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A generalized acoustic analogy model is implemented for the hot and cold static high-speed jet cases corresponding to conditions of the Strategic Investment in Low-carbon Engine Technology (SILOET)...

We conduct large-eddy simulations to investigate the role of thermal nonuniformity on the development of instability modes in the shear layer of supersonic M=1.5 jets. We develop an arbitrary high-...

Holographic reconstructions of the sound field in the vicinity of a tied-down F-35 aircraft were achieved by applying multisource statistically optimized near-field acoustical holography to measure...

Crackle is a perceptual feature of supersonic jet noise that is related to the presence of acoustic shocks. This study investigates the apparent source locations of the steepest shocklike events in...

The directivity of noise from three large-eddy simulations of turbulent jets at Mach 0.7, 0.9, and 1.5 is investigated using spectral proper orthogonal decomposition (SPOD). The most energetic patt...

Flow and noise solutions of the two Large-Eddy Simulation (LES) approaches are evaluated for the jet flow conditions corresponding to a benchmark coaxial jet case from the European Union Computatio...

Measurements of full-scale high-performance military aircraft reveal phenomena that are not widely seen at laboratory scales. However, recent modifications to large eddy-simulation (LES) methods al...

The current state of the use of large-eddy simulations (LESs) in jet aeroacoustics is reviewed. Various computational techniques are discussed, as well as methods to extend the LES results to the a...

In many areas of data science, deep neural networks (DNNs) have shown a remarkable ability to learn complex, nonlinear relationships between sets of variables. In this paper, this network architect...

The fan broadband noise of a rotor–stator system representing a turbofan aeroengine is investigated using the improved delayed detached-eddy-simulation approach. The baseline configuration of the N...

The relation between different time scales of the finite-time Lyapunov exponent (FTLE) and the acoustic wave is studied. Gonzalez et al.’s proposition (“Finite-Time Lyapunov Exponent-Based Analysis...

The paper presents a lower-cost higher-accuracy algorithm for numerical simulation of jet aerodynamics and acoustics on unstructured meshes. To provide a higher-accuracy prediction of unsteady jet ...

It is shown experimentally that thermal nonuniformity can reduce peak supersonic jet noise while keeping static thrust levels equivalent. A thermal nonuniformity is generated by introducing an unhe...

The dominant acoustic radiation from turbulent jets has been associated with coherent wave-packet structures in the plume. Jet noise models are therefore often designed using the statistics of deco...

An experimental study of the azimuthal modes of the sound field generated by a single-stream round jet installed near a flat plate is presented. The investigation is conducted at subsonic Mach numb...

The acoustic analogy provides a general framework for predicting broadband jet noise. The accuracy of the noise predictions is strongly dependent on the second- and fourth-order integral time and l...

A numerical approach to predict broadband trailing-edge noise for low Mach number flows is presented. It is based on the combination of a Helmholtz solver to propagate sound waves with a sound source term computed stochastically. The sound propagation is performed in the frequency domain using a high-order finite element solver. The stochastic approach is based on a random particle-mesh method. The performance of this numerical approach is first examined for a Gaussian source. The numerical approach is then applied to a NACA0012 airfoil for flow Mach numbers of 0.11 and 0.16 and to a controlled-diffusion airfoil for a Mach number of 0.047. The predicted sound levels are compared with experimental data and acoustic results obtained from the acoustic perturbations equations. The mean flow does not significantly modify the acoustic propagation. Using a no-flow propagation model like the Helmholtz equation is therefore a valid approach for low Mach numbers. The reduced computational cost of a Helmholtz calculation, together with the speed of the random-particle mesh turbulence synthesis, allows for fast predictions. While this approach provides relative assessments between different configurations (in particular the spectrum shape), it often requires the inclusion of calibration factors determined from reference measurement or numerical data.

The Paper deals with the investigation of the effect of multiple facets of pulse detonation combustors on its acoustic signature. First, the acoustic signature is characterized in both the near- an...

A rectangular single-expansion ramp nozzle (SERN) with an aft deck is used to model the exhaust from a three-stream engine. At the nozzle exit two unmixed streams exist. The first is assumed to be ...

Recently, Horne et al. (“Measurements of Unsteady Pressure Fluctuations in the Near-Field of a Solid Rocket Motor Plume,” International Journal of Aeroacoustics,Vol. 15, Nos. 4–5, July 2016, pp. 55...

