ncar-tn/edd-82 wind tunnel and field investigations of ...74/datastream/p… · wind tunnel and...

NfCAR-TN/EDD-82 ^5NCAR TECHNICAL NOTE

February 1973

Wind Tunnel and FieldInvestigations of Shapes afor Balloon Shelters )

Robert H. Meroney >/Erich J. Plate 0

FACILITIES LABORATORY

CNATIONAL CENTER FOR ATMOSPHERIC RESEARCH) 4i/sBOULDER, COLORADO

IH

iii

PREFACE

This report discusses the results of a series of wind tunnel and

field tests performed in support of a program to develop a balloon in-

flation and launch shelter. The study was conducted for the NCAR Scien-

tific Balloon Facility by the Fluid Dynamics and Diffusion Laboratory,

Colorado State University; subcontract work was also carried out by

EG & G, Inc., Environmental Service Operation, who supplied scale models

of a partially inflated balloon (for wind tunnel tests) and a wind

screen (for outdoor tests). The study discusses scaling criteria for

simulating a shelter; velocity, turbulence, and frequency spectra down-

wind of four basic shelter shapes; the effects of screen materials on a

shelter's efficiency; and the influence of the presence of a simulated

balloon upon the effectiveness of the shelter.

Robert H. Meroney

Erich J. Plate

18 August 1972

v

CONTENTS

Preface ...... ......... ............. iii

List of Figures ...................... vii

List of Tables .............. ........... ix

I. INTRODUCTION ........................ 1

II. GENERAL CONSIDERATIONS OF SIMILARITY AND WINDBREAKDYNAMICS .......................... 2

The Drag Coefficient CD for Solid Shelters . . . . . . 2

The Drag Coefficient CD for Porous Shelters . . . . . . 3

The Effect of h/6 . . . . . . . . . . . . . . . . . . . . . 5

Pulsatory Forces on the Balloon . . . . . . . . . . . . . . 5

III. DESCRIPTION OF EXPERIMENTS . . . . . . . . . . . . . . . . . 7

Wind Tunnel Simulations . . . . . . . . . . . . . . . . . . 7

Velocity Distributions ................. 7

Turbulence Intensity ............. ..... 8

Turbulence Spectra ............. ...... 8

Pressure Drop Coefficients . . . . . . . . . . . . . . . 9

Wind-Screen Field Tests ............. ..... 9

IV. RESULTS AND DISCUSSION ................... 11

Wind Tunnel Simulation-Visualization . . . . . . . . . . . 11

Velocity Profiles . . . . . . . . . . . . . . . . . . . . 12

Turbulence Intensity .................. 12

Spectral Analysis .................... 13

Quantitative Effects of Included Angle, ScreenMaterial, and Balloon Presence . . . . . . . . . . . . . 13

Wind-Screen Field Tests .................. 14

V. CONCLUSIONS ......................... 16

References . . . . . . . . . . . . . . . . . . . . . . . . 17

Figures . . . . . . . . . . . . . . . . . . . .19

Tables .......................... 39

vii

FIGURES

1. Experimental balloon ready for launch . . . . . . . . ..... 21

2. Wind screen at Marshall Field Site .............. 22

3. Smoke visualization past 120° wedge (UO 5 ft/s) ....... 23

4. Screen models . ....... ....... 24

5. Approach wind profile (at x = 5W upstream from model) . . . . 25

6. Centerline velocity isotachs behind square plate(porosity = 0) ........................ 26

7. Cross-section velocity isotachs behind square plate(porosity - 0) . ....................... 27

8. Centerline velocity isotachs behind square-plate screen(porosity ~ .67) ....................... 28

9. Cross-section velocity isotachs behind square-plate bugscreens (porosity - .67) ................... 29

10. Centerline velocity isotachs behind 90° wedge screen(porosity ~ .67) ....................... 30

11. Cross-section velocity isotachs behind 90° wedge screen(porosity - .67; UX = 30 fps) . . . . . . . . . . . . ..... 31

12. Centerline velocity isotachs behind 90° wedge screen(porosity ~ .15) ....................... 32

13. Cross-section velocity isotachs behind 90° wedge screen(porosity - .15; UO = 30 fps) . . . . . . . . . . . . ..... 33

14. Turbulent intensity profiles downstream of squareand 90° wedge screens (porosity ~ .15) ............ 34

15. Upwind, midwind, and downwind sensor locationsfor field measurements ................. .. 35

16. Plot of windspeeds for various sensor locations,one layer of bug screen (sensor height 9 ft 4 in.) ...... 36

17. Plot of wind speeds for various sensor locations,two layers of bug screen (sensor height 5 ft) . . . . . . ... 37

ix

TABLES

1. Velocity Profiles (Feet/Second) ....... ......... 41

2. Turbulence Intensity (Dimensionless) ............. 49

3. Mass Flux Parameter E (Dimensionless) ............. 54

4. Momentum Flux Parameter ) (Dimensionless) . . . . . . . . . . . 55

1

I. INTRODUCTION

Tests were conducted both in a wind tunnel and in the field to cal-

culate possible launch shelters for large meteorological balloons. The

purpose of the investigation was to provide basic criteria for the de-

sign of a screen which would reduce the wind from 25 ft/s to below

14 ft/s over a volume sufficiently large to protect large balloons

(about 70 ft in height, width, and depth) during inflation and launch

(see Fig. 1). The emphasis of the study was upon the effects of basic

shelter configurations and screen materials upon the sheltered area.

The study examined the following aspects of shelter dynamics:

* Height, length, and breadth of the sheltered areas for

various shelter configurations

* Comparative effectiveness of a solid shelter and various

porous screen materials

* Quantitative measurement of the frequency and intensity of

gusts shed from the upper edge and sides of the proposed

shelter configurations

* The effect of changes in the angle of wedge-shaped shelters

on the sheltered area and on gusting

* The effect of the presence of a balloon shape within the

sheltered area upon the effectiveness of the shelter

Four basic shelter shapes were tested at a scale of 1:70 in the

meteorological wind tunnel of the Fluid Dynamics and Diffusion Labora-

tory of Colorado State University. Various shelter angles, screen ma-

terials, and the effect of the presence of a partially inflated balloon

were examined. Subsequently, in outdoor experiments, wind velocities

were measured behind a 1:7-scale maodel for one shelter angle and two

screen materials.

