Acoustic source localization on a model engine jet with different nozzle configurations and wing installation

Abstract

The source distribution in the dual jets of a 1:10 scale jet engine model was investigated in a large anechoic facility. A linear microphone array was used to record the data and the analysis was performed with the SODIX method, an optimization technique that fits a model distribution of sound sources with non-uniform directivity to the measured cross-spectral matrix. Configurations with a plain secondary nozzle and a secondary nozzle with a serrated trailing edge were investigated with and without a wing installed above the engine. The results show that the SODIX method is able to resolve the different source distributions of the jet with the plain and the serrated nozzle as well as the additional sources that occur when a wing is installed.

Keywords

Acoustic beamforming jet noise installation effects

Introduction

The work presented here was performed within the framework of the European project OPENAIR.¹ This project focused on the reduction of noise sources from both the engine and the airframe, with the aim of generating solutions that will bring a reduction of sound pressure levels of 2.5 dB towards the ACARE objectives. In the work package dealing with turbomachinery noise, the project partners Rolls-Royce UK, Rolls-Royce Deutschland, Snecma, Airbus, and German Aerospace Center (DLR) cooperated for the experiments in the Noise Test Facility (NTF) of QinetiQ in Farnborough, UK. These experiments were performed in the summer of 2012 over a period of several weeks.

Acoustic and aerodynamic measurements were performed with a high bypass ratio jet engine at model scale. The engine model had an approximate scale of 1:10 and featured a dual-stream nozzle with a heated core, a simulated flight stream and the option to install a wing above the engine. Several engine builds were tested, including a short cowl engine model which could be fitted with a standard design, dual-stream baseline nozzle and an alternative serrated nozzle. Broadband fan noise could be simulated by an array of loudspeakers that was integrated in the casing of the model and radiated into the bypass duct. A wing with a flap system could be mounted over the engine in order to investigate installation effects on the sound field.

Configurations with and without an external flight stream with different velocity settings were tested. The core, bypass, and flight stream jets could be matched to the conditions that correspond to ICAO certification points approach, cutback, and sideline.²

The main focus of the acoustic measurements lay on far-field directivities in the sideline and fly-over directions. Additionally, DLR set up a linear microphone array for source localization measurements. This paper reports the source localization analysis of the data acquired with the linear microphone array. Preliminary results have already been presented by Siller et al.³

Experiments

The QinetiQ NTF incorporates one of the largest anechoic chambers in the world. It is approximately 27 m long, 26 m wide, and 16 m high, with the sidewalls and ceiling covered with absorbent wedges. On the floor, mobile wedges can be placed after the experimental set-up is completed.

The model is a dual jet nozzle with the option of a wing and flap system to be installed. A heated core and a cold bypass stream can be simulated and a flight stream can be generated. The diameters of the core, bypass, and flight stream nozzles are approximately 136 mm, 264 mm, and 1780 mm, respectively.

Figure 1 shows a photograph of the experimental set-up in the NTF. The linear microphone array is mounted on a scaffolding with the axis parallel to the jet. The microphone array consisted of 100 electret microphones that were distributed over a length of 5.94 m with an equidistant spacing of 60 mm. The microphones were mounted 3 m above ground level and 5.7 m below the engine shaft axis, offset by approximately 1 m to the port side. A schematic of the set-up is presented in Figure 2. The coordinate system originates on the engine axis at the primary nozzle with the x-axis pointing downstream along the jet. The geometrical emission angle θ of a source (in the absence of a flight stream which would shift the observed source position) is defined in jet coordinates as the angle between the engine shaft axis and a line connecting the position of the source on the engine shaft axis to an observer (i.e. a microphone) position. In this convention, an emission angle of $θ = 0^{\circ}$ corresponds to an observer position in the rear arc, downstream of the engine on the engine axis, and an emission angle of $θ = 180^{\circ}$ to an observer position in the forward arc, upstream of the engine. For a source at the secondary nozzle exit plane, the linear microphone array covers a range of emission angles between $θ = 50^{\circ}$ for the microphone position furthest downstream at $x = 4.86$ m and $θ = 100^{\circ}$ for the microphone positioned 1.08 m upstream of the nozzle.

