

and a broad picture of the flow stmcture emerged. However, its detailed flow pattems

were still undefined. Consequently, actual representations of the atmospheric boundary

layer remained in much debate. Accumulation of a considerable amount of data from

various sites during the 1960s and 1970s allowed better agreement on descriptions of the

ABL. The empirical formulae developed from these measurements have guided

simulations of atmospheric flows.

In simulating the ABL flow, it is important to accurately describe the ABL and its

characteristics. The most general and current accepted wind characteristics, as stated

by Simiu and Scanlan (1996), include the following:

• The variation of the mean wind speed with height.

• The variation of turbulence intensities and mtegral length scales with

height.

• The spectra and cross-spectra of turbulence in the along-wind, across

wind, and vertical directions.

The details of the mean velocity characteristics and the turbulence characteristics are

discussed m the following sections.

2.2.1 Mean Velocity Characteristics

Many laws for the variation of the mean velocity yvith height have been

suggested. Stevenson (1880) proposed a parabolic law which provided a fair

representation of the measured velocity profiles:

u u ref

f z + 22 ^ V^«f+22^

(2.4)

where the units of z and Zref are m meters. However, it did not apply to the lower 10m of

the measured velocities. A better empirical model for mean gust wind speed profiles

utilized a power law model of the following form [Hellman, 1916]:

u z ^

M, ref v^ref y (2.5)

The longitudinal velocity, w(z) ,is the time averaged velocity at a distance, z ,above the

surface. The velocity distribution is generally normalized against at a meteorological

standard height of z^f = 10m above the surface. The exponent of the power law, a, is a

function of the terrain roughness. While an improvement over Stevenson's parabolic

law, this model is also not analytically correct for the measured velocities in the bottom

10m of the ABL. Nonetheless, it is still widely used due to its simplicity [Cook, 1985].

Sutton (1949) developed a more suitable representation of the mean velocity profile that

was applicable to the lower 10m heights and the inertial sublayer. Within this region, the

descriptions for the outer and inner layer scaling are valid simultaneously. Sutton's

mean velocity profile follows a logarithmic relation as follows:

u=—iU'Ln k

{ z-z^

V ^0 J

k=fSA (2.6)

where k is the Von Karman constant which is based on experiments m vsdnd tunnels and

m the atmosphere. Equation (2.6) is commonly known as the logarithmic law-model.

This law takes into account the zero plane displacement ,z^, when considering large

roughness elements. Equation (2.6) also mcludes two other important parameters: the

aerodynamic roughness, Zo, and the fiiction velocity, M, .

The aerodynamic roughness length is defined as the height above the ground

where the wind speed becomes zero due to the effects of vegetation. Typical roughness

heights defined by Shniu and Scanlan (1996) are the following:

Urban

Suburban

Rural

Smooth

2 m < Z j j < 3 m

0.2m<Zj,< 1.2 m

0.001 m<z„< 0.2 m

0.0001 m<z„< 0.006 m.

The fiiction velocity is defined as the square root of the wall shear stress divided

by the ambient density.

".=J— (2.7)

and the wall shear stress, r^, is defined by,

The Reynolds stress (-p u'w'] is a maximum at the zero-plane displacement, and is the

surface shear stress transferred through the surface layer. The definition of friction

velocity at the zero plane displacement can, therefore, be extended to the following

equation:

M. =^-(wV') . (2.9)

8

2.2.2 Turbulent Characteristir.s

Turbulent velocity fluctuations from the viewpoint of a fixed observer may be

conceptually understood as resulting from the passage of a sequence of eddies, each

characterized by a periodic motion of circular frequency W = 2-KIN {oiby 3. wave

number K^l- njX, where X is the wave length). The total kinetic energy of the

turbulent motion may, correspondingly, be regarded as a sum of contributions by each of

the eddies of the flow. The fimction representing the dependence upon wave number of

these energy contributions is defined as the energy spectrum of the turbulent motion

[Simiu and Scanlan, 1996]. Therefore, by performing spectral analysis on turbulent

motion, an understanding of the energy distribution in turbulent flows are possible.

The statistical theory of turbulence, related to problems of diffusion and the scale

and spectrum of turbulence is largely credited to G.I. Taylor in the period 1915-1938

[Garratt, 1992]. Taylor noted that since velocities vary spatially as well as temporally,

correlation measurements can give an indication of the size of gusts or eddies. Taylor's

hypothesis can be shown in the following form:

00

0

where Ruiui^^ the cross-covariance of the fluctuating longitudinal velocity components

separated at a spatial distance. If it is assumed that the flow disturbance travels vyith the

mean velocity Hir^t^, then the following relation can be written as follows,

00

Lu, = u-JR{T)dT. (2.11) 0

R(T) is the autocorrelation of the fluctuating longitudinal velocity and r is the time lag

between measured velocities.

Kolmogorov (1941) made important contributions to the understanding of the

small-scale stmcture of turbulence and the energy transfer process from large to small

scales through his similarity theory of turbulence. The hypothesis of Kolmogorov (1941)

showed that the small scale turbulent eddies were associated v^th the high frequency end

of the spectrum having mdependent properties, and are isotropic. After Kolmogorov's

similarity theory, h was generally accepted that the eddy sizes represented in the

spectrum could be divided mto three categories as follows [Counihan, 1975]:

a. A low frequency range, containing most of the turbulent energy: this

energy being transferred by inertial forces to higher frequencies.

b. An intermediate range, or vertical subrange, which follows Kolmogorov's

-5/3 law, and

c. A high frequency range, where viscous forces dominate and dissipation

occurs.

Figure 2.3 shows a representation of the energy spectrum of turbulence and where

Kolmogorov's -5/3 law applies.

I Equitt)hum Range

Energy

ir>ertial Subrange i ^

I I Vscous Dissipation

I ^

Ffequency or VVavenwrt)er

Figure 2.3. Energy Spectrum of Turbulence [Garratt, 1992]

Before 1925, it was thought that all eddy velocities were isotropic. Observations

by Goldie (1925) proved otherwise. Goldie showed that the air near the ground consisted

of partially formed, rapidly dismtegrating eddies. Best (1935) demonstrated that the

longitudinal, lateral, and vertical eddy velocities were not the same in magnitude; he also

10

showed that the flow consisted of eddies of various sizes. Schmidt (1935) suggested that

this turbulence, in the form of vortices at ground level, was spread upwards m the

boundary layer. As described by Counihan (1975), it became clear m the 1960s that

these vortices were in fact "projected" into the main flow above the ground, thus

distributing the turbulence from ground level into the upper part of the boundary layer.

The simplest and most commonly used descriptor of atmospheric turbulence is the

turbulence intensity. Equations 2.12-2.14 define the turbulence intensity for the

longitudinal (w), lateral (v), and the vertical (w) components, respectively.

h=^ = Ji-ti^,-wf-^ (2.14) u \ N ^ u

where cr„, cr , cr are the standard deviations for the longitudinal, lateral, and vertical

fluctuating velocity components, respectively.

2.3 Simulation of the ABL

When work first began in vsind tunnel simulation of the ABL it was noted that

the structure of the atmospheric boundary layer showed many similarities to the two-

dimensional turbulent boundary layer. Both had distinctive inner and outer regions. In

the inner layer, the flow is mainly dependent on the surface characteristics and is virtually

independent of Coriolis forces (due to Earth's rotation). In contrast, the outer region flow

shows little dependence on the nature of the surface and is strongly influenced by the

Earth's rotation. The transition between the irmer and outer layers is not abrupt, but is

characterized by an overlap region. The influence of the surface is directly felt in the

interfacial sublayer, which is the layer of air within and just above the roughness

elements comprising the land or sea surface [Garratt, 1992].

11

Although meteorological investigations have established many theoretical and

empirical formulae for the ABLs' characteristics, there are many complex effects

associated v^th fluid flow that remain uncertain. Efforts have been made to duplicate a

'typical' ABL in a wind tuimel. Successful sunulation in a wind tunnel may explain

some of the complexities associated with fluid flow. Neutral flow, in which buoyancy

effects are absent, is readily produced in a vsind tunnel, and may be closely approximated

m the atmosphere m windy conditions with complete cloud cover (see Fig. 2.1) [Garratt,

1992]. The ABL's flow characteristics on or around buildings and other structures are

especially of interest to a structural engineer. Therefore, for convenience and economic

reasons, physical experimentation in a flow facility is necessary.

The significance of using the wind tunnel for boundary layer testing was

recognized as early as 1943. It was recognized that in order to produce a wind gradient,

models had to be mounted on a thin flat plate, parallel with dkection of air flow. Studies

showed that the wind gradient magnitude depends on the length and roughness of the

surface on the upstream side of the model [Cook, 1985]. Subsequently, the length of the

test section is used as a classification of the boundary layer wind tuimel (BLWT).

