Suppression of refraction in jet noise by cooling

May, 1975

by

J. T. Ke1sa11

Kluy'ri. EJ 1

UTIAS Technica1 Note No. 194 CN ISSN 0082-5263

SUPPRESSION OF REFRACTION IN JET NOISE BY COOLING

by

J. T. Ke1sa11

Grateful mention must be made of the advice and encouragement given by Dr. H. S. Ribner and everyone at the Institute for Aerospace Studies.

Contributions by my fami1y and by Mr. M. Haacke must a1so be acknow1edged.

This work was funded by N.R.C. (Canada) and the U.S. Air Force Office of Scientific Research under grant AFOSR 70-1885. Much of this work was carried out under a fellowship from the Transportation Deve10pment Agency (Canada).

SUMMARY

Liquid nitrogen .was used to cool a jet of air (at no.zz1e speeds 0;21 to 0.2S'of·ámbient sound speed) down te temperatures (between -110°C and -155°C) at which the' refraction from the opposed veloei ty and temperature gradient,s 1argely cancelled: thedirectivities both 01; a'pure toneinjected into the

flow-and of narrow band filtered jet noise ,exqibited near elimination of the

refraetive valley observed along the axis of room tempe~~ture'and hot·jets.

·There'·remained in. the jet .noise pattern the "bulge" in downstream emission

owing to convection of the ·eddies. Thus theexperiment demonstrated a

sub-stantial degree of separation of refraction and convection effects in jet noise.

Acknow1edgements Summary

Tab1e of Contents INTRODUCTION EXPERIMENT

The Jet Faci1ity Measurement OBSERVATIONS REFERENCES APPENDIX TABLE FIGURES

tv

i i i i i iv 1 3 3 4 7 9 10 12

INTRODUCTION

Noise generation in an air jet çan,be'modelled bya set .ofsound sourees·with joint directivity as shown in Figyre l(a) following Ribner l .

·The.'effèct of motion of the sources can ·be described in terms·of a convection . faetorc'that modifies the basic directivity to give the pattern shown in Figure

1 flY) .;. -The velocity gradients in the flow refract the sound, gi ving rise tq th'e"eommonly observed final directivity .pict~red in Figure 1 (c). If the air in-t~e j ët flow is at a temperature ot her than ambient, the temperattlre

grad-ient~' likewise alterthe pattern through refraction. In a hot jet the increased

saand speed in the jet flow tends to bend the sound rays away from the axis, deepening the refraction valley. The reverse occurs in a cold jet: . the

··temperature gradients tend to bend the sound rays toward the axis; The refraction valley thus tends to be "filied in',', and in extreme ·cases even

beeomes a peak in intensity. '

In theexperiments to be outlined here the effects of convection and refraction on directivity are examined for a jet of very cold air. More particularly, an attempt is made to determine whethe~ anyinterdependence

betwe~n convection and refraction can be detected experimentally in nay;row

band jet noise data.

The Mach numbers and frequencies used in this work were to some extent arbitrary. This cannot be said for the temperature: it was chosen

tO,minimize or if possibleeliminate tneeffect of refraction. The concept

involved opposing temperaturegradients to velocity gradiEmts in such a way that their effects on refraction tend to cancel,as was done in earlier work.by Keisall for low speed jets. In other words, a balanceis reached between the peak in intensity on·axisfor very cold jets and.the refractive valley of warm jets. Elimination of refraction would allow the direct~vity due to convection alone to b~ measured. .

The choice of temperature was made by means of the simple condition

(1)

which was taken as a crude approximation of the condition for zero refraction effect; the purpose was to provide a roughguide to the required amount of jet cooling which would be dictated bycj~ It did not seem worthwhile even to attempt calculations by ray acoustics, since they become invalid at these · lóng wavel~ngths,and the great complications of wave acoustics (cf. Schubert 3) did not see~ warranted. '

The convection factor by which the basic directivity of the jet ,is multiplied is

c-

s where

(2)

and Mc

=

_{Uj/2co is taken as the convection Mach number of the} ,noise producing

eddies and a is taken a~ 0.55. For Mc <.2 the factor a2Mc2 is negligtbIe.