The dynamics of a supersonic jet emanating from a high-aspect-ratio rectangular nozzle is investigated using dynamic mode decomposition. In the analysis, dynamic mode decomposition is applied to flowfield data obtained from a large-eddy simulation and the significance of each extracted mode to the snapshot data series is determined using a sparsity-promoting method. Furthermore, in order to assess the validity of the resulting modes, they are compared with the unprocessed large-eddy simulation data in terms of frequency content, sound pressure levels, and space–time correlations. It is found that the accuracy of the modes predicted by the dynamic mode decomposition is sufficient and that the modes deemed as most important give a good representation of the jet flow dynamics. Moreover, the jet flow, at the simulated conditions, is known (from experiments) to be dominated by a screech mechanism. The dynamic mode decomposition methodology, as set up in the present study, is able to isolate the jet screech mode, and the frequency of the extracted mode is in excellent agreement with previous observations.

形態: カラー図版あり, Note: Presented as Paper 2012-2069 at the 18th AIAA/CEAS Aeroacoustics Conference (33rd AIAA Aeroacoustics Conference) (June 4-6, 2012. Antlers Hilton), Colorado Springs, Colorado, USA, Physical characteristics: Original contains color illustrations, Accepted: 2017-01-04, 資料番号: PA1720002000

Experience gained from previous jet noise studies with the unstructured large-eddy simulation flow solver “Charles” is summarized and put to practice for the predictions of supersonic jets issued f...

To develop jet noise-reduction concepts, it is necessary to understand the physics underlying the noise-generation process. Decomposition of the sound field into azimuthal modes is one of the effective experimental methods that allows extracting subtle features of different types of noise sources. For turbulent flows, the interpretation of the noise analysis results using the azimuthal decomposition technique was developed by the authors in previous papers. In the present paper, a generalization of this method allowing the decomposition of the jet acoustic field into azimuthal modes by means of reduced number of microphones is developed. It is shown that jet noise measurements by only three microphones in each cross section allow reconstruction of three azimuthal mode directivities (axisymmetric, first, and second) for both low and moderate frequency bands. Furthermore, two-microphone measurements, provided the microphones are properly located, make it possible to reconstruct directivity of the axisymmetr...

High-pressure jet injection into quiescent air is a challenging fluid dynamics problem in the field of aerospace engineering. Although plenty of experimental, theoretical, and numerical studies have been conducted to explore this flow, there is a dearth of literature detailing the flow evolution and instability characteristics, which is vital to the mixing enhancement design and jet noise reduction. In this paper, a density-based solver for compressible supersonic flow, astroFoam, is developed based on the OpenFOAM library. Large-eddy simulations of highly underexpanded jets with nozzle pressure ratios from 5.60 to 11.21 at a Reynolds number around 105 are carried out with a high-resolution grid. A grid-convergence study has been conducted to confirm the fidelity of the large-eddy simulation results. The large-eddy simulation results have also been validated against available literature data in terms of the time-averaged near-field properties of underexpanded jets. The turbulent transition processes are r...

A method for calculating the effective Gol’dberg number for diverging waveforms is presented, which leverages known features of a high-speed jet and its associated sound field. The approach employs a ray tube situated along the Mach wave angle where the sound field is not only most intense, but advances from undergoing cylindrical decay to spherical decay. Unlike other efforts, a “piecewise-spreading regime” model is employed, which yields, separately, effective Gol’dberg numbers for the cylindrically and spherically spreading regions in the far field. The new approach is applied to a plethora of experimental databases, encompassing both laboratory- and full-scale jet noise studies. The findings demonstrate how cumulative nonlinear distortion is expected to form in the acoustic near field of laboratory-scale round jets where pressure amplitudes decay cylindrically; waveform distortion is not expected in the acoustic far field where waveform amplitudes diverge spherically. On the other hand, where full-sca...

The spatial evolution of acoustic waveforms produced by a laboratory-scale Mach 3 jet are investigated using both 1/4 and 1/8 in. pressure field microphones located along rays emanating from the postpotential core where the peak sound emission is found to occur. The measurements are acquired in a fully anechoic chamber, where ground or other large surface reflections are minimal. Various statistical metrics are examined along the peak emission path, where they are shown to undergo rapid changes within 2 m from the source region. An experimentally validated wave-packet model is then used to confirm the location where the pressure amplitude along the peak emission path transitions from cylindrical to spherical decay. Various source amplitudes, provided by the wave-packet model, are then used to estimate shock formation distance and Gol’dberg numbers for diverging waves. The findings suggest that cumulative nonlinear distortion is likely to occur at laboratory scale near the jet flow, where the waveform amp...

