Aircraft noise reduction technologies: A bibliographic review

There is a wide body of evidence that suggests that the turbulent mixing noise from high-speed jets consists of two components. These two components are generated by sources/mechanisms associated with the fine-scale turbulence and the large-scale structures of the jet plume. The noise generated by the large-scale structures radiates predominantly to the aft quadrant and typically peaks at angles close to the jet axis. The noise from the fine-scale turbulence is dominant in the forward quadrant and at near-normal angles to the jet axis. The initial objective of this study is to find a means to alter this basic radiation process in a high-speed jet. The ultimate goal of this study, however, is the achievement of noise reduction by accomplishing the preceding objective. A simple concept, ab eveled nozzle, is proposed, and detailed aeroacoustic measurements are performed on two nozzles of different bevel angles. Noise measurements, over a wide range of polar angles, are made at several azimuthal angles to map the azimuthal variations. The performances of the beveled nozzles are assessed against a reference round nozzle. The beveled nozzles introduce significant azimuthal variations in the spectra, resulting in major differences in the polar directivities of the overall sound pressure levels at different azimuthal angles; these differences become pronounced when the jet velocity is increased. It is demonstrated that significant noise reduction is achieved in the azimuthal directions below the longer lip of the beveled nozzle, principally in the polar angular range of ∼110 to ∼140 deg. Furthermore, this reduction is observed at all frequencies, with a low performance penalty of ∼1%. The magnitude of the noise reduction is a strong function of the jet velocity, with progressively higher reductions as the jet velocity is increased. With proper orientation of the nozzle, the noise footprint on the ground can be reduced. It is shown that the noise reduction is caused by the modification of the noise generated by the large-scale structures in the jet plume. Thus, noise reduction is achieved through the manipulation of the generation mechanisms. HERE is a wide body of evidence that suggests that the turbulent mixing noise from high-speed jets consists of two components. These two components are generated by sources/mechanisms associated with the fine-scale turbulence and the large-scale structures of the jet plume. The noise generated by the large-scale structures radiates predominantly to the aft quadrant and typically peaks at angles close to the jet axis. The noise from the fine-scale turbulence is dominant in the forward quadrant and at near-normal angles to the jet axis. As the jet velocity is increased, there is a broadening of the angular sector in which peak radiation occurs, with more noise being radiated to lower polar angles. (All polar angles are measured from the jet inlet axis, with the jet exhaust axis corresponding to

Nozzle Shaping for Reduction of Jet Noise from Single Jets

Aeronautics and astronautics are two closely related disciplines, sometimes found under the joint term aerospace, that have progressed at a formidable pace during the 20th century. Their im...

Aeronautics and astronautics: Recent progress and future trends

Numerical simulations of the flowfield and noise of single and staggered dual, round, and beveled nozzles are carried out, with the goal of gaining insights into the flow features that are responsible for noise generation and mitigation, and ultimately arriving at better designs. For aircraft applications, the geometry of the nozzles must be satisfactory both for aerodynamic and acoustic performance. The importance of taking the nozzle internal flow into account in the simulations instead of using simple inflow profiles, especially for complex geometries of current interest, is emphasized. A two-step Reynolds-averaged Navier-Stokes/large-eddy simulation methodology is developed and applied to several nozzle geometries allowing a new level of complexity in the geometry. The predictions of the gross thrust measures by Reynolds-averaged Navier-Stokes computations are in very good agreement with experimental measurements. The spectral predictions from large-eddy simulation are also in good agreement with measured data, for a wide range of jet conditions. The measured azimuthal variations in the noise field from beveled nozzles are reproduced by large-eddy simulation. The spectra from dual-stream nozzles are also well predicted. Preliminary efforts at establishing the link between flow and noise are presented. The interpretation of the flow/noise connection is not straightforward and is a big challenge; it is far from clear how one can establish cause and effect. The ability to relate changes in flow to noise may remain an open issue for the foreseeable future.

