Unheated Jets Research Articles

Eduction of coherent structures from schlieren images of twin jets using SPOD informed with momentum potential theory in the spectral domain

Abstract This work presents a methodology to extract coherent structures from high-speed schlieren images of turbulent twin jets which are more physically interpretable than those obtained with currently existing techniques. Recently, Prasad and Gaitonde (J Fluid Mech 940:1–11, 2022) introduced an approach which employs the momentum potential theory of Doak (J Sound Vib 131(1):67–90, 1989) to compute potential (acoustic and thermal) energy fluctuations from the schlieren images by solving a Poisson equation, and combines it with spectral proper orthogonal decomposition (SPOD) to educe coherent structures from the momentum potential field instead of the original schlieren field. While the latter field is dominated by a broad range of vortical fluctuations in the turbulent mixing region of unheated high-speed jets, the momentum potential field is governed by fluctuations which are intimately related to acoustic emission, and its spatial structure in the frequency domain is very organized. The proposed methodology in this paper improves the technique of Prasad and Gaitonde (J Fluid Mech 940:1–11, 2022) in three new ways. First, the solution of the Poisson equation is carried out in the frequency-wavenumber domain instead of the time-space domain, which simplifies and integrates the solution of the Poisson equation within the SPOD framework based on momentum potential fluctuations. Second, the issue of solving the Poisson equation on a finite domain with ad hoc boundary conditions is explicitly addressed, identifying and removing those unphysical harmonic components introduced in the solution process. Third, the solution of the SPOD problem in terms of momentum potential fluctuations is used to reconstruct schlieren SPOD fields associated with each mode, allowing the visualization of the obtained coherent structures also in terms of the density gradient. The method is applied here to schlieren images of a twin-jet configuration with a small jet separation at two supersonic operation conditions: a perfectly-expanded and an overexpanded one. The SPOD modes based on momentum potential fluctuations retain the wavepacket structure including the direct Mach-wave radiation, together with upstream- and downstream-traveling acoustic waves, similar to SPOD modes based on the schlieren images. However, for the same dataset, they result in a lower-rank decomposition than schlieren-based SPOD and provide an effective separation of twin-jet fluctuations into independent toroidal and flapping oscillations that are recovered as different SPOD modes. These coherent structures are more consistent with twin-jet wavepacket models available in the literature than those originally obtained with direct schlieren-based SPOD, facilitating their interpretation and comparison against theoretical analyses. Graphical abstract

A Comparative Study of Scarfed Nozzle for Jet-Installation Noise Reduction

The experimental investigation of the effects of scarfed nozzles on jet-installation noise was conducted using unheated subsonic jets in an anechoic jet noise facility. Four different types of scarfed nozzles with increasing nozzle lip angles were examined to study the installation effects at various plate distances away from the jet. Mach numbers ranging from 0.3 to 0.8 were investigated in the experiments. The use of scarfed nozzles is known to result in the deflection of flow away from the centre axis, inducing asymmetry in the jet shears, leading to azimuthal variation in the spectra, and ultimately, noise reduction. This study aims to explore the possibility of reducing jet-installation noise using scarfed nozzles at subsonic flow conditions. The characteristics of jet hydrodynamic pressure fluctuations were investigated in the axial direction using far-field measurements. The near-field flow features were studied using surface pressure transducers installed on the flat plate for the installed configurations. Detailed spectral, coherence, and correlation analyses were carried out to determine the noise reduction mechanisms associated with scarfed nozzles in the proximity of a flat plate. The results of the study showed that the use of scarfed nozzles significantly reduced the jet-installation noise. The reduction was attributed to the generation of an asymmetric flow field induced by the nozzle geometry. The reduction in noise levels was also observed to increase with increasing nozzle lip angle. The detailed analyses revealed that the noise reduction mechanism was associated with a decrease in the acoustic power generated by the jet. Overall, the results suggest that scarfed nozzles can be an effective means of reducing jet-installation noise in subsonic flow conditions.

Is an elliptic jet quieter than a round jet?

