Jet Noise Research Articles

Cost-effective hybrid RANS-LES type method for jet turbulence and noise prediction

Jets at higher Reynolds numbers have a high concentration of energy in small scales in the nozzle vicinity. This is challenging for large-eddy simulation, potentially placing severe demands on grid density. To circumvent this, we propose a novel procedure based on well-known Reynolds number (Re) independent of jets. We reduce the jet Re while rescaling the boundary layer properties to maintain incoming boundary layer thickness consistent with high Re jet. The simulations are carried out using hybrid large-eddy simulation type of approach which is incorporated by using near-wall turbulence model with modified properties. No subgrid scale model is used in these simulations. Hence, they effectively become numerical large-eddy simulation with Reynolds-averaged Navier–Stokes covering the full boundary layer region. The noise post-processing is carried out using the Ffowcs-Williams-Hawking approach. The simulations are made for Mach numbers (M) of 0.75 and 0.875 (cold and hot). The results for the overall sound pressure level are observed to be within 2–3% of the measurements, and directivity of sound is also captured accurately for both the cases. Hence, the low Re simulations can be more beneficial in saving time and cost while providing reasonably accurate results.

Unstructured Large-Eddy Simulations of Supersonic Jets

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 from a converging–diverging round nozzle. In this work, the nozzle geometry is explicitly included in the computational domain using an unstructured body-fitted mesh. Two different mesh topologies are investigated, with emphasis on grid isotropy in the acoustic source-containing region, either directly or through the use of adaptive refinement, with grid size ranging from 42 to control volumes. Three different operating conditions are considered: isothermal ideally expanded (fully expanded jet Mach number of , temperature of , and Reynolds number of ), heated ideally expanded (, , and ), and heated overexpanded (, , and ). Blind comparisons with the available experimental measurements carried out at the United Technologies Research Center for the same nozzle and operating conditions are presented. The results show good agreement for both the flow and sound fields. In particular, the spectra shape and levels are accurately captured in the simulations for both near-field and far-field noise. In these studies, sound radiation from the jet is computed using an efficient permeable formulation of the Ffowcs Williams–Hawkings equation in the frequency domain. Its parallel implementation is reviewed and parametric studies of the far-field noise predictions are presented. As an additional step toward best practices for jet aeroacoustics with unstructured large-eddy simulations, guidelines and suggestions for the mesh design, numerical setup, and acoustic postprocessing steps are discussed.

Prediction of jet mixing noise with Lighthill's Acoustic Analogy and geometrical acoustics.

A computational aeroacoustics prediction tool based on the application of Lighthill's theory is presented to compute noise from subsonic turbulent jets. The sources of sound are modeled by expressing Lighthill's source term as two-point correlations of the velocity fluctuations and the sound refraction effects are taken into account by a ray tracing methodology. Both the source and refraction models use the flow information collected from a solution of the Reynolds Averaged Navier-Stokes equations with a standard k-epsilon turbulence model. By adopting the ray tracing method to compute the refraction effects a high-frequency approximation is implied, while no assumption about the mean flow is needed, enabling the application of the method to jet noise problems with inherently three-dimensional propagation effects. Predictions show good agreement with narrowband measurements for the overall sound pressure levels and spectrum shape in polar angles between 60° and 110° for isothermal and hot jets with acoustic Mach number ranging from 0.5 to 1.0. The method presented herein can be applied as a relatively low cost and robust engineering tool for industrial optimization purposes.

Design and Qualification of an Anechoic Facility in PNRPU

The paper describes the anechoic facility built in Perm National Research Polytechnic University (PNRPU) in 2014-2015 with the objective of studying noise of turbulent jets, vortex rings, and airframe noise. At present, the only other analogous facility in Russia is anechoic chamber AC-2 of TsAGI built in 1970s. One of the most important characteristics of an anechoic facility is a region where inverse square law spreading holds within some tolerance, i.e. where the free-field conditions are observed. Acoustic qualification is aimed at determination of this region in the facility. The present paper provides the results of acoustic qualification of the anechoic chamber in PNRPU, which show that the free-field conditions in the facility are realized within 3 m radius from the sound source for broadband noise in the frequency range 125 Hz – 20 kHz. Therefore, the study demonstrates that the anechoic facility in PNRPU allows aeroacoustic experiments to be performed for obtaining quantitative results. The present work restores domestic competences in design of aeroacoustic experimental facilities, which is important in view of the plans to build in Russia an anechoic wind tunnel with hot flow AKT TsAGI for large-scale aeroacoustic tests.

