Richard F. Bozak and Brenda S. Henderson

Glenn Research Center, Cleveland, Ohio

Aeroacoustic Experiments With Twin Jets

NASA/TM—2012-217249

February 2012

AIAA–2011–2790
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Richard F. Bozak and Brenda S. Henderson

Glenn Research Center, Cleveland, Ohio

Aeroacoustic Experiments With Twin Jets

NASA/TM—2012-217249

February 2012

AIAA–2011–2790

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

17th Aeroacoustics Conference

cosponsored by the American Institute of Aeronautics and Astronautics and 

the Confederation of European Aerospace Societies

Portland, Oregon, June 5–8, 2011
Acknowledgments

This work was funded by the Subsonic Fixed Wing and Supersonics Projects of NASA’s Fundamental Aeronautics Program.

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NASA/TM—2012-217249 1

Aeroacoustic Experiments With Twin Jets

Richard F. Bozak and Brenda S. Henderson

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

While the noise produced by a single jet is azimuthally symmetric, multiple jets produce azimuthally 

varying far-field noise. The ability of one jet to shield another reduces the noise radiated in the plane of 

the jets, while often increasing the noise radiated out of the plane containing the jets. The present study 

investigates the shielding potential of twin jet configurations over subsonic and over-expanded supersonic 

jet conditions with simulated forward flight. The experiments were conducted with 2 in. throat diameter 

nozzles at four jet spacings from 2.6d to 5.5d in center-to-center distance, where d is the nozzle throat 

diameter. The current study found a maximum of 3 dB reduction in overall sound pressure level relative 

to two incoherent jets in the peak jet noise direction in the plane containing the jets. However, an increase 

of 3 dB was found perpendicular to the plane containing the jets. In the sideline direction, shielding is 

observed for all jet spacings in this study.

Introduction

Several subsonic and supersonic vehicles in the current fleet have multiple engines mounted near one 

another. Some future vehicle concepts may use innovative propulsion systems such as distributed 

propulsion which will result in multiple jets mounted in close proximity. Engine configurations with 

multiple jets have the ability to exploit jet-by-jet shielding which may significantly reduce noise. Jet-byjet shielding is the ability of one jet to shield noise that is emitted by another jet. The current study 

investigates the noise shielding potential of a twin jet configuration.

Full and small scale twin jet aeroacoustic experiments were documented throughout the 1970s. Full 

scale static tests of the XB-70 found that effects of shielding depended mainly on the observation angle. 

The results showed that shielding occurs at high frequencies. For subsonic jets, engine spacing had no

shielding effect (Ref. 1). However, extensive small scale static experiments showed that the frequencies 

where shielding occurred depended on the jet spacing (Ref. 2). The apparent discrepancy between the full 

and small scale results may be due to the limited full scale data set. Additionally in small scale 

experiments, azimuthal measurements show mixing suppression in addition to the shielding for closely 

spaced jets (Ref. 2).

In the 1980s, an analytical model was used to explain the directivity of jet-by-jet shielding when jet 

spreading was taken into account (Refs. 3 and 4). At angles near the jet axis, shielding was most effective

due to the spreading of the jet and the lack of sound transmission through the jet. The theoretical limit 

of transmission through the jet was given by the transmission cut-off angle (Ref. 5). At polar angles 

between the jet axis and the cut-off angle no sound is transmitted from one jet through the shear layer of 

the other jet.

The present study investigates the aeroacoustics of twin jets over subsonic and over-expanded 

supersonic jet conditions for simulated forward flight Mach numbers from 0 to 0.3 and jet spacings from 

2.6d to 5.5d, where d is the nozzle throat diameter. Two inch throat diameter jets were used in the 

experiments. The magnitudes of jet-by-jet shielding and interaction are analyzed. The frequency 

dependence of the jet-by-jet shielding is also investigated.
NASA/TM—2012-217249 2

Experimental Setup

The experiments were conducted in the Aero-Acoustic Propulsion Laboratory (AAPL) at NASA 

Glenn Research Center shown in Figure 1. The AAPL is a 65 ft radius hemi-anechoic dome containing 

the Nozzle Acoustic Test Rig (NATR). The NATR (shown in Figure 2) is a 53 in. free jet wind tunnel, 

used to simulate forward flight, capable of achieving a maximum Mach number of 0.35. The High Flow 

Jet Exit Rig (HFJER) is a dual stream jet engine simulator mounted in the center of the NATR. The 

HFJER provides heated core and bypass flow to simulate common engine exhausts (Ref. 6).

