Jet Noise Research Articles

Elliptical Tabs as a Mixing Enrichment Tool for Jet Nozzles

Mixing efficiency helps to thrust gain and jet noise reduction. Aerospace and aviation research communities around the world are constantly demanding cleaner, quieter and more efficient commercial aircrafts. Though there are many sources of noise during flight, one such is deficient mixing of air at the rear of the nozzle, uneven expansion of nozzle and corresponding engine noise is quite dominant. The mixing proficiency is achieved by mixing of cold air at the surroundings with the hot air that exits through the nozzle. To reduce the engine noise, elliptical tabs are employed at the exit of convergent nozzle. These are passive noise controllers which create vortices to the existing flow and entrains cold mass from the surroundings in order to reduce noise level. This paper is concerned with the various shapes and sizes of tabs to test the mixing efficiency at different Mach numbers. Further, it is evaluated by both qualitative and quantitative analysis of shadowgraph system and the pressure identifying method using some of the propitious pressure scanner.

Source Localization of Crackle-Related Events in Military Aircraft Jet Noise

Crackle is a perceptual feature of supersonic jet noise that is related to the presence of acoustic shocks. This study investigates the apparent source locations of the steepest shocklike events in the noise field of a high-performance military jet aircraft using an event-based time-domain beamforming method. This method uses the cross correlation between adjacent microphones to determine the angle of propagation of an ensemble of shock-related events within the time waveform. This angle of propagation is then traced back toward the source to find the apparent source location. Based on the propagation angle, derivative skewness, and overall sound pressure level, the microphone positions along the array are sorted into six groups. Shock events from groups related to crackle perception in the near field originate anywhere from 2 to 14.5 m downstream along the nozzle lip line, with distributions that shift downstream and broaden with increasing engine power. The crackle-related events appear to be generated by Mach wave radiation and large-scale turbulence structure noise.

A proposed wavy shield for suppression of supersonic jet noise utilizing reflections

In this work we propose replacing the conventional flat-surface airframe that shields the engine by a wavy surface. The basic principle is to design a wavy pattern to reflect the incoming near-field flow and acoustic perturbations into waves of a particular dominant frequency. The reflected waves will then excite the corresponding frequency of the large-scale structure in the initial region of the jet’s shear layer. By designing the frequency of the reflected waves to be the harmonic of the fundamental frequency that corresponds to the radiated peak noise, the two frequency-modes interact nonlinearly. With the appropriate phase difference, the harmonic dampens the fundamental as it extracts energy from it to amplify. The outcome is a reduction in the peak noise. To evaluate this concept, we conducted Detached Eddy Simulations for a rectangular supersonic jet with and without the wavy shield and verified our numerical results with experimental data for a free jet, as well as, for a jet with an adjacent flat surface. Results show that the proposed wavy surface reduces the jet noise as compared to that of the corresponding flat surface by as much as 4 dB.

Influence of microjets on flow development for diameter ratio of 1.6 for correctly expanded nozzles

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.

Aeroacoustic modal analysis of underexpanded pipe jets with and without an upstream cavity

The investigation of the aeroacoustics of an underexpanded pipe-cavity jet is carried out experimentally. Two different aspect ratios of the cavity are tested for a wide range of nozzle pressure ratios. Both internal and externally radiated pipe-cavity acoustics are studied. Linear and higher-order spectral analyses are implemented on the unsteady cavity pressure to comprehend the nature of the cavity acoustics and nonlinear interactions of different acoustic modes of the pipe–cavity system. Results show that an increase in depth leads to an enhancement in the nonlinear interactions. Furthermore, the power spectral and overall sound pressure level analyses of pipe and pipe-cavity jet noise radiation are carried out. High-speed schlieren imaging techniques are used to understand jet dynamics. Highly unsteady motion of the jet initial shear layer is observed due to an upstream disturbance of the cavity. In addition, proper orthogonal and dynamic mode decomposition methods are used to extract the spatial and dynamic modes of the jet structure. These methods are used to segregate the cavity associated jet dynamics and screech dynamics.

