EXPERIMENTAL STUDY ON A NOTCHED NOZZLE FOR JET NOISE REDUCTION

Transcription

1 Proceedings of ASME Turbo Expo 2011 GT2011 June 6-10, 2011, Vancouver, British Columbia, Canada GT EXPERIMENTAL STUDY ON A NOTCHED NOZZLE FOR JET NOISE REDUCTION T. Ishii*, H. Oinuma, K. Nagai Aviation Program Group Japan Aerospace Exploration Agency Jindaijihigashi-machi, Chofu, Tokyo, Japan ishii.tatsuya@jaxa.jp, oinuma.hideshi.jaxa.jp, nagai.kenichiro@jaxa.jp N. Tanaka, Y. Oba and T. Oishi Aero-Engine & Space Operations IHI Corporation 229 Tonogaya Nishitama-gun, Tokyo, Japan Nozomi_tanaka@ihi.co.jp, yoshinori_ooba@ihi.co.jp, tsutomu_ooishi@ihi.co.jp ABSTRACT This paper describes an experimental study on a notched nozzle for jet noise reduction. The notch, a tiny tetrahedral dent formed at the edge of a nozzle, is expected to enhance mixing within a limited region downstream of the nozzle. The enhanced mixing leads to the suppression of broadband peak components of jet noise with little effect on the engine performance. To investigate the noise reduction performances of a six-notch nozzle, a series of experiments have been performed at an outdoor test site. Tests on the engine include acoustic measurement in the far field to evaluate the noise reduction level with and without the notched nozzle, and pressure measurement near the jet plume to obtain information on noise sources. The far-field measurement indicated the noise reduction by as much as 3 db in terms of overall sound pressure level in the rear direction of the engine. The use of the six-notch nozzle though decreased the noise-benefit in the side direction. Experimental data indicate that the high-frequency components deteriorate the noise reduction performance at wider angles of radiation. Although the increase in noise is partly because of the increase in velocity, the penetration of the notches into the jet plume is attributed to the increase in sound pressure level in higher frequencies. The results of near-field measurement suggest that an additional sound source appears up to x/d=4 due to the notches. In addition, the total pressure maps downstream of the nozzle edge, obtained using a pressure rake, show that the notched nozzle deforms the shape of the mixing layer, causing it to become wavy within a limited distance from the nozzle. This deformation of the mixing layer implies strong vortex shedding and thus additional noise sources. To improve the noise characteristics, we proposed a revised version of the nozzle on the basis of a computational prediction, which contained 18 notches that were smaller than those in the 6-notched nozzle. Ongoing tests indicate greater noise reduction in agreement with the computational prediction. NOMENCLATURE D Diameter of Nozzle Exit EPNL Effective Perceived Noise Level SPL Sound Pressure Level OASPL Overall Sound Pressure Level R Distance between Microphone and Center Position Note that the center position is the center of a nozzle or an engine. S282, S286, S287 Serial Numbers of Engine Operation X Axial Position Relative to Nozzle Edge θ Radiation Angle from Jet Axis 1. INTRODUCTION 1.1 Noise Regulation and Jet Noise Aircraft noise regulations, due to growing attention being focused on environmental issues, require aircraft and engine manufacturers to further reduce noise. The present noise standard, which is prescribed in chapter 4 of the International Civil Aviation Organization (ICAO) Annex-16 [1], has introduced the concept of the cumulative noise margin as a primary condition in evaluating the noise of the subsonic-jet aircraft newly certified after The cumulative noise margin is the arithmetic sum of the margins from the effective perceived noise level (EPNL) at three measurement points [2], namely the approach, lateral and flyover points illustrated in Fig.1. In addition, as a supplementary condition, the present regulations require a cumulative noise margin of more than 2 EPN [db] at any two points instead of imposing a direct stringency at each measurement point. *Address all correspondence to this author. 1 Copyright 2011 by ASME