Thus the calculated'difference in decibels between the noise levels at

e and_those at 900

to the jetaxis due to the effect of convection is

approximately 50 loglO (1 - Mccose). This 'convection factor" is tabulated

in Table 1 for a variety of Mach numbers at e

=

35? The corresponding

temperature estimates for refraction cancellation from (1) are also tabulated.

In ,part of the investigation an injected pure tone,at the centre

frequency of the frequency bands of jet noise studied was introduced into

'the j et flow througl1 a narrow probe. This "point sO'\lrce" of sound was used

earlier'in our laborat;pry by Atvars et 'al.S6 , and others 7 ,8, to explore'the

role of refraction in the directivity of jet noise. As ~he probe is

station-ary the pure tone directivity will not be affected by convection, but will be

changed by refraction by the jet flow. MacGregor et al. 7 h~ve pointed out

that since the spherical directivity of the injected pure tone does not match

the _basic directivity of the jet noise (Fig. IA), it will not be refracted

in exactly the same way. However, in the case of refraction cancellation by

cooling, it will be seen that the directivity of the jet noise is within 3dB

of being spherical. In MacGregor et al. 7 a 5dB variation introduced a corre'

c-tion of less than 1/2 dB. This is considered insignificant for the

3

EXPERIMENT

The Jet Facility.

The jet apparatus used in this experiment was described by Atvars 6 .

Compressed air was arranged ,to flow into a settling chamber and out through

a 3/4" (1.9 cm) diameter nozz1e. The inside of the sett1ing chamber was insulated at different points with either 3/8" fiberg1ass or neoprene foam. Absorption of the liquid J)itrogen entering the jet was prevented by a 1ayer of

,sheet a1uminum covering_~lle ,in~1l+ati0n, in the intake region. Liquid nitrogen ,

inj ected into the compressed air , line 'V 1/2 m upstream of the jet flowed up with theair through an' insulated corrugated pipe into, the ,settling chamber. The corrugations provided extrasurface area and roughness to facilitate mixing.

The nitrogen was·fed in through two plastic tubes from a,25 t dewar

which was' kept ata regu1ated pressure wi th dry compressed air. The pressure

could be adjusted and was usua11y kept ,between 15 and 35 cm of mercury. Nitrogen was consumed at a rateof between 1 and 3 liters/minute.

The air pressure was controlled by a series of regulators in the compressed air 1ine, the last of which was connected to tqesettling chamber in a feedback loop. This was necessary to compensate for instabi1ities in the nitrogen feed which were causing the pressure to oscillate and to drift. A1though the jet pressure still occasiona11y varied sinusoida11y about an

average va1ue, the drift was removed. More will be said about this problem

later.

Noise from 1eaks in the jet app~ratus was measured with the nozz1e

plugged and found to be more than 10dB lower than the jet noise over the

range of pressures used. This, however, was not sufficient. It was discovered

that under dynamic·conditions the turbu1ence in the corrugated plastic intake

1ine also generated disturbing noise. The intake was according1y wrapped in '

fiberg1ass and lead, which effective1y eliminated this source of noise.

There are two effects that cool the jet. First of all thecompressed

air (and the jet p1umbing) vapourizes the nitrogen which ,then.addsvery cold

. gas-tG the' flow. ' Ni trogen and compressed air both contribute about, the same volume

of

gas tO,the 'flow. Beyond 'a certain 'point extra.1iquid nitrogen

added'simp1y isnJt vapourized. The'compressed air mass flow is decreased by

'increasednitrogen flow and thismeans that less heat is availab1e fr om the

air to vapourize the nitrogen. Thus the jet temperature is self limiting at a

particular sett1ing chamber pressure. Indeed, the on1y way tocool the jet down

further is to increase the sett1ing chamber pressure. Natura11y this increases

the jetvelocity, 50 that a still lower temperature is required to cancel

refraction. The observations were carried out down to this limit·for settling

chamher 'p'res'sures of 70 and 90 cm of water, and 12 cm of mercury.

Loca1 Mach number (jet exit velocity divided by the speed of sound at the jet exit temperature) was ca1cu1ated from the pressure in ,the jet .

settling chamber, which was measured with a water or mercury manometer. The regulators kept this pressure constant within 2%. A differential manometer was used to demonstratethat this pressure equalled the nozzle stagriation pressure measured with a small probe inserted into the potential core.

