Reference

A11104

__ NBSIR 78-1585

Low Velocity Performance of A

Vortex-Shedding Anemometer

L. P. Purteil

Fluid Engineering Division

Center for Mechanical Engineering

and Process Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234.

f

December 1978

Task Report

• QC —100

,1156

78-1565

Prepared for United States Department of the Interior

Bureau of Mines

iaWoiwi Esireaa of Standards MAY 1 4 197?

NBSIR 78-1565

LOW VELOCITY PERFORMANCE OF A

VORTEX-SHEDDING ANEMOMETER

L. P. Purteil

National Bureau of Standards

Fluid Engineering Division

Washington, D.C. 20234

December 1978

Task Report

on

Contract No. HOI 661 98

Evaluation of the Behavior of Mine Anemometers in the NBS Low

Velocity Calibration Facility

Prepared for United States Department of the Interior

Bureau of Mines

U.3. DEPARTMENT OF COMMERCE, Juanita M. Kraps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director
DISCLAIMER NOTICE

The views and conclusions contained in this document should not be

interpreted as necessarily representing the official policies or recommendations of the Interior Department's Bureau of Mines.
NBS-1 1.4 A (REV. 5-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 79-1565

2. GovN Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

LOW VELOCITY PERFORMANCE OF A VORTEX-SHEDDING ANEMOMETER

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

L. P. Purtell 8. Performing Organ. Report No. NBSIR 79-1565

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. H0166198

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

Feb. 28, 1978-Apr. 30,1978

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.)

Performance of a vortex-shedding anemometer is evaluated over the speed range

of 58.1 feet per minute to 836 feet per minute. Tests on two units were

performed in the NBS Low Velocity Airflow Facility which provides a uniform

flow of low turbulence and utilizes a laser velocimeter as the velocity

standard

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; anemometer; laser velocimeter; low velocity, mine ventilation; wind

tunnel

18. AVAILABILITY ,F _ Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

KX For Official Distribution. Do Not Release to NTIS

UNCL ASSIFIED

1" 1 Order From Sup. of Doc., U.S. Government Printing Office Washington. D.C. 20402. SD Stock No. SN 00 3-0 03

20. SECURITY CLASS

(THIS PAGE)

22. Price

! Order From National Technical Information Service (NTIS)

Springfield, Virginia 22161 UNCLASSIFIED

USCOMM-DC 0 6 5 8 0 -P7 8 1
FOREWORD

This report was prepared by the National Bureau of Standards, Fluid

Engineering Division, Washington, D. C. 20234, under USBM Contract

Number H0166198. The contract was initiated under the Coal Mine Health

and Safety Program. It was administered under the technical direction

of PM&SRC, With Dr. George H. Schnakenberg , Jr., acting as the Technical

Project Officer. Mr. H. R. Eveland was the contract administrator for

the Bureau of Mines. This report is a summary of the work recently

completed as part of this contract during the period February 28, 1978

to April 30, 1978. This report was submitted by the author December

1978.

2
LIST OF SYMBOLS

velocity measured by laser velocimeter

frequency indicated by anemometer under test

line segments fitted to U, f data

group mean true velocity

group mean indicated frequency

standard deviation of f data from f

standard deviation of f data expressed as true

velocity

Q adjusted for known variance in laser

velocimeter measurements
LOW VELOCITY PERFORMANCE OF A VORTEX-SHEDDING ANEMOMETER

L. P. Purtell

1.

INTRODUCTION

The National Bureau of Standards in order to meet the need for a calibration capability with adequate accuracy at low air velocities,

i.e., below 500 feet per minute (fpm) undertook the development of a low-velocity calibration facility for wind speed measuring instruments

which would provide a capability down to 3 meters per minute (approximately 10 fpm) with an accuracy of plus or minus one percent. It was a natural consequence therefore that when said facility became operational

to undertake an evaluation of the state-of-the-art and to provide the

information needed as to the reliability and performance of instrumentation for such measurement. Accordingly, a number of prototypes of

various types of instruments for low velocity air measurements are undergoing test at NBS, and this report is concerned specifically with

the results of one such test.

