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Turbulent jet flows at the Mach number equal to 0.25 from an industrial nozzle are studied using the large eddy simulation (LES). The feasibility of controlling the jet flow and noise by using chevrons is explored. A comparative study is carried out between the baseline and chevrons nozzles with the focus paid to the influences of the chevrons on the flow mixing and noise. The results show that the chevrons can significantly change the flow field. Especially the jet potential core length is reduced. In addition, the contours of velocity in the cross sections of the jet change from circular into daisy shaped structures by the chevrons. Flow visualization using the vorticity and the Q criterion shows that large vortical structures in the shear layer are fragmented into small-scale eddies. Meanwhile, the sound pressure level of the jet noises is weakened by chevrons.

This paper aims to study the microjet’s efficacy as a management tool for the duct’s flow field. The nozzle was correctly expanded for a diameter ratio of 1.6 (i.e., area ratio = 2.56). The Mach numbers considered were from 1.25 to 2. The investigation shows that the development and recovery of the duct flow are smooth at lower Mach numbers. At Mach 1.48, jet noise was reduced considerably when the control is initiated. For higher Mach numbers of the study, namely Mach 1.6, 1.8, and 2.0, the flow’s oscillatory nature was noticed. This phenomenon reiterates that the nozzles flow is wave-dominated. For most of the flow, the flowing nature remains unaltered due to control. The flow remained connected with the duct for duct length twice the nozzle exit diameter.

The noise sources of the tip-jet driven rotor are mainly classified as rotor blade noise and jet noise. The rotor blade noise includes thickness noise, loading noise, and nonlinear quadrupole noise, while the jet noise includes nozzle momentum noise and jet radiation noise. Each noise component has different noise source mechanisms related to flow characteristics and the rotating motion. Especially, the nozzle momentum noise has two noise source mechanisms based on the rotating steady aerodynamic load and the nozzle mass flow rate. The details of the time signal and spectrum are discussed to study the unique spectral characteristics. The tip-jet driven rotor noise has both tonal and broadband spectral characteristics. The periodicity of each instance of jet radiation noise is discussed, while the total jet radiation noise also has a tonal noise component. The noise source mechanisms and spectral characteristics are important considerations for an acoustically efficient tip-jet driven rotor.

This paper deals with the topic of upper surface blowing noise. Using a model-scale rectangular nozzle of an aspect ratio of 10 and a sharp trailing edge, detailed noise contours were acquired with and without a subsonic jet blowing over a flat surface to determine the noise source location as a function of frequency. Additionally, velocity scaling of the upper surface blowing noise was carried out. It was found that the upper surface blowing increases the noise significantly. This is a result of both the trailing edge noise and turbulence downstream of the trailing edge, referred to as wake noise in the paper. It was found that low-frequency noise with a peak Strouhal number of 0.02 originates from the trailing edge whereas the high-frequency noise with the peak in the vicinity of Strouhal number of 0.2 originates near the nozzle exit. Low frequency (low Strouhal number) follows a velocity scaling corresponding to a dipole source where as the high Strouhal numbers as quadrupole sources. The culmination of these two effects is a cardioid-shaped directivity pattern. On the shielded side, the most dominant noise sources were at the trailing edge and in the near wake. The trailing edge mounting geometry also created anomalous acoustic diffraction indicating that not only is the geometry of the edge itself important, but also all geometry near the trailing edge.

An experimental study is conducted on unsteady pressure fluctuations occurring near the nozzle exit and just outside the shear layer of compressible jets. These fluctuations are related to ‘trapped waves’ within the jet's potential core, as investigated and reported recently by other researchers. Round nozzles of three different diameters and rectangular nozzles of various aspect ratios are studied. The fluctuations manifest as a series of peaks in the spectra of the fluctuating pressure. Usually the first peak at the lowest frequency (fundamental) has the highest amplitude and the amplitude decreases progressively for successive peaks at higher frequencies. These ‘trapped wave spectral peaks’ are found to occur with all jets at high subsonic conditions and persist into the supersonic regime. Their characteristics and variations with axial and radial distances, jet Mach number and aspect ratio of the nozzle are documented. For round nozzles, the frequency of the fundamental is found to be independent of the jet's exit boundary layer characteristics and scales with the nozzle diameter. On a Strouhal number (based on diameter) versus jet Mach number plot it is represented by a unique curve. Relative to the fundamental the frequencies of the successive peaks are found to bear the ratios of 5/3, 7/3, 9/3 and so on, at a given Mach number. For rectangular nozzles, the number of peaks observed on the major axis is found to be greater than that observed on the minor axis by a factor approximately equal to the nozzle's aspect ratio; the fundamental is the same on either edge. For all nozzles the onset of screech tones appears as a continuation of the evolution of these peaks; it is as if one of these peaks abruptly increases in amplitude and turns into a screech tone as the jet Mach number is increased.