Flow and Noise Predictions for Single and Dual-Stream Beveled Nozzles

The characteristics of the flow and the noise of shock-containing jets have been studied for nearly three decades. It is now established that broadband shock-associated noise is generated by the interaction of the downstream-convecting coherent structures of the jet flow with the shock cells in the jet plume. Past analyses of far-field data have been carried out with the total measured noise, which contains both the turbulent mixing noise and shock noise. In this study, these two components are first separated and extracted from the total spectra. Both convergent and convergent-divergent nozzles are considered. The decomposition is made possible by a recently developed scaling methodology for turbulent mixing noise, which provides excellent collapse of the mixing noise spectra from jets at all velocities but at a fixed temperature ratio. The characteristics of the shock component alone are investigated. A surprising effect of jet temperature on shock noise is established for the first time: the levels increase as the jet is first heated; however, the levels do not increase with further increase in jet temperature. The physical phenomenon responsible for this saturation of levels is not known at this time. The intensity for shock noise in the forward quadrant does not scale as the fourth power (shock exponent) of √|M 2 j -M 2 D | but spans a range from 2.9 to 6.17, depending on the radiation angle and the jet temperature ratio. It is not straightforward to collapse the shock spectra. It is also established for the first time that nonlinear propagation effects are manifested at lower radiation angles, in which the shock component is dominant. The physical phenomenon that triggers the onset of nonlinear propagation for the shock noise could not be identified. The characteristics of the correlation functions at the lower inlet angles for subsonic and supersonic jets are different, attesting to the different noise generation mechanisms.

Characteristics of the Shock Noise Component of Jet Noise

Jet noise is a major component of total aircraft noise at takeoff, even for modern aircraft powered by high-bypass-ratio (BPR) turbofan engines with BPR of around five. The reduction of jet noise at fixed BPR has proven to be a formidable challenge, and no practical design that produces substantial aeroacoustic benefit has evolved despite significant effort. A simple concept of a beveled nozzle is adapted for realistic engine geometries. The strategy for noise reduction in the peak sector of noise radiation is based on our understanding of the characteristics of the noise sources in a dual-stream exhaust system. Specifically, the noise generation mechanisms of the inner shear layer are modified by beveling the primary nozzle to achieve noise reduction. Detailed aeroacoustic measurements are carried out statically and in the presence of a flight stream to assess the noise benefit of the modified exhaust system, relative to a conventional arrangement, at realistic engine cycle conditions. Two beveled nozzles of bevel angles 24 (bevel24) and 45 deg have been evaluated. The bevel24 nozzle has a low performance penalty (∼0.2%) at cruise power and produces substantial reduction in noise levels in the aft quadrant. The reduction in overall sound pressure levels is ∼5 to ∼7 dB near the spectral peak in the angular sector >110 deg. There is a slight increase (∼1 dB) in the overall sound pressure levels at the lower polar angles. The magnitude of noise reduction is strongly dependent on the velocity of the inner stream. The reduction in levels occurs over a wide frequency range; there is no increase at the higher frequencies, which is a typical trend for other noise reduction concepts. The effects of forward flight on the measured spectra are different at different azimuthal angles; further, the flight effects in the azimuthal plane are a function of the bevel angle. A total noise reduction of ∼4 dB effective perceived noise level is demonstrated at takeoff (sideline and overhead) at realistic flight Mach numbers with the bevel24 nozzle.

Elegant concept for reduction of jet noise from turbofan engines

The nozzles of a dual-stream turbofan engine are operated invariably at supercritical pressure ratios at cruise, thereby producing shocks in the jet plume. Consequently, broadband shock-associated noise is generated in addition to turbulent mixing noise. The jet exhaust noise impinges on the fuselage and is then transmitted into the interior of the aircraft cabin. In a companion paper, the beveled nozzle has been shown to provide a community noise benefit during takeoff. (Viswanathan, K., "Elegant Concept for Reduction of Jet Noise from Turbofan Engines," Journal of Aircraft, Vol. 43, No. 3, 2006, pp. 616-626.) Here, the aeroacoustic characteristics of two beveled nozzles, with bevel angles of 24 deg (bevel24) and 45 deg (bevel45), operated at supercritical pressure ratios, are presented under static conditions as well as in the presence of a Mach 0.32 flight stream. The intended application is for the control of aft-cabin noise. The velocity of the primary jet plays a strong role in the reduction of noise at the aft polar angles, with a substantial benefit at higher jet velocities. The magnitude of reduction in overall levels for bevel45 at the aft angles ranges from ∼5 to ∼12 dB under static conditions, depending on the Mach number of the secondary stream. In the presence of a flight stream, the jet conditions, as well as the bevel angle, control the flight effects at different azimuthal angles. It is demonstrated that at typical cruise power settings, the current concept with bevel24 yields a reduction of ∼4 dB in overall levels; the reduction occurs over a wide range of lower frequencies without any increase at the higher frequencies. Given this reduction in impingement levels on the rear fuselage, this design could yield a cabin noise benefit in addition to the benefit for community noise at takeoff conditions.

Krishna Viswanathan

Papers

Elegant concept for reduction of jet noise from turbofan engines

Noise of Dual-Stream Beveled Nozzles at Supercritical Pressure Ratios