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The effect of external nozzle geometry on the emission of screech tones was studied experimentally. Four conical reflector surfaces, with half-angles ranging from 60° to 90°, were installed around the exit of a round convergent nozzle. The investigation focused on two closely spaced fully-expanded Mach numbers, M j = 1.32 and 1.34. The acoustic far-field was surveyed by a microphone phased array that included a continuously-scanning microphone, the latter enabling high spatial resolution. The isolated jets contained well-known screech mode B and its harmonics. Addition of the reflectors caused significant changes in the modal emission pattern, with tones traditionally linked to mode C occurring at M j = 1.34 but not at M j = 1.32. Tonal components associated with new modes E and F emerge at both Mach numbers when the cone half-angle is 60° or 70°. The noise source distribution generally elongates with decreasing cone angle. Some modes show clear scattering from the reflectors, while others do not. The study underscores the complexity that initial conditions can impart on the modal structure of screech and demonstrates the capability of the continuous-scan beamforming technique in resolving fine features of the source. [ABSTRACT FROM AUTHOR]

The paper adapts the delay-and-sum beamforming method and related deconvolution techniques for microphone phased arrays comprising fixed and continuously scanning sensors. Processing of the nonstationary signals introduced by the scanning sensors requires division of all the signals into quasi-stationary blocks and application of a frequency-dependent window within each block. Three distinct approaches are developed: application of delay and sum to each block of the divided signals, followed by assembly of the outputs of the blocks; a cross-spectral matrix completion process; and a partial fields decomposition method. The last two approaches result in a unified cross-spectral matrix that facilitates deconvolution of the array output. The methodologies are applied to a synthetic noise source and to a supersonic underexpanded jet that presented the phenomenon of screech. Introduction of a single scanning sensor to a far-field array comprising 13 fixed sensors significantly improves the fidelity of beamforming. The deconvolved images of the jet noise source resolve fine shock-cell features for which the dimensions are similar to those imaged by optical techniques in past investigations. [ABSTRACT FROM AUTHOR]

We present a computational study, validated by mean-flow experiments, of a dual-stream nozzle simulating the exit conditions of a supersonic turbofan engine with noise-suppressing fan flow deflectors. The study is conducted for eight nozzle configurations and two operating conditions: a cold condition at which mean velocity surveys were conducted and against which the computational code was validated and a hot condition that corresponds to the takeoff engine cycle and at which acoustic data were collected. The code predictions successfully replicate the mean velocity fields and the inflectional layers of the experimental flows. The code is then extended to the conditions of the actual engine cycle. The computations reveal a similar velocity profile for the hot and cold conditions when the axial distance is normalized by the potential core length. For both conditions, the vane deflectors reduce the turbulent kinetic energy k on the underside of the jet. An overall noise source strength is modeled as the axial integral of [k.sup.7/2]. A significant correlation is found between the reduction in the noise source strength and the reduction in the peak level of the overall sound pressure level. DOI: 10.2514/1.J050331

The noise source distribution of coaxial jets with a diameter ratio of 1.6 and variable velocity ratio is investigated with a small-aperture microphone phased array. The array design enables differentiation of noise emitted by largescale and fine-scale turbulence structures, which have different directivities. The acoustic data are complemented by pitot surveys of the mean flow, which yield measurements of the primary and secondary cores of the jet. For zero velocity ratio (single-stream jet), the region near the nozzle emits strong high-frequency noise. Increasing the secondary-to-primary velocity ratio suppresses the near-nozzle noise and extends the location of the peak noise downstream, which increases moderately. The axial location of peak noise is approximately situated at the end of the primary core. The suppression of high-frequency noise is explained by the creation and elongation of the secondary core as the velocity ratio increases. The acoustic trends with velocity ratio are similar for small and large array observation angles from which large-scale and fine-scale turbulence noise, respectively, have been shown to radiate. However, the increase in peak noise is more pronounced for the large-scale noise. DOI: 10.2514/1.J050140