The temperature in the settling chamber of the jet was measured with a differential iron-constantan thermocouple with the reference·junction measuring the ambient temp~~ature in the anechoic chamber. Ambient temperature was.also measured with a mercury .thermometer.

The differential arrangement was used because it is the difference in temperature with respect to ambient which determines the refraction. As the thermocouples are almost ·linear, an error of 10°C in determining the ambient temperature would only affect the differential measurement by 2°C. Variations of this amount were allowed in the jet temperature. For this reason changes in ambient temperature (usually about lO°C) during a run c0uld be ignored. The thermocouple voltage was measured with a radiometer spot galvanometer. The system was calibrated by placing one thermocouple in liquid nitrogen at -196°C.

The reference thermocouple was placed at the nozzle while running and showed that, as expected, isentropic cooling was.negligible at the .pressures used in this work. An oscilloscope was attached to thenormal therrnocouple

arrangement while cooling the jetand no liquid nitrogen was reaching the nozzle, as no sudden voltage changes were observed.

With the pressure and temperature measured, Mach number with respect to arnbient could be calculated easily.

Measurement

The pure tone,point source of sound described by Atvars et al. 5 was inserted into the flow to rneasure.the effect of refraction alone. For measure-ment of jet noise (in a narrow frequency band centered at the frequency of the pure tone) the point source was removed, as the flow of air past the point · source probe generated excessive unwanted noise.

The remotely controlled microphone boom could be rotated in SO increments in the horizontal plane about the point source probe located on axis 8.5 cm downstream of the jet nozzle. At no point did the measured rate· of change of intensity of jet noise with angle .exceed 1/2 decibel per degree. Usually it was weIl below this value. In these cases an accuracy of 2° in microphone positioning was considered adequate. Whenever this rate of change was large enough to cause an appreciable error such as, for instanee, in the refractive valley of a room temperature jet, special care was taken to verify the measurements.

An effect dueto negative buoyancy was pointed out by GrandeS;. a horizontal cold jet tends to droop due to its increased density relative to the air in the anechoic chamber. This downward curvature can lower the

5

entire refractive valley so that a microphone .on the axis of the jet nozzle is no longer in its centre. Moreover the lack ofaxisymmetry probably reduces the de.pth of the refractive valley. These problems should not be serious for the experiments described here. Grande used a colder jet with a maximum Mach number of .112 .. The jet described here is usually run at at least twice this Machnumber. Extrapolation of Grande's data indicates that the droop should be less than one degree in the present case. Visual observation of condensed water vapour in the cold jet confirmed this conclusion. With intensity changing by less. than 1/2 dB/degree thedroop effect should not affect the results

significantly. Manually varying the microphone position around 0° showed this to be .true ..

A 1/2" Bruel and Kjaer type 4133 condenser microphone with nose cone was mounted with the diaphragm 179 cm from the point source probe, aboutwhich it was rotated. Th~ flow speed at the microphone was found by extrapolating stagnation pressure measurements at the nozzle and a short distance downstream to be .~ 5.6 m/sec. The nose cone is supposed to reduce this wind noise within a 1/3 octave .band sound level ·below 25dB9 over the frequency range used. The jet noise was .more,than 10dB .above this level at the f~equencies of measurement.

Tha microphone signal was passed through apreamplifier and a power .supply before entering an HP 3580A spectrum analyzer. The analyzer was set to examine a single .frequency in the linear mode. For jet noise data a band-width of 300 Hz was used. For pure tone point source data the pointsource was driven with a signal from the analyzer set in the cent re of the 10 Hz bandwidth examined. The output for the y-axis of the analyzer display was fed into a Bruel and Kjaer 2305 level recorder with a 50dB potentiometer. Setto record DC levels, this recorder converts the linear signal to a log plot. By setting the writing speed to 4 mm/sec an effective averaging time of 5 sec is achieved lO which removes short term variations caused by jet.turbulence and instabilities. Timing pulses from the recorder rotated the boom once every 5 seconds. Comparison of jet noise.data taken with these settings and data taken with longer gaps between rotations and higher writing speed showed no significant change. The data were also found to be consistent and reproducible.

