tltO'(Ak A ! ;:", : 7- ~'-, "',': '- !7i'::; ~ ,",':, ( ,

M I N I S T R Y OF A V I A T I O N

R. & M. No. 3317

A E R O N A U T I C A L R E S E A R C H C O U N C I L

R E P O R T S A N D M E M O R A N D A

Measurements of Free-Stream Turbulence in some Low-Speed Tunnels at N.P.L.

By P. BRADSHAW, B.A. and D. H. FERRISS, B.Sc.

OF THE AERODYNAMICS DIVISION, N.P.L.

LONDON: HER MAJESTY'S STATIONERY OFFICE

x963 NINE SHILLINGS NET

Measurements of Free-Stream Turbulence in some Low-Speed Tunnels at N.P.L.

By P. BRADSHAW, B.A. and D. H. FERRISS, B.Sc.

OF THE AERODYNAMICS DIVISION, N . P . L .

Reports and Memoranda No. 33z7" January, z962

Summary. The r.m.s, fluctuation levels in the older low-speed tunnels at N.P.L. are found to be as follows: Compressed

Air Tunnel, 0.3O/o rising to 0"6~o at maximum speed and pressure; 13 ft x 9 ft Tunnel, 0.05 -+ 0.20/o; 9 ft x 7 ft (South) Tunnel, 0.18%; 18 in. Anemometer Tunnel, 0-2°/o in the unrestricted tunnel rising to 1~/o when running with diaphragms at low speed.

Introduction.

Detailed measurements of the turbulence in the N.P.L. low-speed tunnels have not previously

been made, and when information about the turbulence in the Compressed Air Tunnel was needed,

the opportunity was taken to investigate some of the other tunnels as well. The results are presented

here with a description of the tunnels and a short discussion of the likely sources of turbulence in each. Although intended chiefly for the information of those who use the tunnels it is hoped that the

results will also be useful to tunnel designers. The authors wish to acknowledge the help of Mr. M. T. Gee, formerly of the Aerodynamics

Division, N.P.L., in the planning and direction of the experiments. Messrs. R. J. Easton and K. W. Stuart assisted with the experimelltal work.

Apparatus.

The measurements in the 9 ft × 7 ft, 13 ft × 9 ft and 18 in. Anemometer Tunnels, and some

earlier measurements in the C.A.T., were made with constant-current hot-wire apparatus, purchased

from Dr. G. D~twyler of Ztirich. The amplifier has resistance-inductance frequency compensation

and a bandwidth of about 30 kc/s. With the vcire sensitivities and time constants of the present

experiment, the noise level corresponded to about 0.07~/o turbulence. The wires were 0. 0002 in.

diameter platinum. 0-0003 in. platinum and 0-00015 in. platinum-rhodium were also used in an

unsuccessful attempt to avoid wire breakage due to vibration at high pressures in the C.A.T.

Single-wire probes were calibrated in the tunnel where they were used, but the cross-wire probes were calibrated in mean speed and in yaw in a 1 in. square pipe in turbulent flow.

The second set of measurements in the C.A.T., reported here, was made with DISA 55A01 constant-temperature anemometers. The noise level is quoted by the makers as equivalent to 0.06% r.m.s, turbulence with a 6 kc/s bandwidth, and was measured as 2mV with approximately

this bandwidth. It probably increases proportional to the square root of the bandwidth, but should

* Replaces N.P.L. Aero Report No. 1001--A.R.C. 23,483. Published with the permission of the Director, National Physical Laboratory.

in any case be small compared with the turbulence in the C.A.T.: a good signal-to-noise ratio was

maintained up to about 15 kc/s as can be seen from the spectra. 0.0002 in. diameter tungsten

wires were used. The single-wire probes were calibrated in the tunnel, and the cross-wire probes were calibrated in mean speed and yaw in the core of a 2 in. diameter jet.

