Screech Tones Research Articles

Noise of an Overexpanded Mach 3.3 Jet: Non-Linear Propagation Effects and Correlations with Flow

The noise emitted by an overexpanded round jet at a Mach number of 3.3 and a Reynolds number of 105, computed in a previous study using large-eddy simulation (LES), is investigated. In a first step, the non-linear sound propagation effects are quantified by performing two far-field wave extrapolations from the LES near-field data. The extrapolations are carried out by solving the linearized Euler equations in one case and the full Euler equations in the other, without atmospheric absorption, up to a distance of 240 radii from the jet nozzle exit. The non-linear effects are shown to be quite significant, resulting in a series of N-shaped waves in the pressure signals, and in weaker mid-frequency components and stronger high-frequency components in the spectra. Close to the peak directivity radiation angle, for instance, they lead to about a 8 dB loss and a 6 dB gain at the Strouhal numbers of 0.2 and 1, respectively. In a second step, noise generation mechanisms are discussed by calculating correlations between far-field pressure fluctuations and turbulent quantities in the jet. High levels of correlation are found with the centerline flow fluctuations at the end of the potential core, with the shear-layer flow fluctuations over a large axial distance, and with the centerline density fluctuations between the 3rd and the 5th shock cells. They are attributed to the intermittent intrusion of low-speed vortical structures in the potential core, to the supersonic convection of turbulent structures, and to the shock motions at the screech tone frequency.

Development and application of a point Doppler velocimeter featuring two-beam multiplexing for time-resolved measurements of high-speed flow

A novel point Doppler velocimeter (pDV) based upon the Doppler global velocimetry principle is presented, which is capable of three-component velocity vector measurements at 100 kHz mean rates over extended time periods. In this implementation, two laser beams are multiplexed to illuminate the flow over alternating time windows, providing for a reduction in the number of sensors required. The implications of this multiplexing paradigm coupled with the fundamental limits set by the optical absorption filter are examined in detail, and uncertainties are predicted via instrumentation modeling and representative synthetic flow data. The results indicate that the multiplexing pDV instrument provides the required temporal and velocity resolution for turbulent shear flows at velocities of nominally 500 m/s. As a demonstration and validation of this time-resolved technique, statistics of three-velocity component measurements in a cold, supersonic, over-expanded jet at jet exit Mach number M j = 1.4 (design Mach number M d = 1.65) are presented. Time resolution up to 250 kHz and instantaneous velocity uncertainties between 6.6 and 11.1 m/s were obtained. Comparisons of mean pDV data with laser Doppler velocimetry data are consistent with uncertainty predictions for the technique. The ultimate value of the instrument is exhibited in the analysis of Reynolds stress spectra in the screeching jet, exposing the spatial development of motions at the harmonics of the screech tone, variable phase-coordinated shock motions, and growth of turbulent fluctuations in the developing shear layer of the jet. From the data presented, the screech tone phenomenon is suspected to be linked to the production of radial–azimuthal shear stresses in extended regions beyond the potential core.

The underexpanded jet Mach disk and its associated shear layer

High resolution planar particle image velocimetry is used to measure turbulent quantities in the region downstream of the Mach disk in an axisymmetric underexpanded jet issuing from a convergent nozzle. The internal annular shear layer generated by the slip line emanating from the triple point is shown to persist across multiple shock cells downstream. A triple decomposition based on Proper Orthogonal Decomposition shows that the external helical structure associated with the screech tone generated by the jet exerts a strong influence on velocity fluctuations in the initial region of the annular shear layer. This influence manifests as the external vortices producing oscillatory motion of the Mach disk, and thus a forcing of the internal annular shear layer. The internal shear layer is characterized by a number of azimuthal modes of varying wavenumber and type, including both helical and axisymmetric modes. Finally, the possibility of a previously hypothesized recirculation region behind the Mach disk is investigated, with no evidence found to support its existence.