2

II. GENERAL CONSIDERATIONS OF SIMILARITY ANDFLUID DYNAMICS OF WINDBREAKS

Earlier work by Plate and Lin (1965) showed that laboratory model-

ing of a field windbreak can be accomplished if CD (the drag coefficient

of the shelter) and the ratio h/6 are the same in both the field and the

laboratory (h denotes the structure's height and 6 the thickness of the

boundary layer). Although these requirements were for two-dimensional

flow fields, it can be expected that only minor modifications would be

required for the three-dimensional counterpart.

THE DRAG COEFFICIENT CD FOR SOLID SHELTERS

Approximately constant drag coefficients CD can be obtained if the

model and prototype shelters both have sharp edges. Then the drag coef-

ficient, defined by

C -D (1)pu h-W

(where D is the drag on the shelter), becomes independent of the Rey-

nolds number uih/V. In this equation, h is the height and W is the

breadth of the projection of the shelter on a plane perpendicular to the

direction of the ambient air flow up (at some reference height). Ordi-

narily C would be a function of the Reynolds number. However, if the

shelter's edges are sharpened, the separation line of the boundary layer

on the shelter becomes fixed, resulting in a CD which is independent of

the Reynolds number. It does, however, depend slightly on h/6, but this

dependency is not critical and can be adjusted by making the boundary

layer of the approach flow as thick as possible.

The drag coefficient determines not only the drag on the shelter

but also the shape of the flow field downstream from the shelter. In

general, the larger CD is, the larger the sheltered area will be, but

evidently at the price of a larger drag force and higher turbulence

levels.

3

For a solid screen, or a square flat plate, it is possible to ob-

tain the drag coefficient, to a first approximation, from the relation

CD rectangular plate

CD infinite 2d plate in boundary layerD in boundary layer

(2)

CD rectangular plate

CD infinite 2d plate in free stream

or

CD rectangular plate 1.16

0.8 1.90

where the free-stream values are from Rouse (1950) and the value of 0.8

for the drag coefficient of the infinite plate in a boundary layer has

been taken from experimental results of Plate (1964). Consequently

CD = 0.5 to a first approximation.

Measurements by Vickery (1968) for a plate which was only partially

imbedded in the wall boundary layer yielded CD = 1.0; this value falls

between the assumed free-stream value of 1.16 and the calculated boun-

dary layer value of 0.5. A safe value to be used in calculation might,

therefore, be taken as CDD 0.7.

Vickery points out that in addition to the mean drag there also

occurs a fluctuating drag whose RMS value might be as much as 10% of the

mean. He does not give a peak value, but suggests that a suitable

safety factor should be used. In view of the fact that the structure of

the shelter will be very light, a safety factor of at least two is recom-

mended; thus, for the design of the structure, CD = 1.0-1.2 should be

used.

THE DRAG COEFFICIENT CD FOR POROUS SHELTERS

It is very likely that the effect of porosity is also a Reynolds

number effect, but this time the Reynolds number should be based on the

4

properties of the screen material. Since the same air flow and viscos-

ity apply to both the model and prototype screens, the screens must be

the same in order to produce similar Reynolds numbers. Actually, how-

ever, it is found that for a given screen material the aerodynamic be-

havior is practically independent of the Reynolds number. A measure of

the aerodynamic behavior can be obtained by determining the pressure

drop Ap across a screen which passes a velocity of u fps. The pressure

drop coefficient

c =- p (4)P hpu2

should become independent of the Reynolds number.

For a porous screen, the pressure drop coefficient yields a measure

of the force exerted on the screen. Let u be the velocity observed di-

rectly downstream of the model screen. Then, to a rough approximation,

1 -2D = cp 2 u 2 w h (5)

or, if the reduction factor c is introduced,

uc = - (6)00

which signifies the reduction of velocity caused by a screen. Then

D = c c2 w h (7)

For a given screen material and shelter shape, the coefficients c andp

c are found from wind tunnel experiments.

Comparison of Eqs. (1) and (7) shows that for a porous screen

CD c c 2 (8)p

The experiments show that for a porous screen both c and c are approxi-

mately independent of velocity, so that CD is found to be independent of

the Reynolds number for porous screens also, provided that the model and

prototype screens are the same.

5

For the bug-screen material used, we find a value of c = 0.62 andP

a wind velocity reduction factor of c = 0.5. Consequently, the equiva-

lent drag coefficient, according to Eq. (8) is

CD = 0.62 . - = 0.16 (8a)

It goes without saying that Eq. (8) is valid only for CD < 0.5-0.7.

Once CD = 0.5-0.7 is reached, a screen behaves like a solid screen

regardless of its actual porosity.

THE EFFECT OF h/6

The parameter h/6 determines mainly the velocity distribution down-

stream of the shelter and outside the sheltered region. For the shel-

tered region its effect is mainly on the drag coefficient. For thick

boundary layers, CD varies approximately proportionally to (h/6)217 in

the case of an infinitely wide shelter. For a finitely wide shelter,

the effect should be even smaller. Thus, if we make the profile ap-

proaching the shelter roughly logarithmic and as thick as possible, the

values of CD obtained in the experiments should be applicable with min-

imal error to atmospheric conditions, which leads to the proposed value

of CD 0.5-0.7.