In the 13 ft x 9 ft and Anemometer Tunnels, spectra were'measured on a Muirhead D788A

analyser with a frequency range of 3 c/s to 3 kc/s. This analyser is of the Wien bridge constant- percentage-bandwidth type with a Q of 10, and its reliability at the low-frequency end of a wide-bandwidtk spectrum is exceedingly doubtful as the off-tune rejection never exceeds about

40 dB. However, a high-Q analyser is preferable if discrete frequencies are expected. In order to show up discrete frequencies, the analyser r.m.s, output, proportional to ~/{f[u(f)] ~} has been plotted against logf. Spectra in the C.A.T. and 9 f t x 7 ft Tunnel were measured with a Bruel and Kjaer 2111 }-octave analyser with a frequency range of 16 c/s to 31-5 kc/s: it had been found previously

that there were no sharp peaks in the spectrum and as accurate readings at low frequency were desired the passive-filter analyser was preferable to the Muirhead type. These spectra are recorded

here in the usual form for turbulence spectra, F = [u(f)]2/u 2, instead of .v'{f[u(f)] 2} oc v ' ( fF) , being plotted against log f.

More details of the apparatus and measurement techniques are given in Ref. 1.

In the Anemometer Tunnel tests the probes were mounted on a rod secured outside the tunnel

structure: in all the other tests the probes were mounted on a heavy steel bar, freely suspended within

a hollow steel strut screwed to the tunnel structure, as shown in Fig. 5.

Description of Tunnels and Results.

1. The Compressed Air Tunnel (Fig. 1).

This tunnel was built in 1930. It has a 6 ft diameter open-jet working section and an annular

return circuit, the whole being enclosed in a pressure shell. The maximum working pressure

is 25 atmospheres absolute and the maximum speed 90 ft/sec, giving a Reynolds number of

1.4 x 107 per foot. A 2-bladed fan is driven by a 400 h.p., 800 r.p.m, electric motor. The tunnel is

largely used for tests of models of prototype aircraft. Since its construction improvements to the flow have been made by fitting vanes to prevent separation at the annular 180 ° bend ahead of the

contraction, and by fitting a 2 in. deep precision honeycomb with right-angled triangular cells

having an apex to hypotenuse spacing of ~ in., but the mean velocity variations and turbulence intensity are worse than in more modern tunnels, owing to the relatively sharp bends and the small contraction ratio. As far as it is possible to judge, these flow conditions have not caused serious discrepancies between C.A.T. model tests and full-scale results.

Two sets of measurements of the tunnel turbulence have been made. The first tests were conducted in May, 1960 with constant-current hot-wire apparatus: trouble was experienced with probe vibration and wire breakage at the higher pressures, and the compensation time constant on the hot-wire apparatus could not be reduced below 0.2 milliseconds. T h e results of these tests are not presented. The second tests were conducted in May, 1961 with constant-temperature

apparatus, stronger probes and tungsten wire. The probes again tended to vibrate at high dynamic pressures, and results obtained with severe vibration have been omitted from the graphs. Large-

amplitude vibrations of the probe could be seen on the closed-circuit television screen when they

occurred. The results have been plotted against Reynolds number per foot at the different pressures

but there is no particular reason why they should all collapse together on this scale: the scatter in

the results under similar conditions of Reynolds number per foot, U/v, and working pressure is

probably due to contamination of the wires by dust, as there has never been any indication of non-repeatability in force measurements on models. Differences in the turbulence measurements

at the same value of U/v but at different working pressures are in part due to the vibration of the

probe relative to the heavy main-tunnel structure, and in part to the effect of vibration of some lighter

parts of the tunnel structure.

The longitudinal component of turbulence lies between 0.3 and 0.4% at the lower speeds and

pressures but rises to 0.6% at the maximum speed at pressures of about 200 p.s.i, and above (Fig. 6).

