Screech Tones Research Articles

DMD-based spatiotemporal superresolution measurement of a supersonic jet using dual planar PIV and acoustic data

The present study applies a framework of the spatiotemporal superresolution measurement based on the total-least-squares dynamic mode decomposition, the Kalman filter and the Rauch-Tung-Striebel smoother to an axisymmetric underexpanded supersonic jet of a jet Mach number of 1.35. Dual planar particle image velocimetry was utilized, and paired velocity fields of the flow with a short time interval were obtained at a temporal resolution of 5000 Hz. High-frequency acoustic data of 200,000 Hz were simultaneously obtained. Then, the time-resolved velocity fields of the supersonic jet were reconstructed at a temporal resolution of 200,000 Hz. Also, time coefficients of dynamic modes in high temporal resolution were calculated. The correlation between time coefficients implies that the mixing promotion by screech tone causes the lift-up of the high-velocity fluid from the jet center and accelerates at the downstream side.

Influence of the swirl vanes in convergent-divergent nozzle on screech tones and mixing efficiency at subsonic and supersonic jet flow

PurposeThe purpose of this study is to investigate the effect of coaxial swirlers on acoustic emission and reduction of potential core length in jet engines.Design/methodology/approachThe swirlers are introduced in the form of curved vanes with angles varied from 0° to 130°, corresponding to swirl numbers of 0–1.5. These swirlers are fixed in the annular chamber and tested at different nozzle pressure ratios of 2, 4 and 6.FindingsThe study finds that transonic tones exist for the nonswirl jet, creating an unfavorable effect. However, these screech tones are eliminated by introducing a swirl jet at the nozzle exit. Weak swirl shows a greater reduction in noise than strong swirl at subsonic conditions. In addition, the introduction of swirl jets at all pressure ratios significantly reduces jet noise and core length in supersonic conditions, mitigating the noise created by shockwaves and leading to screech tone-free jet mixing.Originality/valueThe paper provides valuable insights into the use of coaxial swirlers for noise reduction and core length reduction in jet engines, particularly in supersonic conditions.

A closure mechanism for screech coupling in rectangular twin jets

The twin-jet configuration allows two different scenarios to close the screech feedback. For each jet, there is one loop involving disturbances which originate in that jet and arrive at its own receptivity point in phase (self-excitation). The other loop is associated with free-stream acoustic waves that radiate from the other jet, reinforcing the self-excited screech (cross-excitation). In this work, the role of the free-stream acoustic mode and the guided-jet mode as a closure mechanism for twin rectangular jet screech is explored by identifying eligible points of return for each path, where upstream waves propagating from such a point arrive at the receptivity location with an appropriate phase relation. Screech tones generated by these jets are found to be intermittent with an out-of-phase coupling as a dominant coupling mode. The instantaneous phase difference between the twin jets computed by the Hilbert transform suggests that a competition between out-of-phase and in-phase coupling is responsible for the intermittency. To model wave components of the screech feedback while ensuring perfect phase locking, an ensemble average of leading spectral proper orthogonal decomposition modes is obtained from several segments of large-eddy simulation data that correspond to periods of invariant phase difference between the two jets. Each mode is then extracted by retaining relevant wavenumber components produced via a streamwise Fourier transform. Spatial cross-correlation analysis of the resulting modes shows that most of the identified points of return for the cross-excitation are synchronised with the guided jet mode self-excitation, supporting that it is preferred in closing rectangular twin-jet screech coupling.

Experimental study on acoustic resonance of subsonic and slightly underexpanded impinging jets

The aeroacoustic feedback loops in high-speed circular jets that impinge on a large flat plate are investigated via acoustic measurements and schlieren visualizations. In the present experiments, the nozzle pressure ratio ranges from 1.39 to 2.20, the corresponding ideally expanded jet Mach number $M_j$ is from 0.70 to 1.12 and the nozzle-to-plate distance ( $H$ ) is from 4.0 $D$ to 6.0 $D$ , where $D$ is the nozzle exit diameter. The results of acoustic measurements show that the strongest tones are generated in a limited frequency band. The empirical dispersion relations obtained from the fluctuating greyscales along the jet centreline of time-resolved schlieren images have good agreement with the dispersion relations from the vortex-sheet model. The coherent flow structures at tonal frequencies are extracted by spectral proper orthogonal decomposition and are analysed in detail. For the $M_j<0.82$ jets, the upstream-propagating guided jet mode is progressively confined to the potential core of jets with increasing tonal frequency, which provides the first direct experimental support for theoretical results. The evolution in the structures of acoustic resonance loops is studied along a single frequency stage of axisymmetric impinging tones. When the acoustic resonance between the upstream- and downstream-propagating guided jet modes is formed at tonal frequencies, the impinging tones are intenser. Slightly underexpanded impinging jets can simultaneously produce impingement tones and screech tones. Shock-cell structures have modulatory effects on the downstream-propagating Kelvin–Helmholtz wavepacket and the upstream- and downstream-propagating guided jet modes. Due to the interaction between the flow structures at the frequencies of impinging and screech tones, tones of axisymmetric modes can be produced outside the frequency ranges in which the axisymmetric upstream-propagating guided jet modes are supported by jets.