Short test-section boundary-layer wind tunnels (SBLWT) may be defined as a

boundary layer wind tunnel of length such that a naturally developed boundary layer will

just reach a state of equilibrium. The floor of the SBLWT test section is usually on the

order of 5m long. Long test-section boundary layer wind tunnels (LBLWT) may be

defined as a wind tunnel with sufficient length to produce a naturally developed

simulated ABL with a nominal thickness in the range of 0.5-1 m at an ambient wind

speed of about 10 m/s [Cermak, 1984]. The majority of wind tunnels are most often

intermediate between a SBLWT and a LBLWT and have insufficient fetch length (test

section length) for a simulated ABL to naturally develop. As a result, various types,

shapes, and combinations of passive devices have been used to artificially increase the

boundary layer depth.

In the late 1960s augmentation devices were introduced in an attempt to create

realistic wind profiles and turbulence profiles. Previous simulation methods could not

12

achieve correct representation of all of the flow properties simultaneously, and in most

cases reproduced only the velocity profile. Counihan (1969) developed an improved

method of simulating an ABL in a wind tunnel. He found that a working section length

of between four and five boundary layer heights was required to produce the simulated

flow. The characteristics of four simple generator shapes were examined in a boundary

layer vsind tunnel. The tunnel had a working section of 0.609m wide, 0.185m high, and

1.52 m fetch length. The generators consisted of triangular, cranked triangular, plane

elliptic and elliptic wedge shapes as shovm in Figure 2.4. Since the wakes behind the

triangular and elliptic generators were distinctly different, Counihan elected to explore

those two shapes in greater detail.

iMlt VltWOf T«MM6Ut.AN AMO C««<mtC CCMCRATOeS

4 i

- f . / jiWLBaEd".

CLLirnc Mcoec ceMCRAToe NOMZONTAL SCCTKM • a' scM-eift

Figure 2.4. Types of Turbulent Generators Investigated [Counihan, 1969]

With the triangular generator, Counihan noticed a tendency for an excessive

momentum loss in the inner region of the boundary layer and msufficient loss in the outer

13

section. The elliptic wedge generators, however, produced adequate results for a rural

terrain. Counihan (1973) modified his rural simulation method for an urban environment.

He did this by mtroducing floor roughness elements down the entire length of the test

section floor. The roughness elements consisted of "LEGO" bricks measure 9.5 mm

high, and 5.9 mm square which were fitted to a "LEGO" baseboard on the wind tunnel

floor. This simulation w£is considered to be an 'adequate' representation of an urban

boundary layer. However, at the time only a small amount of full-scale measurements for

urban boundary layers were available for comparisons.

Several other noted authors such as N.J. Cook (1978), Irwin (1980), Cermak

(1982), and Farell (1999) have made progressive improvements in simulation methods.

Irwin (1980) developed simple design formulae for the use of tapered flat plate spires in

simulating the ABL. The significance of the design formulae was that it allowed research

to be conducted with the same augmentation devices in varying size wind tunnels.

Similarity criteria applicable to both ABL flow characteristics and test models are

still a matter of debate. Matching velocity profiles, turbulence intensities, power

spectrum, and Reynolds numbers (in non-dimensional form), for both the test model and

fiiU scale is more trial and error than an exact science. When simulation is reduced to

geometric scale, the above parameters generally do not coincide to a unifying scale

factor. For example, the power spectrum may have a 1000:1 scale factor whereas the

velocity profile may have a 100:1 scale factor. Therefore, continued research in

simulation techniques is necessary. This thesis is an attempt to simulate a rural and

suburban type terrain by matching key parameters to a reasonable unifying scale factor.

14

CHAPTER 3

EXPERIMENTAL ARRANGEMENT AND PROCEDURE

3.1 Texas Tech Wind Tunnel

Boundary layer sunulations were conducted in the Texas Tech University wind

tunnel which is shown in Figures 3.1 and 3.2. The total span from the contraction outlet

to the boundary layer test section is 15.2 m. This lengtii is sufficient for the wind tunnel

to be classified as a LBLWT rather than a SBLWT. It is a closed cu-cuit tunnel with two

test sections for classical aerodynamic tests and boundary layer testing. All velocity

measurements were taken at the center span of the boundary layer section. The nominal

test section dimensions are 1.82 m wide by 1.21 m high (6 ft. x 4 ft.). The tuimel has an

adjustable ceiling height dovynstream from the aerodynamic section to the boundary layer

section. A vent is located on both sides of each test section to force the section static

pressure to be equal to the ambient pressure.

A plane of aluminum honeycomb material is installed in the contraction inlet

followed by two planes of fine mesh screen. The honeycomb reduces the scale of the

turbulent eddies and the magnitude of the turbulence intensity. The screens further

reduce the turbulence intensity and force a near uniform velocity distribution.

The wind tunnel is powered by a 250 hp fan allowing wind speeds to reach 49.2

m/s (110 mph). For the current study, all test wind speeds were maintained at a constant

13.4 m/s (30 mph) evaluated at the contraction outlet. The corresponding peak velocities

at the boundary layer test section varied depending upon the types of boundary layer

generation devices that were instedled.

15

Figure 3.1. Wind Tunnel

AtiPio«

- r r - ^ ^

^ bpwird '-foL.qtiress :zene"ts

^ ^

16 2 m Test Section

TunOllte

Figure 3.2. Wind Tunnel Schematic

3.2 Physical Modeling

A wide variety of devices have been applied by numerous researchers over the

last 50 years to generate simulations of ABL's. In spite of this cumulative effort, no clear

consensus on the best approach has been established. It appears that the best simulation

procedure depends on the specific characteristics of the individual wind tunnel and the

objectives for the ABL testing. For the current investigation, the objectives were to

16

develop a reasonably accurate simulation of the ABL for a rural (Exposure C) and

suburban or urban (Expostu*e B) terrains.

As mentioned in the previous chapter, various augmentation devices are often

used for ABL sunulation such as spues and fence barriers. The purpose of the spires is to

cause the generation of vertically oriented eddies through the abmpt separation of flow

around the spires' edges. The role of the fence barrier is to provide an initial momentum

deficit and depth to the boundary layer which is mixed into the developing flow by

turbulence generated from the spires. This helps to simulate a longer fetch in which the

boundary layer has the characteristics of growing naturally over the same surface

roughness. Therefore, the barrier helps to establish the boundary layer height.

The boundary layer simulation configurations evaluated in the current

investigation consist of various combinations of spires, fence barriers, and different types

of floor roughness. The location of each mixing device is shown in Figure 3.3.

Downstream Fence Barrier

N // ^ I n n n r-in n n n n n / / n n n n n r-i r

Floor Roughness

'" 'r- i I"! f i n

2.9 m 11m

15.2 m Test Section

Figure 3.3. Location of Mixing Devices

A total of four equally spaced spires are located at the contraction outlet. The

spire design along with the size and location offence barriers were modeled after those

used at Colorado State University [Ham and Bienkiewicz, 1998]. A downstream view of

the spire apparatus, with a 95 mm high fence barrier attached to its base, is shown in

Figure 3.4. The orientation of each spire is normal to the flow. For support, a splitter

17

plate is attached on the downwind side of each spire (Figure 3.5). A more detailed

schematic of the spire apparatus is shown in Figure 3.6.

Figure 3.4. Downstream View of Spires

Figure 3.5. Upstream View of Spires

18

100

40

95

730

Splitter Plate

Spire

1140

154

/ / 13

Figure 3.6. Spire Design (units in mm)

Three types of roughness fetch were evaluated. The first type consisted of

wooden strip panels attached to removable 12.7 mm (Yz") thick floor boards. The strip

panels lay cross stream to the flow and characterized a "rough terrain." A later

modification added 50 mm blocks to the floor boards in order to increase the effective

roughness of the strip panels. The square blocks were applied in a diamond-shape pattem

on the floor board next to the wooden strip panels. Carpet was utilized as the third type

of roughness fetch to characterize a smoother type terrain. The carpet extended from the

downstream fence barrier to the boundary layer test section area. The carpet and cross

strip panels (with the addition of blocks) are illustrated in Figures 3.7 and 3.8,

respectively.

19

1

M

1

Figure 3.7. Upstream View of Carpeted Tuimel

Figure 3.8. Upstream View of Roughness Blocks

20

Each mixing device was tested m several different combmations with the other

devices, while some of the devices were introduced individually. The test configuration

descriptions are shown in Table 3.1. The main variations in the table are for the variation

of the fence barriers and spires with the floor roughness.

