NBSIR 78-1536

*V A Scaled-Down Version of the

National Bureau of Standards

Low Velocity Airflow Facility

L. P. Purtell

Fluid Engineering Division

Center for Mechanical Engineering

and Process Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D C. 20234

September 1978

Final

Prepared for Department of Defense

Calibration Coordination Group

Working Group on Flow

NBSIR 78-1536

A SCALED-DOWN VERSION OF THE

NATIONAL BUREAU OF STANDARDS

LOW VELOCITY AIRFLOW FACILITY

L. P. Purtell

Fluid Engineering Division

Center for Mechanical Engineering

and Process Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

September 1978

Final

Prepared for Department of Defense

Calibration Coordination Group

Working Group on Flow

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Undar Secretary

Jordan J. Baruch, Assistant Saeratary for Sc/anca and Taehnology

NATIONAL BUREAU OF STANDARDS, Emaat Ambler. Diractor

A SCALED-DOWN VERSION OF THE

NATIONAL BUREAU OF STANDARDS

LOW VELOCITY AIRFLOW FACILITY

Le P. Purtell

ABSTRACT

A scaled-down version of the National Bureau of

Standards Low Velocity Facility for calibration of

anemometers at low air velocities is presented.

Included is an analysis of the quality of air flow to

be expected, the instrumentation (a laser velocimeter)

necessary for the velocity measurements, and estimates

of the cost of components and materials. Enough

detail is included to serve as a . preliminary design

of the proposed facility.

O • 1. INTRODUCTION

In response to a need for air flow measurements at very low

velocities generated by concern for environmental quality and

occupational safety and health, the National Bureau of Standards (NBS)

has developed a calibration facility for the measurement of these low

velocities. The construction of the facility required many design

innovations since standard design guidelines are strictly applicable

only at air speeds higher than those required of this facility. Likewise standard anemometers do not operate well at the lower velocities

desired which required turning to the new technology of laser

velocimetry. The laser velocimeter, linear from first principles and

having high resolution, was thus chosen to be the velocity standard for

the facility.

The successful construction of the NBS Low Velocity Airflow

Facility led to an inquiry by the Department of Defense Calibration

Coordination Group, Working Group on Flow, concerning the possibility

of constructing a scaled-down version of the NBS facility for use in

their own extensive calibration operations. This report, a feasibility

study conducted by NBS for the Working Group on Flow, presents an

analysis of the estimated performance of the wind tunnel, a study of

the required laser velocimeter velocity standard, and a preliminary

design of the wind tunnel and laser velocimeter. The basic goal of

this study was to determine whether or not a scaled-down version of

the NBS facility within the size limitations imposed (a volume, 30

feet long by 12 feet wide by 7 feet high) would perform satisfactorily
as a calibration facility. The design thus contains detail sufficient

for this purpose but should also serve as a preliminary design since

only some minor construction details were omitted. Estimations of

equipment and special material costs are included, but construction

(labor) costs are omitted since they would require the detailed final

design.

2. THE WIND TUNNEL

The basic approach taken toward a wind tunnel design based on the

NBS Low Velocity Airflow Facility was to scale down the size of all

components of the facility except the mesh and cell sizes and streamwise dimensions and separations of the screens and honeycomb. Thus

only the size of the air stream should be affected and not its quality

(e.g. , turbulence intensity) . In addition the test section length was

reduced to a size appropriate for calibrations and the entrance section

was not tilted with respect to the centerline of the tunnel (as is the

NBS tunnel) since this tilt was necessary in the NBS tunnel only to

avoid excavating the floor of the building. Since the scaled-down

tunnel would be elevated off the floor for most of its length, the

supporting structures would be somewhat different in detail from the

NBS tunnel but generally quite similar in construction.

Calibrating an intrusive anemometer requires, ideally , placing the

instrument in an airstream of infinite lateral extent so that the

presence of the instrument in the airstream does not affect the air

velocity far from the instrument. In practice, however, the airstream

is confined to a finite width, either in a duct or a jet, and thus only

approximates the ideal situation. This approximation becomes increasingly better as the size of the airstream increases relative to the

size of the intruding anemometer. For a proposed calibration facility,

then, the magnitude of this approximation must be estimated and subsequently a judgment made on the adequacy of the facility for

calibrations.

