Far-field Jet Noise Research Articles

Analysis of aeroacoustic characteristics of a supersonic jet at designed conditions based on numerical simulation

The work is devoted to the numerical simulation of aeroacoustic characteristics of a supersonic jet issuing from a Laval nozzle at the design Mach number M=2. The results of the large-eddy simulations (LES) are presented. Characteristics of mean jet flow and its fluctuations, as well as the characteristics of the far-field jet noise, including its azimuthal content, are obtained. The results of the simulation are compared with experimental data and their acceptable agreement is shown. It is concluded that there are various noise generation mechanisms in the considered jet.

A GPU-Based Lattice Boltzmann Method for Predicting Near- and Far-Field Jet Noise

Jet flow and noise are simulated using the lattice Boltzmann method (LBM). The D3Q19 lattice model calculates the flow field with a high-order distribution function to overcome LBM limitations in solving high Mach number flows. Multiple relaxation times (MRT) and large eddy simulation (LES) are used in this study. Compressible LBM was applied to the simulation jet flow to demonstrate the ability of the fifth-order equilibrium distribution function to simulate compressible flows efficiently. The results from the LBM simulation were then employed in Kirchhoff's surface integral approach to predict far-field jet noise. In this research, we used the compute unified device architecture (CUDA) to implement the LBM in a graphics processing unit (GPU), thus creating a hybrid code, LBM-MRT-LES. The obtained results agreed with the experimental and numerical results in the literature.

Development of a far-field prediction model for tactical jet noise spectra

Semi-empirical models for predicting far-field jet noise radiation are computationally efficient. One historical example is NASA SP-8072 (1971), which couples source sound power spectra with frequency-dependent directivity indices to obtain far-field radiated spectra for rockets. This paper applies the SP-8072 framework to develop spectra for the T-7A trainer aircraft. Data at 38 and 76 m arcs are used to obtain sound power spectra and frequency-dependent directivity indices for four different engine conditions ranging from 36% thrust through maximum afterburner. Application of a ground reflection correction model is discussed. The model is applied to obtain results at distances between 19 and 229 m and the performance of the model is evaluated using measured data. [Work supported by ONR Grant No. N00014-21-1-2069.]

Jet noise predictions by time marching of single-snapshot tomographic PIV fields

This work combines the latest advancements in time marching of 3D vector fields from tomographic particle image velocimetry, with an adapted version of Lighthill’s formulation, for the prediction of far-field jet noise. Three-dimensional velocity vector fields of the jet flow are first reconstructed from a tomographic volume of 4times3times9.5 D_{j}^3, with D_{j} = 5 cm being the jet-exit diameter. (The jet-exit Mach number M_{j} ranges from 0.10 to 0.20.) The obtained vector fields are then used as input to a recently developed procedure for the time marching of the vorticity field, which relies upon the vortex-in-cell methodology. This yields time series of each three-dimensional velocity field, from which the far-field pressure is computed via Lilley’s acoustic analogy (through evaluation of the Lighthill’s stress tensor). It is shown that the estimate of the far-field noise spectrum compares well with the spectrum measured directly from a far-field microphone in the anechoic A-tunnel facility of TU Delft, in the Strouhal number range from approximately 1 to 12.Graphical

Supersonic jet noise source distributions.

The purpose of this work is to examine the noise source distributions of shock-containing supersonic jets at various pressure ratios corresponding to fully expanded Mach numbers ranging from 1.0 to 1.4 in intervals of 0.2 for various nozzle exit diameters. Source location measurements using a phased array (beamforming), farfield jet noise measurements, and schlieren flow visualization are presented. It is found that supersonic noise source distributions are more complex than those of subsonic jets. The source distributions for supersonic jets can be divided into three different Strouhal regions. At low Strouhal numbers ( fD/U≤0.3), the noise source distributions appear very similar to those of a subsonic jet, as reported in open literature. This Strouhal region is dominated by jet-mixing noise associated with small-scale turbulence mixing. At high Strouhal numbers ( fD/U≥1.0), the noise source distributions are comprised of several repetitive sources at various discrete downstream jet locations that produce noise at all frequencies. The locations of these sources roughly correspond to the shock cells in the jet, and thus, vary with jet Mach number. Another region exists at Strouhal numbers between these two regions ( 0.3<fD/U<1.0) for which the precise location of the sources as a function of Strouhal number was determined to be ambiguous due to a limitation of the phased array used. This region roughly corresponds to the frequencies of noise where jet-mixing noise and shock noise are of similar levels. The spacing of the shock sources in this region are smaller than the beam width of the array measuring them. Their locations can no longer can be separately recorded; and instead, they are averaged together and their centroid location is plotted.