Configuration

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Rural

Suburban

Spires

-

-

-

-

-

-

-

-

Table 3.1 . Configurat] ion of Mixing Devices

Fence Barriers X = 0 m X = 2.9 m X = 2.9 m

Upstream 3.7" Fence

-

-

-

-

Downstream 3.7" Fence

-

-

-

-

-

Downstream 10.6" Fence

-

-

-

-

-

Floor Roughness

Cross-Strip Panels

-

-

-

-

-

-

Strip Panels w/Blocks

-

Carpet

-

-

-

-

-

-

-

The final two arrangements in the table appeared to be most suitable for a Rural

and Suburban simulation. They include the mstallation of all the mixing devices with the

95 mm (3.7") fence installed upstream at the base of the spues and the 270 mm (10.6")

fence located downstream. The last configuration in Table 3.1 was the modification from

the cross strip panels by the addition of 50 mm (2 in.) square blocks. The final two

21

configurations were used for simulation comparison and will be the primary focus in the

Results and Discussion chapter.

3.3 Method of Profile Measurements

Velocity measurements were acquired using a thermal anemometry system. The

thermal anemometer measures air velocity by sensing changes in heat transfer from a

smsdl, electrically heated sensor exposed to the air flow. The sensor is maintained at a

constant temperature by use of a bridge and amplifier circuit. The amplifier senses the

bridge off-balance and adjusts the bridge voltage thus keeping the bridge in balance. The

bridge voltage can then be related to the velocity of the flow. An illustration of the

anemometer circuit is shown in Figure 3.9.

There is also a second order dependence of the bridge voltage on the air

temperature. A thermocouple circuit is included in the anemometer system to correct for

this effect.

r Out D.C-Oitfwwtial L

Figure 3.9. Anemometer Circuit [TSI Manual]

Longitudinal velocities were measured by using single hot-fihn probes (TSI 1210-

60). Lateral and vertical velocities were obtained through the use of X-film probes (TSI

1241-20). The probes consist of sensors made of quartz rod(s) with a diameter of 50

microns (0.05 mm). The rods are coated with a thin film of platinum approximately 0.1

micron (.0001 mm) thick. The sensor(s) serve as the carrier material. Due to their high

22

sensitivity and fast response to velocity changes, they are well suited for turbulence

meeisurements.

The X-film probe was positioned either in a vertical or a horizontal orientation as

depicted in Figure 3.10. The vertical orientation allowed measurements for the vertical

(w) component, while the horizontal orientation allowed measurements for the lateral (v)

components. The longitudinal velocity could be acquired in either configuration. Details

of the flow and sensor coordinate system can be found in the TSI manual (1997).

Wind

w

K - • u

Vertical Orientation

or

- • u

Horizontal Orientation

Figure 3.10. X-Fihn Sensor Orientation

3.3.1 Data Acquisition

The data acquisition system consisted of a MS Wmdows ® based personal

computer and a Thermal Systems Lie. (TSI) anemometer system (IFA300). The IFA300

contamed the basic bridge cucuits, amplifiers and signal conditioning devices required to

operate the anemometer probes. The IFA300 produced an analog output voltage for each

probe which was sampled with a data acquisition board mounted in the personal

23

computer. Overall control of the IFA300, calibration, 12 bit data acquisition and data

reduction were accomplished with TSI thermal pro software. The control communication

between the IFA300 and the personal computer was through an RS232 interface.

3.3.2 Calibration Method

An au velocity calibrator (TSI Model 1127) was utilized for calibration of the hot

film sensors. A compressed air supply was connected to the calibrator through a

filter/regulator assembly. Static pressure readings in the calibrator's internal flow settling

chamber were obtained from a manometer and used to determine the calibration velocity.

The TSI model drawing was modified to represent the calibration setup used in the

current study and is shown in Figure 3.11.

Prob*

FHter/Reguiator Assambty

A/D Board

CharwHM 1 Output

Figure 3.11. Calibration Setup [TSI Manual]

24

Calibrations of the hot-film probes were obtained by exposing the sensors to

uniform flows. Yaw angle coefficients of the X-probes were obtained by sweeping

angles fi'om -30 to +30 degrees with the probe axis. Full details of the calibrations and

the look-up inversion procedure used to convert probe voltages to velocities can be found

in the IF A 300 manual (1997). An example of the relationship of velocity with respect to

bridge voltage is shown in Figure 3.12.

Calibration Curve

2.7

2.6

2.5 V = K+AE + BE' + CE' + DE 2 I /^ r-J . r> r"*

K= 46.2310 A= -7.12847 B=-44.409401 |C= 24.86530 D= -3.05691

Figure 3.12. Calibration Curve

The equation generated to calculate the velocity from the calibration curve in Figure 3.12

used the following fourth-order polynomial:

F = 46.231-7.12847 •£:-44.4094-£'+24.8653-£'-3.05691 • £ ' , (3.1)

where F represents velocity and E is the bridge voltage. In comparing the known

velocity to the calculated velocity, the mean square error (MSE) was 0.024%.

25

3.3.3 Traversing Mechanism

Velocity and turbulence profiles were obtained through the use of a traversing

mechanism, which is illustrated in Figure 3.13. The traversing mechanism is driven by

two stepper motors for the horizontal and vertical axes which were oriented in a cross-

stream plane. The base of the mechanism was recessed mto the floor to minimize frontal

area and wind tunnel blockage. Also, a 0.9 m (3ft.) length cantilever arm placed the

velocity sensor ahead of the flow disturbance caused by the mechanism.

The range of motion of the traversing mechanism was limited within the tunnel.

A manually positioned slider arm that could be oriented in the vertical or horizontal plane

was used to increase the range of motion. This allowed measurements to be made in a

much greater area in both horizontal and vertical directions. The various slider arm

positions for the vertical measurements are shown in Figures 3.14 and 3.15.

Figure 3.13. Traversing Mechanism

26

Figure 3.14 shows a side view of the traversing mechanism with the probe support

normal to the flow. The probe was positioned near the floor at a height of 6.35 mm (0.25

in.). Velocity measurements were recorded beginning at this height and traversed

incrementally to 500 mm. As depicted in the figure, the probe was traversed in small

incremental spacings nearest the floor and then increased to larger increments. The

maximum allowable height was 500 mm without manual adjustment of the slider arm.

As illustrated m Figure 3.15, the slider arm was flipped 180 degrees and lowered

back to floor. In this position, the probe was located at a minimum height of 550 mm.

The probe support was then traversed, with incremental spacing of 100 mm to a

maximum height of 1052 mm.

27

Heights Imm)

500 450 400 350 300 250 200 175 150 125 100 75 50 26 10 6,35

Inc.

spacing

50 rnm

Ceding

Motor

SRdefAnn

Candlevei Ann

^ Floo(

Threaded Screv^

Figure 3.14. Slider Arm Setup 1

Heights fmml

1052

950

850

750

650

550

Inc.

spacing

100 mm

CH

Figure 3.15. Slider Arm Setup 2

28

CHAPTER 4

VELOCITY AND TURBULENCE MEASUREMENTS

This chapter contains a discussion on the results of velocity and turbulence

measurements for the configurations listed previously in Table 3.1. It is of interest to see

how the flow characteristics are altered by systematically introducing the various mixing

devices individually and in combmations. Two of the complete configurations, identified

as "rural" and "suburban," were evaluated in greater detail and the results of those

measurements are presented in the next chapter.

4.1 Empty Wind Tunnel

The flow characteristics were first investigated with an empty wind tuimel and

then later with the addition of the mixing devices. Cross-stream velocity traverses were

performed in the horizontal midplane of the test section and in three equally spaced

vertical planes as illustrated in Figure 4.1. All other measurements, which include the

boundary layer augmentation devices, were taken at the centerline of the boundary layer

section and acquired in the vertical plane only.

The traverses were undertaken at nominal wind tunnel speed of 13.4 m/s (30

mph). Vertical traverses were repeated at the centerline of the boundary layer test section

with wind speeds of 17.9 m/s and 22.4 m/s (40 mph and 50 mph). Wind tunnel speeds

were set according to the exit velocity located at the contraction outlet.

29

Positions Traversed in Wind Tunnel

1200

E

N

z

1000

800

600

400

200

0

}

)

!

J

:

3

3

3

3

3

3

3

¥

: 3:

c 3 :

c ;:

c 3 :

: 3

: 3

: 3

: : : 3

c 3

: 3

C ^ ^ ^ ^ ^ J E ^ 7%—

( 3 ;

: 3 :

: 3E

: 3 :

: 3C

: 3 :

: 3 :

y^ 'k it : : : : 3 ; : : : : 3 : ; : : : 3 : !: :: :: !! :: :: ;; ;; ;;

— 1 1 ' i — 1 1 — i ' 1 1 1 1 f 1 1

!