PULSATORY FORCES ON THE BALLOON

A sharp-edged shelter might be expected to shed regular eddies (of

Karman-type vortices) which would be the dominant feature in the large-

scale turbulence. On the other hand, flow through the porous screen may

never allow significant lateral pressure excursions to occur such that

an alternating structure would be observable. Recent measurements by

Castro (1971) of wake characteristics of two-dimensional perforated

plates in a uniform stream indicate that at an open-area ratio of about

0.2 there is a radical change in the flow structure. Above a porosity

of 0.2 the formation of vortices apparently ceases altogether and the

energy thus preserved is evident in the abrupt reduction in drag coeffi-

cient. One may perform a similar analysis to that of Eq. (2) for the

6

ratio of drags of a porous rectangular plate to a solid rectangular

plate in a shear flow as compared with the ratio of two-dimensional por-

ous plates to a solid plate in a uniform stream. A value of CD for the

bug-screen material of 0.16 is found which agrees exactly with the pre-

diction of Eq. (8a). Indeed, even for solid prisms in a turbulent shear

flow, the presence of regular shedding is indefinite, according to Cer-

mak.1 Jensen and Franck (1963) remark from their experiments on wind

screens of large aspect ratio that behind porous surfaces the vortex

layer will certainly oscillate somewhat, since all discontinuity surfaces

are unstable. However, they observed that these small motions will cause

only minute variations in the velocities. Nevertheless, one might wish

to estimate the likelihood of resonance of any gusts generated around a

filling balloon. Vickery (1968) did obtain load fluctuation results for

prisms in uniform turbulent flow. Thus the frequency f of the dominant

eddies may be given approximately by the Strouhal frequency obtained

from the relation

St = fW = 0.08-0.11 (9)

where St is the Strouhal number (which according to results of Vickery

is approximately constant and lies within the indicated range) and f is

the peak frequency. Typically, for a shelter 70 ft wide, one would ex-

pect a dominant frequency (at 30 ft/s) of about

U c 30f = ~ St = 7- (0.1) = 0.45 Hz (10)

For the 1-ft-wide model examined in the wind tunnel, a similar cal-

culation would suggest a dominant frequency (at 30 ft/s) of about

·30f = -- (0.1) 3 Hz (11)

Unfortunately this value is at the lower range of reliability for wind

tunnel instrumentation and may not be easily determined.

1Cermak, J. E., private communication, 1971.

7

III. DESCRIPTION OF EXPERIMENTS

WIND TUNNEL SIMULATIONS

The meteorological wind tunnel has a 6 x 6 x 80 ft test section and

generates a shear layer on the order of 2 to 3 ft in thickness (Plate

and Cermak, 1963). Four basic shelter shapes were tested. One con-

sisted of a 1 x 1 ft square plate (scale 1:70) set perpendicular to the

wind; the other three were of a wedge type of the same height and pro-

jection on the plane normal to the flow and had apex angles of 90, 120,

and 150°. The models consisted of steel frames over which the screen

materials had been stretched, as shown in Fig. 4c. They were designed

with sharp outer edges, so that separation would always occur at the

edges. Two different screen materials were tested: ordinary fiber-

glass bug screen (.010-in. wire, 18 mesh, - .67 open area, and a special2

dense-mesh fiber-glass material provided by NCAR (- .15 open area) .

The presence of a partially inflated balloon was simulated by a

rigid wooden model. The model was 10 in. high and tapered from a 4-in.-

diameter hemispherical cap to a 0.75-in.-diameter base. The model was

not, of course, pliant when struck by gusts (see Fig. 3).

Velocity Distributions

Vertical distributions of horizontal mean velocities were taken to

map out the sheltered region. Using a coordinate system where X is the

distance downstream measured from the shelter's trailing edge, Y is the

lateral distance from the screen's centerline, and Z is the vertical

height, velocity measurements were taken for X = 0(3)18, Y = (3)12, and

Z = 0(5)15 in. (Only one side of the shelter need be considered in view

of the symmetry of the shelter about the XZ plane.) Velocities were

measured by a pitot-static tube with a Transonic pressure transducer.

2Style 9710 - 18 x 56 in. mesh fiber-glass cloth screen (manufacturedby J. P. Stevens and Co., Waterboro, S.C.)

8

A free-stream velocity range of 20-50 ft/s was used on the four differ-

ent shelters.

Turbulence Intensity

A gross measure of the tendency of the balloon shelter to dissipate

the kinetic energy of the unrestrained wind is the turbulence intensity

u'2 where u' is the fluctuating velocity component (with time mean zero)

in the direction of the mean local flow velocity; the overbar denotes

the time mean. Due to the limitations of the RMS analyzers utilized,

these data are of frequencies higher than 2 Hz; they are thus not repre-

sentative of the low-frequency end of the spectrum, which is the region

of greatest importance for balloon sheltering. It also became clear

that one area of major interest was the side edges of a shelter. Fur-

ther examination of this area involved turbulence intensity measurements

and frequency analyses of the eddy shedding at the edges. Turbulence

intensities were measured with a Disa constant-temperature anemometer,

type 55A01.

Turbulence Spectra

Two types of turbulence spectra data were obtained. The first was

pressure fluctuations of a pitot-static tube 3 in. from the centerline

and 6 in. above the floor (6 in. = 2h) at four different downstream dis-

tances from the NCAR dense-mesh square plate and wedge. These data,

recorded on strip charts, give an indication of the low-frequency turbu-

lence which is likely to affect the balloons. However, one cannot de-

tect any low-frequency component in the recordings which might be sig-

nificant. A second set of turbulence data was obtained from a constant-

temperature hot-wire anemometer. Special attention was given to the

shear flow at the shelter edge. The signal from the anemometer was

analyzed using General Radio's graphic level recorder, type 1510-A,

coupled to a sound and vibration analyzer, type 1911-A. This equipment

provides a frequency response from 2.5 to 25 Hz.

