***T °F CoA \

<- ^T^ \ «F

Q NBS TECHNICAL NOTE 989

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

RLE COPY

DO NOT TAKE

A Low-Velocity

Airflow Calibration

and Research Facility
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory,

and the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

Agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities2 — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical

services to users in the public and private sectors to address national needs and to solve national problems in the public interest; conducts research in engineering and applied science

in support of objectives in these efforts; builds and maintains competence in the necessary

disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities; provides engineering measurement traceability services; develops test methods and proposes engineering standards and code changes; develops and

proposes new engineering practices; and develops and improves mechanisms to transfer

results of its research to the utlimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering2 — Mechanical

Engineering and Process Technology2 — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal Agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal Agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following divisions:

Systems and Software — Computer Systems Engineering — Information Technology.

'Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Some divisions within the center are located at Boulder, Colorado, 80303.

The National Bureau of Standards was reorganized, effective April 9, 1978.
>

=H*«

Hi

3*H cr

m

X 4

CD 4

fH pj

CD r+ X c

>n *0 3 4 * • cd

ir> T3

Ul c =

HH > o r+ 3 CD f1 M O

Hi »- s O 1 H p < 3 CD > D* h-" X O

-0

O o H> 3 rt </>X

13 « P > OP ?S H-

(5M Hj

CD H}

i— cr h- O pi o 3 C W O

3* Hi

n

O Hi P c <• I—"

l—l Hex. z cr

CO 4

cr </> P

0) rt H H- « CD O H O 3

H- 3*

rt 3 P

pt H- 3

CDO D- 3 P • • - so

CD Z W O CD pt P

CD 4O

KD 31 00

<o -n • Po

H>

t—Hrt X

A Low-Velocity Airflow Calibration

and Research Facility

L. P. Purtell

P. S. Klebanoff

Fluid Engineering Division

Center for Mechanical Engineering

and Process Technology

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

Sponsored by: Office of the Assistant Director - Mining

Bureau of Mines

U.S. Department of the Interior Washington, DC 20241

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

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued March 1979
National Bureau of Standards Technical Note 989

Nat. Bur. Stand. (U.S.), Tech. Note 989, 21 pages (Mar. 1979)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1979

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Stock No. 003-003-02038-9 Price $1.20

(Add 25 percent additional for other than U.S. mailing)
PREFACE

National concern with problems of the environment, occupational health and safety,

mine ventilation, etc., and accompanying regulatory standards, have imposed requirements

for instrumentation and calibrations that would provide for more accurate airflow measurements at very low velocities. In order to meet this need the National Bureau of Standards

undertook to develop an airflow facility that would provide the technical basis for accurate

and uniform aerodynamic measurements at very low velocities. Such a facility can also play

an important role in providing significant data in a number of technical areas bearing on the nation's technology. It can be used to good advantage, for example, in conducting aero- dynamic research on low Reynolds number flows, boundary layers, flow stability, turbulence,

fluid mechanical modeling, etc.

The development of the facility described herein was carried out with support from the

United States Bureau of Mines under Contract No. H0133024. The contract was initiated under

the Coal Mine Health and Safety Program, and administered by the Pittsburgh Mining and

Safety Research Center. The support of the USBM and the cooperation of Dr. George

Schnakenberg, Jr., Technical Project Officer of PM§SRC, is gratefully acknowledged.

iii

A LOW-VELOCITY AIRFLOW CALIBRATION AND RESEARCH FACILITY

L. P. Purtell and P. S. Klebanoff

A low velocity airflow facility suitable for the calibration of

wind speed measuring instruments and research in aerodynamics is

described. The flow facility is of the open-return type with a test section 20 feet (6.1 meters) long, and nominally 3x3 feet

(0.91 x 0.91 m) in cross section. Special attention was given to obtaining an air stream with a high degree of spatial uniformity and

low turbulence with excellent speed control over the range from 10 to 3300 feet (3 to 1000 meters) per minute. Laser-optical methods with

appropriate signal processing electronics are employed to establish

a primary standard for the measurement of very low velocities. A crossed-beam dual-scattering laser-optical system is used that can operate with either forward or backscattering, and in a frequency

shifted Bragg cell mode, or in a non-frequency shifted optical beamsplitter block mode. Detailed performance characteristics of the flow

facility and the laser velocity standard are presented.

Key Words: Airflow, calibration, facility, laser optics, low velocity,

velocity standard.

1 . INTRODUCTION

In recent years concern for environmental quality, and occupational safety and

health have generated a need for air flow

measurements in lower velocity ranges than

previously dealt with. Regulations are being

established which are setting more and more

rigorous standards as to the levels of airflow to be maintained in connection with

problems of health and safety in mines;

industrial ventilation, indoor circulation,

th«=> "clean bench" nrnMem. etc. \s. d result,

there has been generated the need for more

accurate measurements of airflow in a range

of velocities which hitherto has not re- ceived much attention, i.e., below 152.4

meters (500 feet) per minute with particular

emphasis on velocities less than 30.5 meters

per minute.

Inherent to these measurement problems

is the need for equipment and procedures to provide accurate calibrations which in turn

will provide the basis for accurate and re- liable instrumentation required for setting

appropriate and unambiguous flow standards.

It was to meet this need that the National

Bureau of Standards, with support from the

U. S. Bureau of Mines, undertook to develop

a suitable calibration facility for the

measurement of very low velocities. In addition, there is also a need for quality technical data for turbulent flows at low

Reynolds numbers. It is thus highly desirable that in the development of a capability

for low-velocity measurements, consideration

I

also be given to the dual purpose of providing a capability for low Reynolds numbers

research on turbulent flows and associated

instrumentation.

There are two essential factors to be

considered in the proper development of the

forementioned capabilities. One is the

quality of the air stream, and the other is

the method to be used as the primary stan- dard for the measurement of velocity. A suitable facility must avoid blocking

effects, have good speed control, and provide an air stream with spatial uniformity

and steadiness of flow. The approach in

this regard was to apply existing design

practices that have been established for

relatively high velocity flows in wind

tunnels to the development of an airflow

facility with very low velocities.

In general, instruments used to measure

air flow velocity use some definite and

reproducible effect related to the motion,

such as a pressure differential or thrust,

convective cooling, sound transmission,

transmit time of a tracer particle, and more

recently light scattering. The Pitot-static

tube which utilizes a difference in pressure

produced by the motion of the fluid when con- structed according to appropriate dimensions

has served as a satisfactory primary standard

over a wide range of air speeds, but its use- fulness is limited to the higher velocities.