Width, y (mm)

Figure 4.1. Traversing Positions Looking Upstream

A single horizontal traverse was conducted at the midplane of the test section area

(600 mm above the floor). Longitudinal velocities were averaged over a 10-second

interval with a 100 Hz sampling rate. A plot of the resulting velocity distribution is

shown in Figure 4.2 (a). It may be noted that the wall boundary layer thickness is

approxunately 250 mm (10 in.) m depth, but otherwise the mean flow is very uniform

across the wind tunnel with a percent standard deviation of 0.5%.

The correspondmg plot of longitudinal turbulence intensity (lu %) for the

horizontal traverse is shown m Figure 4.2 (b). Outside of the wall boundary layers, the

turbulence intensity was approximately 0.1% with a percent standard deviation of 0.18.

30

Mean Velocity Profile

18 16 14 12

I 10

4 -

2

0

• Z=600mm

—1 1 1 1 1 1 1 1 (— —1 1 r-

182 364 546 728 910 1092 1274 1456 1638 1820

Width, y (mm)

(a)

0 -

7 :

6 :

3 :

2 \

1 ^

0 -(

X

X

| — I 1 r —

X

X

X

1 — 1 — 1 1 1

182

X 3 1 1 1 1

364

' X X, 1 1 1 1 " 1

546

Turbulence Intensity

X » * X X X X

728 910 1092

Width, y (mm)

•K X *

1274

X Z = 600mm

X

X

X

[ X X

1456 1638 16 20

(b)

Figure 4.2. Profiles of Cross-Stream Traverse

31

The results of the vertical velocity and turbulence intensity profiles are compared

in non-dimensional form in Figure 4.3. The mean velocity plots have been normalized to

eliminate the effects of small changes in wind tuimel speed settings.

By inspection of the velocity profile in Figure 4.3, it may be noted that the data

coincide for all three traverses with a floor boundary layer depth of approximately 250

mm (10 in.), similar to the wall boundary layer seen earlier in Figure 4.2. The percent

standard deviation of the freestream velocities was 0.25%. The vertical turbulence

intensity profiles are similar to those measured on the horizontal traverse with centerline

values in the range of 0.11-0.24%. Within the boundary layer, at a distance of 25.4 mm

from each wall, both profiles showed turbulence intensities were in the range of 7-10%.

The effects of higher wind speeds on velocity and turbulence intensity profiles

were evaluated. Comparison of profiles obtained at wind speeds of 13.4,17.9, and 22.4

m/s (30,40,50 mph) are shown in Figures 4.4. It is seen that the data coincides for all

three traverses, indicating that these normalized profiles are independent of wind tuimel

speed in this range.

32


1200

1000-

800-

E ^ 600 a> ' 5 X

400

200

0.2

«]&

BL-«^

• n A ftA ^ ^°-"

- r 0.4 0.6

u/umax 0.8

oy = 1365mm ay = 910mm Ay = 455mm

1.2

(a)


1200

1000

800 +

E ^ 600 g> 0) X

400

200 AO D

*ô-f o A. .a <g

A a I I I I

5

, r ^ A g. 0^ A ip a , , , , , , r-

10 15

lu%

20

- 1 1 1 1

25

«y = 1365mm ny = 910mm Ay = 455mm

(b)

Figure 4.3. Profile Comparison of High Wind Speeds

33

Mean Velocity Profiles

1200

1000

800

;£ 600

0) X

400

200

0.0 0.2

"5SE" ca « i \

- ^ ^ 3 ^ — < ^ 0.4 0.6

u/umax

0.8

-et

a

1.0 1.2

Ou=30mph nu=40mph Au=50mph

(a)


1200

1000

800 E" E.

- 600 g)

X 400

200

10 15

l u %

Ou=30 mph g u=40 mph A u=50 mph

1 I

1

a a Si

, * ^ ^ Oo/P aao 20 25

(b)

Figure 4.4. Profile Comparisons of Wind Speeds

34

4.2 Effects of Boundary Layer Augmentation Devices

This section contains the velocity profiles and turbulence intensities resulting

from the introduction of various mixing devices. The measurements were taken on the

centerline of the test section with a samplmg frequency of 50 Hz and a time duration of

20.48 seconds.

Figure 4.5 shows the flow characteristics for the empty wind tuimel and the

effects caused by each respective roughness fetch. As can be seen, the addition of either

roughness fetch created a larger momentum deficit than the smooth floor empty wind

tunnel. The addition of the strip-panels increased the boundary layer thickness to 500

mm and a turbulence intensity of 18.2% near the floor. The carpet produced a boundary

layer depth of 350 mm and a maximum turbulence intensity of 16.6% near the floor.

An mdependent comparison was also made between the 270 mm (10.6 in.) and 95

mm (3.7 in.) fence barriers and is shown in Figure 4.6. As expected, each fence barrier

produced a significant increase in boundary layer depth and turbulence intensity

compared to the empty wind tunnel. The mean velocity profile for the 95 mm fence

barrier actually shows a slight increase in boundary layer depth as opposed to the 270

mm fence barrier. The boimdary layer depth produced by the 95 mm fence barrier was

750 mm, and only 550 mm for the 270 mm fence barrier. The decrease in boundary layer

depth with the taller fence barrier may be due to flow separation occurring at the ceiling.

Although the boundary layer depth decreased near the floor, it may be noted that the

boundary layer depth caused by the ceiling increased by the same difference of 200 mm.

A more noticeable difference between each fence barrier can be seen in the turbulence

intensity profile. The turbulence intensities caused by the 270 mm fence barrier were

approximately doubled in magnitude compared to the 95 mm fence barrier.

35


1200

1000

-^ 800

2 600

3: 400

200

0

<o.

o Strip-Panels

n Carpet

• - Empty tunnel

-I—I I I I I

0.0 0.2

t

i

i

- 4 > - ^ ' ^ M ^ . vV:..---

0.4 0.6

u/umax

- I — I — I —

0.8 1.0 1.2

(a)


1200

1000-

-^ 800

E ^ 600 O)

X 400

200

•

'.t»

' I

•

To • «

- — 1 1

o D

a

1 1 1 —

D

• « _

«

° D

o Strip-Panels n Carpet

- - - Empty Tunnel

I

* a ** * *

0.0 5.0 10.0 15.0

lu% 20.0 25.0

(b)

Figure 4.5. Effects of Roughness Elements

36


1200

1000

-^ 800

^ 600-1-g> o ^ 400

200

0.0

•

-

•

•

o 270 mm Fence

a 95 mm Fence

- - - Empty tunnel

•

1 1 1 1 1 1 1 1

OD .

4 . _ _ I

no 4^ DOO 1 D « I

1 1 1 1 r " M » * » r ~ - i 1 — T ' — 1 1 1 1 1 1 1 1 — -1 1 1

0.2 0.4 0.6

u/umax

0.8 1.0 1.2

(a)


1200

1000

^ 800-1

E £ 600 g> 0) ^ 400

200

-

D

D

• • D • 1

:• a . 1 , . . _ __ - i DD ' • D

i • I o • • D

: ••-- °

• 1 1 1 1 1 1 1 1 1

«

,• -P-- T . T - n q » ,

o 270 mm Fence a 95 mm Fence

- - - Empty Tunnel

—

— I — ^ 1 ^ 1 1 1 1 1 1 1

0.0 5.0 10.0 15.0

lu%

20.0 25.0

(b)

Figure 4.6. Effects of Downstream Fence Barriers

37

In Figures 4.7 and 4.8, the comparison of the effects caused by varying roughness

fetch with different size fence barriers are shown. For a comparison, the effects caused

by the roughness element configurations without the fence barriers are also shown. Each

fence barrier was located downstream at x = 2.9 m in the wind tunnel (see Figure 3.3).

The velocity profile in Figure 4.7 shows that each roughness fetch increased m

boundary layer thickness and turbulence intensities with the addition of the 95 mm fence

barrier. Both configurations mcreased the boundary layer depth to 750 mm (29.5 in).

The turbulence intensities increased in the outer layer but remained approximately the

same near the floor.

The effects caused by the roughness elements and the 270 mm fence barrier

combination are depicted in Figure 4.8. For each configuration, the velocity profile

shows an increase in momentum near the floor and an increase in boundary layer depth to

650 mm. The turbulence intensities near the floor were increased from around 16.5% to

20% for the carpet and fence barrier combination. The turbulence intensities for the strip

panels and fence barrier combination increased from approxunately 18% to 21%. Both

configurations decreased asymptotically to a minimum of around 10%. The strip panel

and carpet configurations had less of an effect in the outer boundary layer with turbulence

intensities around 0.2%.

