SHOCK ASSOCIATED NOISE

1998

Data from the direct numerical simulation (DNS) of a turbulent, compressible (Mach=1.92) jet has been analyzed to investigate the process of sound generation. The overall goals are to understand how the di erent scales of turbulence contribute to the acoustic eld and to understand the role that linear instability waves play in the noise produced by supersonic turbulent jets. Lighthill's acoustic analogy was used to predict the radiate sound from turbulent source terms computed from the DNS data. Preliminary computations (for the axisymmetric mode of the acoustic eld) show good agreement between the acoustic eld determined from DNS and acoustic analogy. Further work is needed to re ne the calculations and investigate the source terms. Work was also begun to test the validity of linear stability wave models of sound generation in supersonic jets. An adjoint-based method was developed to project the DNS data onto the most unstable linear stability mode at di erent streamwise positions. This will allow the evolution of the wave and its radiated acoustic eld, determined by solving the linear equations, to be compared directly with the evolution of the near-and far-eld uctuations in the DNS.

Journal of Fluid Mechanics, 2008

The primary objective of this investigation is to determine experimentally the sources of jet mixing noise. In the present study, four different approaches are used. It is reasonable to assume that the characteristics of the noise sources are imprinted on their radiation fields. Under this assumption, it becomes possible to analyse the characteristics of the far-field sound and then infer back to the characteristics of the sources. The first approach is to make use of the spectral and directional information measured by a single microphone in the far field. A detailed analysis of a large collection of far-field noise data has been carried out. The purpose is to identify special characteristics that can be linked directly to those of the sources. The second approach is to measure the coherence of the sound field using two microphones. The autocorrelations and cross-correlations of these measurements offer not only valuable information on the spatial structure of the noise field in th...

15th AIAA/CEAS Aeroacoustics Conference (30th AIAA Aeroacoustics Conference), 2009

High speed exhaust noise reduction continues to be a research challenge for supersonic cruise business jets as well as for current and future tactical military aircraft. Significant noise reduction may be possible from advanced concepts for controlling instability generated large-scale turbulence structures in the jet shear layer, generally accepted to be the source of aft-angle noise. In response to this opportunity, our team is focused on experimental diagnostic studies and unique instability modeling suited for identifying control strategies to reduce large scale structure noise. The current paper benchmarks the jet noise from supersonic nozzles designed to provide the supporting experimental data and validation of the modeling. Laboratory scale jet noise experiments are presented for a Mach number of Mj = 1.5 with stagnation temperature ratios ranging from Tr=0.75 to 2. The baseline configuration is represented by a round converging-diverging (CD) ideal expansion nozzle. A round CD nozzle with chevrons is included as the first of several planned non-circular geometries directed at demonstrating the impact on large scale structure noise and validating noise prediction methods for geometries of future technological interest. Overexpanded and underexpanded conditions were tested on both nozzle configurations. The resulting data base provides an opportunity to benchmark the statistical characteristics of round and chevron nozzle data. The current paper examines far field spectra, directivity patterns, and overall sound pressure level dependence comparing observed characteristics with the fine scale turbulence noise and large-scale turbulence structure noise characteristics identified by Tam. In addition, the paper probes the effect of chevrons on the developing flow field and suppression of screech tones. Measurements are also reported from a far-field narrow aperture phased array system used to map the acoustic source distribution on the jet axis. The dominant source region, situated between the end of the potential core and the sonic point, was found to agree with the peak amplitude location of the jet near field wavepackets measured using a unique near field array. This observation supports the cause-effect link between large-scale turbulence structures in the shear layer and their dominant contribution to aft radiated far field noise.

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.

Comptes Rendus Mécanique, 2005

The pressure field generated by a cylindrical boundary pressure wave with spatial modulation is calculated. This model is confronted to the experimental results in order to explain the two observed radiation patterns associated with subharmonic fluctuations in an excited jet shear-layer, namely either a superdirective emission, or a vortex pairing type of radiation with extinction angle. Regarding the far field, the superdirective pattern is recovered by using a generalized Gaussian as a modulation function and the vortex pairing features by a two lobe sinusoidal modulation, in agreement with the experimental description of the near pressure field. Regarding the near field, its exponential decay is also recovered in both cases. To cite this article: V.