A value of 0.8% was recorded on one occasion at 350 p.s.i, and high speed, when the probe and its

support bar were vibrating noticeably: this was the worst fit of vibration noticed, but there is

undoubtedly some contribution from probe vibration in the results at the higher pressures. As there

was no trace of discrete frequencies in the spectra (Fig. 8) it may be assumed that the probe was

responding to random excitation as a solid body rather than suffering from prong resonance as was

found with the weaker probes in the first tests. The probe stand was attached to a lighter part of the

tunnel structure than that normally used for mounting models: it is unlikely that buffeting of the

stand contributed any greater proportion of the apparent turbulence reading in the C.A.T. than in

the other tunnels for which this stand was used. The tests were made with a large swept-wing model in the working section, well behind the probe.

As seen in Fig. 6b, no very significant increase in turbulence resulted from an increase in model

incidence from zero to 22 ° .

The lateral component was also measured, on an axis parallel to the axis of the probe stand so that

any vibration of the probe arm with respect to the stand should have had little effect. Again, the

recorded turbulence increases with increasing pressure for any given fan speed (Fig. 7), and again the spectrum is largely composed of low-frequency contributions (Fig. 9). It is felt that these

low-frequency components of turbulence and vibration might cause discrepancies between flight tests and C.A.T. model tests in respect of maximum lift. A 0.5 % vertical-component r.m.s, intensity implies an r.m.s, incidence fluctuation of 1o ~° will be not , and deviations of incidence up to +

uncommon, so that an aerofoil set'at a mean incidence just below that for maximum lift will stall

prematurely: the average lift obtained will therefore be too low. At high working pressures but low

wind speeds the v-component r.m.s, intensity rose to 0.5 or 0 . 6 ~ , but the observations have been

omitted from Fig. 7 because in practice the corresponding Reynolds numbers would be obtained by

running the tunnel at a higher speed and a lower pressure. The u-component, however, did not rise

to a similar extent. The conclusion from the C.A.T. tests is therefore that the apparent turbulence rises considerably

at the higher speeds and pressures chiefly because of vibration.

2. 13 f t x 9 f t Tunnel 2 (Fig. 2).

This is the largest tunnel at N.P.L., with a 13 ft x 9 ft octagonal working section. The maximum

speed is about 240 ft/sec, but during the laminar-flow stability tests for which the tunnel has been

frequently employed it has been found necessary to keep below 180 ft/sec in order to avoid

premature transition. A honeycomb is fitted but there are no screens: the contraction ratio is only

4:1. The tunnel's unusual features are twin two-bladed fans in parallel circular ducts, and a freely

rotating four-bladed windmill mounted in an octagonal section: the windmill supports have given

trouble on several occasions and the windmill itself has recently had to be re-balanced.

3

(86834) A*

The longitudinal component of turbulence rises from 0.04% at 10 ft/sec to 0.16% at 240 ft/sec (Fig. 10). There are pronounced peaks in the curve at 72, 120, 144, 180 and 208 ft/sec, the last three of which were found to be caused by windmill vibration as shown by the output from a resistance strain gauge bonded to one of the support struts. It is of course probable that the 72 ft/sec and 120 ft/sec peaks were also caused by the windmill, but the strain-gauge output at the lower speeds was difficult to measure. A frequency spectrum at 141 ft/sec is shown in Fig. 12.

Frequency spectra showed that the windmill contribution at twice and four times the shaft frequency is appreciable even at airspeeds away from the peaks in the intensity graph. The fan contribution is also noticeable but in this case no resonances were detected. The two motors are remotely controlled through a single pilot motor, with manual adjustment for synchronization which has only to be made occasionally. No tendency to beating was noticed and small departures from synchronism deliberately introduced had no effect on the turbulence: the effect on mean speed variations in the working section was not measured.

The lateral- and vertical-component intensities (w, v) rise steadily from 0.06~o at 10 ft/sec to 0.23~o at 210 ft/sec (Fig. 11), without any marked increase at the critical speeds mentioned above. A frequency spectrum at one of these critical speeds showed a fairly small windmill contribution (Fig. 13).

