Search

The hydrodynamic characteristics and associated far-field acoustics of installed elliptical jets were studied using linear models based on the Navier-Stokes equations. A compressible elliptical vortex-sheet model predicted that the growth rates of the varicose ce0 and wagging ce1 modes decreased with the aspect ratio, whereas the growth rate of the flapping se1 mode increased, which was consistent with previous studies using finite-thickness stability models. Using a propagation model adapted from the Lighthill acoustic analogy, it was found that the acoustic efficiency of the "varicose" ce0 mode is significantly higher than the "flapping" se1 and "wagging" ce1 modes for both free- and installed jets. Increasing the aspect ratio of the elliptical jet produces a slight reduction in sound for the ce0 mode, a significant decrease in the se1 mode, and a significant increase for the ce1 mode. A similar trend was observed for the installed case. An exponential relationship between the jet-plate distance and scattered sound was found for elliptical jets, as observed for circular jets. Other geometric parameters such as the orientation angle of the ellipse were also analyzed. [ABSTRACT FROM AUTHOR]

Implementing measures for the reduction of aircraft noise impact, such as optimization of flight paths and aircraft, requires sophisticated simulation capabilities. These tools have to incorporate simulation of aircraft noise generation at the source (i.e., emission) and account for prevailing sound propagation effects to ultimately predict noise levels as received on the ground. Obviously, understanding the associated uncertainties is crucial when aiming at a reliable and meaningful assessment. It also becomes essential when comparing different technologies, mixing experimental and numerical data, or using simulation methods of different fidelity (e.g., semi-empirical and first-order methods). This research focuses on quantifying uncertainties in the first step in aircraft noise simulation (i.e., the prediction of the emission situation). The quantified uncertainties reflect imperfections of models for different aircraft noise sources and variability of model input parameters at different operating conditions. A general framework is presented, which also accounts for limited knowledge of the underlying data distributions, and quantitative comparisons of different uncertainty methods are provided. In particular, the first-order second-moment analysis is compared to higher-order polynomial chaos methods, and the advantages and disadvantages of the different methods are discussed. [ABSTRACT FROM AUTHOR]

The distributions of turbulent scales in a Mach 0.9 isothermal round jet are investigated for the purpose of developing linear surface-based models for the noise source that can be informed by a Reynolds-averaged Navier-Stokes (RANS) solution of the flowfield. The jet is calculated by large-eddy simulation (LES), which enables the computation of two-point space-time correlations throughout the jet and its near-acoustic field. Time, length, and convective-velocity scales are examined on the surface of peak Reynolds stress (SPS), representing the location of the most energetic eddies, and on a "radiator surface" at the boundary between the rotational and irrotational fields. The nature of the space-time correlations is different for axial velocity fluctuations and pressure fluctuations. Velocity-based correlations appear to capture localized turbulent events, whereas pressure-based correlations appear dominated by the interaction of large eddies with the surrounding potential flow. The correlation length scales are larger on the radiator surface than on the SPS, thus indicating that small-scale eddies do not make a significant imprint on the radiator surface. The scales associated with an emulated RANS solution of the flow are compared to the LES-based scales. Simple relationships are inferred that may aid the development of rapid predictive models. [ABSTRACT FROM AUTHOR]