2 In addition to the regulations, the environmentally competitive aircraft and aero-engines are expected to be driving forces for those in noise-related fields attempting to promote low-noise technologies. Environmental benefits such as lownoise emission are predicted to become motivating factors for customers when choosing an aircraft or engine, leading to increased research and development. Among the noise sources of subsonic aircraft, gas-turbine engines are major noise sources during takeoff and approach. Focusing on the engine exhaust section during takeoff, jet noise with its extremely high sound level disturbs people in the neighborhood of airports. Jet mixing noise, caused by the highspeed plume interacting with the surrounding air, has been reduced by introducing the bypass engine concept in recent decades. However, modern high-bypass engines still require further jet noise reduction to improve noise margins at the lateral and flyover points. 1.2 Previous Studies on Jet Noise Reduction Regarding jet noise reduction, various devices to enhance mixing have been studied since the onset of jet-powered planes [3]. Noise reduction devices include lobed mixers, a chevrontype nozzles and more recently nozzles equipped with a secondary jet injection. The conventional multilobed mixer, which has a wavy chute at the trailing edge of the nozzle, is designed to mix the core flow with the bypassed flow. The mixing process induced by these wavy chutes contributes to a rapid reduction in the mean velocity and the suppression of large-scale vortices downstream of the nozzle. As a result, a large reduction in jet mixing noise is expected if it is used with a surrounding mixed-flow duct. From a practical viewpoint, the complicated geometry of the lobes makes it difficult to manufacture the nozzle with a reasonable cost and weight increase. Compared with a lobed mixer combined with a mixed-flow duct, a chevron-type mixer is easier to manufacture. The chevron, composed of triangular serrations along the nozzle edge, is inclined slightly toward the core flow and induces vortex shedding. A number of experimental studies on chevron nozzles have been carried out during the past decade [4-8]. A parametric study pointed out that the degree of penetration by a serration is related to the strength of vortex shedding and the amount of mixing [4]. Considering that the enhancement of mixing affects the thrust performance of the engine, the penetration of the chevron into the main stream may result in noise-related benefits exceeding thrust losses under the cruise condition. Unresolved issues still remain regarding the trade-off between noise reduction at lower and middle frequencies and that at higher frequencies. To optimize both noise-related benefits and thrust loss, a variable geometry combined with additional air-blowing has been proposed [6-8]. As an actively adjustable device, attention has been focused on devices that blow secondary air around the nozzle, known as a microjet [7-11]. A small but high-speed jet, if injected appropriately into the main stream, has the capability of promoting vortex shedding such that large-scale vortices downstream are weakened. In some experimental studies, the acoustic characteristics are discussed in terms of the ratio of the secondary air flow to the main flow [9, 10]. However, highspeed air injection is also a potential noise source and degrades the acoustic performance in some cases. 1.3 Proposed Notched Nozzle With the above background, a simple but efficient device for jet noise reduction, called a notch, has been proposed and studied as part of a Japanese research and development project on engines with lower environmental impact for small aircraft, known as the ECO engine project [12, 13]. A notch is a tetrahedral dent, formed at the trailing edge of a nozzle. The outer surface of the nozzle has dents for introducing the outer low-speed flow into the inner high-speed flow. The difference between a notch and a chute is that each notch lies between nondented parts, whereas a chute lies between neighboring Figure 1: A Schematic view of aircraft noise evaluation. Aircraft noise during take-off and approach is measured at three points, known as approach, lateral and flyover. Figure 2: A 6-notched nozzle installed on a demonstrator engine. The notch is located with an equal interval in circumferential direction of the nozzle edge. 2 Copyright 2011 by ASME

3 chutes. Compared with the other devices such as the chevron, it is expected that the notch has the following merits: 1) Easiness to manufacture the simple tetrahedral structure. 2) Slight weight increase in installing the notches on the original nozzle. 3) Flexibility of the acoustic performance by selecting the shape and position of each notch. 