Useof the Bruel and Kjaer recorder involves error in that the aver-aging of the p2 signal takes place af ter the log conversion to dB, from damping of:the pen.motion. A rough analysis in the Appendix.predicts an error of

~4 tQ -SdB when the double amplitude of signal fluctuation is lSdB. Signal swings of this order were indeed characteristic of the jet noise data; however, theiF amplitude was reasonably constant over the range of survey angles .

. Hence, it was conc~uded that the readings would be uniformly low by 4 to SdB so that the shapes{).~ the measured directivity polars would have little. error.

As a check on the validity of this notion, the p2 averaging was accornplished electronically Cby an RC filter) prior to the log conversion to dB in the Bruel and Kjaer recorder. The curve of dB v.

e,

although shifted upward, showed no significant difference in shape.

At .the higher settling chamber pressures instabilities in the

This ha~p~ned quite of ten at a settling chamber pressure of 12 cm of mercury, and occasionally at 90 cm of water. Careful examination of the output indi-cates that t~~s . p),l.1~it:lK m~~_es very . .1i tUe difference to the resul tso The

averagingJ::ime of the recorder is much longer than the .period of the oscillations . The feedback loop, on the regulatqr keep$ the averag~. air pre~s~re at the set

level. Observations with and without this pulsation appear identical and no observablechange is seen whenthese pulses start. For these reasons no

attempt was made to differentiate between .the data from pulsing and from stabIe

7

OBSERVATIONS

In what follows, jets at ambient temperaturewill, for brevity~ be referred to as "warm" to distinguish them fröm the "cold" jets. Figures SA

.. and' B show spectra taken at ang~es of 0°, 25° and 90° for warm jets. Figure 4 shows the directivity of these jets. They show the effect of refraction in.

lowering the intensity at 0° in the higher frequencies. The lower frequencies

'show the effect .of convection inraising theintensity at 25° over that'at

90~. As these spectra were taken only to serve for comparison with results

for a cold jet, no extraordinary care was taken to ensure exact placement of themicrophone; For the cold jet, and for the 25° and 90° spectra of the warm jet, this inexact placement should have introduced no appreciable error, as outlined in the last section. At 0°, however, the spectrum which would be measured exactly on.the jet axis could be significantly.lower than a spectrum taken slightly offaxis. This is especially true at higher frequencies.

At 3000 Hz and below, the microphone was not cQmpletely in the far field at 0° so that .pseudosound the turbulent pressure field within the jet -contributed significantly to the measurements. This is apparent inthe sharp rise in intensity at low frequencies. Figure 4E shows the effect of this pseudosound for a plot of intensity vs angle taken at.3000 Hz. This pseudo-sound problem was the main,reason that 4000 Hz was chosen for most measurements, rather than a lower value. Lower frequencies would have been preferred as

thesmaller effect of refraction at these frequencies make it more manageable. Convection effects are also more marked at the lower frequencies.

Figure SC shows the effect of refraction cancellation Cby opposed temperature-velocity gradients) on the 0° measurement for a cold jet. The hashed area on this and Figure SA indicate the range within which the 0°, 25° and 90° measurements feIl for this attempt at refraction cancellation. The setup used for this measurement was not sensitive enough to exhibit the

effect of .convection. The substantial agreement of the 0°, 25° and 90° curves indicates approximate cancellation of the refraction.valley centered at 0° occurring over a large frequency range.

Figure 6 shows the effect of cooling the jet in attempting to achieve refraction cancellation of the narrow band jet noise. Sufficient cooling was notpossible; however, the refractive valley has been greatly reduced in depth. The convectivebulge can be seen to remain, and at 35°, where refraction effects can be neglected, the agreement is very close.between the warm and cold jet data. Table I compares the ·convective bulge in cold jets with the theoretièal value calculated at 35°, The temperatures used are compared with the crudely estimated temperatures for refraction cancellation. They are found to be 25-50% lower than anticipated.

