K.B.M.Q. Zaman

Glenn Research Center, Cleveland, Ohio

Jet Noise Reduction by Microjets—A Parametric Study

NASA/TM—2010-215846

February 2010

AIAA–2009–3129
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K.B.M.Q. Zaman

Glenn Research Center, Cleveland, Ohio

Jet Noise Reduction by Microjets—A Parametric Study

NASA/TM—2010-215846

February 2010

AIAA–2009–3129

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

15th AIAA/CEAS Aeroacoustics Conference (30th AIAA Aeroacoustics Conference)

cosponsored by AIAA and CEAS

Miami, Florida, May 11–13, 2009
Acknowledgments

The author would like to thank Professor Anjaneyulu Krothapalli of Florida State University and colleagues, John Abbott and 

Nicholas Georgiadis for helpful comments. Support from the Subsonic Fixed Wing Project and the Supersonics Project, under 

NASA Fundamental Aeronautics Program, is gratefully acknowledged.

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Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

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. 

This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
NASA/TM—2010-215846 1 

Jet Noise Reduction by Microjets—A Parametric Study 

K.B.M.Q. Zaman 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The effect of injecting tiny secondary jets (‘jets’) on the radiated noise from a subsonic primary jet 

is studied experimentally. The jets are injected on to the primary jet near the nozzle exit with variable 

port geometry, working fluid and driving pressure. A clear noise reduction is observed that improves with 

increasing jet pressure. It is found that smaller diameter ports with higher driving pressure, but involving 

less thrust and mass fraction, can produce better noise reduction. A collection of data from the present as 

well as past experiments is examined in an attempt to correlate the noise reduction with the operating 

parameters. The results indicate that turbulent mixing noise reduction, as monitored by OASPL at a 

shallow angle, correlates with the ratio of jet to primary jet driving pressures normalized by the ratio of 

corresponding diameters (pd /pjD). With gaseous injection, the spectral amplitudes decrease at lower 

frequencies while an increase is noted at higher frequencies. It is apparent that this amplitude ‘crossover’ 

is at least partly due to shock-associated noise from the underexpanded jets themselves. Such crossover 

is not seen with water injection since the flow in that case is incompressible and there is no shockassociated noise. Centerline velocity data show that larger noise reduction is accompanied by faster jet 

decay as well as significant reduction in turbulence intensities. While a physical understanding of the 

dependence of noise reduction on pd /pjD remains unclear, given this correlation, an analysis explains the 

observed dependence of the effect on various other parameters. 

Nomenclature 

d microjet (jet) port exit diameter 

D primary nozzle exit diameter 

I overall sound pressure level (OASPL) 

J momentum flux ratio = U

2

/ jUj 

2 

k specific heat ratio 

m mass fraction = mass flow rate of all jets / mass flow rate of primary jet 

M Mach number 

MJ      1/2

1 /

1

2 1 



 





   k p p

k k

j a (“Fully expanded jet Mach number”) 

Mej Mach number at exit of primary nozzle 

n number of equally spaced jets 

p supply (total) pressure for primary jet or jet 

rm mass fraction per jet = m/n 

SPL sound pressure level 

t T /Tj (ratio of total thrust of all jets to thrust of primary jet) 

U mean velocity at exit of jet nozzle or jet port 

 density at exit of jet nozzle or jet port 

 microphone angular (polar) location, relative to downstream jet axis 

 jet injection yaw angle relative to radial direction 

 jet injection pitch angle relative to primary nozzle exit plane 
NASA/TM—2010-215846 2 

Subscripts 

a ambient 

j primary jet 

 jet 

1.0 Introduction 

Stricter regulations on aircraft noise have prompted numerous efforts addressing jet noise reduction. 

One technique receiving renewed attention is the use of ‘microjets’ (referred to as ‘jets’). It evolved 

from past studies on noise reduction with water as well as gaseous injection and simply involves 

impingement of tiny secondary jets on to the primary jet near the nozzle lip (Refs. 1 to 13). The concept 

dates back to the 1950s (e.g., Refs. 12 and 13) provides a literature review covering the five decades 

since. While a detailed review of past work is not central to this paper, a few pertinent points are 

important to note. 

First, water injection, used routinely in noise suppression for rocket nozzles during launch, requires a 

large mass fraction (water to jet exhaust mass ratio in the order of unity) (Ref. 11). This is obviously not 

feasible in aircraft flight where the mass fraction must be kept to an absolute minimum. There is growing 

evidence that noise benefit with small mass fraction may be possible with higher injection pressures 

(Refs. 6 and 13). Pushing this aspect of the technique to its limits, within the facility constraints, is an 

objective of the present study. 

Second, as with some other noise reduction techniques, jets have been shown to produce large 

effects on supersonic primary jets. This is apparently due to a weakening of shock structure resulting in an 

attenuation of shock-associated noise. Reference 3 reports up to 6 dB noise reduction, with mitigation of 

all of Mach wave radiation. A reduction of turbulent mixing noise, as occurring in a high subsonic jet, 

however, is far more challenging and holds the key to noise abatement in many aircraft. With the jets, a 

moderate but consistent noise reduction has been observed in high subsonic jets (Refs. 6, 9, 10, and 14). 

The scope of the present investigation is limited to the latter flow regime for an unambiguous study of the 

effect on turbulent mixing noise. 

Furthermore, very little has been known about the dependence of the noise attenuation by the jets on 

the operating parameters. Reference 3 noted a correlation with the momentum flux ratio (J), however, the 

data were limited. The study pertained to supersonic jets, leaving it unclear how the mixing noise and the 

shock-associated noise components were individually affected. Results in Reference 14 suggested that J

might not be the best parameter to correlate mixing noise attenuation. In this study, an attempt is made to 

find such correlations and underlying trends in turbulent mixing noise attenuation by the jets, by 

scrutinizing a collection of experimental data. 

The present study was initiated a few years ago in an effort to explore the full potential of the jet 

effect and advance the understanding of its mechanisms. First, the effect of air jets was explored with 

variable port geometry and driving pressure while paying attention to estimated thrust and mass fraction 

requirements. The results were summarized in Reference 14. A key observation was that the reduction in 

OASPL in the direction of peak noise radiation scaled with the ratio of the density-velocity-diameter 

product of the jet to that of the primary jet. In following experiments, gaseous CO2 and liquid H2O were 

also used as working fluid. The limit of jet driving pressure was pushed to twice the value covered 

earlier. The new results confirmed the scaling law noted in Reference 14 but only for gaseous injection. 

With water injection, large deviation from the correlation occurred. A normalized pressure parameter was 

then found to correlate all the data. Even though a complete physical understanding of this correlation 

remains from being clear, the experimental findings are deemed significant enough to merit 

documentation. The objective of this paper is to describe these results and provide an analysis and 

discussion. 
NASA/TM—2010-215846 3 

2.0 Experimental Facility 

The experiments were conducted in an open jet facility at the NASA Glenn Research Center 

(Fig. 1(a)). The primary jet issued from a convergent nozzle with exit diameter, D = 37.6 mm. The jets 

were injected via a ring manifold mounted on the primary nozzle (Fig. 1(b)). The surfaces were machined 

such that when placed all the way in the manifold sat flush with the lip of the nozzle. It was supplied with 

compressed air from a pressurized source (‘K-bottle’). There were six threaded outlets which could be 

fitted with tiny screws (similar in size as used with eyeglasses). The jets issued from ports and passages 

machined in these screws. Different sets of screws provided different port geometries. A picture of the 

manifold, with one of the screws in the foreground, is shown in Figure 1(c). 