Since the largest instrument to be considered for the scaled-down

tunnel is a four-inch vane anemometer, such an instrument was chosen to

evaluate the effects of a finite sized test section. Sufficiently far

from an obstacle in an ideal flow the flow field may be approximated by

that over a sphere of radius chosen to characterize the object. For

the four-inch vane anemometer this radius was chosen to optimize the

fit of actual velocity measurements upstream of the anemometer on the

centerline of the tunnel to theoretical velocities upstream of a sphere

having that radius. Knowing the radius of the sphere then permits

computing velocities anywhere in the flow field around the sphere and

using them as approximations to flow around the anemometer.

2
The velocity upstream of a sphere in an ideal flow is given by

Uu

= 1 - (1 + £)"3

r

(1 )

where U is the streamwise component of velocity, the velocity far

upstream of the sphere, r the radius of the sphere, and x the distance

upstream from the surface of the sphere. Figure 1 presents some measurements at two air speeds of the streamwise velocity upstream of

the anemometer and on the centerline of the tunnel and of the anemometer.

Also plotted (x's) are theoretical velocities upstream of a sphere

(radius 5.63 cm) at the same location as the anemometer. Note that the

curves are quite similar for distances beyond about 5 cm from the

surface of the sphere.

To investigate the effects of a finite flow field, then, the flow

around the 5.63 cm sphere can be examined since it approximates the

flow around the anemometer. The streamwise velocity at a distance y

from the centerline measured from the center of the sphere is given by

2- . 1 + i(y/r)‘3

. (2)

That this expression with r = 5.63 cm is also a good approximation to

the flow around the anemometer was verified by computing the streamwise velocity at y

= 22.86 cm (nine inches) and comparing that to a measured value at that position. The common result was U/u^ = 1.007.

Equation (2) is plotted in Figure 2 which shows the rapid fall-off of

U/U from high values near the sphere toward 1.0 far from the sphere.

Since in an infinite flow field U/U^ = 1.007 at y

= 9 inches, some

interaction between a wall at that position and the flow nearer the

sphere would be expected, but the effect on the calibration should be

small, and since (U/U - 1) varies with the characteristic radius

cubed, the effect on anemometers smaller than the four-inch vane

anemometer should be imperceptible.

Based on the above considerations a test section of eighteen

inches in width should be the minimum, and since this is also the

maximum that is to be considered, the basic linear scaling for the

tunnel design was fixed at one—half since the NBS tunnel has a test

section 36 x 36 inches.

Figure 3 presents the overall layout of the proposed wind tunnel.

Though the design is meant to be schematic, it should easily serve as

3
a preliminary design since it lacks only some details for the

construction. The basic goal of the layout was to determine whether

or not a 1/2-scale design could fit within the limits imposed on the

size of the facility. All aspects of the NBS tunnel not pertinent to

the proposed tunnel (such as an extensive test section) were omitted

from the design.

The proposed tunnel has three sections: the flow conditioning

section, the test section, and the drive section. The flow conditioning section consists of the entrance, honeycomb, screens, and

contraction. The drive section consists of a square-to-round transition, the fan, and a diffuser. The following paragraphs detail

construction considerations necessary to ascertain the feasibility of

the proposed tunnel and to serve as a preliminary design and basis for

rough cost estimation.

3. THE ENTRANCE

The entire flow conditioning section should be constructed of

wood lined with tempered hardboard. The interior joints should be

sealed airtight with glass fiber cloth and sanded smooth. Wood is the

preferred material because of its natural vibration absorption quality

and because of the ease of forming the curved supports for the

hardboard

.

The entrance should be fitted with a framework to hold a bank of

air filters (Figure 4) to control dust entry into the flow. The

smaller particles of ambient dust are necessary for laser velocimeter

operation, but the larger particles should be removed. Ordinary household furnace filters are used in the NBS facility.

