Screech Amplitude Research Articles

Heat effects on supersonic jet screech: a linear stability analysis based on parabolized stability equations

In this paper, we use the parabolized stability equations (PSEs) to investigate heat effects on supersonic jet screech. The PSE is derived from the linear Euler equations, and the computations are performed on an empirical mean flow profile. Employing PSE, we can examine several important characteristics of the shear-layer instability waves, including the convection velocity, the growth rate, and the location where the instability waves attain the maximal total amplification. Equipped with these knowledge, we further explore their impacts on the jet screech. Specifically, using a newly proposed iteration scheme, we first identify a more suitable convective Mach number to predict the screech frequency in both heated and cold jets. Second, we find that the transition of the screech mode from the axisymmetric to the helical mode may be explained by the shift in the most amplified instability modes. Using this criterion, we can predict a transition Mach number for the mode staging. Third, by assuming that the effective source location is the position where the instability wave attains its maximal amplitude and using the newly obtained convective Mach number, we can predict the screech mode staging in cold and heated jets using Gao's model. The predictions agree well with the experimental data. Finally, we investigate the heat effects on screech amplitude. It is found that compared to cold jets, the difference between the frequency of the most amplified instability wave and the frequency of the jet screech is much bigger in heated jets, which may lead to a lower screech amplitude.

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.

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.

Structural excitation of SERNs during supersonic operation: A source of screech amplitude modulation

This paper investigates the structural response of rectangular nozzles during their supersonic operation. The nozzles in this study are known as Single Expansion Ramp Nozzles (SERNs), with Throat Aspect Ratios (TARs) ranging from 6 to 9 and a designed Mach number of 1.5. Under and approaching the strongest off-design flow conditions, in both underexpanded and overexpanded flow regimes, periodic modulation of screech amplitude is observed. The nearfield pressure signature of the modulation phenomenon and its frequency characteristics show that its origin is the excitation of a structural mode of the nozzle wall and not a fluid–acoustic phenomenon, like screech with Strouhal number characteristics. The modulation behavior corresponds to the wide side of the converging–diverging channel, which forms the non-expanding wall of the SERN. The structures are stable when not under these flow conditions and thus lack this modulation.

Shock oscillation in highly underexpanded jets**Project supported by the National Natural Science Foundation of China (Grant No. 11602028) and the Science and Technology Project of General Administration of Quality Supervision Inspection and Quarantine of China (Grant Nos. 2017QK119 and 2017QK188).

The oscillatory motions of shocks in highly underexpanded jets with nozzle pressure ratios of 5.60, 7.47, 9.34, and 11.21 are quantitatively studied by using large eddy simulation. Two types of shock oscillations are observed: one is the Mach disk oscillation in the streamwise direction and the other is the shock oscillation in the radial direction. It is found that the Mach disk moves quickly in the middle of the oscillatory region but slowly at the top or bottom boundaries. The oscillation cycles of Mach disk are the same for different cases, and are all dominated by an axisymmetric mode of 5.298 kHz. For the oscillation in the radial direction, the shocks oscillate more toward the jet centerline but less in the jet shear layer, and the oscillation magnitude is an increasing function of screech amplitude. The cycles of the radial shock oscillation switch randomly between the two screech frequencies for the first two cases. However, the oscillation periodicity is more complex for the jets with high nozzle pressure ratios of 9.34 and 11.21 than for the jets with the low nozzle pressure ratios of 5.6 and 7.47. In addition, the shock oscillation characteristics are also captured by coarse mesh and Smagorinsky model, but the coarse mesh tends to predict a slower and weaker shock oscillation.

Flow field and acoustic characteristics of elliptical throat CD nozzle

ABSTRACTThe mixing characteristic of circular jet with elliptical throat is studied at different levels of expansion of the nozzle. Two kinds of configurations are studied in this paper, elliptical throat and circular throat jet. At all pressure ratios, the jet with non-circular throat experiences superior mixing than circular throat. The core length of elliptical throat jet is shorter than circular jet. Also, at all pressure ratios, the profile of both configurations looks identical in oscillation; however, the efficiency of the mixing promotion is high in non-circular throat. The promotion of mixing is most efficient at nozzle pressure ratio (NPR) level of 8. As the NPR increases, the core length of the jet reduces nearly 5–10% at each range of NPR. The centreline Pitot pressure profile of non-circular and circular throat shows that the decay of the jet from elliptical throat nozzle is faster than the jet from circular jet. At different levels of expansion, the shock structure shows notable changes with elliptical shape. The amplitude of screech frequency of the jet noise decreases with an increase in NPR. Introduction of elliptical throat to circular throat modifies the shock-cell structures, which significantly change the screech amplitude due to weakening of waves.

