Jet Screech Tones Research Articles

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.

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.

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.

Numerical simulation of the three-dimensional screech phenomenon from a circular jet

Despite great advances on jet screech research during the past few decades, the prediction of jet screech amplitudes still remains a challenging task. The main objective of this paper is to develop an accurate three-dimensional computational aeroacoustic procedure for the simulation of screech phenomenon from an underexpanded supersonic circular jet. The three-dimensional Navier–Stokes equations and a modified two-equation standard k-ϵ turbulence model are solved in the generalized curvilinear coordinate system. A dispersion-relation-preserving scheme is utilized for space discretization. The 2N storage low-dissipation and low-dispersion Runge–Kutta scheme is applied for time marching. Numerical results are presented and compared with available experimental data over Mach number range from 1.17 to 1.60. The predicted shock cell structure and radial density profiles agree very well with the measured results by Panda and Seasholtz [“Measurement of shock structure and shock-vortex interaction in underexpanded jets using Rayleigh scattering,” Phys. Fluids 11, 3761 (1999)]. In particular, it is shown that not only the predicted wavelengths, but also the amplitudes of the flapping and helical modes are in good agreement with experimental data by Ponton et al. [“Near field pressure fluctuations in the exit plane of a choked axisymmetric nozzle,” NASA Tech. Memo TM-113137, 1997]. It is found that although the instantaneous flow fields accompanying the flapping and helical jet screech tones are not axisymmetric yet, the long time average mean flow field is still almost axisymmetric. This is because of the slow rotation of the flapping plane around the jet axis. The real time pressure signal of a Maj=1.30 screeching jet at r∕D=2 in the nozzle exit plane is analyzed. The results indicate that the acoustic field of the simulated flapping and rotating motions of the screeching jet agree well with Ponton and Seiner’s experimental measurements [“Acoustic study of B helical mode for choked axisymmetric nozzle,” AIAA J. 33, 413 (1995)].

An experimental study of the nozzle lip thickness effect on supersonic jet screech tones

It is well known that screech tones of supersonic jet are generated by a feedback loop driven by the instability waves. Near the nozzle lip where the supersonic jet mixing layer is receptive to external excitation, acoustic disturbances impinging on this area excite the instability waves. This fact implies that the nozzle lip thickness can influence the screech tones of supersonic jet. The objective of the present study is to experimentally investigate the effect of nozzle-lip thickness on screech tones of supersonic jets issuing from a convergent-divergent nozzle. A baffle plate was installed at the nozzle exit to change the nozzle-lip thickness. Detailed acoustic measurement and flow visualization were made to specify the screech tones. The results obtained obviously show that nozzle-lip thickness significantly affects the screech tones of supersonic jet, strongly depending on whether the jet at the nozzle exit is over-expanded or under-expanded.

Numerical simulation of the generation mechanism of axisymmetric supersonic jet screech tones

In this paper an axisymmetric computational aeroacoustic procedure is developed to investigate the generation mechanism of axisymmetric supersonic jet screech tones. The axisymmetric Navier-Stokes equations and the two equations standard k-ϵ turbulence model modified by Turpin and Troyes [“Validation of a two-equation turbulence model for axisymmetric reacting and non-reaction flows,” AIAA Paper No. 2000-3463 (2000)] are solved in the generalized curvilinear coordinate system. A generalized wall function is applied in the nozzle exit wall region. The dispersion-relation-preserving scheme is applied for space discretization. The 2N storage low-dissipation and low-dispersion Runge-Kutta scheme is employed for time integration. Much attention is paid to far-field boundary conditions and turbulence model. The underexpanded axisymmetric supersonic jet screech tones are simulated over the Mach number from 1.05 to 1.2. Numerical results are presented and compared with the experimental data by other researchers. The simulated wavelengths of A0, A1, A2, and B modes and part of simulated amplitudes agree very well with the measurement data by Ponton and Seiner [“The effects of nozzle exit lip thickness on plume resonance,” J. Sound Vib. 154, 531 (1992)]. In particular, the phenomena of modes jumping have been captured correctly although the numerical procedure has to be improved to predict the amplitudes of supersonic jet screech tones more accurately. Furthermore, the phenomena of shock motions are analyzed. The predicted splitting and combination of shock cells are similar with the experimental observations of Panda [“Shock oscillation in underexpanded screeching jets,” J. Fluid. Mech. 363, 173 (1998)]. Finally, the receptivity process is numerically studied and analyzed. It is shown that the receptivity zone is associated with the initial thin shear layer, and the incoming and reflected sound waves.

Numerical simulation of jet screech tones

Screech tones are discrete frequency sounds radiated from imperfectly expanded supersonic jets. The tones are generated by a feedback loop driven by the large-scale instability waves of the jet flow. In the plume of an imperfectly expanded jet is a quasiperiodic shock cell structure. Screech tones are produced by the interaction of the instability waves and the shock cells as the former passes through the latter. Although the screech phenomenon has been studied over many years, many of its features are still not understood. Presently, there are accurate tone frequency prediction formulas. However, there is no known way to predict tone intensity. In this study, the screech phenomenon is investigated through numerical simulation using CAA methods. The simulation reproduces all the characteristic features of jet screech, including the staging phenomenon. The computed screech frequencies and intensities are found to compare well with experimental measurements. Numerical results on the effects of jet temperature also compare well with experiments. The favorable agreements indicate that numerical simulation is a valuable tool for investigating the jet screech phenomenon. [Work supported by NASA Lewis Research Center Grant NAG 3–2102.]

Screech Tone Noise and Mode Switching in Supersonic Swirling Jets

An experimental study of mode switching phenomena of supersonic jets with swirl is reported. A convergent nozzle with four tangential inlets was used. The pressure ratio corresponds to a fully expanded Mach number up to 1.80. Schlieren visualization of shock structures and pressure measurements indicated effects of swirl, such as e ow recirculation. Screech tone frequencies and phase information were measured for both nonswirling and strongly swirling jets. Results suggest existence of quasiperiodic shock structure in strongly swirling jets in spite of swirl-generated recirculation. However, pressure e elds are quite different between swirling and nonswirling jets. Both helical and toroidal screech tones were found to exist in strongly swirling jets. Strouhal numbers of the swirling jet screech tones are compared with published theory for nonswirling jets.

Numerical simulation of the generation of axisymmetric mode jet screech tones

Numerical simulation of the generation of axisymmetric mode jet screech tones

Supersonic Jet Screech Tone Cancellation

A new method of supersonic jet screech tone reduction is presented. The method utilizes a sound reflecting surface positioned upstream of the nozzle exit a distance of one-quarter wavelength of the fundamental screech tone. The reflector establishes a standing wave pattern of acoustic waves with a node at the nozzle exit plane. The pressure minimum at the exit halts the screech tone feedback mechanism. Experimental results indicate that the method eliminates supersonic jet screech as effectively as the currently accepted technique using an intrusive tab, but without distortion of the jet flow. The change in shock cell spacing, which occurs with an intrusive tab, does not occur when screech is cancelled with the new technique. The broadband shock-associated noise is also influenced much less when the jet screech tones are eliminated by the new method.