The uncertainty in reading small pressure

differences increases the uncertainty of

speed measurements with decreasing speeds,

and the Pitot-static tube cannot meet the

desired accuracy requirements at the very low
velocities. It was felt that of the various

forementioned possibilities, the lightscattering method was the most promising for

achieving a velocity standard of sufficient

accuracy for a calibration capability at low

velocities. The advent of the continuous

wave laser provided the coherence, monochromaticity, and intensity of radiation to make this method feasible. It is a method

which provides certain advantages in that it

is nonintrusive, has a linear response with

velocity which can be derived from first

principles, and has good spatial resolution.

The laser velocimeter is a complex lightscattering system dependent on appropriately

processing the light scattered from particles

moving with the fluid. However, the programmatic content and associated instrumentation envisaged for the facility made it

desirable that it be made to operate without

artificial seeding, and that it respond with

sufficient sensitivity to ambient scatterers.

With both of the foregoing aspects, i.e.,

the quality of the air stream, and laseroptics as the primary velocity standard, incorporated as design concepts, the objective

was to develop a calibration and research

facility over the velocity range of 3-1000

meters per minute. The progress made toward

the development of such a facility is summa- rized herein, and a detailed description and

performance evaluation of the airflow facility and laser velocimeter are presented.

2. FLOW FACILITY

2.1 General Description

A special purpose duct, capable of

accommodating the more common types of windspeed measuring instruments without blocking

effects, is shown schematically in figure 1. It is an open-circuit duct with room return

which pulls air through the duct and exhausts

it into the room. It was designed for low

velocity flow, i.e., in the velocity range

of 3 meters per minute to 1000 meters per

minute, and to be able to accommodate laser

anemometry. The upper end of the velocity

range was selected so as to overlap an existing capability for the calibration of

anemometers and Pitot-static tubes. Particular emphasis was given to obtaining a low

free-stream turbulence level, and a uniform

mean velocity within the test region. The

flow facility may be considered to consist

of three sections: 1) an upstream or flow

conditioning section, 2) a test section, and

3) a downstream or fan-diffuser section.

Each section requires special attention in

its design in order to achieve the desired

performance.

2.2 Flow Conditioning Section

This section is the most important from

the design point of view in achieving the

desired flow characteristic of low turbulence

Fan-Diffuser Section

6.1 m

Test Section Flow Conditioning Section

Diffuser Fan

Side View

Figure 1. Schematic of flow facility.
level and uniform mean flow (references 1,2).

It consists of three subsections: 1) an entrance cone, 2) a settling chamber with

honeycomb and damping screens, and 3) a con- traction section. All three subsections are made of wood framing with a masonite liner,

and are square in cross section.

The design of the facility was con- strained to some extent by the available

space, and this limitation reflected itself

particularly in the unusual design of the

entrance cone. There was not sufficient

height available to have a conventional

entrance cone axisymmetric to the centerline

of the facility, and it was tilted up at 12 degrees to the centerline in order to clear

the floor. It was felt that this was more desirable than excavating, and relining, and

that it could be done without adversely

affecting the flow. The entrance cone, however, is axisymmetric to its own centerline,

and has an area ratio of 2.14 to 1. The

primary function of the entrance cone is to collect the air entering the facility in such

a manner as to avoid parasitic vortices, and

as shown in section 4, this condition is

satisfied and the change in centerline had no noticeable effect on the flow. In order to minimize the stream contamination that may

result with an open room-return facility,

furnace filters, nominally 20 x 20 x 1 inch

(51 x 51 x 2.54 cm), were installed in a lattice- like frame at the front of the en- trance cone. Although the laser velocimeter

depends on ambient scatterers for its operation, the filters have the desirable features

of providing particles that are smaller and more uniform in size than would otherwise

have been present, and except for reducing

the particle count, the performance of the

laser velocimeter is not adversely affected.

The use of a honeycomb and screens as flow-conditioning devices is well-established

and adequately documented in the forementioned references. The honeycomb is used to reduce swirl and lateral mean velocity variations. It is made of glass fabric with re- inforced plastic with 3/8-inch hexagonal

cells, 3 inches long, and is located 21 inches upstream of the first screen. Ten 24-

mesh per inch stainless steel damping

screens with 0.0075-inch diameter wire were

used to reduce the turbulence level. The

open-area ratio of 0.672 is more than adequate to avoid the small variations in flow

direction in the test section that may be

introduced by screens having too high a solidity (reference 3) . The screens are

spaced 4 inches apart, and are mounted in

wooden frames that are recessed in order to avoid disturbance to the flow. The screens

can be removed for cleaning through an access

door in the side of the settling chamber.

The contraction is perhaps the most

critical of the various subsections from the

design point of view. Since there is a region in a contraction in which an adverse

pressure gradient exists a major design problem is to avoid flow separation. This problem is further complicated by secondary flow

in the corners. Another problem is to avoid

accelerating the flow in such a manner as to produce "ears" of higher velocity in the mean flow near the walls of the test section.

Design information for contractions in very

low velocity facilities, i.e., a few feet

per minute in the settling chamber, is nonexistent, and a procedure of trial and error

was anticipated in obtaining a contraction

having good flow characteristics. All four

walls of the contraction were constructed

with radii of 3.33 feet and 5.0 feet at the

upstream and downstream curvatures, respectively, and false adjustable walls were superposed which by screw adjustment permitted

modifying the contours by trial and error,

as discussed in section 4, so as to prevent

flow separation at the very low speeds. The

contraction ratio was 5.36 to 1, and the

initial configuration consisting of two circular arcs and the contours finally arrived

at are shown in figures 2a and 2b. Figure 3 shows a view of the contraction and settling

chamber looking upstream from the test section.

Distance

to Wall

(cm)

Original Wall

I I I I I I I I 100 120 140 160 180 200 220 240 260 DISTANCE FROM TEST SECTION (cm)

Original Wall

Figure 2a. Contraction contours of

sidewalls (top view)
Dlscance

to Wall

(cm) 160 180 200

DISTANCE FROM TEST SECTION (cm)

beginning of the test section. The test

section is nominally 3 feet by 3 feet in cross section, 20 feet long, and is supported

from above by two steel trusses, one on each

side, extending along its length. This

support arrangement allows the traverser for

the laser velocimeter, and associated tracks,

to be located on the floor. The test section

is constructed of plexiglass panels, 3/8-inch

thick, mounted in a steel framework. The

sidewalls diverge from a separation of 36

inches at the entrance to 39 inches at the

exit with the top and bottom walls remaining

parallel. Three doors, spaced along its

length, on one side, provide access, and at the entrance to the test section access is

also provided for the insertion, if desired,

of turbulence generating grids.

Figure 2b. Contraction contours of

upper and lower walls

.

Figure 3. The contraction and settling

chamber viewed looking upstream.

Two wall-pressure taps, one located 12 inches downstream from the last screen, and

the other located at the entrance to the test section are available to monitor the air

speed at the higher velocities, where accurate pressure measurements can be made.