4) Fixed throat position without any adjustment devices. The inner surface of a notched nozzle forms blockages, the tetrahedral shape of which separates the inner flow. After some experimental trials in the ECO engine project [13], a proof-ofconcept test has started using a turbojet engine as a joint collaboration between Japan Aerospace Exploration Agency (JAXA) and IHI Corporation. A nozzle with six notches was manufactured and installed on a demonstrator engine. In Fig.2, the tested notched nozzle is shown, the notches of which are located uniformly at its edge. Noise tests on the notched nozzle were performed [14], which were followed fundamental model tests and computational predictions to verify the test results and improve the nozzle design. In these noise tests, a notched nozzle is compared with a baseline conical nozzle, the cross section of which is identical to that of the notched nozzle. This paper begins with an explanation of engine noise tests performed using a turbojet engine. The authors discuss the results for the notched nozzle, focusing on the noise at far-field points, the noise along the jet plume and the pressure distribution perpendicular to the jet plume. Strategies for improving the suppression of high-frequency components are discussed together with some results of scale model tests. 2. ENGINE NOISE TESTS 2.1 Turbojet Engine For the engine noise tests, a turbojet engine is employed as a jet noise source [15]. This single-shaft engine, which has a simple structure consisting of an axial-flow compressor, a subsequent centrifugal compressor, a combustor and a singlestage turbine, is suitable for multipurpose research and development. Its maximum thrust exceeds 0 kg under static conditions. Figure 3 shows typical performances in nondimensional form plotted against the engine rating. Data on the figure are obtained using a baseline nozzle. A 50% rating corresponds to an idling condition. Ratings of more than % are avoided for maintenance reasons. The corrected thrust and the corrected exhaust gas temperature increase steeply above a rating of %. In static tests, a % rating approaches the choke condition. In tests, a rating of between % and % is set, considering the thrust curve and the danger speed. In the operation of the engine, a simulation-based controller is used to stabilize the rating. Engine parameters, including rotational speed, thrust, pressure, temperature and vibration, are monitored and recorded for later analysis. 2.2 Attachments Used in Noise Tests Figure 4 illustrates the attachments used in the noise tests [14, 15]. They are an inlet screen, an inlet duct section and an exhaust duct section. The inlet screen is attached on the bellmouth of the inlet duct section. The primary role of the screen is to prevent damage from foreign objects during outdoor Nondimensional Parameters (%) Conical Nozzle (S103) Engine Rating (%) Figure 3: Typical performances of the demonstrator engine. The corrected thrust, rotational speed and exhaust gas temperature are plotted in nondimensional form against the engine rating. Inlet Screen Engine Inlet Section Exhaust Section Corrected Rotation Speed (%) Corrected Thrust (%) Corrected Exhaust Gas Temperature (%) Rake Turbojet Engine Exhaust Section Figure 4: Attachments for engine operations. The top figure illustrates a full configuration for the outdoor noise test. The bottom shows a detailed exhaust section. A A A-A Rake Nozzle Lip 3 Copyright 2011 by ASME

4 operation. The screen, however, is not expected to act as an inflow control device against turbulence and inlet distortion, although it is clear that the inlet distortion, through the interaction with rotating blades, is a potential cause of undesired noise emission [15-17]. Recognizing this possibility, we attempted to minimize inlet distortion due to ground vortices by raising the engine position. This will be focused on when discussing the obtained data. The exhaust duct section includes a pressure rake used to obtain the nozzle pressure ratio (NPR) inside the section (see the bottom figure in Fig.4). In the noise tests, a conical nozzle or notched nozzle is installed on the end of the exhaust section. The edge of the nozzle is 260 mm in equivalent diameter. The height of the investigated notch is 12 mm. 2.3 Test Stand The engine is rigidly fixed on a test stand, which comprises an upper stand and a lower stand [14]. The upper stand supports the engine together with the thrust measurement equipment. This stand is set directly on the ground during operation in the low-mounted mode. Measurements are made by traversing a microphone or a rake to investigate the noise along the jet plume and the downstream distributions of pressure and temperature. The lower stand is used for operation in the highmounted mode, during which far-field noise is measured. This lower stand provides a distance of 3 m between the ground surface and the engine axis, which is ten times the inlet diameter of the engine. This distance is expected to weaken the distortion at the inlet by ground vortices to some extent. 