A far-field overhead microphone array contains 24 microphones on a 45 ft arc covering polar angles 

from 45° to 160° relative to the inlet axis of the nozzle. For all data presented, microphone sensitivity, 

actuator, and diffraction corrections have been made. Background noise has been subtracted and free

jet shear layer corrections have been applied using Amiet’s method (Ref. 7). Atmospheric attenuation 

effects have been removed (Ref. 8) and the data are presented in 1/12th octave bands propagated to a 1-ft 

lossless arc. 

The twin jet model was mounted on the core stream of the HFJER. A model of the twin jet model is 

shown in Figure 3. The y-duct divides the core flow from the dual stream HFJER into two streams. The 

internal pressure taps shown at the inlet to the s-ducts monitor the pressure in each leg of the twin jet 

model to ensure equal flow through each nozzle. The s-ducts following the y-duct are used to vary the 

spacing between the nozzles. The straight adapter was used to obtain parallel flow through the nozzles. 

For subsonic jet exhausts, 2 in. throat diameter convergent conical nozzles are attached to the straight 

adapter, while 2 in. throat diameter contoured convergent-divergent nozzles with a design Mach number 

of 1.52 were used for supersonic exhausts. The four nozzle spacings used in the experiments are shown in 

Table I as a ratio of the center-to-center spacing, s, to the nozzle throat diameter, d. Single jet 

measurements were made with a conical plug on one jet.

Figure 1.—Aero-Acoustic Propulsion 

Laboratory (AAPL).

Figure 2.—Twin jet model installed on the High Flow Jet Exit Rig 

(HFJER) in the Nozzle Acoustic Test Rig (NATR).

TABLE I.—TWIN JET SPACINGS

Spacing 

designation

s/d

S1 2.625

S3 3.245

S5 4.39

S8 5.54
NASA/TM—2012-217249 3

Figure 3.—Twin jet model components.

25°

s

d

Sideline

In-plane

Out-of-plane

Figure 4.—Azimuthal directivity microphone 

measurement positions, relative to the jet throats.

Acoustic data were acquired at the three azimuthal angles shown in Figure 4, where the black dots 

represent the axis of the microphone array. In the plane of the two jets, the acoustic radiation from one jet 

can be shielded by the other jet. The acoustic radiation from both jets and the acoustic effects of plume 

interaction can be measured out of the plane containing the two jets. The sideline position represents an 

orientation used for Federal Aviation Administration sideline takeoff noise measurements, which is 

approximately 25° from the shielding direction. 

The jet operating conditions used in the experiments are shown in Table II. Nozzle pressure ratios 

(NPR) from 1.5 to 3.7, nozzle temperature ratios (NTR) from 1.0 to 3.1, and free jet Mach numbers (Mfj) 

from 0 to 0.3 were used in the experiments. The nozzle pressure ratios were calculated as a ratio of the jet 

stagnation pressure to the atmospheric pressure. The nozzle temperature ratios were calculated as the ratio 

of the jet stagnation temperature to the ambient temperature. The jet operating conditions used with the 

convergent nozzles include over-expanded, under-expanded, and ideally expanded conditions. The 

conditions used with the contoured convergent-divergent nozzles cover a range of over-expanded 

conditions and an ideally expanded condition. Since the single stream model was mounted to a dual 

stream rig, the conditions of the bypass stream were matched to those of the free jet.
NASA/TM—2012-217249 4

TABLE II.—JET OPERATING CONDITIONS

NPR NTR Mfj

Convergent nozzles

1.86 1.00 0.00

1.86 1.00 0.10

1.70 3.11 0.00

1.70 3.11 0.10

1.52 1.95 0.10

1.51 2.57 0.10

1.69 2.08 0.10

1.67 2.85 0.10

1.89 1.00 0.10

1.88 2.37 0.10

1.87 3.12 0.10

2.01 1.00 0.10

2.00 2.51 0.10

1.87 3.12 0.30

2.00 2.51 0.30

Contoured convergent-divergent 

nozzles

2.00 2.40 0.10

2.50 2.80 0.10

3.00 3.00 0.10

3.50 3.00 0.10

3.70 3.10 0.10

2.00 2.40 0.30

2.50 2.80 0.30

3.00 3.00 0.30

3.50 3.00 0.30

3.70 3.10 0.30

Results

The shielding and interaction effects are first analyzed at the in-plane and out-of plane positions 