2. 3 Test Section

Figure 4 shows a view of the flow

facility looking downstream from the

2.4 Fan and Diffuser

The fan is a 36-inch diameter, variable

pitch, twelve-bladed, axial fan directly

coupled to a variable speed, 30 HP, DC motor.

The DC voltage is supplied by a motor generator set that incorporates a feedback

control from a tachometer-generator mounted

on the shaft of the fan motor in order to maintain a constant speed. The pitch of the

fan and voltage to the motor are remotely

controlled, and the resulting arrangement

provides excellent control over the range

from zero rpm to maximum operating rpm. The

fan and motor assembly is mounted on a con- crete base and is isolated from the test section by a flexible coupling to minimize

vibration. Attached to the exit of the fan

is a relatively short diffuser, the length of

which was limited by the space available. It has a 5.8 degree included angle and is 6.7

feet long.

3. VELOCITY STANDARD

3.1 General Description

Laser-optic methods, employed in a crossed-beam dual -scattering mode were used

to develop a primary standard for the

measurement of low velocities. In such a system two coherent beams, of nominally equal

intensity intersect to form an interference

fringe pattern within a probe volume, and by

appropriately processing the scattered light

from a particle passing through the volume a measure of the flow velocity is obtained.

Involved herein is the assumption that the

scattering particle is traveling with the

velocity of the fluid. In order to obtain

as clean an air stream as possible, compatible with the need for scatterers, no artificial seeding is provided, and ambient
particles are used as scatters. In the

dual-scattering mode, if there is sufficient

sensitivity, light scattered from the probe

volume in any direction may be collected for

measuring the velocity of the scatterers.

In the present system provision has been made

for operating in either forward or backscattering as required.

The laser and associated optics are mounted on a traverser capable of threedimensional positioning of the probe volume

and signal processing is performed by individual realization, counter-type processors

interfaced to a data acquisition system. A photograph illustrating the arrangement of

the laser-optic system, associated electronics and traverser is shown in figure 5.

Figure 4. The flow facility viewed looking downstream from the

beginning of the test section.

A stationary fringe pattern, however,

does not permit resolution of flow direction

in certain flow situations, as for example,

in the measurement of the velocity component

transverse to the mean flow, or where flow

reversal may occur, as in separated flows.

This directional ambiguity may be resolved by

acoustic modulation of the laser beam

(reference 4) . Acoustic modulation of the

laser beam at a known frequency creates a moving fringe pattern with the result that

the observed frequency is greater or less

than the modulation frequency depending on whether the particle traverses the probe

volume in a direction opposite to the moving

fringes, or in the same direction. Motion

of the fringes also results in an increase

in the number of effective fringes that the

particle traverses. In order to have this

capability the laser-optic system, in addition to employing a beam-splitting optical

block that provides a stationary fringe

pattern, also incorporates a Bragg cell for

acoustic modulation of the laser beam.

Figure 5. The laser-optic system and

signal processing electronics,
3.2 Optics

The optical system for a laser velocimeter capable of being used in a duct large

enough for instrument calibration has

imposed on it certain requirements beyond

those usually encountered in small, laboratory configurations. To position the probe

volume virtually anywhere in the duct re- quired a focal length of the projecting and

receiving optics of about 3.3 feet. This in

turn required large diameter projecting and

receiving optics to allow a sufficiently

large beam intersection angle and to collect

sufficient scattered light for a good signal

to noise ratio. In addition, space restrictions required that, in the forward-scattering mode, the scattered light be returned by

a spherical mirror to the projecting side of

the tunnel in order to focus it on the photomultiplier tube. This arrangement, however,

did provide the advantage of having all the

optical adjustments on one side of the

facility.

The optical configuration in the forward and back-scatter modes is shown schematically in figures 6(a) and 6(b),

respectively.

Collimator/Beam

Expander Cell Colli

Figure 6a. Optics - forward scatter.

The projecting lens and spherical mirror are

12 inches in diameter with the latter having

a 4-inch diameter central opening to avoid

having the scattered light interfere with the

projected beams. The configuration has been

designed to facilitate the change from one mode of operation to the other. To change

the system from forward scatter to backscatter requires only the insertion of a second 12-inch diameter lens, as indicated

in the figure, and covering the spherical

mirror. To change from acoustic modulation,

and a moving fringe pattern, to a stationary

fringe pattern requires that the beamsplitter optical block replace the Bragg cell

in the optical path. It should be noted,

however, that adjustment of the various

optical components is usually required when

changing the mode of operation.

Figure 6b. Optics - backscatter.

The system in the Bragg cell mode of

operation is a two-component system, i.e., a two-dimensional Bragg cell, Spectron Development Laboratory, Incorporated, Model 1522, is

used which splits the laser beam in such a manner as to permit the establishment of two sets of moving interference fringes, orthogonal to one another, for measuring the longitudinal and transverse components of velocity.

In order to be able to separate the two com- ponents in the processing of the resulting

signal the modulation frequencies for the two sets of fringes are different, being 25 MHz and 15 MHz for the longitudinal and transverse

components, respectively. Although operation

with the beam-splitter optical block can in

principle be designed for multi -component

measurement, at the present time it provides

parallel beams for only one set of fringes,

permitting only the longitudinal component of

velocity to be measured.

A Spectra-Physics, Model 164, 2 -watt Argon ion laser is used to provide a laser

beam with a wave length of 514.5 nm. The

power is varied only as necessary for a particular mode of operation, and in general

has ranged from 100 mw to 200 mw. The primary function of the initial collimator is to adjust the location of the beam waist with

respect to the intersection of the beams

within the probe volume. The ideal condition would be for the waist and intersection

point to coincide. The main function of the

collimator/beam expander is to produce

Brand names of equipment are used solely to provide a reference for performance characteristics and do not represent an endorsement.
enlarged parallel beams sufficiently separated to be focused at an appropriate angle

by the projecting lens. However, since the

beams entering the collimator/beam expander

may be either parallel from the beamsplitter optical block or diverging from the

Bragg cell, the input lens must be selected

accordingly.

An important parameter in the optical

arrangement is the focusing angle, i.e., the

angle between the intersecting beams. This

angle was determined by scanning the projected beams at two locations along their

path, with a photodetector behind a 25 micron precision pinhole for resolution. A

typical scan of the distributions of beam

intensity at one location for a measured

separation of the intersecting beams, is

shown in figure 7 for the beam-splitter

optical block mode of operation.

change in acoustic velocity of water is

approximately 0.16 percent per degree Celcius, and the resulting percent change in

angle is equal to the percent change in

acoustic velocity. Thus, it is important

that Bragg cell transducers be efficient to prevent unnecessary heating of the fluid.

The final step in the optical processing is to have the scattered light focused

on a EMI 9781 photomultiplier tube through

an adjustable aperture mounted on a threedimensional positioner.

