James E. Bridges

Glenn Research Center, Cleveland, Ohio

Noise Measurements of High Aspect Ratio

Distributed Exhaust Systems

NASA/TM—2015-218901

December 2015

AIAA–2015–3119
NASA STI Program . . . in Profi le

Since its founding, NASA has been dedicated 

to the advancement of aeronautics and space science. 

The NASA Scientifi c and Technical Information (STI) 

Program plays a key part in helping NASA maintain 

this important role.

The NASA STI Program operates under the auspices 

of the Agency Chief Information Offi cer. It collects, 

organizes, provides for archiving, and disseminates 

NASA’s STI. The NASA STI Program provides access 

to the NASA Technical Report Server—Registered 

(NTRS Reg) and NASA Technical Report Server—

Public (NTRS) thus providing one of the largest 

collections of aeronautical and space science STI in 

the world. Results are published in both non-NASA 

channels and by NASA in the NASA STI Report 

Series, which includes the following report types:

• TECHNICAL PUBLICATION. Reports of 

completed research or a major signifi cant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of signifi cant 

scientifi c and technical data and information 

deemed to be of continuing reference value. 

NASA counter-part of peer-reviewed formal 

professional papers, but has less stringent 

limitations on manuscript length and extent of 

graphic presentations.

• TECHNICAL MEMORANDUM. Scientifi c 

and technical fi ndings that are preliminary or of 

specialized interest, e.g., “quick-release” reports, 

working papers, and bibliographies that contain 

minimal annotation. Does not contain extensive 

analysis.

• CONTRACTOR REPORT. Scientifi c and 

technical fi ndings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scientifi c and technical 

conferences, symposia, seminars, or other 

meetings sponsored or co-sponsored by NASA.

• SPECIAL PUBLICATION. Scientifi c, 

technical, or historical information from 

NASA programs, projects, and missions, often 

concerned with subjects having substantial 

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientifi c and 

technical material pertinent to NASA’s mission.

For more information about the NASA STI 

program, see the following:

• Access the NASA STI program home page at 

http://www.sti.nasa.gov

• E-mail your question to help@sti.nasa.gov

• Fax your question to the NASA STI 

Information Desk at 757-864-6500

• Telephone the NASA STI Information Desk at

 757-864-9658

• Write to:

NASA STI Program

 Mail Stop 148

 NASA Langley Research Center

 Hampton, VA 23681-2199
James E. Bridges

Glenn Research Center, Cleveland, Ohio

Noise Measurements of High Aspect Ratio

Distributed Exhaust Systems

NASA/TM—2015-218901

December 2015

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

AIAA–2015–3119

Prepared for the

Aviation 2015

sponsored by the American Institute of Aeronautics and Astronautics

Dallas, Texas, June 22–26, 2015
Acknowledgments

The High Speed Project of the NASA Fundamental Aeronautics Program supported this work. Cliff Brown and Khairul Zaman 

acquired the far-fi eld data. Adam Wroblewski printed the septae inserts from the author’s solid models that were in turn based on 

geometries from Hyun Dae Kim. Many thanks to all for their contributions!

Available from

This work was sponsored by the Fundamental Aeronautics Program 

at the NASA Glenn Research Center.

Level of Review: This material has been technically reviewed by technical management. 

NASA STI Program

Mail Stop 148

NASA Langley Research Center

Hampton, VA 23681-2199

National Technical Information Service

5285 Port Royal Road

Springfi eld, VA 22161

703-605-6000

This report is available in electronic form at http://www.sti.nasa.gov/ and http://ntrs.nasa.gov/
NASA/TM—2015-218901 1 

Noise Measurements of High Aspect Ratio 

Distributed Exhaust Systems 

James E. Bridges 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

This paper covers far-field acoustic measurements of a family of rectangular nozzles with aspect 

ratio 8, in the high subsonic flow regime. Several variations of nozzle geometry, commonly proposed for 

embedded exhaust systems, are explored, including bevels, slants, single broad chevrons and notches, and 

internal septae. Far-field acoustic results, presented previously for the simple rectangular nozzle, showed 

that increasing aspect ratio increases the high frequency noise, especially directed in the plane containing 

the minor axis of the nozzle. Detailed changes to the nozzle geometry generally made little difference in 

the noise, and the differences were greatest at low speed. Having an extended lip on one broad side 