9

Pressure Drop Coefficients

Pressure drop coefficients c were obtained by stretching screens

across the whole cross section of a small duct and measuring velocity

and pressure drop across the screen with one pitot-static tube located

upstream and one downstream of the screen. For the NCAR screen, we

found a pressure drop coefficient c of 22; this value implies an almost-p

solid screen, independent of the Reynolds number. For the bug screen,

the pressure drop coefficient was found to be 0.62. A double layer of

bug screen with a 45° angle between the mesh orientation yielded a pres-

sure drop coefficient of about 1.25. Again, all Reynolds number depen-

dencies, if existent, were hidden in the scatter of the experimental

results. These values of pressure coefficients agree qualitatively with

the estimating expressions suggested by Hoerner (1965). For high solid-

ity, a, he recommends c = (a/(l-a)) 2 and for low solidity, a < 0.5,

cp = CD/(1-a) where CD is the drag coefficient of the screen element.

For the dense NCAR screen a = 0.85 and for the open bug screen a = 0.33;

thus one obtains c = 32 and 0.73, respectively, which are on the order

of those measured.

WIND-SCREEN FIELD TESTS

A 1:7-scale model of the proposed shelter was constructed by E.G.

& G., Inc., of Boulder, Colo., and tested under the direction of NCAR

at the NCAR Marshall Field Site south of Boulder. The shelter was con-

structed by stretching fiber-glass bug screen over a frame of 2 x 4 in.

building planks. The shelter consisted of two flat sections, 10 ft high

by 5.75 ft wide, hinged together along the 10-ft common edges. When

standing with an apex angle of 120°, the shelter exposes a 10 x 10 ft

frontal area to the incident wind (see Fig. 2).

Measurements were made on a day with ambient winds in the range of

17 to 34 ft/s; the shelter was positioned with the apex pointing into

the wind. Reference cup-anemometers were located 40 ft upwind and 28 ft

downwind of the shelter, each 5 ft above the ground. Data were moni-

tored continuously from these sensors while a midwind sensor was placed

alternately in various regions of the sheltered zone. The midwind sensor

10

was operated both at 5 ft and at 9 ft 4 in. above the ground, with first

two layers and then one layer of bug screen attached to the shelter.

Data were recorded for at least 2 min for each midwind location, each

height, and each layer of the screen. Figure 15 shows the sensor

locations.

11

IV. RESULTS AND DISCUSSION

WIND TUNNEL SIMULATION-VISUALIZATION

To provide an estimate of the amount of flow deflected around the

shelters, smoke tracers were released upstream of the shelters. At very

low free-stream velocities the majority of the smoke was deflected right

along the upstream face. At higher free-stream velocities most of the

smoke passed through the screen about halfway along the screen's surface.

With the balloon model placed behind the screen the smoke pattern was

not observably changed.

The other visualization technique involved looking at the motion of

a small cork ball suspended by a thread from a long wire rod held out-

side the flow field. The ball was suspended at various positions about

the shelter, primarily at the sides and top edges of the shelter. The

ball did not rotate over the top edges, but rotated vigorously at the

side edges. As the ball was drawn across a vertical side support (in

the direction of decreasing Y), its rapid rotation abruptly changed di-

rection. A short distance inside the sheltered region the ball's rota-

tion slowed and ceased. It is felt that this vortex phenomenon at the

shelter's edges is due entirely to the vertical supports. The outer vor-

tex is produced by the flow deflected along the screen and the free-

stream flow as it sweeps around the trailing edges of the supports; the

inner vortex, of opposing direction, is produced by the flow that passes

through the screen close to the supports (see following sketch).

Z

10 inches

\2 ̂^ Shelter Frame Disturbance

12

Velocity Profiles

A typical profile of the approach velocity for the shelters is

shown in Fig. 5. Isotachs were constructed from a complete set of such

profiles. Two typical types of figures were prepared (Figs. 6-13): One

type is a profile along the centerline, showing the reduction of wind

velocity along this plane; the other type is a cross section through the

sheltered region. Downwind distances from the wedge models are measured

from the downwind edges.

From the techniques described, it appears that all shelter angles

give approximately the same sheltered region with a velocity reduction

of about 50%. Eddy shedding from the shelter supports is an important

feature but is confined to the side edges of the shelter and does not

affect the sheltered area. No other large-scale vortices related to

the shelter's geometry were observed.

With the wooden balloon model in place, a marked decrease in veloc-

ity on the centerline with an increase around the sides and over the top

of the balloon was observed. A short distance downstream these two

effects seemed to combine to produce a flow pattern similar to that of

the "no balloon" case.

Turbulence Intensity

The turbulence intensity distribution around the shelters was in-

vestigated with the Disa hot-wire anemometer. The free-stream turbu-

lence level was about 3%. Behind the screen section of a shelter the

turbulence level was very low (4%), but on the centerline (i.e., behind

the center support) the level rose to 20%. The effect was most marked

at the edges, 40% intensity being the general value. These high values

at the edges are consistent with observations of the cork ball in the

vortex. The ball failed, however, to indicate the relatively high tur-

bulence level behind the center support. Typical profiles of u'2 along

a distance l/4w off the centerline are shown in Fig. 14.

13

Spectral Analysis

The signal from the anemometer was subjected to a frequency spec-

trum analysis, the eddy shedding frequency at the edges being the major

area of interest. The possible frequency for dominant eddies, given

approximately by the Strouhal frequency, was on the order of 10 Hz. Ex-

tensive investigation failed to isolate this frequency. In addition to

the arguments cited previously, there is the possibility that for porous

structures the realm of regular frequencies ceases for Re > 10 5 . Indeed,

even for cylinders, measurements for high Reynolds numbers are very

sparse. Roshko has suggested that a Strouhal gap exists between 10 5 and

107 for eddy shedding from cylinders (Morkovin, 1964). Though the Rey-

nolds number for the prototype shelter size may well be greater than

107 , one may have difficulty in identifying a specific maximum shedding

frequency since the maximum becomes increasingly blurred at high Rey-

nolds numbers, and since the flow through the porous screen may never

allow significant lateral pressure excursions to occur such that an al-

ternating structure may be observable.