For comparison, we explored the case of an injected pure tone. Figure 7 shows,the effect of refraction on its directivity'in a cold jet at a representative Mach .number and' temperature, The intensity at 0° equals the · intensity at 90°, resembling the jet noise data. However, a marked bulge extending out to about 20° indicates that the cancellation at small angles is imperfect. Th~s distortion does.not seem to be present in the jet ,noise

data, due perhaps to the difference between the unrefracted directivities of

the two kinds, of sourees. In addition, the convective bulge found in the

jet noise pattern (most evident in the 20°-60° range) is lacking; this is as it should be, since the injected souree is stationary.

9

REFERENCES

1. Ribner, H. S., Turnbull Lecture. Jets and Noise, Canaqian Aeronautica1

and Space Journal, 14, pp. 281-298, (1968).

2. Kelsa,.ll, J. T., Suppression of Refraction in Jet Noise, Proceedings of

Second Interagency' Symposium on University Research in,Transportation

Noise (June, 1974).

3. Schubert, L. K., Numerical Study of Sound Refraction by a Jet Flow. 11

Wave Acoustics. Journalof Acoustica1 Society of America. 51, pp.

447-463 (1972).

4. Ffowcs Wi11iams, J. E., The Noise from Turbu1ence Convected at High Speed, Phi1osophica1 Transactions of the Roya1 Society, A 255, 469-503, (1963).

5. Atvars, J., Schubert, L. K., Grande, E., Ribner, H. S., Refraction of

Sound by Jet ,Flow or Jet Temperature. Univ. of Toronto, Institute fer

Aerospace Studies, TN 109 (NASA CR-494 (1966)). (1965) .

6. Atvars, J., Refraction of Sound by a Jet Velocity Field, M.A.Sc. Thesis, Univ. of Toronto, Institute for Aerospace Studies (Oct. 1964).

7. MacGregor, G. R., Ribner, H. S., Lam, H., "Basic" Jet Noise Patterns Af ter De1eti.on of Convection and Refraction Effects: Experiments vs. Theory,

Journalof Sound and Vibration, 27, (4), pp. 437-454. (1973).

8. Grande, E., Refraction of Sound by Jet Flow and Jet Temperature. Extension

of Temperature Range Parameters and Deve10pment of Theery. Univ. of Toronto

UTIAS TN 110, NASA CR-840. (1967).

9. Brue1, ,P. V., Aerodynamically Induced Noise of Microphones and Windscreens ,

Brue1

&

Kjaer Technica1 Review, 1960 #2.

10. Broch,'J. T. and Wahrman, C. G., Effective Averaging Time of theLevel

APPENDIX

With a single ,level recorder an error can be introduced into the recording depending on·the method of smoothing or averaging. There are two main possibilities of averaging the signal (~p2) and arriving at ,a decibel or logarithmic average:

dB_AV

=

10 log

PT

Cl)

(2)

(with a suitable choice of the reference for zero dB). Option Cl) is correct and (2) - implemented in .some ,level recorders '- is at best an approximation.

In order to estimate roughly the order of error in dBAV incurred in option

(2),

we model thefluctuation in p2 as a square wave:

-l - r-- l - r - -

Pt

p.

-

" - >--

>-f,

dB2

=

10 log P""Cl+e) dBl

=

10 log P""Cl-e) e

=

(n-l)jn+l dBl + dB2 10

10gl~1-e2)

(dB) AV = = 2 2 = true dB AV + 5 loglO Cl-e 2) Error in dB

₌

5 loglOCl-e 2)

APPENDIX (continued) n E 4 3/5 5 4/6 . . . 10 . 9/11

.

, 20 19/21 30 29/31 100 99/101 Signa1 -Pluct. dB 11 10 log n. 6.0 7.0 10.0 13.0 14.8 20.0 : 5 Error in Mean dB log(l , --1.0 -1.3 -2.4 -3.7 -4.5 -7.0 E2 )

TABLE 1

·Mach . T ambient

-

Tjet Effect of Convection

Figure

_No.

_{(CO) at 35° (dB re 90°)}

Estimate Expt. Theory Expt.