shown in Figure 4. The results obtained at these model orientations are then used to determine the polar 

angle directivity at three free jet Mach numbers. The azimuthal angle directivity is determined by 

analyzing the data from all three model orientations shown in Figure 4. To evaluate the shielding and 

interaction effects, twin jet data are compared against data for a single jet and a single jet plus a level 

increase of 3 dB, referred to as “single jet plus 3 dB.” A single jet plus 3 dB represents the addition of two 

incoherent jets. When one jet is entirely shielded by the other jet, the levels measured for the twin jets will 

equal those for the singe jet. When no shielding or interaction effects are observed, the levels for the twin 

jets will equal those for the single jet plus 3 dB. Interaction effects are defined as a change in sound 

pressure level from the single jet plus 3 dB level which can be attributed to jet plume interactions or 

acoustic interactions between the jets. 

Subsonic conditions are evaluated at polar angles of 90° and 150° to show the effects of the twin jet 

configuration on acoustic radiation at the broad side and peak jet noise directions. Supersonic conditions 

are evaluated at polar angles of 60° and 150° to show the effects of the twin jet configuration on acoustic 

radiation at the broadband shock noise and the peak jet noise directions. The 1/12th octave band sound 

pressure levels obtained at each spacing in Table I are compared to the levels of a single jet and single jet 

plus 3 dB to evaluate shielding effects and compared only to the single jet plus 3 dB levels to evaluate

interaction effects.
NASA/TM—2012-217249 5

Overall sound pressure levels are used to evaluate both polar and azimuthal directivity. The difference 

in OASPL between the twin configuration and a single jet plus 3 dB is referred to as ∆OASPL. The polar 

directivity from both the shielding and interaction positions are evaluated between free jet Mach numbers

of 0 and 0.3. The azimuthal directivity is evaluated by comparing all three positions shown in Figure 4 to 

a single jet plus 3 dB for each jet spacing.

Shielding 

Shielding effects are evaluated for heated subsonic and over-expanded supersonic jet conditions with 

a free jet Mach number of 0.1 at the in-plane azimuthal angle. Figure 5 shows the spectra of heated 

subsonic jets in the shielding position. At a polar angle of 90°, no shielding is seen below 4 kHz while 

partial shielding is evident above 4 kHz. Acoustic radiation is not sensitive to jet spacing at this polar 

angle. In the peak polar jet noise direction of 150°, sound pressure levels at frequencies below 3 kHz are 

partially shielded, while the levels for frequencies over 3 kHz are completely shielded. More shielding is 

observed for a s/d of 2.625 (S1) than any other jet spacing. The shielding obtained at this spacing 

surpasses complete shielding for frequencies above 2.5 kHz by approximately 1 dB. A variation in sound 

pressure level with spacing is evident over all frequencies. The shielding trends shown in Figure 5 are

consistent over all subsonic jet conditions in Table II.

Figure 6 shows the spectra of heated over-expanded supersonic jets in the plane containing the jets. 

At a polar angle of 60°, the broadband shock noise peak is seen at 7 kHz. The twin jet noise levels are 

unshielded over all frequencies with the exception of the s/d of 5.54 (S8). For the S8 spacing, 1 dB of 

shielding is evident for frequencies over 9 kHz. At a polar angle of 150°, complete shielding exists below 

3 kHz for the all jet spacings, while partial shielding of 2 dB is seen over all other frequencies. The 

shielding trends shown in Figure 6 are consistent with results obtained at all supersonic jet conditions 

shown in Table II.

(a) (b)

Figure 5.—Shielding of heated subsonic jets with NPR=1.88, NTR=2.37, and Mfj=0.1 at polar angles (a) 90°

and (b) 150°. 
NASA/TM—2012-217249 6

(a) (b)

Figure 6.—Shielding of heated supersonic jets with NPR=3.0, NTR=3.0, and Mfj=0.1 at polar angles (a) 60°

and (b) 150°.

(a) (b)

Figure 7.—Interaction of heated subsonic jets with NPR=1.88, NTR=2.37, and Mfj=0.1 at polar angles (a) 

90° and (b) 150°. 