Figure 1.

Photograph of the experimental set-up in the Noise Test Facility (NTF) of QinetiQ, blue circle: model nozzle, red box: linear microphone array (QinetiQ).

Figure 2.

Schematic of the experimental set-up in the Noise Test Facility (NTF) of QinetiQ.

The test matrix included experiments with the plain baseline nozzle and a serrated secondary nozzle, both in the isolated configuration and with the wing installed. The plain nozzle configuration is shown in Figure 3, the serrated nozzle in Figure 4. A schematic of the arrangement of the plain nozzle model with the wing and flap is presented in Figure 5. A loudspeaker array mounted in the bypass duct (see Figure 3) could be used to simulate rearward fan broadband noise, which required very high sound pressure levels from the loudspeakers. In order to make a clear impact on the far field acoustic measurements, the broadband noise levels had to be 10 dB above the jet noise levels. In order not to overload the loudspeakers and to provide high amplitudes in the frequency bands of interest, the loudspeakers were externally cooled and band limited broadband noise was generated in order to maximize the output power in a specific frequency band.

Figure 3.

Schematic of the engine model with the plain nozzle and the loudspeaker system for the simulation of fan broadband noise in the bypass (QinetiQ).

Figure 4.

Schematic of the engine model with the serrated secondary nozzle (QinetiQ).

Figure 5.

Schematic of the wing installation above the nozzle.

These configurations were tested for the dual jets alone and with an external flight stream. The most realistic configurations tested were those with a flight stream, simulated fan broadband noise, and the core and bypass jets matched to the conditions that correspond to ICAO certification points. This paper presents measurements under flow conditions that simulate the sideline certification point with a Mach number of Ma = 0.75 in the core jet, 0.85 in the bypass jet, and 0.26 in the flight stream. The heated core jet had a temperature of 727 K, the bypass 333 K, and the flight stream 304 K.

Data analysis

The microphone array data were processed with the source localization method SODIX (Source Directivity Modeling in the Cross-Spectral Matrix). SODIX fits a distribution of equivalent sources by comparing the measured cross-spectral matrix with the cross-spectral matrix calculated from a model distribution of sources, iterating the individual source strengths with an optimization method. The directional characteristics of the sound sources are taken into account by calculating a different distribution of monopole sources for every microphone position and emission direction.

SODIX has been developed by Michel and Funke⁴ and Funke et al.⁵ and is a generalization of the spectral estimation method of Blacondon and Elias.⁶ It is a general source localization method that has been successfully compared against state-of-the-art localization methods.⁷ SODIX has been applied mainly to problems of aircraft engine noise with highly directive sound sources.^3,8,9 The main benefit of SODIX over conventional source localization methods is that it determines the directivity of the sources by modeling a different distribution of equivalent sources for every microphone, see Figure 6.

Figure 6.

Schematic of the SODIX source model with source strengths D_jm from every source position j to every microphone position m.

The SODIX solution is computed on a linear grid on the engine axis with a spatial resolution of $Δ x = 0.25 λ$ that has a low density of points in the low-frequency range and a spatial resolution that increases with the frequency. Different grids with uniformly spaced sources were used for different one-third-octave bands. For lower frequencies, in the range from 50 Hz to 500 Hz, grids with 35 sources were used. From the 630 Hz one-third-octave band upwards, the number of sources in the grid was steadily increased starting with 45 sources and going up to 1408 sources for the 20 kHz band by approximately doubling the number of sources from one octave to the next.

The output of the SODIX routine is the matrix of the source amplitudes $D_{jm} (f)$ between every source j and every microphone m for every frequency band. The SODIX results are presented as source directivity plots for individual one-third-octave bands. These are contour plots of the source strengths in decibels, as shown in Figure 7, where positions on the x-axis represent the axial source positions normalized with the bypass nozzle diameter. The y-axis represents the emission angle θ from each source position towards the microphone positions. The emission angle is defined as the angle between the engine axis and a line connecting a source on the axis and a microphone position in jet coordinates, i.e. increasing from microphone positions downstream of the source towards microphone positions upstream of the source. The nozzle exit is used as the origin of the coordinate system and the x-axis is positive in the downstream direction.

