Screech Tone Frequency Research Articles

The source localization and dynamical evolution of axisymmetric screech modes in underexpanded supersonic jets

High order numerical simulations of underexpanded supersonic cold jets issuing from a circular sonic nozzle are conducted to investigate the effective source location of axisymmetric mode screech tones. The frequency and sound pressure level of screech tone agree well with the experimental results in literature. The detailed generation processes of axisymmetric screech modes in a complete cycle are exhibited through investigating the dynamic evolutions of the flow structures and acoustic waves. By implementing spectral proper orthogonal decomposition (SPOD) of jet velocity field and pressure field, the corresponding SPOD modes to the A1 and A2 screech modes are extracted respectively and their evolutions along streamwise direction are analyzed. The source location of A1 mode and A2 mode screech tones were revealed based on the dynamic evolution analysis and the Fourier mode decomposition. It is worth noting that the screech waves of each axisymmetric screech mode radiate from an isolated discrete source rather than the distributed sources at a given jet Mach number. Another interesting finding is that the effective source position of screech A2 mode varies with jet Mach number.

Structure and Screech Tone of Radially Underexpanded Jet Emitted by Facing Cylinders

An underexpanded jet has typical shock-cell structure and strongly oscillates, its behavior being known to cause many industrial problems. An underexpanded jet radially issues from intake and exhaust valves of an internal combustion engine, a pressure control valve and so on. When a supersonic jet exhausted from a circular nozzle impinges on a flat plate, the wall jet formed on the plate often becomes underexpanded and spreads out radially. Such underexpanded impinging jet is one of models of supersonic jets on laser cutting process and glass tempering process. In this study, an underexpanded jet radially discharged from a circular slit nozzle, which consists of two cylinders, was experimentally examined for different nozzle pressure ratios and for different diameters of cylinder. Jet structure was analyzed by means of a visualization e.g. Schlieren method. A noise emitted from the jet was measured and the frequency of screech tone was analyzed. The experimental results were compared with those of a two dimensional jet issuing from a rectangular nozzle. Furthermore, a comparison of visualized sound waves with the screech tone frequency reveals that the sound source measured was in the vicinity of the end of the second cell and that the length of the second or third cell was one of the most important parameters to determine the frequency of the emitted screech tone.

Underexpanded Supersonic Jets from Elliptical Nozzle with Aft Deck

An experimental investigation of an underexpanded supersonic jet issuing from a circular nozzle, an elliptical nozzle, and an elliptical nozzle with an aft deck at different nozzle pressure ratios (NPRs) is reported in this paper. Static-pressure measurements over the nozzle walls were analyzed to understand the internal flow structure. Schlieren visualization and conical probe measurements were conducted to elucidate the development of the jets issuing from the nozzles. The sound-pressure level in the near field was measured, and it was found that the screech amplitudes were 26 dB lower in the elliptical nozzle with an aft deck. The screech-tone frequencies were observed to be above and near audible region (16 kHz) in the presence of the aft deck at NPRs 3 and 4. The jets emanating from the circular and truncated elliptical nozzles were observed to be flapping about the jet axis at NPRs 3 and 4. The unsteadiness of the jet was reduced by locking the jet onto the aft deck. The centerline Mach-number decay revealed that, due to the screeching effect, the truncated elliptical nozzle has a shorter potential core length and larger jet spreading. The flow separation and reattachment were observed over the aft-deck surface for higher NPR conditions.

Transforming the Shock Pattern of Supersonic Jets Using Fluidic Injection

Double shock diamonds establish in the exhaust of modular convergent–divergent nozzles. These consist of two shock structures: one originating from the nozzle throat, and another from its exit. Analyzing the shock pattern developing for different fluidic injection operating conditions, it is shown that fluidic injection allows the rearrangement of the shock structures relative to each other. Overlapping the two structures causes large pressure oscillations in the exhaust and high amplitudes of shock associated noise, whereas staggering the shock structures mitigates these effects. The screech tone frequency does not change for all injection operating configurations, although the shock diamonds are shifted drastically with respect to each other. Hence, the screech phenomenon is dominated by the primary shock spacing originating from the nozzle throat.