2.4 Test Site Outdoor noise tests using a jet engine should be implemented in an ideal environment. Taking into account several factors, the following are the conditions that must be satisfied by a test site. 1) Flat topography, on which the test stand, additional machines and acoustic sensors may be set. 2) Moderate climate with low rainfall. 3) Location far from residential areas and roads with heavy traffic. To satisfy these constraints, JAXA s Noshiro Test Center (NTC) was chosen which is located more than km north of Tokyo and has long been utilized for ground firing tests of solid propellant boosters. The engine stand, microphones and other equipment were set on one of the firing test site. Because this test site faces the sea, attention should be paid to the wind and salt-containing breeze during the tests. The wind direction and speed were continually monitored not only to evaluate background noise but also to determine their effect on engine operation. When there is a salt-containing breeze, the time duration of microphone deployment and the necessity of repeated cleaning of the microphones should be kept in mind. 2.5 Acoustic Measurement Two different acoustic setups were used in the engine tests. One was for far-field measurements in the high-mounted mode. The far-field acoustic data directly provide the difference in SPL due to the notched nozzle relative to that of the baseline nozzle. In the far-field measurements, 16 microphones are placed on an arc around the engine stand as shown in Fig.5. The distance between each microphone and the center of the engine, which was limited by the size of the test site, was 18 m, more than 65 times the nozzle exit diameter. The position of each microphone is listed in Table 1. The radiation angle is defined relative to the inlet axis. At each location, a microphone is inversely installed on a metal plate, assuming an ideal reflection. A 1/4-inch microphone cartridge, a preamplifier and a signal conditioner are allotted to each channel. Each channel is calibrated by imposing a sinusoidal signal of 124 db amplitude with 250 Hz frequency through a B&K type 4228 or a similar device. At a constant engine rating, the sound signal at each position is transformed into a 24-bit digital signal at 51.2 khz simultaneously and stored in a hard-disk unit. After the tests, data reduction is conducted and all signals under all operating conditions are automatically converted into narrow Figure 5: The outdoor test site. The engine is mounted on the test stand (this photo shows the high-mounted mode). Around the test stand, microphones are located in the far field (small plates on the upper side of the photo). Each microphone is inversely placed on the reflective plate. Table 1: Microphone positions in the far field and the near field. Distance from the Engine (m) 18 Angle from Engine Inlet (deg.) FarField Microphone Position Near-field Microphone Position 60,,,,,110,115,120,,130,135,140, 145,150,155,160 Radial Position from Jet Axis (mm) 600 Circumferential Rotation (deg.) 0-1 Axial Position from Nozzle Lip (X/D) Copyright 2011 by ASME

5 band frequency responses and one-third octave band frequency responses. The other setup is used for measurements along the jet plume in the low-mounted mode. Traversing equipment is employed so that a microphone can be placed at any position along the plume. A piezoelectric microphone is installed on the ring, the radial position of which is 600 mm from the jet axis. The axial position relative to the nozzle edge ranges from 0.5D to 6D (see Table 1). During acoustic measurements, the rake used for measuring the total pressure and temperature is removed to avoid interaction noise. Digitized signals are recorded together with signals at the circumferential and axial positions. Postprocessing is identical to that in the far-field case. 2.6 Pressure and Temperature Measurement A cross-shaped rake is attached on the traversing equipment to investigate the distributions of total pressure downstream of the nozzle [14]. The cross-shaped rake, which is used for pressure and temperature measurements, is rotated around the jet axis at the desired axial positions. The crossshaped rake installed on the traversing system is shown in Fig.6. The digitized pressure and temperature signals combined with positioning signals are accumulated for analysis and transformed into maps of pressure and temperature. These maps are useful for better clarifying the mechanism of noise emission, by providing on the mixing layer, and thus the sound sources near the nozzle. Figure 6: The pressure and temperature rake and its traversing system located in the aft of the low-mounted engine. OA N% Conical (S287) N% Notch (S282) N% Conical (S287) N% Notch (S282) N% Conical (S287) N% Notch (S282) Engine θ R Microphone Jet Angle from Jet Axis (deg.) Figure 7: Overall sound pressure levels (OASPLs) with regard to the radiation angle from jet axis. Solid lines indicate data of a conical nozzle and dotted lines a 6-notched nozzle. 