2.

THE INSTRUMENT

The anemometer tested for this report (two units, serial numbers 48

and 49) is a commercially available instrument (J-TEC Associates,

Incorporated, VA-216 Air Draft Sensor) 1 supplied for test by the U. S.

Bureau of Mines. The instrument is approximately 12 x 4 x 5-1/2 inches

in size (see Figure 1) and includes a probe head and the necessary

electronics for operation. The probe head consists of a rod approximately 0.25 inches in diameter and 1.38 inches in length supported at

the ends and oriented normal to the air stream. An ultrasonic transmitter and receiver pair located downstream of the cylinder detect the

vortex street shed by the cylinder as an amplitude modulation of the

ultrasonic signal. This modulation is converted by the electronics

section to a pulse train (approximately a square wave signal) as an output, A frequency- to-voltage converter section also provides an analog output. Since the frequency of shedding of a vortex street is

roughly proportional to velocity, the output signals can be used to

measure the air velocity. A separate power supply is necessary to

operate the instrument.

3.

THE TESTS

The NBS Low Velocity Airflow Facility [1] used to test this instrument generates a low velocity air stream having a low turbulence intensity (less than 0.05%) and a large region of uniform flow (at least

i —-—

ihis particular instrument was selected as being representative of

this type of anemometer and its selection does not represent an endorsement

.

4
75 x 75 cm) . A laser velocimeter is employed as a primary velocity

standard. It is nonintrusive , has a linear response with velocity, and

has good spatial resolution. Adequate sensitivity is obtained without

the artificial seeding of scattering particles. Thus the difficulties

and inconvenience associated with seeding and the possible effect of

such seeding on the performance of the device under test are avoided.

Each anemometer was mounted with the probe head on the centerline

of the tunnel test section one meter downstream of the entrance to the

test section in a manner to minimize the effect of the support on the

air stream around the anemometer (Figure 1) . Since the anemometer

itself modifies the airflow in the tunnel, the velocity should be

measured at a location in the flow which has the same velocity in the

presence of the anemometer as it does in the absence of the anemometer.

The velocity upstream of the anemometer was measured to find the position

where deceleration of the flow due to the presence of the anemometer was

no longer detectable within the scatter of the measurement. These measurements were performed at the free-stream velocity of 990 fpm. As shown

in a previous report [2] the variation of the ratio of the local velocity

to the free stream velocity with distance upstream of the anemometer is

independent of free-stream velocity. A distance of 35 cm upstream of

the anemometer was chosen as the position for velocity measurement by

the laser velocimeter. With no anemometer in the tunnel, variation in

velocity is imperceptible over the distance traversed (35 cm)

.

The frequency of the anemometer output was recorded during the time

interval required for the measurement by the laser velocimeter. Five

separate test runs were made for each instrument, a run consisting of

thirteen different velocities. The lowest velocities were limited by a minimum operating speed of 54.1 fpm for S/N 49 and 58.8 fpm for S/N 48.

The data are presented in chronological order in Tables 1A to IE and 2A

to 2E for instruments with S/N 49 and 48 respectively.

4. TESTS RESULTS

Since a particular air speed in the wind tunnel cannot be exactly

reset from run to run, scatter in the test data in the form of frequency

against speed is distributed along a curve, thus prohibiting computing

the standard deviation of the data from a simple average. Instead,

deviations from a curve fit to the data were computed and the standard

deviation approximated by the r.m.s. value of these deviations within a group. The groups are (fpm):

U < 65 110 < U < 150 450 < U < 550

65 < U < 75 150 < u < 200 550 < u < 750

75 < u < 85 200 < u < 250 750 < u

85 < u

< 95 250 < TJ < 350

95 < u < 110 350 < u < 450

5
Since the groups of data are compact (small range of U within a group; see Figures 2 and 3) , a straight line segment is used to approxima_te_the curve within a group. The line segment passes through the point (U,f),

the group mean true velocity and the group mean frequency. The slope of

the line segment is computed as the average of the slopes of two lines,

both passing through (U,f) of the group being considered, one line

passing through the (U,f) of_the adjacent group higher in velocity, and one line passing through (U,f) of the adjacent group lower in velocity.