Quantitative Effects of Included Angle, Screen Material, and Balloon

Presence

The general character of velocity reduction behind the shelters

for the different configurations was discussed above. For the single-

screen shelter the velocity reduction for all angles was over 50%. For

the double-screen shelter the reduction was about 75%, with the pressure

coefficient increasing to twice that of the single-screen case.

Two dimensionless parameters, i, a mass flux parameter, and i, a

momentum flux parameter, were defined

fgYpu dy

= pOL'

and

fopu (U-u) dy

14

where u is the velocity behind the shelter, U, the free-stream velocity,

and L the total width of the shelter (L = 12 in.). The momentum flux

parameter i may be viewed as a pseudo-drag coefficient in the sense that

it is a measure of the blocking effect of the shelter. It is, of

course, not exactly a drag coefficient, since the fluid motion is three-

dimensional and corrections must be made for static pressure variations

when a measured traverse is close to the shelter. Schlichting (1968)

discusses correction methods to be applied to drag calculations from

measurements in the mean wake vicinity. These parameters were calcu-

lated for the wake of the different shelters and are tabulated in Tables

3 and 4. It can be seen that there is little variation in the parameters

over the set of single-screen shelters, suggesting that shelter shape

has little effect on the downstream region. For the double-screen shel-

ters, the decrease in these parameters is consistent with intuition.

Again, little variation is seen over the range of free-stream velocities.

Intuition suggests that the shelter angle should influence the

sheltered zone since at small angles the wind must approach the screen

obliquely and will experience a decreased effective permeability.

Eimern (1964) reports the results of numerious investigators concerned

with windbreak effectiveness at oblique angles of wind incidence. Al-

though a number of authors report marked changes when wind angles exceed

30°, others assert that variations in angle of incidence to 45° cause no

important change in sheltering. In any event, all authors suggest that

any suggested influence is in the extent rather than the quality of the

sheltered zone; hence the absence of any strong effect of the shelter's

shape in our experiments is a reasonable finding.

Wind-Screen Field Tests

Wind velocity ratios were extracted from the records of the field

experiment and compared with those from the wind tunnel tests. Ten of

the fourteen measuring sites in the field were directly comparable to

the wind tunnel results. The time-varying wind records were averaged

and standard durations calculated. The general shapes of the sheltered

zones were equivalent. The single screen, with a 120° apex angle,

15

reduced the wind velocity by - .35 with a variance of ~ .025. The double

screen, with a 120° apex angle, reduced the velocity by - .50 with a

variance of - .035. The instantaneous upstream to mid-region velocity

ratio varied from .16 to 1.0 and .07 to 1.0 for the single and double

screens, respectively. This wide variation may result from wind veering

or the lack of spatial correlation of velocity between anemometers.

The field measurements revealed a generally smaller sheltering ef-

fect than the wind tunnel measurements. The single screen provided 50%

wind reduction in the wind tunnel but only 35% reduction in the field

(typical plots are presented in Fig. 16). Values for the double screen

were 75 and 50%, respectively (see Fig. 17). The proportionate increase

in the added shelter of the double screen was about the same for both

experiments.

16

CONCLUSIONS

On the basis of the reported experiments, the following conclusions

about the design of a balloon shelter are drawn:

1. Porous shelter surfaces have a considerably lower turbulence

level than solid or almost-solid surfaces, but a higher mean

wind velocity in the sheltered regions. Furthermore, the forces

on a porous screen are much smaller. Rough estimates for the

square plate give drag coefficients of 0.5-0.7 for solid sur-

faces, and 0.16 for bug-screen surfaces.

2. The shelter angle has no noticeable effect on wind velocity,

turbulence level, or flow pattern.

3. Velocity reduction for all angles with a single screen is over

50%; for a double screen it is 75%. The pressure coefficient

is doubled for the double screen. Neither the flow pattern nor

the percentage reductions depended on the ambient velocity U .

Consequently, it is felt that prototype and model screens

should be the same. It is recommended that a material should

be used for the screens which is slightly denser than the bug

screen, such as a double layer of bug screen or its equivalent.

4. The presence of a balloon produces higher velocities around the

balloon surface. Downstream the flow pattern returns to the

"no balloon" case.

5. Eddies shed from the wind screen's edges could interact with

the balloon if the shelter were too narrow.

6. The vertical side supports may cause a blockage near the struc-

ture. The velocity decrease behind these supports recovers

quickly with distance downstream.

7. A square-plate shelter provides a larger sheltered area, but a

greater turbulence intensity than a wedge-shaped design. For

this reason, and because of greater convenience in construc-

tion, it is recommended that a wedge be used, with suitable

modifications to meet structural requirements.

17

REFERENCES

Castro, I. P., 1971: Wake characteristics of two-dimensional perforatedplates normal to an airstream. J. FZuid Mech. 46(3), 599-609.

Eimern, J. von (Chairman), 1964: Windbreaks and Shelterbelts, WorldMeteorological Organization Technical Note No. 59 (WMO-No. 147, TP,70), Geneva, Switzerland, 188 pp.

EG&G, Inc., 1971: Report on Balloon Shelter Tests, NCAR PurchaseOrder 156-70, EG&G, Inc., Environmental Services Operation, Boul-der, Colo., 43 pp.

Hoerner, S. F., 1965: Fluid Dynamic Drag, published by author, MidlandPark, N. J. (various paging).

Jensen, M., and N. Franck, 1963: Model Scale Tests in Turbulent Wind,Danish Technical Press, Copenhagen, Denmark, 97 pp.

Morkovin, M. V., 1964: Flow around a circular cylinder--a kaleidoscopeof challenging fluid phenomena. In Proc. of Symposium on Fully Sepa-rated Flows, American Society of Mechanical Engineering, May 18-20,102-118.

Plate, E. J., 1964: The drag on a smooth flat plate with a fence im-mersed in its turbulent boundary layer. American Society of Mechani-cal Engineering Paper No. 64-FE-17, 12 pp.