38


1200

1000

-p 800

E, J 600 g>

^ 400

200

0

o 95 mm Fence, Strip-Panels

a 95 mm Fence, Carpet

- - Strip-Panels

Carpet ^

0.0 0.2 -I 1 1 1 1 ( * T '

0.4 1.2

(a)


1200

1000

-^ 800 E, £ 600 g>

^ 400

200

0 0.0

o 95 mm Fence, Strip-Panels a 95 mm Fence, Carpet

- - - strip Panels Carpet

< <>. o

20.0 25.0

(b)

Figure 4.7. Effects of 95 mm Fence Downstream and Roughness Elements

39


1200

1000

•p> 800

E ^ 600 O) 0) ^ 400

200

-

-

o 270 mm Fence, Strip Panels n 270 mm Fence, Carpet

- - - Strip Panels

Carpet

-

-

— 1 1 1 — i — 1 1 — 1 — 1 — 1 ( 1 • - S ^

D O ^

a<' 1

a 1

^ . • • j^^'''^^^

•p-"—1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

0.0 0.2 0.4 0.6

u/unnax

0.8 1.0 1.2

(a)


25.0

lu%

(b)

Figure 4.8. Effects of 270 mm Fence Downstream and Roughness Elements

40

Figures 4.9-4.11 demonstrate the effects caused by different spire combinations

with varying roughness elements. Velocity and turbulence intensity profiles for the spire

and roughness element combinations are shown in Figure 4.9. The velocity distributions

are approxunately the same as the flow distributions for the roughness elements despite

the addition of the spues. Surprisingly the spues also had little effect m the lower

turbulence intensity profile. However, from z = 400 mm the turbulence intensities

decreased to a minimum of only 4% as opposed to 0.2%.

Figures 4.10-4.11 show the velocity and turbulence intensity profiles for the

spires, roughness elements, and 95 mm fence barrier. Different locations of the 95 mm

fence barrier were also investigated in these configurations. The results for the spire

configuration with the 95 mm fence barrier located downstream, at x = 2.9 m, are shown

in Figure 4.10. Velocity and turbulence profiles shown in Figure 4.11 are for the 95 mm

fence barrier located upstream at x = 0 m (attached to the base of the spires).

In Figure 4.10, the velocity profile shows that the boundary layer depth increases

with the addition of the downstream fence barrier. Both configurations converge aroxmd

a height of 650 mm and have a boimdary layer depth of 750 mm. Turbulence intensities

increased slightly in the outer boundary layer as shown in the turbulent intensity profile.

By inspection of Figure 4.11, the spire combination with the location of the 95

mm fence barrier upstream (x = 0 m) also increased the boundary layer depth. The spire

configuration produced a boundary layer depth of 650 mm with the strip panels as the

roughness fetch; 100 mm less than the 95 mm fence barrier located downstream (x = 2.9

m). With the carpet as the roughness fetch a boundary layer thickness of 500 mm was

produced; 250 mm less than the 95 mm fence barrier located downstream (x = 2.9 m).

Despite the location of the fence barrier, the turbulence intensities remained consistent

with the previous configurations in Figure 4.10.

41


1200

o Spires, Strip-Panels a Spires, Carpet - - Strip-Parjels

Carpet

0.0 0.2 0.4 0.6

u/umax

0.8 1.0 1.2

(a)


o Spires, Strip-Panels • Spires, Carpet - - Strip Panels

Carpet

10.0 15.0

l u%

20.0 25.0

(b)

Figure 4.9. Effect of Spires with Roughness Elements

42

1200

1000

-p> 800

E ^ 600 O) 0)

I 400

200

0.0


-

-

o Spires, 95 mm Fence, Strip-Panels n Spires, 95 mm Fence, Carpet

- - - Spires, Strip-Panels

Spires, Carpet

-

;

1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 ; ft P * " ! ^

»a

I I

«1

o

— r — 1 1 1 1 1 -

on,* /

0.2 0.4 0.6

u/umax

0.8 1.0 1.2

(a)


o Spires, 95 mm Fence, Strip-Panels n Spires, 95 mm Fence, Carpet - - Spires, Strip Panels

Spires, Carpet

lu%

(b)

Figure 4.10. Effects of Spires with Roughness Elements and Downstream Fence Barrier

43


1200

1000

^ 800

5' 600 O) 0) 3: 400

200

0 0.0

o Spires, 95 mm Fence, Strip-Panels

a Spires, 95 mm Fence, Carpet

- - Spires, Strip-Panels

Spires, Carpet

0.2 - I — I — I — I — I — I — I — ~

0.4 - I — I — I — I -

1.2

(a)

1200

1000

- - 800

E

5' 600

I 400

200

0 0.0


o Spires, 95 mm Fence, Strip-Panels a Spires, 95 mm Fence, Carpet • - Spires, Strip Panels

Spires, Carpet

iJ^ -I 1 1 I

20.0 25.0

lu%

(b)

Figure 4.11. Effects of Spires with Roughness Elements and Upstream Fence Barrier

44

4.3 Summary

The flow characteristics were altered by systematically introducing the various

mixmg devices individually and in combinations. To summarize some of the main

results above, they are the following:

• The empty wind tuimel produced a boundary layer thickness of 300 mm and

maintained a uniform low turbulence (0.1-0.2%) flow region outside the boundary

layer.

• The roughness elements were effective in increasing both boundary layer depth

and turbulence intensities.

• The 95 mm downstream fence barrier was effective in producing a boundary layer

depth of 750 mm, and produced greater turbulence intensities above lOOmm but

had little effect below 100 mm.

• The 270 mm downstream fence barrier produced a boundary layer of 550 mm,

200 mm less than the 95 mm fence; however, the turbulence intensities within the

entire boimdary layer region approximately doubled in magnitude.

• The combination of the 270 mm downstream fence barrier with the roughness

elements created higher turbulence intensities in the lower 500 mm compared to

the fence barrier without the roughness elements.

• The 95 mm downstream fence barrier combined with the roughness elements

increased turbulence intensities only in the upper region above 250 mm. This

combination was not as significant as the larger fence, especially in the lower

region of the boundary layer.

• The spires with the roughness elements had little impact on the flow distribution

with virtually lio change in boundary layer thickness. The turbulence intensities

started to increase around 300 mm for both configurations.

• It appeared that the spires had little effect when combined with the downstream

95 nun fence barrier and the various roughness elements with the exception of

increased turbulence intensities above 550 mm.

45

• The location of the 95 mm fence barrier was investigated with the spire and

roughness combination. The fence location appeared to have little effect on the

turbulence intensities. However, with the fence barrier located upstream (x =0 m)

the boundary layer decreased by 250 mm with the strip panel as the roughness

fetch and 100 mm for the carpet as the roughness fetch.

In Chapter 5, two specific combinations are examined in greater detail and assessed as

suitable ABL simulations.

46

CHAPTER 5

RESULTS AND DISCUSSION

This chapter provides the results of a more thorough evaluation on the rural and

suburban configurations mtroduced in chapter three and identify them as having the

greatest potential for ABL sunulations. As will be illustrated, a reasonable representation

of a rural exposure is achieved with the installation of the fences and spires with the

carpet as the roughness fetch. For a suburban exposure, an adequate simulation can be

achieved with the installation of the mixing devices and the modified cross-strip panels.

Based on the previous results in chapter four it was decided to modify the cross strip

panels by the addition of 50 mm square blocks. The purpose of this was to increase the

effective roughness of the cross-strip panels. For simplicity, this modification will be

referred to as "roughness blocks" throughout the rest of this paper.

All velocity measurements were obtained using a single hot-film probe for

longitudinal velocities, while measurements involving lateral and vertical velocities were

obtained using a cross-film probe. Velocity signals were sampled at 200 Hz for 20.48 s.

Each measurement was made on the vertical centerline of the boundary layer section.

Three different velocity traverses were undertaken to ascertain the longitudinal,

lateral, and vertical components (u, v, and w components, respectively) for each

simulation. A single sensor probe was used initially to investigate the longitudinal

component in the flow. Later tests utilized a cross film sensor to simultaneously

determine either the longitudinal and lateral components or the longitudinal and vertical

components with a 90° rotation of the sensor plane.

5.1 Mean Velocity Profiles

Shown m Figure 5.1, are the mean velocity profiles for the rural simulation.

Figure 5.2 shows the velocity profiles obtained for the suburban simulation. Each figure

contains three profiles obtained from (1) a single fihn probe, (2) a cross-film probe

oriented to measure longitudinal and vertical velocity components, and (3) a cross-film

47

probe oriented to measure longitudinal and lateral velocity components. As can be seen

in Figures 5.1 and 5.2, the longitudinal component from all three traverses lends support

to the use of a single probe to collect longitudinal velocity data in turbulent flow.