For the highest groups (U > 750) there is only one adjacent group, and

thus the line segment for this highest group passes through (U,f) of

that adjacent group. The line segment for the lowest groups (U < 65

fpm) is similarly formed.

Designating the above line segments as f^, the standard deviation,

G

f of the frequency, f, about the fitted segments is determined by

squaring the differences between the f data and f , i.e.,

[f (U) - ff (U)] 2

. Since the data within the specified groups are reasonably compact, the mean of the squared differences within a group

is taken as an estimate of the variance of f about_f^ within that group

and specified at that group’s mean true velocity, U. To convert this to

a standard deviation in terms of true velocity , designated G,

each a (U) is divjlded by the slope (df^/dU) of the line segment associated with the a (U) . Note that this O does not include the "scatter"

in the U measurements (due to the inability to exactly reset wind tunnel

to a specified speed) , but does include the uncertainty in a particular

laser velocimeter measurement. This uncertainty may be estimated from

repeated measurements of velocity at a particular fan setting, thus also

including any unsteadiness in the velocity, and is estimated as 0.001U

for this report. A standard deviation, G , corrected for the laser

velocimeter uncertainty may thus be computed from

G

2 = G2 - (0.001U) 2 c for any given U. G and G^/f are presented in Figures 4 through 7. O

and G are presented in Figures 8 and 9 as velocity and in Figures 10

and l5 as percentage of U. Since ± 2 g^ is extremely close to the 95

percent confidence interval for one measurement, curves of ± 2G

f are also included in Figures 2 and 3 as dashed lines. The curves shown in

each figure have been drawn for reference only.

5. DISCUSSION OF RESULTS

Since the primary output of this type of instrument ideally is a square wave with frequency related to the air speed, the analysis was concentrated on the measurements of that frequency directly rather than

on the analog voltage output, since the latter merely reflects the

quality of the f requency-to-voltage converter. The resolution in reading the frequency was always kept better than 0.1 percent to avoid

6
affecting a^. The difference in the values of a for the two units

is not readily attributable to any known difference between the

units; qualitatively the instruments performed the same.

The instruments in general performed with no erratic behavior.

Some general comments concerning application of the instrument follow.

With any measurement problem the instrument's capabilities should be

matched to the required measurement.

This anemometer is intrusive, i.e., it must be placed in the flow.

This anemometer requires an outside source of power and either a frequency counter or a voltmeter.

Many other factors that can affect the suitability of an instrument

for a particular application, such as turbulence or unsteadiness of the

air stream, rough handling (shock and vibration), dirt and other en- vironmental factors, time, orientation to the velocity and gravity

vectors, etc., have not been tested herein but should be considered.

6 . SUMMARY

The performance of a vortex-shedding anemometer (two units) has

been evaluated at air speeds up to 836 fpm. Figures are presented

showing the output frequency against true velocity and the standard

deviation of repeated runs about the mean curves. The lowest velocities

measurable were limited by a minimum operating speed of 54.1 fpm for one unit and 58.8 fpm for the other unit.

7. REFERENCES

1. L. P. Purtell and P. S. Klebanoff, The NBS Low velocity Airflow

Facility, in preparation.

2. L. P. Purtell, Low Velocity Performance of a Bronze Bearing Vane

Anemometer, NBSIR 78-1433.