--- --, and J. E. Cermak, 1963: Micrometeorological Wind-Tunnel Facil-ity: Description and Characteristics, Technical Report CER63EJP-JEC9,Fluid Dynamics and Diffusion Laboratory, Colorado State University,Ft. Collins, Colo., 65 pp.

, and C. Y. Lin, 1965: The Velocity Field Downstream from a Two-Dimensional Model Hill, Colorado State University Final Report(Part I), Contract DA-AMC-36-039-63-G7, U.S. Army Materiel Command,81 pp.

Rouse, H. (Editor), 1950: Engineering Hydraulics, John Wiley & Sons, Inc.,New York, N. Y., 1053 pp.

Schlichting, H., 1968: Boundary Layer Theory, McGraw-Hill Book Co.,New York, N. Y., 747 pp.

Vickery, B. J., 1968: Load fluctuations in turbulent flow. In Proc.American Society of Civil Engineers Vol. 94, Paper No. Em 1, Journalof Engineering Mechanics Division, 31-46.

19

FIGURES

21

4' � p.�./,'.A.-. 'trp.�.., .. �.

'/4...' 14 � . 4'

. � 4% ��A' '�% �

7' ,.'.�...7",..#.'@ t�77'44'44' 34/4.#4V�t4�tj;z'�' � #4Atr7't4%z"t'

7 ''4 5 7'.'.7.

7" �. 7 /7 .7 . 7 7 .7,. "7 . '.7 . 7.

7 '.7 . p..

�¾� 7 . 47 ....� .; .�7 ii. . /4 . .p. 7 ./4 .

7' ,.� / / 4..;" 7'4�....7.

.7..., p *J.97777 �

.'7777.. 4 "p77"' 77747' 777/7• � / .';477; 4'7'�pp7/,.��§34 7'7'7 7777.7777.7777 �77',7.7'77<

.77.77.. 7.7 47. 2�77.7',. 77,77,7,7,77777777777/777/�.�7'�7 9 / 7. 77�� 4t.'' .':9V7•7""�"" '7

'77, 773447•.t777.'.77�7p7. .777' .7.77';7'�'.. ' 7, "4rt....77.7.'777 777 777 7'� 7'.,... ....

.7.777/7.4 'flfl"77'/r'''"7' 77 77�4'>77777774474.77>. ,,�,.y,, 7'7'7"'7 7.'777*' t.7 7 A � � ,7,<77��.77 7'. 7.. �, 77777'7�7

P7

77!77•77777'7'77$7'./

77 7 77/7.7777.777.7 77, 7777777 77. ,7, �. 777�' .77. . 4

77777.7 .77W.'.

>4" 7'... >�p4'•s'v :m "477 "":. � 7 7.77,77.777.7777777.' 7 '' .7,777. 744.7.774' '77,7' 7,." ,7,77'77'. 77777 7' "''�""'�"'�.

.77'.�777'77'77.7' ',,.,,,,,.,7,.,.4.. 777777.77' 7777 7777." "'�". 7�'Ž7'.777.77'77 .777.7.777777§77774777J7777/ '.7777.77777'..

7777777777 ,' 7777777' � .777% 77 7 7777.".7 747

Fig 1 Experimental balloon is inflated and read for launch

22

Fig. 2 Wind screen erected at Marshall Field Site, Boulder, Colorado.

23

Fig. 3 Smoke visualization past 120° wedge with balloon modelU ~ 5 ft/s.

24

Tunnel A"__ ~

Schematic of Square Screen

~ Sharp Edge(Milled)

Uco.J h=W 1\/// x

Schematic of Wedge Screen

1/2

Fig. IC ConstructionDetails (Notto Scale)

Fig. 4 Screen models.

25

(in,) , Y

16 -Uo, =2 9.8 fps

14 -

12-

10

c| 8 -

0 6

2 -

00

Fig. 5 Approach wind profile (at x = 5W upstream of model).

26

__----I

T- f/'YS-^---- -50% Reduction4

10 - N .Y

Uoo=10fps - Ix

5-

6 12 18

^^^^ ^ -- = - '-----~ 150% Reduction

10-

Uoo=20fps

6 12 18

Velocity at Y=O (0)

)44------ 40

f.5~g^^----16 50% Reduction

Uoo=40fps

6 12 18

Fig. 6 Centerline velocity isotachs behind square plate(porosity = 0).

27

z

x=3in. x=6in.

3 6 75 13 6 7.5 15

z x=12in z x=18in.

30612 18 424

6 9 12 3 6 9

Fig. 7 Cross-section velocity isotachs behind square plate(porosity 0).

28

X

15

53 6 ~ ~ 12 In16

4120

10 8.6I0 //Y \22 /8.6 8 = 20 fps

1 0 ©5

1 1~~~~~~~~~~~~x

3 6 12 18 in.

z

i40

Fig. 8 Centerline velocity isotachs behind square-plate screen(porosity ~ .67).

3224

20

18 C' U.= 40 f ps

Fig 8 entrlie vlocty soach beindsqure-lat scee

29-

15 X 3" _ = 6

30

10 W^ I 1 6r,oHigh /Low

5 4

18 24 12 24

vI =__YY

0 3 6 9 12 in. 0 3 6 9 12 in.

X =12"

30 UOO =U30 fps

10

0 3 6 9 12

Fig. 9 Cross-section velocity isotachs behind square-plate bug screens(porosity ~ .67).

30

z ,10

15 - Bug ScreenWedge

3 6 9 12 18in.

. z

15 6

H^^^- 4!

U=20fps

3 6 9 12 18 in.

Velocity at Y=0 (()

20

U -20fps

._-x

3 6 12 18in.

Fig. 10 Centerline velocity isotachs behind 90° wedge screen(porosity t .67).

