44th AIAA Aerospace Sciences Meeting and Exhibit

Turbulent jet noise is a controversial fluid mechanical puzzle that has amused and bewildered researchers for more than half a century. Whereas numerical simulations are now capable of simultaneously predicting turbulence and its radiated sound, the theoretical framework that would guide noise-control efforts is incomplete. Wave packets are intermittent, advecting disturbances that are correlated over distances far exceeding the integral scales of turbulence. Their signatures are readily distinguished in the vortical, turbulent region; the irrotational, evanescent near field; and the propagating far field. We review evidence of the existence, energetics, dynamics, and acoustic efficiency of wave packets. We highlight how extensive data available from simulations and modern measurement techniques can be used to distill acoustically relevant turbulent motions. The evidence supports theories that seek to represent wave packets as instability waves, or more general modal solutions of the governing equations, and confirms the acoustic importance of these structures in the aft-angle radiation of high subsonic and supersonic jets. The resulting unified view of wave packets provides insights that can help guide control strategies.

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Wave Packets and Turbulent Jet Noise

Round jets originating from a pipe nozzle are computed by large-eddy simulations (LES) to investigate the effects of the nozzle-exit conditions on the flow and sound fields of initially laminar jets. The jets are at Mach number 0.9 and Reynolds number 105, and exhibit exit boundary layers characterized by Blasius velocity profiles, maximum root-mean-square (r.m.s.) axial velocity fluctuations between 0.2 and 1.9% of the jet velocity, and momentum thicknesses varying from 0.003 to 0.023 times the jet radius. The far-field noise is determined from the LES data on a cylindrical surface by solving the acoustic equations. Jets with a thinner boundary layer develop earlier but at a slower rate, yielding longer potential cores and lower centreline turbulent intensities. Adding random pressure disturbances of low magnitude in the nozzle also increases the potential core length and reduces peak r.m.s. radial velocity fluctuations in the shear layer. In all the jets, the shear-layer transition is dominated by vortex rolling-ups and pairings, which generate strong additional acoustic components, but also amplify the downstream-dominant low-frequency noise component when the exit boundary layer is thick. The introduction of inlet noise however results in weaker pairings, thus spectacularly reducing their contributions to the sound field. This high sensitivity to the initial conditions is in good agreement with experimental observations.

/pdf/influence-of-nozzle-exit-boundary-layer-conditions-on-the-497wfykzms.pdf

Influence of nozzle-exit boundary-layer conditions on the flow and acoustic fields of initially laminar jets

A low Mach number rod-airfoil experiment is shown to be a good benchmark for numerical and theoretical broadband noise modeling. The benchmarking approach is applied to a sound computation from a 2D unsteady-Reynolds-averaged Navier–Stokes (U-RANS) flow field, where 3D effects are partially compensated for by a spanwise statistical model and by a 3D large eddy simulation. The experiment was conducted in the large anechoic wind tunnel of the Ecole Centrale de Lyon. Measurements taken included particle image velocity (PIV) around the airfoil, single hot wire, wall pressure coherence, and far field pressure. These measurements highlight the strong 3D effects responsible for spectral broadening around the rod vortex shedding frequency in the subcritical regime, and the dominance of the noise generated around the airfoil leading edge. The benchmarking approach is illustrated by two examples: In both cases, the ability of computational fluid dynamics to model the source mechanisms and of the CAA approach to predict the far field are assessed separately.

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A rod-airfoil experiment as a benchmark for broadband noise modeling