Research on the underwater noise radiation of high pressure water jet propulsion

High pressure water jet propulsion is a new type of propulsion, which has the characteristics of simple system structure, small energy loss in the water inlet duct, easy vectored control, without additional drag force and so on. Underwater jet noise radiation of high pressure waterjet propulsion is an important factor that would directly determine the stealth performance of underwater vehicles. A co-simulation method combining the CFD (Computational Fluid Dynamics) and CAA (Computational Aeroacoustics) was used in this paper to study and analyze the underwater jet noise radiation characteristics of high pressure water jet propulsion. The Realizable k-ε turbulence model was used to numerically simulate the flow field of waterjet. And the FFT processing of the flow field results were implemented by use of the Actran iCFD module in order to obtain the sound source of the waterjet radiation. Then, the Lighthill acoustic analogy theory was utilized to achieve the underwater jet noise radiation characteristic. Consequently, the effects of inlet velocity on the underwater jet noise radiation characteristics of high pressure waterjet propulsion were researched. The results indicated that the underwater jet noise radiation of high pressure water jet propulsion mainly comes from the mixing area where is the 8–10 times diameter away from the nozzle outlet, and the increase of the inlet flow velocity would enlarge the sound pressure level of the underwater jet noise radiation. In addition, the underwater noise jet radiation is directional and mainly propagates in the axial direction. However, the inlet velocity has little influence on the frequency domain characteristics of underwater jet noise radiation and the underwater jet noise radiation has high energy below the frequency 200 Hz. The research provides theoretical guidance for reducing the underwater jet noise radiation of high pressure waterjet propulsion and improving the stealth performance of underwater vehicles.

Jet Noise Reduction in Co-Flowing Jets with Finite Lip Thickness

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.

Aeroacoustics Analysis of a Dual-Stream Subsonic 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.

Study of the Possibility Of Replacing an Integrated Airframe with a Flat Polygonal Screen to Estimate The Efficiency of Noise Screening of Engines Using Geometric Diffraction Theory

Using the example of an integrated airframe, we performed a computational and experimental study of the possibility of using geometric diffraction theory (GDT) to calculate the sound diffraction when the airframe is replaced with simulating flat polygonal screen as pertains to the effectiveness of noise screening of aircraft power plants. We compared (1) impulse responses experimentally measured with maximum length sequences and the reciprocity theorem for both a three-dimensional model of the airframe and its plane model with (2) the responses calculated using GDT; the results showed good qualitative agreement for observation points in the geometric shadow zone and in the illuminated area, as well as quantitative coincidence in the frequency range characteristic of jet noise and the first harmonics of the blade passing frequency of a propeller or fan.

Flow and Noise Predictions of Coaxial Jets

Flow and noise solutions of the two Large-Eddy Simulation (LES) approaches are evaluated for the jet flow conditions corresponding to a benchmark coaxial jet case from the European Union Computation of Coaxial Jet Noise experiment. The jet is heated and issues for a short-cowl axisymmetric nozzle with a central body at a transonic speed. The first LES method is based on the Compact Accurately Boundary-Adjusting High-Resolution Technique (CABARET) scheme, for which implementation features include asynchronous time stepping at an optimal Courant–Friedrichs–Lewy number, a wall model, and a synthetic turbulence inflow boundary condition. The CABARET LES is implemented on Graphics Processing Units. The second LES approach is based on the hybrid Reynolds-average Navier–Stokes (RANS)/implicit LES method that uses a mixture of a high-order Roe and Monotonicity-Preserving reconstruction of the 9th order (MP9 scheme) and a wall distance model of the improved delayed detached-eddy simulation type. The RANS/implicit LES method is run on a Message Passing Interface (MPI) cluster. Two grid generation approaches are considered: the unstructured grid using OpenFOAM utility snappyHexMesh and the conventional structured multiblock body-fitted curvilinear grid. The LES flow solutions are compared with the experiment and also with solutions obtained from the standard axisymmetric RANS method using the turbulence model. For noise predictions, the LES solutions are coupled with the penetrable surface formation of the Ffowcs Williams–Hawkings method. The results of noise predictions are compared with the experiment, and the effect of different LES grids and acoustic integration surfaces is discussed.

Near to far field relationship of crackle-related events in military aircraft jet noise

Crackle perception in supersonic jet noise is attributed to the presence of acoustic shocks in the waveform. This study uses an event-based beamforming method to track shock events as they propagate from the near to the far field of a high-performance military aircraft operating at afterburner. Near-field events are propagated via a nonlinear model and compared with far-field measurements. Comparisons of overall sound pressure level and spectra validate the use of the nonlinear model. The skewness of the time-derivative pressure waveform, or derivative skewness, a metric indicative of jet crackle perception, is greatly overpredicted for nonlinearly propagated waveforms. Cross-correlation coefficients of waveform segments centered about the near-field beamformed events reveal that for farther aft angles, near-field events are more related to far-field measurements. Waveform observations show that shock-like events in the near field that are more spiked in nature tend not propagate into the far field. However, near-field, large-derivative events with broader, high-pressure peaks nonlinearly steepen and form shocks in the far field that are likely responsible for crackle perception.