Screech receptivity control using exit lip surface roughness for under-expanded jet noise reduction

This study presents an experimental investigation on altering the receptivity of the screech phenomenon toward the goal of jet noise reduction. The screech receptivity refers to the propensity to sustain the screech feedback loop through the reflection/scattering of feedback waves at the jet exit lip and their coupling with hydrodynamic instabilities near the jet exit. The receptivity is modified by attaching sandpapers of different roughnesses on the exit lip of pipes and nozzles. First, the pipe exit lip thickness is varied to determine variations in screech staging, frequency, and amplitude. As lip thickness varies, the screech mode staging varies with minimal changes in frequency, whereas screech amplitude depends on screech mode. The jet noise reduces as the lip surface roughness increases due to screech amplitude reduction or elimination because of the diffuse reflection of screech tone feedback waves by the lip surface. The coarse sandpaper on the exit lip eliminated the screech, while others reduced screech amplitude at high under-expansion levels. The coarse sandpaper reduced more than 50% of the maximum acoustic power observed with the plain lip at the corresponding under-expansion level. The surface roughness does not affect the screech tone at all the under-expansion levels or screech frequencies. The effect begins at a particular frequency (cutoff) and continues for frequencies lower than the cutoff frequency. The cutoff frequency increases with the lip surface roughness and varies with the lip thickness. By properly designing the surface roughness of the lip, the effect becomes a better passive technique for jet noise control.

Supersonic jet noise and screech tone suppression using cross-wire

This experimental investigation is aimed at assessing how the introduction of a cross-wire at the exit of a CD nozzle influences the performance of a supersonic nozzle. The study focuses on cold air jets generated by De Laval nozzles equipped with cross-wires and baseline configurations, particularly at design Mach numbers of 1.5 and 1.75. The investigation involves collecting measurements from the noise field emitted by the cross-wire nozzle with a 2% obstruction at the exit. This passive control approach effectively reduces the occurrence of screech tones in both over-expanded and under-expanded conditions in the azimuthal plane at appropriate operating pressures. Various acoustic parameters, including sound pressure levels (SPL), Strouhal numbers, and the overall sound pressure level spectra (OASPL) are recorded. Schlieren imaging captures images of shock cell patterns, illustrating the impact of shock-associated noise. In comparison to a baseline nozzle, the results demonstrate that a CD nozzle equipped with a cross-wire proves to be a proficient screech tone suppressor, leading to an average reduction of up to 5 dB in OASPL in under-expanded and over-expanded scenarios.

Analysis of the effect of turbulent inflow on the flow-field and noise of supersonic jet flow using large eddy simulation

The impact of inflow flow state on the flow-field and far-field noise of supersonic jet flow has been studied. A new turbulent inflow boundary condition based on the synthetic eddy method is implemented in an in-house LES code. The implementation was validated using a turbulent channel flow. Simulation results of an under-expanded supersonic jet with a converging nozzle have shown that turbulent inflow leads to thicker and turbulent initial shear layer with rich vortical structures. The thick initial shear layer hinders the growth of flow instability and break the acoustic feedback loop, which is vital to the generation of screech tone. The alternation of the flow-field by the turbulent inflow also leads to changes in the far-field noise radiation. The screech tone is eliminated, the noise in the downstream direction is considerably suppressed, and the noise radiates in the upstream direction is slightly amplified.