Non-axisymmetric geometries, mainly elliptic and rectangular, have been proposed for the reduction of jet noise vis-à-vis round nozzles. Most of the studies of these nozzles are from unheated jets and are restricted to nozzles of very small size. Furthermore, all of them have been carried out at static conditions, thereby rendering their value to insignificance for practical applications. All engines in service with long ducts and a confluent nozzle incorporate an internal lobed mixer. The aeroacoustic characteristics of an elliptic compound nozzle that represents the geometry of an existing low bypass ratio (BPR) turbofan engine, is investigated at 1/7th scale in this study. Typical engine cycle conditions are chosen; data are acquired statically and in the presence of a flight stream. The aspect ratio of the nozzle is 2.0; higher aspect ratios are not suitable for engine applications. The results are compared with a round compound nozzle with the same internal geometry, so as to assess the acoustic benefit, if any, of the elliptic nozzle. Both a simple internal splitter and an in-service lobed mixer have been considered. The elliptic nozzle introduces azimuthal asymmetry even for an unheated jet; the magnitude of azimuthal variation becomes pronounced for heated jets. Typically, the lowest level of noise is observed towards the narrow side of the elliptic nozzle (ϕ = 0°); the noise level gradually increases and reaches a maximum towards the broader side (ϕ = 90°). Though there are some superficial similarities between the elliptic and beveled nozzles, it is shown that the noise characteristics are very different. A systematic study is carried out, with step-by-step build up to realistic geometry, with forward flight. A large noise reduction of ∼3 to ∼4 EPNdB is observed for the splitter nozzle under static conditions. The introduction of a realistic lobed mixer reduces this benefit to close to zero. Finally, there is a noise increase at all azimuthal angles with forward flight. Therefore, the elliptic nozzle does not provide any EPNL benefit for actual nozzle geometry and consequently does not constitute a viable design for noise reduction. The importance of evaluating noise reduction concepts using appropriate geometry and under realistic forward flight conditions is emphasized once again.

Turbulent diffusion of scalar and heat in an off-source heated steady round jet

We study, using direct numerical simulation, the characteristics of an off-source heated jet (OSHJ), the rate of heat addition being proportional to the local concentration of a scalar. The heating disturbs the self-similar state of the unheated jet (UJ) entering the heat injection zone (HIZ). Based on the characteristics of the velocity and scalar statistics, the evolution of the OSHJ is demarcated into three axial zones. Zone I, which is dynamically the most active region, exhibits rapid variations in quantities like the mean radial velocity, the scalar-to-velocity width ratio and the second-order correlations. Zone II represents the relaxation of these quantities towards a new self-similar state which is realized in Zone III above the HIZ. We find that the temperature–velocity correlation is negative in the core of the OSHJ in Zone I, which becomes positive for all radial locations in Zones II and III. We interpret this behavior by decomposing the effect of heat addition into “heating” and “buoyancy” factors; the former describes the effect of axial velocity on heating while the latter describes the effect of added buoyancy on flow acceleration. The heating factor dominates in Zone I leading to the negative temperature–velocity correlation, which we conjecture to be the reason for the “disruption” of coherent structures in the OSHJ reported in the literature. Based on wavelet analysis of the scalar field we identify the length scale of the ring-like coherent structure that gets disrupted upon heat addition, which enhances the scalar homogeneity in the core of the OSHJ near the base of the HIZ. Where possible, we compare our results with the literature on heated jets within the context of the three demarcated zones. We find that external heating decreases the scalar-to-velocity flow widths relative to the UJ to a value in Zone III that is largely insensitive to the precise details of heat addition , such as the amount of heat added and the extent of the HIZ. We also present the effect of heat addition on turbulent Schmidt and Prandtl numbers. Our findings are relevant to the dynamics of cumulus clouds which are governed by a similar interplay of scalar, velocity and temperature fields as reported here.