Harmonics of Jet Screech Tones

It is known that imperfectly expanded supersonic jets undergoing strong screech emit not only the fundamental screech tone but also multiple tones at the harmonics of the fundamental frequency. The dominant direction of radiation of the fundamental screech tone is in the upstream direction toward the nozzle exit. This is necessary to complete the feedback loop responsible for the generation of the screech tone. The harmonic tones, however, do not generally radiate in the same direction as the fundamental. For the th harmonic screech tone, there are distinct beams of radiation. Many of these beams are radiated in the sideline and downstream directions. The purpose of this study is to investigate the generation mechanism of the screech harmonics. It is proposed that the screech tone harmonics are produced by Mach wave radiation from supersonic traveling waves in the jet flow. These supersonic traveling waves are created by the interaction of the nonlinear instability wave of the jet flow and the shock-cell structure in the jet plume as well as by nonlinear wave–wave interaction. Based on the proposed generation mechanism, a formula is derived for the principal directions of radiation of the screech tone harmonics. The agreement of these predicted principal directions of radiation with the experimental measurements of Norum is used to validate the proposed tone generation mechanism.

Aeroacoustics and aerodynamics of impinging supersonic jets: Analysis of the screech tones

The interaction between acoustics and aerodynamics of a supersonic jet is an actual fundamental topic which has been a matter of discussion in the last decades. The present paper is devoted to the experimental analysis of free and impinging jets with particular attention on the effect of an impinging surface on screech tones. The acoustics is studied using free-field microphones, while Particle Image Velocimetry is used to investigate the velocity field. The analysis of acquired data allowed to verify and explain the coupling between acoustic discrete tones and mean and fluctuating flow velocities.

Analyzing the impact of the inlet temperature on the acoustic noise production form a supersonic jet using large eddy simulations

Non-ideal expanded supersonic jets emerging from a convergent-divergent nozzle produce three different types of noise, i.e., the shock-associated broadband noise, the screech noise, and the turbulent mixing noise. Interesting to note that the screech tone outside of the nozzle was exposed mostly in downscaled laboratory experiments, while under realistic conditions the exhaust jet of a gas turbine engine does not show this phenomena. Apart from a geometric scaling difference, usually a lower temperature is employed in experimental studies. It is believed that the screech tone occurs due to self-excitation of the shear-layer in a feedback-loop. Acoustic waves generated by vortical structures interacting with a shock are propagating upstream within the subsonic region of the shear-layer or outside of it. They eventually hit the nozzle's lip and excite instabilities at a certain frequency. The compressible Navier-Stokes equations are simulated numerically by using Large Eddy Simulation approach. The effect of supersonic jet operation temperature onto the associated noise is investigated. The jet-exit Mach-number is 1.56, while the total temperature ratios considered at the inlet plane of the nozzle are 1.27, 2.05, and 3.65. The differences in the near-field acoustics will be presented in each of the cases and the flow-acoustic interaction will be analyzed and quantified.

Reducing the noise emanating from a twin jet nozzle using flexible filaments

A twin jet was tested in anechoic facilities at the University of Arizona and NASA Langley Research Center to determine the effectiveness of flexible filaments in jet noise reduction. Results were strongly dependent on filament diameter and material, the most effective of which was found to be Tex 800 Kevlar. In the best configurations, the filaments consistently eliminated screech tones and reduced overall sound pressure level by 3 dB or more. Additionally, broadband shock noise was diminished by more than 5 dB over certain audible frequency ranges. Larger-scale tests run at NASA showed comparable reductions in overall sound pressure level and broadband shock-associated noise.

Broadband Shock-Associated Noise in Screeching and Non-Screeching Underexpanded Supersonic Jets

The effect of screech tones on the broadband shock-associated noise of underexpanded jets is investigated experimentally. Screech is removed by means of a notched nozzle, and the properties of the broadband shock-associated noise in the screech-free configuration are compared to that in a screeching flow. It is first demonstrated that the suppressing technique used is nonintrusive in that it does not alter the shock-cell structure of the jet plume. It is then shown that screech has an effect on the aerodynamics of the jet, which induces changes in the broadband shock-associated noise. Indeed, screech accelerates the damping of the shock-cell pattern, leading to an attenuation of the broadband shock-associated noise and a shifting of this noise component to higher frequencies. Moreover, a tuning between the peak frequency of the broadband shock-associated noise and the screech frequency is observed. It is also deduced from the directivity of the broadband shock-associated noise in the far field that the convective velocity in the shear layer is modified in the presence of screech tones.