Spectral proper orthogonal decomposition (SPOD) is a major modal analysis that extracts a dominant structure at each frequency from complicated flow data, such as the wavepacket hidden under turbulent jets. This study investigated the physical interpretation of subdominant multilobe structures in the SPOD modes. A high-speed schlieren movie that exhibited multilobe structures in the SPOD was analyzed using a wavelet-based conditional sampling and averaging (WB-CSA) method. The results of WB-CSA showed spatiotemporally localized wavepackets, which were formed intermittently and convected downstream through growth and decay. Such intermittent convection causes multilobe structures in the SPOD, as illustrated by simple synthetic data that model the results of WB-CSA. Therefore, multilobe structures, together with the most dominant single-lobe wavepackets, can be interpreted as representing the intermittent convection of wavepackets. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental investigation of the flow-induced sound produced by a smooth, wall-mounted finite length airfoil with a flat-ended tip and a tripped turbulent boundary layer. Acoustic measurements have been taken in the Stability Wind Tunnel at Virginia Polytechnic Institute and State University with a microphone array at a range of Reynolds numbers (ReC=7.9×105-1.6×106, based on chord), angles of attack (a=0-12 deg), and for a variety of airfoil aspect ratios (airfoil length-to-chord ratio of). Spectral data show the dominant noise sources are airfoil trailing-edge noise and tip-vortex-formation noise. Acoustic data are also used to evaluate semi-empirical prediction of wall-mounted finite airfoil trailing edge and tip noise with the so-called Brooks, Pope, and Marcolini model. The prediction method employs the Brooks, Pope, and Marcolini trailing-edge-noise model modified to incorporate spanwise variations in flow properties in combination with the Brooks, Pope, and Marcolini flat-tip-noise model. Three-dimensional trailing-edge-noise predictions agree well with measured spectra at a Strouhal number of SrC<18, based on airfoil chord. The Brooks, Pope, and Marcolini tip-noise model underpredicts the peak level and frequency of tip-noise contributions at SrC>18. A new empirical model of flat-tip-airfoil noise is presented that provides an accurate estimation of the wall-mounted finite airfoil dominant tip-noise contribution to within 1.7 dB. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study on open rotor noise shielding by hybrid-wing-body aircraft, with the objective of understanding the noise shielding features and establishing methods of applying the shielding data for system noise studies of hybrid-wing-body aircraft. By studying the directivity patterns of individual tones, it is shown that, although the tonal energy distribution and the spectral content of the wind-tunnel test model (and thus its total noise) may differ from those of more advanced rotor designs, the individual tones follow directivity patterns that characterize far-field radiations of modern open rotors, establishing the validity of the use of this shielding database, provided that the shielding effects are applied on individual tones. To this end, open rotor tonal noise shielding is categorized into front rotor tones, aft rotor tones, and interaction tones: not only because of their different directivities but also due to the differences in their source locations and coherence features, which make their respective shielding characteristics distinctly different from each other. To reveal the parametric trends of the hybrid-wing-body shielding effects, results are presented with variations in frequency, far-field emission angle, rotor operational condition, engine installation, and local airframe features. [ABSTRACT FROM AUTHOR]

This paper represents the continuation of the works previously published in Chong et al. ("Self-Noise Produced by an Airfoil with Nonflat Plate Trailing-Edge Serrations," AIAA Journal, Vol. 51, No. 11, 2013, pp. 2665-2677), who used several nonflat plate serrated trailing edges for the reduction of airfoil self-noise. The poro-serrated concept developed in the current work improves substantially the overall noise performance of the nonflat plate trailing-edge serration type. The use of porous metal, synthetic foams, or thin brush bundles to fill the gaps between adjacent members of the sawtooth can completely suppress the bluntness-induced vortex shedding tonal noise. Most important, up to 7 dB turbulent boundary layer-trailing-edge broadband noise reduction can simultaneously be achieved without compromising the aerodynamic performances in lift and drag. The poro-serrated trailing edges do not cause any noise increase throughout the frequency range investigated here. The reduction of the turbulent broadband noise is primarily caused by the serration effect, but under a condition that the sawtooth surface must be solid and nonporous. The primary role of the porous metal foams in a poro-serrated trailing edge is to suppress the vortex shedding tonal noise. However, an optimum selection of the porous material is also found to be able to further reduce the broadband noise level. The new serrated trailing-edge concept developed here has the potential to improve the industrial worthiness of the serration technology in achieving low noise radiation in fan and turbine blades. [ABSTRACT FROM AUTHOR]

Ongoing research toward the reduction of environmental noise from aircraft is investigating the possible shielding of engine-noise sources by novel airframe configurations. To assess the noise-reduction benefits attainable from such configurations, it is necessary to develop appropriate acoustic evaluation tools. In this paper, a jet-noise-shieldingprediction methodology is described. The Tam-Auriault ("Jet Mixing Noise from Fine-Scale Turbulence," AIAA Journal, Vol. 37, No. 2,1999, pp. 145-153) jet-noise model with a Reynolds-averaged Navier-Stokes solution input, together with a Fresnel-Kirchhoff diffraction method (Fundamentals of Physical Acoustics, Wiley-Interscience, New York, 2000, pp. 472-494), is used to make isolated and shielded far-field jet-noise predictions. This methodology is employed as a sensitivity-analysis tool to establish the relative importance of the source location, spatial extent, and directivity in jet-noise-shielding predictions. Predictions have been made for a shielded single-stream Mach 0.9 jet, and compared with experimental data. Good qualitative agreement is observed, and the disagreement in the shielding levels is most likely due to underestimation of the source axial extent by the jet-noise model. [ABSTRACT FROM AUTHOR]

The article focuses on a study which investigates the jet noise sources in a hypersonic nozzle at airplane takeoff. Topics discussed include the use of a laser schlieren optical system that realizes a small spot size to gain a high sensitivity capable of detecting the acoustic wave propagation in the ambient, the behavior of the source in a supersonic jet issuing from a realistic hypersonic nozzle, and the cross correlation between light intensity fluctuation and far-field noise.

