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The paper presents a framework for the rapid acoustic optimization of three-dimensional, multi-stream nozzles for the propulsion of supersonic aircraft. The framework is based on Reynolds-Averaged Navier Stokes solutions combined ...
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The paper presents a framework for the rapid acoustic optimization of three-dimensional, multi-stream nozzles for the propulsion of supersonic aircraft. The framework is based on Reynolds-Averaged Navier Stokes solutions combined with a Design of Experiments approach. The RANS flow field is collapsed to a surface that is expected to contain the strongest contributions to far-field sound emission. Differential noise predictions are enabled by a modified acoustic analogy, with emphasis on the change in effective perceived noise level (EPNL). Response surfaces for EPNL and thrust loss are obtained after parameterization of the nozzle geometry. The optimal configuration was tested experimentally.
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The acoustic near-field of a supersonic underexpanded jet at M_j=1.34 was studied experimentally. A near-field azimuthal microphone array was utilized to study the jet oscillation dynamics and infer the geometrical pattern of the ...
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The acoustic near-field of a supersonic underexpanded jet at M_j=1.34 was studied experimentally. A near-field azimuthal microphone array was utilized to study the jet oscillation dynamics and infer the geometrical pattern of the upstream-traveling acoustic waves involved in the jet screech mechanism. Operation of the jet in isolation resulted in a lateral oscillation (mode B) with an acoustic field containing strong screech tones. Addition of a conical reflector of 60° cone half-angle changed the oscillation dynamics, with new mode E appearing prominently and coexisting with mode B. The cross-spectral densities at the distinct tone frequencies are utilized to discern the jet oscillation dynamics and screech frequency and wavelength prediction formulas are utilized to assess if mode E is an extension of previously observed screech modes.
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This study evaluated the feasibility of simulating the aerodynamics and acoustics of ducted fans using small-scale components (approximately 1/38th scale). Enabling technologies include high-definition stereolithography and high-p...
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This study evaluated the feasibility of simulating the aerodynamics and acoustics of ducted fans using small-scale components (approximately 1/38th scale). Enabling technologies include high-definition stereolithography and high-performance, compact DC brushless motors used in model aircraft. The 70-mm ducted fan featured a 14-bladed rotor and 24 stators, and operated at fan pressure ratio of 1.15 and rotor tip Mach number of 0.61. Its operation was thus representative of turbofan engines with very large bypass ratio. The far-field acoustics were surveyed in the downward and sideline planes. The acoustic results of the isolated ducted fan compare favorably with past large-scale experiments. In addition, the project provided initial data on the shielding of ducted-fan noise using a surface that approximated the shape of the blended wing body airplane. The BWB-shaped shield featured removable inboard fins and a moveable elevon, and translated axially to enable various positions of the nacelle relative to the elevon trailing edge. The EPNL reduction on the downward plane ranged from 7.6 dB with the engine at its nominal location to 10.4 dB with the engine moved forward by one fan diameter. Sideline EPNL reduction was marginal with the fins off and reached about 3.0 dB with the fins installed. Upward deflection of the elevon improved moderately the shielding.
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摘要 :
This study evaluated the feasibility of simulating the aerodynamics and acoustics of ducted fans using small-scale components (approximately 1/38th scale). Enabling technologies include high-definition stereolithography and high-p...
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This study evaluated the feasibility of simulating the aerodynamics and acoustics of ducted fans using small-scale components (approximately 1/38th scale). Enabling technologies include high-definition stereolithography and high-performance, compact DC brushless motors used in model aircraft. The 70-mm ducted fan featured a 14-bladed rotor and 24 stators, and operated at fan pressure ratio of 1.15 and rotor tip Mach number of 0.61. Its operation was thus representative of turbofan engines with very large bypass ratio. The far-field acoustics were surveyed in the downward and sideline planes. The acoustic results of the isolated ducted fan compare favorably with past large-scale experiments. In addition, the project provided initial data on the shielding of ducted-fan noise using a surface that approximated the shape of the blended wing body airplane. The BWB-shaped shield featured removable inboard fins and a moveable elevon, and translated axially to enable various positions of the nacelle relative to the elevon trailing edge. The EPNL reduction on the downward plane ranged from 7.6 dB with the engine at its nominal location to 10.4 dB with the engine moved forward by one fan diameter. Sideline EPNL reduction was marginal with the fins off and reached about 3.0 dB with the fins installed. Upward deflection of the elevon improved moderately the shielding.