(‘bevel’) did produce up to 3 dB more noise in all directions, while extending the lip on the narrow side 

(‘slant’) produced up to 2 dB more noise, primarily on the side with the extension. Adding a single, 

nonintrusive chevron, made no significant change to the noise, while inverting the chevron (‘notch’) 

produced up to 2 dB increase in the noise. Having internal walls (‘septae’) within the nozzle, such as 

would be required for structural support or when multiple fan ducts are aggregated, reduced the noise of 

the rectangular jet, but could produce a highly directional shedding tone from the septae trailing edges. 

Finally, a nozzle with both septae and a beveled nozzle, representative of the exhaust system envisioned 

for a distributed propulsion aircraft with a common rectangular duct, produced almost as much noise as 

the beveled nozzle, with the septae not contributing much reduction in noise. 

Nomenclature 

Dj diameter, equivalent area 

h short dimension of rectangular nozzle exit 

L length of bevel, slant, chevron beyond exit of baseline nozzle 

Ma acoustic Mach number, Uj/c∞

c∞ speed of sound, ambient 

T∞ temperature, ambient 

t thickness of trailing edge of septum 

Uj jet exit velocity, ideally expanded 

PSD power spectral density of sound 

Stt Strouhal number based on trailing edge of septum, frequency*t/Uj

StD Strouhal number based on equivalent jet diameter, frequency*Dj/Uj

I. Introduction 

NASA’s Advanced Air Vehicles Program is exploring aircraft designs that take entirely new 

approaches to the problems of aero transport. Several of these conceptual aircraft require that the 

propulsion system be tightly integrated with the airframe, and often the nozzles have a high aspect ratio as 

well as other geometric features that make the nozzle very different from an axisymmetric nacelle. 

Currently there is a lack of adequate noise prediction methods that are known to successfully address the 
NASA/TM—2015-218901 2 

noise of high aspect ratio nozzles. In addition there is a desire to use aspects of the nozzle design to 

reduce the noise of the propulsive flow, such as shielding or enhanced mixing. As part of its exploration 

of nozzle concepts for quiet civilian supersonic aircraft, the NASA Commercial Supersonic Technology 

Project created a family of high aspect ratio nozzles, the Extensible Rectangular Nozzles (ERN). This 

paper presents acoustic results for an array of geometrical variants of rectangular nozzle such as is found 

in many proposed advanced aircraft. 

The initial goal of testing of the ERN was to determine whether high aspect ratio nozzles showed 

promise for noise reduction, to gather basic noise data for creating empirical models, and to provide flow 

data for advanced noise prediction code development. The noise of rectangular nozzles has been reported in 

the past, but in surveying the existing and proposed exhaust systems there have been several variations on a 

simple rectangular nozzle, variations that might cause the jet to produce more or less noise than a simple 

rectangular nozzle. Figure 1 is an example of one such proposed aircraft (Ref. 1) which features a very high 

aspect ratio exhaust nozzle with a trailing edge at a slant to the flow direction, and an aft deck, and septae 

between the fan ducts which comprise the internal passages of the main nozzle. It is a significant assumption 

that the noise from this exhaust system is the same as from a simple rectangular nozzle. 

Design of the ERN model system started with a series of basic rectangular nozzles using 

computational fluid dynamics as documented in Frate & Bridges (Ref. 2). The main design criterion was 

to create a family of rectangular nozzles that had uniform velocity at their exit plane. Far-field noise from 

these basic rectangular nozzles was measured and acoustic results reported by Bridges (Ref. 3). Detailed 

velocity field measurements were also measured and presented in Bridges & Wernet (Ref. 4). In the latter 

reference, the flow fields of the rectangular nozzles were found to be uniform at their exit planes, as 

designed, and the flow and turbulence fields were documented. 