The properties of the flow near the plate and in the wall jets have been investigated from large-eddy simulation data of round impinging jets. Four jets are underexpanded and four jets are ideally expanded, which allowed examination of the influence of the presence of shock-cell structures. The underexpanded jets are characterized by a fully expanded Mach number of 1.56 and an exit Mach number of 1. The ideally expanded jets have a Mach number of 1.5. The Reynolds number of the eight jets is equal to 6 × 104. The jets impinge normally on a flat plate located from 4.16r0 to 12r0 downstream of the nozzle and generate acoustic tones due to an aeroacoustic feedback mechanism. In this paper, the near pressure and density fields of the jets are characterized using Fourier transform on the nozzle exit plane, the plate, and an azimuthal plane. First, mean and rms radial velocities of the wall jets are examined. The impact of the shock-cell structure on the wall jet is discussed. The pressure spectra on the plate are then shown as a function of the radial coordinate. The tone frequencies are all visible where the jet shear layers impinge the plate, but only some of them emerge in the wall jet created after the impact. For the ideally expanded jets, the temporal organization of the wall jet along the frequencies of the feedback mechanism decreases with the nozzle-to-plate distance, but for the non-ideally expanded jets, this organization is linked to the oscillation of the Mach disk located just upstream of the plate. Consecutively, the amplitude and the phase fields at the tone frequencies are represented on the three planes mentioned earlier. Similar spatial organizations of the turbulent structures are found in the jet shear layers and in the wall jets. Thus, axisymmetric and helical arrangements of the structures in the jet shear layers lead to concentric and spiral distributions of the structures on the plate, respectively. In particular, for one of the underexpanded jets, a spiral shape and concentric rings, associated with two tone frequencies generated simultaneously, are observed on the flat plate in the pressure and density phase fields. Finally, the convection velocity of the turbulent structures at the tone frequencies in the wall jets are evaluated based on phase fields, and the mean convection velocity is computed using cross correlations of radial velocity. The results are in good agreement with those from a recent experimental study of ideally expanded impinging jets.

Fan-wake/outlet-guide-vane interaction broadband noise in turbofan jet engines is studied. The mechanism and some issues are first discussed using a two-dimensional gust-prediction model. An oblique gust-prediction model is then developed. Quasi-three-dimensional unsteady lift is calculated using a two-dimensional equivalence method. It is coupled with annular duct modes to obtain the sound power spectrum density. Spanwise turbulence integral length scales and their impact on power spectrum density predictions are investigated. A spanwise integration limit suitable for the complete frequency range is proposed. The model is validated using the NASA Source Diagnostic Test data. Sound power scaling with vane count B is examined. If solidity is maintained, the cascade response does not converge on the single-airfoil response, even for low vane counts. The sound power varies inversely with B at low frequency; it scales with B at very high frequency. The power spectrum density trend with the fan tip Mach number...

An experimental study of the spatial distribution of integral eddy speeds in heated, supersonic free jets is presented. The physics of such jets are of immediate interest to noise-reduction approaches for tactical aircraft jet noise. In these flows, the noise produced has a strong influence from eddy speeds, particularly for radiation in peak noise directions, due to Mach wave contributions. The measurements obtained using a time-resolved Doppler global velocimetry instrument experimentally indicate a key mechanism of the role of heating in jet noise reduction relative to the classical Lighthill U8 scaling. Measurements are presented in jets issuing from an axisymmetric biconic nozzle at two conditions: total temperatures of 1.6 and 2.0 times that of the ambient and both at approximately 106 diameter Reynolds number and pressure-matched exit conditions. For the higher total temperature case, the scaled streamwise eddy convective velocities within the shear layer were measurably reduced compared to the low...

A prediction method is developed based on the acoustic analogy for the cross-power spectral density in the convecting near field of compressible fluid turbulence. Equivalent source near-field, midfield, and far-field terms within the model integrand create corresponding near-field, midfield, and far-field radiating waves. These equivalent sources are modeled with a single equation for the two-point cross correlation of the Lighthill stress tensor that is dependent on the jet operating conditions. An alternative equivalent source model based on steady Reynolds-averaged Navier–Stokes solutions is proposed. The cross-power spectral density model automatically reduces to a traditional autopower spectral density model when observers are at the same location. Predictions of radiation intensity and coherence compare favorably with measurements in the near field, midfield, and far field for a wide range of jet Mach numbers and temperature ratios.