A model-scale experimental investigation of an installed jet–pylon–wing configuration was conducted at the University of Southampton, with the scope to study the effect a pylon has on noise generation and to clarify its impact on the fluctuating wall-pressure load. The set-up consisted of two single-stream nozzles, a baseline axisymmetric annular nozzle and a partially blocked annular pylon nozzle. The nozzles were tested first isolated and then installed next to a NACA4415 aerofoil ‘wing’ at a single nozzle–wing position. The jet Mach number was varied between $0.5 \leq M_{{j}} \leq 0.8$ and measurements were performed both under static and in-flight ambient flow conditions up to ${M_{{f}} = 0.2}$ . The jet flow-field qualification was carried out using a single-velocity-component hot-wire anemometer probe. The pressure field on the wing surface was investigated using two miniature wall-pressure transducers that were flush-mounted in the streamwise and spanwise directions within the pressure side of the wing. A linear ‘flyover’ microphone array was used to record the noise radiated to the far field. The unsteady pressure data were analysed in both time and frequency domains using multi-variate statistics, highlighting a far-field noise reduction provided by the presence of the pylon only in the installed case. Furthermore, the wake field generated behind the pylon is seen to significantly modify the wall-pressure fluctuations, particularly at streamwise locations close to the pylon trailing edge.

Fine-scale mixing noise (FSMN) and broadband shock-associated noise (BBSAN) are the dominant components of supersonic jet noise in the sideline and upstream directions. We use the previously developed statistical FSMN and BBSAN models to compare the noise radiated from three different nozzles, i.e., a method of characteristics nozzle, a bi-conic nozzle, and a faceted nozzle at different operating conditions. A numerical sensitivity analysis is performed using the models by perturbing various model parameters and conditions such as nozzle pressure ratio (NPR), total temperature ratio, area ratio, and boundary layer thickness. We observed that FSMN is most sensitive to NPR and BBSAN is most sensitive to area ratio. We also examine the changes in source statistics and corresponding correlations of the radiated noise using the fluidic injection noise reduction technique. Noise reduction predictions relative to the baseline cases are compared at different operating conditions and similar reduction as the experimental measurements were obtained at over-expanded conditions. Finally, we analyze the noise source locations for both components of jet noise in the sideline direction. The trends predicted in this study increase understanding of the changes in source statistics and radiated noise for different nozzles over a range of operating conditions.

We study a turbulent jet issuing from a cylindrical nozzle to characterise coherent structures evolving in the turbulent boundary layer. The analysis is performed using data from a large-eddy simulation of a Mach 0.4 jet. Azimuthal decomposition of the velocity field in the nozzle shows that turbulent kinetic energy predominantly resides in high azimuthal wavenumbers; the first three azimuthal wavenumbers, that are important for sound generation, contain much lower, but non-zero amplitudes. Using two-point statistics, low azimuthal modes in the nozzle boundary layer are shown to exhibit significant correlations with modes of same order in the free-jet region. Spectral Proper Orthogonal Decomposition (SPOD) is used to distill a low-rank approximation of the flow dynamics. This reveals the existence of tilted coherent structures within the nozzle boundary layer and shows that these are coupled with wavepackets in the jet. The educed nozzle boundary-layer structures are modelled using a local linear stability analysis of the nozzle mean flow. Projection of the leading SPOD modes on the stability eigenmodes shows that the organised boundary-layer structures can be modelled using a small number of stable eigenmodes of the boundary-layer branch of the eigenspectrum, indicating the prevalence of non-modal effects. Finally local and global resolvent analysis of the mean-flow are performed. It is shown that the most-energetic nozzle structures can be successfully described with optimal resolvent response modes, whose associated forcing modes are observed to tilt against the nozzle boundary-layer, suggesting that the Orr mechanism underpins these organised, turbulent, boundary-layer structures., 38 pages, 31 figures. Submitted to Journal of Fluid Mechanics

Rectangular propulsion nozzles have advantages over circular nozzles, including easier thrust-vectoring and air-frame-integration. Jet noise is easier to study with circular jets (CJ), however, due to azimuthal homogeneity, which, together with low-rank acoustic dynamics enables simpler acoustic models. Using Large Eddy Simulations of rectangular jets (RJ) of various aspect ratios we show that acoustic fluctuation components exhibit comparably rapid convergence in azimuthal Fourier space even for high aspect ratios. A reduced-order model for RJ that retains near-field acoustic asymmetry can be constructed using only three leading azimuthal modes, but with two additional terms relative to CJ.

The split-type conical nozzles with replaceable exit sections with diameters of 30, 40 and 50 mm were designed and produced from steel by machine turning. In addition, the replaceable output parts of the nozzles with the same diameters were produced by additive technology (fused deposition manufacturing) from ABS plastic. In the acoustic anechoic chamber, the noise measurements of a single-stream cold air jet for all the nozzles at jet velocities in the range of 0.3-0.7 Mach numbers were carried out. The noise measurements were performed on distance of 2 m from the center of the nozzle exit section at angles from 30 to 105o. For different directions of noise radiation and different velocities of the jet, the power spectral density and overall sound pressure level were determined. The obtained results demonstrates that the jet noise for nozzles with diameter of 40 and 50 mm from steel and ABS plastic differs by no more than 1 dB, which is within the measurement error for these types of experiments. The nozzles with diameter 30 mm have a higher difference in noise, which can be explained by the more sensitivity of a nozzle with a small diameter to the deviations of geometric parameters when it produced by additive technology.