Interaction 

Interaction effects are evaluated for heated subsonic and over-expanded supersonic jet conditions 

with a free jet Mach number of 0.1. Figure 7 shows the spectra of heated subsonic jets out of the plane 

containing the jets. At a polar angle of 90°, no interaction effects are evident for frequencies less than 

6 kHz. However, from 6 to 30 kHz interaction effects increase with increasing frequency. The interaction 

effects increase the noise levels with decreasing jet spacing. At a polar angle of 150°, interaction effects 

are seen for all frequencies greater than 1 kHz at all jet spacings. At a s/d of 5.54 (S8), a secondary peak 

appears at 4 kHz and no interaction effects are seen above 6 kHz. The interaction effects are concentrated 

near the peak frequency where an increase of up to 4 dB is evident. The peak frequencies for the S1 and 

S3 jet spacings are slightly higher than that of the single jet plus 3 dB while those of the S5 and S8 jet 

spacings are slightly lower. The interaction effects in the peak jet noise direction are sensitive to jet 

spacing. The interaction trends shown in Figure 7 are representative of all subsonic jet conditions in 

Table II.

Broadband 

Shock Noise
NASA/TM—2012-217249 7

(a) (b)

Figure 8.—Interaction of heated supersonic jets with NPR=3.0, NTR=3.0, and Mfj=0.1 at polar angles (a) 60°

and (b) 150°. 

Figure 8 shows the spectra of heated over-expanded supersonic jets at the out-of-plane azimuthal 

angle. At a polar angle of 60°, all twin jet noise levels, including the broadband shock noise, are not 

affected by interaction effects for any jet spacing. At a polar angle of 150°, the noise levels of the S1 and 

S3 jet spacings are below those for the single jet plus 3 dB for frequencies below 3 kHz. This reduction in 

noise level may be attributed to enhanced mixing, as seen in similar experiments (Ref. 2). The noise level 

increase for the S5 and S8 jet spacings show interaction effects up to 2 dB between 1.5 and 3 kHz. Above 

3 kHz an interaction effect increase of up to 2 dB are evident for all jet spacings. Data acquired at other 

supersonic conditions in Table II show similar interaction effects.

Polar Directivity

In the plane of the jets, some of the sound measured at a polar angle of 90° emitted by the far jet 

travels through the near jet. In a simplified model, rays from the sound wave emitted by the far jet make a 

right angles with the jet axis of the near jet. As the observation angle increases, the incidence angle of the 

sound wave decreases to a theoretical limit where all incident sound is reflected. In the region where all 

incident sound is reflected, shielding and interaction effects are maximized. The transmission cut-off 

angle, θc, refers to the maximum polar angle at which sound from the far jet can propagate into the near 

jet (Refs. 3 and 5). The transmission cut-off angle is calculated via the equation below where c0 and c1

refer to the ambient speed of sound and the speed of sound within the jets, respectively, and ∆M is the 

difference in Mach numbers of the free jet and twin jet.





 





+ ∆

θ = −

M

c c c 1

cos 1 0 1

Overall sound pressure levels are used to evaluate polar directivity. The single jet plus 3 dB overall 

sound pressure levels were subtracted from the twin jet levels to give a change in overall sound pressure 

level (ΔOASPL). A negative ΔOASPL represents a reduction in noise level. Figure 9 shows the polar 

directivity of heated subsonic jets at three free jet Mach numbers at the in-plane and out-of-plane 

azimuthal angles for a s/d of 3.245. For upstream polar angles from 40° to 80° little shielding and
NASA/TM—2012-217249 8

Figure 9.—Polar directivity of heated subsonic jets at a s/d of 

3.245 with a NPR=1.7, NTR=3.11, Mfj=0, NPR=1.88 , 

NTR=3.12, Mfj=0.1, and NPR=1.88, NTR=3.12, Mfj=0.3.

interaction effects are evident. For polar angles between 80° and 140° both shielding and interaction 

effects increase to a maximum ΔOASPL of about 3 dB. For angles greater than 140° the ∆OASPL 

magnitude decreases with increasing polar angle. In a simple 2D model, maximum shielding will occur

for polar angles above θc. In these experiments a slight increase in shielding is observed above θc. For 

angles greater than θc, interaction effects increase sharply with increasing polar angle up to 140°. As the 

free jet Mach number increases, shielding effects are decreased over all polar angles while the interaction 

effects are either maintained or increased. The polar directivity shown in Figure 9 is typical over the jet 

spacings in Table I and the subsonic and supersonic conditions in Table II.

Azimuthal Directivity

Acoustic data were acquired in the three positions shown in Figure 4 to evaluate azimuthal directivity. 