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.

Holographic reconstructions of large-eddy simulations of a highly heated laboratory-scale jet

Near-field acoustical holography allows for the reconstruction of a sound field from a limited measurement. For aeroacoustic noise generated by military aircraft, reconstructions of the pressure field within the jet plume have provided insight into equivalent acoustic noise source distributions, though the relationship between these equivalent sources and actual flow pressures and velocities within the plume remain unknown. This study reconstructs the sound field of a highly heated laboratory-scale jet simulated by the Naval Research Lab's Jet Noise Reduction (JENRE) large eddy simulation solver. The simulated data allow for comparisons of the holographic reconstructions of the pressure and particle velocity fields to points along the nozzle lipline, to the Ffowcs Williams and Hawkings integration surface (FWHS), and to the far field. Pressure field reconstructions outside and inside of the FWHS match well, while particle velocity reconstructions follow similar trends as the velocities generated by the LES but severly underestimate the time harmonic complex amplitdues.

Large‐eddy simulation of subsonic turbulent jets using the compressible lattice Boltzmann method

SummaryThe lattice Boltzmann method (LBM) is a powerful technique for the computational modeling of a wide variety of single‐s and multiphase flows involving complex geometries. Although the LBM has been demonstrated to be effective for the solution of incompressible flow problems, there are limitations when this methodology is applied to the solution of compressible flows, especially for flows at high Mach numbers. In this article, we investigate strategies to overcome some of the limitations associated with the application of LBM to compressible flows. To this purpose, one of the key contributions of this study is the synthesis and integration of previous efforts concerning the formulation of LBM for the large‐eddy simulation (LES) of compressible turbulent flows in the subsonic flow regime. It is shown how certain limitations of applying the LBM to compressible flows can be addressed by using either a higher order Taylor series expansion of the Maxwell–Boltzmann equilibrium distribution function or using the Kataoka and Tsutahara (KT) LBM model formulation for compressible flows. The proposed LBM/LES methodology for compressible flows has been combined with the Kirchhoff integral formulation for computational aeroacoustics and used to simulate the flow and acoustic fields of compressible jet flows at high subsonic speeds with practical relevance for providing a better understanding of problems associated with jet noise. In this context, simulations of the physics associated with the jet flow and concomitant noise in the near‐ and far‐field regimes were conducted using the proposed framework of a compressible LBM/LES and Kirchhoff integral method. The results of the subsonic isothermal and nonisothermal jet flow simulations for the flow and acoustic fields have been compared with available numerical and experimental results with generally good to excellent agreement.

Using the Correlation Model of Random Quadrupoles of Sources to Calculate the Efficiency of Turbulent Jet Noise Screening with Geometric Diffraction Theory

The paper presents the results of a computational–experimental study of jet noise reduction using the screening effect. According to correlation theory, the noise sources of a single circular jet are represented by a set of uncorrelated quadrupoles. Using the basic relations of geometric diffraction theory (GDT), expressions are obtained for the sound field emitted by a point quadrupole source located near an acoustically rigid infinite half-plane. Using the expressions obtained in the framework of the GDT, the correlation model of the jet noise sources was adapted to the calculation of the sound field in the presence of a flat rectangular screen. A comparison of the calculated and experimentally measured spectra of sound pressure levels, performed at a given jet velocity and various screen positions, showed good qualitative, and for certain observation angles, quantitative agreement.

Numerical investigations on acoustic environment of multi-nozzle launch vehicle at lift-off

The acoustic environment significantly influences the safety and reliability of the launch vehicle at lift-off. Aeroacoustic has always been the focus of attention in the field of aerospace. In the paper, the flow field and acoustic environment caused by multiple jets of the launch vehicle at lift-off are numerically simulated using the three-dimensional compressible Navier-Stokes equation, second-order Roe scheme, scale-adaptive simulation (SAS) and acoustic analogy method (FW-H). The jet noise of single-nozzle launch vehicle are numerically simulated for the verification of numerical methods solving the acoustic, and the relative error in between the numerical results and the experimental data is less than 1.6%. Then the influence of the position of the tower, the vehicle's altitude and the configuration of the deflector on the acoustic environment of the vehicle is studied. The results show that the overall sound pressure level (OASPL) reaching the vehicle with the one-sided jet deflector is larger than that of the vehicle with the two-sided jet deflector. Compared with the initial lift-off altitude, the vehicle's altitude at 12 times of the nozzle-exit's diameter can increase the acoustic load around the vehicle. However, the position of the tower has little effect on the OASPL reaching the vehicle with the one-sided jet deflector. The analysis method provides reference for noise prediction and acoustic environment control of the launch vehicle.