*

6a .21 106 145 2.0 1.5 6b .22 115 162 2.1 0.5 6c .22 106 134 2.1 1 6d .23 115 155 2.2 2.2 6e .23 106 123 2.2 1.5 6f .24 . 115 145 2.3 1.5 ~g .25 115 134 2.3 2 6h .28 140 162 2.7 2

A B C ____ COLD JET " . -... BASIC FIGURE 1 WARM,JEl' /

'

/

'

'"

... _... CONVECTION REFRACTION

JET NorSE AS A BASIC PATTERN NOT VERY DIRECTIONAL -WHICH IS POWERFULLY MODIFIED BY CONVECTION OF THE

EDDY SOURCES AND REFRACTION OF THE SOUND WAVES BY THE MEAN FLOW,

F~2: EXPERIMENTAL SET UP SPOT WATER MANOMETER GALVANOMETER L..--_..J ROTAWR CONTROL : ..

_{. .}

~ . I : . . .. I . ..• I _,

.

11, SCREENS

- - - - l

BOOM ROTATOR BOOM

I

I

I

STEEL WOOL INSULATED SBTTLING CHAMBER III III 111 MICROPHONE WITä~---"T---' NOSE CONE I COMPRESSED _ AIR LINE _ _ _ .J. JET POINT .SOURCE

I

I

I

I

I

I

LIQUID

I

DRY NITROGEN COMPRESSED -~---_... DEWAR AIR

I

I

~ ANECHOIC CHAMBER

I

~---~---.---~ SPECTRUM ANALYZER LEVEL RECORDER

2....@. DIVISION ~ DIVISION .0° 3A

[0: " · --.."

=0,

3B FlGURE 3, 4KHZ. PURE TONE AVERAGE OF 3.9, 4.0, 4.1 KHZ. DIREX;TIVITIES 4KHZ. NARROW BAND (300 HZ.) WHITE NOISE

DIREX;TIVITY OF POINT SOIJRCE WITHOur JE]'

3e

JidB DIVISION

30

FlGURE 3:

DIREX;TIVITY OF POINT SOIJRCE WITHOur JE]'

90°

10 KHZ. NARROW BAND WHITE NOISE (300 HZ.) WITH TREATED DUeT

loó

~

.

DIVISION • ~ ~. DIVISION ~

10

0 DIVISION • 5dB DIVISION 0° •• • • • • • _• • • • • f t 4A

.

_.

·

. ·

PSl!lJOOSOUND 4B •

. · . . . . · .

• • 4C • • • • • _{• • •} • •••• 4D FlGURE 4: •

.

. M ... 43 90° 4000 "Hz. M = .43

•

•

90° 3000 HZ. • I • M~; .43 90° 0000 Hz • oM . . . 34 90 4000 Hz. +

ROJM Tn4PmATURE (+10°0) JE!' NOrsE DIREm'IVITY

1

0' • 5dB DIVISION

,

l'

~OO

5dB DIVISION • 5dB

lOd

DIVISION 5dB DIVISION

• • • •

~"

••

g~! & , . Q.t. 90° 4E •

.

.

.

.

_• • _{• •} • • •

•

f t 90° 4F

. .

. . .

.

.

_.

_{. .}

_,

_.

_, 90° 4G

. . .

.

.

90° 4H FlGURE 4: M • • 31 5000 Hz • M ... 31 4000 Hz. M = .31 3000 Hz. MOl .23 4000 Hz.

~~r>S;

~V.aU

1 KHz _11KHz 5B 0° 10dB 25° > DrViSIëiN 90°

f

<:::::::::::: I KHz II KHz 5C 10dB DIViS'ÏON 0° 250&9OO::~~ 11KHz 1 KHz FlGURE 5: U. = 115 m/sec J U.= 105 m/sec J U.= 74 m!sec TJ= _ 1520C

SPECTRA OF JEl' NOISE: Linear Scale, 300 Hz. Bandwidth

5dB DIVISION COLD JEl' AVERAGED DIRECTIVITY FROM MEASUREMENTS

OF NARRO.i BAND JEl' NOISE

90° GASES AVERAGED: 6A U. = 71 m/sec J T = -135°C (COLD) U_j = 74 m/sec T = -152°C (COLD) 6B U_j = 74 m/sec T = -124°C (COLD) 6C U. = 78 m/sec J 0 T = -145 c 6D

CASES e-h PLOTTED SEPARATELY

5dB DIVISION 5dB DIVISION 5dB ·DIVISION 5dB DIVISION 5dB DIVISION 6E 6F 6G 6H FIGURE 7: 6el M ... 23 6f: M ... 24 6g: M ... 25 6h: M ... 28 FlGURE 6:

NARRCM BAND (300 Hz) JEr NOISE D:nm::TIVITY AT 4000 Hz FOR WARM & COLD JErS

AVERAGE OF DIRECTIVTIIES FOR 3.9, 4.0, 4.1 Khz IN.m:TED PURE TONES IN WARM AND COLD JErS

REPORT DOCUMENTATION PAGE _{BEFORE COMPLETING FORM} READ INSTRUCTIONS

I. REPORT NUMBER 2. GOVT ACCESSION NO.

I

f'

RECIPIENT'S CATALOG NUMBER

AFOSR-TR-75-0908 ADA012015

4. TITLE (and SubtIlIe) 5. TYPE OF REPORT a PERIOD COVERED

INTERIM SUPPRESSION OF REFRACTION IN JET NOISE BY _6.

PERFORMING ORG. REPORT NUMBER

COOLING

Technical Note No, 194

7. AUTHOR(s) 8. CONTRACT OR GRANT NUMBER(s)

J. T. KELSALL _{AFOSR 70-1885}

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT, PROJECT, TASK

AREA a WORK UNIT NUMBERS

UNIVERSITY OF TORONTO

681307 INSTITUTE FOR AEROSPACE STUDIES

4925 DUFFER IN ST., DOWNSVIEW , ONT. CM ADA M3R 5T6 9781-02 61102F

I!. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORl tn>.ïE""

M~ 1975

13. NUMBER OF PAGES

:::>1

14. MONITORING AGENCY NAME a ADDRESS(if different lrom Controlling Office) 15. SECURITY CLASS. (ol thls report)

UNCLASSIFIED

15a. DECL ASSI FICATION/ DOWN GRADING SCHEDULE

16. DISTRIBUTION STATEMENT (ol thls Report)

Approved for public release, distribution unlimited.

17. DISTRIBUTION STATEMENT (ol the abstract entered In B/ock 20, if diflerent lrom Report) •

18. SUPPLEMENTARY NOTES

19. KEY WORDS (Continue on reverse slde il necessary and ldentify by b/ock number)

1. Jet Noise 2. Refraction 3. Aeroacoustics 4. Acoustics

20. ABSTRACT (Continue on reverse side Jf necessary end ldentlly by b/ock number)

Liquid nitrogen was used to cool a jet of air ( at nozzle speeds 0.21 to 0.28 of ambient sound speed) down to temperatures (between -110°C and -155°C) at which the refraction from the opposed velocity and temperature gradients largely cancelled: the directivities both of a pure tone inj~cted into the flow and of narrow band filtered jet noise exhibited near elimination of the refractive valley o1:>.served along the · axis of room temperature and hot jets. There

remained in the jet noise pattern the ''bulge'' in downstream emission owing to convection of the eddieR ThllR t.hp pxnp'Y'impnt. npmnn"'+ .... "'+."'" '" '"',,'h"'+~,...+~ ~l

ITNCT,ASSTFTED

DO 1 JAN 73 FORM 1473 EDITION OF 1 NOV 65 IS OBSOLETE

UNCLASSIFIED

SECURITY CLASSIFICATION OF TH IS PAGE(When Data Ente,ed)

degree of separation of refraction and convection effects in jet noise.

.

'

tJNCLASSTFIED

~

SlIPPI!ESSIOB OF REFRACTIOB IB JET liOISE BI COOLJ:IG

Kel.sall, J. T.

1.. Jet BoiBe

I. Kel.sall, J. T.

12 pages 7 figures 1. tabl.e

2. Ref'ractiOll 3. Aeroacoustic. 4. Acoustics

II. urIAS Technical. Kote Ilo. 1.91<

Liquid nitrogen YIIS used to cool. a jet of air (at nozzle speeds 0.21 to 0.28 of aDilient sound speed) down to teDperatures (between -llO·C and -1.55·C) at wbich the re!ractiOll trom the opposed vel.ocity and teq>erature grsdients 1.argel.y cancelled: the directiv1ties both of a pure tone injected into the nOl< and of narrow band fil.tered jet noise exh1bited near el.ilIIination of

the ref'ractive valley observed a.long the a.x1s of room teDperature and hot jets. There rema.1ned.

in the jet noise pattern the "bUlge" in downstream em1ssion awing to convection ot tbe edd1es. Thus the experiment d.emonstrated a substaatl&1. degree or separation ot ref"ractian and coavectlan eff'ects in jet nolse.