Acoustic and mixing characteristic of CD nozzle with inverted triangular tabs

ABSTRACTIn this paper, we study the pressure energy decay, acoustic and mixing characteristics of non-circular jet with inverted triangular tabs at different ranges of aspect ratio. The centre line pitot pressure profile of the inverted triangular tab and ordinary tab shows that the decay of the jet from elliptical jet with inverted tab is faster than the jet from circular jet. When the aspect ratio of nozzle increased, the core length of the jet changes significantly due to the pressure loss. At different aspect ratios, the shock structure shows a prominent change with respect to the mixing promotion. The screech frequency of the jet noise gives appreciable change as the nozzle pressure is increased, and nearly 20–30% of decrease is witnessed in the magnitude of screech frequency. Introduction of inverted triangular tab jet modifies the shock cell formation which extensively changes the screech amplitude due to axis switching.

On the Existence of Subharmonic Screech in Choked Circular Jets from a Sharp-Edged Orifice

Experiments are performed in choked circular hot and cold nitrogen jets issuing from a 2.44 cm diameter sharp-edged orifice at a fully expanded jet Mach number of 1.85 in an effort to investigate the character of screech phenomenon. The stagnation temperature of the cold and the hot jets are 299 K and 319 K respectively. The axial distribution of the centerline Mach number was obtained with a pitot tube, while the screech data (frequency and amplitude) at different axial and radial stations were measured with the aid of microphones. The fundamental screech frequency of the hot jet is slightly increased relative to that of the cold jet. It is concluded that temperature effects on the screech amplitude are manifested with regard to the fundamental and the subharmonic even at relatively small temperature range considered.

Shock Oscillations in a Supersonic Jet Exhibiting Antisymmetrical Screech

T HIS contribution follows a Technical Note published by the authors inAIAAJournal [1], inwhich a shock-trackingalgorithm was applied to a screeching underexpanded jet. It was shown that the shocks oscillate at the screech frequency, in a symmetrical or antisymmetrical manner, according to the screech mode. The temporal signals of twodiametrically opposed near-fieldmicrophones were also considered. At a nozzle pressure ratio (NPR) of 2.54, corresponding to a flapping mode B, both microphones showed a simultaneous modulation of the screech amplitude at a frequency of the order of 1=1000 of the screech frequency. A schlieren video was extracted from the complete recording for each one of the weak and strong screech spells. The tracking algorithm was applied on the resulting movies, and it was deduced that the shock oscillation amplitude increases with an increase in the screech level, which is in agreement with the analytical model proposed by Panda [2]. The picture of the flapping screech viewed by two diametrically opposed microphones appears, however, too simplistic, and a property of screech that remained unseen at that time is highlighted in the present work. Screech is a tonal component of shock-associated noise arising in incorrectly expanded supersonic jets. It was first studied by Powell [3], who already mentioned the existence of modes. Of particular importance for the present study is the flapping mode B, whose pressure signal in the near field is antisymmetrical about a plane. The modal behavior of screech was later extensively studied, especially by Powell et al. [4] as well as Ponton and Seiner [5], who noted that the plane of antisymmetry can slowly rotate or oscillate. This feature, referred to in the following as plane rotation, was investigated in some detail by the authors [6] using a near-field azimuthal microphone antenna comprising 18 transducers located every 20 deg. It was shown that the plane rotation was related to the simultaneous presence of two counter-rotating helices of slightly different frequencies in the screech azimuthal-mode content. It was also explained that the screech level is minimum near the plane of antisymmetry and maximum perpendicular to that direction. In light of [6], it is believed that the strong amplitude modulation in time of the microphone signals in the original study [1] was caused by the rotation of the plane of antisymmetry. The plane rotation was then interpreted as a time modulation of screech strength due to the small number of microphones used. The identification of the plane rotation in the acoustic field has suggested a possible rotation of the plane of antisymmetry of the shock motion. This question is considered in the present paper. The experimental facility is first presented. Then, results for two cases of flapping screech are discussed.