31

Uoo = 30 fps

iz 7z

-33030

-24I0O ^^^ ^l--H8 _________ \\|X 3 "X = 6"

12 a

14

0 3 6 9 12 in. Y = 3 6 9 12 in. y

Fig. 11 Cross-section vel y i s b d 90° w e 15

(porosity .67) U = 30 fps.24

- 18 1 ~~~~~~~~~~~~X 18

0 3 6 9 12 in. y 0 3 6 9 12 in. y

32

y

15

5 h i 3!6 U,-=20fps

15 40

10 4 I x

3 6 12 18 3

Fig. 12 Centerline velocity isotachs behind 90° wedge screen(porosity .15).

33

15 "x=3in. 15 x=6in.

10 I0

(prst .1530

12 18 24

6 9 y 3 6 9 y

zx=12in. x=18in,

15 5

I0

3 6 9 3 6 9

Fig. 13 Cross-section velocity isotachs behind 90° wedge screen(porosity ~ .15) U = 30 fps.

34

Z Peak: I (fps) 2

15 -

5

0 - -.---

x

3 6 9 12 15 18 in.

Z Peak: ~8(fps)2

15

Y 1/4W=3"

5

0 3 6 12 18 in.

Fig. 14 Turbulent intensity profiles downstream of square and 90°

wedge screens (porosity ~ .15).

Upwind ________Sensor ~~~-

Wind Direction----_ 6Note:Sensor Height =5' 6

C-64+1 -- I

Balloon 5 6Shelter

L-36 68"

28'

L-68-i R-68

1-10'--

DownwindSensor------

Fig. 15 Upwind, midwind, and downwind sensor locations for field measurements.

36

14 Location C-32

12

'8 t' \ A?° A / --- L\ Upwind-o Downwind

. . b o Midwind4-

2 -0 1020304050

1435 36 37 38Time

18 - Location C-64

16

14 -(,)1- 12-

· , 8 ~ - ' '

U) 6 -t^> _ -o -

20 1020304050

14 40 41 42 43Time

16 " Location C-9614

12 -10 \

8 X

4

2 -0 10203040500 I I II I

14 45 46 47 48Time

Fig. 16 Plot of wind speeds for various sensor locations, one layer

of bug screen (sensor height 9 ft 4 in.).

37

24 ' Location C-3222

20 -

18

16 -

14 _ \ 1^ - a- - Upwind4 \ A A - o Downwind

12 - f ----- Midwind10 2 j000/

6 - \ p4 -

2 _ 0 o. n.0 10 20 304050. n

I134 35 36 40 41 42

Time

268 -Pltowidsedfovaouseno Location C-64

bug ^VW/ \ (n

16 -

0 14 -/

1020S04050

18 - I \ '* 0 '

6- ?

4 -

2 - /

0 10 20 304050

4 44II75 505 592

Time

Fig. 17 Plot of wind speeds for various sensor locations, two layers ofC-96

39

TABLES

41

TABLE 1

VELOCITY PROFILES (FEET/SECOND)

X,Y,Z Co-ordinates in Inches

90 Degree Shelter (No Balloon)

Uo = 19.0 Ft./Sec.

Z 5 Inches

15 12 9 6 3

6.83 5.94 - 7.04 7.24 UX

9.04 8.01 - 9.66 9.66 3

10.10 8.37 - 8.01 2.41 6

16.20 15.27 16.20 15.74 9

Z = 10 Inches

15 12 9 6 3

10.80 9.04 - 7.04 7.04

9.35 9.35 - 9.66 10.10 3

10.80 10.10 10.10 4.52 6

17.91 18.71 - 17.91 17.91 9

U.,> = 31.5 Ft./Sec.

Z = 5 Inches

15 12 9 6 3

11.46 11.46 - 12.19 13.23

15.74 16.20 - 17.08 18.71 3

17.08 16.20 - 13.77 8.71 6

27.53 27.00 - 28.06 28.08 9

42

Z = 10 Inches

15 12 9 6 3

19. 47 18.71 - 13.23 12.55

17.08 17.08 - 17.91 18.71 3

18.71 19.47 17.91 10.80 6

31.02 30.55 29.58 30.55 9

= 48.6 Ft./Sec.

Z = 5 Inches

15 12 9 6 3

17.08 17.08 - 18.71 18.71

24.15 24.15 25.90 26.45 3

24.75 24.15 20.91 10.80 6

40 41 39.68 - 40.77 39.68 9

Z 10 Inches

15 12 9 6 3

28,06 25.33 19.47 17.91 y

25.33 25.33 26.45 27.53 3

27.53 27,53 28.06 18.71 6

43.20 42.52 - 43.20 43.20 9

43

90 Degree Shelter (with Balloon)

U=o =19.0 Ft./Sec.

Z = 5 Inches

15 12 9 6 3

2.41 3.82 2.53 -x

6.39 6.16 7.04 12.07 3

9.04 8.87 8.87 7.64 6

16.64 16.73 16.64 9

Z = 10 Inches

15 12 9 6 3

10.25 9.81 10.25

10.10 - 10.66 11.33 12.07 3

9.96 10.25 10.25 10.10 6

18.31 18.47 18.55 9

44


co = 19.0 Ft./Sec.

Z = 5 Inches

12 9 6 3

5.66 5.92 6.39 7.24

8.0l 8.37 8.71 9.96 3

8.17 8.37 7.83 3.82 6

16.11 16.02 16.20 16.38 9

Z = 10 Inches

12 9 6 3

6.39 6.39 6.61 6.61

8.54 8.87 9.35 9.96 3

8.54 8.19 7.44 3.82 6

16.90 17.50 17.50 18.15 9

45

120 Degree Double Screen Shelter (No Balloon)

Uo = 19.0 Ft./Sec.

Z = 5 Inches

12 9 6 3

3.82 4.00 4.52 5.40

4.83 5.26 5.79 6.61 3

4.97 5.12 5.53 2.96 6

13,. 77 14.79 16.20 16.64 9

Z = 10 Inches

12 9 6 3

3.82 4.18 4.83 6.27 y

4.83 4.83 5.12 5.66 3

5.26 5.26 5.26 2.41 6

15.74 17.08 17.66 17.50 9

Ua = 31.5 Ft./Sec.