4. THE HONEYCOMB AND SCREENS

The honeycomb f low-straightener consists of a 3 inch thick sheet

of phenolic with 3/8 inch hexagonal cells (Figure 5). This sheet

should be mounted in grooves in the walls as indicated in Figure 5 to

minimize disturbance of the flow.

The screens (to reduce the scales of the turbulence) should be

24-inch mesh 7.5 mil stainless steel wire mounted in wooden frames

(Figure 5). These frames should also be mounted in grooves in the

walls as indicated. Though the details of the construction are subject

to modification, the spacing between the screens and between the first

screen and the honeycomb should not be changed.

4
5. CONTRACTION *The contours of the interior surface of the contraction are

presented in Figure 6. These are scaled down by one-half from the NBS

tunnel. The interior surface should be faired smoothly between the

curves and the adjoining flat sections (the screen housing and the test

section)

.

6.

TEST SECTION

To accomodate a laser velocimeter the walls of the test section

should be constructed of a good quality clear acrylic. The section is

square in cross section, 18 inches between walls (at all locations) and

48 inches long (Figure 7). The framework should be metal to minimize

its size since the frame blocks light transmission through the test

section. For user convenience both side walls should be removable for

access to the interior. The test section is suspended from beams

which rest on columns near the fan and on the framework of the

contraction. This allows clearance for the laser velocimeter traverser

to be located beneath the tunnel. Support of the test section at the

downstream end simply by the transition section and fan is not possible

because of a flexible coupling between the transition section and the

fan to isolate fan vibrations from the test section.

7.

SQUARE-TO-ROUND TRANSITION

Since the test section is square and the fan housing is round, a transition section fabricated from sheet metal is necessary. Flanges

should be attached around both perimeters and along its length (Figure

8) to maintain its shape. The downstream end should be attached to a flexible coupling (e.g., neoprene gasket material) attached in turn to a flange on the fan.

8

.

THE FAN

Since the pressure drops through the screens and honeycomb are

the major factors in the pressure drop through the tunnel and since

these pressure drops will be approximately the same for the proposed

tunnel and for the existing NBS tunnel (at a given speed) the required

pressure rise across the fan for the proposed tunnel was computed by

scaling the pressure rise in the existing tunnel. Thus the following

specifications were compiled for the fan:

1.5 inches water pressure differential at 5000 cfm

3.0 inches water pressure differential at 6750 cfm

5
The first specification would apply to a fan providing a top speed of

approximately 37 fps and the second to a fan providing a top speed of

approximately 50 fps. Since the required powers are approximately 2 and

5 HP, respectively, the costs will be quite different. Also the fan may

be direct drive or belt driven which will also affect the cost. Experience at NBS has only been with the direct drive, but no problems are

foreseen with using the belt drive, which may be much less expensive.

The units should be powered from an AC to DC solid-state converter

employing 1% speed control.

9.

THE DIFFUSER

A diffuser downstream of the fan will serve to reduce noise and

improve the performance of the fan. Assuming that a wall may be

located approximately six feet from the exit of the fan, little

improvement (and perhaps degradation) of the flow could be expected by

a diffuser longer than shown in Figure 9. The construction of the

diffuser should be of wood and hardboard similar to the entrance

section, though here the glass cloth may be omitted from the joints.

10.

LASER VELOCIMETER VELOCITY STANDARD

For a number of reasons the laser velocimeter is by far the best

instrument for measuring very low fluid velocities in the laboratory.

It is nonintrusive, linear, and can have high resolution. A calibration determined at a high, and thus easily resolvable, speed is valid

throughout the measuring range of the instrument. Today laser velocimeters in configurations suitable for various flow configurations are commercially available which provide for convenient maintenance and

replacement (or expansion) parts if needed.

The particular type of system analyzed here for application to

mean flow measurements in the proposed tunnel consists of a single

channel, dual beam, forward scatter optical system and a frequency

tracking (PLL) signal processor with digital data validation and analog

output meter. Once installed the system should be easy to operate and

reasonably maintenance free. Various types of systems are commercially

available and many may be suitable for the proposed facility. The one analyzed here was chosen simply to assess the feasibility of assembling

a laser velocimeter from commercially available items with capabilities

sufficient for the desired calibrations.