We study the velocity fields of unforced, high Reynolds number, subsonic jets, issuing from round nozzles with turbulent boundary layers. The objective of the study is to educe wavepackets in such flows and to explore their relationship with the radiated sound. The velocity field is measured using a hot-wire anemometer and a stereoscopic, time-resolved PIV system. The field can be decomposed into frequency and azimuthal Fourier modes. The low-angle sound radiation is measured synchronously with a microphone ring array. Consistent with previous observations, the azimuthal wavenumber spectra of the velocity and acoustic pressure fields are distinct. The velocity spectrum of the initial mixing layer exhibits a peak at azimuthal wavenumbers ranging from 4 to 11, and the peak is found to scale with the local momentum thickness of the mixing layer. The acoustic pressure field is, on the other hand, predominantly axisymmetric, suggesting an increased relative acoustic efficiency of the axisymmetric mode of the velocity field, a characteristic that can be shown theoretically to be caused by the radial compactness of the sound source. This is confirmed by significant correlations, as high as 10 %, between the axisymmetric modes of the velocity and acoustic pressure fields, these values being significantly higher than those reported for two-point flow–acoustic correlations in subsonic jets. The axisymmetric and first helical modes of the velocity field are then compared with solutions of linear parabolized stability equations (PSE) to ascertain if these modes correspond to linear wavepackets. For all but the lowest frequencies close agreement is obtained for the spatial amplification, up to the end of the potential core. The radial shapes of the linear PSE solutions also agree with the experimental results over the same region. The results suggests that, despite the broadband character of the turbulence, the evolution of Strouhal numbers 0.3 ≤ St ≤ 0.9 and azimuthal modes 0 and 1 can be modelled as linear wavepackets, and these are associated with the sound radiated to low polar angles.

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Wavepackets in the velocity field of turbulent jets

Filtered azimuthal correlations in the acoustic far field of a subsonic jet

The effects of microjets on the aerodynamic characteristics of a Mach 09 high-Reynolds axisymmetric jet are investigated and are interpreted in the light of previous acoustic results These measurements are obtained by means of Stereoscopic Particle Image Velocimetry in planes normal to the jet axis Three parameters of the microjets system are varied: the outgoing mass flux per microjet, the number of microjets and their layout in the azimuth of the main jet The aerodynamic results indicate a strong correlation between the maximum level of turbulence just behind the nozzle exit and the high-frequency noise, previously shown to potentially balance the acoustic benefits obtained for lower frequencies The maximum level of turbulence measured at the longitudinal position corresponding to half the potential core length is also highly correlated to the jet noise reduction, which is highlighted by the similar evolution of these two quantities regarding the mass flux per microjet and the number of microjets For low values of the number of microjets, the microjets are shown to act independently, and their contributions to the turbulence reduction are retrieved far downstream the impinging point without any noticeable azimuthal diffusion

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Jet-Noise Reduction by Impinging Microjets : An Acoustic Investigation Testing Microjet Parameters

This experimental study examines the efficiency of wall pulsing control that is applied to a confined flow going through a two-dimensional diffuser continued by a duct. One- and dual-side pulsing generated by synthetic jet actuators are tested as control means. Particle image velocimetry is used to investigate instantaneous velocity fields and obtain quantitative information. One-side pulsing provokes vectoring of the primary flow in the same direction as the periodic excitation and increases its lateral expansion. Dual-side pulsing neutralizes vectoring but is more efficient for spreading. In both cases, the inner structure of the primary flow was found to be strongly modified, with the generation of highly periodic large-scale structures which kept the flow attached to the diffuser walls. The fluctuating velocity maps point out that the high-fluctuation zone greatly spreads out under control. The flow is considerably widened and the mixing is enhanced.

Synthetic jet control for flows in a diffuser: vectoring, spreading and mixing enhancement

Oscillating jet actuators have been implemented and tested on a circular cylinder. Their action on the separation of turbulent boundary layers is investigated using complementary approaches. Wall pressure distribution shows that a large lift is generated, at the expense, however, of a slightly increased drag. Particle image velocimetry measurements provide the mean and fluctuating velocity fields in the near-wake. The control jet deflects the mean flux lines towards the wall, illustrating that the separation is delayed. This effect appears more and more powerful as the pulsed jet velocity increases. Phase averaging of the PIV fields shows that periodic structures are generated by the control, and how these structures modify the aerodynamic forces by entraining the external flow towards the wall. Finally, a few comparisons are made with laminar boundary layers and some general mechanisms are presented for the lift increase.

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Changing lift and drag by jet oscillation: experiments on a circular cylinder with turbulent separation

https://acoustique.ec-lyon.fr/publi/castelain_cras06.pdf

Michel Sunyach

Papers

Filtered azimuthal correlations in the acoustic far field of a subsonic jet

Jet-Noise Reduction by Impinging Microjets : An Acoustic Investigation Testing Microjet Parameters

Synthetic jet control for flows in a diffuser: vectoring, spreading and mixing enhancement

Changing lift and drag by jet oscillation: experiments on a circular cylinder with turbulent separation

Noise reduction of a Mach 0.7–0.9 jet by impinging microjets