A means of measuring the lateral distribution of the noise sources of air jets using two micropho... more A means of measuring the lateral distribution of the noise sources of air jets using two microphones placed on opposing sides of the jet in the geometric far-field is presented. The method is derived from the Polar Correlation Technique but, unlike the latter, its use is restricted to source distributions that are not a strong function of frequency. Its application to jet mixing noise is considered plausible as the majority of the noise of interest is produced in the shear layer of the jet. The basis of polar correlation is first briefly reviewed and its application to three-dimensional sources considered. For an axially symmetric source, whose radial distribution is independent of frequency, the cross-power spectral coefficient, for 180° microphone polar separation, is shown to equal a Hankel transform, the inverse of which is equal to the radial source distribution. Experimental data are presented demonstrating the application of the method on a round unheated subsonic jet for U/ao =0.4 and 0.8. Using numerical simulation, the application of the method to a coaxial jet is considered and shown to be feasible.
The eventual aim of this project is to establish the mechanism of jet noise generation in hot and... more The eventual aim of this project is to establish the mechanism of jet noise generation in hot and cold supersonic jets, using the remote optical Crossed Beam Schlieren Method. The work reviewed in this report covers the period October 1969 to October, 1970. During this period the remote Crossed Beam Schlieren Apparatus was built and its performance assessed on a 1 inch dia. cold free air jet run at both subsonic and supersonic speeds. In order to check the viability of the fundamental crossed beam Schlieren theory, an extensive survey of the cold subsonic jet was undertaken; the supersonic programme was initiated during the latter part of the period. The results verify the suitability of the system to remotely extract fundamental turbulence information and it appears that little difffculty should be encountered on applying the apparatus to the high temperature jet measurements programme to follow.
This paper describes hot-wire anemometer measurements made in a low speed air jet and their use t... more This paper describes hot-wire anemometer measurements made in a low speed air jet and their use to model jet mixing noise using Lighthill’s acoustic analogy evaluated in a fixed frame of reference. The measurements include second and fourth order two-point space-time correlations for the axial velocity, associated cross-spectra and spectral length scales of the axial and lateral velocity components. The two-point measurements are made in three orthogonal axis about a mid point in the mixing layer and for different Strouhal numbers, thereby avoiding the customary assumption of separation of space and time variables. Predicted fixed frame convective amplification is compared with acoustic measurements made specifically for this purpose.
At stations very close to the boundary of a turbulent jet, the pressure fluctuations measured usi... more At stations very close to the boundary of a turbulent jet, the pressure fluctuations measured using a microphone, or sensed by a structure, contain a hydrodynamic component that is a part of the near noise field of the turbulence. Measurements of this component are presented for different external stations of the jet and for different jet velocities. Using Lighthill’s equation, adapted for the quadrupole near field, a semi-empirical mathematical model of the hydrodynamic noise, including its spectral distribution and level, is developed, without recourse to turbulence measurements. Finally, a comparison is made with full-scale engine noise measurements.
During cruise, the jet mixing noise sources of commercial airliners are relatively weak. However,... more During cruise, the jet mixing noise sources of commercial airliners are relatively weak. However, the jet exhausts of turbofan engines become supercritcal in cruise and quite strong shock noise can be generated. Under these conditions shocks form in a regular pattern in the exhaust plume and their interaction with the jet turbulence generates shock-associated noise, the presence of which is well known on military aircraft. To satisfy the cruise noise requirements of airlines and aircraft manufacturers, methods of reducing or alleviating the shock noise, which is broadband in character and of relatively high frequency, are being pursued. As little is known of shock noise specific to modern high-bypass ratio turbofan engines, an experimental research programme has been carried out at model-scale in the enhanced Noise Test Facility at QinetiQ, Farnborough. This paper describes the design concepts and constraints together with commissioning tests, including noise cancellation using hot-films, source location measurements, and measurements of shock noise spectra at cruise conditions.
QinetiQ (formerly the Defence Evaluation and Research Agency) is currently undertaking a programm... more QinetiQ (formerly the Defence Evaluation and Research Agency) is currently undertaking a programme of applied research to develop improved prediction methods for acoustic fatigue assessment on next generation combat aircraft for the UK MOD. This paper describes ongoing work to develop practical methods of predicting the near-field noise of the highspeed exhausts of military jet engines, for use at an engineeering level. The work is focussed on predicting intensity, spectra and spatial coherence of jet noise at the airframe, using a semi-empirical basis that marries aeroacoustic theory with source location data. The study encompasses both jet mixing noise and broadband shock noise for single and twin jets. Preliminary research on extending the prediction capability to the off-design operation of convergentdivergent nozzles, which is particularly relevant to modern combat aircraft, is described. The background to the development of the methodology is also described, through heated model noise tests in the QinetiQ Noise Test Facility, and validation tests on a full-scale engine in the QinetiQ GLEN sea-level engine test facility.