Other researchers have investigated rectangular nozzles previously. One example is the work of 

General Electric Co., who acquired data on a 6:1 ‘slot’ nozzle at their Corporate Research and 

Development labs in the late 1970s (Ref. 5). Georgia Tech Research Institute (GTRI) made a more 

thorough investigation (Ref. 6) of rectangular jets, reporting results for aspect ratios 1.5, 4, and 8. Munro 

& Ahuja (Ref. 7) explored the noise of rectangular nozzles of very high aspect ratio (100 to 3000) and 

created empirical models for the noise (Ref. 8). Little flow field information was available in the earlier 

tests of lower aspect ratios, either because of their age or because of the facility. The high aspect ratio jets 

were documented aerodynamically in Reference 9. 

In addition to documenting the noise of the basic rectangular nozzle, our previous work also explored 

the how the jet noise changed as one long edge of the nozzle was extended, a nozzle variation we called a 

‘bevel’ in keeping with the nomenclature of Viswanathan et al. (Ref. 10). In application, having a nozzle 

exhaust across an aft deck of an aircraft results in a bevel nozzle, as seen in Figure 1. In subsequent work 

looking at the acoustic impact of jet-surface interaction by Brown (Ref. 11), these beveled nozzles were 

extended, literally, to include the impact of very long surfaces (Ref. 12). By extending the concept of a 

bevel nozzle to become more like that of a blown flap it became clear that the increase in noise found in 

the bevel nozzle was due to the extended lip exhibiting dipole noise as the shear layers from the shorter 

portions of the lip passed over the longer lip. This exemplifies how a concept that was supposed to reduce 

noise by shielding an observer from noise actually increases noise by creating a new aero noise source. 

Figure 1.—Proposed Turboelectric Distributed Propulsion System. 

 
NASA/TM—2015-218901 3 

In conceptual vehicles the rectangular or slot nozzle often has the trailing edge of the nozzle slanted at 

an angle to the flight and exhaust direction in the horizontal direction to better blend in with the aircraft 

body. Does this create less noise from shear layers with reduced spanwise coherence, or does it create 

another jet-surface trailing edge noise source as the beveled configuration did? 

Trailing edges that are not normal to the flow have often been used to produce enhance mixing, in the 

form of chevron. The F-22, for example, features a single large chevron on the upper and lower edges of 

its low-aspect ratio nozzle. While the details of an articulated nozzle operating at various over-expanded 

conditions are not easily captured, the basic behavior of a single large chevron exit is of interest if it 

produces a measureable noise impact. And inverting the chevron shape, creating a notch geometry, might 

produce a large-scale variation in the plume cross-section, leading to reduced noise from downstream 

noise sources. 

Finally, practical considerations of noncircular pressure vessels require that rectangular nozzles either 

have significantly stronger shells, or internal bracing to maintain the shape of the nozzle. Or, as in the 

case of the Turboelectric Distributed Propulsion concept (Fig. 1) the high-aspect-ratio nozzle is the 

accumulation of many fan ducts, each separated from its neighbor by a septum. These series of septae, or 

a series of internal bracing, could have significant acoustic impact on the exhaust system, impacts that 

need to be understood at least at a system study level, to allow proper assessment of a vehicle’s viability. 

The nozzle designs examined in this paper are intended to provide one degree of realism beyond that 

of a simple rectangular nozzle, itself a level of detail beyond the one-dimensional (axisymmetric) noise 

models often used in aircraft system design. The extent to which these variations on a rectangular design 

have an acoustic impact is intended to only give a slightly more refined answer in a system level study. 

II. Facility, Model Hardware, Flow Conditions, and Instrumentation 

A. Facility 

The nozzle tests were conducted on the Small Hot Jet Acoustic Rig (SHJAR, pronounced with a silent 

‘J’) located in the Aeroacoustic Propulsion Laboratory (AAPL) at the NASA Glenn Research Center in 

Cleveland, Ohio. The SHJAR was developed to test jet noise reduction concepts at a low technology 

readiness level (TRL 1-3) and at minimum expense, and to conduct fundamental studies of jet noise and 

jet turbulent flow fields. The rig is described and baseline acoustic results for single-stream round jets are 

given in Reference 13. 

B. Model Hardware 

The model hardware being considered in this paper is all based on an 8:1 rectangular nozzle that has 

been documented in a previous conference paper (Ref. 2). The 8:1 nozzle had height h = 0.668 in. 