2.7 Other Measurement Items Engine parameters, such as the rotating speed of the shaft, engine thrust, exhaust gas temperature and total pressure inside the exhaust section are also digitized and recorded. In addition, environmental conditions [18], including wind velocity, atmospheric temperature, pressure and humidity, are monitored through a portable DAVIS weather station. Their average values are referred to in order to correct the engine data. 3. RESULTS AND DISCUSSION 3.1 Noise Measurements in Far-field Overall sound pressure levels (OASPLs) for engine ratings of %, % and % are plotted in Fig.7. In the figure, the horizontal axis represents the radiation angle from the jet axis and the vertical axis indicates the OASPL in decibel. For each rating, dotted and solid lines show the cases of using the notched nozzle and the baseline nozzle, respectively. It is found that the notched nozzle reduces the OASPL by as much as 2-3 db. The higher the engine rating, the larger the reduction in the OASPL compared with the baseline case. The reduction in the OASPL using the notched nozzle, however, is limited to the rear direction, and tends to decrease toward the side direction. Moreover, an increase in noise occurs when the radiation angle exceeds approximately 45. According to the two source model [19], the observed noise in the far field is divided into two components. One component is due to the large scale vortex structure far downstream of the nozzle. The other component origins from the small turbulent structure distributed along the shear layer. Considering the directivity of these components and the vortices generated immediately after the notches, it is anticipated that an additional noise source is generated due to the penetration of the notches into the jet plume. 5 Copyright 2011 by ASME

6 Figure 8 shows frequency responses at angles of 30, 60 and from the jet axis. The engine rating is set at %. In the figure, the horizontal axis indicates the center frequency of the one-third octave band in Hz, and the vertical axis indicates the sound pressure level (SPL) in decibel. The frequency response at 30 position shows a sufficient reduction of mixing noise by as much as 3 db from the Hz band to the 0 Hz band. This noise reduction is explained by the following processes. 1) The penetration of the notches into the main stream results in a small amount of shedding immediately after the nozzle edge. 2) This vortex shedding triggers more mixing between the jet and the surrounding air than that occurring in the baseline conical nozzle. 3) The enhanced mixing lowers the mean velocity of the jet and prevents the formation of large-scale vortices downstream. At the 30 position, there are frequencies at which the SPLs is increased by the mixing device, for instance, frequencies above Hz. It has also been reported in experimental studies on other penetration-type devices that the noise increase at higher frequencies can be attributed to enhanced vortex shedding [4]. At other positions such as the 60 and positions, the broadband peak decreases in intensity and the peak frequency increases. The effect of the high-frequency components on the OASPL becomes increasingly dominant as the position of observation is moved forward. This leads to an increase in the OASPL in the side direction due to the notches. A similar tendency can be seen under other operating conditions such as at % and % ratings. 3.2 Effect of Increase in Velocity on Noise Emission Before investigating the effect of the penetration of notches and the resulting vortex shedding on the additional noise increase using the result in Fig.8, the velocity increase is taken into account. By considering the total pressure ratio inside the exhaust duct section, the exit Mach number is expressed as Here, Mj, κ and NPR represent the Mach number at the nozzle exit, the specific heat ratio and the nozzle pressure ratio, respectively. Figure 9 shows the estimated exit Mach number versus the engine rating. The Mach numbers of the conical nozzle are all lower than those of the 6-notched nozzle. Although the engine rating, based on the mechanical rotational speed, is kept constant