Dimensions of the manifold and the ports are shown schematically in Figure 2(a). Data from 

cylindrical ports with diameters, d = 0.203 and 0.102 mm will be presented in this paper. The ports were 

drilled in the brass screws with a tolerance of 0.005 mm. For a given set, the number of ports could be 

varied by blanking off some of the outlets with ‘blind’ screws. Each screw could also be turned to inject 

the jet at a desired yaw angle () relative to the radial direction (Fig. 2(b)). The injection pitch angle () 

was zero in the present experiment. Note that there is a total stand-off distance of 0.89 mm in the radial 

direction between the port exit and the periphery of the jet, 0.76 mm being the thickness of the nozzle lip. 

The manifold could also be positioned with a stand-off distance, x, in the axial direction with respect to 

the nozzle lip. 

For the smaller ports in the present experiment the diameter ratio d/D is 0.0027 which is smaller than 

that in previous studies. For example, the smallest d/D was 0.0038, 0.012, 0.018, and 0.020 in 

References 5, 4, 6, and 10, respectively. Based on the diameter ratio, the attribute ‘microjet’ is obviously 

a misnomer but it has prevailed in the literature. Also, much higher injection pressures, up to 12.3 mPa 

(1770 psig), were investigated in the present study compared to, for example, 5.6 mPa in References 3 

and 14. Note that the spectral data acquisition took about 35 sec during which the bottle pressure would 

drop by several hundred kPa depending on regulator pressure and port size. Thus, the choice of maximum 

working pressure was somewhat lower even though bottle pressure up to 14.5 mPa was available, in order 

to ensure a constant pressure during runs. It should also be noted that the data presented in this paper were 

obtained in many sessions over a period of several months. The chosen pressure (p) for various sets of 

data was sometimes dictated by available bottle pressure. Thus, the reader may notice some 

inconsistencies in the choice of p from figure to figure but this did not impact the conclusions. 

The CO2 injection was done with the same hardware covering pressures up to about 5.5 mPa (limited 

by liquefaction at room temperature). For water injection, a 1.9 cm i.d., U-shaped, stainless steel tube was 

placed in the supply line (Fig. 1(a)). The 4 m long tube was filled with distilled water making sure there 

were no air pockets. The column of water was driven by compressed air and the rest of the hardware was 

unchanged. Unfortunately, clogging occurred with water injection possibly due to some sort of residues 

depositing within the tiny ports. The problem aggravated with the smaller 0.1 mm ports and impacted 

data repeatability as discussed later. Water injection data from the 0.2 mm ports only are to be presented. 

The exit flow properties of the jets including the mass fraction were calculated from the supply 

pressure p and the port diameter, using isentropic nozzle flow equations. The pressure was measured at 

the regulator just downstream of the K-bottle. With the larger ports there was a pressure drop in the 

supply line, as determined by separate measurements, by about 10 percent. All pressures (p) were 

corrected to represent the pressure within the manifold. Some of the data from the literature, to be 

compared with in the following, involved direct measurement of the mass fraction m. The pressure p and 

other exit properties for these cases were calculated from the m data using the nozzle flow equations. Note 

that in all calculations boundary layer effects were neglected, i.e., the discharge coefficient was assumed 

unity. 

The far-field noise was measured with (B&K model 4135) microphones located at 37 and 50 D from 

the nozzle exit for  = 90° and 25°, respectively. The microphone angular location  is referenced with 

respect to the downstream jet axis in this paper (Fig. 2(b)). Most of the data presented are for  = 25°, the 
NASA/TM—2010-215846 4 

direction of peak noise radiation. The spectral data reported earlier in Reference 14 were obtained with a 

Nicolet 660B analyzer. Later, the data acquisition was done by a PC based system together with National 

Instruments hardware and Labview software. Spectral analysis was done digitally and the OASPL data 

were obtained by integration of the spectra. Figure 1(a) indicates that the test environment was less than 

ideal for noise measurement. However, a comparison with data from other facilities showed that the 

spectral amplitudes were within a facility-to-facility scatter band of about 2 dB (Ref. 15). The data quality 

is considered adequate for studying the effect of the jets relative to the baseline (no-jet) case. Note, 

however, that the differences in the OASPL values in some cases bordered on the limits of data 

repeatability. The test chamber was climate-controlled and care was taken to ensure best possible 

measurement conditions. The data for the baseline as well as a few other cases were repeated several 

times over a period of several months. The scatter in the OASPL values is discussed shortly in the 

following sections. 

Standard Pitot probe and single hot-wire techniques were used to measure centerline variation of flow 

properties. Caveats in these measurements will be discussed together with the results. Flow visualization 

pictures were obtained for the water injection case. All data acquisition, activation of the jet and setting 

its pressure as well as flow visualization were done under remote computer control. All results presented in 

this paper are for cold flow, i.e., with the total temperature the same everywhere as in the ambient. 

3.0 Results 

For the earlier results reported in Reference 14, the jet parameters were varied within the constraints 

of the experimental facility, while looking for maximum noise reduction. Noise spectra were compared 

while varying injector yaw, arrangement, number, geometry, etc. Key results are narrated here and an 

interested reader may find details in the cited paper. It was found that yawing the jets improved noise 

reduction relative to the case of radial injection; this is discussed further with Figure 3. In one 

arrangement, the yaw angle was alternated (positive and negative) for successive ports. The noise 

reduction with this geometry was smaller compared to that produced by six ports yawed in the same 

direction. Reducing the number of ports from six to three approximately halved the OASPL reduction at 

 = 25°. Reference 10 reported optimum noise reduction with 18 ports when the number of ports was 

varied from 3 to 36. In fact, their OASPL reduction data (in dB) exhibited almost a linear variation with n, 

up to 18. This trend will be invoked when comparing the present results with data from the literature. 

Figure 3 shows an important result from Reference 14. OASPL reduction is plotted as a function of 

the yaw angle. The circular symbols are for the 0.2 mm ports with p = 3410 kPa. For each data point, all 

six ports are yawed by the same angle (). The optimum reduction has occurred at  = 45°. The three 

diamond symbols are for the 0.1 mm ports with p = 6930 kPa. The calculated mass fraction and thrust of 

the jets as well as momentum flux ratio for the two port cases are listed in Table 1. First, it is observed 

that for both ports a small additional reduction has been achieved with yaw relative to the zero-yaw case. 

For the smaller ports the optimum has occurred at  = 25° as compared to 45° with the larger ports. This 

shift is likely to be due to the stand-off distance between the port exit and the periphery of the jet 

(Fig. 2(a)). At 45° yaw, with the smaller port, the jet has to travel a longer relative distance (about 18 d, 

including the estimated shear layer thickness) before impinging on the core of the jet. Thus, the impact is 

diminished because of turbulent diffusion of the jet. This also explains the rapid drop-off of the effect 

for  > 45° with either port cases. It is evident that the noise reduction has improved with the smaller 

ports. By inspecting the properties listed in Table 1, the following trend is noted: smaller ports with 

higher pressure, involving less jet mass fraction and thrust, can produce better noise reduction. Thus, for 

radial injection ( = 0°) an additional 0.4 dB reduction has been achieved with the smaller ports. This is 

achieved with 50 percent less mass fraction and thrust. This trend is confirmed with further data in the 

following. 