3.3 Signal Processing

The signal from the photomultiplier tube

may be shifted in frequency from the un- shifted Doppler frequency by the modulation

frequencies of the Bragg cell, or it may be

the unshifted Doppler frequency when the

beam-splitter optical block is used.

i 1 1 1- 6.1 6.2 6.3 6.4 6.5 6.6 6.7 Distance Across Beams (mm) Figure 7. Comparison of intensity profiles of laser beams with Gaussian distribution.

It is seen that the measurements of the

light intensity within the beam by this method agree fairly well with the expected

Gaussian distribution. The angles for the

beam-splitter optical block and Bragg cell

modes of operation were 3.85 degrees ± 0.5

percent, and 1.3 degrees ±1.0 percent,

respectively. The larger uncertainty in the

angle for the latter can result from imperfections in the optics and beam motion asso- ciated with the Bragg cell. Beam wander, for

example, may be caused by density variations

in the fluid of the Bragg cell. At 22°C the

These two types of signals are quite

different and require separate conditioning

channels, although the function of each

channel is similar. In addition, the signal

from the photomultiplier tube also contains

a pedestal or amplitude modulating pulse re- sulting from the Gaussian distribution of

light intensity within the laser beam. A block diagram illustrating the signal con- ditioning electronics is shown in figure 8. In the Bragg cell, two-component mode of

operation, two separate processing channels

are required. The Bragg cell frequencies of
25 MHz and 15 MHz which have been frequency

shifted by the horizontal and vertical components of the Doppler frequency require a wide

band, low noise preamplifier for amplification of the signal from the photomultiplier

tube, and are sufficiently separated in frequency so as to conveniently permit the

isolation of the two components by bandpass

filtering. The pedestal frequency is very

much lower than the shifted frequencies and

is readily removed by this filtering process.

In order to obtain adequate system resolution

in the Bragg cell mode of operation, particularly at low velocities, as discussed in

section 4, it is necessary to have a variable

heterodyning capability. This capability is

provided by a Tektronix SG-503 variable frequency oscillator and an appropriate mixing

circuit. Low-pass filters are used to obtain the difference signals associated with

heterodyning, and the resulting signals are appropriately conditioned for data processing.

(~)PMT

14/16 MHz

3. P. Filter

I

[Variable

Band-pass [_ Filter

I

Bragg Cell Mode

Mlxer 0*- Tc Heterodyning

illator Heterodyning

Oscillator

HI

Oscilloscope

Error Reset

Trigger

Data Controller

Processor No. 1 Trigger

Error Reset

Data

i Trigger

Processor No. 2

Data Acquisition System

Figure 8. Signal conditioning electronics;

Bragg cell mode (solid lines)

,

beam splitter mode (dashed lines)

The procedure for obtaining the signal

conditioned for data processing is less involved and more straightforward when the

beam-splitter optical block is used. The

procedure is further simplified inasmuch as provision has been made for measuring only

the longitudinal component of velocity. The

signal conditioning channel for this mode of

operation consists only of a preamplifier

for amplifying the signal from the photomultiplier tube, and a bandpass filter for

removing the pedestal frequency. However,

since the signal is the Doppler frequency

that varies over the velocity range, a variable bandpass filter (Krohn-Hite Model

3103R) is used.

An oscilloscope acts as both a monitor

and gating circuit for the conditioned

signal. The oscilloscope is adjusted to trigger when sufficiently large signals are present, and routes a gate pulse to the data

processors initiating measurement of a "burst" frequency that results from a particle traversing the fringes . The

oscilloscope also serves the customary function of system adjustment for maximizing

signal quality.

The processors which were constructed

for the National Bureau of Standards under

contract by the Arnold Research Organization,

Incorporated, Arnold Air Force Station,

Tennessee, use digital counting and validation circuits to measure an average period

between cycles of the "burst." The validation consists of comparing durations of five

and eight cycles, respectively, and rejecting any signal for which the comparison is not within a specified limit (reference 5)

.

The signal associated with a "burst" that is

accepted appears at the output of the pro- cessor in both binary, and binary coded

decimal form.

An important aspect of the logistics of

data management is that served by the data

controller which is an intermediate step

between the processor and the data acquisition system. The data controller performs

several functions including control of the

trigger signal to the processors, control

rate of data transfer to the data acquisition

system, and determination of simultaneity of

processor signals when operating in the simultaneity mode. In the simultaneity mode the

data controller will transmit to the data ac- quisition system only those signals that are processed after one trigger pulse from the

oscilloscope. In the event either processor

fails to accept a signal, the system resets
automatically to sample again. In the individual mode either or both processors may

independently initiate data transfer to the

data acquisition system.

Data transmitted to the data acquisition system by the data-controller are re- ceived by two interface modules (Wang

705-1A) for translation to a Wang Model 720C

programmable calculator. The data are then

converted from period to velocity, statistically analyzed, and recorded. Peripherals

available to the Wang 720C include plotter,

typewriter and card reader.

3.4 Traverser

A three-dimensional traverser which

permits positioning the focal volume at any

desired location within the test section is

shown schematically in figure 9.

rides also by means of ball bushings, thus

providing motion in a direction transverse

to the test section.

Attached at each end of the upper platform, i.e., on opposite sides of the test section, are vertical assemblies that provide for vertical positioning of the focal

volume. These vertical assemblies also consist of a ball bushing and ground rod

arrangement on which ride shelves driven by

ball screws. On one side the shelf contains

the projecting lens, the back-scatter focusing lens, when used, the photomultiplier

tube assembly, and various mirrors to appropriately orient the laser beams. On

the opposite side of the test section, the

shelf contains the spherical mirror mounted

on a gimbal mechanism to allow aligning the

mirror with the optical axis of the projecting lens.

Figure 9. Schematic of traverser for laser-optic system.

Two ground rod tracks mounted on the floor,

and running the length of the test section,

support and guide a "car" which consists of

two platforms that can move relative to one

another. The lower platform is attached to ball bushings that ride on the floor rails

providing for traversing capability in the

longitudinal direction. Mounted to the lower

platform are two additional ground rod tracks

that run transverse and horizontal to the test section on which the upper platform

The two vertical assemblies are linked by a common drive mechanism so that they move up

and down as a unit. Both shelves are partially counter-weighted to provide ease of

movement and reduce torsional distortion

of the linkage shafts.

4. PERFORMANCE OF LASER-VELOCITY STANDARD

Laser-optic methods have broad capabilities for fluid flow measurement. However,
to fully utilize these capabilities requires

attention to numerous details of design and

operation, and achieving a proper balance

among many parameters. Critical to the reso- lution and accuracy of the system, and its

overall performance, is the focal volume.