The suburban configiu-ation in Figure 5.2 produced a slightly thicker boundary

layer depth than produced by the rural configuration. The suburban simulation, usmg

roughness blocks along the fetch length, had a 650 mm boundary layer depth. A

boundary layer depth of 550 mm was produced by the rural configuration (with the carpet

as the roughness fetch).

Mean velocity Profiles

1200

1000-

800

E E £ 600 -I-gi

400

200

Oft

DOft

A <D AIO

— I —

0.2 — I —

0.4

- • — ^ •

0.6

u/umax

0.8 1.2

Figure 5.1. Mean Velocity Profile of Rural Simulation

48

200


1000-

8 0 0 -

Hei

ght

(mm

)

§

400 J

B O

BO

OA

A

•

MJ

Figure 5.2. Mean Velocity Profile of Suburban Sunulation

The velocity profiles from the single film probe are compared for each simulation

to the ASCE 7-98 standard exposures in Figure 5.3 at frill scale. Each exposure is

represented by mean hourly wind speeds within the ABL and were determined by using

the following ASCE 7-98 empirical formula:

A V'^'=bTc-\r^

a Tc

lOm.C • (5.1)

In non-dimensional form, the mean hourly wind speeds are divided by the mean velocity

at a referenced 10 m height, and therefore Equation 5.1 becomes

Vz,Tc

10,7b

Z

10 (5.2)

The necessary model scale was 1:100 to match full-scale exposure profiles. This scale

factor gave the best alignment of results from die rural configuration to the characteristics

of an exposure C category; which is representative of a rural terrain. Also, with a 1:100

scale ratio, the flow characteristics for the suburban simulation matched an exposure B

profile, defining a suburban type terrain.

49

120

100

80-

g> '« I 60

s

40

20-

0-1-0


Scale 1:100

o Rural Simulation

A Suburban simulation

D C

-CA

0.5 U/Uref

2.5

Figure 5.3. ASCE 7-98 Velocity Profile Comparison

5.1.1 Characterization of Mean Velocity Profiles

As discussed earlier in the literature survey, velocity distributions are generally

represented by either the logarithmic or power law equation (Equations 2.5 and 2.6). In

this section, both representations were investigated and used to characterize the velocity

profiles. For convenience. Equations (2.5) and (2.6) are repeated here as follows:

• Power Law

Log-Law

u ^ z^"

u. ref v^ref y

1 r u=—iU'Ln

k z-z^

V ^ o

For characterization of the mean velocity profile the aerodynamic roughness, Zo,

shear velocity M, and power law exponent, a, were determined for each simulation. The

roughness length parameter Zo and fiiction velocity M* were determined by fitting the

50

measured mean velocity profile near the ground surface to the logarithmic law-of-the

wall; the zero plane displacement, z^, was assumed to be zero. The power law exponent,

a, was calculated from a power law fit to the data. From Equation 2.6, and using a best

line fit method-approach, the parameters w. and Zo may be written as follows:

«• = m X (A:) ; m = slope of line (5.3)

-kxb

ZQ = exp "• ; b= line intercept. (5.4)

The logarithmic fit had a correlation coefFicient of 0.992 for the rural sunulation and

0.996 for the suburban simulation. The power law fit for the rural simulation had a

correlation coefficient of 0.987 and 0.997 for the suburban sunulation. The results for

u,, Zo and a, are shown at actual model scale in the table below.

Table 5.1. Results for w*, Zo, and a

Configurations Rural Simulation

Suburban Simulation

M» (m/s)

0.587 1.145

Zo(mm)

0.03 3.39

a

0.137 0.239

Figures 5.4-5.5 show the comparisons between the logarithmic and power law

representations for each simulation. The heights at which a best line fit occurred can be

observed in both figures. For the rural simulation, the logarithmic law and power law

gave a best fit near the floor surface at z = 6.35 mm up to z = 650 mm. For the suburban

simulation in Figure 5.5, the best line fits were found at a height of 75 mm to 650 mm.

Heights below 75 mm produced a power index of a = 0.114, which is typical of a smooth

boundary layer. Consequently, the gap between the roughness elements and the smooth

surface measurement location allowed an inner smooth-boundary layer to develop.

Overall, the use of either formula gives a fair representation of the actual velocity

distribution.

51

2.0-1

1.8-

1.6-

1.4-

1.2-

£

5 1.0-

0 .8 -

0.6-

0.4-

0 .2-

0.0-

Semi-Log Plot

Log Law: u.= 0.587 ,zo = 0.03mm

Power Fit a = 0.137

.^•j^'ii^

^^^^"^'^'^^"'^

^ . . ^ • • ^ ^ - ^ ^ r * »

o Data log Law

1 10 100

Z(mm)

1 1 1 1 1 1 1

1000 10000

Figure 5.4. Logarithmic and Power Law Representations of Rural Simulation

2.0

1.8

1.6

1.4

1.2

2 10

0.8

0.6

0.4

0.2

0.0 10

Semi-Log Plot

Log Law: u.=1.145, 20 = 3.39mm

Power Fit: a = 0.235

o o

. . . • • ' •

o ^

• •'

« < < ^ ^

J^

yf-

V^* o «

« Data

log law

Power fit

100

Z(mm)

1000 10000

Figure 5.5. Logarithmic and Power Law Representations of Suburban Simulation

52

The indirect procedure mentioned above is one method for finding the fiiction

velocity 14. Another method to find an estunate of the fiiction velocity is by direct

measurement of the maximum Reynolds stress, when u, = J-lu'-w'). Where the

fluctuating longitudmal and vertical (w' and w') components are defined as follows:

u'=u-u (5.5)

w'=w-w. (5.6)

The Reynolds stress distribution is shown in Figure 5.6. For each simulation, the

maximum Reynolds stress was determined and used to calculate the fiiction velocity. A

comparison of fiiction velocities obtained from the direct and indirect procedure is shown

in Table 5.2. The indirect procedure gave similar results for the single sensor and dual

sensor probe. However, the fiiction velocities determined by the maximum Reynolds

stress show a large difference in comparison to the indirect procedure. The reason for the

discrepancy may be due to the limited directional response with the cross film sensor

which only has a ± 45 degree range.

1200

1000

800

E"

E £ 600 O)

<u X

400

200

Reynolds Stress Distnbution

• •

i

I '

i i !

X X

•

' T * .

o o

i

X X o

X 3

D

-p -

o

AxJp

o

—1 r—

i

I (

1 1

i

-

X Rural simulation

o Suburban simulation

i

i

^ i

' ^mBxJp

— ! — I — 1 — 1 — 1 1 1 — 1 — I — i

0.0 0.2 0.4 0.6

-i?w' (m' 2/s' 2)

0.8 1.0

Figure 5.6. Reynolds Stress Distribution

53

Table 5.2. Method Comparison of M, (m/s) Values

Probe Sensors

1. Single 2. Cross

Rural Simulation Log-law, lu

0.587 0.581

Reynolds, u.

N/A 0.471

Suburban Simulation Log-law, u»

1.145 1.112

Reynolds, lu N/A

0.651

5.2 Turbulence Profiles

The measured longitudinal lateral and vertical turbulence intensities are shown for

the rural simulation in Figure 5.7 and the suburban simulation is depicted in Figure 5.8.

The longitudinal, latered, and vertical turbulence Intensities are denoted lu, Iv, Iw

respectively. From the figures, it is apparent that the longitudinal velocity fluctuations

are the dominant components in the turbulent flow. For the rural simulation, the lateral

and vertical turbulence mtensities are essentially the same in magnitude and are

approxunately 70% of the longitudinal mtensities. There seems to be a discrepancy,

however, in which the lateral turbulence intensities for the suburban simulation decreased

near the floor. The magnitudes of the v and w components merge near the outer edge of

the boundary layer.