Except for Figure 3, all data presented in this paper were obtained subsequent to the experiments of 

Reference 14. All results from the literature, to be discussed in the following, pertained to zero yaw. 
NASA/TM—2010-215846 5 

Thus, for direct comparison the subsequent experiments were also done for zero yaw, foregoing the 

additional gain noted for   45°. Figure 4 shows the effect of injection location, x (Fig. 2(a)). OASPL 

data for  = 25° are shown with and without injection from six 0.2 mm ports at p = 4175 kPa. For 

no-injection, the level has not changed with varying x except for some scatter and an average line is 

drawn through the data. With injection (diamond symbols) the levels are lower. Difference from the 

average line for the no-injection case denotes OASPL reduction. For x = 0 (manifold face flush with 

nozzle lip), a reduction by approximately 1.5 dB has been achieved. However, with even a slight increase 

in x, the reduction drops significantly. Norum (Ref. 4) also noted an increased noise reduction when the 

injection was done closer to the nozzle; however, his experiment addressed effect on shock-associated 

noise and involved large steps in the axial location of the injectors (in fractions of D rather than d). In 

Figure 4 the reduction is seen to level off for x > 1 mm. For larger x, the primary jet adjusts to the 

cylindrical passage of the manifold that effectively becomes the nozzle for the ensuing jet of slightly 

larger diameter. Since the jets are injected just downstream of the manifold face the relative injection 

location for this larger jet remains unchanged. This apparently explains the leveling off of the amplitude. 

It should be mentioned here that the data of Figure 4 were actually obtained at the very end of the 

investigation but are shown first to also provide the reader with an idea about the scatter involved (the 

issue is further discussed at the end of Section 3.1). Data taken over several days are shown in Figure 4 

exhibiting a scatter band of about 0.4 dB. However, the sets of OASPL reduction results to be presented 

in the following were obtained in single runs each and are thought to involve much less scatter, within 

0.2 dB. Note that all subsequent results are for x = 0. 

3.1 Noise Attenuation at  = 25° and Correlation With Operating Parameters 

The most significant result of the investigation is introduced in Figure 5 and it calls for a step by step 

discussion. The OASPL reduction at  = 25° is shown as a function of mass fraction per injector in 

percent. First, let us focus on the data points bunched on a line on the left (diamond, right- and lefttriangle symbols). These are for six 0.1 mm ports, the diamond symbol being for CO2 injection and the 

other two for air. The diamond and right-triangles are for injection at a fixed MJ (= 0.95) with variable p

while the left-triangles are for a fixed p (= 7000 kPa) and variable MJ. These data are congruent and the 

observed OASPL reduction is simply a function of the mass fraction for the given port diameter. Next to 

these on the right is another set of data for six 0.2 mm ports (circle, gradient and delta symbols). Here, the 

circles (CO2) and the gradients (air) are for injection with variable p at MJ = 0.95, while the deltas (air) 

are for variable MJ with p = 5130 kPa. As with the 0.1 mm ports, the data are also congruent. In spite of 

two working fluids the noise attenuation is dependent only on the injection mass fraction, for a given port 

diameter. By drawing a vertical line through these two sets of data it is also evident that more noise 

reduction is achieved with the smaller ports for a given mass fraction, corroborating the result discussed 

with Figure 3. 

Also shown in Figure 5 are a few sets of data from the literature. Let us first consider the air injection 

cases. The data from Reference 10 (denoted ‘CSJB’) were approximately read off from their Figure 13. 

Reference 6 (denoted ‘AKSL’) quoted about 2 dB decrease in OASPL at  = 30°. These data are shown and 

identified in the legend. Note that both experiments are for cold flows and air-in-air injection in high subsonic 

jets, as in the present case. Table 2 compares the operating parameters. In both References 6 and 10, 18 jets 

were used as opposed to 6 in the present experiment. In order to make a proper comparison, first one needs to 

consider the amplitudes. The collection of data involves different number of jets, observation angle, primary 

jet Mach number as well as pitch angle ( = 0° in the present case versus 45° in the others). Unfortunately, the 

available database is not enough to understand and fully account for the differences in the effects of these 

parameters. It appears that by far the largest effect is due to the difference in the number (n) of jets. As stated in 

the foregoing, Reference 10 reported almost a linear variation of OASPL reduction with n, up to 18. The 

OASPL values, for six versus three ports, reported in Reference 14 also followed this trend. Thus, if 6 jets 

were used instead of 18 in the two cited experiments, one might expect the reductions to be approximately 3 
NASA/TM—2010-215846 6 

times smaller. This rule has been followed in presenting those data in Figure 5. Additional effects due to 

differences in pitch (), jet Mach number (MJ) and polar location () are ignored. While the latter parameters 

may affect the amplitudes somewhat, it should become evident shortly that the ‘collapse’ of the data hinges 

largely on the choice of the abscissa and exact scaling of the amplitudes may not be critical in the exercise that 

we embark on in the following. 

Upon an inspection it should be evident that all three curves from ‘CSJB’ exhibit a peak followed by 

a dip and then again an increase with increasing rm. (An interested reader may look up either Reference 10 

or 14 to see these trends clearly with an amplified vertical scale.) The authors reasoned that the dip 

occurred when the jet transitioned from subsonic to supersonic regime. Specifically, the dips occurred 

just past the condition where the jets became supersonic when additional shock associated noise from 

themselves caused a drop in the measured noise reduction. However, with further increase in rm

eventually the trend of increasing noise reduction resumed. The present data do not exhibit such peaks 

and dips. This is simply because the jets are supersonic for the entire parametric range covered. 

The solid symbols in Figure 5 are for water injection. The gradient data points are taken from 

Reference 4 that reports a detailed study covering subsonic and supersonic primary jets including high 

temperature effect. Two types of injection (spray versus jet) were explored while varying the number of 

injectors. Here, data for a condition close to the present case are compared—six injectors for a cold 

primary jet at MJ = 0.8 (case ‘J2/06/45’). The raw directivity data (30° <  < 135°), kindly provided by 

the author, showed noise reduction at most angular locations with some scatter. Data for  = 32.5° are 

shown. As stated in Section 2, while the mass fraction for the present experiment was estimated from the 

driving pressure, it was measured directly in Reference 4 as well as Reference 10. On the other hand, 

while p was measured directly in the present experiment it was deduced from the mass fraction data for 

the cited cases. The solid triangular symbols in Figure 5 represent water injection result from the present 

experiment. These fall to the left of the data of Reference 4. Since Reference 4 involves much larger ports 

(1.6 mm), it can be inferred that water injection with the smaller (0.2 mm) ports has also yielded better 

noise reduction at a given rm, in a similar manner as seen with the gaseous injection cases. It is also noted 

that the data for water and gaseous injection from the present experiment, with the same set of (0.2 mm) 

ports, are separated by an order of magnitude in the abscissa. Clearly the noise reduction does not depend 

on the parameter rm alone. Neither would the data ‘collapse’ if plotted as a function of the total mass 

fraction (m = rmxn). For example, the present air-injection data falling to the left of the data from 

References 6 and 10 would move further to the left since only 6 ports are used as opposed to 18 in the 

latter cases. 

