Broadband shock-associated noise is a component of jet noise encountered in imperfectly expanded supersonic jets. It is going to be responsible for a dominant part of cabin noise levels at cruise condition for the next-generation commercial aircraft including composite fuselage. A flight simulating facility has been built in the anechoic room of the Centre Acoustique at Ecole Centrale de Lyon and extensive aero-acoustical measurements have been performed to study flight effects on broadband shock-associated noise. In the final paper, flight effects will first be characterized by far field acoustic measurements. In a second step, the main features will be linked to modifications in the jet dynamics due to the secondary stream, with aid of schlieren visualizations, static pressure measurements and particle image velocimetry. The representation of shock-associated noise as the result of an interaction between vortical structures and shocks will be assessed and modelling will be addressed.

The present experimental study focuses on some properties of the turbulence and the shock-cell structure in underexpanded supersonic jets, which are of practical relevance in air transport. Choked jets at fully expanded Mach numbers M-j = 1.10, 1.15, 1.35 and 1.50 are investigated using particle image velocimetry. The strength of the shock-cell structure is studied from mean velocity profiles, both in the jet core and in the mixing layer. The general geometry of the latter and its location relatively to the mean shock-cell structure are established. Furthermore, detailed accounts of mixing layer thickness, turbulence levels, spatial correlations and intrinsic turbulence length scales are given. While the mean velocity variations related to the shock-cell structure extend up to the subsonic part of the studied jets, their mixing layer is found to be mostly located in the subsonic region. Some of the observed turbulence properties, like the mixing layer thickness and turbulence levels, are close to what is found for subsonic jets. The effect of the shock-cell structure on turbulence is however visible for M-j >= 1.35. The spatial correlations of turbulence are used to estimate intrinsic turbulence length scales and these are found to be of the order of the shock-cell length. These data are used to make some comments upon the generation mechanism of shock-associated noise, a noise component produced by imperfectly expanded supersonic jets. (C) 2014 Elsevier Inc. All rights reserved.

Two underexpanded free jets at fully expanded Mach numbers M-j = 1.15 and 1.50 are studied. Schlieren visualizations as well as measurements of static pressure, Pitot pressure and velocity are performed. All these experimental techniques are associated to obtain an accurate picture of the jet flow development. In particular, expansion, compression and neutral zones have been identified in each shock cell. Particle lag is considered by integrating the equation of motion for particles in a fluid flow and it is found that the laser Doppler velocimetry is suitable for investigating shock-containing jets. Even downstream of the normal shock arising in the M-j = 1.50 jet, the measured gradual velocity decrease is shown to be relevant.

Screech and broadband shock-associated noise linked to the presence of a shockcell structure in supersonic jets are reviewed in this paper. Only underexpanded supersonic circular jets issued from a convergent nozzle are considered here. An overview of the flow and of these two noise components is presented, based on recent experimental and numerical work. Flight effects on broadband shock-associated noise are also introduced, within the framework of aeronautical applications.

A slightly underexpanded supersonic jet at a Mach number Mj of 1.10 is studied experimentally. Schlieren visualizations and particle image velocimetry are applied in order to characterise the shock-cell structure and turbulence in the mixing layer, which are the two elements at the origin of the shock-associated noise emitted by such a jet. It is found in particular that the velocity gradients typical of the shock-cell structure still exist in the subsonic part of the mixing layer. From the evaluation of some turbulence properties (turbulence level, momentum thickness and spatial correlation), it is shown that the jet behaves very similarly to jets at high subsonic Mach numbers. It is believed that such data could shed light on the shock-associated noise source.

Mechanical tabs are often used as a screech-suppressing device in order to study broadband shock-associated noise (BBSAN). In this study, various experimental methods are used to characterise the effect of a tab on the development and noise of an underexpanded supersonic round jet. It is shown that the jet development is considerably modified by the introduction of the tab in that the shock spacing is shortened and the shock-cell pattern loses axisymmetry and strength. Moreover, the broadband shock-associated noise radiated by the tabbed jet is compared to that of a jet in which screech was suppressed by means of a notched nozzle. The peak frequency of BBSAN for the tabbed jet approximately matches that of the notched counterpart, but the amplitude of this noise component was seen to be smaller for the former jet. More importantly, the axisymmetry of the acoustic field is lost when using a tab, resulting in a dependence of measurements on the location of the tab relatively to the microphones. It is concluded that the use of a tab should be avoided to remove screech when studying broadband shock-associated noise.