Figure 10 shows the azimuthal directivity of subsonic heated jets for each jet spacing in Table I. Overall 

sound pressure levels measured from the sideline azimuthal angle are expected to fall between 

measurements from the in-plane and out-of-plane angles. The sideline azimuthal angle acoustic levels fall 

between those of the in-plane and out-of-plane angles near the peak jet noise angle for all jet spacings 

except S3. For polar angles greater than 120° and the S3 spacing, the sideline acoustic levels are equal to 

the levels measured in the plane of the jets. For angles less than 140°, polar directivity is highly affected 

by jet spacing. For the largest spacings (S5 and S8), sideline acoustic levels are greater than those for inplane and out-of-plane measurements for angles less than 110°. At the sideline angle, shielding effects are

observed in the peak jet noise direction for all jet spacings. The azimuthal directivity shown in Figure 10

is consistent for all subsonic conditions in Table II.
NASA/TM—2012-217249 9

(a) (b)

(c) (d)

Figure 10.—Azimuthal directivity of heated subsonic jets at a NPR=1.88, NTR=2.37, and Mfj=0.1 with a s/d

of (a) 2.625 (S1), (b) 3.245 (S3), (c) 4.39 (S5), and (d) 5.54 (S8).

Conclusions

Shielding and interaction effects are concentrated in the peak jet noise direction where a 3 dB OASPL 

reduction or elevation is seen relative to single jet plus 3 dB levels, respectively. Twin jet interaction 

effects increase the sound pressure level up to 4 dB relative to a single jet plus 3 dB near the peak 

frequency in the peak direction. The increase is evident at jet spacings of 4.39d and 5.54d in the out-ofplane azimuthal angle. The out-of-plane measurements show that twin jet interaction effects can increase 

or decrease the radiated sound pressure level in the peak jet noise direction for a range of frequencies. As 

the free jet Mach number increases, shielding effects decrease over all polar angles while interaction 

effects either maintain or increase noise levels for all jet spacings. In the sideline orientation, shielding is 

observed in the peak jet noise direction for all jet spacings.
NASA/TM—2012-217249 10

References

1. Lasagna, P. L., and Putnam, T. W., “Engine Exhaust Noise During Ground Operation of the XB-70 

Airplane,” NASA TN D-7043, 1971.

2. Balsa, T. F., Gliebe, P. R., Kantola, R. A., Mani, R., Stringus, E. J., and Wang, J. C. F., “High 

Velocity Jet Noise Source Location and Reduction Task 2 – Theoretical Developments and Basic 

Experiments,” FAA-RD-76-79, 1978.

3. Gerhold, C. H., “Analytical Model of Jet Shielding,” AIAA Journal, Vol. 21, No. 5, 1983, pp. 694-

698, 1873.

4. Gerhold, C. H., and Kim, C. H., “Modified Jet Noise Source Model for Twin-Jet Shielding Analysis,” 

Texas A&M University, AIAA-83-0776, Atlanta, GA, Apr. 1983.

5. Yeh, C., “A Further Note of the Reflection and Transmission of Sound Waves by a Moving Fluid 

Layer,” Journal of the Acoustical Society of America, Vol. 43, No. 6, pp. 1454, 1968.

6. Soeder, R. H., Wnuk, S. P., and Loew, R. A., “Nozzle Acoustic Test Rig User Manual,” NASA TM

2006-212939, 2006.

7. Amiet, R. K., “Correction of Open Jet Wind Tunnel Measurements for Shear Layer Refraction,” 

United Aircraft Research Laboratories, AIAA 75-532, Hampton, VA, Mar. 1975.

8. ANSI S1.26-1995 (R2004), “Method for the Calculation of the Absorption of Sound by the 

Atmosphere.”

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14. ABSTRACT

While the noise produced by a single jet is azimuthally symmetric, multiple jets produce azimuthally varying far-field noise. The ability of 

one jet to shield another reduces the noise radiated in the plane of the jets, while often increasing the noise radiated out of the plane 

containing the jets. The present study investigates the shielding potential of twin jet configurations over subsonic and over-expanded 

supersonic jet conditions with simulated forward flight. The experiments were conducted with 2 in. throat diameter nozzles at four jet 

spacings from 2.6d to 5.5d in center-to-center distance, where d is the nozzle throat diameter. The current study found a maximum of 3 dB 

reduction in overall sound pressure level relative to two incoherent jets in the peak jet noise direction in the plane containing the jets. 

However, an increase of 3 dB was found perpendicular to the plane containing the jets. In the sideline direction, shielding is observed for all 

jet spacings in this study. 

15. SUBJECT TERMS

Twin; Jet; Aeroacoustics 

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