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The paper presents a low-order prediction scheme for the noise change in multistream jets when the nozzle geometry is altered from a known baseline. The essence of the mode] is to predict the changes in acoustics due to the redist...
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The paper presents a low-order prediction scheme for the noise change in multistream jets when the nozzle geometry is altered from a known baseline. The essence of the mode] is to predict the changes in acoustics due to the redistribution of the mean flow as computed by a Reynolds-Averaged Navier Stokes (RANS) solver. A RANS-based acoustic analogy framework is developed that addresses the noise in the polar direction of peak emission and uses the Reynolds stress as a time-averaged representation of the action of the coherent turbulent structures. The framework preserves the simplicity of the Lighthill acoustic analogy, using the free-space Green's function, while accounting for azimuthal effects via special forms for the space-time correlation combined with source-observer relations based on the Reynolds stress distribution in the jet plume. Results are presented for three-stream jets with offset tertiary flow that reduces noise in specific azimuthal directions. The model reproduces well the experimental noise reduction trends.
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摘要 :
The paper presents a low-order prediction scheme for the noise change in multistream jets when the nozzle geometry is altered from a known baseline. The essence of the mode] is to predict the changes in acoustics due to the redist...
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The paper presents a low-order prediction scheme for the noise change in multistream jets when the nozzle geometry is altered from a known baseline. The essence of the mode] is to predict the changes in acoustics due to the redistribution of the mean flow as computed by a Reynolds-Averaged Navier Stokes (RANS) solver. A RANS-based acoustic analogy framework is developed that addresses the noise in the polar direction of peak emission and uses the Reynolds stress as a time-averaged representation of the action of the coherent turbulent structures. The framework preserves the simplicity of the Lighthill acoustic analogy, using the free-space Green's function, while accounting for azimuthal effects via special forms for the space-time correlation combined with source-observer relations based on the Reynolds stress distribution in the jet plume. Results are presented for three-stream jets with offset tertiary flow that reduces noise in specific azimuthal directions. The model reproduces well the experimental noise reduction trends.
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We assess the potential of three-stream nozzle concepts to reduce the takeoff perceived noise level from future supersonic aircraft. The study encompasses the effects of non-axisymmetric exhaust configurations as well as the effec...
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We assess the potential of three-stream nozzle concepts to reduce the takeoff perceived noise level from future supersonic aircraft. The study encompasses the effects of non-axisymmetric exhaust configurations as well as the effect of an enlarged plug. Asymmetry in the plume was created via eccentric tertiary and secondary ducts, in combination with a wedge-shaped deflector in the tertiary duct; and by reshaping the primary plug from circular to elliptical cross-section. The eccentricities were directed to increase the thickness of the lower-speed tertiary and secondary flows underneath the fast primary stream, thereby reducing noise in the general downward direction. The plug ellipticity was designed to flatten the primary stream along the major axis of the ellipse, thereby improving its coverage by the lower-speed streams with the goal of improving sideline noise reduction. The enlarged plug design was motivated by sonic-boom signature considerations. Acoustic measurements using helium-air mixture jets from small-scale, rapid-prototyped nozzles generated sound pressure level spectra at a number of polar and azimuthal angles for each configuration. These were converted to estimates of flyover perceived noise level and effective perceived noise level (EPNL) assuming a typical takeoff profile. The effect of the enlarged plug is to reduce the takeoff (downward) and sideline EPNLs, each by about 1.7 dB. Configurations with eccentricity in the tertiary flow only, all other components being axisymmetric, reduced the downward EPNL by as much as 5.1 dB but did not reduce the sideline EPNL. Adding eccentricity to the secondary ducts results in sideline noise reduction of about 2 EPNdB while providing downward reduction of around 5.6 EPNdB. The effect of the elliptical plug is to add another 1.0 dB to the sideline reduction. The best configuration involves the combination of eccentricity in the secondary and tertiary ducts with ellipticity of the primary plug; it yields noise reductions of 5.8 EPNdB and 2.9 EPNdB in downward and sideline directions, respectively. Including the effect of the enlarged plug, a cumulative (takeoff plus sideline) noise reduction of 12 EPNdB is estimated.