Operational Control of Mixing Enhancement and Far-Field Jet Noise Suppression Using Downstream Fluidic Injection

Abstract A novel fluid injection scheme is analyzed for its ability to suppress the far-field jet noise using multiple radial microjets placed downstream from the nozzle exhaust. Contrary to other previous research studies, which injected fluid either inside the nozzle or just at the nozzle exhaust, this injection scheme uses a coaxial injector tube to inject multiple equally spaced microjets perpendicular to the jet axis at an axial location downstream from the nozzle exhaust. Microjet injection closer to the jet axis leads to the formation of a counter rotating vortex pair (CVP) close to the injection location which subsequently breaks down into streamwise vortices as the microjet bends and follows the flow direction. Microjet injection pressure and angle of injection play a crucial role in determining the jet trajectory, penetration, and the strength of the CVP formed. The effect of these parameters on the far-field acoustic radiation is evaluated in this study. Detailed large eddy simulations are performed for a nozzle–injector setup operating at Mach 0.9 jet with Reynolds number ≈106 to help understand the aerodynamic and acoustic features of the interaction of this fluid injection scheme with the main jet.

Influence of Turbulence Statistics on Stochastic Jet-Noise Prediction with Synthetic Eddy Method

The separation algorithms that solve a flowfield and a sound field separately are used to estimate the jet noise accurately while suppressing the increase of computational cost. In this study, the synthetic eddy method (SEM) was applied to a strong shear flow to simulate a major sound source of jet noise, and the influence of turbulence statistics used in the SEM on the predicted far-field jet noise was investigated. The divergence-free SEM was employed as a reconstruction method of the turbulent field, and the shear effect due to the background flow was provided to the generated velocity field. In addition, the dissipation effect of the turbulence in the shear layer was modeled by decorrelating the generated velocity fluctuations. The estimated noise using the linearized Euler simulations appeared larger than the experimental value when parameters of the SEM were set based on actual flow conditions. In contrast, the sound pressure spectra agreed well with the experiment by increasing the time scale of the noise source. With the present stochastic noise prediction framework based on the SEM, turbulence statistics and far-field sound pressure spectra were correlated, which would help to modify parameters in stochastic noise generation methods to achieve desired far-field sound pressure spectra.

Experimental investigation on noise radiation characteristics of pulse detonation turbine engine

The noise radiation characteristics of two-phase pulse detonation combustor and pulse detonation turbine engine were investigated under different operating frequencies utilizing gasoline as fuel and air as oxidizer. The sound pressure data of noise radiation were presented for both single-pulse detonation combustor tube and pulse detonation turbine engine. The experimental results implied that the peak sound pressure level of PDTE exit with inner diameter being 60 mm was about 157 dB under the operating frequencies which varied from 5 Hz to 25 Hz, while the peak sound pressure level of single-pulse detonation combustor tube exit was about 170 dB under the same condition. The far-field jet-noise measurements of the pulse detonation turbine engine showed that radial turbine interacting with the pulse detonation combustor could decrease the peak sound pressure level of pulse detonation combustor with the maximum acoustic attenuation being approximately 14.2 dB for the current test conditions, which could be contributed to the energy extraction by the radial turbine from the pulse detonation combustor exhaust flow. The sound pressure level of both pulse detonation combustor and pulse detonation turbine engine exit was function of directivity angle from the exhaust centerline. In all the experiments, the equivalence ratio of gasoline/air mixture and the fill fraction were 1.2 and 1.0, respectively.