This experimental investigation examined the potential of pylon-based deployable flaps to reduce jet noise of separate-flow turbofan engines with a bypass ratio of eight. The main function of the flap deflectors is to thicken the low-speed region surrounding the core jet in the downward and sideline directions. The study encompassed acoustic measurements, noise-source imaging, mean-velocity surveys, and aerodynamic estimates. Three types of deflectors were tested: solid flaps, porous flaps made of coarse perforation, and porous flaps made of fine perforation. It is shown that all the deflectors reduce noise sources near the end of the primary potential core. However, the solid flaps create excess noise in the vicinity of their location that can overwhelm this noise benefit, particularly at large polar angles. Porous flaps significantly reduce velocity gradients that cause excess noise. Noise generation from the perforations themselves can be shifted to very high frequency (rapidly attenuated by atmospheric absorption) by reducing the size of the perforation. Accordingly, the fine-perforation flaps provided superior acoustic results yielding effective perceived noise level benefits of 2.1 dB in the downward direction and 1.0 dB in the sideline direction. The static-thrust loss of these flaps is estimated at 0.7%. DOI: 10.2514/1.37780

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]

Fan flow deflection for jet noise reduction was applied to subscale nozzles simulating the geometry and exhaust conditions of separate-flow turbofan engines. Two types of deflectors were tested, one comprising two pairs of vanes internal to the fan duct and the other consisting of a wedge positioned outside the fan duct. The noise reduction achieved by the vanes was strong in the downward direction and moderate in the sideline direction. The wedge generated significant attenuation in both directions. The acoustic results are consistent with the measured distortion of the mean velocity field. An approach for the treatment of nonaxisymmetric jets vis-a-vis perceived noise prediction is introduced. DOI: 10.2514/1.22552

A new method for reducing large-scale mixing noise from dual-stream jets is presented. The principle is reduction of the convective Mach number of turbulent eddies that produce intense downward sound radiation. In a jet representing the coaxial exhaust of a turbofan engine, this is achieved by tilting downward, by a few degrees, the bypass (secondary) plume relative to the core (primary) plume. The misalignment of the two flows creates a thick low-speed secondary core on the underside of the high-speed primary flow. The secondary core reduces the convective Mach number of primary eddies, thus hindering their ability to generate sound that travels to the downward acoustic far field. Tilting of the bypass stream is possible by means of fixed or variable vanes installed near the exit of the bypass duct. Subscale aeroacoustic experiments simulated the exhaust flow of a turbofan engine with bypass ratio 6.0. Deflection of the bypass stream resulted in suppression of the peak overall sound pressure level by 4.5 dB and the effective perceived noise level by 2.8 dB. For the nozzle configuration used, the thrust loss is estimated at around 0.5% with the vanes activated and 0.15% with the vanes deactivated.

The connections between the vortical and near-acoustic fields of three-stream, high-speed jets are investigated for the ultimate purpose of developing linear surface-based models for the noise source. Those models would be informed by low-cost, Reynolds-averaged Navier-Stokes (RANS) computations of the flow field. The study uses two triple-stream jets, one is coaxial and the other has eccentric tertiary flow that yields noise suppression in preferred directions. Large eddy simulations (LES) validate the RANS-based models for the convective velocity Uc of the noise-generating turbulent eddies. In addition, the LES results help define a "radiator surface" on which the jet noise source model would be prescribed. The radiator surface is located near the boundary between the rotational and irrotational fields and defined as the surface on which the Uc distribution, obtained from the space-time correlations of the pressure, matches that inferred from the RANS model. The edge of the mean vorticity field is nearly coincident with the radiator surface, which suggests a RANS-based criterion for locating this surface. The two-dimensional space-time correlations show how the asymmetry of the tertiary stream and the resulting thicker low-speed flow weakens the generation of acoustic disturbances from the vortical field. [ABSTRACT FROM AUTHOR]

We present a brief account of Anatol Roshko's research and educational contributions to fluid mechanics, focusing on the spirit of his transformative ideas and legacy. [ABSTRACT FROM AUTHOR]

The paper presents a methodology for the direct estimation of the spatiospectral distribution of an acoustic source from microphone measurements that comprise fixed and continuously scanning sensors. The nonstationarity introduced by the sensor motion is quantified by means of the Wigner-Ville spectrum. Its strongest effect is on the correlations of the sensor signals. Suppression of the nonstationarity in the signal processing involves division of the signals into blocks and application of a frequency-dependent window within each block. The direct estimation approach entails the inversion of an integral that connects the source distribution to the measured coherence of the acoustic field. A Bayesian estimation approach is developed that allows for efficient inversion of the integral and performs similarly to the much costlier conjugate-gradient method. The methodology is applied to acoustic fields emitted by impinging jets approximating a point source and an overexpanded supersonic jet. The measurement setup comprises one continuously scanning microphone and a number of fixed microphones, which are all arranged on a linear array. Comparisons are made between array configurations with fixed microphones only and with the scanning microphone: all having the same sensor count. The noise source maps with the scanning microphone have improved spatial fidelity and suppressed side lobes. The ability of the continuous-scan paradigm to provide high-definition noise source maps with a low sensor count is demonstrated. [ABSTRACT FROM AUTHOR]