Jet Flow and Noise Predictions for the Doak Laboratory Experiment

Large-eddy simulations (LESs) are performed for two isolated unheated jet flows corresponding to a Doak Laboratory experiment performed at the University of Southampton. The jet speeds studied correspond to acoustic Mach numbers of 0.6 and 0.8 as well as Reynolds numbers based on the nozzle exit diameter of about one million. The LES method is based on the compact accurately boundary-adjusting high-resolution technique (CABARET) and is implemented on graphics processing units (GPUs) to obtain 1000–1100 convective time units for statistical averaging with reasonable run times. In comparison with the previous jet LES calculations with the GPU CABARET method, the mean-flow velocity and turbulent intensity profiles are matched with the hot-wire measurements just downstream of the nozzle exit. The far-field noise spectra of the Doak jets are evaluated using two methods: the Ffowcs Williams–Hawkings approach and a reduced-order implementation of the Goldstein generalized acoustic analogy. The flow and noise results are compared with hot-wire and acoustic microphone measurements of the Doak Laboratory and critically analyzed in comparison with the NASA small hot jet acoustic rig database.

Features of far-downstream asymptotic velocity fluctuations in a round jet: A one-dimensional turbulence study

An accurate prediction of the turbulent jet noise is usually only possible with direct numerical simulation (DNS) or high-resolution large-eddy simulation (LES) of the turbulent sources in the acoustic near field. The required level of fidelity comes at the price of high numerical resolution requirements, a severe restriction of the accessible parameter space, and high computational costs in general. These limitations can be partially mitigated by reduced-order models. In the present work, the stochastic one-dimensional turbulence (ODT) model is utilized as a stand-alone tool in order to study turbulent fluctuations in the far downstream region of turbulent round jets with finite co-flow velocity. ODT is a dimensionally reduced turbulence model that aims to resolve flow-field over a broad range of scales and, thus, the turbulent noise sources at all relevant scales, but only for a single, radially oriented, physical coordinate that is advected downstream with the flow during a simulation run. Here, unheated round jets with nozzle diameter D, nominal Mach number Ma = 0.9 but Reynolds number ReD∈{9×104,2×105,4×105} are studied as a canonical problem. An ensemble of ODT realizations is used to obtain flow statistics from a detailed representation of fluctuations that may be used to estimate turbulent noise by small-scale resolved sources in the near future. As the first step in this direction, we analyze the model representation of the flow field and the participating flow scales in detail. This is done even far downstream of the nozzle, which is not possible with high-resolution LES or DNS. The present ODT results agree well with the available reference data. The model accurately reproduces the asymptotic mean and fluctuating velocity behavior, and radial turbulence spectra of the jet that approximately obey large-scale jet similarity but are modified by axially decreasing the turbulence intensity. Based on these results, an outlook on the model application for turbulent jet noise prediction is given.

Subsonic jet noise source location as a function of nozzle exit boundary layer

Over the years, there have been numerous studies on determining subsonic jet noise source locations, typically plotted as Strouhal number as a function of distance from the nozzle exit. A comparison of the results of various studies yields a spread of about two nozzle diameters in measured source location. This work examines how boundary layer thickness, which can vary from nozzle to nozzle, could be the cause of observed differences in different studies in subsonic jet noise source location. Source location measurements of unheated jets from ASME nozzles, which have comparably thinner nozzle exit boundary layers, and conical nozzles, which have comparably thicker nozzle exit boundary layers, are compared. These results are substantiated with the use of schlieren flow visualization and velocity profile measurements. It is found that the nozzles with thinner nozzle exit boundary layers have noise source distributions that are 0.25–2 diameters upstream of those with thicker nozzle exit boundary layers. Thinner nozzle exit boundary layers result in higher growth rates of instability waves, increasing mixing and thereby moving noise sources upstream.

Acoustic measurement of under-expanded jet and its numerical prediction

AbstractIntense acoustic loads from jet noise cause noise pollution and induce failures, such as the malfunctioning of electronic devices and fatigue failure of internal/external structures. Consequently, the prediction of jet noise characteristics is crucial in the development of high-speed vehicles. This study presents acoustic experiments and predictions for an under-expanded, unheated jet using a small-scale prototype. Outdoor measurements are carried out using a vertical ejection setup. Acoustic characteristics are measured using both linear and circular microphone arrays. Additionally, numerical prediction of the same jet noise is performed using a detached eddy simulation and the permeable Ffowcs-Williams and Hawkings acoustic analogy. The vertical experimental setup exhibits the typical acoustic characteristics of a supersonic jet in terms of directivity and broadband shock-associated noise. Moreover, the numerical prediction exhibits satisfactory accuracy for the jet downstream, where the large-scale turbulence structures of the directivity predominate. However, discrepancy increases in the domain of lower directivity. The presented experiment and prediction will be extended to future studies regarding the noise of various deflector duct configurations impinging on supersonic jets.