Experimental investigation on noise radiation characteristics of pulse detonation engine–driven ejector

The noise radiation characteristics of multi-cycle pulse detonation engine with and without ejector were investigated under different operating frequencies utilizing gasoline as fuel and air as oxidizer. The straight cylindrical ejector with convergent inlet geometry was coaxially installed at different axial locations relative to the exit of the detonation tube. In all the experiments, the equivalence ratios of gasoline–air mixture and the fill fraction were 1.2 and 1.0, respectively. The experimental results implied that the addition of ejector could drastically change the far-field acoustic performance of pulse detonation engine exit and the peak sound pressure level of noise radiation was a strong function of the ejector axial position. But the peak sound pressure level was not sensitive to the operating frequencies which varied from 10 to 25 Hz. The pulse sound pressure level, however, increased with the increase in operating frequencies. The far-field jet-noise measurements of the pulse detonation engine-ejector system also showed that ejector could decrease the peak sound pressure level of pulse detonation engine. The maximum reduction was approximately 8.5 dB. For the current pulse detonation engine test conditions, an optimum ejector position was found to be a downstream axial placement of x/ DPDE = 0.5.

Time-Domain Analysis of Excited Subsonic Jet Noise

Building on previous work showing that far-field jet noise has significant intermittent aspects and the statistical analysis of those intermittent aspects, the present work applies the same analysis techniques to an excited jet. Using an experimental database covering several operating conditions [jet Mach number ( Mj = 0.9), nozzle diameter ( D = 2.54 cm), and jet stagnation temperature ratios (TTR = 1.0 – 2.5)] and a wide array of excitation parameters (azimuthal mode mF = 0, 1, &amp; 3 and Strouhal number StDF = 0.09 – 3.0), these events are extracted from the far-field noise signals measured in an anechoic chamber. This database is analyzed to determine how the noise event characteristics are altered by excitation. The relationship between the noise events and the flow-field dynamics of the excited jet are discussed. Analysis of the excited jet reveals the existence of a resonance condition. When excited at the resonance condition, large noise amplification occurs – this is associated with nearly every large-scale structure producing a noise event. Conversely, noise reduction occurs when only one noise event occurs per several large-scale structures. The impact of the azimuthal extent of large-scale structures is explored using a wave-packet model to provide support to the discussion of azimuthal mode radiation efficiency. The results indicate that there is a competition for flow energy among neighboring structures that dictates if and how their dynamics will produce noise that radiates to the far-field. Furthermore, the azimuthal model supports the conclusion that higher modes are less efficient radiators. These various factors provide support for the conclusion that the mechanism of noise reduction involves inducing the jet into a condition where the naturally occurring structures are suppressed by excited structures which are less efficient radiators (i.e. higher azimuthal modes at frequencies where neighboring structures destructively compete for flow energy).

Intermittent Nature of Subsonic Jet Noise

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 (), nozzle diameters (, 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 described in terms of three parameters (the variance of the original signal, the mean width of the events, and the mean time between events) and two universal statistical-distribution curves. The variation of these parameters with noise-radiation direction, nozzle diameter, and jet velocity and temperature are discussed. There is a strong correlation between the mean width of the intermittent events and the mean time between the events () in all the cases investigated in this work, implying a strong link between dynamics governing the two quantities.

Effect of non-parallel mean flow on the Green’s function for predicting the low-frequency sound from turbulent air jets