Traditional installations of turbofan intake liners include acoustically "hard" axial splices between liner segments for ease of fabrication and assembly. The splices scatter energy from strong rotor-locked tones into adjacent azimuthal orders for which the liner is less effective, thereby degrading the liner performance. The significance of this "splice effect" has led to the adoption of "zero-splice" liners in recent turbofan nacelles. However, damage can occur to such liners in service, and the extent to which local liner repairs reduce the effectiveness of the zero-splice design then becomes an issue. In the current paper, the acoustic effect of damage and repair in a zero-splice liner is simulated numerically. The effects of the extent and the location of a hard patch, representing the liner damage and repair, on the overall performance of the liner are predicted. A close agreement is demonstrated with results from an asymptotic analytical model valid for small patch widths. An approximate method is also described to account for the effect of the nonlinear propagation on linear predictions of the liner performance when repairs are present. [ABSTRACT FROM AUTHOR]

An acoustic analogy is developed to predict the noise from jet flows. It contains two source models that independently predict the noise from turbulence and shock wave shear layer interactions. The acoustic analogy is based on the Euler equations and separates the sources from propagation. Propagation effects are taken into account by approximating the vector Green's function of the linearized Euler equations with the use of a locally parallel mean flow assumption. A statistical model of the two-point cross correlation of the velocity fluctuations is used to describe the turbulence. The acoustic analogy attempts to take into account the correct scaling of the sources for a wide range of nozzle pressures and temperature ratios. It does not make assumptions regarding fine- or large-scale turbulent noise sources, self- or shear noise, or convective amplification. The acoustic analogy is partially informed by three-dimensional steady Reynolds-averaged Navier-Stokes solutions that include the nozzle geometry. The predictions are compared with experiments of jets operating subsonically through supersonically and at unheated and heated temperatures. Predictions generally capture the scaling of both mixing noise and broadband shock-associated noise for the conditions examined, but some discrepancies remain, which are due to the accuracy of the steady Reynolds-averaged Navier-Stokes turbulence model closure, the equivalent sources, and the use of a simplified vector Green's function solver of the linearized Euler equations using a locally parallel mean flow. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of the slat noise for hybrid wing-body aircraft. It is shown that the hybrid wing-body slat noise is characterized by its broad spectral shapes with frequencies depending on both the mean flow velocity and the aircraft angle of attack, with the former following the conventional Strouhal number scaling and the latter explainable by the dependence of the coherence length of the unsteady flows on the angle of attack. Although the overall noise levels approximately follow the fifth power law in Mach number, the Mach number effects manifest themselves spectrally in both amplitudes and spectral shapes. The noise amplitude is shown to also depend on the angle of attack, assuming a minimum in the range of 3 to 5 deg. These features are all modeled and incorporated in slat noise-prediction methodologies, extending the prediction capability from conventional to hybrid wing-body configurations. Comparisons between predictions and data show very good agreements in both various parametric trends and the absolute levels. The hybrid wing-body aircraft is designed to operate at angles of attack higher than those of conventional aircraft. This is shown to significantly increase the hybrid wing-body slat noise. To further illustrate, the test data are extrapolated to full scale and compared with the slat noise of the Boeing 777 aircraft, showing that the former is higher than the latter. [ABSTRACT FROM AUTHOR]

The measurement of airframe noise on small-scale models is a well-known and common practice in conventional wind tunnels. Because conventional wind tunnels cannot generally achieve full-scale Reynolds numbers, measurements during the development process of modern aircraft are performed in cryogenic and/or pressurized wind tunnels that are capable of higher-Reynolds-number flows. Thus, the characteristics of the moving fluid are better adapted to the scale model. At the Göttingen Institute of Aerodynamics and Flow Technology of DLR, German Aerospace Center, the microphone-array measurement technique was further developed to perform measurements in a cryogenic wind tunnel at temperatures down to 100 K. A microphone array consisting of 144 microphones was designed and constructed for this purpose. In this paper, acoustic-array measurements performed in a cryogenic wind tunnel are described for various Reynolds numbers using a 9.24% Dornier 728 half-model. Our results showed a significant Reynolds-number dependency of the measured equivalent sound power for various sources. [ABSTRACT FROM AUTHOR]

