Acoustic Power Spectral Density Research Articles

Passive acoustic glider for marine acoustic environment characterization

Acoustic payload-equipped underwater gliders have great potential for assessing the marine soundscape (oceanic acoustic environment), including characterizing properties of the sound field, of acoustic sources, and of the marine environment (water column and seabed). This talk presents observations and analysis of passive acoustic recordings carried out by gliders during the 2017 Seabed Characterization Experiment (SBCEX17). Two Teledyne Webb Research Slocum gliders equipped with omni-directional hydrophones were deployed as virtual acoustic moorings in 72 m deep water on the New England Mud Patch for time periods of ∼6 days for one glider and ∼3 days for the other. Acoustic recordings collected by the gliders captured the marine soundscape, including natural ambient noise, marine-life sounds, ship noise, and acoustic signals transmitted by the SBCEX17 participants. The acoustic environment in the water column is characterized in terms of the acoustic power spectral density as a function of time, depth, and frequency. As an example of the use of passive acoustic gliders for environmental characterization, Bayesian geoacoustic inversion for seabed properties is presented, making use of recordings of controlled sources associated with SBCEX17. [Work supported by the Office of Naval Research.]

Investigation of Cavitation Noise in Cavitating Flows around an NACA0015 Hydrofoil

To provide theoretical basis for cavitation noise control, the cavitation evolution around a hydrofoil and its induced noise were numerically investigated. A modified turbulence model and Zwart cavitation model were employed to calculate the flow field and predict the cavitation phenomenon accurately. Then, the acoustic analogy method based on the Ffowcs Williams-Hawking (FW-H) equation was applied to analyze the cavitation-induced noise. Seven cavitation numbers were selected for analysis. Acoustic power spectral density (PSD) and acoustic pressure were investigated to establish the relationship between cavitation number and their acoustic characteristics. It was indicated that as cavitation number decreases, cavitation cycle length gets shorter and the magnitude of acoustic power spectral density increases dramatically. One peak value of acoustic power spectral density induced by the extending and retracting of leading-edge cavitation can be obtained under sheet cavitation conditions, while under cloud cavitation, two peak values of acoustic power spectral density can be obtained and are induced by superposition from leading-edge cavitation and trailing vortex.

Interaction between self-excited oscillations and fuel–air mixing in a dual swirl combustor

A partially premixed gas turbine model combustor close to an industrial design is investigated using Large Eddy Simulation (LES). Two flames, one stable and another unstable with self-excited oscillations are computed. In particular, this study addresses the previously unexplained transition of flame shape in the experiments, from V-shaped to flat when the flame becomes acoustically unstable, suggesting a notable change of the important convective delay in the thermoacoustic feedback loop. The LES results show good agreement with the measured velocities, temperature and mass fractions. The acoustic power spectral density (PSD) obtained from the LES of the unstable flame also agrees well with the measured amplitudes in the air plenum and combustion chamber, and reasonably captures the frequency with a slight under-prediction. A comparison of the stable and unstable cases shows different mixing and reaction behaviours despite similar mean velocity fields. Further detailed analysis shows that the different mixing behaviour is driven by the significantly varying air mass split between the two air passages during a thermoacoustic oscillation cycle. This variation is due to the different impedances experienced by the pressure oscillations propagating through the two swirling injector passages with different internal geometries. This causes a periodic variation of the radial momentum of the fuel jets injected between the two swirling air flows. The resulting flapping of the fuel jets creates an enhanced radial fuel–air mixing that leads to a flattened flame in the unstable case. This provides a new physical explanation for the transitions of flame shape observed in the experiments.

Sound Radiation From Point Acoustic Sources With Shield of Large Prolate Spheroidal Baffles

Sound radiation from stationary and rotating point acoustic sources with shield of rigid prolate spheroidal baffles is explored in the prolate spheroidal coordinate system. The formulae of far-field sound pressure and acoustic power are derived and acoustic power spectral density (PSD) in terms of circumferential and azimuthal wavenumber is manifested from the low frequency range to high frequency range. Acoustic wave propagation features in the spherical coordinate system as a particular case of the prolate spheroidal coordinate system are presented. Rotating sound sources cause the frequency veering phenomenon and change the patterns of PSD. Some spheroidal harmonic waves with lower and higher wavenumber for the large prolate spheroids cannot contribute to far-field sound radiation in the high frequency range when sound sources are close to the axes of the spheroids. Sound pressure directivity and acoustic power of stationary point sound sources are also analyzed with the variation of source location.

On the computation of low-subsonic turbulent pipe flow noise with a hybrid LES/LPCE method

Aeroacoustic computation of a fully-developed turbulent pipe flow at Reτ = 175 and M = 0.1 is conducted by LES/LPCE hybrid method. The generation and propagation of acoustic waves are computed by solving the linearized perturbed compressible equations (LPCE), with acoustic source DP(x,t)/Dt attained by the incompressible large eddy simulation (LES). The computed acoustic power spectral density is closely compared with the wall shear-stress dipole source of a turbulent channel flow at Re τ = 175. A constant decaying rate of the acoustic power spectrum, f -8/5 is found to be related to the turbulent bursts of the correlated longitudinal structures such as hairpin vortex and their merged structures (or hairpin packets). The power spectra of the streamwise velocity fluctuations across the turbulent boundary layer indicate that the most intensive noise at ω + z R < 1.5).

Acoustic radiation from an airfoil in a turbulent stream

A theoretical expression for the far-field acoustic power spectral density produced by an airfoil in a subsonic turbulent stream is given in terms of quantities characteristics of the turbulence. For an observer directly above the airfoil the relevant quantities are the spanwise correlation length of the turbulence as a function of frequency and the PSD of the vertical velocity fluctuations, both quantities being measured in an airfoil fixed co-ordinate system. In the derivation it is assumed that the spanwise correlation length is much smaller than the airfoil span. A more solid theoretical foundation is developed for a lift expression given by Liepmann for an airfoil in turbulence. Also, the analysis shows why it is not necessary, within certain limitations, to integrate over all gust wavenumbers in order to calculate the sound at a given observer location. Rather, for small turbulence length scales the observer hears only the sound produced by a particular wavenumber component of the turbulence, and thus the analysis can be simplified considerably. A comparison of the theory with experimental acoustic and lift measurements available in the literature shows good agreement.