It is clear that the 'turbulence' in this tunnel is chiefly an inviscid mode of longitudinal pulsation

caused by the windmill and, to a lesser extent, the fans. The windmill contribution is undoubtedly

increased by the peculiar siting of the windmill in an octagonal section so that at the sides of the tunnel the blade tips are well clear of the walls whereas troughs are needed in roof and floor to

prevent fouling: the airspeed over the tips of the blades therefore fluctuates severely during a

revolution. If the windmill were removed, the turbulence level would be expected to decrease

appreciably and might permit laminar-flow tests up to the maximum speed of the tunnel, but some

attention would have to be given to the corner vanes in order to produce an acceptable mean velocity distribution. The components normal to the airstream are probably caused largely by worldng-section vibration.

3. 9 f t x 7 f t Tunnel (South) 2 (Fig. 3).

There are two 9 f t x 7 ft Tunnels in the same building, identical except that one has right-hand corners and the other left-hand. The maximum speed is 230 ft/sec. The contraction ratio is 4:1, the same as that of the 13 f t x 9 ft Tunnel, but the 9 ft x 7 ft Tunnels have practically no settling length after the honeycomb. The longitudinal-component intensity rises only slightly with speed from about 0.17% at 60 ft/sec to 0.2~o at 200 ft/sec (Fig. 14), and no resonant peaks were detected even though this tunnel, like the 13 ft x 9 ft, has audible resonances at several speeds. Clearly the chief sources of the velocity fluctuation are unsteadiness in the return circuit (the corner vanes are known to be inefficient) and turbulence shed from the honeycomb. In the 13 ft x 9 ft Tunnel, the long settling chamber reduces the latter, and the higher power factor (0.32 as against 0.22 for the 9 ft x 7 ft Tunnel, due to the absence of a diffuser before the first corner) may help to reduce the former.

The lateral-component intensity also rises slightly with speed, from about 0 .18% at 70 ft/sec to 0.22% at 200 ft/sec (Fig. 15). The spectra (Figs. 16, 17) are unremarkable: the longitudinal component has a large low-frequency contribution absent from the lateral component, so that the return-circuit unsteadiness, which doubtless originates as a fluctuation in flow direction caused

by separations from corner vanes and diffuser walls, is reduced by the honeycomb to a longitudinal pulsation without any large fluctuations in flow direction. A similar but less pronounced effect is noted in the C.A.T.

4. 18 in. Anemometer Tunnel 8.

This tunnel has an open circuit, with a contraction ratio of 3.2 to 1. A honeycomb is fitted in the settling chamber. The fan has two blades. The maximum speed is 85 ft/sec: as the tunnel is frequently used for anemometer calibration down to very low airspeeds, perforated diaphragms can be fitted at the rear of the working section to improve speed control by using a low-blade-angle fan and keeping its thrust and speed high; sensitivity to draughts in the room is also reduced.

The longitudinal-component intensity in the unrestricted tunnel (Fig. 18) has a peak of about 0" 28% at 25 ft/sec and thereafter remains approximately constant wi th speed at about 0.2% : slight peaks are noticeable at about 45 if/see and 67 ft/sec, caused by resonances at the fan-blade frequency as seen from the measured spectra (Fig. 21).

The lateral- and vertical-component intensities (Fig..~20) decreased with increasing speed from 0.35% at 20 ft/sec to 0.27% at 75 ft/sec. The spectrum (Fig. 22) indicates one predominant source of turbulence, dearly the honeycomb.

The longitudinal-component intensity with diaphragm 2 fitted is about 0.1% at the lowest speeds, less than that of the empty tunnel at the same fan speed, but rises to 0.25% at the maximum speed, which is now 13" 5 if/see.The spectrum at this speed shows a very large contribution at the fan-blade frequency, with noticeable components at twice and four times that frequency.

The longitudinal component of turbulence with diaphragm 3 fitted rises from 0.2% at low speed, with occasional peaks, to 0.5% at the maximum speed of 3.8 if/see. The fan-blade frequency and its harmonics again make the largest contribution to the spectrum.