(16.9 mm) and width w = 5.336 in. (135.5 mm) giving an equivalent diameter of 2.13 in. (54.1 mm). The 

flow field was measured extensively with PIV and these results were reported in Reference 4. 

Beyond the basic rectangular nozzle, several variants were considered that embodied aspects of 

embedded exhaust systems found in existing and proposed aircraft. Table 1 gives the dimensions and 

visual sketch of each of the nozzles tested and reported. Adding an additional piece to the base 

rectangular nozzle created most of these variants, although the bevel was created as an integral nozzle. In 

earlier work (Ref. 3) two extension lengths L/h were explored for rectangular nozzles of aspect ratios of 2, 

4, and 8. In the present paper only the 8:1 nozzle is presented for comparison to other geometric variants 

on the rectangular nozzle. 

Adding metal parts to the outer mold line of the basic rectangular nozzle created the slant, chevron, 

and notch nozzles. The surfaces of the added parts were carefully machined where they met the nozzle to 

produce a smooth surface continuing the inner surface of the nozzle. In all cases the additional parts 

maintained the same dimensions as the exit perimeter, so for instance the chevrons had zero penetration 

into the shear layer. 
NASA/TM—2015-218901 4 

TABLE 1.—DESIGN FEATURES OF NOZZLE HARDWARE 

Base nozzle, 8:1, 

0.668”5.336” 

Bevel, Short 

L/h = 1 

Bevel, 

Long 

L/h = 2 

Slant, Short 

L/h = 2 

Slant, 

Long 

L/h = 4 

Single chevron, 

Short 

L/h = 0.5 

0 penetration 

Single 

chevron, 

Long 

L/h = 2 

0 

penetration 

Notch, Short 

L/h = 2 

Notch, 

Long 

L/h = 4 

Septae 

6 chambers, 

flush exit 

Septae + 

Bevel, 

L/h = 2 
NASA/TM—2015-218901 5 

Figure 2.—Septae insert. Passages are rectangular to 

round (fan plane) and back to rectangular at exit. 

The septae nozzle was fabricated by printing an insert for the basic rectangular nozzle using Fused 

Deposition Modeling of ABS plastic. The insert maintained a continuously decreasing area as it started with 

equal area rectangular ducts, transitioned to round cross-sections as appropriate for individual fans, and then 

back to rectangular ducts. The septum dividing each duct terminated in a 0.030 in. thick trailing edge. To 

reach the most realistic geometry of the Turboelectric Distributed Propulsion concept possible, the insert 

was also put in the beveled nozzle and tested. A transparent sketch of the septae is given in Figure 2. 

C. Flow Conditions 

Because the test program had such a large number of geometries and a large number of orientations 

for each of these geometries, only a cursory number of unheated subsonic flows were measured, 0.5 < 

Uj/c∞ < 0.9. For most of the paper only the highest speed will be shown. When results vary significantly 

with acoustic Mach number then results at lower speeds will be addressed. 

D. Instrumentation 

Acoustic data were acquired simultaneously on 53 1⁄4-in. Bruel & Kjaer condenser microphones 

located along three arcs of 150-in. radius (3.8 m radius) from the jet exit, as shown in Figure 3. Relative 

to the jet nozzle diameter, the microphones were located approximately 70 equivalent jet diameters away 

from the jet exit. The microphones were placed along two polar arrays spaced 90° apart azimuthally. The 

horizontal polar array, located at azimuthal angle 90°, spanned polar angles from 50° to 165° relative to 

the forward flight direction at 5° increments. The vertical polar array, azimuthal angle 0°, spanned polar 

angles from 80° to 165° at 5° increments. The azimuthal array was located in the plane of the nozzle exit, 

polar angle = 90°, and spanned azimuthal angles from –50° to 90° at 10° increments. 

All data presented have had atmospheric attenuation added back to the spectrum and have been 

transformed to an arc with radius of 100 equivalent jet diameters assuming spherical spreading from the 

jet exit. The flow conditions were controlled to maintain jet velocity relative to ambient speed of sound 

(acoustic Mach number Ma) to within 0.5 percent of stated values. 