This study is motivated by the need for physical models for the jet noise source to be used in practical noise prediction schemes for propulsion–airframe integration concepts. The basis for the source model is an amplitude-modulated traveling wave—the wavepacket. The source is parameterized and the parameters are determined by minimizing the difference between the modeled and experimental sound intensity distributions in the far field. Even though the pressure signal that reaches the far field is highly filtered, sufficient information is available to construct a wavepacket with reasonable physical characteristics. A simple stochastic extension of this concept shows a connection between the shape of the far-field sound pressure level spectrum and the emission polar angle. It suggests that the broadening of the spectrum with increasing polar angle from the downstream axis can be explained on the basis of a single noise source (the wavepacket), rather than the prevailing model of two distinct noise sources, one coherent and the other incoherent. [ABSTRACT FROM AUTHOR]

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 model 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 secondary and tertiary flows that reduce noise in specific azimuthal directions. The model reproduces well the experimental noise reduction trends. Principal mechanisms of noise reduction are elucidated. [ABSTRACT FROM AUTHOR]

The transmission of sound at normal incidence through perforated plates with bias flow is investigated experimentally and theoretically over a large parameter space. A specially designed experimental apparatus enabled the measurement of insertion loss with bias flow Mach number up to 0.25. A theoretical model for insertion loss was constructed based on inviscid, one-dimensional wave propagation with mean flow through a single contraction/expansion chamber. The mass end correction of the contraction is modified for hole interaction effects and mean flow. Hydrodynamic losses are modeled using a vena contracta coefficient dependent on both perforation geometry and Reynolds number. Losses in acoustic energy that occur in the mixing region downstream of the perforations are modeled as fluctuations in entropy. The proposed model was validated experimentally over a range of plate thickness, porosity, and hole size. The experimental results indicate an increase in insertion loss with increasing frequency, followed by saturation and decline as resonant conditions are established in the perforations. The insertion loss at low frequency increases with increasing Mach number through the perforation. The proposed model captures these trends and its predictions are shown to be more accurate than those of past models. [ABSTRACT FROM AUTHOR]

A study has been conducted on the transmission loss of perforated plates at normal incidence. The investigation includes a theoretical analysis of the problem with validation through experimentation. The experiments comprised microphone measurements of transmission loss for 11 perforated plates with variable thickness, hole size, and porosity. The theoretical model is based on planar wave propagation through a single contraction/expansion chamber with modifications to account for hole interaction effects. The resulting formula for transmission loss yields superior predictions over past theories for the range of properties investigated. Deviations between experimental measurements and theoretical predictions of transmission loss are less than about 1.5 dB for dimensionless hole diameter d/λ < 0.5. The accuracy of the model does not show a strong dependence on plate thickness-to-diameter ratio or porosity. [ABSTRACT FROM AUTHOR]

Flow exiting a convergent-divergent nozzle operated at severely overexpanded conditions, such that a shock wave is formed inside the nozzle, exhibits a strong instability that causes mixing enhancement in the flow itself and can destabilize an adjacent flow. The latter property enables mixing enhancement of an arbitrary jet via axial injection of a secondary gas flow. A systematic investigation of this phenomenon reveals that the instability is associated with shock-induced nozzle flow separation. In this paper we review key results that demonstrate the potential of this mixing technique and provide insight into the relevant physical processes. In addition, we establish criteria for the occurrence and location of the separation shock, and construct correlations between mixing enhancement and shock strength. [ABSTRACT FROM AUTHOR]

An experimental study of simultaneous multipoint measurements in the flowfield and acoustic field of a Mach 1.75 cold-air jet is presented. A series of four optical-deflectometer probes measured turbulent fluctuations in or near the jet flow, and eight microphones recorded the far-field pressure in the direction of peak emission. The correlation methodology involves the coherence between the delay-and-sum beamformer outputs of the optical-deflectometer probes and the microphones. This procedure yields results with greater fidelity and higher coherence levels than obtained with individual optical-deflectometer-to-microphone correlations. With the optical-deflectometer probes in the jet shear layer, there is a significant correlation, on the order of 0.1, between the turbulent fluctuations and farfield noise. As the optical-deflectometer probe moves transversely away from the jet, its correlations with the microphone beamformer first drops and then increases. This trend signifies the transition from hydrodynamic to acoustic pressure fields. In the vicinity of the nozzle exit, the peak coherence between the beamformed optical-deflectometer and microphone signals coincides with the physical location of the optical-deflectometer probe. However, as the shear layer thickens downstream, the peak coherence generally lags the probe location, which is a probable result of acoustic refraction by the mean flow. [ABSTRACT FROM AUTHOR]