Wind tunnel experiments on smoke diffusion from a chimney

The diffusion of heated and unheated jets from a chimney was investigated in cross flows with large-scale turbulence generated by a Makita-type active turbulence generator and with gird generated turbulence. The behavior of jet with smoke was observed by a high-speed camera. The smoke was dispersed more widely by meandering motion in the large-scale turbulence. The turbulent eddies corresponding to the integral scale should effectively contribute to the meandering smoke diffusion. The time-averaged smoke concentration distribution was obtained from the brightness values of instantaneous images. The stream-wise variations of the smoke diffusion width and the smoke flow center were evaluated from the distribution. Obtained data were compared with previous correlations.

Azimuthal decomposition of the radiated noise from supersonic shock-containing jets.

Acoustic measurements of unheated supersonic underexpanded jets with ideally expanded Mach numbers of 1.14, 1.38, and 1.50 are presented. Of the three components of supersonic jet noise, the focus is on the broadband shock-associated noise (BBSAN) component. Motivated by the modelling of BBSAN using the wavepacket framework, a traversable microphone ring is used to decompose the acoustic pressure into azimuthal Fourier modes. Unlike noise radiated downstream, BBSAN is dominated by azimuthal modes 1-3, which are approximately 3-4 dB/St stronger than the axisymmetric component. Crucially, the relative contribution of successive modes to BBSAN is sensitive to the observer angle and jet operating condition. Four azimuthal modes are necessary to reconstruct the total BBSAN signal to within 1 dB/St accuracy for the conditions presented here. The analysis suggests, however, that the number of modes required to maintain this accuracy increases as the peak frequency shifts upward. The results demonstrate the need to carefully consider the azimuthal content of BBSAN when comparing acoustic measurements to predictions made by jet noise models built on instability theory.

Evaluation of Quasi-One-Dimensional Unstructured Method for Jet Noise Prediction

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 flow, the scale-resolving delayed detached-eddy simulation approach is implemented on unstructured meshes using the edge-based reconstruction schemes. The jet far-field noise is calculated with the help of the Ffowcs-Williams and Hawkings method. The efficiency of the resulting “unstructured” algorithm is demonstrated on two popular cases: a subsonic unheated jet of Mach number 0.9, and a sonic underexpanded hot jet.

Measurements of the single-point and joint turbulence statistics of high subsonic jets using hot-wire anemometry

This paper describes a systematic and comprehensive hot-wire investigation into the turbulent statistics of low-, moderate- and high-speed subsonic jets. Experiments were performed to obtain the one-point and two-point statistics of a single-stream, unheated jet turbulence field over a broad region of the jet plume. Results show that hot-wires can be used to measure both the one-point and two-point statistics of the high turbulence intensity, noise-producing regions of unheated, compressible, subsonic jets. For the two-point measurements, probe pairings are performed over the three orthogonal axes. Analysis of the experimental data reveals four main conclusions: (1) both the statistical and joint moments of the turbulence scale well with the local jet shear layer half-width; (2) a simple relationship exists between the statistics of the velocity fluctuations and the square of the velocity fluctuations; (3) a simple relationship exists between the longitudinal and transverse length-scales, and (4) a semi-empirical model has been developed to predict the cross-correlation coefficients, power spectral density, frequency-dependent length-scales and coherence decay of the turbulent velocity field. From the second and third conclusions, it is shown that, in the locations near an eddy’s centre of rotation (i.e. the midpoint of the jet shear layer), the turbulence statistics can be described as quasi-homogeneous and quasi-frozen. The joint statistical moments, therefore, can be inferred simply from single-point tests. These results will help to develop models for predicting jet mixing noise, highlighting the situations in which the simplifying assumptions are inadequate.Graphical abstract

Correlation Analysis of High-Resolution Particle Image Velocimetry Data of Screeching Jets