An attempt is made to explore if a certain combination of parameters would collapse the data of 

Figure 5, with the expectation that such an exercise might provide insight into the scaling as well as the 

underlying mechanism. The data are plotted and compared with various parameters for the abscissa. In 

order to make a ‘fair’ comparison the abscissa scales in all plots are chosen to span 4 decades. Figure 6 

shows the result as a function of the ratio of density-velocity-diameter product. It is apparent that all the 

gaseous injection data have collapsed reasonably. This was a key observation in Reference 14 that is 

confirmed with a larger variation in p and an additional working fluid for the jet. It is also immediately 

clear that the water injection data have not followed the correlation. In Figure 7, the data are plotted as a 

function of the thrust fraction. Here, Tμ represents calculated total thrust of the jets. Clearly, the data are 

far from collapsed. Similarly, a lack of collapse is noted when the data are plotted as a function of the 

momentum flux ratio J (Fig. 8); the effect of J will be further discussed in Sections 3.4 and 3.5. Similar 

exercise with velocity ratio, U /Uj, velocity-density product ratio, U /jUj, or the quantity rm/d2

 (as 

explored in Reference 10) failed to produce a reasonable collapse. 

The best collapse occurred when the data were plotted as a function of the ratio of the driving 

pressures normalized by the ratio of the diameters, as shown in Figure 9. Practically all data now fall in a 

continuous band. Thus, the noise reduction appears to correlate with the parameter p d/pjD. Note that the 

database for reaching this correlation pertains to high subsonic jets and in the direction of peak noise 

radiation. Thus, the correlation applies to turbulent mixing noise reduction. 
NASA/TM—2010-215846 7 

As stated before, for comparing the relative collapse, the data in Figure 9 are shown with the abscissa 

spanning four decades. Thus, the curves are squeezed together and might hide finer trends. In Figure 10, 

the same data are examined with an expanded linear abscissa. There are conspicuous differences among 

the different sets; however, no clear trend is apparent. The water injection data from the present 

experiment exhibit the largest deviation. The lower amplitudes for this case are thought to be partly due to 

the clogging of the ports as discussed in Section 2. Some of the data points within this set are repeats from 

later runs and exhibit the largest scatter. Ignoring this set, the rest of the data fall within a cluster that 

could be represented by a linear correlation, 

I 12.0 pd / p jD (1) 

where I is OASPL reduction at a shallow angle. Note again that p d/pjD applies to the condition for each 

jet and I represents noise reduction achieved by using six jets. 

With regards to the repeatability of the data, referring back to the discussion of Figure 4, it should be 

noted that after the water injection experiment discrepancies occurred even with air injection when trying 

to repeat some measurements. Although the injectors were cleansed in ultrasonic baths, the amplitudes 

especially with the 0.1 mm ports were lower. The larger ports could be restored mechanically by inserting 

a drill bit under microscope but this was not possible with the more fragile drill bit for the smaller ports. 

A new set of injectors was fabricated but it turned out that the stand-off distance of the ports was 

different, (  0.25 instead of 0.13 mm; Fig. 2), and the results could not be reproduced. However, the 

basic trends in the relative amplitudes remained valid. These deliberations prompted the experiment 

described with Figure 4. Subsequent flow-field experiments described in Section 3.3 were conducted only 

with the 0.2 mm ports. 

3.2 Noise Attenuation at  = 90° and Self Noise From jets 

In previous investigations, the noise reduction by the jets was noted at shallow as well as large 

angles. This is seen from the directivity plots of References 4 and 5 for high subsonic jets and 

Reference 3 for imperfectly expanded supersonic jets. In the present experiment data were acquired only 

at 25° and 90°. While the 25° data, discussed so far, exhibit consistent noise reduction the results for 90° 

are mixed. This can be seen in Figure 11 where data for 90° corresponding to the present cases of 

Figure 10 are plotted. The magnitudes are small exacerbating the scatter. Nevertheless, overall trends can 

still be discerned. For smaller values of p d/pjD, there is a noise reduction (positive ordinates) but at 

higher values many of the cases exhibit a noise increase (negative ordinates). 

It is noteworthy that most of the data from the literature pertain to lower values of p d/pjD. In 

Reference 5, for example, noise reduction was shown to be practically uniform over all  in two different 

facilities. With p = 17.2 bar and d/D  0.0075 for either facility, p d/pjD turns out to be 0.075. At this 

abscissa value a reduction in the noise is also indicated in Figure 11 for all cases. Thus, it is possible that 

the noise increase at  = 90° may be characteristic of operation at high p d/pjD.

At the highest value of p d/pjD  0.16 in Reference 4, the directivity data also showed a noise 

reduction practically over all  (private communication). At corresponding location in Figure 11, the 

gaseous injection cases exhibit a noise increase. However, the present water injection data also exhibit a 

noise reduction (solid delta symbols). It was shown earlier in Reference 14 that with air injection often 

there is an increase in high frequency noise even though there was a reduction at lower frequencies and in 

OASPL. Comparison of spectra for water and air injection suggests a difference in the high-frequency 

‘crossover’. This is shown by the spectra in Figures 12 and 13. The injection pressure was p  4500 kPa 

for both air and water and the OASPL reduction at 25° was about the same. While the effect at low 

frequencies is similar there is a clear difference on the high frequency end. A crossover occurs under air 

injection but none with water. A similar observation is made from the 90° data in Figure 13. It appears that 
NASA/TM—2010-215846 8 

the high frequency crossover is at least partly due to self noise from the jets. With gaseous flow the jets are in 

highly underexpanded supersonic condition and noisier. Comparatively, with water the flow is incompressible 

and there are no shocks or shock-associated noise. The self noise of the air jets is illustrated in Figure 14 

illustrating elevated levels in the frequency range where the crossover takes place. Note, however, that simply a 

vector summation of the self noise with the baseline noise does not account for the total increase in the 

amplitudes. It appears that the flow interactions generate further high frequency noise which, intriguingly, is 

absent with the water injection. 

3.3 Flowfields 

A few aspects of the flowfield are now explored. Figure 15 shows pictures of the jet with water 

injection. In Figure 15(a), six 0.2 mm jets are injected at p = 3200 kPa. The individual jets can be seen 

near the nozzle that fills the core of the jet by about two diameters downstream. Subsequently, a jet of 

mist remains visible over a distance of tens of diameters. The trajectories of the jets in the vicinity of the 

nozzle were explored. Five of the ports were closed off and only one port was allowed to inject. The flow 

field was photographed with a camera-angle of 15° with respect to the nozzle exit plane; an example is 

shown in Figure 15(b). The trajectories of the outer edge of the jet were read off from the pixel counts in 

the digital pictures. Appropriate correction for the viewing angle was applied. The trajectories are shown 

in Figure 16 for different injection pressure (mPa), indicated in the legend. The lines through the data are 

‘power function’ curve-fits. As expected, an increasing penetration with increasing p is evident. Note 

that even at the highest p (= 10.6 mPa) the jet is far from ‘piercing’ the primary jet. The jet centerline is 

at y/d = 93. 