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.

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.

A SHOCK-CONTAINING supersonic jet radiates two so-called shock-associated noise components in addition to the classical mixing noise. These two components are the broadband shockassociated noise and the screech. The latter was studied in the early 1950s by Powell [1]. Screech is a tonal noise component. Its generation mechanism has been explained with some success as a feedback loop involving sound production through shock– turbulence interaction. The broadband shock-associated noise component has been investigated at least since Martlew [2]. HarperBourne and Fisher [3] have adapted Powell’s stationary source array model to derive some observed properties of this noise component. In his review about high-speed jet aeroacoustics, Seiner [4] points out the necessity of studying shockmotion to confirmPowell’s [1]model of stationary sources. Furthermore, fluid disturbances having a relative motion to shocks may be responsible for broadband shock noise [4]. A certain insight into shock behavior within a jet plume is thus essential to accurately understand shock-associated noise generation. The screech phenomenon in a round jet involves different modes. They have been studied extensively in the past, for example, by Davies and Oldfield [5], Merle [6], or Powell et al. [7]. Mode switching appears as a sudden change of jet plume structure and screech frequency with operating conditions. Five modes are traditionally quoted: A1 and A2 are axisymmetric, B is sinuous or flapping, C is helical, andD is again sinuous. The reader is referred to Raman [8,9] for a review of screech properties. The coincidence between shock oscillation frequency and screech frequency has been first observed by Lassiter and Hubbard [10] from a shadowgraph technique. Sherman et al. [11] havemade use of highspeed schlieren recording to investigate the shock distortion during screech. They have also found the coincidence between oscillation and screech frequencies and estimated oscillation amplitudes for the third shock. Using an optical shock detection technique based on laser light scattering by a shock, Panda [12,13] has significantly contributed to the characterization of shock motion during screech. He has reported that every shock oscillates at the screech frequency andmotion amplitudes have been assessed. In the latter reference, an analyticalmodel for shock oscillation is also proposed and is found to be in good agreement with measurements. To the authors’ knowledge, Sherman et al.’s [11] and Panda’s [12,13] contributions are the only occurrences of shock oscillation measurements in the screech literature. In the present Note, oscillation frequency and amplitude are examined by means of high-speed schlieren images and simultaneous near-field acoustic recordings. A procedure for shock tracking is developed and applied on two different screech modes. The experimental setup is presented in Sec. II. The procedure is explained in Sec. III, and some results are discussed. Conclusions are finally given in Sec. IV.

Flight effects on screech from an underexpanded supersonic jet have been experimentally investigated in a free jet facility. Screech frequency prediction is addressed, and some conclusions about the convection velocity under flight conditions are drawn. An azimuthal near field acoustic antenna is used to investigate the modal behaviour of screech with forward flight effects. Several mode switchings are identified as the flight velocity is increased but none can be related to a change in the screech azimuthal mode content. Screech is enhanced by flight at high fully expanded Mach number Mj. This conclusion, established from acoustic far field measurements, is supported by the analysis of schlieren visualizations.

A coaxial supersonic facility has been installed in the anechoic room of the Centre Acoustique (LMFA, Ecole Centrale de Lyon) to reproduce a supersonic fan flow in forward flight. This paper first presents this new facility and some validation tests. To get further insight into the shock-associated noise, Schlieren visualizations as well as near- and far-field acoustic measurements have been performed. Shock cell lengths are shown to be in agreement with the literature, as also the effect of forward flight. A and B screech modes are investigated. It is found that near-field microphones can well discriminate very well between those two, and that mode B has a chaotic behaviour. The effect of secondary flow on screech tones, broadband shock-associated noise and turbulent mixing noise is investigated too. It is shown that forward flight has an effect on both frequencies and amplitudes of supersonic jet noise components. Finally, a shock tracking procedure has been developed to estimate shock oscillation amplitudes and frequency, which is found to be the screech frequency.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Experimental investigation of screech by an underexpanded supersonic jet with forward flight effect Benoît André, Thomas Castelain, Christophe Bailly, Daniel Juvé