Z = 5 Inches

12 9 6 3

5.40 5.92 6.83 8.87 y

8.37 9.35 10.10 11.20 3

8.87 9.20 9.20 5.4 6

24.75 26.45 27.53 27.00 9

Z = 10 Inches

12 9 6 3

5.92 6.61 8.01 9.81 Xy

8.20 8.87 9.81 11.33 3

9.35 9.35 9.35 3.19 6

25.90 26.45 29.08 29.08 9

46

Uc 48.6 Ft./Sec.

Z = 5 Inches

12 9 6 3

6.61 8.01 9.35 11.33

11.83 13.77 15.27 17.08 3

13.12 13.77 13.77 14.08 6

36.62 37.41 39.68 39.68 9

Z = 10 Inches

12 9 6 3

8.54 9.20 10.80 13.23 -x-

12.55 14.29 14.69 17.50 3

14.39 14.69 15.08 13.77 6

40.77 40.05 42.86 42.86 9

47


U0 = 19.0 Ft./Sec.

Z = 5 Inches

No Measurements taken

Z = 10 Inches

15 12 9 6 3

-7.64 6.61 6.16

7.83 8.19 8.37. 8.71 8.87 3

9.35 - 8.37 7.44 5.40 6

18.31 18.31 18.23 9

48

150 DereeShelter (With Balloon)

U = 19.0 Ft./Sec.

Z = 5 Inches

No Measurements Taken Xy

Z = 10 Inches

12 9 6 3

8.54 8.87 10.39y

9.35 10.10 11.20 - 3

9.66 9.35 8.71 6

19.02 19. 09 9.099

49

TABLE 2

TURBULENCE INTENSITY (DIMENSIONLESS)


U, = 9.96 Ft./Sec.

Z = 5 INCHES

12 9 6 3

0.1387 0y

0.0380 3

0.4374 6

0.0453 9

Z = 10 INCHES

12 9 6 3 Xy

0.1904

0.0332 3

0.4582 6

0.0380 9

Z = 15 INCHES

3X0y

0.0254

(For all Y)

50


U = 19.0 Ft./Sec.

Z = 5 INCHES

12 9 6 3 XO

0. 1291 0.1460 0.1508 0.1497

0.0619 0.0517 0.0446 0.0447 3

0. 1538 0.1896 0.2701 0.4688 6

0.0722 0.0579 0.0525 0.047 9

Z = 10 INCHES

12 9 6 3 x0

0.2649 0.2398 0.2011 0.1953

0.0926 0.0659 0.0479 0.0386 3

0.1532 0.1799 0.2352 0.3756 6

0.0567 0.0451 0.0391 0.0379 9

Z = 15 INCHES

12 9 6 3 x0

0. 0303 0.0285 0.0285 0.0270

(For all Y)

51


Ut : 19.0 Ft./Sec.

Z 5 INCHES

0y

No measurements taken

Z = 10 INCHES

15 12 9 6 3 10

0.2338 0.3214 0.3437

0.1056 0.1002 0.0814 0.0714 0.0667 3

0.1504 - 0.2148 0.2721 0.3580 6

-- 0.1107 0.0913 0.0410 9

52

90 Degree Shelter (With Balloon)

U= 19.0 Ft./Sec.

Z = 5 INCHES

15 12 9 6 3 0

0.5704 0.4300 0.4221

0.3181 - 0.4029 0.3571 0.1247 3

0.1679 - 0.1673 0.2286 0.3893 6

0.0517 0.0478 0.0454 9

Z = 10 INCHES

15 12 9 6 3 xy

0.3287 0.3295 0.3721

0.1880 - 0.1661 0.1146 0.0464 3

0.1536 - 0.1750 0.2196 0.3304 6

- - 0.0478 0.0401 0.0568 9

53


U = 19.0 Ft./Sec.00

Z = 5 INCHES

x0

No measurements taken

Z 10 INCHES

15 12 9 6 3

.2954 0.3108 0.3613 -

0.1969 0.1910 0.1163 - 3

0.1468 0.1450 0.1880 - 6

0.0726 0.0517 0.0431 - 9

54

TABLE 3

MASS FLUX PARAMETER E (DIMENSIONLESS)

g is calculated for the various freestreamvelocities at Z = 10 and X coordinate shown below.


X = 3 X = 6 X = 12 X = 15

U = 19.0 0.48 0.57 0.58 0.61

U = 31.5 0.54 0.61 0.65 0.65oo

U = 48.6 0.52 0.59 0.59 0.61O0


X = 3 X = 6 X = 9

U =19.0 0.64 0.63 0.62co


X = 3 X = 6 X = 9 X =12

U = 19.0 0.46 0.51 0.51 0.5000


X = 3 X =6 X = 9 X =12

U 19.0 0.35 0.38 0.38 0.35

U = 31.5 0.36 0.40 0.37 0.35

U = 48.6 0.41 0.39 0.37 0.35


X =3 X= 6 X= 9

U = 19.0 0.46 0.50 0.52


X=6 X=9 X=12

U = 19.0 0.61 0.59 0.58CO

55

TABLE 4

MOMENTUM FLUX PARAM/ETER ~ (DIMENSIONLESS)

- calculated for the various freestream velocitiesat Z = 10 and X coordinate shown below.


X 6 X= 12 X= 15

U 19.0 0.21 0.21 0.22

U = 31.5 0.21 0.21

U = 48.6 0.23 0.22

S0 Degree Shelter (With Balloon)

X = 6 X = 9

U 19.0 0.21 0.21


X = 12 X = 15

U = 19.0 0.21 0.22U 0 21


X = 12 X =15

U = 19.0 0.17 0'.18

U = 31.5 0.19 0.18

U = 48.6 0.19 0.18


X = 6 X + 9

U = 19.0 0.21 0.21


X = 6 X = 9 X = 12

U = 19.0 0.20 0.21 0.21

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