The penetration of the jet into the primary jet may be compared to available correlations for jet-in-acrossflow. A set of data for J = 20 is shown in Figure 17. The (red) dotted curve represents text book 

correlation for subsonic air-in-air injection, y/d = J

0.33 (x/d)0.43 (Ref. 16). Note that a similar correlation 

was found to apply even with supersonic jets injected in supersonic crossflow (Ref. 17). Obviously, the 

water jet does not follow this correlation. The mixing rate of a liquid jet in a gaseous medium is slower, 

yielding much deeper penetration. The dashed curve for liquid-in-air injection follows the equation, 

y/d = 1.55J

0.53 ln[1 + 1.66(x/d)], given in Reference 18. The present data are reasonably close to this 

curve. 

The correlations noted above should also apply to the 0.1 mm port case. For J = 20, the value of 

pd/pjD with the 0.1 mm port is one-half of that with the 0.2 port (0.038 as opposed to 0.077). 

Conversely, for a constant p d/pjD, the value of J is double with the smaller port. Using the correlation of 

Reference 18, the penetration at a fixed downstream distance of x/D = 0.5 can be calculated. For the 

0.2 mm port with J = 20 the penetration is y/D = 0.207, whereas for the 0.1 mm port with J = 40 it would 

be y/D = 0.169. Since pd/pjD remains the same, this suggests that a given noise reduction may be 

achieved with different jet penetration depending on the port size. In other words, a larger noise 

reduction may not necessarily be due to a higher penetration of the jet. 

Pitot probe surveys were conducted along the centerline of the jet. The data were converted to 

velocity assuming the static pressure to be the same as the ambient pressure. The results are shown in 

Figure 18. For the case without jets, a detailed profile was obtained under computer control. With the 

jets, only a few data points were acquired manually in order to conserve the bottle pressure. Two sets of 

data are shown with the pressures indicated in the legend. With jet injection a faster centerline decay of 

mean velocity is apparent; higher pressure causes faster jet decay. 

A hot-film probe (TSI 1260-20) was used to measure the centerline variations of turbulence intensities. Note that in compressible flow a hot-wire responds to a combination of velocity and density 

fluctuations (with the high overheat ratio of 1.5 it is relatively insensitive to temperature fluctuation). 

First, variations in mean ρu are shown in Figure 19. These profiles correspond to the conditions of 

Figure 18 and exhibit distinct differences. The ‘potential core length’ for the baseline case turns out 

shorter with the hot-wire data. The Pitot probe velocity calculation assumes a constant ρ that actually 
NASA/TM—2010-215846 9 

decreases with downstream distance relaxing to ambient level after several jet diameters. Moreover, the 

static pressure close to the nozzle usually is not the same as in the ambient—an assumption also made in 

the velocity calculation. On the other hand, the hot-film, which is calibrated against the plenum pressure 

by placing it at the nozzle exit, has its own problems. Besides sensor breakage in the high-speed flow, 

problems such as strain-gauging is often encountered that, in particular, affects the turbulence data. 

Nevertheless, the ρu data from the hot-wire are thought to be well represented whereas the ‘U’ data from 

the Pitot probe clearly have errors primarily due to variation of ρ with x. In Figure 19, the jet decay with 

the jets at the lower pressure is found to be practically indistinguishable from the baseline case. 

However, at the higher pressure, a trend similar to that noted in Figure 18 is observed. 

The root mean square (rms) fluctuation of ρu is shown in Figure 20. Relative to the baseline case, a 

distinct reduction in the turbulence is noted for both injection cases. A larger reduction in the peak level 

has taken place at the higher pressure. The turbulence data are commensurate with observations made by 

Arakari et al. (Ref. 6) using particle image velocimetry. As noted in that reference, the jets apparently 

disrupt the evolution of the large scale coherent structures causing a reduction in turbulence intensities that is 

accompanied by a reduction in jet noise. 

3.4 A Few Implications of the Results 

Recall that the parameter p d/pjD collapsed all data and the gaseous injection data also collapsed 

equally well with the parameter Ud /jUjD. Also, by comparing Figures 6 and 9 it can be seen that the 

abscissa value for a given level of OASPL reduction with the gaseous cases is almost identical in the two 

figures, i.e., p d/pjD Ud /jUjD. Let us examine this. Starting with compressible flow equations for 

pressure, temperature, and the equation of state, one can derive, 

2( 1)

1

2

1 2

1 1 

 





 



    

k

k

j j j ej Mej

k M

R

k U p (2) 

Here, j, Uj, and Mej are density, velocity and Mach number at the exit of the primary jet, pj is the total 

pressure driving the primary jet. (Note that the subscript ‘e’ is dropped for the exit values of  and U for 

convenience). Using corresponding equation for the jet and assuming air-in-air injection one finds, 

2( 1)

1

2

2( 1)

1

2

2

1 1

2

1 1



 



 



   





 



  





 



  

 



k

k

ej

k

k

e

ej

e

j j j

M k

M k

M

M

p D

p d

U D

U d

(3) 

For a subsonic primary jet, Mej = MJ by definition, and with injection from convergent ports, Me = 1. 

Therefore, for MJ = 1, 

p D

p d

U D

U d

j j j

    

 (4) 

At lower values of MJ, Ud /jUjD becomes slightly larger than p d/pjD. For example, at MJ = 0.95 

and 0.7 the ratio of the two parameters for air can be calculated from Equation (3) as 1.002 and 1.094, 

respectively. Thus, for the high subsonic jets considered here the two parameters are nearly equal. This is 
NASA/TM—2010-215846 10 

why the abscissa values for the air injection cases are almost identical in Figures 6 and 9. For CO2

injection, with k = 1.3 and MJ = Me = 1, one finds, 

p D

p d

U D

U d

j j j

    

 0.9744 (5) 

Thus, there is not much difference between the two parameters for CO2 injection either. With water 

injection, on the other hand, the flow is incompressible and the relationship changes. With, 

    U 2 p / (6) 

one can find, 

p D

p d

U D

U d

C

j j j

    



2

2

1 (7) 

Where, 

2 1

2

1 2

1 1

2

 





 



   

k

k

J

J M kM k C (8) 

For a given primary jet, C1 is a constant. Thus, in contrast to Equations (3) to (5), the LHS of Equation (7) 

involves velocity-ratio-squared. Therefore, given that the data collapse with pd /pjD, a deviation occurs 

with the water injection data when plotted as a function of Ud /jUjD (Fig. 6). 

With regards to the dependence of the effect on J the following may be noted. For air-in-air injection, 

starting with Equation (3), and with MJ = Me = 1, one can show, pd /pjD = Jd/D. With water injection, 

and for MJ = 1, it can be similarly shown that, pd /pjD = 0.37 Jd/D. Thus, if the data of Figure 5 were 

plotted as a function of Jd/D the cases for air (as well as CO2) would collapse while the water injection 

data would still remain separated. However, the separation would be only by a factor of about 0.37 and 

not by an order of magnitude as seen in Figure 6. The fact that the data would collapse with Jd/D once 

again implies the following. Depending on the port diameter, the same noise reduction would be achieved at 

different values of J. Since the trajectory of the jet is a unique function of J, one infers that the noise 

reduction may not be a direct function of the jet penetration. 