Figure 7.

Example of a SODIX source directivity plot with the source distribution on the horizontal and the source directivity on the vertical axis. The upper limit of the blue area is defined by the data of the first microphone in the array, the lower limit by the data of the last microphone on the downstream end of the array.

Because the length of the linear microphone array is limited, it cannot cover all source positions and emission directions, and the source region in the plots shows as a wide diagonal band. The contribution of an individual microphone to the source plot is a curve running from the lower left to the upper right. The upper limit of the SODIX solution in the map plots is defined by the data from the microphone at the upstream end of the array, the lower limit by the microphone at the downstream end. The microphone at the upstream end of the array looks at the source at the nozzle exit plane at an angle of slightly larger than $θ = 90^{\circ}$ and it sees the sources further downstream in the jet at emission angles that increase up to $θ > 130^{\circ}$ for the last source position of the grid used for the calculation at $x / d_{bypass} = 18$ . The microphone on the downstream end of the array sees the nozzle source at an angle of $θ \approx 45^{\circ}$ and the source at $x / d_{bypass} = 18$ at $θ \approx 90^{\circ}$ .

The source directivity plots show the directivity of a specific source along the vertical line at the source position (see Figure 7), while the source distribution seen from a particular direction can be found along the horizontal line that corresponds to the emission angle. In Figure 7, the positions of the engine nozzle and the leading and trailing edges of the wing flap are indicated as vertical gray dashed lines. The position of the wing leading edge corresponds with the nozzle exit position. The frequencies are shown as measured, and the corresponding frequencies for a full size commercial aircraft engine have to be calculated by dividing the measured frequency by the model scale factor of 10.

Results

From the experimental database, some examples will be shown to illustrate different configurations: 1) for the baseline nozzle with broadband noise simulation in the bypass duct with and without a jet flow, and a simulated flight stream, with and without wing installation, and 2) a nozzle with a serrated trailing edge with and without wing installation.

Simulated rearward fan broadband noise

Fan broadband noise emitted from the bypass duct was simulated with an array of loudspeakers (see Figure 3). The configuration with simulated broadband noise alone, without a jet and flight stream simulation flow, is a good example to demonstrate the capability of SODIX to localize the source at the nozzle and to detect the additional sources and reflections at the wing.

Isolated engine with broadband noise simulation

Figure 8(a) shows the source directivity for the isolated case in the 2 kHz one-third-octave band. The source at the bypass nozzle is correctly detected at the axial position of the nozzle and it radiates with almost uniform directivity in the up- and downstream directions. A secondary source appears about one nozzle diameter downstream of the main source and radiates into the rear arc between $50^{\circ} < θ < 70^{\circ}$ . This source is most likely caused by reflections from the pylon and the rig.

Figure 8.

Source directivity maps for simulated broadband noise from the bypass nozzle in the 2 kHz one-third-octave band. (a) Isolated engine, no flow. (b) Wing installed, no flow. (c) Isolated engine, sideline condition with flight stream. (d) Wing installed, sideline condition with flight stream. (e) Isolated engine, sideline condition with flight stream and shear layer correction. (f) Wing installed, sideline condition with flight stream and shear layer correction.

The high accuracy of the source localization and the high dynamic range of over 20 dB is remarkable, considering that the distance between the microphone array and the engine axis is equivalent to almost 30 nozzle diameters.

Engine installed underneath a wing with broadband noise simulation

Figure 8(b) presents the results for the engine model mounted under the wing with simulated fan broadband noise in the 2 kHz one-third-octave band. The maximum sound pressure level is about 1 dB higher than for the isolated engine. The directivity of the source at the nozzle is changed, with the radiation in the upstream direction being slightly lower. Additional sound sources, which are about 15 dB below the nozzle sources, appear at the wing flap (marked by the two dashed vertical lines). One flap source radiates downstream, between $θ = 50^{\circ}$ and $60^{\circ}$ , the other from $θ = 75^{\circ}$ to $100^{\circ}$ , with a gap between the two sources. The flap sources can be caused by direct reflections of the broadband noise or by refraction effects.