AbstractIt is now well-known that there is an exact formula relating the far-field jet noise spectrum to the convolution product of a propagator (that accounts for the mean flow interactions) and a generalized Reynolds stress autocovariance tensor (that accounts for the turbulence fluctuations). The propagator depends only on the mean flow and an adjoint vector Green’s function for a particular form of the linearized Euler equations. Recent numerical calculations of Karabasov, Bogey &amp; Hynes (AIAA Paper 2011-2929) for a Mach 0.9 jet show use of the true non-parallel flow Green’s function rather than the more conventional locally parallel flow result leads to a significant increase in the predicted low-frequency sound radiation at observation angles close to the downstream jet axis. But the non-parallel flow appears to have little effect on the sound radiated at $9{0}^{\ensuremath{\circ} } $ to the downstream axis. The present paper is concerned with the effects of non-parallel mean flows on the adjoint vector Green’s function. We obtain a low-frequency asymptotic solution for that function by solving a very simple second-order hyperbolic equation for a composite dependent variable (which is directly proportional to a pressure-like component of this Green’s function and roughly corresponds to the strength of a monopole source within the jet). Our numerical calculations show that this quantity remains fairly close to the corresponding parallel flow result at low Mach numbers and that, as expected, it converges to that result when an appropriately scaled frequency parameter is increased. But the convergence occurs at progressively higher frequencies as the Mach number increases and the supersonic solution never actually converges to the parallel flow result in the vicinity of a critical- layer singularity that occurs in that solution. The dominant contribution to the propagator comes from the radial derivative of a certain component of the adjoint vector Green’s function. The non-parallel flow has a large effect on this quantity, causing it (and, therefore, the radiated sound) to increase at subsonic speeds and decrease at supersonic speeds. The effects of acoustic source location can be visualized by plotting the magnitude of this quantity, as function of position. These ‘altitude plots’ (which represent the intensity of the radiated sound as a function of source location) show that while the parallel flow solutions exhibit a single peak at subsonic speeds (when the source point is centred on the initial shear layer), the non-parallel solutions exhibit a double peak structure, with the second peak occurring about two potential core lengths downstream of the nozzle. These results are qualitatively consistent with the numerical calculations reported in Karabasov et al. (2011).

Experimental Study of the Spectral Properties of Near-Field and Far-Field Jet Noise

The near and far pressure fields generated by round, isothermal and cold jets of diameter D = 38 mm with Mach numbers varying over the range 0.6 ≤ M j ≤ 1.6 are investigated experimentally, and characterized in terms of sound spectra and levels. Properties of near-field jet noise, obtained in particular at 7.5 diameters from the jet centerline, are documented. They differ appreciably from properties of far-field noise, and form a database that can be used for the validation of the acoustic fields determined by compressible Navier-Stokes computations. The near pressure fields originating from simulations can thus be directly compared, without resorting to extrapolation methods which might lead to uncertainties in the far pressure fields. In the present paper, sound source localizations are also carried out from the near-field pressure signals. The experiments provide in addition far-field results evaluated at 52 diameters from the nozzle exit, in good agreement with the data of the literature. The classical dependence of jet noise features with the emission angle is observed. The level and frequency scalings of the pressure spectra obtained for subsonic jets in the sideline and downstream directions are also studied. For small radiation angles, the narrow-banded sound spectra measured are found to scale as the Strouhal number, whereas, as previously noted by Zaman &amp; Yu [1], the one-third octave spectra seem to scale as the Helmholtz number.

Reduced-order representation of turbulent jet flow and its noise source

In the present study, subsonic turbulent jet noise is investigated employing reduced-order representations of the flow field and its noise source targeting 'least-order' approximations of the key processes. These representations utilize LES data for a compressible jet at Mach number 0.9 and Reynolds number 3600. The fluctuations of the velocity field and of the Lamb vector as noise source are investigated with three methods. Firstly, the streamwise development is characterised by a statistical analysis. Thus, the most active region of the flow field and the Lamb vector are observed at 11 and 8 jet diameters downstream, respectively. Secondly, an azimuthal mode decomposition is carried out. The first five azimuthal modes resolve most of the flow field and Lamb vector fluctuation. Thirdly, the dimension of the dynamics phase space is estimated by the proper orthogonal decomposition (POD). About 350 modes are necessary to resolve at least 50% of the fluctuation level of the hydrodynamics and even more modes are required for the noise source. As expected, the end of the potential core correlates with the location of a distinct peak in the noise source magnitude, thus indicating a highly active acoustical region. Intriguingly, the noise source efficiency per unit energy increases with higher azimuthal modes. The comparison of the compressible jet results with the incompressible LES at the same Reynolds number reveals a significant smaller energy concentration in the first azimuthal modes, i.e. the incompressible flow is dynamically more complex. The current results are part of an ongoing effort to predict far-field jet noise by reduced- order modelling of its hydrodynamic source.