With diaphragm 4 fitted, the longitudinal component (Fig. 19) rises from about 0.4% to 1.3% at the maximum speed of 1.27 ft/sec: the speed ranges of the various diaphragms overlap so that the regions of very high turbulence need not be used for normal running. The fan shaft frequency now appears in the spectrum, indicating purely mechanical vibration due to unbalance of the fan or motor.

Spectrum peaks were also observed at certain fixed frequencies with all the diaphragms. The most important of these are at 33, 50, 60, 75 and 130 c/s approximately, and probably correspond to structural vibration frequencies or alternatively organ-pipe acoustic oscillations. The distance from the diaphragm position to the fan is 9" 2 ft and that to the bellmouth 11 ft, corresponding to half-wavelengths of 61 and 50 c/s standing waves.

Conclusions.

Tunnel

C.A.T. 13f tx 9ff 9ft x 7ft 18 in. Anemometer

U U -+ : increasing speed

0"3 ->0"6 0'05 -+0"18

0"18 1 "0 ~ 0"2

0-3 ->0-7 0"06 -+ 0-23

0-20 0"3

Special Causes

vibration at high pressures windmill (z7 component only)

fan vibration when diaphragms installed for low-speed running

(86834) A* 2

Tunnel Hot-wire System Spectrum Analyser

C.A.T. 13ft x 9f t 9f t x 7 f t 18 in. Anemometer

const, temperature const, current const, current const, current

B + K passive fikers Muirhead Wien bridge B + K passive filters Muirhead Wien bridge

In all these tunnels, particularly the C.A.T. and 9 ft x 7 ft Tunnels , the turbulence is high by

modern standards because of the absence of damping screens, compensated in the case of the

13 ft x 9 ft Tunne l by an unusually long settling chamber.

Installation of screens would be very difficult in the C.A.T. but less so in the 9 ft x 7 ft or

13 ft x 9 ft Tunnels : the 18 in. Tunne l was designed to accommodate screens if required.

T h e analysis of the turbulence in the 13 ft × 9 ft and 18 in. Tunnels also illustrates the importance

of the fan and]or windmill as a source of disturbances. This is a difficulty common to all fan-powered

wind tunnels, even when a low random-turbulence level is not specially required, as unpleasant noise

or even structural fatigue may result: unfortunately considerations of space and expense often

prohibit the use of a sufficiently low fan speed to avoid vibration.

6

No. Author(s)

1 P. Bradshaw and R. F. Johnson . .

2 E . J . Richards and F. Cheers . .

3 C. Salter . . . . . . . .

REFERENCES

Title, etc.

Turbulence measurements with hot wire anemometers.

N.P.L. Note on Applied Science, No. 33. H.M.S.O. 1963.

Notes on the N.P.L. 13 ft x 9 ft and 9 ft x 7 ft Wind Tunnels.

A . R . C . R . & M. 2136. July, 1945.

Low speed wind tunnels for special purposes.

NPL/Aero/155. 1947.

O0

! )

FIG. 1. Plan view of Compressed Air Tunnel.

f

-~ oi.io,°g

~k. " [ Working I J ~_~" I section ~ I Contraction

rotio I ~.~- .I ~ 1 4:1 I

k . " I

(9 Hot-wire stat ion j

~a,,-,ectiono, ?"/XII h, ~'~'~'~ wi'ndmill /I / ~ )k I\ ,"~

L_-~--=-z-~7 1 _-~.~_ ,_~ .~7

40J ~jj ~ j Corner

. ) j ] VQ nl~s

Honeycomb Jjj • %

J

130

FIG. 2. 13 f t x 9 ft Wind Tunnel.

, , , w°0m,, / 1 ~ Mot°.

f ( . r . 11

~-"- l I ~j,~anao '2- Working I 1 k. ~- R ra t io 4 : I i ~¢ctJon J j , I Ho.~y°omb ~

2 " ~ ~ ' ' " Q Hot-wire station

831

FIG. S. 9 ft × 7 ft Wind Tunnel.