The noise characteristics of a dual-stream jet of bypass ratios greater than 10 were investigated in response to area ratio, nozzle-operating conditions, installation effect, and chevron effect. The key conclusions were as follows. (1) The area ratio impacted the noise by up to 3 dB at aft angles and at mid-frequency for heated conditions. (2) The spectral variation due to the change of primary stream parameters was constant at low Strouhal numbers and diminished at high Strouhal numbers. (3) The impact of the secondary stream pressure was described by the scaling law of single-stream jets for unheated conditions. The flow parameters of other jets were required to describe the spectra of Strouhal numbers between 0.2 and 2.0, based on the secondary jet for heated conditions. (4) The installed jet noise was quantified for various wing configurations and was more pronounced at forward angles. (5) The chevron nozzle reduced overall noise by up to 4 dB, though it became less attractive when installed. The quantified information is expected to improve the estimation of noise from a dual-stream jet of high bypass ratios.

A fluid injection scheme consisting of multiple radial microjets located downstream from the nozzle exhaust is analyzed for its ability to suppress far field jet noise. Microjets in cross flow are ...

The role of the external boundary conditions of the nozzle surface on the azimuthal mode selection of impinging supersonic jets is demonstrated for the first time. Jets emanating from thin- and infinite-lipped nozzles at a nozzle pressure ratio of $3.4$ and plate spacing of $5.0D$, where $D$ is the nozzle exit diameter, are investigated using high resolution particle image velocimetry (PIV) and acoustic measurements. Proper orthogonal decomposition is applied to the PIV fields and a difference in dominant instability mode is found. To investigate possible explanations for the change in instability mode, additional nozzle external boundary conditions are investigated, including the addition of acoustic dampening foam. A difference in acoustic feedback path is suggested to be the cause for the change in dominant azimuthal modes between the flows. This is due to the thin-lip case containing a feedback path that is concluded to be closed exclusively by a reflection from the nozzle base surface, rather than directly to the nozzle lip. The ability of the flow to form a feedback path that maximises the impingement tone gain is discussed with consideration of the numerous acoustic feedback paths possible for the given nozzle external boundary conditions.

The outflow of a cold supersonic off-design jet with the nozzle pressure ratio of 21.8 is calculated using the high-resolution RANS/ILES method. For the nozzle under consideration and at the given nozzle pressure ratio the exit-to-ambient pressure ratio is 0.6. The jet is characterized by a long supersonic region with many “barrels.” The calculations are validated against the experimental data. The fluctuating and acoustic characteristics of the jet flow under consideration are compared with the characteristics of other supersonic jets. The geometric dimensions of the barrels and the acoustic properties of the jet are estimated from the known empirical relations for supersonic jets.

Experimental studies in the acoustic far field and flow visualization are carried out on circular and non-circular impinging jets at a nozzle to plate distance ratio of 5, for the nozzle pressure ratios of 1.8, 4, and 6. Spectra and directivity of the overall sound pressure level of circular jets are compared with those of non-circular jets for the same nozzle pressure ratios, at various emission angles. Non-circular jets mitigate the transonic impinging tones at around the critical pressure ratio, compared to the circular jet. Square jet is quieter at low nozzle pressure ratios by 5–8 dB compared to the circular jet. At higher under expansion conditions, the triangular and square jets almost completely eliminate the supersonic impinging tones. Elliptical jet is noisy at a nozzle pressure ratio of 6, even compared to the circular jet. Elliptical minor axis plane is quieter at lower nozzle pressure ratios compared to the major axis plane and the trend is reversed for the higher nozzle pressure ratios. Schlieren flow visualization studies reveal that the shock cell structures are weakened and symmetry of the shock cells are lost for the non-circular jets compared to the circular jet.

International audience; The characteristics of acoustic tones near the nozzle of jets are investigated for Mach numbers between M = 0.50 and 2 using large-eddy simulations. Peaks are observed in all cases. They are tonal for M ≥ 0.75 and broaden for lower Mach numbers. They are associated with the azimuthal modes nθ = 0 to nθmax, with nθmax = 8 for M = 0.75 and 1 for M = 2, for example. Their frequencies do not appreciably vary with the nozzle-exit boundary-layer thickness and turbulence and fall in the frequency bands predicted for the upstream-propagating guided jet waves using a vortex-sheet model. For all azimuthal modes, the peak intensities are highest for the first radial guided jet mode. They increase approximately as M^8 for M ≤ 1 and as M^3 for M ≥ 1, following the scaling laws of jet noise, suggesting that they mainly result from a band-pass filtering of the upstream-travelling sound waves by the guided jet modes. In support of this, the Mach number variations of the peak width and sharpness are explained by the eigenfunctions of the guided waves. Moreover, it appears that, for high subsonic Mach numbers, among the waves possibly resonating in the potential core, only those close to the cutoff frequencies of the guided jet modes can contribute to the near-nozzle peaks. Finally, the peaks are detectable in the far field for large radiation angles. For M = 0.90, for instance, they emerge for angles higher than 135° .