Staging Behaviour in Screeching Elliptical Jets

The staging behaviour of jet screech in an elliptical nozzle with with an aspect ratio AR = 2.0 is examined across a range of pressure ratios. Acoustic measurements and high resolution imaging are used to assess the staging behaviour. Based on the frequency of the dominant screech tone, the jet staging behaviour can be separated into five distinct modes over the range of pressures studied. A statistical analysis of the schlieren images suggests that the first mode is varicose, the next three modes are flapping instabilities, and the final mode is helical in nature. A comparison of characteristic length scales in the jet is undertaken to explain the mode switches. A shift from the classical “weakest link” feedback model to an acoustic waveguide model, associated with a shift in characteristic length scale of the jet, is used to explain the discontinuous jumps in screech tone frequency. The results indicate that the different acoustic feedback mechanisms need not necessarily be associated with a particular instability mode shape.

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.

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.

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.

A multi-mode screech frequency prediction formula for circular supersonic jets

A time evolution analysis is presented for the interaction between the instability waves, shock cells, and screech tones based on the authors' previous numerical simulation database. An attachment and reinforcement process of the upstream propagating screech waves with the downstream hydrodynamic waves is identified and recognized as part of the screech loop. The first five shock cells are recognized as the effective sound source region. Through an analysis of the phase variation in the dominant pressure fluctuations for several typical Mach number screeching jets, it is found that the total number of the instability waves and the upstream feedback sound waves in the effective source region can be identified as 5 for the A1, B, and D modes and 6 for A2 and C modes, respectively. A screech tone frequency prediction formula is thus proposed based on this relation. The predicted screech wavelengths or Strouhal numbers of cold and hot jets all agree well with the experimental data by other researchers, except for a small discrepancy for the B mode. It is also noticed that the measured two A0 modes by Ponton et al. [NASA Technical Memorandum No. 113137, Langley Research Center (1997)] can be classified to A1 and A2 modes, respectively, according to the proposed formula.

Effect of Nozzle Lip Shape on Screech Tone in a Supersonic Jet

The effect of nozzle lip shape on screech tone is investigated by a computational aeroacoustic approach. First, the scattered patterns of screech tone are examined without a jet mean flow, where plane waves interact with a nozzle lip wall. Then, an axisymmetric jet screech is simulated in the Mach number range from 1.07 to 1.2, where the axisymmetric mode is a dominant screech mode. The conservative form of the axisymmetric Euler equations written in generalized coordinates are used to treat the complex nozzle lip geometry for the scattering problem as well as the Reynolds Averaged Navier-Stokes equations with the modified Spalart-Allmaras turbulence model. The computed shock-cell structure, screech tone frequency, and sound pressure levels in the near field are in good agreement with existing experimental data.

Jet Screech Noise Computation

The near-field screech-tone noise of a typical underexpanded circular jet issuing from a sonic nozzle is simulated numerically. The self-sustained feedback loop is automatically established in the simulation. The computed shock-cell structure, acoustic wave length, screech tone frequencies, and sound pressure levels in the near field are in good agreement with existing experimental results.

Linear Solutions for the Emission of Screech Tone in Supersonic Jets

The article presents a critical analysis of the solutions of the linear screech tone problem obtained by the author in the 1970s and compares them with modern experimental results. The comparison shows that the proposed model, despite its many simplifications, on the whole provides a faithful description of the screech tone phenomenon, including fine perturbed flow structures in a pulsating jet and its external acoustic field. The modified formula for the screech tone frequency fits the experimental observations.

Three-Dimensional Numerical Simulation of the Jet Screech Phenomenon

Three-dimensional numerical simulations of the jet screech phenomenon are carried out using the multiple-size-mesh, multiple-time-step, dispersion-relation-preserving scheme. The simulations reproduce all of the experimentally observed characteristic features of the screech tones, including the axisymmetric A 1 and A 2 modes, the flapping/helical B and C modes, and the staging phenomenon. Screech tone frequencies as well as intensities calculated by the numerical simulations are in excellent agreement with experimental measurements. Power spectral analysis of the numerical simulation data provides irrefutable evidence that, whenever two tones are measured in the noise spectrum, it is not because there is a continuous switching from one tone to another, but because the two tones actually coexist simultaneously. A first explanation of why there are two axisymmetric and two flapping/helical screech modes based on two feedback mechanisms is offered. Strong evidence is provided to support the proposed mechanisms.