We investigate experimentally the causes of jet plume instability and enhanced mixing observed in the exhaust of shock-containing convergent-divergent nozzles. Key features of the internal flow are the separation shock, separation shear layers, and pattern of alternating expansion and compression waves downstream of the shock. We focus on two possible reasons for this instability—the motion of the separation shock and the wave pattern downstream of the shock. The nozzle flow was generated in a planar facility with variable area ratio and pressure ratio, and the motion of the shock was tracked using time-resolved wall pressure measurements. The isolated effect of the wave pattern was investigated in a separate facility wherein a sonic shear layer, simulating the nozzle separation shear layer, was disturbed with compression and expansion waves emanating from a wavy wall. In both instances, the instability of the shear layer was characterized by time-resolved measurements of the total pressure. In the nozzle flow, the amplitude of shock motion increases with shock strength. Correlation of shock motion with shear layer total pressure is virtually absent for weak shocks but becomes significant for strong shocks. However, impingement of stationary waves on the shear layer had no impact on its growth rate. We conclude that the enhanced shear layer instability is strongly coupled to shock motion, and that the wave pattern by itself is not a cause of enhanced mixing. The occurrence of asymmetric separation at large shock strengths is a further contributor to the enhancement of instability. [ABSTRACT FROM AUTHOR]

Focuses on a study which examined the mean flow development in dual-stream compressible jets. Description of the planar shear layer model used; Potential core lengths and sonic lengths; Mass flow rates.

Presents a study which characterized the acoustics of axisymmetric high-speed jets at a variety of Mach numbers and velocities and at pressure-matched, overexpanded, and underexpanded conditions. Components of supersonic jet noise; Noise measurements; Correlations of jet noise with engine operating parameters.

Presents a study which demonstrated directional suppression of noise from a high-speed jet using an asymmetric parallel secondary stream. Methodology; Results and discussion; Conclusions.

Provides information on a study which proposed an empirical model for eddy convection in coaxial supersonic jets. Experimental methods used; Results and discussion; Conclusion.

Provides information on a study which measured the noise radiated from perfectly expanded supersonic coaxial jets in an anechoic chamber for operating conditions with the same total mass flow and thrust and with the same temperature ratio. Experimental method used; Numerical results and calculations; Conclusion.

Presents information on a study that investigated the noise measurements for perfectly expanded coaxial jets composed of a supersonic primary stream at conditions designed to prevent formation of Mach waves. Description of the flow apparatus; Global features of the jet; Noise characteristics.

The disturbance field induced by a small isolated vortex in a compressible shear layer is studied using direct simulation in a convected frame. The convective Mach number, Mc, is varied from 0.1 to 1.25. The vorticity perturbation is rapidly sheared by the mean velocity gradient. The resulting disturbance pressure field is observed to decrease both in magnitude and extent with increasing Mc, becoming a narrow transverse zone for Mc>=0.8. A similar trend is seen for the perturbation velocity magnitude and for the Reynolds shear stress. By varying the vortex size, it was verified that the decrease in perturbation levels is due to the mean-flow Mach number and not the Mach number across the vortex. At high Mc, the vortex still communicates with the edges of the shear layer, although communication in the mean-flow direction is strongly inhibited. The growth rate of perturbation kinetic energy declines with Mc primarily due to the reduction in shear stress. For Mc≥0.6, the pressure dilatation also contributes to the decrease of growth rates. Calculation of the perturbation field induced by a vortex doublet revealed the same trends as in the single-vortex case, illustrating the insensitivity of the Mach-number effect to the specific form of initial conditions. [ABSTRACT FROM AUTHOR]

Investigates the evolution of large turbulent eddies in seven supersonic shear layers with average convective Mach numbers and with large variation in density and velocity ratios. Use of a two-laser, single-detector planar laser-induced fluorescence technique for flow visualization and its evolution.

Presents experimental results on a method that eliminates Mach waves from the exhaust of supersonic jets. Removal of supersonic jet noise; Increase of takeoff thrust with minimal impact on overall fuel consumption; Definition of the principle of Mach wave elimination.