during measurement, a difference in the nozzle exit geometry may change the engine cycle slightly. As a result, the total pressure in the case of the notched nozzle is increased by %, and thus the Mach numbers are increased as shown in the figure. To discuss the effect of a velocity increase on noise emission, it is essential to obtain noise features using an (1) SPL(dB) SPL(dB) SPL(dB) θ=30deg., R=18m N% Conical (S287) N% Notch (S282) OA Frequency(Hz) θ=60deg., R=18m N% Conical (S287) N% Notch (S282) OA Frequency(Hz) θ=deg., R=18m N% Conical (S287) N% Notch (S282) OA Frequency(Hz) Figure 8: Frequency responses at % rating. Data with and without the 6 notches are compared at angles of 30 (top), 60 (middle) and (bottom) from the jet axis. 6 Copyright 2011 by ASME

7 identical NPR. Scale model tests, instead of further engine tests, are conducted under cold-jet conditions. A scaled 6- notched nozzle of 25mm diameter is compared with a conical nozzle with an identical cross section. The model tests are implemented in JAXA s anechoic room [20, 21]. Figure 10 shows examples of the results of noise tests. Frequency responses observed at 30, 60 and from the jet axis for the baseline model and the 6-notched model are compared. The NPR is kept constant to realize a Mach number of 0.9. The results are in agreement with the engine test results in that the broadband peaks of mixing noise are reduced more than in the engine tests, the results of which are shown in Fig.8. The scale model tests at an identical NPR show improved noise reduction performance for the 6-notched nozzle at low and middle frequencies even in the 60 and directions. These results suggest that the SPLs of the 6-notched nozzle in the engine test might have been overestimated. However, the noise increase at higher frequencies still remains even if the exit Mach number is adjusted. This implies that the decreased noise reduction at higher frequencies is attributed to not only an increase in mean velocity but also other factors. 3.3 Acoustic Measurements along Jet Plume To obtain a better understanding of the noise increase at higher frequencies, SPLs along the plumes are next discussed. There is a slight variation in the OASPL at the notch position in the circumferential direction when the axial position is less than the diameter of the nozzle. Figure 11 shows the averaged OASPLs with respect to the axial position at % and % ratings. The average OASPL exceeds that of the baseline nozzle Mj=0.9, θ=30deg. 5dB Mj=0.9, θ=60deg. 5dB Conical (Baseline), D=25mm 6-Notch, D=25mm Conical (Baseline), D=25mm 6-Notch, D=25mm Exit Mach Number, Mj Conical Nozzle (S287) Notched Nozzle (S286) Notched Nozzle (S282) Mj=0.9, θ=deg. 5dB Conical (Baseline), D=25mm 6-Notch, D=25mm 0. Engine Rating (%) Figure 9: Mach numbers at the nozzle exit. Data are estimated by the nozzle pressure ratio Figure 10: Examples of the noise tests using the scaled 6- notched nozzle under the cold-jet condition in the anechoic facility. Mach number is kept Copyright 2011 by ASME

8 160 Conical Nozzle, N% Notched Nozzle, N% Conical Nozzle, N% Notched Nozzle, N% OA Microphone Position, X/D. Figure 11: OASPLs obtained by the traversed microphone along jet plume. The microphone position X is defined by X/D= Conical Nozzle, N% Notched Nozzle, N% X/D= Conical Nozzle, N% Notched Nozzle, N% Figure 12: Frequency responses obtained by the traversed microphone at X/D=2 (top) and 6 (bottom). Figure 13: Pressure maps obtained by a pressure rake in the engine test. The conical nozzle (left) and the 6-notched nozzle (right) are compared. until X/D= This indicates that the additional noise sources due to the notch arise within a limited region from the nozzle exit and that the enhanced mixing due to vortex shedding becomes effective in the subsequent region, corresponding to downstream of X/D=6. Frequency responses are detected at typical axial positions of X/D=2 and 6 and are plotted in Fig.12. Upstream, higherfrequency components are dominant and are amplified by the notches. After the crossover position, as shown in Fig.11, the broadband peak is shifted to a lower frequency and is attenuated over a wide range of frequencies. These source features along the jet plume are associated with the far-field directivity, resulting in a small reduction of the OASPL in the side directions as shown in Fig Pressure Maps across Jet Plume The sound sources along the jet plume originate from the mixing layer. To explain the above near-field results, maps showing the total pressure are discussed. The contours in Fig.13 are experimentally obtained using the pressure rake. In the figure, the left and right columns show maps of the conical 8 Copyright 2011 by ASME