The dimensions of the focal volume, and the

nature of the fringes within, whether sta- tionary or moving, determine the quality of

the signal generated by a particle traversing the focal volume. The focal volume is

defined by an ellipsoidal surface at which

the beam intensity is 1/e2 of its maximum

value. Implicit in this definition is that

the diameter of a Gaussian beam is given by

the 1/e2 intensity.

The dimensions of the focal volume are

a function of the wavelength of the laser

light, A, the focal length of the projecting

lens fr , the diameter of the beams at the

projecting lens, d, and the angle of intersection of the beams, 6, (reference 6]. The

three axes, Ay, Ax, and Az, where Ay is vertical and transverse to the optical axis, Ax is

horizontal and transverse to the optical axis,

and Az is in the direction of the optical

axis, are given by

Ay

Ax

4 f

L

Ay

Az =

cos 6/2

Ay

sin 6/2

The fringe spacing, L, is given by

L = X

2 sin 9/2

and from the above formulations, the number

of fringes, N, in the volume is

N

4 D

v d

where D is the beam separation at the projecting lens. The pertinent characteristics of

the focal volume in the Bragg cell, and

optical beam splitter block modes of operation are given in Tables 1 and 2 below.

Table 1 Characteristics of the Bragg Cell Mode of Operation

Frequency Shifting:

Projecting Lens: Beam Diameter at Lens: Beam Separation at Lens: Angle of Beam Intersection:

Focal Volume:

Fringe Spacing

Two -component, waterfilled Bragg Cell (Spectron

Development Laboratories), 15 and 25 MHz.

Focal length = 1 m; diameter = 30 cm.

6. 2 mm. 2.27 cm. 1.3°.

Ax = 0.10 mm

Ay = 0.10 mm

Az = 9.3 mm

22 pm.

Projecting Lens: Beam Diameter at Lens: Beam Separation at Lens

Angle of Beam Intersection, 6: Focal Volume:

Fringe Spacing:

Table 2 Characteristics of the Beam-Splitter Mode of Operation

Focal length = 1 m; diameter = 30 cm.

4.6 mm. 6.7 cm

.

3.85°.

Ax = 0.14 mm

Ay = 0.14 mm

Az = 4.2 mm

7.65 pm.

10
In the optical beam-splitter block mode of

operation, i.e., without frequency shifting,

the longitudinal component of velocity, U, is

simply

U L/T

where T is the average period of a particle

crossing the focal volume as measured by the

processor. Consequently, the resolution in

this mode, of operation depends solely on the

resolution in determining L and T. The un- certainty in measuring the average burst

period, T, depends on the processor setting,

and is 0.1 nsec for frequencies above 500 kHz

and 0.01 ysec for frequencies below 500 kHz.

The maximum uncertainty in the measurement of

T is 0.5 percent, which is well within the

desired accuracy of ± 1.0 percent for the

speed range covered by the flow facility.

The major uncertainty over the speed range,

therefore, in the stationary fringe mode of

operation arises from the uncertainty in the

fringe spacing. Several methods were used to determine L. One method consisted of direct

measurement of the beam intersection angle as described in section 3.2. A second method involved measuring the velocity associated with

a rotating disc. An acrylic disc attached to an electric motor rotating at a measured rate

was located such that one of its sides intersected the focal volume at a specified radial

location. Imperfections and dust on the sur- face served as scatterers for the laser-optic

system which measured their velocity. By

this method the fringe spacing could generally be determined to 0.5 percent. This

method, however, holds the promise of being

improved by greater precision in determining

the rotational speed of the disc and radial

location of the focal volume. The third method used to determine the fringe spacing is

similar in principle to the second. It con- sisted of comparing the air velocity of the

flow facility by the laser-optic system, Ul,

with those obtained by the Pitot-static tube

velocity standard, Up, which has more than

adequate resolution at the higher flow velocities. The Pitot-static tube has a 4 to 1 modified ellipsoidal nose (reference 7) with

an external body diameter of 0.309 inch, and

an impact opening of 0.0625 inch. The results

of such a comparison are shown in figure 10. In this connection, it should be noted that

the measurement of velocity by the laseroptic system was made with fringe spacing as determined by direct measurement of the focusing angle. The relatively large scatter at the lower velocities, i.e., below 100 m/min

is primarily a consequence of the uncertainties associated with Pitot-static measurements

alluded to in the Introduction. The squaresymbols denote the average values, and the

dashed curve indicates the trend of the impact

pressure of the Pitot-static tube to increase

at low Reynolds numbers due to viscous effects,

The Reynolds number based on the internal diameter, a, (i.e., Ua/v, where v is the kinematic viscosity) is 104 at U = 61 m/min.

8\«

I

I

\

H ±a—•-

-i 1 1 1-

II, (m/min)

Figure 10. Comparison of laser velocity standard with Pitot-static tube velocity standard,

11
The data are too limited to draw definitive

quantitative conclusions as to the magnitude

of the viscous effect at low Reynolds numbers but they do demonstrate the utility of

the facility for low Reynolds number investigations. Be that as it may, at the higher

velocities, for example, above 100 m/min,

the agreement between the laser system and

the Pitot-static tube is excellent. Thus,

comparison with the Pitot-static tube at higher velocities, affords a convenient

method for monitoring and determining the

focusing angle within the required accuracy.

It is particularly advantageous when operating in the Bragg cell mode in view of the

greater uncertainty associated with the

direct measurement of a much smaller angle.

It is still desirable, however, that future

effort be directed toward increasing the

focusing angle in the Bragg cell mode of

operation. All three methods of determining

the fringe spacing gave values that agreed

to within 0.5 percent.

Evaluating the resolution and accuracy

in the Bragg cell mode of operation is some- what more complicated than that discussed

above in that an additional consideration

involving the Bragg cell and heterodyning fre- quencies is introduced. In the Bragg cell

mode of operation the fringes are not

stationary and move at a velocity

V. = Lf

f c where fc is the Bragg cell frequency. The

average period, Tf, of a particle crossing

the focal volume thus becomes

L c

depending on whether the fringes are moving

in a direction opposite to, or in the same direction as the traversing particle. However, the period of the signal, To, measured

by the processor has been conditioned by the

heterodyning frequency, fH , with the result

that

T

f

" T

s

+ *H

and the longitudinal component of velocity

is given by

where Af is the difference, f

c

- f„, and the

fringes are moving in a direction opposite

to the traversing particle. The uncertainty

in the measurement of velocity in the

cell mode of operation can therefore be

expressed as

an [r +Af] 2 3U _ JL _ 3T

U U

'

L

in which the fringe spacing or beam angle

and the resolution of the processor, 8Tg,

may be regarded as being predetermined for a given arrangement, and the remaining factor

requiring evaluation is Af and its dependence on U. In order to avoid errors associated with the arithmetic difference of two nearly equal large quantities, the difference f -

ffl, in the present arrangement is

measured directly. This difference is

monitored by a seven digit counter which

provides a resolution of ± 0.1 Hz over the

required range. Thus, the resolution depends

primarily on optimizing Af to a minimum as a function of velocity such that the resulting

signal is within the higher resolution range,

i.e., 1/TS

> 500 kHz, for as much of the

velocity range as possible.