Presents experimental evidence of shocklets in a counterflow Mach 2 shear layer. Shock waves emanating from the turbulent structure; Analysis of the slope of the Mach waves suggesting that the turbulent structures are nonstationary, even though the shear layer is symmetric.

The characteristics and bicharacteristics of the linearized equations of motion for compressible fluid are identical to the wave fronts and rays, respectively, calculated by Fermat's least-time principle of geometric acoustics. The bicharacteristics emanating from the middle of a compressible shear layer curve so as to hinder communication in the axial direction. [ABSTRACT FROM AUTHOR]

Fan flow deflection for jet noise reduction was applied to subscale nozzles simulating the geometry and exhaust conditions of separate-flow turbofan engines. Two types of deflectors were tested, one comprising two pairs of vanes internal to the fan duct and the other consisting of a wedge positioned outside the fan duct. The noise reduction achieved by the vanes was strong in the downward direction and moderate in the sideline direction. The wedge generated significant attenuation in both directions. The acoustic results are consistent with the measured distortion of the mean velocity field. An approach for the treatment of nonaxisymmetric jets vis-à-vis perceived noise prediction is introduced. [ABSTRACT FROM AUTHOR]

Imaging of aeroacoustic noise sources is routinely accomplished with geometrically fixed phased arrays of microphones. Several decades of research have gone into improvement and optimization of sensor layouts, selection of basis models, and deconvolution algorithms to produce sharper and more localized images of sound-producing regions in space. This paper explores an extension to conventional phased array measurements that uses slowly, continuously moving microphone arrays with and without coupling to rigid fixed arrays to improve image quality and better describe noise mechanisms on turbofan engine sources such as jet exhausts and turbomachinery components. Three approaches are compared in the paper: fixed receiver beamforming (FRBF), continuous-scan beamforming (CSBF), and multireference CSBF (MRCSBF). The third takes advantage of transfer function matrices formed between fixed and moving sensors to achieve effective virtual arrays with spatial density one to two orders of magnitude higher, with practical sensor budgets and scan speeds. The MRCSBF technique produces array sidelobe rejection that approaches the theoretical array pattern of a continuous two-dimensional (2D) aperture. The implications of this finding are that better source localization may be achieved with conventional delay and sum (DAS) beamforming (BF) with practical sensor budgets, and that an improved starting image of the sound source can be provided to deconvolution algorithms. These findings are demonstrated on analytical and experimental examples from a low-cost rotating phased array using point sound sources, as well as linear scanning array experiments of an impinging jets point source and a near-sonic jet nozzle exhaust. [ABSTRACT FROM AUTHOR]

This is a joint experimental and computational research effort on the aerodynamics of perforated surfaces inclined to a freestream. The goal is to characterize the key parameters affecting the fiowfield in order to construct a macroscopic computational model that obviates resolution of the detailed perforations. The model simulates the effects of the perforated surfaces by locally applying a body force term in the momentum equation. The study considers wedge-shaped deflectors used for jet noise reduction in turbofan engine nozzles in the form of flaps with varying angles and perforation shapes. The experiments measured the mean velocity field inside and in the wake of the flaps while the numerical analysis comprised direct computations of simplified perforated flaps and computations using the body force model. It is found that the effective porosity of the surface is affected by flow separation within the perforations. The flow blockage of the separation regions is primarily a function of the angle to the freestream and the ratio of the thickness to hole diameter. Accordingly, a formulation for the body force model is proposed that incorporates this finding. The resulting velocity field is in good agreement with the experiments and with the direct computations. [ABSTRACT FROM AUTHOR]

We present a computational study, validated by mean-flow experiments, of a dual-stream nozzle simulating the exit conditions of a supersonic turbofan engine with noise-suppressing fan flow deflectors. The study is conducted for eight nozzle configurations and two operating conditions: a cold condition at which mean velocity surveys were conducted and against which the computational code was validated and a hot condition that corresponds to the takeoff engine cycle and at which acoustic data were collected. The code predictions successfully replicate the mean velocity fields and the inflectional layers of the experimental flows. The code is then extended to the conditions of the actual engine cycle. The computations reveal a similar velocity profile for the hot and cold conditions when the axial distance is normalized by the potential core length. For both conditions, the vane deflectors reduce the turbulent kinetic energy k on the underside of the jet. An overall noise source strength is modeled as the axial integral of k7/2. A significant correlation is found between the reduction in the noise source strength and the reduction in the peak level of the overall sound pressure level. [ABSTRACT FROM AUTHOR]