This paper describes the design and characterization of a micromachined microphone for aircraft fuselage arrays utilized by aeroacousticians to help identify aircraft noise sources and/or assess the effectiveness of noise-reduction technologies. The developed microphone utilizes piezoelectric transduction via an integrated aluminum nitride layer in a thin-film composite diaphragm fabricated using a combination of surface and bulk micromachining. The experimental characterization of several microphones is presented. Measured performance was in line with the Boeing Company specifications for the fuselage array application, including sensitivities of 32.1 µ V/Pa to 43.7 µ V/Pa, minimum detectable pressures as low as 40 dB (1 Hz bin at 1 kHz), confirmed bandwidths up to 20 kHz, > 100 kHz resonant frequencies, and 3% distortion limits between 160 and 172 dB sound pressure level. With this performance, in addition to the small sizes, these microphones are shown to be a viable enabling technology for low-cost, high-resolution fuselage array measurements. [ABSTRACT FROM AUTHOR]

One of the major environmental problems facing the aviation industry is that of aircraft noise. The work presented in this paper, done as part of the European Union's Opfimisation for Low Environmental Noise Impact Project, looks at reducing spoiler noise while maintaining aerodynamic performance, through means of large-scale fractal porosity. It is hypothesized that the highly turbulent flow generated by fractal grids from the way the multiple-length scales are organized in space, would reduce the impact of the recirculation region and, with it, the low-frequency noise it generates. In its place, a higher frequency noise is introduced, which is more susceptible to atmospheric attenuation and is less offensive to the human ear. A total of nine laboratory-scaled spoilers were looked at, seven of which had a fractal design, one with a regular grid design, and one solid for reference. The spoilers were inclined at an angle of 30 deg. Force, acoustic, and flow visualization experiments on a flat plate were carried out and it was found that the present fractai spoilers reduce the low-frequency noise by 2.5 dB. Results show that it is possible to improve the acoustic performance by modifying a number of parameters defining the fractal spoiler, some of them very sensitively. From these experiments, two fractal spoilers were chosen for a detailed aeroacousfic study on a three-element wing system, where it was found that the fractal spoilers had a reduction of up to 4 dB in the sound pressure level, while maintaining similar aerodynamic performances as conventional solid spoilers on the measured wing system. [ABSTRACT FROM AUTHOR]

A source localization and reconstruction technique is applied to analyze wall pressure measurements undertaken on the surface of a space launcher mock-up. The technique is based on the combined use of a beam-forming technique and elementary acoustic solutions tailored to the space launcher geometry. Two classes of acoustic elementary solutions are considered: plane waves impinging on the surface of an infinite cylinder computed analytically through a literature formula, and plane waves impinging on the real space launcher computed numerically through a computational acoustic technique. These two classes of basis functions are proven to provide consistent source localization results in the addressed frequency range. Moreover, the source reconstruction technique is shown to recover the measured wall pressure cross-spectra with a good confidence level in magnitude and fair confidence level in phase. The proposed launcher-tailored source localization and reconstruction techniques are therefore promising useful tools for the vibroacoustic design of future launch vehicles. [ABSTRACT FROM AUTHOR]

The article discusses the problem of noise pollution during the takeoff and landing of aircrafts. It mentions that a new method has been proposed for the detection of spinning modes using a single sensor. It also talks about several topics including in-duct circular microphone arrays, spinning modes and noise source of turbofan engine which generates between rotating fan and nearby stator vanes.

Accurate prediction of jet noise from dual-stream nozzle exhaust geometries typical of high bypass ratio turbofan engines has practical relevance for aircraft design studies and in aircraft noise certification. Existing empirical methods do not provide good predictions against measured data; in addition, they are restricted to lower area ratio (secondary to primary, As /Ap) nozzle geometries and lower bypass ratios. The area ratio and bypass ratios of newer engines have increased substantially, well beyond the range of validity of existing prediction methods. Therefore, there is a dire need to develop an accurate empirical prediction method that is valid over a wider range of jet operating conditions and nozzle geometries that are representative of current engines. A wind-tunnel test has been completed and a comprehensive database from realistic geometries and operating conditions has been generated. The data have been analyzed and investigated with an eye toward modeling that would lead to an empirical prediction method. The area ratio and bypass ratios cover wide ranges that encompass all current engines and potential ultrahigh bypass ratio engines of the future. The spectral characteristics have been examined and the effects of the following parametric variations are reported in this paper: 1) impact of area ratio at several power settings; 2) at a given As/Ap, fixed primary jet and systematically varying secondary conditions; 3) at a given As/Ap, fixed secondary jet and systematically varying primary conditions; and 4) the effects of forward flight for the above situations. It is established that the characteristics of dual-stream jets with velocity ratio less than 0.5 are similar to those for a single jet, with the velocity ratio being the main controlling parameter and the area ratio playing a lesser role. At higher velocity ratios, contributions from the secondary shear layer is dominant at higher frequencies and the contributions from the mixed jet component controls the peak spectral levels in the peak radiation sector at large aft angles. At low inlet angles, the spectral shape remains invariant for all jet and geometric conditions. [ABSTRACT FROM AUTHOR]