11.

OPTICAL SYSTEM

e The optical system considered (Figure 10) consists of a He-Ne

laser, of at least 15 mw power, projecting and receiving optics, and a photomultiplier tube assembly, all standard commercial items. The

6
incident laser beam is split by a glass beam-splitter, the resulting

beams focused to an intersection within the test section, and light

scattered from the intersection collected by the receiving optics. For

the recommended optics with a 600 mm focusing lens the intersection

volume has approximate dimensions of 0.7 mm in diameter by 26 mm in

length, the long dimensions along the optical axis of the system (i.e.,

trnasverse to the flow). The optics need to be mounted rigidly on supports such as discussed in Section 13.

12. SIGNAL PROCESSING

The signal from the photomultiplier tube consists of bursts of

frequency data which must be converted to velocity data (the velocity

is proportional to the frequency) . The processor analyzed here

utilizes phase locked loop circuitry to "capture" a burst which is then

converted from frequency to voltage. A digital counter guarantees a minimum number of cycles without drop-out.- The analog output meter may

be adjusted to read the voltage in any desired units of "velocity" for

convenient reading of data though this may reduce the resolution of the

system. The meter also averages data with time (two settings of damping).

The proportionality factor between frequency and velocity should

remain very nearly constant with time providing the optics are not

moved. Thus, knowing this factor, the electronics can be checked and

adjusted as desired by simply connecting a known frequency to the

input of the processor and adjusting the meter to read the appropriate

velocity.

The resolution of the recommended system is presented in Figure 11.

It was computed assuming a direct reading of voltage which gives the

maximum attainable resolution for the system. A resolution of 1% is

attained at 15 fpm, improves rapidly above this speed, and reaches 1.5%

at 10 fpm. If the meter is adjusted for "velocity" units the resolution

will be improved by using a separate voltmeter with higher resolution

limited only by the frequency to voltage conversion within the

processor. An oscilloscope is a highly desirable accessory for determining and adjusting the quality of the laser velocimeter performance.

They are readily available with specifications adequate to handle all

signals to be encountered and can include storage capability which is

necessary to view the random bursts of data.

13. TRAVERSER

The optics for the laser velocimeter must be held rigidly in place

to hold the intersection angle fixed and to maintain proper focusing of

the scattered light onto the photomultiplier tube. However, the probe

volume needs to be movable to measure velocity profiles within the duct

both with and without anemometers mounted in it. Thus a traverser is

7
desirable which will hold the optical components fixed with respect to

the duct . A preliminary design of such an apparatus is presented in

Figures 12 and 13. The primary goal of the design was to determine the

space required for such a device yet provide enough detail to permit a final design to be completed. The traverser as designed could locate

the focal volume virtually anywhere in the duct. The optical system,

including the laser, is mounted on a rigid platform which can be moved

vertically on ball bushings/shafts driven by a screw. This assemble is

mounted on a frame which rides on ball bushings/shafts transverse to the

duct, all of which rides on ball bushings /shafts laid on the floor

beneath the tunnel parallel to the flow. The two horizontal motions

could be either motor or hand drive.

14. LIST OF EQUIPMENT AND POSSIBLE SUPPLIERS

The calibration facility may be conveniently divided into two subsystems: the wind tunnel and the laser velocimeter. The cost of the

latter can be estimated except for the labor necessary to construct the

traverser and to set up and adjust the optical components. The cost of

the wind tunnel, however, is mostly in the labor to construct it and is

difficult to estimate without a final design and consultation with

carpentry estimators. The following estimates, therefore, are of

materials and equipment only. Several possible suppliers are listed,

but others may be available; the listings are meant to be representative

and do not imply an endorsement.

14.1 The Wind Tunnel

14.1.1 The Fan

The choice of the type of fan (belt driveor direct drive) and of

the maximum air speed (37 or 50 fps) will determine the cost of the fan.