III. Results 

Acoustic data will be presented as spectral directivity of power spectral density of sound using a carpet 

plot format. As shown in the example plot in Figure 4, the height of the carpet is the power spectral density 

(PSD) as a function of polar angle and frequency (normalized as StD, in log scale). The color of the surface is a 

delta-dB between the spectral density of the nozzle flow and a reference case. In Figure 4 the nozzle flow is the 

basic rectangular nozzle at Ma = 0.9 and the reference case is a round jet of the same flow conditions. This 

figure shows how the noise of a rectangular nozzle compared with that of a standard round jet. In all other 

cases in this paper the reference case is the basic 8:1 rectangular nozzle. For most of the paper the range of the 

color bars is held fixed at –4 < dB < 4. This convention allows easy cross-comparisons of all the 

configurations. A small graphic is included in each figure to note the nozzle and the plane of measurement. 
NASA/TM—2015-218901 6 

Figure 3.—Far-field acoustic array locations relative to SHJAR. Blue arc is 

horizontal polar array, green arc is vertical polar array, red arc is azimuthal 

array. Yellow arc is at constant polar angle = 150°, for visual reference. 

Figure 4.—Spectral directivity of PSD for base rectangular nozzle relative to round nozzle, in major and minor 

azimuthal planes. Ma = 0.9, unheated flow. Color indicates difference in dB relative to equivalent round jet. 

Inset sketches give orientation of nozzle and measurement arc. 
NASA/TM—2015-218901 7 

A. Bevel 

The impact of lengthening a broad side of a rectangular nozzle is to increase the noise produced by a 

given flow, as demonstrated in Figures 5 and 6. The short bevel added 1 to 2 dB to most all angles at the 

peak frequency and above. The long bevel added 2 to 3 dB. At the lowest frequencies the bevel nozzle is 

1 dB or less quieter than the base rectangular nozzle. In this case the nozzle had a triangular side wall which 

makes it somewhat different than a simple rectangular nozzle mounted on an aft deck; however, previous 

work (Ref. 3) showed that the presence of the sidewall made little difference on the noise. 

The main parameter that controls the noise of the bevel, or aft deck, configuration is the length of the 

long lip. Note that the noise increase is greatest in the minor axis planes and is the same on both the long 

and short sides of the bevel. In Reference 11 it is shown that the additional noise has a strong dipole 

behavior, which becomes more evident as the nozzle lip is extended. Thus the extra noise is a separate 

source from the jet mixing noise and scales differently with velocity, as demonstrated in Figure 7. By 

looking at the how the effect of the bevel changes with velocity it is clear that the effect is not a change in 

efficiency of the same jet mixing noise source, but a separate noise source. At Ma = 0.5, the dipole source 

produced by the turbulent shear layer passing over the extended edge dominates the quadrupole jet 

mixing source and the impact of the extended lip is roughly 10 dB at its maximum. At Ma = 0.9, the 

dipole source, scaling as Ma (Ref. 6), is not as dominant over the Ma (Ref. 8) jet mixing noise, and the 

impact of the extended lip is roughly 4 dB. 

In summary, ignoring the complexity of the additional noise source, it is safe to say that the bevel 

primarily adds noise, and does not create ‘shielding’ of the jet noise. 

 

Figure 5.—Spectral directivity of PSD for L/h = 1 bevel nozzle relative to base rectangular nozzle, in three azimuthal 

planes. Ma = 0.9, unheated flow. 
NASA/TM—2015-218901 8 

 

Figure 6.—Spectral directivity of PSD for L/h = 2 bevel nozzle relative to base rectangular nozzle, in three azimuthal 

planes. Ma = 0.9, unheated flow. 

 

Figure 7.—Effect of jet velocity on spectral directivity of PSD for L/h = 2 bevel nozzle, acquired on minor axis, open 

side. Ma = 0.5 (left), 0.7 (middle), 0.9 (right). Note color bar range is twice that of other figures! 
NASA/TM—2015-218901 9 

B. Slant 

If lengthening the broad side of the rectangular nozzle creates a bevel nozzle, then lengthening one of 

the short sides creates the slant nozzle. Two lengths of slant nozzle were tested: a short slant with length 

two times the nozzle height, and a long slant with length four times the nozzle height. The spectral 

directivities of these two configurations are given in Figures 8 and 9, respectively. 