形態: 図版あり, Physical characteristics: Original contains illustrations, 資料番号: PA1520029000

For the last 10 years, large-eddy simulations have become a major tool for investigating jet noise sources because of their intrinsic ability to capture broadband turbulent features. However, many challenges still arise when dealing with complex geometries in terms of method accuracy and computational costs. Two different approaches to compute jet noise in an industrial context are here validated and compared. Both approaches are based on a hybrid methodology combining large-eddy simulation of jet flows for sources computations and Ffowcs Williams and Hawkings’s analogy for far-field noise prediction, but they differ on their grid topologies. The first approach uses classical block structured grids. The numerical scheme is a low-dispersive, low-dissipative finite-volume compact scheme. The second approach uses fully unstructured tetrahedral grids with a low-dispersive, low-dissipative Taylor–Galerkin finite-element scheme. Both approaches are used to compute a Mach 0.9 cold jet at the moderate Reynolds nu...

In this study, the noise generated from a cold supersonic circular twin jet is simulated with a high-order finite difference solver. The three-dimensional compressible Favre-filtered Navier–Stokes ...

Flow structure oscillations and tone generation mechanisms in an underexpanded round jet impinging on a flat plate normally have been investigated using compressible large-eddy simulations. At the exit of a pipe nozzle of diameter D, the jet is characterized by a nozzle pressure ratio of 4.03, an exit Mach number of 1, a fully expanded Mach number of 1.56, and a Reynolds number of 60000. Four distances between the nozzle and the plate are considered. Snapshots of vorticity, density, pressure, and mean velocity flowfields are first presented. The latter results compare well with data of the literature. In three cases, in particular, a Mach disk appears to form just upstream from the plate. The convection velocity of flow structures between the nozzle and the plate, and its dependence on the nozzle-to-plate distance, are then examined. The properties of the jet near pressure fields are subsequently described using Fourier analysis. Tones emerge in the spectra at frequencies consistent with those expected for an aeroacoustic feedback loop between the nozzle and the plate as well as with measurements. Their amplitudes are particularly high in the presence of a near-wall Mach disk. The axisymmetric or helical natures of the jet oscillations at the tone frequencies are determined. The motions of the Mach disk found just upstream from the plate for certain nozzle-to-plate distances are then explored. As noted for the jet oscillations, axially pulsing and helical motions are observed, in agreement with experiments. Finally, the intermittency of the tone intensities is studied. They significantly vary in time, except for the two cases where the near-wall Mach disk has a nearly periodic motion at the dominant tone frequency.

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. ...

An experimental and computational investigation of a combination of fluidic injection and chevrons (fluidically enhanced chevrons) for supersonic jet noise reduction was performed. Previous studies have shown that chevrons underperform in overexpanded flow due to reductions in effective penetration. Consequently, this study focuses on improvements gained by fluidic enhancement in the overexpanded regime. Acoustic results indicated the fluidically enhanced chevrons outperform the chevrons, with additional overall sound pressure level reductions of 2 dB in the upstream direction and nearly 1.5 dB in the downstream direction. Spectral results indicated that primary benefits were in shock and low-frequency noise. A small high-frequency penalty was the only observed detriment. Time-averaged velocity and turbulence quantities as well as shock cell spacing, were computed by large-eddy simulation and then compared to particle imaging velocimetry and shadowgraph results. These flowfield measurements indicated that...

In this study, a method for reducing the jet noise of a high-speed jet is presented. An array of microjets was immersed within the centerline of an underexpanded Mach 1.6 jet in an effort to reduce the overall sound pressure level and alter its spectral composition. Based on the mass flux through the microjets and the location of the microjets along the centerline of the jet, reductions of up to 5.81 dB with respect to the downstream direction and 7.99 dB with respect to the perpendicular direction can be achieved. An interesting phenomenon is discovered whereby the maximum noise reduction in the downstream direction is found with injection activated, whereas the maximum noise reduction in the perpendicular direction is found with injection deactivated. Placement of the injection tube within the flow is shown to eradicate distinct tones while flow visualizations depicted a strong augmentation in the jet plume via a modification of the shear layer and the internal shock cell structure.