Studying the effect of flap angle on the noise of interaction of a high-bypass jet with a swept wing in a co-flow

Jet–flap interaction noise for a small-scale swept wing model of a trendsetter plane has been studied experimentally, with a double-stream nozzle installed nearby. The effect of the flap angle on the noise of jet–wing interaction has been analyzed. It has been discovered that the flap angle considerably affects the interaction noise in a wide frequency range in such a way that the noise intensity in each frequency band grows exponentially as the flap edge approaches the shear-layer boundary (on a logarithmic scale, this corresponds to linear scaling of the spectrum depending on the flap angle). The exponential noise reduction as the flap angle decreases agrees with the known theoretical simplified-configuration models that are based on the effect of interaction of the near field of instability wave with the edge. Not only does this agreement show that noise increase/reduction mechanism may be associated with similar processes, it also provides an effective tool for controlling the interaction noise.

О развитии методов снижения шума турбулентных струй

О развитии методов снижения шума турбулентных струй

スプライン断面を有する可変ノズルの音響・推力特性

This paper proposes a new concept of a noise reduction nozzle and describes the sensitivity analysis of the nozzle's acoustic and thrust characteristics. The proposed concept envisages a variable nozzle that enables a change in cross-section and reduction of noise by a single mechanism. Under takeoff conditions, the nozzle has a convergent shape with teeth and slots similar to those of a spline hub, which enables noise reduction. On the other hand, under cruise conditions, the shape of the nozzle can be altered from convergent to a clean convergent-divergent configuration. This concept has been validated by scale model tests in a jet noise facility, and the sensitivity of the acoustic and thrust characteristics has been analyzed using numerical analysis. The analysis revealed that the acoustic benefit depends on the depth of slot, and its frequency characteristics depend on the number of slot. Thrust loss strongly depends on the nozzle throat position and weakly depends on the number of slot. The proposed concept has the potential to provide significant acoustic benefits relative to the thrust loss, and the results of the sensitivity analysis enable us to design appropriate nozzle geometry.

Computational analysis of subsonic jets from round and bevelled nozzles

Numerical simulations of turbulent compressible subsonic jets at Mach number 0.75 emanating from round and bevelled nozzles were carried out using a commercially available CFD code. The modelling and calculations were performed on a three-dimensional computational domain using RANS equations and SST k-ω turbulence model. The computational domain was discretised using hexahedral/tetrahedral mesh with approximately two million cells. The flow was investigated for axial and radial profiles of velocity components, shear layer thickness, self preserving nature, turbulent intensity, turbulent viscosity and Reynolds's stresses. The results were found to be in reasonable agreement with the available data in the literature. It was observed that the bevelling of nozzle can significantly change both flow pattern and turbulence structure and thus act as a passive method for jet noise reduction. Moreover, flow characteristics of jets issuing from various nozzle configurations intended to reduce noise levels may be easily evaluated using this computationally less expensive method.

Two-dimensional model of the interaction of a plane acoustic wave with nozzle edge and wing trailing edge.

The interaction of a plane acoustic wave with two-dimensional model of nozzle edge and trailing edge is investigated theoretically by means of the Wiener-Hopf technique. The nozzle edge and the trailing edge are simulated by two half-planes with offset edges. Shear layer behind the nozzle edge is represented by a vortex sheet supporting Kelvin-Helmholtz instability waves. The considered configuration combines two well-known models (nozzle edge and trailing edge), and reveals additional interesting physical aspects. To obtain the solution, the matrix Wiener-Hopf equation is solved in conjunction with a requirement that the full Kutta condition is imposed at the edges. Factorization of the kernel matrix is performed by the combination of Padé approximation and the pole removal technique. This procedure is used to obtain numerical results. The results indicate that the diffracted acoustic field may be significantly intensified due to scattering of hydrodynamic instability waves into sound waves provided that the trailing edge is close enough to the vortex sheet. Similar mechanism may be responsible for the intensification of jet noise near a wing.