An experimental study of rectangular under-expanded supersonic jets collision

Rectangular under-expanded supersonic jet collision experiment is carried out under different nozzle distances and jet pressures and compared with that in the case of free jet. Experiments indicate that there are four screech tone modes of supersonic jet collision, switched from one mode to another depending on the nozzle distance and jet pressure. Two normal shock waves are present between nozzles as jet pressure is more than 0.5 MPa and nozzle distance is less than 50 mm, radiating a stable screech tone with a frequency of about 3 kHz. With nozzle distance increasing or jet pressure decreasing, a bow shock is present at one nozzle exit and a normal shock wave appears at the other exit with the collision surface oscillating between them. Collision surface might be kept balanced in the centre of two nozzles with a 9 kHz frequency screech tone, however, it is vulnerable to disturbance and would return to the equilibrium position near nozzle exit or oscillate between nozzles with large amplitude. When jet pressure is less than 0.36 MPa and nozzle distance greater than 70 mm, the collision surface substantially oscillates between the nozzles, radiating a screech tone with a frequency of about 1 kHz which decreases with jet pressure decreasing and nozzle distance increasing.

Image Processing-Based Technique for Computation of Axial Sound Source Distribution in a Chocked Screech Jet

A proposed technique to characterize the acoustic and flow fields during screech noise emission of a screech sonic jet is presented. Time-resolved visualization of the flow and acoustic fields under investigations are obtained using a high-speed schlieren optical system. Digital image processing of the acquired visualization is conducted and information on both the acoustic and flow fields is subsequently derived with Fourier analysis. The amplitude and phase of the screech tone in the near field are used to compute the screech-producing sound source distribution along the jet axis using the point source model proposed by Lee and Westley . The technique enables simultaneously the measurements of the screech tone frequencies emitted from the jet and characterization of the associated acoustic and flow fields.

Supersonic Jet Noise Reduction by Microjet Injection

The effect of microjet (μjet) injection on the noise from supersonic jets is investigated. One convergent and three convergent-divergent (C–D) nozzles, covering design Mach numbers ( MD) 1.0 to 2.2, are used in the study; all nozzles have the same exit diameter ( D). The μjets are injected perpendicular to the primary jet at the nozzle lip from six equally-spaced ports; each port has a diameter of 0.0054 D. Effects in the over-expanded and under-expanded regimes as well as one case of fully expanded condition are explored. Relative to the effect on subsonic jets, larger reduction in the overall sound pressure level (OASPL) is achieved in most supersonic conditions. While the injection readily eliminates screech tones with the convergent nozzle it becomes increasingly ineffective in screech reduction with nozzles of higher MD. With all nozzles, the injection is often found to amplify the broadband shock-associated noise; the overall noise reduction occurs due to a suppression of broadband levels especially at lower frequencies. With the shock still within the divergent section of the C–D nozzles, there is ‘excess broadband noise’ and sometimes there are ‘transonic tones’; the injection is found to affect the tones very little but reduce the broadband noise component significantly. For shock-free, fully expanded condition, the OASPL reduction in the peak noise radiation direction is comparable to that in a subsonic case; the same correlation, found earlier for subsonic cases, applies.

Experimental Investigation on Supersonic Jet Screech Tones

In order to investigate the characteristic of the supersonic jet screech tones, an experimental bench of the supersonic jet was designed and a free field noise signal acquisition system was established. Effects of the nozzle size and jet Mach number on jet noise sound field distribution was analyzed, through the result comparison of supersonic jet noise experimental measurement. Results indicate that the field distribution of supersonic jet screech tones is characterized with very strong directivity. Peak value of the screech tones decrease and occurrence frequency of the screech tones increase with the decreasing jet exit Mach number; occurrence frequency of the screech tones decrease with the increasing nozzle size, but the peak value change very less. The experimental measurement of supersonic jet noise provides mechanism research of sound production with data supports and references; and also provides the numerical modeling of supersonic jet noise with validation criteria.