SIlPPRESSIOB Ol REFRACTIOB IB JET !lOISE BI COOLJ:IG JrelJIall, J. T.

1.. Jet Boise

I. Jrel.8~, J. T.

12 page. 7 f~e. 1 tab1.e

2. Ref'raction 3. Aeroacoustics 4. AcoUBt1cs II. UXIAS Technic&l Nate No. 191<

Liquid nitrogen YIIS used to cool. a jet of air (at nozz1e speeds 0.21 to 0.28 of aDilient SOWld

speed) down to teDperatures (between -llO·C and -155·C) at wbich the refract10n !rom the

~

apposed vel.ocity and teDperature grsdients 1arge1y cancelled, the directivities both of a pure

tone injected into the flOW' and of narraw band tlltered jet noise exhiblted near el.lm1nation of

the ref'ractlve vaJ.l.ey observed along tbe axis or room teuperature and hot jets. There rema1ned

in the jet nolse pattern the ''b\Üge'' in downstream em1ss1on ow1ng to convectlon of the edd1es.

Thus the experiment deIIIonatrated a .ubatant1al degree of separation of refraetion and convection ef'f'ects in jet no1se.

Available copies of th is report: are limite(J. Return this card to UTIAS, if you require a copy. Available copies of this report: are limited. Return this card to UTIAS, if you require a copy. urIAS TU:JIlIlCAL 1IO.rI! 110. 1.91<

~

Institute for Aerospace Studies, University of T oronl:o

SUPPHESSION OF REFRACTIOB IN JET !lOISE BI COOLJ:IG

Kel.sall, J. T. 12 pages 7 f~es 1. tab1.e

1.. Jet Boise 2. Ref'raction 3. Aeroacoustics 4. Acoustics

I. Kel.sall, J. T. II. IJrIAS Technical. Nate Bo. 191<

Liquid nitrogen YIIS used to cool. a jet of air (at nozz1e speeds 0.21 to 0.28 of aDilient sound speed) down to teDperatures (between -llO·C and -155·C) at wbich the ref'ractiOll from the opposed vel.ocity and teDperature grsdients 1argel.y cancel.led: the directiv1ties both of a pure tone injected into the nOl< and of narrOl< band fil.tered jet noise exh1bited near el.ilIIination of the retractive valley observed &long the axis of room teDperature and hot jets. There remained

in the jet noise pattern the "bulge" in dawnstream emI..sion oor:I.ng to ccmvection of the edd1es.

Thua the experiment deIIIonatrated a .ubstanti&l. degree of separation of refraetiOll and ccmveetiOll effecta in jet noise.

Available copies of I:his report: are limited: Return this card to UTIAS, if you require a copy.

urIAS TU:JIlIlCAL 1IO.rI! 110. 1.91<

Institul:e for Aerospace Studies, University of T oronto

SUPFRESSION OF REFRACTION IN JET NOIBE BI COOLIN:l

Kel.sall, J. T.

1. Jet Boise I. Kelsall, J. T.

1.2 pages 7 figures 1 tab1.

2. Refract10n 3. Aeroacoust1cs 4. Acoustics

n. UXIAS Technical Nate No. 194

~

Liquid nitrogen was used to cool a jet of air (at nozz1e speeds 0.21 to 0.28 of amb1ent aOWld

speed) down to te>peratures (between -llO·C and -155·C) at which the refract10n from the

opposed veloclty and teuperature gradients large1y cancelled: the d1rectivitles both of a pure

"tone injected into the flOW' and of narraw band filtered jet nolse exh1blted ne&I' ellm1.natlon of

the refractive valley obaerved along the axis of room teDperature and hot jets. There remained in the jet noiae pattern the ''bulge'' in dawnstream eml.ss10n oor:I.ng to convect10n of the eddiea.

Thua the experiment demonstrated a sub.tanti&l degree of separat10n of refraction and ccmvection

.effecta in Jet noi.e.