Development and evaluation of laser acoustic measurement system for supersonic jet noise

Acoustic measurements in the near field are significant to suppress the noise generated by a supersonic jet. The microphone measurement is commonly used to measure noise, but it is not appropriate for the jet noise measurement in the near field, because it may be broken or disturb the sound field. In this study, we propose an optical acoustic measurement method using a laser and a 2-D position sensitive detector. This system can measure the propagating direction and the spectra of acoustic waves by detecting the angle and direction of laser path deflection by acoustic wave passing. To evaluate this laser measurement method, it was applied to various supersonic jet noise phenomena, such as Mach waves and screech tones, and then the results were compared with those of microphone measurements. The spectra measured by the laser measurement show good agreement with those by microphone measurements, and the difference in OASPL was less than 0.7 dB. As for the propagating direction, it is calculated using the principal component analysis in the laser measurement, and its results also show good agreement with those of acoustic intensity vector measurement.

Influence of nozzle external geometry on the emission of screech tones

The effect of external nozzle geometry on the emission of screech tones was studied experimentally. Four conical reflector surfaces, with half-angles ranging from 60° to 90°, were installed around the exit of a round convergent nozzle. The investigation focused on two closely spaced fully-expanded Mach numbers, M j = 1.32 and 1.34. The acoustic far-field was surveyed by a microphone phased array that included a continuously-scanning microphone, the latter enabling high spatial resolution. The isolated jets contained well-known screech mode B and its harmonics. Addition of the reflectors caused significant changes in the modal emission pattern, with tones traditionally linked to mode C occurring at M j = 1.34 but not at M j = 1.32. Tonal components associated with new modes E and F emerge at both Mach numbers when the cone half-angle is 60° or 70°. The noise source distribution generally elongates with decreasing cone angle. Some modes show clear scattering from the reflectors, while others do not. The study underscores the complexity that initial conditions can impart on the modal structure of screech and demonstrates the capability of the continuous-scan beamforming technique in resolving fine features of the source.

Nature of unsteady and acoustic behavior in an elliptic sonic jet with aft-deck

An experimental investigation is carried out on the under expanded sonic jet issuing from an elliptic nozzle with aft-deck at two different Mach numbers (Mj). Flow characteristics including shock-cell structure, jet separation, and pressure variation over the aft-deck wall are investigated with the help of schlieren images, oil flow visualization, and steady pressure measurements. A complex shock-structure is found to exist in the x-z and x-y planes due to the shape and curvature of the aft-deck for Mj = 1.35 and 1.56. The shock-induced jet separation is identified at x/L ∼ 0.75 for Mj = 1.56, whereas no separation is identified for Mj = 1.35. Synchronized acoustic and unsteady measurements reveal the coupling between the shock-induced jet separation on the aft-deck wall and the far-field noise emissions. Acoustic spectra elucidate the strong tonal noise component corresponding to the flapping Mode-B at St ∼ 0.68 for Mj = 1.56. Moreover, the noise components including screech tones, broad-band shock-associated noise, and turbulent mixing noise are dominant for the azimuthal angle ϕ = 0°. The dominant flow mode from proper orthogonal decomposition analysis shows the existence of strong jet flapping and upstream-traveling acoustic waves for the separated jet.

The axisymmetric screech tones of round twin jets examined via linear stability theory

Spatial linear stability analysis is used to study the axisymmetric screech tones generated by twin converging round nozzles at low supersonic Mach numbers. Vortex-sheet and finite-thickness models allow for identification of the different waves supported by the flow at different conditions. Regions of the frequency–wavenumber domain for which the upstream-propagating guided jet modes are observed to be neutrally stable are observed to vary as a function of solution symmetry, jet separation,$S$, and the velocity profile used. Screech-frequency predictions performed using wavenumbers obtained from both models agree well with experimental data. Predictions obtained from the finite-thickness model better align with the screech tones measured experimentally and so are seen to be an improvement on predictions made with the vortex sheet. Additionally, results from the finite-thickness model predict both symmetric and antisymmetric screech tones for low$S$that are found in the vortex-sheet model only at greater$S$. The present results indicate that the feedback loop generating these screech tones is similar to that observed for single-jet resonance, with equivalent upstream and downstream modes.

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.