Unlike the bevel nozzle, the slant produced a relatively small impact on the noise. For the Ma = 0.9 

flow (Figs. 8 and 9) the impact was generally less than 1 dB at all angles and all frequencies. Also, unlike 

the bevel nozzle, the noise was not similar for both long and short sides of the nozzle, leading to the 

expectation that any dipole source produced must be small since it would be symmetric in the major axis 

of the nozzle. By comparing the short and long slant nozzle results, it can be seen that the impact of the 

slant slightly increases with slant length, but is a small effect at either length. 

Looking at lower velocities, however, gives a slightly different picture. Figure 10 shows how the noise 

on the long side of the long slant nozzle varies with jet velocity. The effect of the slant, as observed in the 

color of the surfaces representing delta-dB from the base rectangular nozzle, is much stronger at the lower 

speeds. At lower speeds the difference is 2 to 3 dB rather than 1 dB, and it is fairly uniform across the polar 

angles and frequencies except for the high frequencies and forward angles. It is not clear whether this is due 

to a separate source, as in the case of the bevel nozzles, or a change in the jet mixing noise. 

In summary, the slant nozzle generally produces slightly more noise than the base rectangular nozzle, 

but the effect is 2 to 3 dB for mid-Ma flows decreasing to 1 dB for high Ma flows. 

 

Figure 8.—Spectral directivity of PSD for L/h = 2 slant nozzle relative to base rectangular nozzle, in three azimuthal 

planes. Ma = 0.9, unheated flow. 
NASA/TM—2015-218901 10 

 

Figure 9.—Spectral directivity of PSD for L/h = 4 slant nozzle relative to base rectangular nozzle, in three azimuthal 

planes. Ma = 0.9, unheated flow. 

 

Figure 10.—Effect of jet velocity on spectral directivity of power spectral density for L/h = 4 slant nozzle relative to 

base rectangular nozzle, acquired on major axis, closed side. Ma = 0.5 (left), 0.7 (middle), 0.9 (right). 
NASA/TM—2015-218901 11 

C. Single Chevron 

The single chevron designs are in some ways related to the slant nozzle in that they both feature 

nozzle trailing edges that are not normal to the flow. The single chevron nozzles tested here had very 

different lengths, the short chevron being one-half the nozzle height while the long chevron was twice the 

nozzle height. In both cases the chevron inner surface was parallel to the jet centerline, e.g., no 

penetration. However, given the initial jet spread usually found, and the induced change in the static 

pressure by the presence of the chevron, the chevrons were expected to produce a change in the flow. 

However, as seen in Figure 11 (short chevron) and Figure 12 (long chevron), the acoustic impact was less 

than 1 dB at all angles and frequencies. At this level of approximation to a realistic nozzle geometry the 

acoustic impact can be considered null. 

 

Figure 11.—Spectral directivity of PSD for L/h = 0.5 single-chevron nozzle relative to base rectangular nozzle, in two 

azimuthal planes. Ma = 0.9, unheated flow. 

 

Figure 12.—Spectral directivity of PSD for L/h = 2 single-chevron nozzle relative to base rectangular nozzle, in two 

azimuthal planes. Ma = 0.9, unheated flow. 

 
NASA/TM—2015-218901 12 

D. Notch 

Continuing with the idea of having trailing edges that are not normal to the flow, the notch is 

essentially the inverse of the chevron design. The angle is twice that of the slant for a given length since 

the notch reaches the base rectangular nozzle lip halfway across the nozzle. It was originally thought that 

the notch might encourage the rapid expansion of the jet cross-section in the minor axis plane, thereby 

enhancing an axis switch in the jet cross-section and the reduction of jet velocity downstream. 

In retrospect, after seeing the acoustic impact of the slant nozzle, which was to increase the noise on 

the long side of the nozzle, it is not surprising to find that the notch produced significant noise in the 

plane containing the major axis since both sides now are the ‘long’ side of the slants that comprise the 

notch (Fig. 14). This is exactly what is found for the long notch. The short notch (Fig. 13) seems to follow 

the short slant in not producing much if any acoustic impact relative to the basic rectangular nozzle. 