Prediction of the far-field jet noise using Kirchhoff's formulation

The Kirchhoff method consists of the calculation of the nonlinear near- and midfields, usually numerically, with the far-field solutions found from a linear Kirchhoff formulation evaluated on a surface D surrounding the nonlinear field. The surface S is assumed to include all the nonlinear flow effects and noise sources

Numerical simulation of jet noise

Jet noise and jet-induced structural loads have become key issues in the design of commercial and military aircraft. Computational Fluid Dynamics (CFD) can be of use in predicting the underlying jet shear-layer instabilities and, in conjunction with classical acoustic theory, jet noise. The computational issues involved in the resolution of high Reynolds number unsteady jet flows are addressed in this paper. Once these jet flows can be accurately resolved, it should be possible to use acoustic theory to extract, for example, the far-field jet noise. An assessment of future work and computational resources required for directly computing far-field jet noise is also presented.

Preliminary analysis of tone‐excited two‐stream jet velocity decay

Acoustic research related to jet flows has established that sound, by amplifying the naturally occurring large‐scale structures in turbulent shear layers, can cause a more rapid decay of the jet plume velocity and temperature and jet spread. One possible application of this sound‐flow interaction phenomenon is to future STOL aircraft that may require modified jet plume characteristics in order to reduce the loads and temperatures on the deflected flaps during takeoff and landing operations. A preliminary analysis is made herein of the tone‐excitation effect on the velocity decay of model‐scale, two‐stream jet plumes. Measured data are correlated in terms of parameters that include excitation sound level and outer‐to‐inner steam velocity ratio. The effect of plume tone‐excitation on farfield jet noise is examined briefly and its implication for large‐scale two‐stream jets is discussed.

Turbulence suppression in free turbulent shear flows under controlled excitation. Part 2. Jet-noise reduction

It is shown that reduction of broadband (even total) far-field jet noise can be achieved via controlled excitation of a jet at a frequency in the range 0.01 &lt; Stθ &lt; 0.02, where Stθ is the Strouhal number based on the exit momentum thickness of the shear layer. Hot-wire measurements in the noise-producing region of the jet reveal that the noise suppression is a direct consequence of turbulence suppression, produced by early saturation and breakdown of maximally growing instability modes.

Contaminating effect of probe noise on jet pressure correlations

When a fast-responding static pressure probe is inserted into a flow, there are several possible mechanisms for the generation of extraneous noise. If the probe signal is cross correlated with the farfield sound, then the ’’probe noise’’ may contribute a dominant portion of the total correlatable noise from the local source region. A theoretical model is developed for predicting the distortion of ’’causality’’ correlation signatures (obtained when in-flow probes are cross correlated with the farfield sound), due to the parasitic probe noise. The predicted correlation functions agree with experiments conducted using probes of various shapes and sizes. In practice, the contaminated portion of the causality correlation signature is expected to be displaced in time from the ’’true’’ jet pressure correlation. An unexpected result of the present experiments is the absence of any legitimate correlation between the turbulent pressure fluctuations and the farfield jet noise. This leads to the conclusion that the extent and strength of the jet pressure fluctuations as noise sources must be very weak. Therefore, any probing device inserted into the flow will generally produce a large and potentially misleading portion of the total correlation.

Two point correlations of jet noise

A large body of careful experimental measurements of two-point broad band correlations of far-field jet noise has been carried out and was briefly reported recently by Lucio Maestrello in NASA TM X-72835. The rather sharp directional lobes and marked absence of axisymmetry were striking and motivated the present effort to bring theory to bear. The model of jet-noise generation is an approximate version of an earlier work of Ribner, based on the foundations of Lighthill. The model incorporates isotropic turbulence superimposed on a specified mean shear flow, with assumed space-time velocity correlations, but with source convection neglected. The particular vehicle is the Proudman format, and the previous work (mean-square pressure) is extended to display the two-point space-time correlations of pressure. The shape of polar plots of correlation is found to derive from two main factors: (1) the non-compactness of the source region, which allows differences in travel times to the two microphones—the dominant effect—and (2) the directivities of the constituent quadrupoles —a weak effect. The non-compactness effect causes the directional lobes in a polar plot to have pointed tips (cusps) and to be especially narrow in the plane of the jet axis. In these respects, and in the quantitative shapes of the normalized correlation curves, results of the theory show generally good agreement with Maestrello's experimental measurements.