Several factors, however, impose con- straints on Af . In measuring time dependent

flows, for example, where the velocity may

change sign, the frequency shift should not

be removed or directional ambiguity may

occur. In addition, in time dependent flows,

attention should be given to not having the

signal too close in frequency to a filter

cut-off frequency or information may be lost.

The important constraint imposed on Af in

the present arrangement arises from the

small focusing angle. As a result there are only five fringes in the focusing volume and

this number is not sufficient for operation

of the processor. The processor requires

for its logic nominally ten cycles of information, and in general it is desirable that more than ten be available to permit triggering at higher level signals thus improving

the signal to noise ratio. The number of

effective fringes, N§, with frequency shifting, is given by

N

s

= N[— 1] (3)

and the minimum threshold value of Af is

L( TSAf) CD

12
for 9T 10" Us, and

Af = (tt~ - l)r m ^N L (4) Af- Af„ (7)

where Nm is the specified minimum number of

fringes that the particle crosses. It is

noted from equation (2) that for a given

system there will be a value of U above

which (U/L + Afm) is greater than 500 kHz

and 3Tg will be 10

_,+ ys. The threshold

value, Um , above which Af, = Afm

is

for 9T = 10" 2 ys. The ratio, R, of the un- certainties 9U/U, for the conditions ex- pressed in (6) and (7) is

U

lO" 2 ^) 2 (8)

5 x 10 3 LN

(5)

In the event U < Um

, the uncertainty

in Tg will become 10~ 2 ys rather than 10

_,+ ys unless Af is increased above Afm

such that

(U/L + Af) is greater than 500 kHz. In view

of the emphasis placed on low velocities in

the present facility it is desirable to resolve the question as to which setting of

the processor will produce the least un- certainty, 9U/U.

Since for either value of 9T$ the mini- mum value of 9U/U is obtained by minimizing

Af, the uncertainties, 9U/U, may be compared

by setting

Af

x = 5 x 10- (6)

Um

< U < Um

and that at

It is seen from equation (8) , and

illustrated in figure 11, that only for a limited range of U, i.e., 0.1

should Af be greater than Af , values of U < 0.1 U less uncertainty, 9U/U,

is obtained with 9T„ = 10~ 2 ys rather than

10

-1* ys. The resulting uncertainty in U using an optimum setting for Af is small,

being 0.15 percent. When the uncertainty in

U includes the contribution from L, arising

from the uncertainty in determining the

focusing angle, the contribution from L is

dominant. As a result, the uncertainty in U

in the Bragg cell mode of operation with Af

and L considered as random variables, is

approximately 1.0 percent over the speed

range. Similar considerations for the beamsplitter mode of operation give an uncertainty in U that varies from 0.5 to 0.7 percent

over the speed range.

Figure 11. Optimum difference between Bragg cell and heterodyning frequencies

13
The foregoing comments as to the effect

of Af and L on an individual realization of

velocity are particular to the present

arrangement but the approach should be generally applicable. However there are many

factors contributing to scatter in velocity

measurements by laser-optic methods other

than the two discussed above. Some of the more important of these are optical abberations, improper focusing of the laser beam,

impurities or density variations in the Bragg

cell, particle effects and vibration. Inas- much as the airflow in the facility, as discussed in section 5 is uniform and has low

turbulence level, it provides an excellent

medium for assessing the composite effect of

the variables affecting the scatter in. the

velocity data obtained with the laser system.

The magnitude of the random variations

in individual realizations of velocity, at a fixed speed, resulting from the above

effects, can best be characterized by the

standard deviation of the data. Assuming no systematic error, the standard deviation,

coupled to the number of realizations and the

probability distribution function determine

the degree of confidence that can be placed

in measuring a limiting mean value. The

number density of scattering particles, since

seeding is not employed, varies greatly from

day to day, and the number of individual

realizations per minute, for example, ranges

from 30 to 300 at a velocity of one meter per

second.

The data are processed by the data acquisition system described in section 3.3 to provide the velocity of individual realizations, their number, the cumulative mean, and

the standard deviation. The random variations

about the mean have a normal (Gaussian) probability distribution function with standard

deviations over the speed range in the beamsplitter and Bragg cell modes of operation of

1 and 5 percent, respectively. As a result,

operation in the beam-splitter mode, for

example, would require 30 realizations for a 99 percent confidence level that the sample mean velocity be within ± 0.5 percent of the

true mean value. In the Bragg cell mode 180

realizations would be required for a 99 percent confidence level that the sample mean be

within ± 1.0 percent of the true mean velocity. The mean velocity can therefore be

measured to a high order of accuracy, assuming no systematic error, solely by increasing

the number of realizations constituting the

sample mean. However, measurement of turbulence intensity, i.e., the rms value of fluctuations in velocity relative to the mean velocity is limited by the magnitude of the

"noise" or base level standard deviation.

The present system, therefore, is limited to the measurement of relatively high turbulence

levels, with perhaps a correction for the

"noise" level when appropriate. Considerable

reduction in the standard deviation would be

required to make the system competitive with

hot-wire anemometry for the measurement of

low turbulence levels. It should be noted,

however, that the laser method for measuring

turbulence can find application in a variety

of flow situations in which hot-wire anemometry cannot be satisfactorily applied.

5. PERFORMANCE OF FLOW FACILITY

The important factors in evaluating the

performance of the flow facility, other than

assuring that the velocity range meets design

requirements, are the uniformity of the flow,

the turbulence level, and the degree of un- steadiness of the airstream. The latter is

regarded as distinct from the turbulence, and

its occurrence is generally associated with

fan performance or the presence of large

scale, intermittent disturbances in the flow

circuit.

The fan drive was monitored with a tachometer and no difficulty was encountered in

maintaining a constant rpm over the speed

range. However, at a velocity below 0.6 m/s

in the test section, and with the contraction

in its initial configuration (see section

2.2), severe fluctuations in velocity were

observed. These fluctuations were highly

intermittent in time and location, and thus were difficult to characterize. Measurements

made with a hot-wire anemometer indicated

that the fluctuations in velocity were not due to problems with fan pitch or rpm, but

that they resulted from an unsteadiness in

the flow upstream of the test section. One

of the factors that played a role in producing this unsteadiness, and perhaps the most

critical, was the upstream concave curvature

of the contraction, and the associated intermittent separation of the flow from the walls

of the contraction. In regard to this effect,

the procedure was to adjust the adjustable

panels which were attached to the inside of

all four walls of the contraction in such a manner as to decrease the upstream curvature

until the speed at which fluctuations in

velocity were observed was a minimum. Below

this minimum speed, which was about 0.3 m/s,

disturbances due to leaks were also observed

to occur. The flow facility operates at below atmospheric pressure and it was observed that leaks into the facility around

the door leading to the screens in the

settling chamber were sufficient to cause fluctuations in velocity in the test section.

