An analysis of shock noise components

17th AIAA/CEAS Aeroacoustics Conference (32nd AIAA Aeroacoustics Conference), 2011

In this paper, the data obtained by LES for an initially laminar and overexpanded jet at Mach number 3.3 and Reynolds number 10 5 are reexamined in order to investigate the nonlinear effects on the propagation of the acoustic waves, and the normalized flow/acoustics cross-correlations. To study the non-linear propagation effects at the direction φ = 60 • and up to 240 radii from the nozzle exit, the LES near field is propagated in far-field by solving either the isentropic linearized Euler equations or the full Euler equations. The comparisons of the acoustic data obtained from the two methods clearly show that the non-linear effects are strong up to about 240 radii from the nozzle exit. Using the far-field acoustic results from the non-linear propagation, the normalized cross-correlations between the turbulent flow quantities and the acoustic pressure signals at the direction φ = 60 • are then evaluated to give some information on sound generation. A sound source which may be similar to that one observed in subsonic jets is first found on the jet axis in the vicinity of the end of the potential core. Other sound sources attributed to the supersonic convection of turbulent vortices are noticed. Finally, the normalized cross-correlations between the fluctuating density along the jet axis and the acoustic pressure display correlation bands between the 3rd and the 5th shock cells. These bands might be linked with the screech generation mechanism.

2010

Supersonic jets operating off-design produce broadband shock-associated noise. Broadband shock-associated noise is characterized by multiple broadband peaks in the far-field and is often the dominant source of noise towards the sideline and upstream direction relative to the jet axis. It is due to large scale coherent turbulence structures in the jet shear layers interacting with the shock cell structure. A broadband shock-associated noise model recently developed by the authors predicts this noise component from solutions to the Reynolds averaged Navier-Stokes equations using a two-equation turbulence model. The broadband shock-associated noise model is applied to dualstream and rectangular nozzles operating supersonically, heated, and off-design. The dualstream jet broadband shock-associated noise predictions are conducted for cases when the core jet is supersonic and the fan jet is subsonic, the core jet is subsonic and the fan jet is supersonic, and when both jet streams operate supersonically. Rectangular jet predictions are shown for a convergent-divergent nozzle operating both over-and under-expanded for cold and heated conditions. The original model implementation has been heavily modified to make accurate predictions for the dualstream jets. It is also argued that for over-expanded jets the oblique shock wave attached to the nozzle lip contributes little to broadband shock-associated noise. All predictions are compared with experiments.

The aim of this work is to investigate the way in which sound is generated and radiated from supersonic jets. The work programme is essentially an experimental one using cold model air jets and has involved a study of the farfield and nearfield sound pressures. Also a relatively new optical technique 'The Crossed Beam Schlieren Apparatus'', is used t o obtain jet turbulence data. This is an important introduction to this field of work as prior to the development of optical probing techniques little information has been available in the correlation of flow fluctuations in supersonic jets , with their radiated sound characteristics.

International Journal of Aeroacoustics, 2012

An acoustic analogy is developed based on the Euler equations for broadband shock-associated noise (BBSAN) that directly incorporates the vector Green's function of the linearized Euler equations and a steady Reynolds-Averaged Navier-Stokes solution (SRANS) to describe the mean flow. The vector Green's function allows the BBSAN propagation through the jet shear layer to be determined. The large-scale coherent turbulence is modeled by two-point second order velocity cross-correlations. Turbulent length and time scales are related to the turbulent kinetic energy and dissipation rate. An adjoint vector Green's function solver is implemented to determine the vector Green's function based on a locally parallel mean flow at different streamwise locations. The newly developed acoustic analogy can be simplified to one that uses the Green's function associated with the Helmholtz equation, which is consistent with a previous formulation by the authors. A large number of predictions are generated using three different nozzles over a wide range of fully-expanded jet Mach numbers and jet stagnation temperatures. These predictions are compared with experimental data from multiple jet noise experimental facilities. In addition, two models for the so-called 'fine-scale' mixing noise are included in the comparisons. Improved BBSAN predictions are obtained relative to other models that do not include propagation effects.