Motivated by design challenges related to aerospace propulsive jets, an experimental investigation has been conducted of the high Mach number jet plume flow field from a round convergent nozzle at under-expanded shock-containing conditions. Hot jets up to a total temperature ratio of 3 were considered. Laser doppler anemometry (LDA) measurements in the jet near field (first 15 nozzle exit diameters) captured the turbulent mixing process in detail, enabling the separate effects of compressibility and static temperature ratio (t) on the development of the velocity and turbulence profiles to be identified. Compressibility dominated in the initial shear layer region, whereas temperature effects controlled the downstream jet merging zone. Analysis of shear layer development demonstrated that, at all temperature ratios, a similar, but significantly stronger, damping effect was observed as in planar shear layers (correlated well by convective Mach number Mc). Consideration of the interaction of compressibility and temperature ratio – which reduce/enhance turbulent mixing respectively – provided for the first time a rational explanation of the observation that increasing jet temperature influenced flow development only up to a static temperature ratio t ~ 1.5, after which further increase has little effect. Measurements of the potential core length (Lp) were analysed to produce an empirical correlation that also illustrated the diminishing effects of heat addition at all jet Mach numbers. The data provide the improved understanding and empirical design techniques essential for developing technologies for jet noise and infra red (IR) signature reduction and represent an important validation test case for computational fluid dynamics (CFD) modelling.

Present experimental investigation reports effect of change in nozzle exit geometry on acoustic characteristics of compressible jet in the near and far field at jet exit Mach number 0.8. Three different nozzle configurations used in this investigation are circular, elliptical, and rectangular. Aspect ratio of elliptical and rectangular nozzle is 3. Equivalent nozzle exit diameter of all nozzle configurations is maintained constant as 40 mm. Results are presented by obtaining cross-correlation between near field and far field simultaneously measured pressure fluctuations. Acoustic characteristic along major and minor axis is different for elliptical and rectangular nozzle. Cross-correlation obtained indicates that noise source shifts closer to nozzle exit for elliptical as well as rectangular nozzle.

Passive control for suppressing mixing noise from Co-Flowing Jets (CFJ) is presented in this study. The idea behind this is to reduce the convective Mach number of turbulent eddies that produce intense sound radiation. The present study analyses co-flowing jets with a bypass ratio of 6.3 and the primary nozzle lip thickness of 10 mm. The aim of the study is to find the jet noise level in finite lip thickness in co-flowing jets. CFJ with finite lip thickness forms a recirculation zone (in the near field). The secondary core and recirculation zone are shielding the primary core by reducing the jet noise. A single free jet with a diameter equal to that of a primary nozzle of co-flowing jet is also studied for comparison. The results show that co-flow jet with finite lip thickness of 10mm for various emission angles and the Overall Sound Pressure Level (OASPL) level gets reduced when compared with the single free jet.

This work was part of a major investigation for coaxial jet noise prediction and refers to a study of geometrical and velocity parameters and their influence on the noise generated by coaxial nozzles. The Reynolds Averaged Navier-Stokes (RANS) approach coupled with a fluctuation synthetization model and the integral formulation of Curle's Acoustic Analogy were employed to calculate the noise spectrum and compare it to experimental data from JEAN EU project.

Abstract Jet noise remains a significant aircraft noise contributor, and for modern high-bypass-ratio aero-engines the strong interaction between the jet and aircraft wing leads to intensified installed jet noise. An experiment is carried out in this paper to study the effects of lobed nozzles on installed jet noise. It is found that the lobed nozzles, compared to round nozzles, have similar effects on installed jet noise for all the plate positions and Mach numbers tested. On the shielded side of the plate, the use of lobed nozzles leads to a noise reduction in the intermediate- and high-frequency regimes, which is thought to be due to a combination of enhanced jet mixing and more effective shielding effects by the flat plate. On the reflected side of the plate, noise reduction is only achieved in the intermediate frequency range; the little noise reduction or a slight noise increase observed in the high-frequency regime is likely due to enhanced jet mixing. On both sides of the plates, little noise reduction is achieved for the low-frequency noise due to the scattering of jet instability waves. This is likely to be caused by the fact that lobed nozzles cause negligible change to the dominant mode 0 (axisymmetric) jet instability waves. That the jet mean flow quickly becomes axisymmetric downstream of the jet exit could also play a role. Graphic abstract

The knowledge of jet mixing and its enhancement of elliptic jet are important in a propulsion system of aircraft, rocket, and missile’s system design for advancement of combustion via fuel-air mixture increment, lowering the jet noise and reduction of the plume infrared (IR) signature. The jet issuing from a twin elliptic orifice is non-uniform in shape that promotes the faster mixing and it influences by orifice exit conditions, so knowledge of absence of boundary layer and jet mixing characteristics is important. Hence, an experimental work helps to study the jet mixing for a twin elliptic orifice of aspect ratio two at nozzle pressure ratios of one, two, and three. The proximity between the orifices kept as one to 3mm in steps of one. The experimental readings were taken using pitot probe. The results revealed that jet mixing is faster and effective when the proximity between the orifices is closer to each other than the faraway distances at measured nozzle pressure ratios. Difference in orifice jet core exerted a noticeable influence at high proximity levels of nozzle pressure ratio of three and four for elliptic orifice.