9 nozzle and the 6-notched nozzle, respectively, for nondimensional axial positions of X/D, 0.5, 1.0, 2.0 and 6.0. In the case of the conical nozzle, the total pressure map immediately after the nozzle (X/D=0.5) has a clear circular distribution. At this position, maps of the temperature also exhibit a similar distribution. For simplicity, our discussion is focused on the total pressure distribution. Pressure maps at other axial positions indicate that the mixing layer expands as it propagates downstream. In contrast, the 6-notched nozzle generates distinct dips in the map, each dip indicating the position of a notch, because of the reduced velocity. The depth of the dip increases gradually downstream, which appears to continue even after X/D=2. However, at a sufficient distance from the nozzle (for example, X/D=6), it is difficult to distinguish the deformation. The dips in the pressure maps suggest that the mixing layer of the jet is deformed within a certain distance from the nozzle [21]. Moreover, this deformation is accompanied by strong vortex shedding that may lead to the additional noise sources. This noise source is associated with the fine-scale turbulence as Tam et al. indicated [19]. If the deformation due to the mixing device continues far downstream, the fine-scale turbulence may exist in wider range along the plume and increase the additional noise sources. 3.5 Redesign of the Notch On the basis of the above results, the notched nozzle is redesigned, with the aim of suppressing the increase in highfrequency noise. Tanaka et al. conducted the large eddy simulation (LES) with the sector model of the notched nozzle [22-24]. Their numerical work demonstrated that counterrotating vortices, resulting in the enhanced mixing downstream of the nozzle, are formed at dips as shown in the above maps. They employed the Reynolds stress and turbulence kinetic energy (TKE) for evaluating the intensity of the fine-scale turbulence within a short distance from the nozzle edge. According to the prediction, the Reynolds stress and TKE differ with the nozzle edge geometry and the number of notches. This implies that if the size and number of notches are appropriately Mj=0.9, θ=30deg. 5dB Conical (Baseline), D=40mm 18-Notch, D=40mm Mj=0.9, θ=60deg. 5dB Mj=0.9, θ=deg. 5dB Conical (Baseline), D=40mm 18-Notch, D=40mm Conical (Baseline), D=40mm 18-Notch, D=40mm Figure 14: The scaled models of the proposed 18-notched nozzle. The model for the cold-jet tests (left) and that for the hot-jet tests (right) Figure 15: Examples of noise tests using the redesigned 18- notched model under the cold-jet condition, Mach number is kept 0.9. Data of the conical nozzle are referred as a baseline case. 9 Copyright 2011 by ASME

10 chosen, the rapid increase in Reynolds stress and TKE can be suppressed compared with those for a chevron nozzle. As a result, the following guidelines were proposed: 1) Reduce the notch size such that it does not exceed the thickness of the boundary layer. 2) Increase the uniformity along the nozzle edge by increasing the number of notches. Figure 14 shows examples of revised notched nozzles. The left nozzle was prepared for cold-jet tests. The height of a notch is reduced by up to 20% compared with that of the former 6- notched nozzle. To increase the uniformity, the number of notches is increased to 18, which is determined by referring to the cases of the microjet [9,25]. This 18-notched nozzle is 40 mm in diameter and attached to the exit of the pressurized chamber. The noise emission in the far field and pressure maps of the region downstream of the nozzle are measured in the anechoic facility. The model on the right of Fig.14 was prepared for hot-jet tests. The equivalent diameter of the nozzle exit is 50 mm. This nozzle is installed on the exhaust side of a model jet engine, the thrust of which is approximately 120 N. 3.6 Examples of Cold-jet Tests on the 18-Notched Nozzle Noise tests under cold-jet conditions are conducted on the 18-notched nozzle, the results of which are presented in Fig.15. Compared with the 6-notched model, it is found that the additional noise increase at higher frequencies almost disappears and that the broadband peak components are reduced at middle and lower frequencies. The total pressure maps at several axial positions are compared among the conical nozzle, the 6-notched nozzle and the 18-notched nozzle in Fig.16. The 6-notched model has a diameter of 25 mm, whereas the others have a diameter of 40 mm. A 15-hole pressure rake is set precisely with