16th AIAA/CEAS Aeroacoustics Conference, 2010

A tool based on Linear Stochastic Estimation and frequency-wavenumber filtering, aimed at clarifying the flow characteristics associated with sideline and downstream sound emission from subsonic jets, is presented. The wavenumber-frequency filter is first used to separate components radiating in two angular sectors: (0 • ≤ θ ≤60 •) and (60 • ≤ θ ≤120 •). Flow variables (pressure and velocity) are correlated with the pressure fluctuations observed in each of these sectors and conditional space-time flow fields associated with the two radiation sectors are then computed by means of stochastic estimation. An analysis of the conditional flow fields is presented. The three main results of the analysis are, for the flow we study: (1) the radiating sub-space of the jet shows a higher degree of organisation than the turbulence which drives it (using the POD eigenspectrum convergence as a metric of this organisation we find an order of magnitude difference between the full flow solution and the radiating flow skeleton); (2) organised large-scale flow motion is important for both downstream and sideline sound emission (though this may be due, in part, and particularly where sideline radiation is concerned, to the overly coherent nature of the LES solution); (3) a wavepacket mechanism associated with coherent structures is observed where downstream radiation is concerned.

Journal of Fluid Mechanics, 2005

22nd AIAA/CEAS Aeroacoustics Conference, 2016

Acoustic waves trapped in the potential core of subsonic turbulent jets have recently been observed and explained by Towne et al. 11, 13, 14 We show that these waves also radiate outside the jet, primarily into the upstream arc. We provide an experimental identification of the Mach-number dependence of the phenomenon, which indicates that the modes are active even when evanescent, probably due to turbulent forcing. Finally, we show that for Mach numbers lower than about 0.8, the strong tonal dynamics and sound radiation (up to 170dB) that occur when a sharp edge is placed close to the jet are related to a resonance mechanism involving convective hydrodynamic wavepackets and a 'slow', upstreampropagating, trapped acoustic mode. A Helmholtz scaling of the resonance at higher Mach number suggests involvement of the 'fast' trapped modes in the range 0.8 ≤ M ≤ 1.

49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2011

The most prominent component of turbulent mixing noise in jets is associated with Mach wave radiation. Large-turbulence structures radiate Mach waves efficiently when they convect supersonic relative to the ambient. An experimental study is conducted on an unheated (T0 = 286.6K) and heated (T0 = 1020.6K) fully expanded Mach 1.553 jet to investigate the effect of heat on this radiation process. The acoustic near-field was captured using a line array that comprised ten microphones situated in the hydrodynamic periphery of the dominant sound producing region, which is located downstream of the collapse of the potential core. Space-time correlations revealed a convective speed of the pressure signatures that was slightly larger than sonic, relative to the ambient, for the unheated jet, while being around M = 1.48 for the heated jet; Mach wave radiation occurred in both cases. A far-field circular arc array with a radius of 58.5 jet exit diameters was centered at the jet exit and consisted of twelve microphones ranging from 20 • to 135 • relative to the jet axis. A linear coherence and temporal correlation study unveiled mechanisms by which the near-and far-field pressures are coupled. Mach wave radiation is a fairly linear process. From arrival times of the acoustic disturbances traveling from near-to far-field it was found that propagation speeds were uniform in the heated case, while variations in speed, up to 15% above the ambient sound speed, were found for the unheated case.

12th AIAA/CEAS Aeroacoustics Conference (27th AIAA Aeroacoustics Conference), 2006

The near and far pressure fields generated by round, isothermal and cold jets of diameter D = 38 mm with Mach numbers varying over the range 0.6 ≤ Mj ≤ 1.6 are investigated experimentally, and characterized in terms of sound spectra and levels. Properties of near-field jet noise, obtained in particular at 7.5 diameters from the jet centerline, are documented. They differ appreciably from properties of far-field noise, and form a database that can be used for the validation of the acoustic fields determined by compressible Navier-Stokes computations. The near pressure fields originating from simulations can thus be directly compared, without resorting to extrapolation methods which might lead to uncertainties in the far pressure fields. In the present paper, sound sources localizations are also carried out from the near-field pressure signals. The experiments provide in addition far-field results evaluated at 52 diameters from the nozzle exit, in good agreement with the data of the literature. The classical dependence of jet noise features with the emission angle is observed. The level and frequency scalings of the pressure spectra obtained for subsonic jets in the sideline and downstream directions are also studied. For small radiation angles, the narrow-banded sound spectra measured are specially found to scale as the Strouhal number, whereas the one-third octave spectra seem to scale as the Helmholtz number, as previously shown by Zaman & Yu. 1