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 length scales of the turbulence quantities in the jet. Two low-order models for the second- and fourth-order integral length scales are examined. The low-order models are defined by locally isotropic scales estimated from two-dimensional particle image velocimetry measurements. These measurements are of screeching underexpanded unheated round jets issuing from a purely converging nozzle at conditions that correspond to ideally expanded Mach numbers of 1.45 and 1.59. The jets are dominated by the helical (C) instability screech mode, which is associated with large-scale coherent periodic fluctuations. These fluctuations are filtered using a proper orthogonal decomposition method to assess low-order models that approximate the length scales associated with the broadband noise mechanisms. The length scale model parameters are shown to be insensitive for the two Mach numbers considered. The root-mean-square error associated with the low-order models indicates that either is sufficient for approximating the integral length scales required to model equivalent sources of broadband jet noise.

Experimental study of the noise radiated by an air flow discharge through diaphragms and perforated plates

An experimental analysis of the noise radiated by a high pressure flow discharge through diaphragms and perforated plates is carried out for a large range of subsonic and supersonic operating conditions (nozzle pressure ratio (NPR) from 1 to 3.6). A parametric study of the geometrical parameters is also achieved to characterize their influence onto the acoustic radiation and ways to reduce it. This reaches from single diaphragms to multi-perforated plates with variable hole diameters and arrangements that are placed at the exit of a duct. Different acoustic behaviors are observed: in all cases the far-field acoustic radiation is dominated by a broadband contribution associated with the turbulent mixing in shear layers. In the diaphragm cases, this broadband noise has similar characteristics as the mixing noise of classical unheated jets while in the multi-perforated plates cases, it is composed of two distinct humps associated with different parts of the jets development. For supersonic regimes, in addition to this broadband radiation, shock associated noise (screech and broadband shock associated noise) appears for all diaphragm cases and for the perforated plate with the closest holes. Finally for the smallest NPR, a high frequency tonal noise has been observed in most of the multi-perforated cases and for the smallest diaphragm. Different regimes of this radiation have also been observed with a possible amplitude modulation of the dominant tone. This radiation may be attributed to vortex shedding due to the sharp section reduction that would trigger a flow resonance between the small ducts of the holes and their sharp edges.

Noise reduction analysis of supersonic unheated jets with fluidic injection using large eddy simulations

A set of large eddy simulations is used to perform a numerical analysis of fluidic injection as a tool for noise reduction. This technique, developed at the Pennsylvania State University, allows one to turn on and off the air injectors in order to reduce the noise during takeoff and landing without penalizing performance in other flight regimes. Numerical simulations are performed on a military-style nozzle based on the GE F400-series engines, with a design Mach number of 1.65, for overexpanded jet conditions. The numerical results are compared and validated with the outcome of experiments performed at the Pennsylvania State University. For the case chosen, the fluidic injection shows the potential of breaking down shock cells into smaller structures with different orientation and strength. This directly reduces the intensity of broadband shock associated noise, with a positive effect of reducing the overall sound pressure level by more than [Formula: see text] along the direction of maximum sound propagation of the baseline case. The maximum noise reduction was found to be almost [Formula: see text] at 55° on the azimuthal plane in between two lines of injectors.

Non-intrusive acoustic measurement of flow velocity and temperature in a high subsonic Mach number jet

In previous studies, sonic anemometry and thermometry have generally been used to measure low subsonic Mach flow conditions. Recently, a novel configuration was proposed and used to measure unheated jet velocities up to Mach 0.83 non-intrusively. The objective of this investigation is to test the novel configuration in higher temperature conditions and explore the effects of fluid temperature on mean velocity and temperature measurement accuracy. The current work presents non-intrusive acoustic measurements of single-stream jet conditions up to Mach 0.7 and total temperatures from 299 K to 700 K. Comparison of acoustically measured velocity and static temperature with probe data indicate root mean square (RMS) velocity errors of 2.6 m s−1 (1.1% of the maximum jet centerline velocity), 4.0 m s−1 (1.2%), and 8.5 m s−1 (2.4%), respectively, for 299, 589, and 700 K total temperature flows up to Mach 0.7. RMS static temperature errors of 7.5 K (2.5% of total temperature), 8.1 K (1.3%), and 23.3 K (3.3%) were observed for the same respective total temperature conditions. To the authors’ knowledge, this is the first time a non-intrusive acoustic technique has been used to simultaneously measure mean fluid velocity and static temperatures in high subsonic Mach numbers up to 0.7. Overall, the findings of this work support the use of acoustics for non-intrusive flow monitoring. The ability to measure mean flow conditions at high subsonic Mach numbers and temperatures makes this technique a viable candidate for gas turbine applications, in particular.