54

Turbulence Intensity Profiles

1200-

1000

800

• lu, single sensor

D lu , cross sensor

A Iv, cross sensor

X Iw, cross sensor

E E. £ 600 9>

• « )

I

400

XA «aa » D O

— X A • - & • -

200

>A

AX

A X

It-

» • DO

XA X A

X A « • •

D

X A XA ^Â_

« « •

D O n, ^.JA-

10 15 20

%lu,Iv, Iw

1200

1000

800 - -

"E E. £ 600 -a '« I

400 4

200

25

Figure 5.7. Turbulence Intensity of Rural Simulation

Turbulence Intensity Profiles

1 1 r

X A O

AXSA

A A

A A 1 1 r* 1 1 r

D

Q «C» Q •» «

« D

X I»

X

A X

\ "x

A

1 1 1 1 1

S

« a

X X

1 1 —

« lu, Single sensor

• lu, cross sensor

A Iv, cross sensor

X Iw, cross sensor

• o

D « ° *„ D O

a «

1 T 1 1 r-"—1-**—1 1 A '

10 15 20 25

% kj, Iv, Iw

Figure 5.8. Turbulence Intensity of Suburban Simulation

55

A comparison of the turbulence intensities for each simulation with ASCE 7-98

standard is shovm in Figure 5.9. The following ASCE equation is the current standard

for calculating turbulence intensities and is as follows:

-* z, Tc ~ ^Tc (5.7)

where c . is the terrain category constant for the respective ABL exposure. Equations

2.12-2.14 in the literature survey were used to determine the turbulence mtensities from

the measured fluctuating velocities. The data was aligned to ASCE profiles through trial

and error. A height scale factor of 1:500 matched the majority of the curve for the

longitudinal turbulence intensities with their respective exposures. At this scale factor,

the rural simulation best aligned with an Exposure C category (a rural-type terrain).

Also, for the suburban simulation, a 1:500-scale ratio gave a good alignment for a

suburban type terrain. Although the majority of the data had good alignment for each

sunulation, both simulations had insufficient turbulence intensities in the lower region of

the curve.

120 Scale 1:500

100 -

E, 80

g> '» X .2 8 (0 = 40

D C

Turbulence Intensity profiles

A Rural simulation

• Suburt>an simulation

0 - l — I — I — ' — I — I — I — I — I — I — I — I — I — ' — ' — I — ' — ' — ' — ' — ' — ' ' ' ' ^ I I I — I 1 — I — I 1 — I r ~ ~ T — i ~ ~ i 1 1 1 — I 1 r -

10 20 30 40 50

lz%

60 70 80 90 100

Figure 5.9. ASCE-97 Turbulence Intensity Comparison

56

5.3 Integral Length Scales

The integral length scales were calculated using Taylor's statistical theory of

turbulence in which correlation measurements can give an indication of the size of gusts

or eddies. Based on the eissumption that the flow disturbance travels with the mean

velocity w(z), the autocorrelation, which is dependent on time, was performed on each

measured component. Using Taylor's hypothesis, the integral length scales for the

longitudinal, lateral, and vertical were then determined by the following equation:

Lu^ = Tu^'U, (5.8)

where Tux is the respective integral time scale.

5.3.1 Autocorrelation

Autocorrelation of the three components were performed for each simulation at

heights of lOOmm and 250mm. These heights were chosen by viewing the logarithmic

law region m the velocity profiles. The 100mm height is at the lower end of the

logarithmic law region, while the 250mm height is located in the central part of the

region.

Figures 5.10 and 5.11 show the autocorrelation of the time series for each

respective simulation at a 100mm height. These are plotted to a tune lag at which

correlation approaches zero. A high correlation has a value of ± 1, a low correlation

approaches zero. This describes the dependency of the signal with respect to time.

Autocorrelations of the tune domain signal at 250mm are shown m Figures 5.12 and

5.13.

57

1 0 •

0.9 -

0.8 -

•£ 0.7 -o e 0.6 -8 "i 0 .5 -o

1 0.4 -t o O 0.3 -

0.2 -

0.1 -

0 0 •

0

Autocorrelation

„A_.__%

—•—u-component

-B—v-component

-A—w-component

0 0.1 0.2 1

0.3

Time (s)

1

0.4 1

0.5 0.6

Figure 5.10. Autocorrelation of Rural Simulation @ lOOmm Height

c « •(3

% o O c o a

t o O

1.0

0.9

0.8

0.7

0.6

0.5 •

^ 0.4

0.3

0.2

0.1

0.0

0.0

Autocorrelation

1 1 ll l\ I \

—•—u-component

—a—v-component

—A—w-component

1 \ 1 \ \ >v

\Xy ^ ^ S w _ , ^

^ - - A A a . A r ' * * ' ' * * ' ^ ^

0.1 0.2 0.3

Time (s)

0.4 0.5 0.6

Figure 5.11. Autocorrelation of Suburban Simulation @ lOOmm Height

58

Autocorrelation

-u-component

v-component

- w-component

0.0 0.1 0.2 1

0.3

Time (s)

— 1 —

0.4 0.5 0.6

Figure 5.12. Autocorrelation of Rural Simulation @ 250nmi

Autocorrelation

1.0

0.9

0.8

0.7

I 0.6 .2

i 0-5

•3 0.4 <

0.3

0.2

0.1

0.0 0.0

i 1

A

i\ I\

- • - u-component

-B- v-component

-A- w-component

\

1 \ • "I \^

% \

" ' \ ' ^ < ^ - - -

" ^ ^ ^ . . ^ ' ' ^ ' - * - — • . . . - - * - - - * ^ .

0.1 0.2 0.3

Time (s)

0.4 0.5 0.6

Figure 5.13. Autocorrelation of Suburban Sunulation @ 250mm

59

5.3.2 Determination of Eddy Size

The length scale of the eddy size was determined by integrating the

autocorrelation function to obtain an integral time scale, Tux. Then using Taylor's

hypothesis, the integral length scale was determined by Equation (5.8). The results are

shown below in Table 5.3. A graphical representation of the eddy size for the suburban

simulation located at 250inm is depicted in Figure 5.14. Generally, it is expected that the

lateral component is larger than the vertical component. However, this is only the case at

250 mm height in the wind tuimel for the suburban type sunulation.

Table 5.3. Integral Length Scales

Model Size Heigm (mm)

100 250

Rural Simulation LUx (m)

0.605 0.581

LVx (m)

0.220 0.182

LWx (m)

0.228 0.207

Suburban Simulation LUx (m)

0.432 0.625

LVx (m)

0.104 0.174

LWx (m)

0.152 0.165

Wind Tunnel Upstream View

1.143 m

0.25 m

Wind Tunnel

Wind 1.143 m ^

Analyzed at 250

250 mm

Figure 5.14. Illustration of Eddy Size @ 250inm m the Wmd Tuimel

60

5.4 Power Spectmm Analysis

Spectral analysis is important in understanding the amount of energy associated

with the flow's turbulent motion. Spectral analysis was performed on the longitudinal

velocity fluctuations at heights of 100 mm and 250 mm in the wind tunnel. All spectral

analysis is fi'om the cross fihn sensor measurements with a sampling fi-equency of 200

Hz. Figure 5.15 is shown as an example of the raw spectrum, plotted on a log-log plot,

for the rural simulation at z =100 mm. A 20-point moving average was performed to

smooth the spectra. With a sampling frequency of 200 Hz, the figure was plotted with a

maximum frequency resolution of 0.0488 Hz.

1.00E- 01

1,00E+00

1.00E-01

^ 1.00E-02 < E

3 CO

^ 1.00E-03

1.00E-04

1.00E-05

1.00E-06 1.00E-02

Raw Power Spectrum

T—' I M r-

1.00E-01 1.00E- 00

Frequency (Hz)

1.00E+01 1.00E+02

20 per. Mov. Avg. (xb u2)

Figure 5.15. Raw Power Spectrum of Suburban Simulation at 100mm

Determination of a scale factor was performed for each spectrum. ASCE 7-98

design standard code was used for the comparison. Normalized spectral amplitudes of

the mral and suburban simulation are shovm in Figures 5.16-5.19. These amplitudes are

61

plotted along with the ASCE 7-98 wind spectrum model for exposure C and exposure B,

respectively. The ASCE wind spectrum equation is defined as follows:

7.47-A ,

^ " ( l + 10.3.7^,f ^^'^^

where the reduced frequency (Ni) is,

^1=-^ (5.10)

and the longitudinal integral length scales are determined by.

^z, Tc ~ ''Tc

f 2 v^' (5.11)

A range of fiill-scale heights were analyzed and compared to 100 mm and 250

mm heights in the wind tunnel. The data was normalized by multiplying the raw power

spectrum, Sw, with the maximum fi-equency resolution and then divided by the

variance (7 • "S jo^ \. The fi*equency along the x-axis was normalized by multiplying it

with height, z, and dividing through by the mean longitudinal velocity, i7 (m/s).

When examining the spectrum at 100 mm height in the wind tuimel a good

comparison can be made for z = 50 m in full scale for the rural simulation. Therefore, the

resulting scale factor is 1:500. At the same scale factor a fair comparison is made with

the 250 mm location. The resulting full-scale height would be at 125 m. The suburban

simulation, however, had a good comparison at a lower scale ratio of 1:300. The

corresponding full-scale heights for 100 mm and 250 mm would then be equivalent to

z =30 m and z =75 m.

