Analysis of noise-controlled shear-layers

Eschricht, Dandy; Jordan, Peter; Wei, Mingjun; Freund, Jonathan B.; Thiele, Frank

doi:10.2514/6.2007-3660

Cited by 4 publications

(4 citation statements)

References 13 publications

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“…During this period, the mutual cancellations, which occur between neighboring vortices, are unbalanced, making possible the generation of a large amplitude sound wave. This assertion is consistent with the conclusions drawn by Wei and Freund 4 and Eschricht et al, 5 but we have here identified the actual flow event driving it and so we are in a position to propose a local explanation for the sound production mechanism and its control, free from the cloudiness of averaging.…”

Section: B Flow Dynamics For the Uncontrolled And Controlled Flowssupporting

confidence: 89%

“…Some subsequent work addressed the differences between the noisy and quiet mixing layers from other perspectives: Eschricht et al 5 showed that there are differences in the wavenumber-frequency spectra of the hydrodynamic pressure fields of the uncontrolled and controlled flows, with less energy being found in the radiating sector of the controlled flow spectrum. In that same effort it was shown that two-point, two-time causality correlations between acoustic pressure and the Lighthill source term were affected by the control: the controlled flow showed more effective source cancellation and was thus a less efficient source of sound.…”

Section: Introductionmentioning

confidence: 98%

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Intermittent sound generation and its control in a free-shear flow

et al. 2010

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Comparisons are made between direct numerical simulations ͑DNS͒ of uncontrolled and optimally noise-controlled two-dimensional mixing layers in order to identify the physical mechanism responsible for the noise reduction. The analysis is carried out in the time domain to identify events that are significant in sound generation and which are acted upon by the control. Results show that a triple vortex interaction in the uncontrolled mixing layer radiates high-amplitude pressure waves to the far acoustic field; the elimination of this triple merging accounts for 70% of the noise reduction accomplished by a body force control applied normal to the shear layer. The effect of this control is shown to comprise vertical acceleration of vortical structures; the acceleration, whose action on the structures is convected across the control volume, leads to changes in their relative convection velocities and a consequent regularization of their evolution, which prevents the triple merger. Analysis of a longer time series for the DNS of the uncontrolled mixing layer using a wavelet transform identifies several similar intermittent, noisy events. The sound production mechanism associated with such noisy events can be understood in terms of cancellation disruption in a noncompact source region, such as described by a retarded-potential formalism. This shows that acoustic analogies formulated from the perspective of quadrupole acoustic sources are, in principle, useful for the modeling of such events. However, this study also illustrates the extent to which time-averaged statistical analysis of sound producing flows can mask the most important source activity, suggesting that intermittency should be explicitly modeled in sound prediction methodologies.

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Section: B Flow Dynamics For the Uncontrolled And Controlled Flowssupporting

confidence: 89%

Section: Introductionmentioning

confidence: 98%

Intermittent sound generation and its control in a free-shear flow

et al. 2010

Self Cite

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show abstract

“…During this period, the mutual cancellations which occur between neighbouring vortices hold to a much lesser degree, making possible the propagation of a large amplitude sound wave. This assertion is consistent with the conclusions drawn by Wei and Freund 4 and Eschricht et al, 5 but in this case we can pinpoint a particular flow event and thus propose a local explanation, free from the cloudiness of averaging. It is interesting that the high-amplitude sound-wave comprises a compression followed by a depression, and not vica-versa.…”

Section: Iib Flow Dynamics For the Uncontrolled And Controlled Flowssupporting

confidence: 93%

“…Some subsequent work addressed the differences between the noisy and quiet mixing-layers from other perspectives. Eschricht et al 5 showed that there are differences in the wavenumber-frequency spectra of the hydrodynamic pressure fields of the uncontrolled and controlled flows, less energy being found in the radiating sector of controlled-flow spectrum. In the same work an analysis based on causality-correlations with the Lighthill source term was presented.…”

Section: Introductionmentioning

confidence: 99%

Intermittent Sound Generation in a Free-Shear Flow

Cavalieri

Jordan

Gervais

et al. 2010

16th AIAA/CEAS Aeroacoustics Conference

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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.

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Adjoint-based control of loud events in a turbulent jet

2014

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Efforts to reduce the noise from turbulent jets at fixed flow conditions, with aircraft noise as the principal technological motivation, have generally involved some degree of parametric empiricism often based upon a series of trial-and-error testing. As a result, it is unclear if the modest reductions found, in rare cases that do not greatly affect the flow field or incur prohibitive losses, are near the limit of what can be accomplished or if there are undiscovered opportunities for more substantive reductions with better designs or active control. We assess this using an adjoint-based optimization procedure in conjunction with an experimentally validated large-eddy simulation of a Mach 1.3 turbulent jet. The adjoint solution provides a definitive direction in which to adjust a model control actuation in order to reduce noise, providing guidance that seems lacking by any other current means. It is found that three conjugate-gradient iterations in the control space provide ∼3.5 dB of reduction, comparable to other reductions found empirically. The control seems to work by disrupting the coherence of acoustically efficient axisymmetric flow structures. The control and noise-reduction mechanisms are informative, but also suggest that any significantly quieter state would not be a simple perturbation from the uncontrolled jet. Additional iterations might reduce noise more significantly, but there might be only modest opportunities to reduce the sound from simple round turbulent jets without radical changes or relatively sophisticated controls. Though it is difficult to prove any behaviour in a global space of actuations, there does not seem to be a direct route based upon a local sensitivity gradient to substantially quieting a jet, even with an unrealistically flexible actuation. More complex jets or other noisy flows may be more amenable to control, in which case the adjoint-based optimization procedure demonstrated here could provide invaluable engineering guidance.

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Analysis of noise-controlled shear-layers

Cited by 4 publications

References 13 publications

Intermittent sound generation and its control in a free-shear flow

Intermittent sound generation and its control in a free-shear flow

Intermittent Sound Generation in a Free-Shear Flow

Adjoint-based control of loud events in a turbulent jet

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