Flow dynamics and noise generation mechanisms in supersonic underexpanded rectangular and planar jets

In this study, large eddy simulations of two rectangular jets with different aspect ratios and one planar jet with spanwise periodic boundaries are carried out in an attempt to investigate and clarify some key issues regarding the flow dynamics and noise generation mechanisms for the screech tone. The substantial effects of the nozzle configuration on shock structures and noise are first revealed in detail. It is found that when the lateral confinement is weakened, the spacing of the shock cells increases and the jet oscillates more intensively in the minor-axis plane, increasing the noise level and altering the screeching frequency. By analyzing the pressure fluctuations of the shear layer in the wavenumber-frequency space, different kinds of waves in these supersonic jets are examined. Importantly, it is revealed that the guided jet wave should play a dominant role in closing the resonance loops rather than the acoustic wave. Moreover, the energy-containing structures with pertinent frequencies are extracted by employing the reduced-order variational mode decomposition, and some underlying flow dynamics are presented. Especially, a novel mechanism has been identified: the low-frequency stretching motions of shock cells have a significant modulation effect on the screeching amplitude in the planar jet. Furthermore, with the aid of the multi-process acoustic theory, the characteristics of physical noise sources are diagnosed, particularly the source mechanisms related to the screech tone. The general structures and distribution of the kinematic noise source Sβ and entropy noise source Se are presented. Sβ mainly exhibits a vortex-like structure near the nozzle, while Se exhibits a lamellar bilayer structure. The spatiotemporal correlations between the physical sources and far-field noise show that the dominant mechanisms for the screech tone rely on the nozzle configuration and the screech tone tends to be produced by multiple sources in all cases.

Large-eddy simulations of flow and aeroacoustics of twin square jets including turbulence tripping

In this study, we investigate the flow and aeroacoustics of twin square (i.e., aspect ratio of 1.0) jets by implicit large-eddy simulations (LESs) under a nozzle pressure ratio of 3.0 and a temperature ratio of 1.0 conditions. A second-order central scheme coupled with a modified Jameson's artificial dissipation is used to resolve acoustics as well as to capture discontinuous solutions, e.g., shock waves. The flow boundary layer inside of the nozzle is tripped, using a small step in the convergent section of the nozzle. The time-averaged axial velocity and turbulent kinetic energy of LES with boundary layer tripping approaches better to particle image velocimetry experimental data than the LES without turbulence tripping case. A two-point space–time cross-correlation analysis suggests that the twin jets are screeching and are coupled to each other in a symmetrical flapping mode. Intense pressure fluctuations and standing waves are observed between the jets. Spectral proper orthogonal decomposition (SPOD) confirms the determined mode and the relevant wave propagation. The upstream propagating mode associated with the shock-cell structures is confined inside jets. Far-field noise obtained by solving Ffowcs Williams and Hawkings equation is in good agreement with the measured acoustic data. The symmetrical flapping mode of twin jets yields different levels of the screech tone depending on observation planes. The tonalities—the fundamental tone, second and third harmonics—appear clearly in the far-field, showing different contributions at angles corresponding to the directivities revealed by SPOD.

Effect of zero penetration angle chevrons in supersonic jet noise and screech tone mitigation

Abstract An experimental study is conducted to determine the effect of chevrons with zero penetration angles at the CD nozzle exit on an emitted noise field. The implication of passive control is to reduce the blockage of the nozzle exit area with minimal engine thrust penalty. The cold air jets issued at design Mach numbers 1.5 and 1.75 from the De Laval nozzles of the circular section were investigated. This passive control eliminates screech tones at the over and ideally expanded conditions at 60° and 90° in the azimuth plane. The acoustic data measurements have also been observed for the chosen jet Mach numbers. The schlieren images reveal the shock cell pattern to eliminate the effect of shock-associated noise levels at supersonic jets. The results show that 10 chevrons with no penetration act as an effective eliminator of screech tone and noise suppression average ∆OASPL value up to 3 dB at Mach number 1.75.

On the application of gradient based reconstruction for flow simulations on generalized curvilinear and dynamic mesh domains

Accurate high-speed flow simulations of practical interest require numerical methods with high-resolution properties. In this paper, we present an extension and demonstration of the high-accuracy Gradient-based reconstruction and α-damping schemes introduced by Chamarthi (2022) for simulating high-speed flows in generalized curvilinear and dynamic mesh domains with the freestream preservation property. In the first part of this paper, the algorithms are detailed within the generalized curvilinear coordinate framework, with a focus on demonstration through stationary and dynamic mesh test cases. It has been shown both theoretically and through the use of test cases that the conservative metrics, including their interpolation to cell interfaces, must be numerically computed using a central scheme that is consistent with the inviscid flux algorithm to achieve the freestream preservation property. The second part of the paper illustrates the efficacy of the algorithm in simulating supersonic jet screech by displaying its capability to capture the screech tones and accurately characterize the unsteady lateral flapping mode of a Mach 1.35 under-expanded supersonic jet, in contrast to the WENO-Z scheme which fails to do so at the same grid resolution. In the final part of the paper, the parallelizability of the schemes on GPU architectures is demonstrated and performance metrics are evaluated. A significant speedup of over 200× (compared to a single core CPU) and a reduction in simulation completion time to 34.5 h per simulation were achieved for the supersonic jet noise case at a grid resolution of 13 million cells.

