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

Abstract

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.

Axisymmetric superdirectivity in subsonic jets

“…Experimental evaluation of the instantaneous interference between coherent structures in a flow is not an easy task, but such endeavours appear worthwhile considering the additional physical insight to be gained in terms of the dynamic law of jet noise source mechanisms. Furthermore, as seen by Cavalieri et al (2010), the details of the mutual interference in the source region can be crucial for the understanding of differences between uncontrolled, noisy flows and their controlled, quieter counterparts. for all m, which is not an accurate result: a uniform distribution in azimuthal modes is obtained only for a function with zero azimuthal coherence length, but for all physical quantities this length will be finite.…”

Wave Packets and Turbulent Jet Noise

“…When a wave-packet ansatz is used (Koenig et al 2012, Morris 2009, Papamoschou 2011, one can obtain parameters such as envelope amplitude, wavelength, position, and convection velocity using far-field measurements. Cavalieri et al (2012a) recently imposed radial structure on the wave-packet ansatz using eigenfunctions from linear stability analysis and used azimuthally decomposed far-field measurements to determine envelope parameters for higher-order azimuthal modes (m = 1 and m = 2).…”

Educing the source mechanism associated with downstream radiation in subsonic jets

“…Following a methodology similar to that of Cavalieri et al (2010Cavalieri et al ( , 2011b and Koenig et al (2011), a wavelet transform is used in order to identify temporally localized, high-amplitude fluctuations in the low-angle sound emission; those fluctuations are then tracked back into the conditional flow field to discern the flow behaviour that led to their emission. The continuous wavelet transform involves a projection of the pressure signal onto a set of basis functions that are localized in both time and time scale, being thus better adapted to the analysis of intermittent events than the Fourier basis.…”