Assessment of Optimization Methods for Aeroacoustic Prediction of Trailing-Edge Interaction Noise in Axisymmetric Jets

Our concern in this paper is in the fine-tuning of the arbitrary parameters within the upstream turbulence structure for the acoustic spectrum of a rapid-distortion theory (RDT)-based model of trailing-edge noise. RDT models are based on an appropriate asymptotic limit of the Linearized Euler Equations and apply when the interaction time of the turbulence with the surface edge discontinuity is small compared to the eddy turnover time. When an arbitrary transversely sheared jet mean flow convects a finite region of nonhomogeneous turbulence, the acoustic spectrum of the pressure field scattered by the trailing-edge depends on (among other things) the upstream turbulence via the Fourier transform of the correlation function, R22 (where subscript 2 refers to a co-ordinate surface normal to the plate). We show that the length and time scale parameters that govern the spatial and temporal de-correlation of R22 can be found using formal optimization methods to avoid any uncertainty in their selection by hand-tuning. We assess various optimization methods that are broadly categorized into an ‘evolutionary’ and ‘non-evolutionary’ paradigm. That is, we optimize the acoustic spectrum using the Multi-Start algorithm, Particle Swarm Optimization and the Multi-Population Adaptive Inflationary Differential Evolution Algorithm. The optimization is based upon different objective functions for the acoustic spectrum and/or turbulence structure. We show that this approach, while resulting in the total modest increase in computation time (on average 2 h), gives excellent prediction over most frequencies (within 2–4 dB) where the trailing-edge noise associated amplification in sound exists.

A Comparison of Mechanical Wind Filters for Infrasound Sensor Noise Reduction

Infrasound sensors record the ambient acoustic field that contains not only signals of interest but also noise and clutter. Noise is defined as atmospheric turbulence that is incoherent over the distances of meters, whereas, signals of interest and clutter are acoustic pressure waves that are coherent over 10s of meters to 100s of kilometers. There is a growing interest in monitoring sources that extend across the acoustic spectrum from infrasound (below 20 Hz) into the low-end audible acoustic (20–1000 Hz). Monitoring of these extended band signals with a single sensor is made possible with the development of contemporary infrasound sensors, such as Hyperion IFS-3000 with a flat response from 0.01 to 1000 Hz. Combining infrasound sensors with seismometers provides opportunity to better assess noise contributions for both sensor types and improve characterization of sources that occur close to the solid earth–atmosphere boundary. Because sensors are installed to target these broadband acoustic sources, considerations need to be made when selecting a mechanical wind filter to mitigate the noise, while minimizing the impacts to the signals of interest across these frequency ranges. Motivated by these opportunities, this article compares traditional infrasound wind filter designs, that is, porous hoses rosettes and domes, in an urban setting for frequencies 0.01–45 Hz. Data analysis compares the filters, in terms of their response to noise and signals with direct comparisons of wind filters, as a function of frequency. The quantification of performance of these filters in an urban setting provides insight into their effects on detection of sources of interest in this environment.

Analysis of Acoustic Signals During the Optical Breakdown of Aqueous Solutions of Fe Nanoparticles

The study investigates the spectra of acoustic signals generating during the optical breakdown of colloidal solutions of iron nanoparticles. A characteristic form of the acoustic spectrum has been experimentally established, a distinctive feature of which is the presence of signals from an expanding and collapsing gas bubble. It is shown that the amplitude and area of these acoustic signals depend on the concentration of nanoparticles in the irradiated colloid. The effect of the concentration of nanoparticles on the bubble lifetime corresponding to the time interval between the acoustic spectrum signals corresponding to the birth and cavitation of a gas bubble has been studied.