Unconventional aircraft propulsion configurations have to be considered in the future to address environmental issues, including air traffic noise that is know to affect communities surrounding airports. One approach involves rectangular jets in the vicinity of flat surfaces that are parallel to the jet axis in an attempt to shield the noise, but previous experimental work indicated that there is an increase in the noise generated by these configurations, mainly associated with the effect that the plate trailing edge exerts on the flow. In this work, we use large eddy simulations to investigate the potential of wall deformations at the plate trailing edge to reduce jet noise. We consider a high aspect ratio rectangular nozzle exhausting a jet over a flat surface in different configurations, and estimate the farfield noise using the Ffowcs Williams and Hawkins acoustic analogy. Because of the high aspect ratio of the rectangular nozzle, we approximate the jet as being two-dimensional, and use periodic boundary conditions in the spanwise direction. For the configurations that we considered here, the trailing edge deformations did not seem to affect the noise significantly; an overall sound pressure level in the order of 1-2 dB was observed for some selected cases.

Jet study lays the proper foundation for future aircraft ad rocket system design and development, for example, reduce jet noise in rocket, minimized nature of infrared signature in fighter airplanes, etc. Keeping these concerns, the present study is focused on the experimental investigation of twin elliptic orifice of aspect ratio two. The aluminum material is used for the twin orifice by rapid prototyping method. The total exit area of the orifice was maintained as equivalent diameter (Deq) of 7 mm. The test has been carried out at nozzle pressure ratios (NPRs) of two, three, and four by using high-speed jet facility. The proximity between the twin orifices was kept 1, 2, and 3 mm. Centerline pressure decay of jets was measured up to 20 Deq, and these results indicate that the mixing rate of jets was effective at lower nozzle pressure ratio compared to higher nozzle pressure ratio of two and three. In addition, the closest proximity of 1 mm of twin orifice enhances the faster mixing along its length of the jet field of the measured NPRs. The amplitude of pressure oscillations was high at 4 Deq than other locations. The jet mixing was started at 2 Deq and ends at 7, 8, and 9 Deq for 1, 2, and 3 mm proximity, respectively, of measured nozzle pressure ratio range. The reason of the above results depends on orifice geometry, aspect ratio, and nozzle pressure ratio of elliptic jet.

Detailed near- and far-field acoustic measurements were conducted for two circular stepped nozzles with 30 deg and 60 deg design inclinations at over- and perfectly-expanded supersonic jet flow conditions and compared to those for a circular nonstepped nozzle. Far-field acoustic results show that stepped nozzles play an insignificant role in altering noise emissions at perfectly expanded condition. At an over-expanded condition, however, the longer stepped nozzle produces significant noise reductions at the sideline and upstream quadrants, while the shorter stepped nozzle does not. Noise spectra analysis and Schlieren visualizations show that noise reduction can be primarily attributed to mitigations in the broadband shock-associated noise (BSAN), due to the ability of the longer stepped nozzle in suppressing shock strengths at downstream region. Near-field acoustic measurements reveal that the source region, as well as the intensity of turbulent and shock noises, are highly sensitive to the stepped nozzle configuration. Furthermore, BSAN seems to be eliminated by the longer stepped nozzle in near-field region due to the shock structure modifications. Ministry of Education (MOE) Nanyang Technological University Submitted/Accepted version This work was funded by the Singapore Ministry of Education AcRF Tier-2 Grant (Grant No. MOE2014-T2-1-002) and School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore.

In this paper the wall pressure fluctuations induced by a compressible jet overflowing a flat plate were analysed with the scope of providing an analytical prediction of the two-points coherence. The reported approach follows the idea formulated by Efimtsov to model the coherence at relatively high Mach numbers and with a good accuracy in the low wave-numbers region. The database used for the model assessment refers to an experimental campaign carried out into the semi-anechoic chamber of the Roma Tre University using pressure transducers flush mounted at the wall of a rigid flat plate. The jet Mach number was varied within the compressible subsonic regime up to 0.9 and two different nozzle exhaust diameters were considered to analyse the effect of the Reynolds number. The tangential flat plate was installed at 0.75 diameters from the jet axis, a position that reproduces well a realistic jet-wing configuration. The ability of the model to predict the coherence function is verified for the whole range of conditions investigated and the achieved results are discussed with reference to the available literature.

The exhaust of the jet engines used in commercial (subsonic) aircrafts produce a lot of noise known as jet noise. This noise has adverse effects on the population living nearby the airports. However, the noise levels are above the permissible limit and thus need more attention. This paper reviews some of the techniques for noise reduction and mainly focuses on the use of chevron nozzle in reducing the jet noise. Large eddy simulations are performed for a simple nozzle and chevron nozzle with triangular wedges at the rear circumference of the nozzle with same boundary conditions. Acoustic data is collected at different receiver locations to understand the effect of chevrons in jet noise reduction. The result of the simulations shows that chevrons are good attenuators of jet noise.

