Laura A. Brooks

Broadband trailing edge noise from a sharp-edged strut

This paper presents experimental data concerning the flow and noise generated by a sharp-edged flat plate at low-to-moderate Reynolds number (Reynolds number based on chord of 2.0 × 10(5) to 5.0 × 10(5)). The data are used to evaluate a variety of semi-empirical trailing edge noise prediction methods. All were found to under-predict noise at lower frequencies. Examination of the velocity spectra in the near wake reveals that there are energetic velocity fluctuations at low frequency about the trailing edge. A semi-empirical model of the surface pressure spectrum is derived for predicting the trailing edge noise at low-to-moderate Reynolds number.

Fundamental and higher‐mode Rayleigh wave characteristics of ambient seismic noise in New Zealand

[1] In order to use ambient seismic noise for mapping Earth’s structure, it is important to understand the spatiotemporal characteristics of the noise field. This study uses data collected during four austral winter months of 2002 to investigate New Zealand’s ambient seismic noise field in the double-ocean-wave-frequency range (0.1‐0.3 Hz). It is shown via beamforming analysis that there are two distinct dispersive waves in the data. These waves can be separated. Their estimated phase velocities (2.5‐2 and 4‐3 km/s in the frequency range 0.14‐0.25 Hz) match well with fundamental and higher-mode Rayleigh dispersion curves. Studies of double-wave-frequency microseisms elsewhere generally show the Rayleigh noise fields to be dominated by fundamental mode waves. The reason why higher-mode signals are observed here may reflect a combination of long ocean wave periods, large waveheights, the direct deep water approach to narrow continental margins, and the proximity of the seismograph array to the source regions. Citation:Brooks, L. A., J. Townend, P. Gerstoft, S. Bannister, and L. Carter (2009), Fundamental and higher-mode Rayleigh wave characteristics of ambient seismic noise in New Zealand, Geophys. Res. Lett.,36, L23303, doi:10.1029/2009GL040434.

Green's function approximation from cross-correlations of 20-100 Hz noise during a tropical storm

Approximation of Greens functions through cross-correlation of acoustic signals in the ocean, a method referred to as ocean acoustic interferometry, is potentially useful for estimating parameters in the ocean environment. Travel times of the main propagation paths between hydrophone pairs were estimated from interferometry of ocean noise data that were collected on three L-shaped arrays off the New Jersey coast while Tropical Storm Ernesto passed nearby. Examination of the individual noise spectra and their mutual coherence reveals that the coherently propagating noise is dominated by signals of less than 100 Hz. Several time and frequency noise normalization techniques were applied to the low frequency data in order to determine the effectiveness of each technique for ocean acoustic applications. Travel times corresponding to the envelope peaks of the noise cross-correlation time derivatives of data were extracted from all three arrays, and are shown to be in agreement with the expected direct, surface-reflected, and surface-bottom-reflected interarray hydrophone travel times. The extracted Greens function depends on the propagating noise. The Greens function paths that propagate horizontally are extracted from long distance shipping noise, and during the storm the more vertical paths are extracted from breaking waves.

Ocean acoustic interferometry

Ocean acoustic interferometry refers to an approach whereby signals recorded from a line of sources are used to infer the Greens function between two receivers. An approximation of the time domain Greens function is obtained by summing, over all source positions (stacking), the cross-correlations between the receivers. Within this paper a stationary phase argument is used to describe the relationship between the stacked cross-correlations from a line of vertical sources, located in the same vertical plane as two receivers, and the Greens function between the receivers. Theory and simulations demonstrate the approach and are in agreement with those of a modal based approach presented by others. Results indicate that the stacked cross-correlations can be directly related to the shaded Greens function, so long as the modal continuum of any sediment layers is negligible.