Why is there a collapse of the data with pd /pjD? Why is the noise reduction (I), for a given primary 

jet, scale with p d? Unfortunately, the basic mechanism for turbulent mixing noise generation is not yet 

completely understood. Therefore, an understanding of the noise reduction with the jets remains even 

farther from being clear. In passing, one may note the following. For subsonic jets, experimentally it is known 

that OASPL (I) at a shallow angle scales as MJ 

9.5 (e.g., Reference 19). Let us assume that the effective Mach 

number of the primary jet has dropped by M under the influence of the jets. With I = C2MJ 

9.5, where C2 is a 

constant, one can show, 































    

9.5

9.5

2 1

J

J

J M

M M I C M (9)

With M << MJ, this reduces to, 
NASA/TM—2010-215846 11 

I  C M J M 8.5

2 (10) 

Thus, if p d were to reduce the jet Mach number by M, the noise reduction by I would be reconciled 

via Equation (10). If and why p d might cause the jet Mach number to drop by M remains unclear. The 

present as well as earlier measurements of Reference 6 indicate a clear reduction in jet turbulence under 

the influence of the jets. This perhaps is the most direct connection to the observed noise reduction. It is 

needless to reiterate that a full understanding of the noise reduction mechanism is considered beyond the 

scope of the present study. 

Finally, let us examine some practical implications. Recall that smaller ports with higher pressure, 

involving less mass fraction and thrust, produce better noise reduction. First, with the observed scaling 

with Ud /jUjD for the gaseous injection, this is readily reconciled. Say, d/D is decreased and the 

pressure is increased such that the parameter Ud /jUjD remains a constant. Since d/D has been 

decreased, mass fraction (= Ud2 /jUjD2

) also has decreased. Thus, with a smaller mass fraction the 

same noise reduction is achieved since Ud /jUjD has been kept constant. Pushing this trend to 

extremes in applications, however, would have obvious limits. Perhaps, injector diameter less than 

0.5 mm is not practical. Let us now consider the applicability of air injection to realistic engine 

conditions. Consider a 0.91 m (3-ft) diameter nozzle with hot flow at 593 °C (1100 °F). If one uses 

18 ports with d = 0.5 mm, Ud /jUjD = 0.06, (yielding 2.2 dB OASPL reduction per Equation (1) and 

recalling that it is for six injectors), would be achieved at about p = 11.2 mPa (1600 psig). With a 

compressed air reservoir of volume 0.0283 m3 (1 ft3

), if one starts with 15.3 mPa (2200 psig), estimates 

show that the supply, that would have lasted 40 min in the model scale experiment (with eighteen 0.1 mm 

ports to yield the same amount of noise reduction), would be exhausted in about 35 sec. Obviously such a 

supply will not be adequate in practice and an alternative source will be required. It might be possible to 

have a small auxiliary compressor to provide high pressure at the small flow rates. Perhaps, the technique 

will work better in conjunction with other devices such as tabs/chevrons (Ref. 20). Periodic injection with 

small duty cycle, if effective, might cut down the flow-rate requirement drastically. As a noise reduction 

technique, the potential of jet injection would obviously require some further exploration and research. 

Conclusions 

The effect of jets injected from ports of different geometry on the radiated noise from a subsonic 

primary jet has been studied experimentally. Air, CO2 and water were used as injection fluid while 

driving pressure up to 12.3 mPa was covered. A clear noise reduction is observed that improves with 

increasing jet pressure. It is found that smaller diameter ports with higher driving pressure, but involving 

less mass fraction and thrust, can produce better noise reduction. The results are examined in combination 

with air- as well as water-injection data from the literature. It is found that OASPL reduction at a shallow 

angle correlates with the ratio of jet to primary jet driving pressures normalized by the ratio of the 

corresponding diameters (pd /pjD). When all data are plotted as a function of this parameter they collapse 

reasonably well. The OASPL reduction (in dB) in such a plot increases almost linearly with this 

parameter and a correlation equation is provided. 

While pd /pjD collapses all data, the parameter Ud /jUjD is found to be equally effective for the 

gaseous cases. The water injection data, however, deviate from the gaseous cases when plotted as a 

function of the latter parameter. An analysis shows that for high subsonic primary jet and choked jets, the 

two parameters are nearly equal for the gaseous cases. With incompressible flow for the water jets, on the 

other hand, the two parameters are not equal. Thus, given that the data collapse with pd /pjD, a deviation is 

noted for the water case when plotted as a function of Ud /jUjD. 

The scaling with pd /pjD is observed towards the jet’s downstream direction and thus applies to 

turbulent mixing noise attenuation. At a measurement location perpendicular to the jet axis, the result is 

mixed and often an increase in the noise is noted. The collection of data suggests that the increase occurs 
NASA/TM—2010-215846 12 

at large values of pd /pjD with the gaseous cases. Spectral analysis show that there is actually noise 

reduction at lower frequencies while a ‘crossover’ takes place at high frequencies, similar to observations 

made in previous studies with other devices such as tabs and chevrons. The increase in the high frequency 

amplitudes may offset the gain at low frequencies causing a net increase in OASPL. For water injection, 

on the other hand, no such high frequency crossover occurs and the attenuation in OASPL is observed 

throughout the parameter range. Spectra taken with air jets alone suggest that the high frequency 

crossover is at least partly due to self noise from the jets. The jets are in a highly underexpanded 

condition and apparently shock-associated noise adds to the total noise. For incompressible flow with 

water jets there is no shock-associated noise and, thus, no crossover at high frequencies. 

Centerline velocity surveys made with a Pitot probe as well as a hot-film probe exhibit jet decay 

commensurate with the noise reduction at shallow angle. That is, larger noise reduction at higher pd /pjD

is accompanied by faster jet decay. A physical basis why the noise reduction data correlate with the 

parameter pd /pjD remains unclear. A connection is seen from the flow data in that the jets substantially 

reduce jet turbulence. With increasing pd /pjD the centerline turbulence profiles exhibit decreasing peak 

amplitude that is accompanied by larger reduction in the mixing noise. Finally, possible application 

potential of the technique is discussed. 

References 

1. Zoppellari, E. and Juve, D., “Reduction of Hot Supersonic Jet Noise by Water Injection,” AIAA Paper

98-2204, 1998. 

2. Krothapalli, A., Venkatakrishnan, L., Lourenco, L.M., Greska, B., and Elavarasan, R., “Turbulence 

and Noise Suppression of a High-Speed Jet by Water Injection,” J. Fluid Mech., vol. 491, 2003. 

3. Greska, B. and Krothapalli, A., “Jet Noise Reduction Using Aqueous Microjet Injection,” 10th 

AIAA/CEAS Aeroacoustics Conference, AIAA Paper 2004–2971, 2004. 

4. Norum, T.D., “Reduction in Multi-component Jet Noise by Water Injection,” 10th AIAA/CEAS 

Aeroacoustics Conference, Manchester, U.K., AIAA Paper 2004–2976, 2004. 

5. Greska, B., Krothapalli, A., Burnside, N., and Horne, W.C., “High-Speed Jet Noise Reduction Using 

Microjets on a Jet Engine,” 10th Aeroacoustics Conference, AIAA Paper 2004–2969, 2004. 

6. Arakeri, V.H., Krothapalli, A., Siddavaram, V., Alkisar, M.B., and Lourenco, L.M., “On the Use of 

Microjets to Suppress Turbulence in a Mach 0.9 Axisymmetric Jet,” J. Fluid Mech., vol. 490, 2003. 