Near-Field Acoustic Characteristics of a Turbulent Axisymmetric Pulsed Jet

Near-Field Acoustic Characteristics of a Turbulent Axisymmetric Pulsed Jet

Experimental Studies on the Limiting Tab

Experiments were carried out to study the effect of the tabs with limiting length, termed Rathakrishnan limit, on controlling the mixing and acoustic characteristics of free jets. The limiting tab is found to be efficient in promoting the jet mixing even in the presence of an adverse pressure gradient. Furthermore, the limiting tab is found to bemost efficient in promoting the mixing near the correct expansion rather than at an underexpanded condition, with a favorable pressure gradient at the nozzle exit. The reason behind theworking of the limiting tab in the formof a cross wire is discussed forMach 1.6, 1.79, and 2.0 jets. As high as 50%reduction in the core length is achieved using a cross wire forMach 1.79 jet operatedwith nozzle pressure ratio 5.66. The combination of thewaves in the jet core and their interaction with the detached shock envelop generated by the wire has a strong influence on the overall jet noise and its spectral content. Even though thewire reduced the overall noise of the jet and the reduction is significant formany cases of the present study, it caused an increase of screech amplitude for some cases. Jet noise reduction was found to be sensitive to the wire orientation.

Shock-Refracted Acoustic Wave Model for Screech Amplitude in Supersonic Jets

A physical model is proposed for the estimation of the screech amplitude in underexpanded supersonic jets. The model is based on the hypothesis that the interaction of a plane acoustic wave with stationary shock waves provides amplification of the transmitted acoustic wave upon traversing the shock. Powell's discrete source model for screech incorporating a stationary array of acoustic monopoles is extended to accommodate variable source strength. The proposed model reveals that the acoustic sources are of increasing strength with downstream distance. It is shown that the screech amplitude increases with the fully expanded jet Mach number. Comparisons of predicted screech amplitude with available test data show satisfactory agreement. The effect of variable source strength on the directivity of the fundamental (first harmonic, lowest frequency mode) and the second harmonic (overtone) is found to be unimportant with regard to the principal lobe (main or major lobe) of considerable relative strength, and is appreciable only in the secondary or minor lobes (of relatively weaker strength).

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.

SUPERSONIC JET SCREECH: HALF-CENTURY FROM POWELL TO THE PRESENT

Under certain conditions, shock-containing jets produce an intense tone referred to as screech. Screech was discovered about half a century ago by Alan Powell in England. Here I recount developments in supersonic jet screech — from Powell's first observation in 1951 to now. During this period more than 200 papers have been published — many offering only incremental advances. This paper provides a concise screech resource including a historical perspective, a summary of recent developments and a critical assessment of the state of the art. Topics include modulation of instability waves by shocks, shock-cell models and screech frequency prediction models, unsteady shock motions and clues about their role in shock noise generation. Also, detailed nearfield measurements and computer simulation methods now available are discussed. However, despite the advances, screech amplitude prediction remains an elusive but increasingly important goal not only due to concerns about sonic fatigue failure of aircraft structures but because knowledge gained by the study of screech can be applied to a variety of resonant flow situations, including jet impingement, cavity resonance, and closed-loop active flow control.

Experiments characterizing nonlinear shear layer dynamics in a supersonic rectangular jet undergoing screech

Jet screech is the result of a resonant aeroacoustic feedback loop. In modern high-performance aircraft jet screech has given rise to in-flight sonic fatigue failure of nozzle components. For this reason, jet screech has been the focus of numerous investigations. However, the prediction of screech amplitude has been a very elusive goal. A primary objective of the work to be reported is to experimentally characterize the nonlinear instability wave development in the shear layer of a rectangular supersonic jet undergoing screech. Hot-film probes are used to measure the instability wave development in the jet shear layers along a constant Mach number ray. Higher-order polyspectral techniques are employed to determine resonant nonlinear phase locking between multiple instability modes. A detailed comparison of the “inner loop” hot-film measurements with “outer loop” acoustic measurements is made. Such a comparison reveals the importance of incorporating nonlinear effects into a jet screech amplitude prediction method.

Screech suppression in supersonic jets

Jet screech from underexpanded sonic nozzles has been investigated experimentally. Multiple screech modes, or stages, are found to be present at some jet operating conditions. The fundamental screech tone of each mode attains a maximum amplitude at about 20 deg from the inlet axis, with higher harmonics exhibiting multiple lobes. The directivity of each harmonic is predicted quite well from a stationary array of acoustic monopoles, with phasing between consecutive monopoles determined by the shock cell spacing and eddy convection velocity. Large reduction of screech amplitude can be obtained from modifications to the nozzle exit, although the extent of this suppression is mode dependent.