This paper presents a fundamental investigation into the validity of the classical model by Amiet for predicting the broadband trailing-edge noise due to rotating blades. This approximate model is compared analytically against a model in which the effects of rotation are treated exactly. Low- and high-frequency limits are identified, within which Amiet's model is in excellent agreement with the exact solution. Estimates of the error of Amiet's model outside of these limits are also provided. The method is illustrated by application to an open propeller, a model cooling fan, and a wind turbine. The use of Amiet's model for treating the effects of rotation can provide substantial reductions in computation time when the unsteady blade response is computed from computationally demanding methods, such as computational fluid dynamics. [ABSTRACT FROM AUTHOR]

The noise of high-speed jets, generated by nozzle exhaust systems that resemble typical jet engine installations on fighter aircraft, has been investigated with the main objective of developing noise reduction designs. To this end, the aeroacoustic characteristics of beveled nozzles have been examined in model scale tests, and their performance relative to a conventional round nozzle system has been established. Two different physical models with appropriate area ratios to simulate the engine operating conditions at different power settings were tested; these correspond to 96% NI (military power) and 91% N1 (cutback power). Aeroacoustic measurements and analyses have been carried out for nozzles at bevel angles of 20, 24, 28, and 35° for military power, and 24 and 32° for cutback power. For the same plenum conditions, the mass flow rates for the beveled nozzles are identical to those of the baseline round nozzle, for a wide range of nozzle pressure ratio. The thrust coefficients are higher for the beveled nozzles for both the nozzle geometries at typical military power and cutback power, respectively. The increase in the thrust coefficient ranges from ~0.75 % for the bevel35 to ~2 % for bevel28, in spite of the slight deflection of the jet plume. The reasons for this phenomenon are examined. The beveled nozzles produce at least the same or greater absolute thrust as the baseline nozzle. The beveled nozzles specifically reduce the noise in the peak polar radiation angles; the maximum noise benefit is observed in the azimuthal direction of the longer lip. The magnitude of noise reduction increases with increasing bevel angle, with the largest reduction for bevel35. An examination of the dBA metric, which is used in noise exposure studies, indicates that there is a noise benefit of ~3 to ~4 d BA in the peak radiation sector. The noise benefit in the azimuthal direction of the shorter lip is only slightly lower than that in the direction of the longer lip, while there is a small noise increase toward the broader side of the beveled nozzle at the lower inlet angles. [ABSTRACT FROM AUTHOR]

Heated high-speed subsonic and supersonic jets operating on- or off-design are a source of noise that is not yet fully understood. Helium-air mixtures can be used to simulate the total temperature ratio of heated jets and hence have the potential to provide inexpensive and reliable flow and acoustic measurements. Accurate knowledge of the mean flow is a crucial initial step for a complete understanding of the noise generation mechanisms. This study focuses on providing close comparisons between measured mean flow properties of helium-air mixture jets and results from simulations of similar mixed jets as well as heated air jets. Axisymmetric supersonic jets issuing from convergent and convergent-divergent nozzles are investigated, and the results show good agreement with heated air jet measurements. The flow properties are examined in detail to demonstrate the validity of simulating heat with the addition of helium. Excellent agreement is obtained in the presented data between the numerical predictions and the experiments, further justifying the helium addition methodology, as well as validating the frozen chemistry model used in the numerical simulations. [ABSTRACT FROM AUTHOR]

The article discusses the effect of deflecting the jet plume on jet noise. It examines whether the canting and the beveling of a round nozzle have the same noise characteristics. It also determines how much of the observed change in noise between beveled and round nozzles is due to the kinematic effect of different plume orientations.