Essential knowledge of elliptic jets creates an importance in the field of aerospace industry, for example, low jet noise in aircraft/rocket engines, reduced nature of plume infrared (IR) signature in fighter airplanes, and high combustion at combustion chamber by increased rate of fuel–air mixing. The jets which come out from a twin elliptic orifice are non-circular shape that initiate the rapid growth of mixing. Therefore, this current study focuses the aerodynamic mixing of elliptic jet flows for twin elliptic orifice. Two types of orifice jets, namely elliptic orifice facing minor axis and elliptic orifice facing major axis, are investigated at nozzle pressure ratios of 2, 3, and 4. Distances between the orifices kept as 1, 2, and 3 mm. Increasing the proximity between the orifices shows jet mixing rate getting slower. The jet facing major axis plane was efficient in mixing compared jet facing minor axis plane at measured nozzle pressure ratios.

In this present work, jet noise level measurements and flow field characteristics of coaxial flat vane Swirlers are investigated out experimentally to calculate the efficiency of the passive control regime. This induces swirl from six flat vanes with vane angles varying from 0 to 60. Experiments were carried out using high-pressure open jet facility and images are captured through visualization technique. The results of noise levels and flow visualization were obtained for nozzle pressure ratios ranging from NPR 2 to NPR 5 and the same are compared between low and high swirler configurations.

A model-scale test with single-stream nozzle exhaust geometries was carried out at the anechoic chamber of Beihang University in Beijing, China. The spectral characteristics are investigated, and the effects of the following parametric variations are reported in this paper: impact of nozzle operating conditions on spectra; impact of the presence of a plug; and effectiveness of chevron configurations for noise mitigation. The measurement shows that the change of pressure values has more impact on spectra than the change of temperature values. The spectral change due to pressure is shown at all band numbers for unheated conditions whereas it is more pronounced at high-frequency ranges for heated conditions. An impact of the presence of a plug is also clearly observed. The reduction of noise is moderate up to band number of 35, and becomes more significant at higher band numbers. It is observed that chevron nozzles are more efficient at high pressure and temperature values. It is expected that the quantified analysis will be used to develop an empirical model of single jet noise, which will be the baseline for the development of prediction of noise from turbofan engines of high bypass ratios. Keywords: Chevron, Engine noise, Engine plug, Jet noise, Single-stream jet

Jet noise prediction is notoriously challenging because only subtle features of the flow turbulence radiate sound. The article by Brès et al. (J. Fluid Mech., vol. 851, 2018, pp. 83–124) shows that a well-constructed modelling procedure for the nozzle turbulence can provide unprecedented sub-dB prediction accuracy with modest-scale large-eddy simulations, as confirmed by detailed comparison with turbulence and sound-field measurements. This both illuminates the essential mechanisms of the flow and facilitates prediction for engineering design.

Purpose The purpose of this paper is to present the results of mixing promotion and screech frequency of controlled elliptical supersonic jet. Design/methodology/approach Flow field characteristics of low-aspect-ratio elliptical jets are examined at over-expanded, under-expanded and correctly expanded conditions. The tabs are placed at elliptical jet exit along the major and minor axes. Findings The results show that the mixing done by the minor axis is superior to the tabs along major axis. At all pressure ratios, the content of jet noise and the frequency are high for the tabs along the major axis because of increase in the amplitude of screech frequency. Further the tabs along minor axis show a dominance of large-scale vertical structures. In under-expanded conditions, the shock cell shows the rapid change because of the presence of tabs. The tabs along minor axis are making the shock weaker, hence no evidence of axis switching. Practical implications To achieve the greater performance of jet, the authors need to reduce the potential core length of the issuing jet. This can be achieved by implementing different types of tabs at the exit of the nozzle. Originality/value The present paper represents the flow of controlled jet using inverted triangular tabs. By achieving the controlled jet flow, the performance of propulsion systems can be improved. This can be used in systems such as combustion chamber, missile’s noise reduction and thrust vector control.

The GPU CABARET method for solving the Navier–Stokes equations coupled with the Ffowcs Williams–Hawkings scheme for far-field noise predictions is applied for conditions of the NASA SHJAR experiment corresponding to Set Point 3 and 7 in accordance with Tanna's classification. The questions addressed include the sensitivity of the flow and noise spectra solutions to the grid resolution and the inflow condition at the nozzle exit. To study the grid sensitivity, several “hand-made” multi-block curvilinear grids are considered along with a simple hanging-nodes-type grid that was automatically generated with OpenFOAM, whose solutions are cross-verified. To study the effect of the inflow jet condition, the flow and noise solutions based on the laminar inflow condition for Set Point 7 case are compared with the same based on modifying the interior nozzle geometry with a turbulence grid to generate the initial unsteadiness inside the nozzle so that both the centerline velocity fluctuations and the jet Mach number at the nozzle exit are preserved in accordance with the experiment. The numerical solutions obtained are compared with the experimental data and reference LES solutions available in the literature.

Serrated jet nozzles are considered to be an efficient and practical passive control approach for jet noise. However, some fundamental mechanisms of serration effects on jet noise are not fully understood, especially in terms of the sound source. In this paper, a high-fidelity simulation framework using large-eddy simulation (LES) is demonstrated to predict near-field turbulence and far-field acoustics from an ultra-high-bypass-ratio engine with round and serrated nozzles. Far-field sound is predicted using Ffowcs Willams–Hawkings (FWH) integration. The results show that the serrated nozzle increases mixing near the nozzle and hence the turbulence decay rate, reducing the turbulence level downstream. The serrations shift the energy from the low frequencies to the high frequencies and decrease overall sound pressure levels by about 3 dB over the low-frequency range. Sound sources are analysed based on fourth-order space–time correlations. There are six major source components ( R 1111 , R 2222 , R 3333 , R 1313 , R 1212 , and R 2323 ) inside the jet shear layers. The serrations are able to reduce the amplitude of these source terms, causing them to decay rapidly to a level below the round nozzle jet within 2D downstream of the nozzle.