7
Table 1A

J-Tec Type VA-216

S/N 49

Instrument Output Frequency Instrument Output Voltage True Air Speed

Hz volts f pm

8.68 .132 60.3

10.9 .160 72.2

12.6 .182 81.0

14.6 ' .208 91.4

16.5 .233 100.6

22.7 .312 128

32.5 .44 170

46.9 .62 219

65.4 .88 284

83.2 1.12 374

110 1.48 492

147 1.95 649

187 2.45 835

242 3.20 1103

278 3.70 1290

318 4.20 1495

T = 21.3 to 21.6 °C

B = 752.4 mm Hg
Table IB

J-Tec Type VA-216

S/N 49

Instrument Output Frequency

Hz

Instrument Output Voltage

volts

True Air Speed

fpm

8.63

10.5

12.3

15.0

16.6

.126 60.8

.154 72.5

.175 81.6

.209 92.9

.229 100.8

22.9

32.6

46.9

66.2

84.0

.308 130

.44 172

.62 220

.86 284

1.13 375

112

148

186

245

276

1.49 492

1.95 647

2.50 833

3.20 1109

3.65 1293

320 4.15 1502

T = 22.5 °C

B = 752.7
Instrument Output Frequency

Hz

True Air Speed

f pm

Table 1C

J-Tec Type VA-216

S/N 49

Instrument Output Voltage

volts

8.70 .130 61.6

10.8 .154 72.5

12.7 .180 81.6

14.9 .210 92.5

16.3 .226 100.2

23.0 .311 131

32.6 .43 171

47.4 .60 219

65.4 .86 285

84.0 1.13 375

112 1.48 492

147 1.96 648

186 2.47 834

245 3.21 1110

282 3.68 1298

318 4.16 1497

T = 22.5 to 22.8 °C

B = 752.8 mm Hg
Instrument Output Frequency

Hz

True Air Speed

fpm

Table ID

J-Tec Type VA-216

S/N 49

Instrument Output Voltage

volts

8.59 .126 59.8

10.6 .154 71.1

12.6 .175 80.5

14.8 .209 91.9

16.3 .227 100.4

22.7 .308 131

32.7 .43 171

46.7 .60 219

64.9 .86 286

82.6 1.12 374

110 1.48 493

148 1.97 648

189 2.46 835

243 3.20 1106

279 3.65 1286

316 4.19 1505

T = 22.8 to 23.0 °C

B = 752.8 mm Hg
Table IE

J-Tec Type VA-216

S/N 49

Instrument Output Frequency

Hz

Instrument Output Voltage

volts

True Air Speed

fpm

8.50

10.8

12.6

14.9

16.4

.126 59.5

.152 71.5

.177 80.9

.207 92.1

.226 100.2

23.3

32.8

46.5

65.4

83.3

.313 130

.43 172

.61 220

.84 284

1.12 375

110

147

188

244

279

1.48 490

1.96 648

2.47 836

3.20 1111

3.65 1292

320 4.12 1502

T = 23.2 to 23.4 °C

B = 751.3 mm Hg
Table 2A

J-Tec Type VA-216

S/N 48

Instrument Output Frequency,

Hz

Instrument Output Voltage,

volts

True Air Speed,

fpm

8.61

10.8

12.6

14.7

16.3

.118 59.6

.146 71.0

.168 80.4

.195 92.0

.216 100.5

22.2

30.8

46.3

59.5

82.6

.29 130

.40 172

.60 220

.78 286

1.09 375

106.8 1.43 494

145.6 1.91 650

185.2 2.44 832

= 23 . 1°C

= 751.0 mm Hg

T

B
Table 2B

J-Tec Type VA-216

S/N 48

Instrument Output Frequency,

Hz

Instrument Output Voltage,

volts

True Air Speed,

fpm

8.48

10. 7 12.5

15.0

16.2

.116 58.1

.144 70.3

.166 80.2

.199 93.0

.215 99.5

22.2

29.7

46.1

57.5

82.0

.295 130