Numerical investigation of the interaction of the turbulent dual-jet and acoustic propagation**Project supported by the Fundamental Research Funds for the Central Universities, China (Grant No. N150204003).

In order to study the interaction between two independent jets, a three-dimensional (3D) transient mathematical model is developed to investigate the flow field and acoustic properties of the two-stream jets. The results are compared with those of the single-stream jet at Mach number 0.9 and Reynolds number 3600. The large eddy simulation (LES) with dynamic Smagorinsky sub-grid scale (SGS) approach is used to simulate the turbulent jet flow structure. The acoustic field is evaluated by the Ffowcs Williams–Hawkings (FW-H) integral equation. Considering the compressibility of high-speed gas jets, the density-based explicit formulation is adopted to solve the governing equations. Meanwhile, the viscosity is approximated by using the Sutherland kinetic theory. The predicted flow characteristics as well as the acoustic properties show that they are in good agreement with the existing experimental and numerical results under the same flow conditions available in the literature. The results indicate that the merging phenomenon of the dual-jet is triggered by the deflection mechanism of the Coanda effect, which sequentially introduces additional complexity and instability of flow structure. One of the main factors affecting the dual-jet merging is the aperture ratio, which has a direct influence on the potential core and surrounding flow fluctuation. The analysis on the noise pollution reveals that the potential core plays a fundamental role in noise emission while the additional mixing noise makes less contribution than the single jet noise. The overall sound pressure level (OASPL) profiles have a directive property, suggesting an approximate 25° deflection from the streamwise direction, however, shifting toward lateral direction of about 10° to 15° in the dual-jet. The conclusion obtained in this study can provide valuable data to guide the development of manufacturing-green technology in the multi-jet applications.

A model problem for sound radiation by an installed jet

A model for sound generation by a jet in the vicinity of a flat plate, mimicking an exhaust jet installed near an aircraft wing, is presented. An earlier model (Cavalieri et al. J. Sound Vib. 333 (2014) 6516—6531) is further simplified by considering that the sound source is an axially-extended, cylindrical wavepacket concentrated on the jet lipline, and that this source is scattered by the trailing edge of a semi-infinite flat plate; the model is shown to match earlier results and considerably simplifies the analysis. It is used to evaluate how the parameters of the problem influence sound radiation by subsonic jets. We show that the axisymmetric mode of the source is the most acoustically efficient, similarly to what is seen for free jets; but unlike the latter problem, the sound scattered by the trailing edge is only weakly dependent on the details of the wavepacket envelope and on the two-point coherence of the source, the wavepacket phase speed being the salient feature for installed jet noise. We then use the model to evaluate how geometrical parameters of jet-plate configurations modify the radiated sound. The acoustic radiation is particularly sensitive to the jet-plate distance due to the exponential radial decay of near-field disturbances; the relative axial position of jet and trailing edge is shown to play a comparably minor role. Finally, changes in the angle of attack of the plate and in the sweep angle of the trailing edge considerably modify the radiated sound, leading to significant reductions of the acoustic intensity in some directions. The various properties of installed jet noise are further explored by appealing to the wavenumber transform of the tailored Green's function used to compute the scattered field; insight is thus provided on how jet-wing configurations might be designed so as to reduce installation noise.

Prediction of installed jet noise

A semianalytical model for installed jet noise is proposed in this paper. We argue and conclude that there exist two distinct sound source mechanisms for installed jet noise, and the model is therefore composed of two parts to account for these different sound source mechanisms. Lighthill’s acoustic analogy and a fourth-order space–time correlation model for the Lighthill stress tensor are used to model the sound induced by the equivalent turbulent quadrupole sources, while the trailing-edge scattering of near-field evanescent instability waves is modelled using Amiet’s approach. A non-zero ambient mean flow is taken into account. It is found that, when the rigid surface is not so close to the jet as to affect the turbulent flow field, the trailing-edge scattering of near-field evanescent waves dominates the low-frequency amplification of installed jet noise in the far-field. The high-frequency noise enhancement on the reflected side is due to the surface reflection effect. The model agrees well with experimental results at different observer angles, apart from deviations caused by the mean-flow refraction effect at high frequencies at low observer angles.

