Intermittent Sound Generation in a Free-Shear Flow

Abstract

Comparisons are made between direct numerical simulations of uncontrolled and optimally controlled mixing layers in order to understand what it is about the controlled flows that makes them substantially quieter. Special attention is paid to the possibility that the essential details of the source mechanism may be spatially and/or temporally localised: such features are hidden when second-order statistics such as spectra are considered; and indeed these are almost identical for the two flows. Analysis is thus performed in the time domain, in order to search for intermittent sound-producing events. The results show that a large-amplitude pressure wave associated with a triple vortex merger in the uncontrolled mixing layer contributes significantly to the farfield, and that this event has been eliminated in the controlled flow. The large amplitude pressure wave associated with this event appears to be due to two things: the axial concentration of a low-pressure zone associated with the merging of the three vortical structures on one hand, and an axially-extended high-pressure region which opens up in the low-vorticity region immediately upstream of the three said structures. These pressure distributions can be mechanistically understood in terms of centripetal forces associated with the vortex dynamics, and the sound production associated with this can be mechanistically understood in terms of the axial imbalance that occurs between the spatially-localised low pressure and the spatially extended highpressure.Having understood the above, we proceed to analyse a longer time-run simulation of the uncontrolled flow, to see if we can objectively extract similar events. We apply a wavelet transform to the radiated pressure field, and by means of this we identify a collection of similar signatures. In each case we find that these correspond to a similar mechanism.The results highlight the importance of considering sound-producing flows in the time domain, and using appropriately adapted signal processing. The implications for noisesource modelling, which are often based on second-order statistics, are also discussed.

Jet preferred mode vs shear layer mode

“…Note that the studies from Kibens 25 and Bechert & Pfitzenmaier 26 presented in the study of Gutmark & Ho 4 are not considered since they only measure the radiated noise spectrum. Spectral peaks in the acoustic field do not necessarily indicate the jet preferred mode since pairing of neighbouring vortices was shown to be the main source of sound production in a jet and there is no subharmonic component for the excitaion of the jet preferred mode 25,27,28 . Moreover, it is more plausible that natural disturbances in individual facilities affect the shear layer mode and hence its subharmonics rather than the jet preferred mode.…”

Reprint of: Using LES to explore sound-source mechanisms in jets

“…Motivated by the results of [4] we choose in this work to study the LES data using azimuthal Fourier decomposition and temporal wavelet transforms. The latter proved to be well suited to the identification of the aforesaid space-time localised source activity.…”

“…We here study a subsonic jet, which we compute by means of Large Eddy Simulation: by means of an analysis methodology similar to that applied to the optimally-controlled two-dimensional mixing-layer data, we endeavour to identify the salient features of the flow structure where sound production is concerned. We decompose the radiated pressure field into azimuthal Fourier modes, and then analyse the temporal structure associated with each of the modes by means of the wavelet transform (this proved particularly useful in the study of both the low Reynolds number two-dimensional numerical data [4] and high Reynolds number experimental data [7]). In this way we filter the pressure field so as to isolate the temporally localised high-amplitude events.…”