62

Semi-Log Plot of Power Spectrum

Scale 1:500 0.35

0.3

i 0.25 Q. E < 2 0.2

«

^ 0.15 •o a> N

1 0.1

0.05

0.0001 0.001

Exposure C Anal ized at Z=50 m

Exposure 0 —Seriesi

Figure 5.16. 100 mm Rural Simulation

Scale 1:500 0.35

0.3 <u t j

1 0.25 -I o. E (0

2 0.2 •c

I 0.15 (U m la E 0.1 i o c

0.05 --

0.0001


Exposures i re analyzed at 12: > meter Heights

Exposure C —Seriesi

100

Figure 5.17. 250mm Rural Sunulation

63


Scale 1:300 0.35

0.3

.1 0.25 Q . E < g 0.2 « f 0.15 N

0.1

0.05

0.0001

Exposur is are analyzed at 30 meter heights

•Exposure B —Ser ies i

Figure 5.18. 1 OOmm Suburban Simulation

0.35

0.3

(D

. i 0.25 Q. E CD

2 0.2

<D

^ 0.15

CO

0.1

0.05

Scale 1:300


Exposures are analyzed at

0 -J 1——1—I I 1 1 I I I 1 — I — i | 1 — I — I 1 1 1 I I I 1 — I — I I I i I I I 1 f

75 meter Heights

0.0001 0.001 0.01 0.1

nz/u

10 100

•Exposures Seriesi

Figure 5.19. 250nim Suburban Simulation

64

5.5 Unified Scale Factors

A thorough evaluation of the rural and suburban configurations showed that both

gave reasonable simulations of ABL exposures. However, neither simulation had a

single scale factor that simultaneously matched the velocity profiles, turbulence

intensities, and power spectrum (m non-dimensional form) to ASCE full-scale profiles.

Therefore, the determination of a single unifying scale factor required some compromise

among the fits to the various parameter distributions. A model scale of 1:350 appeared to

be the best for the rural simulation and 1:300 for the suburban simulation. The results for

the mean velocity profiles, turbulence intensity profiles, and power spectra are shown in

Figures 5.20-5.26 at these scale factors. Plots of the Reynolds stress variation are not

showm due to the large discrepancies in the data near the floor.

100

i" 80 -£ CD

'« I 60 « 8 in 1 40

20

U H

0.

Scale 1:350

« Rural Simulation


D C 1 « 1

B A

/ * / / /

1^1 / y

/*/ / .y^

*/ / .y^^^^"'"'^

DO

1

0.50 1.00 1.50 U/Uref

2.00 2. 50

Figure 5.20. Mean Velocity Profile for Rural Simulation at 1:350 Scale

65


100

90

80

^ 70 E,

D C Scale 1/350

X Rural Simulation

60

50

40

30

20

10

«> I S. S

CO

Zmin

10 20 30 40 50

l z%

60 70 80 90 100

Figure 5.21. Turbulence Intensity Profile for Rural Simulation at 1:350 Scale


Scale Factor 1:350 0.35

0.3

• o

0.25 Q. E < 2 0.2

0.15 Q .

<n

a i 0.1 o

0.05 -

0.0001 0.001

Ex( osures are analyzed at 3i > meter heights

0.01 0.1

nz/V

10 100

•Exposure C

Figure 5.22. Power Spectrum of Rural Simulation at 1:350 Scale

66

100

90

80

^ 70 E. S 60 a

° «

I 50

w 40

•^ 30

20

10

Scale 1/300

• Suburban Simulation

0.00 0.50


D C

2.50

Figure 5.23. Mean Velocity Profile for Suburban Simulation at 1:300 Scale

100 n

90

80

^ 70 E, £ 60 a> '5 X 50 -09

OT 40 -

^ 30

20 -

10

0 (

D C B A


Scale 1/300

X Suburban Simulation

1 1 X I I

1 I * \

1 1 ' \ I I X \

\ \ X \ \

\ \ X Zmin

V "V X

) 1( } 20 30 40 50 60

lz%

70 80 90 1 00

Figure 5.24. Turbulence Intensity Profile for Suburban Simulation at 1:300 Scale

67


Scale 1:300 0.35

0.3 - -o •o

I 0.25 E < 2 0.2 - -

« 0.15 •o «>

I 0.1

0.05

I I I I 1 1 1

Exposures are analyzed at 30 meter heights

0.0001 0.001 0.01 0.1

nz/u

10

1 I I r 11

100

——Exposures —Seriesi

Figure 5.25. Power Spectrum of Suburban Simulation at 1:300 Scale

68

CHAPTER 6

CONCLUSIONS AND RECOMMENDATIONS

Adequate simulations were performed for a rural and suburban sunulation wdth

the exception of the turbulence intensities in the lower boundary of the wind tunnel. The

best single scale model to have a reasonable compromise between the velocity profiles,

turbulence mtensities, and power spectra was 1:350 for the rural sunulation, and 1:300

for the suburban simulation. The following sections in this chapter summarize the above

results based on these model scales.

6.1 Rural Simulation at 1:350 Scale Model

A scale model of 1:350 appeared to be the best compromise between the various

parameter distributions for the rural simulation. The results for the rural simulation at

this scale model are summarized as follows:

• The rural simulation profiles. Figures 5.20-5.23, show a fair comparison between

the simulated values to the predicted ASCE 7-98 full-scale values.

• The fiiction velocity, lu, was determined at 0.587 m/s. At 1:350 scale, the

aerodynamic roughness, Zo, was 0.01 m compared to full-scale values of 0.01-

0.04 m for low grass, steppe. The power law exponent, a, was equal to 0.137

compared to the ASCE full-scale value of 0.154.

• The longitudinal integral length scale analyzed at 100 mm in the wmd tuimel was

LMX = 212 m (scaled at 1:350). Using equation 5.8, the longitudinal mtegral

length scale was predicted at Lz= 196 m for a fiill-scale height of 35 m.

• Analyzing the integral length scale at 250 mm, or 87.5 m fiiU scale, Lwx = 203 m

and Lz = 235 m for the ASCE values.

69

6.2 Suburban Simulation at 1:300 Scale Model

The best suigle scale model to have a reasonable compromise for the suburban

simulation was 1:300. The suburban simulation results at this scale model are

summarized as follows:

• The suburban simulation profiles. Figures 5.24-5.27, also show a fan comparison

between the simulated values to the predicted ASCE 7-98 full-scale values.

• The fiiction velocity, M. , was determined at 1.145 m/s. At 1:300 scale, the

aerodynamic roughness, Zo, was 1.02 m compared to full-scale values of 0.8-1.2

for densely built up suburbs, towns. The power law exponent, a, was equal to

0.239 compared to the ASCE fiill-scale value of 0.25.

• The longitudinal integral length scale analyzed at 100 mm in the wind tuimel was

Lwx = 130 m (scaled at 1:300). Once again using equation 5.8, the longitudinal

mtegral length scale was predicted at Lz= 141 m for a full-scale height of 30 m.

• Analyzing the mtegral length scale at 250 mm, or 75 m full scale, LMX=1 88 m and

Lz= 191 m for the ASCE values.

A further investigation of measurement assessment is suggested for the lateral and

vertical components. In section 5.1.1, a large discrepancy was found between the

comparison of fiiction velocities obtained fi'om the indirect procedure and the direct

measurements. However, the discrepancy may also exist due to 'error in origin' in which

data is chosen through discretion for a best line fit (the indirect procedure). Also,

generally, it is expected that the lateral integral length scale is larger than the vertical

component as discussed in section 5.3.2. The reason for the discrepancies may be due to

the limited directional response with the cross film sensor, which only has a ± 45 degree

range. Further studies should be performed wdth perhaps a coplanar triple-wire probe

which has a better directional response than the cross film probes.

70

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Counihan, J. "Simulation of an Adiabatic Urban Boundary Layer in the Wind Tunnel."

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Goldie A.H.R. "Gustiness of Wind m Particular Cases." Q.J.R. Met. Soc. 51 (1925):

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Invin, H.P.A.H. "The Design of Spires for Wind Simulation." Journal of Wind

Engineering and Industrial Aerodynamics 7 (1981): 361-366.

Kolmogroff N. "The Local Structure of Turbulence in an Incompressible Fluid for Very

Large Wave Numbers." C.R. Acad Set U.S.SR. 30, (1941): 301.

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Suniu, E. and Scanlan, R. Wind Effects on Structures (3'''^ edition). John Wiley & Sons,

Inc., New York, N.Y., 1996

Stevenson T. "Report on the simultaneous observations of the force of the wdnd at

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73

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WIND TUNNEL SIMULATION OF AN ATMOSPHERIC BOUNDARY