AIAA Journal, 2010

Broadband shock-associated noise is a component of jet noise generated by supersonic jets operating offdesign. It is characterized by multiple broadband peaks and dominates the total noise at large angles to the jet downstream axis. A new model is introduced for the prediction of broadband shock-associated noise that uses the solution of the Reynolds-averaged Navier-Stokes equations. The noise model is an acoustic analogy based on the linearized Euler equations. The equivalent source terms depend on the product of the fluctuations associated with the jet's shock-cell structure and the turbulent velocity fluctuations in the jet shear layer. The former are deterministic and are obtained from the Reynolds-averaged Navier-Stokes solution. A statistical model is introduced to describe the properties of the turbulence. Only the geometry and operating conditions of the nozzle need to be known to make noise predictions. This overcomes the limitations and empiricism present in previous broadband shock-associated noise models. Results for various axisymmetric circular nozzles and a rectangular nozzle operating at various conditions are compared with experimental data to validate the model.

Journal of the Acoustical Society of America, 2017

Supersonic crackle, an irritable component of aircraft jet noise, was investigated using model scale measurements. Near-field results showed Gaussian distribution but far-field had high skewness and even higher in its derivative. Skewness, a measure of asymmetry in the waveform, was compared to screech arising from shock associated noise which was also high but in contrast to crackle its skewness derivative had dropped to a much smaller value than its waveform. Both crackle and screech are nonlinear but their nonlinear properties are entirely different. Crackle is quantified when its derivative skewness becomes larger than its waveform skewness which should exceed 0.3.

Shock Waves

Broadband shock-associated noise (BBSAN) is a particular high frequency noise that is generated in imperfectly expanded jets. BBSAN results from the interaction of turbulent structures and the series of expansion and compression waves which appears downstream the convergent nozzle exit of moderately underexpanded jets. This paper focuses on the impact of the pressure waves generated by BBSAN from a large eddy simulation of a non-screeching supersonic round jet in the near-field. The flow is under-expanded and is characterized by a high Reynolds number Re j = 1.25 × 10 6 and a transonic Mach number M j = 1.15. It is shown that BBSAN propagates upstream outside the jet and enters the supersonic region leaving a characteristic pattern in the physical plane. This pattern, also called signature, travels upstream through the shock-cell system with a group velocity between the acoustic speed U c − a ∞ and the sound speed a ∞ in the frequency-wavenumber domain (U c is the convective jet velocity). To investigate these characteristic patterns, the pressure signals in the jet and the near-field are decomposed into waves traveling downstream (p +) and waves traveling upstream (p −). A novel study based on a wavelet technique is finally applied on such signals in order to extract the BBSAN signatures generated by the most energetic events of the supersonic jet.

2017

The sessions described analytical, computational, and experimental approaches to both fundamental and applied problems on model and full-scale jets and rocket exhaust plumes.

22nd AIAA/CEAS Aeroacoustics Conference, 2016

This study presents the shock-cell noise results obtained with a large eddy aeroacoustic simulation from a dual stream jet. The primary stream is cold and subsonic with an exit Mach number of Mp = 0.89 and a Reynolds number of Rep = 0.57 × 10 6. The secondary stream is cold supersonic and under-expanded with a perfectly expanded Mach number of Ms = 1.20 and Res = 1.66 × 10 6. The computations are performed with the structured multiblock solver elsA. The aerodynamics and aeroacoustics of the flow are studied and analyzed in detail showing good agreement with experimental fits for the lengthscale and shear layer development. An acoustic-hydrodynamic filtering is used in order to compute the characteristic wavelengths of each component and estimate the shock-cell noise frequency. Moreover, the broadband shock-cell noise is captured in the near and far fields.

39th AIAA Fluid Dynamics Conference, 2009

This paper examines the flow structures emanating from convergent-divergent nozzles with conical sections like those used in practical high-speed propulsion applications. LES simulations and experimental measurements in the form of PIV and shadowgraph imaging and far-field acoustic measurement are employed. The development of the supersonic jets from these nozzles are examined in underexpanded, perfectly expanded and overexpanded conditions. It is found that in addition to the shock diamond features expected of smoothly contoured nozzles at off-design conditions, these nozzles produce a second set of shock diamonds anchored at the nozzle throat and overlaid on the shock diamonds shed from the nozzle lip. These together form a doublediamond appearance. These nozzles do not exhibit any shock-free condition at or near the expected design condition. As a consequence, there is no absence of shock noise at or near the design condition. Near the design condition there is a diminution in the intensity of the screech tone, but no diminution in the broad-band shock-associated noise. These nozzles produce shock cells 9%-25% shorter than cells from a comparable smoothly contoured nozzle.