Measurements at sideline condition with broadband noise simulation

Figure 8(c) shows the source distribution in the 2 kHz one-third-octave band for the isolated engine at sideline condition with a flight stream and broadband noise generated in the bypass duct. The convection and the shear layer refraction effects have not been compensated. Therefore, the most striking difference, compared to the static case shown in Figure 8(a), is that the sources have convected downstream with the flow. A discontinuity in the lower third of the diagonal band of the SODIX solution results from a defective microphone channel.

In Figure 8(c), the nozzle source appears to move downstream in the rear arc, towards the low-emission angles because of convection and refraction of the sound waves in the shear layer of the flight simulation jet. The further downstream a microphone is located, the lower the emission angle and the longer the transmission path through the shear layer.

The shift of the source positions and the changes in the sound pressure levels that are induced by the shear layer can be compensated according to the theory by Amiet.¹⁰ Here, the corrected emission angles and source amplitudes were calculated by postprocessing the SODIX solution that had been calculated under the assumption of free-field sound propagation.

The basic geometry for the shear layer correction is shown in Figure 9. Sound waves generated by the source S propagate through the flight stream until they reach the shear layer at the point B. The shifted source position is E and the emission angle of the source is now θ, while the microphone ‘sees’ the shifted source at an angle θ_s. In the shear layer, the wave normal direction is changed to the angle ϕ. Outside the shear layer, the wave propagates through the medium at rest until it reaches the microphone M. Amiet¹⁰ provides a set of equations for the iterative calculation of the emission angle θ, the geometrical angle $φ_{m}$ , the angle of the refracted sound wave relative to the shear layer ϕ, and the difference of the sound pressure level received at the microphone M using the parameters of the shear layer, the radius h, the radial distance to the microphones y, and the Mach number Ma. The SODIX solution detects the sound source S at the position E and the original source position S and the emission angle θ are iterated for every microphone position M using the Amiet theory. The source positions, the emission angles, and the sound pressure levels in the SODIX solution are then corrected with these values.

Figure 9.

Shift of source positions from S to E and the geometric angle $φ_{m}$ , the true emission angle θ, and the emission angle θ_s from the SODIX results.

The Amiet theory, however, only applies to the propagation of sound through an infinitely thin shear layer at an angle of $90^{\circ}$ . In an open-jet wind tunnel, the shear layer widens in the downstream direction and the turbulence reduces the coherence of the sound waves. The angles at which the sound waves cross the shear layer can vary significantly for positions up- and downstream in the jet. For these reasons, the shear layer effects cannot be compensated well for very large or very small emission angles (e.g. see Funke et al.,⁸ who demonstrated that the shear layer effects can also be compensated by integrating the Amiet corrections directly into the propagation functions used in SODIX).

Figure 8(e) presents the data from Figure 8(c) with the shear layer corrections applied (and with the discontinuity due to one defective microphone). The corrections of the emission angle and the source levels shift the main source region back to the nozzle position. Compared to the static case presented in Figure 8(a), the peak radiation levels have increased by 4 dB. The source at the nozzle still reaches significantly higher sound pressure levels than the jet mixing noise sources. This confirms that the simulated broadband noise levels are sufficiently high. In the 2 kHz frequency band shown in Figure 8(e), the jet sources would be expected to appear between $5 \leq x / d_{bypass} \leq 9$ , but the broadband source from the nozzle dominates the sound field.

Downstream of the nozzle, a secondary source pattern appears along curved lines. This effect resembles the sidelobe patterns that occur in classical beamforming, even though the SODIX method is very different from the beamforming approach. These patterns are barely discernible in the configuration without flow, where they are about 20 dB below the maximum. In the complex situation with a dual jet flow and a flight stream jet, however, these spurious sources appear at levels that are 12 dB below the main source. The effect depends on the source characteristics and on sound propagation effects through the shear layer of the flight stream jet, and the fact that equivalent sources are placed on the engine axis while the real sources are off-axis. In this situation, the SODIX algorithm reaches a better fit of the measured cross-spectral matrix when it introduces these spurious sources. This effect is not yet well understood and requires further investigation.