Effects of Jet Temperature and Nozzle-Lip Thickness on Screech Tones

Imperfectly expanded supersonic jets, invariably, generate discrete frequency sound known as screech tones. In the low supersonic jet Mach-number range of 1.0-1.25, the screech phenomenon is axisymmetric. In this investigation the effects of jet temperature and nozzle-lip thickness on the axisymmetric jet screech modes are studied by numerical simulation. At the present time there is no known way to predict the intensities of screech tones, even empirically, except by numerical simulation. Numerical results of screech frequencies and directivities of heated jets will be reported. The computed screech frequencies are found to agree well with experimental measurements. Computed tone intensities indicate a gradual decrease in the screech amplitude with an increase in jet temperature. Screech tones are known to be sensitive to the presence of acoustic reflection surfaces. Sound reflected off such surfaces could sometimes affect the feedback loop that drives the screech tones. The nozzle lip is a good reflection surface. As a part of this work, the effect of nozzle-lip thickness on screech tone frequency and intensity is investigated. There are good agreements between the computed tone frequencies, intensities, and experiments at several nozzle-lip thickness to jet diameter ratios. Thick lip nozzles, as expected, are found to generate the loudest tones. But the change in sound-pressure level between a thin and a thick lip nozzle is found to be quite modest.

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.

Ground impingement noise of supersonic jets from nozzles with various exit geometries

Aerodynamc and acoustic measurements of single and twin free and impinging jets were made for circular convergent and CD nozzles, rectangular CD nozzles, and D-shaped convergent nozzles. Measured shock cell lengths and screech tone frequencies compare well with existing theory. Noise due to jet impingement correlates on the basis of the nozzle-to-ground distance relative to the free jet shock structure, with the highest sound pressure levels generally due to jet impingement tones. All nozzle types experienced near-field OASPL of 160 dB at some nozzle-to-ground distances, including the CD nozzles operating at their design pressure. The effect of the second jet was to sometimes increase levels due to twin jet resonance and at other times reduce levels due either to suppression of the screech/impingement feedback process or more simply by acoustic interference of the noise produced by the two jets.

The shock-cell structures and screech tone frequencies of rectangular and non-axisymmetric supersonic jets

The screech tone frequencies and shock cell structures of non-axisymmetric supersonic jets from convergent-divergent nozzles operating at off-design conditions are investigated. For the purpose of providing a simple yet fairly accurate first order estimate of the gross features of these shock structures and screech tone frequencies, a linear shock cell model is developed. In the model the mixing layer of the jet is approximated by a vortex sheet. It is shown that the model problem for a jet with arbitrary cross-sectional geometry can be solved by eigenfunction expansion. In this paper the associated eigenvalue problems for rectangular and elliptical jets are solved and explicit shock cell structure solutions for these jets are found. To test the validity of the model the theoretical shock cell spacings are compared with the experimental measurements of Powell [1] and Hammitt [2]. Good agreements are obtained over a wide range of Mach number. The shock cell spacing formula is then used to compute the screech tone frequencies of rectangular jets by incorporating it into the weakest link theory of Tam, Seiner and Yu [3]. The calculated tone frequencies are found to agree favorably with the measurements of Powell [4] and Krothapalli et al. [5] over the Mach number range of 1·15 to 1·80. The present theory is simple but admittedly crude. If a more accurate description of the shock cell structures of non-axisymmetric supersonic jets is desired a multiple-scales model following the recent work of Tam, Jackson and Seiner [24] may be constructed. In this case the present vortex sheet model solution could be used as the starting solution (at the nozzle exit) of such a multiple-scales expansion. At the nozzle exit region the mixing layers of the jet is usually fairly thin.