 

Figure 13.—Spectral directivity of PSD for L/h = 2 notch nozzle relative to base rectangular nozzle, in major and 

minor azimuthal planes. Ma = 0.9, unheated flow. 

 

Figure 14.—Spectral directivity of PSD for L/h = 4 notch nozzle relative to base rectangular nozzle, in major and 

minor azimuthal planes. Ma = 0.9, unheated flow. 

 
NASA/TM—2015-218901 13 

E. Septae 

The septae nozzle is an interesting case. Upon first seeing distributed exhaust concepts with their 

separated ducts one expects to find a strong increase in noise—common experience with louvers in air 

handling equipment is that these features make significant noise. Drafting model lines using guidelines 

from system studies produced a nozzle with continuously contracting nozzle areas, and the ability to print 

the design in plastic and test as an insert allowed quick testing of the concept. When the initial results 

showed that the septae insert reduced the jet mixing noise, even accounting for the reduced nozzle area, 

the test was repeated on the SHJAR and the findings were confirmed. 

As seen in Figure 15 there is a substantial reduction of noise in the low frequencies with the addition 

of the septae in the rectangular nozzle at Ma = 0.9. This reduction turns to increase at high frequencies, 

especially on the major axis. Measurements at other flow speeds support these preliminary findings. 

Figure 16 shows the same nozzle, same measurement locations, now at lower flow speeds, Ma = 0.5 and 

0.7. When scaled with jet exit velocity and equivalent-area diameter a strong tone at StD = 6.7 is observed 

on the major axis plane, at polar angles near 90°, reducing in amplitude relative to the jet mixing noise 

with increasing Ma until it disappears at Ma = 0.9. However, when scaled with jet exit velocity and 

trailing edge thickness t of the septae, the Strouhal number of the tone Stt is 0.18, a classic value for 

vortex shedding in a wake. The directivity is explained by the dipole nature of the shedding tone, directed 

normal to the trailing edges and the flow. As for the decreased noise at low frequencies it is conjectured 

that the shedding from the trailing edges of the septae are responsible for enhanced jet mixing at a frequency 

well above that of the jet instabilities, giving a classic reduction of noise from enhanced mixing. 

 

Figure 15.—Spectral directivity of PSD for septae nozzle relative to base rectangular nozzle, in two azimuthal 

planes. Ma = 0.9, unheated flow. 
NASA/TM—2015-218901 14 

 

 

Figure 16.—Spectral directivity of PSD for septae nozzle relative to base rectangular nozzle, in two azimuthal planes. 

Ma = 0.5 (top), Ma = 0.7 (bottom). 

F. Bevel With Septae 

Adding septae to the bevel nozzle reduces the noise created by the extension of the bevel lip, shown 

in Section A. Compare the spectral directivities of the bevel nozzle in Figure 6 with that of the bevel 

nozzle with septae in Figure 17 (Ma = 0.9; last set of plots). Relative to a base rectangular nozzle the 

septae in the L/h = 2 bevel produced a noise reduction at Ma = 0.9. At lower speeds, the septae reduced 

the added noise of the bevel (compare Fig. 7 with Fig. 17); however, the septae+bevel combination was 

still louder than the base rectangular nozzle at all frequencies above the peak at all polar angles, as shown 

in Figure 17. (When comparing figures, note that Figure 7 uses a color bar with twice the range.) The 

presence of the bevel did not reduce the shedding tone from the septae in the major axis. 

An interesting view of the effects of the septae+bevel combination is the spectral directivity in the 

azimuthal plane defined by polar angle 90° (Fig. 18). In this view, at the lower speeds, the dipole arising 

from the extended lip of the bevel nozzle shows a distinctive maximum at azimuthal angle 0° (minor 

axis), while the tones from the septae peak strongly at azimuthal angle 90°. Both of these sources are 

subsumed by the jet mixing noise by Ma = 0.9. In high-speed flows, the increase in the high frequency 

region, which likely will scale into the range of human annoyance, is isolated to the major axis, and not 

directed strongly to the ground. At lower speeds, however, the bevel+septae combination nozzle will 

produce more noise on the ground than will the base rectangular nozzle. 
NASA/TM—2015-218901 15 