respect to the nozzle axis and is fixed on a rigid stand. The nozzle turns around the jet axis instead of rotating the rake as in the engine test. Symmetric maps are obtained as shown in the figure. The 6-notched nozzle cuts the plume sufficiently deeply for a distance of more than four times the nozzle diameter to be required to transform the wavy pattern into a circular one. This tendency is consistent with that observed in engine tests. The redesigned notch also deforms the mixing layer at first. However, the wavy pattern becomes circular within a distance Conical (40mmφ) / 6-Notch (25mmφ) / 18-Notch(40mmφ) x/d=4.0 / x/d=2.0 / x/d=1.0 / x/d=0.5 Figure 16: The results of pressure measurement with the pressure rake in the anechoic facility. The conical nozzle (40mm in diameter, on the left), the scaled 6-notched nozzle (25mm in diameter, in the middle) and the redesigned notched nozzle (40mm in diameter, on the right) are compared with axial positions, X=0.5D, 1.0D, 2.0D and 4.0D. 10 Copyright 2011 by ASME

11 of twice the nozzle diameter. This rapid attenuation of the wavy mixing layer is consistent with frequency responses in far-field noise measurement in that the 18-notched nozzle prevents higher-frequency components from exceeding those in the baseline case. These maps are in good agreement with computational results [24]. To summarize, the notch was successfully redesigned as an 18-notched nozzle capable of suppressing additional highfrequency noise. The primary reason for its improved acoustic performance is that the deformation of the mixing layer occurs in smaller region. This result also implies that notches are sensitive to mixing characteristics and require careful design. 3.7 Ongoing and Future Works Owing to the difficulty in tuning the acoustic performance, one of the best methods for upgrading a notched nozzle for a practical use is to gather acoustic data under several conditions. Scale model tests under a single cold jet and dual flow simulating a bypass flow are ongoing and will be performed in an anechoic facility. A wide range of Mach numbers will be used in the tests. For the hot-jet condition, scale model tests using a model jet engine are planned which will be followed by noise tests using the turbojet engine. From the practical viewpoint, the trade-off between noise reduction and thrust loss, interaction with the airframe and noise exposure under flight conditions must be taken into account in parallel with noise tests. These issues remain to be investigated in future works. 4. SUMMARY A notched nozzle has been proposed for use as a mixing device for jet noise reduction. As a proof-of-concept study, an experimental investigation on the notched nozzle has been carried out using a turbojet engine. The tested 6-notched nozzle exhibited the following characteristics: 1) Noise reduction of as much as 3 db in terms of the OASPL is obtained in the far field but within a limited range of directions. 2) Higher-frequency components generated by the notches tend to become increasingly dominant towards the side directions. 3) These amplified components are partly due to the velocity increase induced by the notches. 4) Measured SPLs along the jet axis indicate the existence of additional noise sources up to X/D=4. Pressure maps of the notched nozzle illustrate the deformation of the mixing layer, which leads to additional noise sources. To improve the noise reduction performance, the 6- notched nozzle was redesigned through computational modeling. A revised 18-notched nozzle with reduced penetration of the notches was proposed. In scale model tests, the 18-notched nozzle exhibited a satisfactory reduction in jet mixing noise along with better suppression of the additional noise. ACKNOWLEDGMENTS This study was conducted under cooperative research on jet noise reduction by JAXA and IHI. The ECO engine project, mentioned in the paper, is supported by the New Energy and Industrial Technology Development Organization (NEDO). The authors acknowledge the technical contributions of Jet Engine Technology Center of JAXA. We would like to thank INC Engineering Corporation for operating the turbojet engine. Our thanks also goes to Noshiro Test Center (NTC) for their support in conducting the engine tests. REFERENCES [1] International Civil Aviation Organization (ICAO), Environmental Protection Annex 16 Chapter 4, International Standards and Recommended Practices. [2] For example, ICAO, 19. Environmental Technical Manual on the Use of Procedures in the Noise Certification of Aircraft, Doc. 01-AN/929. [3] Gliebe, P. 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