Experimental Investigation on Supersonic Jet Noise

A systematic study has been undertaken to quantify the effects of jet Mach number and nozzle size on the noise radiated by supersonic jets. All the tests were carried out at an experimental bench of the supersonic jet. Results indicate that the field distribution of supersonic jet screech tones is characterized with very strong directivity. Under the textual experimental conditions, if the jet Mach number remain unchanged, the diameter of nozzle throat increases gradually from 5mm to 8mm or 10mm, and the amplitude values of both the turbulent mixing noise and broadband shockwave correlated noise increase by 2-5dB, and the amplitude value change of the whistler type noise is not obvious, and the occurrence frequency of the whistler type noise decreases by more than 2000Hz; if the jet Mach number increases to 3.0 from 2.0, the amplitude value of the whistler type noise increases by more than 2dB, and the occurrence frequency of the whistler type noise decreases obviously. The experimental measurements of supersonic jet noise provide the sound production mechanism research on the supersonic jet noise with data supports and references and provide the numerical modeling of the supersonic jet noise with validation criteria.

デュアルベルノズルの低高度運転時における流体振動

Unsteady flow characteristics in a dual-bell altitude compensation nozzle at low altitude operation modes are examined. At the low altitude operation mode, a jet separation point is fixed at the nozzle wall inflection. However, experimental and numerical results show that three modes of periodic jet fluctuations take place at certain range of nozzle pressure ratios. The lowest frequency mode is a symmetric modes and is considered to be caused by the acoustic resonance in longitudinal direction in the extension part of the nozzle. Two kinds of asymmetric modes have higher frequency than the symmetric mode. According to the numerical results, one of them is a jet screech tone caused by the convection of large scale vortices beside the separated jet. The other mode has the same frequency with a resonance mode in the diametrical direction of the cross section of the extension part.

Adjoint based noise minimization of a round supersonic jet

Jets with complex shock-cell structures appear in numerous technological applications. The shock/shear-layer interaction emanates a broadband noise component. This may trigger the thin shear layer at the nozzle exit, forming a feedback loop which results in a discrete noise component called screech. Both components are undesirable from structural and environmental (cabin noise) points of view. Screech tones produce sound pressure levels of 160 dB and beyond.The focus of the present research project lies in the minimization of supersonic jet-noise and in particular in the minimization of jet-screech. Since screech - a phenomenon which is not yet understood in all details - seems to be affected by the presence of the jet-nozzle, a porous material will be added to the nozzle exit to suppress the feedback mechanism. Thus, to minimize the emanated noise. It is by no means clear how the shape an characteristic properties of the porous material should be. To this end, an optimization technique, based on adjoint methods will be applied to optimize the material with respect to the emanated noise.

Effect of tube lip thickness on the performance of Hartmann–Sprenger tubes

Experiments were carried out to investigate the effect of tube lip thickness on Hartmann--Sprenger (HS) tones in the moderately underexpanded conditions. It is found that the generation of jet regurgitant (JRG) tones and screech tones are independent of the tube lip thickness. The first shock cell length ( L c) and the correctly expanded speed of sound ( a j) are found to be the characteristic scale for length and velocity, respectively for both JRG and screech tones in the moderately underexpanded jets. The average normalized frequency, f L/ a j ( L is the length of the HS tube) of the JRG tones is 0.25. Two new families of tones are generated by the increase of tube lip thickness. The frequency of tones in one of the two families is the same as the general impingement (IG) tones generated by the supersonic jet impinging on flat plates. The frequency of tones of the other family (induced quarter wave) is equal to the frequency of the quarter wave mode of the tube and these tones are induced by IG tones. The end wall pressure (a measure of compression of the residual fluid) is decreased when the IG tone is present together with the JRG tone. Feedback experiments showed that an acoustic feedback to the nozzle has a role in the self-excitation of screech tones and IG tones but their self-excitation mechanism are found to be different. There is no role of an acoustic feedback in the self-excitation of JRG tones.