Sealing these leaks reduced the level of disturbances considerably.

14
Further hot-wire measurements at these

low velocities indicated that intermittent

flow disturbances still existed but they were

much more infrequent, and at the beginning of

the test section they were confined to the

region of the wall. These disturbances ex- tended to greater and greater distances

laterally in a downstream direction, however,

at one meter downstream from the beginning of

the test section they did not materially

affect the extent of the calibration

"window." These latter disturbances were attributed to temperature inhomogeneities,

and it was possible to prevent or minimize

their occurrence by thoroughly mixing the

air before it entered the facility or by

operating the facility, for a time, at a moderately high speed.

1.02

1.00

.98

.96

1.02

1.00

.92

.90

1.02

1.00

—O300

-*er-a&*

I +10

Figure 12.

y, DISTANCE FROM CENTER LINE (cm)

Velocity distributions (horizontal through centerline) ,©UC = 322 m/min, Pitot-static tube,

ZiUc

= 322 m/min, laser velocity

standard, G]UC = 567 m/min, Pitotstatic tube.

Measurements of distributions of mean

velocity, U, in the horizontal direction, y, and the vertical direction, z, about the

centerline of the duct are shown in figures

12 and 13 , respectively. The distributions

shown are for centerline velocities, U c

> of

322 m/min, and 567 m/min, and at different

downstream distances, x, from the beginning

of the test section of 100 cm, 300 cm, and

500 cm. The decrease in velocity that occurs near the wall reflects the presence of the

boundary layer through which the mean velocity decreases to zero at the surface.

Measurements with both a Pitot-static tube,

and the laser system were made in the process

of evaluating the uniformity of the flow.

1.02

1.00

.98

.96

.94

.92

.90

1.02 •

1.00

.98

.96

.94

.92

.90

1.02 —

1.00 _

.98 —

.96

I |

I

I I I • ' ' -40 -30 -20 -10 (^ +10 20 30 40

z, DISTANCE FROM CENTER LINE (cm)

Figure 13. Velocity distributions (vertical

through centerline) , ©U = 322

m/min, Pitot-static tube,^Uc = 322 m/min, laser velocity

standard, QUC = 567 m/min, Pitotstatic tube.

A comparison, illustrating the very good

agreement obtained using both methods at x = 100 cm, and U c = 322 m/min is also shown in

figures 12 and 13. As has been previously

demonstrated, it is necessary to use the

laser system for an accurate evaluation of

the uniformity of the flow at very low

velocities. Distributions of velocity in the

horizontal and vertical directions about the

15
centerline obtained with the laser system at 100 cm, and U 12.2 m/min are shown in

figure 14,

i.e., the ratio of the rms value of the

longitudinal component of the velocity fluctuations, u' , to U

c

, and its variation with

U , is shown in figure 18. The turbulence

clevel achieved is comparable to levels in

good low turbulence flow facilities, and is

sufficiently low to avoid any significant

statistical bias of the individual realizations of velocity by the laser system.

i -20

y (cm) DISTANCE FROM CENTERLINE (cm)

Figure 14. Velocity distributions measured

with laser velocity standard at x = 100 cm, Uc = 12.2 m/min.

Figure 15. Test "window" at x = 100 cm, U = 322 m/min.

c

It is seen that a good degree of uniformity

in mean velocity exists at this low velocity.

The mean flow characteristics were mapped in

great detail at x = 100, 300, and 500 cm, for

a centerline velocity of 322 m/min. The

results of such measurements are given in

Figures 15, 16 and 17, which show an estimate

of the contour of the test "window," i.e.,

the region over which the flow is uniform to within ± one percent. The shape of the con- tours is as expected, and reflects the

presence of secondary flow in the corners and

the downstream boundary layer growth. The

results illustrate that a satisfactory

"window" exists over the speed range of the

facility, and the length of the test section.

The remaining factor pertaining to the

performance of the flow facility that re- quires comment is the residual turbulence.

Turbulence measurements on the centerline, at

x = 300 cm, were made by constant temperature

hot-wire anemometry that had a low frequency

cutoff of 0.2 Hz. The turbulence intensity,

y (cm)

Figure 16. Test "window" at x = 300 cm, Uc = 322 m/min.

16
z (cm) -

I -40 -30 -20

Figure 17. Test "window" at x = 500 cm,

U„ = 322 m/min.

6. REFERENCES

1. Dryden, H. L. and Abbott, I. H., The

Design of Low-Turbulence Wind Tunnels,

Nat'l. Adv. Comm. Aero. Tech. Note 1755,

November 1948.

2. Bradshaw, P. and Pankhurst, R. C. , Progress in Aeronautical Science, Vol. 5, Pergamon Press, 1964.

3. Bradshaw, P., The Effect of Wind-Tunnel

Screens on Nominally Two-Dimensional

Boundary Layers, Jour. Fluid Mech., Vol.

22, Part 4, p. 679.

4. Farmer, W. M. and Hornkohl, J., TwoComponent Self-Aligning Laser Vector

Velocimeter, Appl. Optics, Vol. 12, No.

11, November 1973, p. 2636.

5. Lennert, A. E. Bray ton, D. B. and

Crosswy, F. L., Summary Report of the

Development of a Laser Velocimeter to be Used in AEDC Wind Tunnels, Arnold

Eng. Dev. Ctr. , TR-70-101, July 1970.

6. Brayton, D. B. and Goethert, W. H. , A New Dual-Scatter Laser Doppler-Shif

t

Velocity Measuring Technique, Instr.

Soc. Amer. Trans., Vol. 10, No. 1, 1971.

7. Salter, C. , Warsap, J. H. and Goodman,

D. G., A New Design of Pitot-Static Tube

with a Discussion of Pitot-Static Tubes

and Their Calibration Factors, Nat'l

Phys. Lab. Aero. Rept . 1013, May 1962.

fr- X 100 .10

Figure 18. Variation of turbulence intensity with centerline velocity.

17
NBS-114A (REV. 5-78)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN 989

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

A LOW-VELOCITY AIRFLOW CALIBRATION AND RESEARCH FACILITY

5. Publication Date March 1979

6. Perfotming Organization Code

7. AUTHOR(S)

L. P. Purtell and P. S. Klebanoff

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 7320483

11. Contract/Grant No. H0133024

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

Office of the Assistant Director - Mining

Bureau of Mines

United States Department of the Interior

Washington, D. C. 20241

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 low-velocity airflow facility suitable for the calibration of wind speed measuring

instruments and research in aerodynamics is described. The flow facility is of the

open-return type with a test section 20 feet (6.1 meters) long, and nominally 3x3

feet (0.91 x 0.91 m) in cross section. Special attention was given to obtaining an

air stream with a high degree of spatial uniformity and low turbulence with excellent

speed control over the range from 3 to 1000 meters (10 to 3300 feet) per minute.