7. Alkislar, M.B., Krothapalli, A., and Butler, G.W., “The Effect of Streamwise Vortices on the 

Aeroacoustics of a Mach 0.9 Jet,” J. Fluid Mech., vol. 578, 2007. 

8. Callender, B., Gutmark, E. and Martens, S., “A Comprehensive Study of Fluidic Injection 

Technology for Jet Noise Reduction,” 13th AIAA/CEAS Aeroacoustics Conference, AIAA Paper 

2007–3608, 2007. 

9. Camussi, R., Guj, G., Tomassi, F., and Sisto, R., “Effect of Air Injection on the Far Field Pressure 

Radiated From a Jet at Subsonic Mach Numbers,” Turbulent Jet Flows,” Int. J. of Aeroacoustics, 

vol. 7, no. 1, pp. 69–82, 2008. 

10. Castelain, T., Sunyach, M., Juve, D. and Bera, J.-C., “Jet-Noise Reduction by Impinging Microjets: 

an Acoustic Investigation Testing Microjet Parameters,” AIAA J., vol. 46, no. 5, May, 2008. 

11. Kandula, M., “Prediction of Turbulent Jet Mixing Noise Reduction by Water Injection,” AIAA J., 

46(11), pp. 2714–2722, 2008. 

12. Lilley, G.M., “Jet Noise Suppression Means,” 1961, U.S. Patent 2990905, (Ser. No. 733889, Claims 

Priority, application Great Britain May 13, 1957). 

13. Henderson, B., “Over 50 Years of Fluidic Injection for Jet Noise Reduction,” Int. J. of Aeroacoustics

(to appear), 2009. 

14. Zaman, K.B.M.Q., 2007, “Subsonic Jet Noise Reduction by Microjets With Various Injection Port 

Geometry,” AIAA Paper 2007–3643, 13th AIAA/CEAS Aeroacoustics Conference, May 21–23, 2007, 

Rome, Italy. 
NASA/TM—2010-215846 13 

15. Zaman, K.B.M.Q. and Dahl, M., “Noise and Spreading of a Subsonic Coannular Jet—Comparison 

With Single Equivalent Jet,” 43rd AIAA Aerospace Sciences Meeting, Reno, NV, Jan. 10–13, AIAA 

Paper 2005–0210, 2005. 

16. Abramovich, G.N., “The Theory of Turbulent Jets,” The M.I.T. Press, 1963. 

17. Papamoschou, D. and Hubbard, D.G., 1993, “Visual Observations of Supersonic Transverse Jets,” 

Experiments in Fluids, 14, pp. 468–476. 

18. Tambe, S.B., Jeng, S.-M., Mongia, H. and Hsiao, G., 2005, “Liquid Jets in Subsonic Crossflow,” 

43rd AIAA Aerospace Sciences Meeting, Reno, NV, Jan. 10–13, AIAA Paper 2005–731. 

19. Zaman, K.B.M.Q. and Yu, J.C., “Power Spectral Density of Subsonic Jet Noise,” J. Sound and Vib., 98, 

pp. 519–537, 1985. 

20. Alkislar, M.B. and Butler, G.W., “Significant Improvements on Jet Noise Reduction by ChevronMicrojet Combination,” AIAA Paper 2007–3598, 13th AIAA/CEAS Aeroacoustics Conference, 

May 21–23, 2007, Rome, Italy. 

TABLE 1.—CALCULATED MASS FRACTION AND 

TOTAL THRUST OF JETS, RELATIVE TO PRIMARY 

JET, CORRESPONDING TO THE DATA OF FIGURE 3 

Number of ports 6 6 

Port diameter, d (mm) 0.203 0.102 

Pressure, p (kPa) 3410 6930 

Momentum flux ratio, J 20 41 

Mass fraction, m (percent) 0.34 0.17 

Thrust fraction, t (percent) 0.60 0.30 

TABLE 2.—OPERATING PARAMETERS FOR THE 

DATA FROM DIFFERENT SOURCES 

INCLUDED IN FIGURE 5 

Work MJ D, 

mm 

d, 

mm 

n 

Present 0.95 37.6 0.203, 

0.102 

6 25 

Reference 10 

(CSJB) 

0.90 50 0.7, 1, 

1.3 

18 30 

Reference 6 

(AKSL) 

0.90 22.2 0.4 18 30 

Reference 4 

(TDN) 

0.80 67.8 1.6 6 32 

 
NASA/TM—2010-215846 14 

(a) 

(b) 

(c) 

Figure 1.—Experimental setup. (a) Jet facility and 

auxiliary components: (1) compressed gas bottle, 

(2) U-tube used for water injection and (3) injector 

manifold. (b) Close-up picture of nozzle. (c) jet 

manifold. 

(a) 

(b) 

Figure 2.—Schematic of nozzle and jet setup. (a) 

manifold details (0.76 mm is nozzle lip thickness, 

thus, jet travels 0.89 mm before impacting the jet), 

(b) definitions of jet yaw, pitch and microphone polar 

angle. Dimensions in mm. 
NASA/TM—2010-215846 15 

Figure 3.—OASPL reduction for varying yaw, for two 

port diameter cases from Reference 14;  = 25°, 

MJ = 0.95. 

Figure 4.—OASPL at  = 25° for varying injection 

location, x, compared to no-injection case; six 

0.2 mm ports, MJ = 0.95. 

Figure 5.—Comparison of OASPL reduction versus 

mass fraction per jet, with data from Reference 6 

(denoted AKSL), Reference 10 (denoted CSJB) and 

Reference 4 (denoted TDN). 

Figure 6.—OASPL reduction data of Figure 5 versus 

density-velocity-diameter-product ratio. 

Injection yaw (, deg)

OASPL reduction (dB)

0 30 60 90 0

0.5

1

1.5

2

6 - 0.2 mm ports, p = 3410 kPa

6 - 0.1 mm ports, p = 6930 kPa

x (mm)

OASPL (dB)

0 1 2 3

108

109

110

6-0.2 p = 0

6-0.2 p = 4175kPa

rm (=Ud2 / jUjD2), %

OASPL Reduction (dB)

10-3 10-2 10-1 100 10 0 1

0.5

1

1.5

2

2.5 18-0.7 CSJB

18-1.0 CSJB

18-1.3 CSJB

18-0.4 AKSL

6-0.2

6-0.1

6-0.1 diff Mj

6-0.1 CO2

6-0.2 CO2

6-1.6 H2O TDN

6-0.2 H2O

6-0.2 diff Mj

Ud / jUjD

OASPL Reduction (dB)

10-3 10-2 10-1 100 10 0 1

0.5

1

1.5

2

2.5 18-0.7 CSJB

18-1.0 CSJB

18-1.3 CSJB

18-0.4 AKSL

6-0.2

6-0.1

6-0.1 diff Mj

6-0.1 CO2

6-0.2 CO2

6-1.6 H2O TDN

6-0.2 H2O

6-0.2 diff Mj
NASA/TM—2010-215846 16 

Figure 7.—OASPL reduction data of Figure 5 versus 

ratio of calculated total jet thrust to primary jet 

thrust. 

Figure 8.—OASPL reduction data of Figure 5 versus 

ratio of calculated momentum flux ratio. 