The cost for a maximum air velocity of 50 fps will be from $5000 to

$8000. The cost for a maximum air velocity of 37 fps will be from

$2500 to $5000. Two possible suppliers are:

Buffalo Forge Company

Buffalo, New York

Joy Manufacturing

New Philadelphia, Ohio

Local offices are in many cities.

8
14.1.2

Honeyc omb

A glass—filled phenolic honeycomb is available which costs

approximately $500 for a sheet 3-1/2 feet by 8 feet. One possible

supplier is:

Hexcel Corporation

11711 Dublin Boulevard

14.1.3 Dublin, CA 94566

Screens

Pieces of screening about 4 feet by 4 feet are available at

approximately $25 each. One possible supplier is:

C—E Tyler, Screening Division

8200 Tyler Boulevard

Mentor, OH 44060

14.2 Laser Velocimeter

14.2.1 Optics

Many suppliers of optical components are available, but ease of

assembly would dictate purchase of optically and mechanically

compatible modules. The total cost will be approximately $5500. Two

possible suppliers are:

Thermo-Systems Incorporated

Box 3394

St. Paul, MN 55165

DISA Electronics

779 Susquehanna Avenue

Franklin Lakes, NJ 07417

14.2.2 Signal Processor

The suppliers listed for optics also supply processors. The cost

will be approximately $8000. Other suppliers may be available.

14.2.3 Laser

Many suppliers of lasers are available selling many different

types and powers of lasers. The total cost depends on* options, power,

and quality, varying from $3000 to $8000. Two possible supplier are:

9
Spectra-Physics Incorporated

1250 W. Middlefield Road

Mountain View, CA 94042

Jodon Engineering Associates, Inc.

145 Enterprise Drive

Ann Arbor, MI 48103

14.2.4 Oscilloscope

The cost will be about $3000. Two possible suppliers are:

Tektronix

P.0. Box 500

Beaverton, Oregon 97005

Hewlett-Packard

1820 Embarcadero Road

Palo Alto, CA 94303

15 . SUMMARY

From the analysis of blockage effects within the 18 x 18 inch

test section, from the study of available laser velocimeter performance,

and from the preliminary design of the wind tunnel, it is concluded that

construction of a calibration facility within the required size and

performance limitations is feasible . As with all design extrapolations

(the tunnel is smaller than any others built for this speed range) there

is some risk that problems may be encountered, but the analysis contained herein indicates that it is minimal.

The preliminary designs are primarily aimed at assessing

feasibility of construction but include enough detail that a final

design can be completed. Estimated costs for the facility are only for

equipment and special materials since construction (labor) costs would

be difficult to estimate without a detailed final design.

10
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NBS-114A (REV. 8-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 78-1536

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

A SCALED-DOWN VERSION OF THE NATIONAL BUREAU OF

STANDARDS LOW VELOCITY AIRFLOW FACILITY

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

L . P . Purtel

1

8. Performing Organ. Report No. NBSIR 78-1536

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2130489

11. Contract/Grant No. CCG Project No.

77-103

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Department of the Air Force

HQ Aerospace Guidance and Metrology Center (AFLC)

ATTN: MLPM (Neu)

Newark Air Force Station, Newark, Ohio 43055

13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

A scaled-down version of the National Bureau of Standards Low Velocity

Airflow Facility for calibration of anemometers at low air velocities

is presented. Included is an analysis of the quality of air flow to

be expected, the instrumentation (a laser velocimeter) necessary for

the velocity measurements, and estimates of the cost of components and

materials. Enough detail is included to serve as a preliminary design

of the proposed facility.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Airflow; anemometers; calibration; facility; laser; low velocity.

18. AVAILABILITY Unlimited

DCt For Official Distribution. Do Not Release to NTIS

I

1 Order From Sup. of Doc., U.S. Government Printing Office

Washington, D.C. 20402, SD Stock No. SNOOCOOj

Order From National Technical Information Service (NTIS)

Springfield, Virginia 22161

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

22. Price

USCOMM-OC 0 6 0 8 0 »P7 8
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