Aeroacoustic jet noise theory describes the relationship between acoustic signals and jet operating parameters. Based on empirical aeroacoustic relationships, a link between peak frequency and amplitude and jet diameter and velocity has been used by volcano acousticians to return the operating parameters of column generating volcanic eruptions. There are however significant differences between the nozzle geometries and structures of flows studied in the aeroacoustic literature and flows emitted from volcanic vents. These differences raise questions as to the validity of applying engineering relationships to volcanic eruptions. Combining particle image velocimetry and acoustic data we investigated the flow structures, acoustic signals and dominant source locations of jet flows emitted from three simple nozzle geometries (convergent, straight and divergent). Flows from the straight and divergent nozzles exited in a more developed flow regime than those emitted from convergent nozzles. While for the convergent nozzles dominant sound sources were located within the jet flow, the dominant signals from the straight and divergent nozzles were located in the region of the nozzle exit. Consequently, for the straight and divergent nozzles it was not possible to differentiate between signals generated by the jet flow and those generated by internal sound sources without beamforming analysis. In order to better replicate the volcanic case and in contrast to studies for industrial applications, our experimental test section was not engineered to minimise internal noise. Without acoustic treatment such as screens, choke plates and paddles, jet noise signals were contaminated by signals generated within the test rig and so, even for the convergent nozzles, the aeroacoustic relationships derived from engineering studies could not be used to retrieve the jet operating parameters. Our results indicate that in the volcanic case, the raw acoustic spectra will contain signals generated by both in-conduit and above-vent processes. Therefore inversions for volcanic jet parameters such as mass eruption rate from standard aeroacoustic relationships may be unreliable.

Jet noise remains the major contributor to airplane noise at take-off. In the past half-century of research, sound generation mechanisms in supersonic jets have been extensively characterized, while the sound sources in subsonic jets are still to be clearly identified. For the last two decades, Large Eddy Simulations (LES) have become a major tool for investigating jet noise sources due to their ability to capture detailed information in turbulent flows and their moderate cost that allows industrial applications. However, many challenges still arise when dealing with complex nozzle geometries and heating effects in the jet core. In this paper, subsonic jets with or without nozzle geometry at Mach number of 0.9 and moderately high Reynolds numbers ranging from 2 × 105 to 1 × 106 are computed using LES, providing a base of validation for different nozzle configurations and operating conditions. In this work, the high-order unstructured LES solver AVBP is combined with Ffowcs Williams and Hawkings’ acoustic analogy on unstructured grids. The vortex pairing phenomenon is evidenced without properly triggering the turbulence in the jet at the nozzle exit. Hence, a non-geometrical tripping, where the firsts prism layers of the mesh at the nozzle wall are removed, is proposed and shown to be a successful method for triggering proper turbulence development in shear layers for the cases with nozzle. Moreover, it is more easily implemented because it does not require any geometrical modifications and it generates more natural turbulence than previous methods, leading the path to actual industrial dual-stream configurations. Both isothermal and heated jet flow cases are performed and validated with existing experimental data in terms of aerodynamics and acoustics, which demonstrates the capacity of an unstructured LES solver to correctly simulate both cold and heated jet noise phenomena.

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 ...

The temporal stability of inviscid parallel shear flows with embedded swirl is investigated. The base flow is divided into three regions: an inner irrotational jet flow, a thin swirl layer on the outer rim of the nozzle, and an external potential flow outside the nozzle. The swirl distribution in the thin shear layer is of solid-body rotation and of Rankine type outside the nozzle. The axial velocity profile in the jet base flow is assumed to be top-hat. The linear dispersion relation was derived and solved numerically. A parametric study on the effect of swirl parameter, S, and the swirl layer thickness fraction, d, on the amplification rates of the unstable modes is performed. A helical instability wave with azimuthal wave number, m = –3, is the most unstable mode with S = 0.8 and = 0.08. Our results indicate that the embedded thin layer swirl is capable of triggering the centrifugal instability waves to promote jet mixing. This powerful technique may be employed to mitigate jet noise without incurring significant thrust loss.

Experimental studies are carried out to reduce the jet noise using co-axial swirlers in the form of curved vanes fixed in an annular passage. The swirl numbers considered for the present work ranged from 0 to 1.31, and the corresponding swirl vane angles ranged from 0 to 60°. The nozzle pressure ratios studied ranged from 1.8 to 6. The acoustic far field study at subsonic conditions revealed the presence of transonic tones for the non-swirl jet. However, swirl eliminates the transonic tones and a weak swirl is most efficient for noise reduction at subsonic conditions. The centerline total pressure measurements indicate the reduced core length for the swirl jets compared to the non-swirl jets. At supersonic conditions, the non-swirl jet emits the highest noise at all the emission angles compared to the swirl jets. The swirl jets are free from screech tones, and have lower amounts of shock associated noise, even at high nozzle pressure ratios. The centerline total pressure measurements and schlieren visualization studies show that shock cell spacing and the number of shock cells are reduced in the swirl jets compared to the non-swirl jet.