Flow and aeroacoustic characteristics evaluation of microjet noise reduction concept in the nozzle design for minimum quantity lubrication

Minimum quantity lubrication (MQL) is an efficient and sustainable technology, which has been considered to be an option of conventional flood cutting process. Even though MQL provides green solution for the cooling and lubrication function during machining, it brings about unendurable aeroacoustic jet noise from the high velocity spray. The acoustic pollution has seriously affected the operator's health and efficiency so that it should be paid much attention to. In this paper, with the utilization of microjets noise reduction concept, a noise cancellation accessory for MQL nozzle has been developed. The flow field and aeroacoustic characteristics of the original nozzle and microjets have been investigated numerically to help understand the noise reduction mechanism. Variables including the main jet flow rates, the air flow rate ratios of the microjets to the main jet and the measuring azimuths on noise directivities and sound pressure level (SPL) have been analysed. It has found the flow ratio is a critical parameter, which has an optimal value in regard to the main jet flux. The simulation results have been partially verified through OASPL measurement. The overall aeroacoustic noise reduction varied from 0.70 to 4.88 dB has been observed experimentally. Finally, the noise cancellation mechanism has been discussed from the flow field and the frequency characteristics points of view. The shrinking effect of microjet injection at appropriate air flow ratios has significantly narrowed the distribution of the turbulent intensity and reduced the SPL of low and medium frequency while increased that of the high frequency in specific directions. The overall amplitude of OASPL has been reduced. However, inappropriate air flow ratios will generate additional jet noise.

Mixing Characteristics and Enhancement of Sonic Elliptic Jets from a Twin Elliptic Orifice

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.

Numerical investigation of the bevelled effects on shock structure and screech noise in planar supersonic jets

Rectangular supersonic jets exist widely in propulsion systems of aircrafts. When they are imperfectly expanded under certain conditions, the upstream traveling waves referred to as screech tones will be produced, which may cause structural fatigue failure. In this work, high fidelity simulations are employed to investigate the bevelled effects due to the asymmetric lips of nozzles on shock structures and screech noise in planar supersonic jets. The present results are in agreement with previous experimental and numerical data for the symmetric case. For asymmetric cases, it is found that the bevelled effects will affect the shear layer transition, noise radiation, and shock cell oscillations. The level of screech noise generally decreases with increasing the length difference of two lips. The maximum 7.9 dB drop is identified, and the deflection angle of the mainstream of 9.35° is achieved when this length difference reaches the height of the nozzle. Moreover, dynamic mode decomposition (DMD) is specifically utilized to analyze shock cell oscillations. The results show that the bevelled effects suppress the most energetic DMD mode, corresponding to the dominant frequency of shock screech. The phenomenon of shock leakage is detected in the symmetric case, which is assumed to be an important screech noise source, while it seems to be weakened when the nozzle is bevelled. The longitudinal flapping motion of shock cells is substantially weakened due to the bevelled effects, which might be responsible for the suppression of shock leakage and the screech noise reduction.

Acoustic Power Prediction of High BPR Turbofan Engine with Chevron Nozzle during Take-off and Landing

Aircraft noise is a major cause of noise pollution around the vicinity of an airport. Jet noise from engines is a common condition, which has considerable impact on overall sound levels. Adding chevron pattern at the fan nozzle is one of the noise reduction enhancements found in current turbofan engine design features. This paper aims to study the power level of sound created in jet shear from the combination of ambient stream, fan and exhaust gas. Computational fluid dynamic modelling of a high bypass turbofan engine with a round shape and chevron nozzle were carried out during takeoff and landing conditions. It was found that during take-off, the acoustic power level of the engine without a chevron nozzle was 105.95 dB, while the chevron nozzle was capable of reducing the sound level to 43.57 dB. During landing, the sound level of the round shape and chevron nozzle model were 145.77 dB and 47.51 dB, respectively. The turbulent intensity area at the fan cowling nozzle, where the ambient air stream and bypassed air from the fan mixed together, produced the highest sound power level. Therefore, slower aircraft speed during final approach produced a higher noise level than during take-off due to higher turbulent intensity and larger jet shear area around the high bypass fan cowling nozzle. The chevron pattern nozzle was capable of reducing turbulent intensity, which in turn reduced the acoustic power level.