An experimental program is carried out to assess the effects of forward flight on both turbulent mixing noise and broadband shock-associated noise from single-stream jets. Far-field spectral measurements are made over a wide range of jet operating conditions and at seven tunnel Mach numbers, in an open freejet anechoic facility. The tunnel Mach number Mt spans a range from 0.0 to 0.32. The values of the flight velocity exponent k, obtained from a least- squares fit of the variation of the reduction in overall sound pressure levels with relative velocity (10 Log[Vj/(Vj - Vt)]), have been calculated at all angles from the database. The flight velocity exponent has no discernible dependence on jet Mach number or temperature ratio. There is a distinct variation with angle: a slow increase from ~2.9 to ~3.5 at the lower polar angles from 50 to ~105° and a steeper increase from ~3.5 to ~7.6 from ~ 105 to 150°. Two different characteristic velocities may be identified for jets in forward flight: jet velocity Vj and relative velocity (Vj - Vt. Though the Strouhal numbers based on both provide good collapse of the spectra at the lower polar angles, the jet velocity Vj rather than (Vj - Vt ) is shown to be the correct characteristic velocity at large aft angles. Thus, it is established that the regular Strouhal number based on ~ is the correct nondimensional frequency for scaling spectra. The scaling of spectra at large aft angles is not straightforward and additional issues need to be considered. For jets in the presence of forward flight, it is established that the phenomenon of nonlinear propagation is observed when the relative velocity (Vj - Vt) becomes convectively supersonic. This is a new result and has not been reported in any prior study. An effective jet velocity, which is a combination of both the jet velocity and the ambient speed of sound, is required to collapse spectra from unheated jets only at large aft angles. The use of an effective velocity is not required for heated jets; the regular Strouhal number is the correct nondimensional frequency at all angles, with or without forward flight. The effect of forward flight on broadband shock-associated spectra is minor. The effects of flight on spectra at various angles need to be modeled for the development of a practical prediction scheme for jet noise. A simple additional term to the recent scaling method is shown to collapse the spectra obtained in the presence of forward flight. It is quite remarkable that the characteristics of the mixing noise spectra from single-stream jets at all angles, with or without the presence of forward flight, can be represented by a single equation. The negligible dependence of the flight velocity exponent on jet conditions allows for the following practical benefit: it is possible to predict the flight effect for lower-velocity jets for which the measurement of flight effect may not be feasible because of high tunnel-noise floor. This key finding perhaps represents the greatest value of this investigation. [ABSTRACT FROM AUTHOR]

An efficient numerical method to solve nonlinear sound propagation is presented. The frequency-domain Burgers equation, which includes nonlinear steepening and atmospheric absorption, is formulated in the form of the real and imaginary parts of the pressure. The new formulation effectively eliminates possible numerical issues associated with zero amplitude at higher frequencies occurring in a previous frequency-domain algorithm. In addition, to circumvent a high-frequency error that can occur in the truncated higher frequencies, a split algorithm is developed. in which the Burgers equation is solved below a cutoff frequency and a recursive analytic expression is used beyond the cutoff frequency. Finally, the Lanczos smoothing filter is incorporated to remove the Gibbs phenomenon. The new method is found to successfully eliminate high-frequency numerical oscillations and to provide excellent agreement with the exact solution for an initially sinusoidal signal with only a few harmonics. The new method is applied to a broad range of applications with a comparison to other methods to assess the robustness and numerical efficiency of the method. These include sonic boom, broadband supersonic jet engine noise, and helicopter high-speed impulsive noise, it is shown that the new method provides the fastest and most accurate predictions compared to the other methods for all the application problems. [ABSTRACT FROM AUTHOR]

A first attempt to numerically simulate broadband noise transmitted through and radiated from a turbofan bypass duct is presented in this paper. Turbulent source assessment on the fan rows is left out here, and a generic power spectral density of in-duct acoustic pressure is imposed as an input to a computational aeroacoustic simulation performed using a time-domain Euler solver. The acoustic field is expanded over Fourier-Bessel modes, and the broadband noise behavior is modeled by introducing a random phase and by performing a quadratic averaging over independent runs. This broadband source generation approach is first validated in a semi-infinite annular duct by comparison with analytical solutions. The exhaust radiation problem from an idealized nozzle is then validated (for a single mode) by comparison to a Wiener-Hopf solution. In a second step, refraction effect through the shear layer is investigated for broadband noise. Computed results appear to be stable even in the presence of vorticity waves created in the shear layer. Then, the computation method is applied to a realistic nozzle. The only problem arising is the computational cost resulting in a limitation of high-frequency range limited here to 2-3 kHz {with around 600 propagating modes). Computed far-field directivities relative to idealized or realistic nozzle with similar input conditions are found to be rather close up to 90° of radiation angle. [ABSTRACT FROM AUTHOR]