A flow resonance accompanied by the emission of acoustic tones occurs in an over-expanded convergent-divergent (C-D) nozzle when operated at comparatively low pressure ratios. This phenomenon is distinguished from conventional screech tones and is referred to as “transonic tones”. In contrast to screech tones, the peak resonant frequency for transonic tones increases with pressure ratio; the peak sound pressure level exceeds 110 dB. In this study, we investigated the basic characteristics of transonic resonance and tones using a circular C-D nozzle in an anechoic room. The effects of the boundary layer trip were also evaluated using a tripping wire for the suppressing transonic resonance and tones. The results of acoustic measurements show that several predominant peaks correspond to transonic tones. However, the boundary layer trip inside the nozzle effectively eliminated these tones and suppressed the unsteadiness of the flow inside the nozzle.

A series of experiments were conducted with nozzle pressure ratios (NPRs) from 2.2 to 8.0 to explore energy transfer mechanisms of under-expanded supersonic jet noise through rectangular nozzles. The results from these experiments resulted in the discovery of a fundamental-frequency-moving phenomenon. The fundamental frequencies were extracted from sound pressure signals for different nozzles, and then by switching modes, energy flows and flow fields' Schlieren structures were compared and analyzed. As a result, two key structural parameters, cross section area and aspect ratio (AR) of the exit, were found to have a contributing effect in jet noise. Single screech tone and the turbulent mixing noise switched, dominated and then disappeared when jet pressure was varied. Besides that, the AR has a more significant effect on the screech tone than another parameter according to the analysis results. Specifically, for AR4 nozzle with cross-section area 20 by 5 mm2, an interesting phenomenon was noticed. As jet pressure decreased after the switching of modes energy moved from the high-frequency tone screech to the low-frequency turbulent noise, and finally gathered in a lower-frequency region, the so called fundamental-frequency-moving phenomenon. Explanations for this fundamental-frequency-moving phenomenon were given based on Powell's acoustic feedback theory and Tam's weakest link theory, as well as analysis on spectra by Schlieren photos.

We report the findings from a theoretical analysis of optimally growing disturbances in an initially turbulent boundary layer. The motivation behind this study originates from the desire to generate organized structures in an initially turbulent boundary layer via excitation by disturbances that are tailored to be preferentially amplified. Such optimally growing disturbances are of interest for implementation in an active flow control strategy that is investigated for effective jet noise control. Details of the optimal perturbation theory implemented in this study are discussed. The relevant stability equations are derived using both the standard decomposition and the triple decomposition. The chosen test case geometry contains a convergent nozzle, which generates a Mach 0.9 round jet, preceded by a circular pipe. Optimally growing disturbances are introduced at various stations within the circular pipe section to facilitate disturbance energy amplification upstream of the favorable pressure gradient zone within the convergent nozzle, which has a stabilizing effect on disturbance growth. Effects of temporal frequency, disturbance input and output plane locations as well as separation distance between output and input planes are investigated. The results indicate that optimally growing disturbances appear in the form of longitudinal counter-rotating vortex pairs, whose size can be on the order of several times the input plane mean boundary layer thickness. The azimuthal wavenumber, which represents the number of counter-rotating vortex pairs, is found to generally decrease with increasing separation distance. Compared to the standard decomposition, the triple decomposition analysis generally predicts relatively lower azimuthal wavenumbers and significantly reduced energy amplification ratios for the optimal disturbances.

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.

International audience; In this study, the methodology relying on LES for the generation of the acoustic sources coupled with an Euler solver for their propagation, previously developed for a free hot supersonic jet, has been applied to a similar jet but impinging on a plate with an exhaust hole. Attention has been paid to the turbulent state of the nozzle exit boundary layer, which is a key parameter for jet noise simulation.

Hydrodynamic instabilities are directly related to large-scale coherent structures that are correlated with jet noise emission. Unravelling and accurately predicting their fundamental dynamics shows a promising direction for designing quieter jet engines. In this study, we analyze high-fidelity large-eddy simulation data of a turbulent Mach 0.4 round jet and a Mach 1.5 chevron jet. Using spectral proper orthogonal decomposition we identify, beyond the well-known Kelvin–Helmoholtz and Orr mechanisms, elongated alternating streamwise streaks of high and low-speed fluid that have been associated with a non-modal lift-up effect in wall-bounded shear flows. In the global three-dimensional domain, the most energetic streaks manifest for azimuthal wavenumber m = 1 and frequency St → 0. Furthermore, for the chevron jet, streaks and streamwise vortices appear due to the presence of the serrated nozzle, and they inherit the periodicity of the nozzle geometry. Finally, local (planar) spectral proper orthogonal decomposition is used to analyze the coherent structures of the chevron jet flow. Near the nozzle exit, antisymmetric and symmetric modes appear to be amplified and linked to the presence of the chevrons/streaks. Further downstream, the most energetic modes share similar characteristics to the ones observed in round jets.