.39 170

.60 219

.76 285

1.09 375

105.7 1.42 494

144.5 1.92 649

186.2 2.45 835

T = 23. 4°C

B = 751.2 mm Hg
Instrument Output Frequency

Hz

True Air Speed

fpm

Table 2C

J-Tec Type VA-216

S/N 48

Instrument Output Voltage,

volts

8.65

10.9

12.7

15.0

16.4

.117 59.6

.147 71.9

.169 81.0

.198 92.9

.218 100.9

23.0

29.2

45.7

57.8

80.0

.29 130

.38 171

.60 220

. 77 285

1.07 375

106.4 1.49 492

143.7 1.91 650

184.8 2.45 836

= 23. 7°C

= 750.8 mm Hg

T

B
Table 2D

J-Tec Type VA-216

S/N 48

Instrument Output Frequency,

Hz

Instrument Output Voltage,

volts

True Air Speed,

fpm

8.50

10.6

12.5

14.7

16.3

.119 63.5

.142 71.2

.166 80.1

.196 91.8

.217 100.2

21.5

28.8

45.2

59.9

78.1

.28 130

.38 170

.59 220

.78 284

1.04 375

107.0 1.40 492

143.5 1.90 647

186.2 2.44 834

T = 23 . 9°C

B = 750.2 mm Hg
Table 2E

J-Tec Type VA-216

S/N 48

Instrument Output Frequency,

Hz

Instrument Output Voltage,

volts

True Air Speed,

fpm

8.40

10.6

12.4

14.6

16.2

.06 63.6

.142 70.4

.165 80.0

.195 91.1

.215 99.4

22.7

27.9

44.8

60.2

78.1

.30 130

.37 170

.59 219

.77 283

1.04 375

103.7 1.58 489

142.9 1.88 642

184.8 2.43 834

T = 24 . 2°C

B = 750.2 mm Hg
7vt;.:

FIGURE 1. THE ANEMOMETER MOUNTED IN THE TUNNEL SHOWING

METHOD OF SUPPORT (VIEWED LOOKING DOWNSTREAM)

.
200

180

160

140

120

100

80

60

40

20

0

4 4 » ^ —? —4 I ) 4——4 f—-4 4- - » 4

i I 4«~—4-

—-4— j « > s < > I- -H

) 100 200 300 400 500 600 700 800 900

U (fpm)

IGURE 2. PULSE FREQUENCY VERSUS TRUE VELOCITY WITH ± 2a CURVES.

INSTRUMENT S/N 49.

1
200

180

160

140

120

100

80

60

40

20

0

< —

«

-

1 > « I 1 > 1 - 4

100 200 300 400 500 600 700 800 900

U (fpm)

3 . PULSE FREQUENCY VERSUS TRUE VELOCITY WITH ± 2 a CURVES.

INSTRUMENT S/N 48.
FIGURE

4.

STANDARD

DEVIATION

OF

PULSE

FREQUENCY.

INSTRUMENT

S/N

49.
+ + t o

o

CT>

^ 8 H ^ +

4-i D

NE

U

(fpm)

FIGURE

5.

STANDARD

DEVIATION

OF

PULSE

FREQUENCY.

INSTRUMENT

S/N

48.
FIGURE

6.

STANDARD

DEVIATION

OF

PULSE

FREQUENCY

AS

PERCENT

OF

GROUP

MEAN

FREQUENCY.

INSTRUMENT

S/N

49.

FIGURE

8.

STANDARD

DEVIATION,

cr,

AND

CORRECTED

STANDARD

DEVIATION,

a

,

EXPRESSED

AS

EQUIVALENT

TRUE

VELOCITY.

INSTRUMENT

S/N

49.

c
FIGURE

9.

STANDARD

DEVIATION,

a,

AND

CORRECTED

STANDARD

DEVIATION

,a

,

EXPRESSED

AS

EQUIVALENT

TRUE

VELOCITY.

INSTRUMENT

S/N

48.
co

O'. oc

COoc

r>» ocooo

LOoooc

COo

CNooc

4-1

IP

G o I p

e> IP

FIGURE

10.a

ANDa

AS

PERCENT

OF

GROUP

MEAN

VELOCITY.

INSTRUMENT

S/N

49.
FIGURE

11.

a

AND

a

AS

PERCENT

OF

GROUP

MEAN

VELOCITY.

INSTRUMENT

S/N

48.