... In the absence of screech, N is typically eight for an underexpanded jet issuing from a conve... more ... In the absence of screech, N is typically eight for an underexpanded jet issuing from a convergent nozzle. To determine the power spectral density of the shock noise we Fourier transform the auto-correlation of eqn.6, which we express in the form: N lYN ,,,xt - 1o)Fx (,t +T--!- AP ...
An alternative approach to the semi-empirical evaluation of jet mixing noise using Lighthill&... more An alternative approach to the semi-empirical evaluation of jet mixing noise using Lighthill's retarded integral equation is presented. In particular, and against the generally accepted precept, turbulence measurements relevant to the mixing noise of a circular jet are analysed in a nozzle fixed frame of reference rather than Lighthill's reference frame moving with the eddies. For the purpose of this fixed-frame analysis, archival filtered fourth-order two-point space-time streamwise velocity correlation measurements are analysed with due consideration to phase and retarded time to build a model of self-noise, which component is found to dominate much of the rear arc. The fixed-frame model is shown to predict the correct level of convective amplification, while avoiding Lighthill's original Mach angle singularity and explains the paradox of the "elusive Doppler shift in jet noise" first raised by Ribner. An estimate of the inherent directivity of self-noise is made from turbulence measurements and the role of refraction is investigated using archival point source experimental acoustic data. An important parameter controlling jet noise emerges from the self-noise model in the form of the aspect ratio of the turbulence correlation volume. While the relative changes in noise levels with respect to directivity and velocity are replicated well by the fixed frame methodology, an attempt to predict an absolute unit slice noise spectrum results in a gross over prediction of the SPL at high frequencies when compared with available source location measurements. Currently, no reasonable explanation has been found for this significant anomaly.
A means of measuring the lateral distribution of the noise sources of air jets using two micropho... more A means of measuring the lateral distribution of the noise sources of air jets using two microphones placed on opposing sides of the jet in the geometric far-field is presented. The method is derived from the Polar Correlation Technique but, unlike the latter, its use is restricted to source distributions that are not a strong function of frequency. Its application to jet mixing noise is considered plausible as the majority of the noise of interest is produced in the shear layer of the jet. The basis of polar correlation is first briefly reviewed and its application to three-dimensional sources considered. For an axially symmetric source, whose radial distribution is independent of frequency, the cross-power spectral coefficient, for 180° microphone polar separation, is shown to equal a Hankel transform, the inverse of which is equal to the radial source distribution. Experimental data are presented demonstrating the application of the method on a round unheated subsonic jet for U/ao =0.4 and 0.8. Using numerical simulation, the application of the method to a coaxial jet is considered and shown to be feasible.
The eventual aim of this project is to establish the mechanism of jet noise generation in hot and... more The eventual aim of this project is to establish the mechanism of jet noise generation in hot and cold supersonic jets, using the remote optical Crossed Beam Schlieren Method. The work reviewed in this report covers the period October 1969 to October, 1970. During this period the remote Crossed Beam Schlieren Apparatus was built and its performance assessed on a 1 inch dia. cold free air jet run at both subsonic and supersonic speeds. In order to check the viability of the fundamental crossed beam Schlieren theory, an extensive survey of the cold subsonic jet was undertaken; the supersonic programme was initiated during the latter part of the period. The results verify the suitability of the system to remotely extract fundamental turbulence information and it appears that little difffculty should be encountered on applying the apparatus to the high temperature jet measurements programme to follow.
This paper describes hot-wire anemometer measurements made in a low speed air jet and their use t... more This paper describes hot-wire anemometer measurements made in a low speed air jet and their use to model jet mixing noise using Lighthill’s acoustic analogy evaluated in a fixed frame of reference. The measurements include second and fourth order two-point space-time correlations for the axial velocity, associated cross-spectra and spectral length scales of the axial and lateral velocity components. The two-point measurements are made in three orthogonal axis about a mid point in the mixing layer and for different Strouhal numbers, thereby avoiding the customary assumption of separation of space and time variables. Predicted fixed frame convective amplification is compared with acoustic measurements made specifically for this purpose.
At stations very close to the boundary of a turbulent jet, the pressure fluctuations measured usi... more At stations very close to the boundary of a turbulent jet, the pressure fluctuations measured using a microphone, or sensed by a structure, contain a hydrodynamic component that is a part of the near noise field of the turbulence. Measurements of this component are presented for different external stations of the jet and for different jet velocities. Using Lighthill’s equation, adapted for the quadrupole near field, a semi-empirical mathematical model of the hydrodynamic noise, including its spectral distribution and level, is developed, without recourse to turbulence measurements. Finally, a comparison is made with full-scale engine noise measurements.