 

Figure 17.—Spectral directivity of PSD for L/h = 2 bevel nozzle with septae relative to base rectangular 

nozzle, in two azimuthal planes. Ma = 0.5 (top), Ma = 0.7 (middle), Ma = 0.9 (bottom). 
NASA/TM—2015-218901 16 

Figure 18.—Spectral directivity of PSD for L/h = 2 bevel nozzle with septae relative to base rectangular nozzle, in 

equatorial plane (polar = 90°). Ma = 0.5 (left), Ma = 0.7 (middle), Ma = 0.9 (right). 

IV. Summary 

The noise of rectangular nozzles of aspect ratio 8:1 with various geometric treatments was explored. 

Extending the wide edge of one side of the rectangular nozzle created a bevel nozzle, which was found to 

increase noise over nearly all frequencies and polar angles, especially at angles broadside to the extended lip 

and at lower jet speeds. Extending the narrow edge of one side of the rectangular nozzle created a slant 

nozzle; this geometric variation increased peak frequency noise by 1 to 2 dB on the long side of the major 

axis, but otherwise made no significant change to the noise. Adding a single chevron with zero penetration 

to the wide edge of the rectangular nozzle made no acoustic impact even when it had a length twice the 

height of the rectangular nozzle. Inverting the chevron to create a single, wide notch on the broad side of the 

nozzle increased the noise by several dB, as the length became four times the nozzle height. Finally, adding 

internal vanes, or septae, to the base rectangular nozzle produced a reduction in noise at low frequencies, 

along with a directional shedding tone off the trailing edges of the septae. Mechanisms for this acoustic 

change are proffered, but more investigation is required to ascertain their validity. 

References 

1. Felder, J.L., Kim, H.D., Brown, G.V. “Turboelectric Distributed Propulsion Engine Cycle Analysis 

for Hybrid-Wing-Body Aircraft,” AIAA Paper 2009-1132, January 2009. 

2. Frate, F.C. and Bridges, J., “Extensible Rectangular Nozzle Model System,” AIAA Paper 2011-0975, 

January 2011. 

3. Bridges, J. “Acoustic Measurements of Rectangular Nozzles with Bevel,” AIAA Paper 2012-2252,

June 2012. 

4. Bridges, J. & Wernet, M.P. “Turbulence Measurements of Rectangular Nozzles with Bevel,” 

AIAA Paper 2015-0228, January 2015. 

5. Balsa, T.F., Gliebe, P.R., Kantola, R.A., Mani, R., Stringas, E.J., and Want, F.C.F., “High Velocity 

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

Experiments,” FAA-RD-76-79, May 1978. 

6. Massey, K.C., Ahuja, K.K., and Gaeta, R., “Noise Scaling for Unheated Low Aspect Ratio 

Rectangular Jets,” AIAA Paper 2004-2946, May 2004. 

7. Munro, S. & Ahuja, K.K., “Aeroacoustics of a High Aspect-Ratio Jet,” AIAA Paper 2003-3323, 

May 2003. 

8. Munro, S. & Ahuja, K.K., “Development of a Prediction Scheme for Noise of High-Aspect Ratio 

Jets,” AIAA Paper 2003-3255, May 2003. 

9. Munro, S. & Ahuja, KK, “Aeroacoustics of a High Aspect-Ratio Jet,” AIAA Paper 2003-3129, 

May 2003. 

10. Viswanathan, K., Spalart, P.R., Czech, M.J., Garbaruk, A., and Shur, M., “Tailored nozzles for jet 

plume control and noise reduction.” AIAA Journal 50.10 (2012), pp. 2115-2134. 
NASA/TM—2015-218901 17 

11. Brown, C.A., “Jet-Surface Interaction Test: Far-Field Noise Results,” J. Eng. Gas Turbines Power, 

135(7), June 2013. 

12. Bridges, J., “Noise from aft deck exhaust nozzles–differences in experimental embodiments.” 

AIAA Paper 2014-0876 (2014). 

13. Brown, C.A. & Bridges, J., “Small Hot Jet Acoustic Rig Validation,” NASA/TM—2006-214234, 2006.