Scattering of turbulent-jet wavepackets by a swept trailing edge.

Installed jet noise is studied by means of a simplified configuration comprising a flat plate in the vicinity of a round jet. The effects of Mach number, jet-plate radial distance, and trailing-edge sweep angle are explored. Acoustic measurements are performed using a traversable 18-microphone azimuthal array, providing pressure data at 360 points on a cylindrical surface surrounding the jet-plate system. Key observations include a decrease, with increasing Mach number, of the relative level of the scattered field in comparison to the uninstalled jet; an exponential dependence of the scattered sound pressure level on the radial jet-plate separation; and considerable sideline noise reductions with increasing sweep angle, with which there is an overall reduction in acoustic efficiency. The measurements are compared with results obtained using a kinematic wavepacket source model, whose radiation is computed in two ways. A TGF for a semi-infinite flat plate is used to provide a low-order approximation of the scattering effect. Use of a more computationally intensive boundary element method provides additional precision. Good agreement between model predictions and experiment, encouraging from the perspective of low-cost prediction strategies, demonstrates that the models comprise the essential sound generation mechanisms.

Acoustics measurements of military-style supersonic beveled nozzle jets with interior corrugations

Increasingly powerful and noisy military aircraft have generated the need for research leading to the development of supersonic jet noise reduction devices. The hot, high speed supersonic jets exhausting from military aircraft during takeoff present a most challenging problem. The present study extends prior research on two methods of noise reduction. The first is the internal nozzle corrugations pioneered by Seiner et al. and the second is the beveled exit plane explored most recently by Viswanathan. A novel research idea of creating fluidic corrugations similar to the nozzle corrugations has been initiated by Penn State. To further the understanding and analysis of the fluidic corrugations, the present study focuses on the flow field and acoustic field of nozzles with two, three, and six conventional, hardwalled corrugations. The effect of the combination of the internal corrugations with a beveled nozzle is explored. The results show that significant noise reductions of over 3 dB of the mixing noise and the broadband shock-associated noise can be achieved. The combination of the beveled nozzle and the internal nozzle corrugations showed that there is less azimuthal dependence of the acoustic field than for the purely beveled nozzle. The combination nozzle was shown to reduce the noise over a wider range of polar angles and operating conditions than either the purely beveled nozzle or the purely corrugated nozzle.

Aeroacoustic Simulations of a Simplified High-Lift Device Accounting for Some Installation Effects

The aeroacoustic noise of a simplified wing-flap configuration placed in an open-jet anechoic wind tunnel is investigated by compressible wall-resolved large-eddy simulations possibly coupled with the Ffowcs-Williams and Hawkings acoustic analogy. The numerical domain includes the nozzle and the anechoic chamber, which allows the direct taking of the installation effects into account. The simulated span (25% of the flap chord) is the largest one simulated so far for this configuration and, more generally, at this high Reynolds number. The results of the simulation are validated with the extensive experimental database for the chosen configuration. A good overall agreement with experimental data is achieved for all mean and fluctuating velocities, as well as wall pressures on the wing and the flap, especially the power spectral densities, the convection velocities, and even the streamwise and spanwise coherences on both elements. Similar good predictions of the far-field acoustic pressure using a hybrid approach are achieved, although the acoustic analogy does not allow taking into account the jet noise given that the Ffowcs-Williams and Hawkings surfaces are placed within the jet core. The acoustic results obtained with the acoustic analogy are also compared with the numerical microphones placed in the far field, showing a good agreement up to the mesh cut-ff frequency. The dilatation field and the sound directivity stress that the flap and wing trailing edges are the dominant noise sources of the wing-flap configuration, with significant diffraction effects on the flap side. Evidence of slight boundary-layer instabilities is also shown close to the flap trailing edge.

Identification of jet noise source using causality method based on large-eddy simulation of a round jet flow

An acoustic analogy analysis based on a decomposition of the source term in Lighthill’s equation is discussed in light of a large-eddy simulation of a subsonic turbulent jet exhausting from a baseline round nozzle at Mach number M = 0.9 with Reynolds number Re = 105. The decomposed sub-terms show the nonlinear reciprocal interactions of density, velocity, vorticity, and dilatation fields. To understand the aerodynamic sound generation mechanism, intrinsic links between turbulent flow and emitted acoustic signals are made and applied to the large-eddy simulation data. Cross-correlation functions are used for the links between the far-field sound signals and the sub-terms as well as major flow variables in the jet flow domain. The spatial distributions of cross-correlations are examined to identify the sound source distribution throughout the domain and observe the mutual interactions and cancellations between the decomposed sub-terms. The contributions of sub-terms are also studied in frequency domain.