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摘要 :
We assess the potential of three-stream nozzle concepts to reduce the takeoff perceived noise level from future supersonic aircraft. The study encompasses the effects of non-axisymmetric exhaust configurations as well as the effec...
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We assess the potential of three-stream nozzle concepts to reduce the takeoff perceived noise level from future supersonic aircraft. The study encompasses the effects of non-axisymmetric exhaust configurations as well as the effect of an enlarged plug. Asymmetry in the plume was created via eccentric tertiary and secondary ducts, in combination with a wedge-shaped deflector in the tertiary duct; and by reshaping the primary plug from circular to elliptical cross-section. The eccentricities were directed to increase the thickness of the lower-speed tertiary and secondary flows underneath the fast primary stream, thereby reducing noise in the general downward direction. The plug ellipticity was designed to flatten the primary stream along the major axis of the ellipse, thereby improving its coverage by the lower-speed streams with the goal of improving sideline noise reduction. The enlarged plug design was motivated by sonic-boom signature considerations. Acoustic measurements using helium-air mixture jets from small-scale, rapid-prototyped nozzles generated sound pressure level spectra at a number of polar and azimuthal angles for each configuration. These were converted to estimates of flyover perceived noise level and effective perceived noise level (EPNL) assuming a typical takeoff profile. The effect of the enlarged plug is to reduce the takeoff (downward) and sideline EPNLs, each by about 1.7 dB. Configurations with eccentricity in the tertiary flow only, all other components being axisymmetric, reduced the downward EPNL by as much as 5.1 dB but did not reduce the sideline EPNL. Adding eccentricity to the secondary ducts results in sideline noise reduction of about 2 EPNdB while providing downward reduction of around 5.6 EPNdB. The effect of the elliptical plug is to add another 1.0 dB to the sideline reduction. The best configuration involves the combination of eccentricity in the secondary and tertiary ducts with ellipticity of the primary plug; it yields noise reductions of 5.8 EPNdB and 2.9 EPNdB in downward and sideline directions, respectively. Including the effect of the enlarged plug, a cumulative (takeoff plus sideline) noise reduction of 12 EPNdB is estimated.
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This paper will show that the matrix of all noise spectra radiated from a jet in a stationary condition, axisymmetric in construction and boundary conditions, has a special invariant structure which simplifies measurements and ena...
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This paper will show that the matrix of all noise spectra radiated from a jet in a stationary condition, axisymmetric in construction and boundary conditions, has a special invariant structure which simplifies measurements and enables the total composite noise field to be expressed as a sum of mutually incoherent partial fields. Each such partial field is described in terms of an azimuthal mode with a integer wave number and polarity. The consequences of this structure include a simple explanation of the nature of azimuthal coherence and a practical procedure for estimating the dominant partial fields of the jet from measurements with a limited azimuthal coverage. It should be noted that the conclusions are only drawn from the assumptions of stationarity and axisymmetry, such that both large and fine scale mixing noise and broadband shock associated noise are covered by this theory. This paper also introduces a concept of sound flares as a model for how random fluid dynamic events in a turbulent flow superpose stochastically to produce measured spectra and crossspectra in the linear hydrodynamic and acoustic fields. This concept may be applied to model both axisymmetric as well as noncircular nozzles.
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摘要 :
This paper will show that the matrix of all noise spectra radiated from a jet in a stationary condition, axisymmetric in construction and boundary conditions, has a special invariant structure which simplifies measurements and ena...
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This paper will show that the matrix of all noise spectra radiated from a jet in a stationary condition, axisymmetric in construction and boundary conditions, has a special invariant structure which simplifies measurements and enables the total composite noise field to be expressed as a sum of mutually incoherent partial fields. Each such partial field is described in terms of an azimuthal mode with a integer wave number and polarity. The consequences of this structure include a simple explanation of the nature of azimuthal coherence and a practical procedure for estimating the dominant partial fields of the jet from measurements with a limited azimuthal coverage. It should be noted that the conclusions are only drawn from the assumptions of stationarity and axisymmetry, such that both large and fine scale mixing noise and broadband shock associated noise are covered by this theory. This paper also introduces a concept of sound flares as a model for how random fluid dynamic events in a turbulent flow superpose stochastically to produce measured spectra and crossspectra in the linear hydrodynamic and acoustic fields. This concept may be applied to model both axisymmetric as well as noncircular nozzles.
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