Supersonic jets, particularly shock-containing jets, often exhibit high-intensity, discrete-frequency acoustic tones. These tones are the signature of an aeroacoustic resonance loop established by ...

https://journals.sagepub.com/doi/pdf/10.1177/1475472X19834521

Aeroacoustic resonance and self-excitation in screeching and impinging supersonic jets – A review:

A class of plasma actuators called localized arc filament plasma actuators for high-speed and high-Reynoldsnumber flow and acoustic control has been developed at the Gas Dynamics and Turbulence Laboratory. Over the past several years, these high-bandwidth (0 to 200 kHz) and individually controlled actuators have been used successfully to excite the jet shear layer, jet column, and azimuthal instabilities in high subsonic and supersonic jets. The focus of this paper is to provide detailed information and sample results highlighting the capabilities and potential of the actuators and the control technique formixing enhancement, noise mitigation, and flow and acoustic diagnostics. The jet, using three different nozzles, is operated over a large range of jet Mach numbers (0.9 to 1.65), stagnation temperature ratios (up to 2.5), and Reynolds numbers (0:2 10 to 1:65 10). Over this space of operating conditions, the jet is found to respond to control with a large range of forcing Strouhal numbers and azimuthalmodes. The results reveal that the jet flowfield and acoustic farfield can be dramatically altered, providing a powerful control tool in these practical high-speed and high-Reynolds-number jets.

High-Speed and High-Reynolds-Number Jet Control Using Localized Arc Filament Plasma Actuators

Mean square estimation methods can be applied to the approximation of conditionally averaged turbulent fields to educe mean structure of the fields. The mathematical procedure is reviewed, and the results obtained by selecting events of increasing complexity are summarized.

Stochastic estimation of conditional structure: a review

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Theory Of Vortex Sound

Synthetic generation of equilibrium boundary layer turbulence from modeled statistics

Free shear layers are building blocks of many flows of interest in applications, including jets, cavity flows, and separated flows. It was found several decades ago that free shear layers are unsta...

Excitation of Free Shear-Layer Instabilities for High-Speed Flow Control

The near-field and acoustic far-field response of an unheated Mach 0.9 jet with Reynolds number 6.2×105 was investigated. The study included both the baseline and controlled jets utilizing plasma actuators. Simultaneous acquisition of the near- and far-field signals with the actuation phase enabled the use of phase averaging to reconstruct the signature of the large-scale coherent structures generated by excitation of instabilities in the shear layer of the jet. Decomposition of the near-field pressure into its constitutive hydrodynamic and acoustic fields is accomplished via the application of a filter in the frequency/wave-number domain. The results showed that both the hydrodynamic and acoustic response to excitation for Strouhal numbers StDF less than 0.50 could be well predicted by a simple linear superposition of the impulse response of the jet. The results appear to indicate that the dominant acoustic radiation reaching the far-field aft angles is being generated over an extended region of the jet ...

/pdf/near-field-and-acoustic-far-field-response-of-a-high-speed-1ul4c1jhnp.pdf

Near-Field and Acoustic Far-Field Response of a High-Speed Jet to Excitation

This work aims to study the dynamics of and noise generated by large-scale structures in a Mach 0.9 turbulent jet of Reynolds number $6.2\\times 10^{5}$ using plasma-based excitation of shear layer instabilities. The excitation frequency is varied to produce individual or periodic coherent ring vortices in the shear layer. First, two-point cross-correlations are used between the acoustic near field and far field in order to identify the dominant noise source region. The large-scale structure interactions are then investigated by stochastically estimating time-resolved velocity fields using time-resolved near-field pressure traces and non-time-resolved planar velocity snapshots (obtained by particle image velocimetry) by means of an artificial neural network. The estimated time-resolved velocity fields show multiple mergings of large-scale structures in the shear layer, and indicate that disintegration of coherent ring vortices is the dominant aeroacoustic source mechanism for the jet studied here. However, the merging of vortices in the initial shear layer is also identified as a non-trivial noise source mechanism.

Vortex dynamics and sound emission in excited high-speed jets

The irrotational near field of an unheated, Mach 0.9 jet (Reynolds number of 6.2x10) was acquired under both baseline and excited states (with the use of plasma actuators). Previous work had decomposed the full near field into its hydrodynamic and acoustic components using a linear filter in Fourier space, in order to analyze the acoustic source location and mechanism. The presence of numerical artifacts in the results prompted the current study, in which a new algorithm is proposed to filter the irrotational pressure field in wavelet space, using a spatio-temporal continuous wavelet transform. Results from the new method are then compared against those obtained using the standard Fourier-based algorithm, as well as another proposed algorithm based upon the Emprical Mode Decomposition. The ability of the wavelet-based algorithm to decompose the broadband irrotational pressure field into its constitutive components, as well as to identify and extract individual events, is shown to be superior to that of the other two decomposition methods.

Decomposition of the Near-Field Pressure in an Excited Subsonic Jet

Localized arc filament plasma actuators (LAFPAs) are used at the Gas Dynamics and Turbulence Laboratory (GDTL) for the purpose of controlling the shear layer development and acoustic radiation from a rectangular jet. Previous studies at the GDTL have shown these actuators to be effective at producing significant mixing enhancement and acoustic mitigation in high Mach number, high Reynolds number heated and unheated axisymmetric jets. In this work preliminary results are presented in an ideally expanded, unheated Mach 1.5 rectangular jet with an aspect ratio of 2, equivalent exit diameter of 2.54 cm (1 inch) and Reynolds number of ~1.5x10 forced using eight LAFPAs. A total of five forcing modes are assessed, with forcing Strouhal numbers ranging from 0.1 to 4.0. Numerous similarities are found between the far-field acoustic response of the rectangular jet to forcing and the response of the axisymmetric jets reported previously. All forcing modes reported here were found to produce a reduction in the far-field at low polar angles, though the amplitude of this reduction and the forcing Strouhal number it occurred at varied between the forcing modes. Forcing at moderate to high Strouhal numbers produces a reduction in the downstream spectral peak, with minimal to significant increase in acoustic radiation towards the sideline polar angles, depending on the forcing azimuthal mode. The response of the rectangular jet is similar for both azimuthal angles (major and minor axes) explored, though all forcing modes were found to produce a greater reduction in acoustic radiation along the major axis of the jet.

Control of a Supersonic Rectangular Jet Using Plasma Actuators

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