During cruise, the jet mixing noise sources of commercial airliners are relatively weak. However,... more During cruise, the jet mixing noise sources of commercial airliners are relatively weak. However, the jet exhausts of turbofan engines become supercritcal in cruise and quite strong shock noise can be generated. Under these conditions shocks form in a regular pattern in the exhaust plume and their interaction with the jet turbulence generates shock-associated noise, the presence of which is well known on military aircraft. To satisfy the cruise noise requirements of airlines and aircraft manufacturers, methods of reducing or alleviating the shock noise, which is broadband in character and of relatively high frequency, are being pursued. As little is known of shock noise specific to modern high-bypass ratio turbofan engines, an experimental research programme has been carried out at model-scale in the enhanced Noise Test Facility at QinetiQ, Farnborough. This paper describes the design concepts and constraints together with commissioning tests, including noise cancellation using hot-films, source location measurements, and measurements of shock noise spectra at cruise conditions.
QinetiQ (formerly the Defence Evaluation and Research Agency) is currently undertaking a programm... more QinetiQ (formerly the Defence Evaluation and Research Agency) is currently undertaking a programme of applied research to develop improved prediction methods for acoustic fatigue assessment on next generation combat aircraft for the UK MOD. This paper describes ongoing work to develop practical methods of predicting the near-field noise of the highspeed exhausts of military jet engines, for use at an engineeering level. The work is focussed on predicting intensity, spectra and spatial coherence of jet noise at the airframe, using a semi-empirical basis that marries aeroacoustic theory with source location data. The study encompasses both jet mixing noise and broadband shock noise for single and twin jets. Preliminary research on extending the prediction capability to the off-design operation of convergentdivergent nozzles, which is particularly relevant to modern combat aircraft, is described. The background to the development of the methodology is also described, through heated model noise tests in the QinetiQ Noise Test Facility, and validation tests on a full-scale engine in the QinetiQ GLEN sea-level engine test facility.
... In the absence of screech, N is typically eight for an underexpanded jet issuing from a conve... more ... In the absence of screech, N is typically eight for an underexpanded jet issuing from a convergent nozzle. To determine the power spectral density of the shock noise we Fourier transform the auto-correlation of eqn.6, which we express in the form: N lYN ,,,xt - 1o)Fx (,t +T--!- AP ...
An alternative approach to the semi-empirical evaluation of jet mixing noise using Lighthill&... more An alternative approach to the semi-empirical evaluation of jet mixing noise using Lighthill's retarded integral equation is presented. In particular, and against the generally accepted precept, turbulence measurements relevant to the mixing noise of a circular jet are analysed in a nozzle fixed frame of reference rather than Lighthill's reference frame moving with the eddies. For the purpose of this fixed-frame analysis, archival filtered fourth-order two-point space-time streamwise velocity correlation measurements are analysed with due consideration to phase and retarded time to build a model of self-noise, which component is found to dominate much of the rear arc. The fixed-frame model is shown to predict the correct level of convective amplification, while avoiding Lighthill's original Mach angle singularity and explains the paradox of the "elusive Doppler shift in jet noise" first raised by Ribner. An estimate of the inherent directivity of self-noise is made from turbulence measurements and the role of refraction is investigated using archival point source experimental acoustic data. An important parameter controlling jet noise emerges from the self-noise model in the form of the aspect ratio of the turbulence correlation volume. While the relative changes in noise levels with respect to directivity and velocity are replicated well by the fixed frame methodology, an attempt to predict an absolute unit slice noise spectrum results in a gross over prediction of the SPL at high frequencies when compared with available source location measurements. Currently, no reasonable explanation has been found for this significant anomaly.
An alternative approach to the semi-empirical evaluation of jet mixing noise using Lighthill's re... more An alternative approach to the semi-empirical evaluation of jet mixing noise using Lighthill's retarded integral equation is presented. In particular, and against the generally accepted precept, turbulence measurements relevant to the mixing noise of a circular jet are analysed in a nozzle fixed frame of reference rather than Lighthill's reference frame moving with the eddies. For the purpose of this fixed-frame analysis, archival filtered fourth-order two-point space-time streamwise velocity correlation measurements are analysed with due consideration to phase and retarded time to build a model of self-noise, which component is found to dominate much of the rear arc. The fixed-frame model is shown to predict the correct level of convective amplification, while avoiding Lighthill's original Mach angle singularity and explains the paradox of the "elusive Doppler shift in jet noise" first raised by Ribner. An estimate of the inherent directivity of self-noise is made from turbulence measurements and the role of refraction is investigated using archival point source experimental acoustic data. An important parameter controlling jet noise emerges from the self-noise model in the form of the aspect ratio of the turbulence correlation volume. While the relative changes in noise levels with respect to directivity and velocity are replicated well by the fixed frame methodology, an attempt to predict an absolute unit slice noise spectrum results in a gross over prediction of the SPL at high frequencies when compared with available source location measurements. Currently, no reasonable explanation has been found for this significant anomaly.
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Papers by Marcus Harper-Bourne