Understanding How Chevrons Modify Noise in Supersonic Jet with Flight Effects

Tests were conducted to determine the effect of chevrons on the sound radiated from the exhaust of a Mach 1.19 core flowwithsimulated flighteffects.Flighteffectswerestudiedbyusingasecondarycoaxial flowvariedfromstatic to Mach 0.50. Shadowgraphy, static pressure, and turbulence measurements were used to complement near-field pressure and far-field acoustic measurements. The presence of a secondary flow causes the shock-cell length to slightly lengthen and the shock cells to persist farther downstream; this was true for both the baseline and chevron nozzles. Compared with the baseline nozzle, the shadowgraphy and near-field static pressure showed that the chevronsreducedtheshock-cellspacingwithminimaleffectontheshock-cellstrength.Higherturbulencelevelsnear thenozzleexitandcomparableshock-cellstrengthledtohighershock-associatednoiseforthechevronconfiguration compared with the baseline. The decrease in shock-cell length caused the peak amplitude of the shock-associated noise to shift to higher frequencies. At the same time, the chevrons significantly reduced any screech tones that were initiallypresentforthebaselinecase.Finally,itwasshownthatthechevronsreducedturbulencelevelsneartheendof the potential core, which resulted in a reduction of the low-frequency mixing noise that dominates the aft angles.

Far-Field Noise Control in Supersonic Jets From Conical and Contoured Nozzles

Plasma actuators are used to control far-field noise in Mach 1.65 jets from contoured and conical supersonic axisymmetric nozzles (henceforth, contoured and conical jets, respectively). The contoured nozzle is designed using the method of characteristics for a shock-free jet. The conical nozzle has converging and diverging conical sections with a sharp throat. Eight plasma actuators, distributed uniformly around the nozzle exit, are used and the jet is forced with azimuthal modes (m) 0–3 and ±4 and forcing Strouhal numbers ranging from 0.09 to 4.0. The far-field acoustic noise is measured by a linear microphone array covering polar angles from 25 deg to 80 deg relative to the jet axis. In both jets, the lower forcing azimuthal modes (m=0 and 1) are less effective than the higher modes (m=2, 3, and ±4), which have similar levels of overall sound pressure level (OASPL) reduction. At shallow angles relative to the jet axis, the reduction in OASPL is about 1.6–1.8 dB at low forcing Strouhal numbers in both jets at the most effective forcing mode of m=3. However, the OASPL in the sideline direction is only slightly increased (about 1 dB) for both the contoured and conical jets at m=3. The reduction at shallow polar angles is related to the decrease in the peak mixing noise level in both jets. The range of forcing Strouhal numbers providing significant noise reduction and the range of polar angles over which the noise is reduced are both much larger in the conical jet compared with the contoured jet. The screech tones are also reduced or suppressed – most likely due to weakening of naturally occurring structures by forcing.

Study on transonic tone in an axisymmetric supersonic nozzle

This paper describes experimental and numerical works to investigate noise phenomenon in supersonic flow discharged from a convergent-divergent nozzle. The noise phenomenon of flow is generated by an emission of ‘transonic tones’. The results obtained show that the frequency of a transonic tone, that differs from the frequency of a screech tone due to the shock-cell structures in a jet and originates in the shock wave in the nozzle, increases in proportion to the nozzle pressure ratio. The high-order transonic tone has the directivity in the direction of the flow. As for the transonic tone’s frequency, the separated zone was calculated by using a simple flow model considering the propagating perturbation. The results of the model corresponded to the results of this experiment well.

축대칭 초음속 제트에서 스크리치 모드 전이현상의 수치적 연구

Mode transition of the axi-symmetric screech tone in the low supersonic Mach number range from 1.0 to 1.20 is numerically analyzed. The axi-symmetric Navier-Stokes equations and the k-e turbulence model are solved in the cylindrical coordinate system. The dispersion-relation-preserving(DRP) scheme is applied for space discretization and the optimized four levels marching method are used for time integration. At low supersonic Mach numbers with an axi-symmetric A1 mode in the simulation, it is shown that acoustic propagation due to the nonlinear effects is seen in the lateral direction and the screech tone frequency is the same as the vortex passing frequency due to the generation of intense large-scale vortical motions.