Laser-optical methods with appropriate signal processing electronics are employed

to establish a primary standard for the measurement of very low velocities. A

crossed-beam dual-scattering laser-optical system is used that can operate with

either forward or backscattering, and in a frequency shifted Bragg cell mode, or

in a non-frequency shifted optical beam-splitter block mode. Detailed performance

characteristics of the flow facility and the laser velocity standard are presented.

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; calibration; facility; laser optics; low velocity; mine ventilation;

velocity standard.

18. AVAILABILITY £2? Unlimited

H For Official Distribution. Do Not Release to NTIS

tXX! Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Stock No. SNOO^OOj -02038-<

KXI Order From National Technical Information Service (NTIS)

Springfield, Virginia 22161

19. SECURITY CLASS

(THIS REPORT)

21. NO. OF P \GES

21

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

22. Price $1.20

UNCLASSIFIED

A

fr U.S. GOVERNMENT PRINTING OFFICE: 1979 O—281-067 (33) USCOMM-DC 86680-P78
There's

anew

look

ri | / ... the monthly / >^^^^^ J

/

magazine of the Nation-

' ^V^^^^^^^^/ a ' ^ureau °f Standards.

/ ^^^^ / Still featured are special ar- ^^^^^k^r'/ tides of general interest on \^^^ ^j / current topics such as consum- \v y / er product safety and building ^^

technology. In addition, new sections are designed to . . . PROVIDE

SCIENTISTS with illustrated discussions

of recent technical developments and

work in progress . . . INFORM INDUSTRIAL

MANAGERS of technology transfer activities in Federal and private labs. . . DESCRIBE TO MANUFACTURERS advances in the field of voluntary and

mandatory standards. The new DIMENSIONS/NBS also

carries complete listings of upcoming conferences to be

held at NBS and reports on all the latest NBS publications,

with information on how to order. Finally, each issue carries

a page of News Briefs, aimed at keeping scientist and consumalike up to date on major developments at the Nation's physical sciences and measurement laboratory.

(please detach here) SUBSCRIPTION ORDER FORM

Enter my Subscription To DIMENSIONS/NBS at $12.50. Add $3.15 for foreign mailing. No additional postage is required for mailing within the United States or its possessions. Domestic remittances should be made either by postal money order, express money order, or check. Foreign remittances should be made either by international money order, draft on an American bank, or by UNESCO

coupons.

Send Subscription to:

O Remittance Enclosed (Make checks payable

to Superintendent of Documents)

CD Charge to my Deposit Account No.

I I I I I I I I

NAME-FIRST, LAST

I I I I I I I I I I I I COMPANY NAME OR ADDITIONAL ADDRESS LINE

STREET ADDRESS

l I I I I I I I I I I I I I I I

I

MAIL ORDER FORM TO:

Superintendent of Documents

Government Printing Office Washington, D.C. 20402

PLEASE PRINT

NBS TECHNICAL PUBLICATIONS

PERIODICALS

JOURNAL OF RESEARCH—The Journal of Research

of the National Bureau of Standards reports NBS research and development in those disciplines of the physical and

engineering sciences in which the Bureau is active. These

include physics, chemistry, engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement methodology, and

the basic technology underlying standardization. Also in- cluded from time to time are survey articles on topics closely

related to the Bureau's technical and scientific programs. As

a special service to subscribers each issue contains complete

citations to all recent NBS publications in NBS and non- NBS media. Issued six times a year. Annual subscription: domestic $17.00; foreign $21.25. Single copy, $3.00 domestic;

$3.75 foreign. Note: The Journal was formerly published in two sections: Section A "Physics and Chemistry" and Section B "Mathematical Sciences." DIMENSIONS/NBS

This monthly magazine is published to inform scientists, engineers, businessmen, industry, teachers, students, and

consumers of the latest advances in science and technology,

with primary emphasis on the work at NBS. The magazine

highlights and reviews such issues as energy research, fire protection, building technology, metric conversion, pollution

abatement, health and safety, and consumer product per- formance. In addition, it reports the results of Bureau pro- grams in measurement standards and techniques, properties

of matter and materials, engineering standards and services, instrumentation, and automatic data processing.

Annual subscription: Domestic, $11.00; Foreign $13.75 . NONPERIODICALS

Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks-—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with interested industries, professional organizations, and regulatory bodies.

Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special

publications appropriate to this grouping such as wall charts, pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists, engineers,

chemists, biologists, mathematicians, computer programmers,

and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of

materials, compiled from the world's literature and critically evaluated. Developed under a world-wide program co- ordinated by NBS. Program under authority of National

Standard Data Act (Public Law 90-396).

NOTE: At present the principal publication outlet for these data is the Journal of Physical and Chemical Reference Data (JPCRD) published quarterly for NBS by the American Chemical Society (ACS) and the American Institute of

Physics (AIP). Subscriptions, reprints, and supplements

available from ACS, 1155 Sixteenth St. N.W., Wash., D.C.

20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research

results, test methods, and performance criteria related to the

structural and environmental functions and the durability and safety characteristics of building elements and systems. Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject.

Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often

serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The purpose

of the standards is to establish nationally recognized require- ments for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS administers this program as a supple- ment to the activities of the private sector standardizing

organizations.

Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest to the consumer. Easily understandable language and

illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order above NBS publications from: Superintendent of Documents, Government Printing Office, Washington, D.C.

20402. Order following NBS publications—NBSIR's and FIPS from

the National Technical Information Services, Springfield, Va. 22161.

Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register.

Register serves as the official source of information in the Federal Government regarding standards issued by NBS

pursuant to the Federal Property and Administrative Services Act of 1949 as amended, Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717

(38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of

interim or final reports on work performed by NBS for outside sponsors (both government and non-government).

In general, initial distribution is handled by the sponsor;

public distribution is by the National Technical Information

Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

ie following current-awareness and literature-survey bibli- raphies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A litera- ture survey issued biweekly. Annual subscription: Domestic, $25.00; Foreign, $30.00.

jquified Natural Gas. A literature survey issued quarterly. Vnnual subscription: $20.00.

Superconducting Devices and Materials. A literature survey

issued quarterly. Annual subscription: $30.00. Send subscription orders and remittances for the preceding bibliographic

services to National Bureau of Standards, Cryogenic Data

Center (275.02) Boulder, Colorado 80302.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D.C. 20234

OFFICIAL BUSINESS

Penalty for Private Use. $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE COM.215

SPECIAL FOURTH-CLASS RATE BOOK