Experimental characterisation and data-driven modelling of unsteady wall pressure fields induced by a supersonic jet over a tangential flat plate

This work deals with the investigation and modelling of wall pressure fluctuations induced by a supersonic jet over a tangential flat plate. The analysis is performed at several nozzle pressure ratios around the nozzle design Mach number, including slightly over-expanded and under-expanded conditions, and for different radial positions of the rigid plate. Pitot measurements and flow visualizations through the background oriented schlieren technique provided a general overview of the aerodynamic interactions between the jet flow and the plate at the different regimes and configurations. Wall pressure fluctuations were measured using a couple of piezoelectric pressure transducers flush mounted over the plate surface. The spectral analysis has been carried out to clarify the effect of the plate position on the single and multivariate wall pressure statistics, including the screech tone amplitude. The experimental dataset is used to assess and validate a surrogate model based on artificial neural networks. Sound pressure levels and coherence functions are modelled by means of a single fully connected network, built on the basis of a recently implemented fully deterministic topology optimization algorithm. The metamodel uncertainty is also quantified using the spatial correlation function. It is shown that the flow behaviour as well as the screech and broadband noise signatures are significantly influenced by the presence of the plate, and the effects on spectral quantities are correctly reproduced by the proposed data-driven model that provides predictions in agreement with the available data.

The role of nozzle exit-lip surface roughness on jet noise

This paper attempts to describe the jet noise sensitivity to the surroundings by considering the surface roughness of the nozzle exit lip. A thick lip convergent nozzle is used, and its lip surface roughness is altered by attaching sandpapers of various grades ranging from fine to coarse. Far-field acoustic measurements for different under-expansion levels are carried out by changing the nozzle pressure ratio from 1.5 to 6.5. The results are compared with that of the smooth lip nozzle. The jet noise (screech tone) varies with the surface roughness of the exit lip due to changes in the receptivity caused by the diffuse reflection of upstream acoustic waves by the lip. The screech tone is eliminated at higher pressure ratios with the coarse sheets, while screech amplitude is decreased for fine-grade ones.

Acoustic emission due to the interaction between shock and instability waves in two-dimensional supersonic jet flows

An analytical model is developed to study the sound produced by the interaction between shock and instability waves in two-dimensional supersonic jet flows. The jet is considered to be of vortex-sheet type and two-dimensional Euler equations are linearized to determine the governing equations for shock and instability waves and their interaction. Pack's model is used to describe shock waves, while instability waves are calculated using spatial stability analysis. The interaction between shock and instability waves can be solved analytically by performing Fourier transform and subsequently using the method of steepest descent. Sound produced by the interaction between the instability wave and a single shock cell is studied first, after which that due to a number of cells follows. We find that the model developed in this study can correctly predict the frequencies of the fundamental screech tone and its first and second harmonics. We show that the predicted sound directivity, even from a single shock cell, is in good agreement with experimental data. In particular, this model shows the strongest noise emission close to the upstream direction but the emitted noise starts to rapidly decay as the observer angle approaches $180^\circ$ , which is in accordance with experimental results; this suggests that the effective noise from a single shock cell is far from of the monopole type as assumed in the classical Powell's model. We find that the noise directivity is very sensitive to the local growth rate of the instability waves and the noise is generated primarily through the Mach wave mechanism.

Reflection and transmission of a Kelvin–Helmholtz wave incident on a shock in a jet

Screech tones in supersonic jets are underpinned by resonance between downstream-travelling Kelvin–Helmholtz waves and upstream-travelling acoustic waves. Specifically, recent works suggest that the relevant acoustic waves are guided within the jet and are described by a discrete mode of the linearised Euler equations. However, the reflection mechanism that converts downstream-travelling waves into upstream-travelling waves, and vice versa, has not been thoroughly addressed, leading to missing physics within most resonance models. In this work, we investigate the reflection and transmission of waves generated by the interaction between a Kelvin–Helmholtz wave and a normal shock in an under-expanded jet using a mode-matching approach. Both vortex-sheet and finite-thickness shear-layer models are explored, quantifying the impact of the shear layer in the reflection process. This approach could enable more quantitative predictions of resonance phenomena in jets and other fluid systems.