G 3"08'to well

I I J Arca ra t io J 3 . 1 6 : I I t /

11 '02 t

FIG, 4,

I Hot-wire I ® s tat.ion I I

Screens

, I \ Diaphragm

Motor

18 in. Anemometer Tunnel,

9 ' 2 0 1 l--

To window 4 ' l O t

, ste~l 3 II x I")

Ho l low steel aero foi l strut

I supports

Screwed to f l oo r

F I G . 5 . Hot-wire mounting strut.

10

0 . 7 "

0 . ' 6

02 5

5-% o 0 " 4

0 " 3

0 " 2

0 ' 1

o o o ~ "._~ • ~ "i I ~" ~ - - o ~ _ ~ . . . . ~x ~ : ~

4

I 2 3 4

xj ; , / - / o a" ~ f / 0

7

%

f /

/

f / 2 - - - - - -

0 >

o Pressure 97"6 p.s.Eg. x ,+ EO0 p.s.i.g. n 208 p.s.i.g. 0 553 p.s.i.g.

The curves indicate the trends at approx. [00, ZOO and 350 p.s..i.g.: the tunnel is not normally run at less than half the maximum speed at any pressure.

I I I . I I 8 9 10 II 12

U Reynolds no. per ft~ ~-- x I0 -6

FIG. 6a. u-component r.m.s, intensity in C.A.T.

0,7

0"~

0'$

U

0"4

0-3

0,2

O-i

x

x

0 I 2

x Zero incidence~L 22 e incidenceJ 200psi9

4 5 5

x ~6 6 u

7 8

Fro. 6b. u-component r.m.s, intensity in C.A.T.: effect of changing model-wing incidence. 49 ° swept wing, area

31- ft ~, aspect ratio 3.

11

(86834) A * *

0"7

0"6

0 5

Vo U~ 0'

O.

o o

/ /

2~ ! /!

o

J

/

J

~ x ~ o 50 I~s,i.g. x 190 l~s.l.g. * 340 Rr~i.g. x

0 .1

2 3 4 5 6 U -67 8 9 x l 0

FIG. 7. v-component r.m.s, intensity in C.A.~ ~.

IZ"

12

0-001

0'0001

0.00001

0"000001

\

Fan b l a d e ~ ~ frequency

I 07 p.sj:~" 78 f t/s~c

700 r.p.m

I I I I I I i I I I I I I I I I I0 I00 I000

t c1$

I I I I I I I I0000

Fla. 8. u-component spectrum in C.A.T.

13

0"001

0"0001

0'00001

0o000001

I0

32 7 p.s.i.g.

45 f t / sec

400 tp.m,

\

I I I f I I f I I i | I I f I I ! f I l i t I00 lO00 IOOO0

f c / t

Fie. 9. v-component spectrum in C.A.T.

1¢

0-2

0.1

/

o u - p r o b e

[]A t x-probe

o o

• L J U o

0 20 40 60 80 I00 120

FIC. 10.

140 160 180 200 220 240

UZ--° f t / see 1/

u-component r.m.s, intensity in 13 ft × 9 ft Tunnel .

0.3

O'Z

0,1 / /

x v - component

1- w -component

X ~

j

/

ZO

FzQ. 11.

40 60 80 - I00 120 140 160 180 Z00 2ZO u 1/o

.-~- f t / s e c

v- and w-component r.m.s, intensities in 13 ft x 9 ft Tunnel .

i5

200

150

(~ F)'/~

I00

50

Blade frequencies I Z 3 4

Windmill

I / III

I 2

I I Fans

arbitrary

scale

Speed 141 R/sec

Fans 540 r.p.m.

Windmill 223 r.p.m.

.-A

S J\ \

, t , i t i l l t i i v i , , ,

I 0 f c / s I 0 0

\ _ _ i , . . . . .