Figure 9.—OASPL reduction data of Figure 5 versus 

normalized ratio of driving pressures. 

Figure 10.—Data of Figure 9 with linear expanded 

abscissa. 

T / Tj (%)

OASPL Reduction (dB)

10-3 10-2 10-1 100 10 0 1

0.5

1

1.5

2

2.5 18-0.7 CSJB

18-1.0 CSJB

18-1.3 CSJB

18-0.4 AKSL

6-0.2

6-0.1

6-0.1 diff Mj

6-0.1 CO2

6-0.2 CO2

6-1.6 H2O TDN

6-0.2 H2O

6-0.2 diff Mj

J (=U

2 / j

Uj

2

)

OASPL Reduction (dB)

10-1 100 101 102 10 0 3

0.5

1

1.5

2

2.5 18-0.7 CSJB

18-1.0 CSJB

18-1.3 CSJB

18-0.4 AKSL

6-0.2

6-0.1

6-0.1 diff Mj

6-0.1 CO2

6-0.2 CO2

6-1.6 H2O TDN

6-0.2 H2O

6-0.2 diff Mj

pd/pjD

OASPL Reduction (dB)

10-3 10-2 10-1 100 10 0 1

0.5

1

1.5

2

2.5 18-0.7 CSJB

18-1.0 CSJB

18-1.3 CSJB

18-0.4 AKSL

6-0.2

6-0.1

6-0.1 diff Mj

6-0.1 CO2

6-0.2 CO2

6-1.6 H2O TDN

6-0.2 H2O

6-0.2 diff Mj

pd/pjD

OASPL Reduction (dB)

0 0.1 0.2 0.3 0

0.5

1

1.5

2

2.5

18-0.7 CSJB

18-1.0 CSJB

18-1.3 CSJB

18-0.4 AKSL

6-0.2

6-0.1

6-0.1 diff Mj

6-0.1 CO2

6-0.2 CO2

6-1.6 H2O TDN

6-0.2 H2O

6-0.2 diff Mj
NASA/TM—2010-215846 17 

Figure 11.—OASPL reduction versus normalized ratio 

of driving pressures at  = 90°, corresponding to a 

few cases of Figure 10. 

Figure 12.—Comparison of SPL spectra at  = 25° for 

air and water injection with the baseline (bsln) case; 

six 0.2 mm ports, p  4500 kPa for both air and 

water. 

Figure 13.—Comparison of SPL spectra at  = 90° for 

air and water injection corresponding to the data of 

Figure 12. 

Figure 14.—SPL spectra at  = 25° with air injection at 

p = 7830 kPa (six 0.2 mm ports, denoted j and J in 

legend) compared to baseline (bsln) case. The other 

two traces are for test chamber ambient condition (amb) 

and with only the jets running at 6840 kPa (j). 

pd/pjD

OASPL Reduction (dB)

0 0.1 0.2 0.3

-0.8

-0.4

0

0.4

6-0.2

6-0.1

6-0.1 CO2

6-0.2 CO2

6-0.2 H2O

6-0.2 diff Mj

f (kHz)

Spl (dB)

100 101

40

50

60

70

80

90

100

bsln MJ=0.95 110.9dB

j&J MJ=0.95 108.3dB

amb MJ=0.00 69.1dB

j MJ=0.00 88.3dB
NASA/TM—2010-215846 18 

(a) 

(b) 

Figure 15.—Picture of jet at MJ = 0.95 with water 

injection; (a) injection from six 0.2 mm ports at p = 

3200 kPa, (b) one 0.2 mm port at p = 5600 kPa. 

Figure 16.—Trajectories of water jet from a 0.2 mm 

port into the MJ = 0.95 jet for various injection 

pressures (mPa). 

Figure 17.—Trajectory of water jet at J = 20 for a 

0.2 mm port; MJ = 0.95. Correlation for H2O-in-air is 

from Reference 18 and that for air-in-air is from 

Reference 16. 

Figure 18.—Centerline velocity as estimated from Pitot 

probe measurement for indicated injection pressures; 

six 0.2 mm (air) jets, MJ = 0.95. 

x/d

y/d

0 10 20 30 40 50 0

10

20

30

40

50

60

d=0.2 p= 10.6 J= 88

d=0.2 p= 5.5 J= 45

d=0.2 p= 2.5 J= 20

d=0.2 p= 1.8 J= 14

x/d

y/d

0 10 20 30 40 50

0

10

20

30

40

d=0.2 p= 2.5 J= 20

J=20 corr H2O-in-air

J=20 corr air-in-air

x/D

U/Uj

0 5 10 15

0.4

0.6

0.8

1

bsln Uj=323.7 p=0

d=0.2 Uj

=324.9 p=5960

d=0.2 Uj

=324.3 p=3010
NASA/TM—2010-215846 19 

Figure 19.—Centerline variations of mean hot-wire 

output for indicated injection pressures corresponding 

to the cases of Figure 18. 

Figure 20.—Centerline variations of the rms of hot-wire 

output corresponding to the cases of Figure 19. 

x/D

U / Uj j

0 5 10 15

0.4

0.6

0.8

1

bsln Uj=324.0 p=0

d=0.2 Uj

=325.3 p=5960

d=0.2 Uj

=324.0 p=2980

x/D

'u' / Uj j

0 5 10 15 0

0.02

0.04

0.06

0.08

0.1

0.12

bsln Uj=324.0 p=0

d=0.2 Uj

=325.3 p=5960

d=0.2 Uj

=324.0 p=2980
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4. TITLE AND SUBTITLE

Jet Noise Reduction by Microjets--A Parametric Study 

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Zaman, K.B.M.Q. 

5d. PROJECT NUMBER

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5f. WORK UNIT NUMBER

WBS 984754.02.07.03.17.04 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

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Subject Categories: 71 and 34 

Available electronically at http://gltrs.grc.nasa.gov 

This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT

The effect of injecting tiny secondary jets (‘μjets’) on the radiated noise from a subsonic primary jet is studied experimentally. The μjets are 

injected on to the primary jet near the nozzle exit with variable port geometry, working fluid and driving pressure. A clear noise reduction is 

observed that improves with increasing μjet pressure. It is found that smaller diameter ports with higher driving pressure, but involving less 

thrust and mass fraction, can produce better noise reduction. A collection of data from the present as well as past experiments is examined in 

an attempt to correlate the noise reduction with the operating parameters. The results indicate that turbulent mixing noise reduction, as 

monitored by OASPL at a shallow angle, correlates with the ratio of μjet to primary jet driving pressures normalized by the ratio of 

corresponding diameters (pμd /pj

D). With gaseous injection, the spectral amplitudes decrease at lower frequencies while an increase is noted 

at higher frequencies. It is apparent that this amplitude ‘crossover’ is at least partly due to shock-associated noise from the underexpanded 

μjets themselves. Such crossover is not seen with water injection since the flow in that case is incompressible and there is no shockassociated noise. Centerline velocity data show that larger noise reduction is accompanied by faster jet decay as well as significant reduction 

in turbulence intensities. While a physical understanding of the dependence of noise reduction on pμd /pj

D remains unclear, given this 

correlation, an analysis explains the observed dependence of the effect on various other parameters.

15. SUBJECT TERMS

Jets; Noise; Shocks; Turbulence 

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