This paper deals with the numerical study of installation effects, a subject which comes under the more general purpose of aircraft noise prediction and reduction, As an example of installation effects, we study here the potential acoustic shield that an empennage airfoil could constitute against the aft fan noise emitted by a coaxial engine. To that end, we make use oft structured computational aeroacoustics solver (sAbrinA code), taking benefits from its coupling functionality in order to handle more easily the solid obstacles that are to be accounted for. Several full-3D simulations are then conducted, which involve an exhaust installed over an empennage airfoil and allotted with two thermodynamics conditions (quiescent medium and takeoff flight). Being validated against boundary element method results, such calculations provide a validation of the numerical strategy adopted here as well as a good illustration of its abilities to solve installed engine noise problems. Coming in addition to such methodological outcomes, these calculations deliver important conceptual insights, demonstrating the potential high efficiency of empennage airfoils in terms of aft fan noise reduction. [ABSTRACT FROM AUTHOR]

The article studies the effect of aerodynamic tabs on the screech problem in Supersonic jets. Environmental compliance is one of the requirements in supersonic transporters particularly the reduction of noise. In the study, tabs are mounted to the nozzle exit in order to reduce the mass flow of the area due to the blockage effect. It was then concluded that when the secondary jet is used, the screech noise is reduced.

Noise from high-lift devices such as slats and flaps can contribute significantly to the overall aircraft sound pressure levels, particularly during approach. The acoustic spectrum of the noise radiated from slats exhibits two distinct features. There is a high-frequency tonal noise component and a high-energy broadband component ranging from low- to midfrequencies. The objective is to predict the broadband slat noise. The broadband noise is predicted using a two-step process. First, the noise sources are modeled based on the local turbulence information. Then, the sound from these sources is propagated by assuming that the flow past the wing is uniform. A boundary element method is used to find Green's function for wave propagation in a moving medium in the presence of the wing. The noise in the far field is then predicted by forming a convolution of Green's function with the modeled sources. The attractive feature of this prediction scheme is the quick computational time, which makes it suitable for new design and control strategies. [ABSTRACT FROM AUTHOR]

Robust design has been developed with the expectation that an insensitive design can be obtained. That is, a product designed by robust design should be insensitive to external noises or tolerances. An insensitive design has more probability to obtain a target value, although there are uncertain noises. Theories of robust design have been developed by adopting the theories of other fields. Based on the theories, robust design can be classified into three methods: 1) the Taguchi method, 2) robust optimization, and 3) robust design with the axiomatic approach. Each method is reviewed and investigated. The methods are examined from a theoretical viewpoint and are discussed from an application viewpoint. The advantages and drawbacks of each method are discussed, and future directions for development are proposed. [ABSTRACT FROM AUTHOR]

Nowadays, commercial aircrafts, invariably, use high-bypass-ratio dual-stream jets for propulsion. As yet, there is still an urgent need for an accurate physics-based noise prediction theory for jets of this configuration. Thus, an investigation is made to determine whether the Tam and Auriault theory (Tam, C. K. W., and Auriault, L., ‘Jet Mixing Noise from Fine Scale Turbulence,’ AIAA Journal, Vol. 37, No. 2, 1999. pp. 145–153), originally developed for predicting the fine-scale turbulence noise of single-stream jets, is capable of predicting accurately the fine-scale turbulence noise of dual-stream jets from separate flow nozzles operating at various bypass ratios. The configuration of a separate flow nozzle is fairly complex. Hence, the jet flow and turbulence in the nozzle region and in the region immediately downstream are also fairly complex. However, these are also the most important noise source regions of the jet. To enable an accurate computation of the mean flow and turbulence level in these regions, a computational aeroacoustics marching algorithm for calculating the parabolized Reynolds averaged Navier-Stokes equations supplemented by the k-ϵ turbulence model is provided. It is shown that the computed mean flow profiles are in good agreement with experiment. Extensive comparisons between computed noise spectra and measurements are reported. They include dual-stream jets from separate flow nozzles with and without an external plug. Jets operating at different combinations of primary and secondary jet Mach number and temperature ratio are considered. The bypass ratios range from 1.5 to 8.0. Effects of forward flight are also included in the comparisons. Good agreements are found not only in spectral levels but also in spectrum shapes and directivities over many sets of data. [ABSTRACT FROM AUTHOR]