The presence of the wing, see Figure 8(d) for the raw SODIX solution and Figure 8(f) for the shear layer compensated results, increases the maximum levels in the maps by 2 dB. Additional sources appear on the flap between $95^{\circ} < θ < 105^{\circ}$ in the upstream direction and between $60^{\circ} < θ < 70^{\circ}$ in the downstream direction. The sources downstream of the nozzle that appear between $70^{\circ} < θ < 80^{\circ}$ in the isolated case are also present when the wing is installed, but they are stronger and extend further downstream. This is most certainly an effect from a superposition of reflections from the fuselage with sources and reflections on the wing itself.

Jet flow at sideline condition with wing installation effects

The jet noise of the short cowl baseline nozzle build was tested with and without wing installation, but without simulated rearward fan noise from the internal loudspeakers.

Isolated engine at sideline condition - shear layer effects

Figure 10 presents the results for the sideline condition with a flight stream of 90 m $/$ s in the 1 kHz, 2 kHz, and 4 kHz one-third-octave bands, respectively. The raw SODIX results for the isolated engine are shown on the left and those after the shear layer compensation on the right-hand side.

Figure 10.

Source directivity maps for the isolated short cowl baseline nozzle at sideline condition with and without the shear layer correction (no wing, no broadband noise simulation). (a) SODIX results for the 1 kHz third-octave band. (b) SODIX results for the 1 kHz third-octave band with shear layer corrections. (c) SODIX results for the 2 kHz third-octave band. (d) SODIX results for the 2 kHz third-octave band with shear layer corrections. (e) SODIX results for the 4 kHz third-octave band. (f) SODIX results for the 4 kHz third-octave band with shear layer corrections.

In the plots without shear layer correction (Figure 10(a), (c), and (e)), the source positions are shifted downstream. The shear layer compensation (Figure 10(b), (d), and (f)) shifts the sources back upstream to their correct positions. The shear layer compensation results in a shorter and weaker jet mixing noise region between four and eight nozzle diameters downstream.

For the isolated engine, the main sources are at the nozzle exit and further downstream in the jet mixing region, between $5 < x / d_{bypass} < 9$ . In the 1 kHz band, the maximum level is about 4 dB higher than in the 2 kHz band, the maximum in the 4 kHz band is another 5 dB lower. The jet source has a maximum at $x / d_{bypass} \approx 6$ to 7.

While the simulated fan noise, shown in Figure 8, radiates into the upstream direction, the noise sources from the nozzle alone, shown in Figure 10, radiate most strongly between $70^{\circ} < θ < 90^{\circ}$ . The relative strength of the nozzle source compared to the jet mixing noise increases from being about 10 dB below the jet noise in the 1 kHz band to about 5 dB in the 2 kHz band until in the 4 kHz band, the peak levels of both sources are about equal.

The nozzle source radiates strongest around $θ \approx 80^{\circ}$ while the jet sources radiate downstream. The strongest sources are found at the downstream boundary of the source map. This indicates that the microphone array was not long enough in the streamwise direction. A microphone array that is longer in the downstream direction would be able to resolve the jet noise sources at smaller emission angles.

Engine with wing installation at sideline condition

The SODIX source maps for the engine model at sideline condition are presented in Figure 11(b), (d), and (f) on the right-hand side, while the left-hand side repeats the isolated results from Figure 10(b), (d), and (f). The shear layer corrections have been applied to all these maps.

Figure 11.

No wing (left) vs. wing (right) source directivity maps for the short cowl baseline nozzle at sideline condition with the shear layer corrections applied and without simulated broadband noise. (a) Isolated jet, 1 kHz third-octave band. (b) Wing installed, 1 kHz third-octave band. (c) Isolated jet, 2 kHz third-octave band. (d) Wing installed, 2 kHz third-octave band. (e) Isolated jet, 4 kHz third-octave band. (f) Wing installed, 4 kHz third-octave band.