Flow Structures Associated with Turbulent Mixing Noise and Screech Tones in Axisymmetric Jets

A jet from an axisymmetric convergent nozzle is studied at ideal and underexpanded conditions using velocity and acoustic data. Two particle imaging velocimetry setups, a 10 kHz system and a multi-camera configuration, were used to capture near-field velocities while simultaneously sampled with far-field microphones. Proper orthogonal decomposition is performed on the velocity data to extract modes representative of physical processes in the flow. The decomposed velocity fields are then correlated with acoustic data to identify modes related to specific noise spectra. Specifically, four modes are associated with noise production in the sonic plume. Selective flow-field reconstruction is carried out, revealing interesting dynamics associated with loud flow states. In the supersonic case, screech-containing and turbulent mixing modes are isolated. The spatial modes of each data set are then compared for similarities in structures.

Generation of Mach waves by supersonic jets

Direct shadowgraphy is used to study the generation of Mach waves and jet noise in a wide range of frequencies by single supersonic jets and by groups of jets in the Mach number range M = 1–4.

Effect of de-correlating turbulence on the low frequency decay of jet-surface interaction noise in sub-sonic unheated air jets using a CFD-based approach

In this paper we extend the Rapid-distortion theory (RDT)-based model derived by Goldstein, Afsar & Leib (J. Fluid Mech., vol. 736, pp. 532-569, 2013) for the sound generated by the interaction of a large-aspect-ratio rectangular jet with the trailing edge of a flat plate to include a more realistic upstream turbulence spectrum that possess a de-correlation (i.e. negative dip) in its space-time structure and use results from three-dimensional Reynolds-Averaged Navier-Stokes (RANS) solutions to determine the mean flow, turbulent kinetic energy and turbulence length & time scales. Since the interaction noise dominates the low-frequency portion of the spectrum, we use an appropriate asymptotic approximation for the Rayleigh equation Green’s function, which enters the analysis, based on a two-dimensional mean flow representation for the jet. We use the model to predict jet-surface interaction noise for a range of subsonic acoustic Mach number jets, nozzle aspect ratios, streamwise and transverse trailing-edge locations and compare them with experimental data. The RANS meanflow computations are also compared with flow data for selected cases to assess their validity. We find that finite de-correlation in the turbulence spectrum increases the low-frequency algebraic decay (the low-frequency “roll-off”) of the acoustic spectrum with angular frequency to give a model that has a pure dipole frequency scaling. This gives better agreement with noise data compared to Goldstein et al. (2013) for Strouhal numbers less than the peak jet-surface interaction noise. For example, through sensitivity analysis we find that there is a difference of 10 dB at the lowest frequency for which data exists (relative to a model without de-correlation effects included) for the highest acoustic Mach number case. Secondly, our results for the planar flow theory provide a first estimate of the low-frequency amplification due to the jet-surface interaction for moderate aspect ratio nozzles when RANS meanflow quantities are used appropriately. This work will hopefully add to noise prediction efforts for aircraft configurations in which the exhaust systems are tightly integrated with the airframe.

Empirical modelling of noise from high aspect ratio rectangular jets

Noise measurements have been performed on rectangular jets of aspect ratios ranging from 49 to 987 with the aim of determining the appropriate velocity and length scaling to be used in an empirical noise prediction model. The results have shown that the velocity exponent is a function of the nozzle aspect ratio, decreasing with increasing nozzle aspect ratio. In an effort to establish a general prediction model, the velocity exponent of 7 was chosen as the best compromise to represent all the measured data. The analysis of the noise measurements from high aspect ratio nozzles of varying jet height and width has shown that, for the range of aspect ratios considered, the jet sound power level scales with the nozzle height to the power of 3 and the nozzle width to the power of 1. The derived jet noise scaling has been validated with independent experimental data and shows good agreement.