The behavior of supersonic impinging jets is dominated by a feedback loop due to the coupling between the fluid and acoustic fields. This leads to many adverse effects when such flows occur in short takeoff and vertical landing aircraft, such as a significant increase in the noise level, very high unsteady loads on the nearby structures, and an appreciable loss in lifting during hover. In earlier studies, it was demonstrated that by using supersonic microjets one could disrupt the feedback loop that leads to substantial reductions in the aforementioned adverse effects. However, the effectiveness of control was found to be strongly dependent on the ground plane distances and the jet-operating conditions. The effect of various microjet control parameters are investigated in some detail to identify their influence on control efficiency and additional insight is provided on the physical mechanism behind this control method. Parameters studied include microjet angle, microjet pressure, and the use of microtabs instead of microjets. These results indicate that by choosing appropriate control parameters it should be possible to devise a control strategy that produces optimal control for the entire operating range of conditions of the supersonic impinging jet. Moreover, the experimental results provide convincing evidence of the generation of significant streamwise vorticity by the activation micro jets. It is postulated that the generation of streamwise vorticity and its evolution in the jet flow might be one of the main physical phenomena responsible for the reduction of flow unsteadiness in impinging jets. [ABSTRACT FROM AUTHOR]

Although civil aircraft environmental performance has been important since the beginnings of commercial aviation, continuously increasing air traffic and a rise in public awareness have made aircraft noise and emissions two of the most pressing issues hampering commercial aviation growth today. This, in turn, has created the demand for an understanding of the impact of noise and emissions requirements on the design of the aircraft. In response, the purpose of this research is to explore the feasibility of integrating noise and emissions as optimization objectives at the aircraft conceptual design stage, thereby allowing a quantitative analysis of the tradeoffs between environmental performance and operating cost. A preliminary design tool that uses a multiobjective genetic algorithm to determine optimal aircraft configurations and to estimate the sensitivities between the conflicting objectives of low noise, Iow emissions, and operating costs was developed. Beyond evaluating the ability of a design to meet regulations and establishing environmental performance trades, the multidisciplinary design tool allows the generation of conventional but extremely low-noise and low-emissions designs that could, in the future, dramatically decrease the environmental impact of commercial aviation, albeit at the expense of increased operating cost. The tool incorporates ANOPP, a noise prediction code developed at NASA Langley Research Center, NASA Glenn Research Center's Engine Performance Program engine simulator, and aircraft design, analysis, and optimization modules developed at Stanford University. The trend that emerges from this research among the seemingly conflicting objectives of noise, fuel consumption, and NOx emissions is the opportunity for significant reductions in environmental impact by designing the aircraft to fly slower and at lower altitude. [ABSTRACT FROM AUTHOR]

The NASA Glenn Research Center recently completed an experimental study to reduce the jet noise from modern turbofan engines. The study concentrated on exhaust nozzle designs for high-bypass-ratio engines. These designs modified the core and fan nozzles individually and simultaneously. In comparison with chevrons, tabs appeared to be an inefficient method for reducing jet noise. Data trends show that interaction between fan flow and the core cowl could strongly impact noise and cruise performance irrespective of the mixing device installed. The study demonstrates that modifications of the core nozzle are generally more advantageous than of the fan nozzle. Even greater advantage in noise reduction and associated cruise thrust penalties is demonstrated through simultaneous modification of both nozzles. The best nozzle design had a 0.06% cruise thrust loss and, corrected for takeoff thrust loss, a 2.7-EPNdB reduction for the effective perceived noise level metric. This design simultaneously employed chevrons on the core and fan nozzles. Last, five nozzle configurations with cruise thrust loss of less than 0.5% and noise reductions of over 2.5 EPNdB are identified as candidates for full-scale engine and flight demonstrations. [ABSTRACT FROM AUTHOR]

Studies acoustic radiation from thin airfoils generated in response to upstream, small amplitude plane vorticity waves computed using a time-domain spectral element method. Solution approach; Description of the noise radiated by a flat plate in response to a plane vorticity wave; Response of a nonlifting finite thickness airfoil; Results for the noise generated by a lifting airfoil.

Presents a comprehensive far-field acoustics database generated for high subsonic, turbulent jets along with mean total pressure and temperature surveys in the flowfield for the purpose of evaluating noise reduction devices. Experimental setup description; Noise metrics; Database results; Flow and noise control results.

Studies the optimal placement and size of disk-shaped piezoelectric actuators to reduce the radiation of sound into the space above a plate structure when excited by an acoustic pressure field produced by a noise source located below the plate. Development of an automatic mesh and boundary condition generation program; Optimal design at the resonance and off-resonance frequencies.

Proposes a vortex-oscillation model of airfoil side-edge noise. Importance of oscillation of the edge vortex; Calculation of vortex oscillation modes; Prediction of surface pressure; Description of far-field pressure; Application of the single-vortex model of the edge vorticity; Components of the vortex-oscillation eigenvector.