Following on previous works showing that far-field jet noise has significant intermittent aspects, the present work assumes that these intermittent events are the dominant feature of jet noise. A definition and method of detection for intermittent noise events is devised and implemented. Using a large experimental database of acoustically subsonic jets with different jet acoustic Mach numbers (Ma=0.5–0.9), nozzle diameters (D=2.54, 5.08, and 7.62 cm), and jet exit temperature ratios (0.84–2.70), these events are extracted from the far-field noise signals measured in the anechoic chamber of the NASA Glenn Aero-Acoustic Propulsion Laboratory. It is shown that a signal containing only these intermittent events retains all of the important aspects of the acoustic spectrum for jet noise radiating to aft angles, validating the assumption that intermittent events are the dominant feature of aft angle jet noise. Statistical analysis of the characteristics of these noise events reveals that these events can be des...

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 flowfield 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 theflapswhile the numerical analysis comprised direct computations of simplifiedperforatedflaps 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. Theflowblockage of the separation regions is primarily a function of the angle to the freestreamand 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.

Significant differences in subsonic jet noise databases have been reported in recent review papers. Specifically, university-type facilities involving higher contraction ratios andpossibly cleanerflows are noted to yield higher levels of noise relative to data from industrial-type facilities. An experimental investigation is carried out in an attempt to understand the sources of the anomaly. It is inferred that differences in jet core turbulencemay not be the source. An observation in a previous study is confirmed showing that two nozzles of the same diameter but different internal geometry can produce a difference in subsonic jet noise. The present measurements demonstrate that the noisier nozzle involves a highly disturbed laminar, or nominally laminar, boundary-layer state as opposed to a turbulent state with the other. The former boundary-layer state with the noisier nozzle is actuallymarked by larger turbulence intensities, consistent with the higher radiated noise. Although the boundary-layer characteristics were not reported with the earlier databases, the present results suggest that differences therein might be a source of the anomaly.

Large-eddy simulations of supersonic jets are performed to validate the development of a second-order finite volume unstructured solver for aeroacoustic applications. Two supersonic jets issuing from an axisymmetric nozzle at Mach number 1.4 are computed: one unheated jet with a Reynolds number of 150,000 and one heated jet with a Reynolds number of 76,000 and a temperature ratio of 1.75. Flow and noise results are compared with the experimental database fromNASAGlennResearchCenter. The nozzle is included in the computational domain.The present study shows that the results from the unstructured solver are in good agreement with the experimental data for time-averaged and fluctuating quantities, velocity spectra in the jet, and the sound obtained in the near field and the far field using the integration of the Ffowcs–Williams and Hawkings equation.

I N THE Japan Aerospace Exploration Agency (JAXA), a precooled turbojet engine specified for hypersonic propulsion is under development [1,2]. This engine uses a precooling cycle using liquid hydrogen fuel, and it is capable of accelerating a vehicle from takeoff toMach 5. It is planned that the vehicle takes off horizontally using existing runways and that the exhaust velocity is supersonic throughout the operation. Thus, jet noise suppression at takeoff is one of the most important issues in the development. At takeoff, the nozzle pressure ratio (NPR) is limited to 2.7 since no ram pressure can be expected in such an airbreather, while at cruise, NPR reaches 200 due to the increase in flight Mach number and altitude [2]. To compensate such drastic change in NPR, a rectangular hypersonic nozzlewith a variable throat realizing the change in throat area by 2.5 times will be employed [2]. However, it is still insufficient to satisfy all operation conditions of the engine, and the nozzle will inevitably be operating under offdesign conditions (overexpansion mode) at takeoff. Because of the complicated geometry and the offdesign operation, it seems to be challenging to apply jet noise suppressors to such an actual hypersonic nozzle. Tabs attached at the nozzle exit are expected to be a promising device for screech and broadband noise reduction in a supersonic jet, and their noise reduction performance and mechanism have been investigated in detail [3–5]. Recently, a new concept of jet noise suppressors using auxiliary gas injection has been proposed [6–8], which achieves similar effect to tabs aerodynamically. Both in mechanical-tab and aerodynamic-tab (AT) concepts, much research is carried out using round/rectangular nozzles that are farther idealized when compared with actual hypersonic nozzles. Furthermore, in theAT concept, bleed air will be used as its working gas, and the total pressure and temperature available in real engines should be taken into account. In the present study, noise reduction performance and feasibility of AT jet noise suppressors in an actual hypersonic nozzle operating under takeoff conditions are investigated experimentally.