The main difference between the isolated and the installed case are additional sources on the wing and a considerable increase of the source levels in the jet noise mixing region. The wing installation effects increase the maximum source amplitudes by 2 to 3 dB. The jet mixing noise is enhanced compared to the isolated case. The amplitudes are higher and the jet mixing noise region extends further upstream. The interaction of the jet with the wing increases the sound source levels not only at the wing but also in the jet region downstream. Additional sources appear at the flap, especially in Figure 11(d), which could be caused by trailing edge noise.

Jet flow with a serrated nozzle at sideline condition with wing installation effects

The short cowl engine model with a serrated trailing edge on the secondary nozzle was tested under the same conditions as the baseline engine. Figure 12 presents the results of the isolated jet on the left-hand and the installed engine model on the right-hand side in the one-third-octave bands with 1, 2, and 4 kHz. These results are shear layer compensated. In order to facilitate a direct comparison, the sound pressure levels are scaled the same way as in Figure 11 for every frequency band. The blue diagonal line in the source maps for the isolated engine (Figure 12(a), (c), and (e)) represents a defective microphone.

Figure 12.

No wing (left) vs. wing (right) source directivity maps with the serrated nozzle at sideline condition with the shear layer corrections applied and without simulated broadband noise. (a) Isolated jet, 1 kHz third-octave band. (b) Wing installed, 1 kHz third-octave band. (c) Isolated jet, 2 kHz third-octave band. (d) Wing installed, 2 kHz third-octave band. (e) Isolated jet, 4 kHz third-octave band. (f) Wing installed, 4 kHz third-octave band.

A comparison of the source maps in Figures 11 and 12 confirms that the serrated nozzle is able to lower the overall sound source levels by about 2 dB in the frequency bands shown here. The greatest sound source reduction occurs in the jet mixing region between five and seven nozzle diameters downstream of the nozzle. While the general shape of the source regions is preserved between the baseline and the serrated nozzle builds, the source levels are lower with the serrated nozzle. The region with strong jet sources is weaker and it radiates more to the downstream direction. The source at the nozzle is between 2 and 3 dB stronger than for the plain nozzle due to the increased shear induced by the serrations. An interesting result is that the serrated nozzle is effective in both the isolated and the installed configuration.

Conclusions

The sound field generated by the coaxial core and bypass jets from an engine model has been investigated using a linear microphone array. The data were analyzed using the SODIX method, which calculates the acoustic source distributions for the different emission angles seen by the array. The results for different nozzle configurations with flight stream simulation and wing installation effects show that the SODIX method is able to resolve the source distribution along the jet axis. This is remarkable, considering the complexity of the arrangement of a microphone array focusing on a scaled model from a distance of about 30 nozzle diameters, with the microphones spread out over a wide angular range with the additional complications of the complex sound propagation from the sources through the shear layer of the flight stream jet to the microphones.

SODIX proved to be a useful tool for investigations of different configurations and set-ups. It is able to resolve wing installation effects – the generation of sources underneath the wing and the increase of jet noise downstream of the wing – and the effects of a serrated nozzle – the increase of nozzle based source amplitudes coupled with a reduction of jet noise in the mixing region.

Further work needs to be done in the postprocessing of the data, especially a more detailed study of the effects of the shear layer compensation. The SODIX sound sources that were calculated on the source grid for the different emission directions could be propagated to far-field microphone positions in order to perform a proper quantitative comparison of different configurations.

Footnotes

Authors' Note

Jonas König is now affiliated with Rolls-Royce Deutschland Ltd. & Co. KG, Steady State Performance Group, Blankenfelde-Mahlow, Germany and Stefan Funke with Rolls-Royce Deutschland Ltd. & Co. KG, Acoustics Group, Blankenfelde-Mahlow, Germany.

Acknowledgements

The authors gratefully acknowledge the support given by Chris Wrighton and his team at QinetiQ during the experiments in the NTF.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research leading to these results has received funding from the European Community’s 7th Framework Program in the framework of the Openair and the JERONIMO projects under the grant agreement numbers 234313 and 314692 and from the European Commission in the framework of the ADEC project, which is part of the Clean Sky 2 Large Passenger Aircraft IAPD, under the grant agreement CS2-LPA-IADP-2014-2015-1.

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