Effect of Scarfing on Rectangular Nozzle Supersonic Jet Plume Flow Characteristics

An experimental and computational fluid dynamics study is reported of supersonic jets issuing from a high-aspect-ratio rectangular convergent–divergent nozzle with and without a scarfed exit. Schlieren visualization and laser Doppler anemometry measurements captured near-field aerodynamic development of an unheated jet at overexpanded, design, and underexpanded conditions. Reynolds-averaged Navier–Stokes computational fluid dynamics predictions using an eddy viscosity closure (Spalart–Allmaras model) for clean and scarfed geometries were compared with measurements to examine the ability to capture nozzle scarfing effects. The measured plume shape for a scarfed nozzle was strongly affected at overexpanded conditions (a distorted four-lobe shape was observed), whereas a rectangular shape was retained for underexpanded flow although plume bifurcation occurred. The development of the plume shape and the mixing rate was a consequence of the strong vortices that occur with rectangular nozzles, with extra vortices introduced by scarfing. The nozzle exit static pressure changed dramatically when scarfing was added, influencing plume secondary flows and near-field development. The main features of scarfed jet development were predicted qualitatively correctly; the four-lobe overexpanded shape was reproduced but the strength of pressure-driven secondary velocities was overpredicted. The experimental data provided represent a challenging validation test case for computational fluid dynamics studies of three-dimensional supersonic jet plumes with scarfed interaction effects.

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.

A trial application of acoustic excitation for jet noise reduction

Jet noise, one of the dominant noise components from an aircraft jet engine is known as the result of developed vortices produced during jet mixing process. In last couple of years, a method for jet noise reduction which acoustically controls the jet mixing layer using a sound excitation is proposed and investigated under the collaboration between The University of Tokyo and JAXA. In this paper, the main idea, experimental apparatus, experimental condition and experimental results will be explained. The experiment was conducted under an unheated and subsonic jet condition. The results show that low frequency sound excitation results in an increase in low frequency noise and a decrease in high frequency noise. The low frequency sound excitation is considered to excite the instability of jet flow and collapse the jet plume. On the other hand, the high frequency sound excitation resulted in a slight increase of high frequency noise and a slight decrease of low frequency noise (0.5 dB reduction in maximum) si...

Jet-Noise-Prediction Model for Chevrons and Microjets

This study develops a jet-noise-prediction model for chevrons and microjets. A novel equation is proposed to express the amplitude of the fourth-order space-time velocity cross correlations, which represent the sources of noise emanated from unheated jets, in terms of mean-flow parameters and turbulence statistics, such as streamwise circulation, axial velocity, and turbulent kinetic energy. The cross correlations based on a Reynolds-averaged Navier-Stokes flowfield showed a good agreement with those based on a large-eddy-simulation flowfield. With the novel acoustic-source description, there is a good agreement between the model’s jet-noise predictions and the experimental data for unheated jets for a wide range of frequencies and observer angles for both chevrons and microjets. As the model provides quick and accurate jet-noise predictions, a parametric study is performed to understand the impact of chevrons and microjets on jet noise. Chevron penetration is the underpinning factor for jet-noise reduction, and its optimum is found to be around one-seventh of the nozzle diameter. The number of chevrons has a considerable effect on jet noise, and six is found to be an optimum number of chevrons. The injected mass-flow rate of a system of microjets has a noticeable impact on jet noise, and for 18 microjets, its optimum is found to be around 0.0072 of the main-jet mass-flow rate. There is a good agreement between predicted and measured optimum values. This establishes that the model is indeed capable of assessing and optimizing jet-noise-reduction concepts, and could contribute toward the development of quieter nozzles for future aircraft.