I000

b itrary tale

/

Blade frequencies 2 3 4 ] I Windmill

, , , ,, , 10

I 2

I I

I Speed 141 f t /sec

Fans 5,42 r.p.m. Windmill 223 r.p.m.

i00 f c/s I000

(

...... i0000

FIG. 12. u-component spectrum in 13 ff x 9 ft Tunnel. FIG. 13. v-component spectrum in 13 f t x 9 ff Tunnel.

0.~

0 . 2

U

0.1

g } u-probe

x x probe

O O

o x

[]

D XO 0

XO n O O

rl x

o n ca

ta o o

o

SO I00 U f t / s¢c 150 ZOO

Fla . 14. u-corn ~onent r.m.s, intensity in 9 ft x 7 ft Tunnel.

0 . ;

U

0 . 1

FIG. 15.

X

X

50 I00 u ft /sec 150

v-component r.m.s, intensity in 9 ft x 7 ft Tunnel.

200

17

O 0

O. 001

0 - 0 0 0 1

0 " 0 0 0 0 1

0.000001

I0

\

Speed 1 6 5 f t / s e c

i i I i r I I I 1 0 0 I I I I I I t i I 0 0 0

f cls

I I i I i I I I I 0 0 0 0

Speed 165 f t / s¢¢

I I I I i I I I I I I I I I I I I I I l I I I 0 0 I 0 0 0 I 0 0 0 0

f c/~

Fla. 16. u-component spectrum in 9 ft x 7 ft Tunnel. FIG. 17. v-component spectrum in 9 ft x 7 ft Tunnel.

0 " 4

0"3

0.2

0"I

0"5

U% 0'2

0"I

I ! -5 -4 DOWN

Vertlcal traverse Jn working section

At U =59.4 f t /sec

I T I I I ! - 3 - 2 - I I 2 4

inches UP

U % (on ~ent~ai~e) ] ~ , , I Ou:put meter reading

f luctuat ing between these I[ml'ts

/ f - - - ,

t t ~ . I

I0

FIc. 18.

20 3 0 4 0 5 0 6 0 7 0 ~ 8 0

u-component r.m.s, intensity in Anemometer Tunnel. Unrestricted tunnel.

O f t / s e ¢

J-2

t . 0

0,8

0,d

0"4

0-2

~-°/o

/

x- ~ x )

/T\ / ' x ~ / x ~ ~-- ×~..

x._~

0-1

FIQ. 19.

0'2 0-3 0"4 0"5 0-6 0'7 0'8 0"9 I '0 I . I 1"2

U t t / s e c

u-component r.m.s, intensity in Anemometer Tunnel. Diaphragm No. 4.

i9

0 . 7

u_,v ,'~ % U U U

0-5

0 " 4

0"~

0,2

0"1

v w I

u - p r o b~ measuremen ts

( f o r comparison)

' f K K T ,5.

.,,. O t,. A

o"'- o . _ O , _ j ~ . o

0 I0 20 30 40 50 60 70 80 U

FIc. 20. v- and w-component r.m.s, intensities in Anemometer Tunnel, and u-component measure-

ments with x-probe. Unrestricted tunnel.

f t ] 5~C

20

150

100

50

arbl+.rary s c a l e

0 A

F I G . 2 1 .

Fan-blade 't'requency

I S p e e d 46.4 f t l ; ¢ c Fan 1500 r.p,m.

. . . . . . i o . . . . . . ;oo f c/s

i i s i , ~ i . . . .

I000 - I0000

u-component spectrum in Anemometer Tunnel. Unrestricted tunnel.

150

I00

50

0 3

a r b i t r a r y scale Fan-blade frequency

I

J IO

• 1 ~ r i i ,,

Speed 45"8 f t /sec

Fan 1538 r.p.rn.

, , r i i , , , , i r l l

f c/s I00 I000 lO000

FIG. 22. v-component spectrum in Anemometer Tunnel. Unrestricted tunnel.

21

(86834) W t . 64/1857 K 5 4 /68 I-Iw.

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Special Volumes Vol. I. Aero and Hydrodynamics, Aerofoils, Controls, Flutter, Kites, Parachutes, Performance, Propulsion,

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