Shallow-water seismoacoustic noise generated by tropical storms Ernesto and Florence

Land-based seismic observations of double frequency (DF) microseisms generated during tropical storms Ernesto and Florence are dominated by signals in the 0.15-0.5 Hz band. In contrast, data from sea floor hydrophones in shallow water (70 m depth, 130 km off the New Jersey coast) show dominant signals in the ocean gravity-wave frequency band, 0.02-0.18 Hz, and low amplitudes from 0.18 to 0.3 Hz, suggesting significant opposing wave components necessary for DF microseism generation were negligible at the site. Florence produced large waves over deep water while Ernesto only generated waves in coastal regions, yet both storms produced similar spectra. This suggests near-coastal shallow water as the dominant region for observed microseism generation.

On the aeroacoustic tonal noise generation mechanism of a sharp-edged plate

This letter presents an experimental study on the tonal noise generated by a sharp-edged flat plate at low-to-moderate Reynolds number. Flow and far-field noise data reveal that, in this particular case, the tonal noise appears to be governed by vortex shedding processes. Also related to the existence of the tonal noise is a region of separated flow slightly upstream of the trailing edge. Hydrodynamic fluctuations at selected vortex shedding frequencies are strongly amplified by the inflectional mean velocity profile in the separated shear layer. The amplified hydrodynamic fluctuations are diffracted by the trailing edge, producing strong tonal noise.

On the noise reduction mechanism of a flat plate serrated trailing edge at low-to-moderate Reynolds number

<4:2 10 ). Acoustic and aerodynamic measurementshave been taken using a at plate with both sharp and serrated trailing edges in theanechoic wind tunnel at the University of Adelaide. Trailing edge serrations are foundto achieve up to 13 dB of attenuation in the narrowband noise levels without modifyingthe directivity of the radiated noise. The noise reduction achieved with trailing edgeserrations is found to be dependent on their geometrical wavelength and Strouhal number,St

A RANS-based Statistical Noise Model for Trailing Edge Noise

ow velocities. The method uses a Green’s function for a semi-innite half-plane to generate a far-eld acoustic autospectrum using a statistical model for the turbulence in the boundary layer in the vicinity of the trailing edge. This statistical model is an assumed turbulent velocity cross spectrum, and is dened in terms of the mean ow data provided by a RANS simulation. Reasonably good agreement is observed between acoustic predictions and experiments for the NACA 0012 cases. For the DU 96-180, the model over-predicts the experimental acoustic spectra.

Airfoil Trailing Edge Noise Source Location at Low to Moderate Reynolds Number

A study of the effects of Reynolds number and angle of attack on the dominant acoustic source location relative to the trailing edge for airfoils at low-to-moderate Reynolds number is presented. This study, which was performed using acoustic beamforming in an anechoic wind tunnel, helps assess the influence of each test parameter on the distance of the acoustic source from the trailing edge thus improving our knowledge of how airfoils produce tonal noise. The results show that the acoustic source location of a NACA0012 airfoil varies along the chord for the frequency range 500 Hz to 3000 Hz at low-to-moderate Reynolds number. Changing the angle of attack and Reynolds number resulted in small changes to the acoustic source location within this frequency range. A modified flat plate, which only radiated broadband noise under similar flow conditions, presented a scattered distribution of source location between the range 300 Hz to 1200 Hz.

On the generation of airfoil tonal noise at zero angle of attack and low to moderate Reynolds number

The generation of tones from a NACA 0012 airfoil at zero angle of attack and Reynolds numbers of 50,000 to 150,000 was investigated using acoustic beamforming and surface flow visualization techniques in an anechoic wind tunnel. Boundary layer separation lengths were obtained using surface flow visualization techniques and XFOIL software and were input into a feedback loop equation to determine if the measured tones were generated by a feedback mechanism. Using the maximum point of velocity on the airfoil surface as a feedback length provided the best agreement with the measured tones. Boundary layer tripping on one airfoil surface resulted in a decrease in the primary tone amplitude and some of the secondary tone amplitudes. Applying a trip to both surfaces resulted in a significant noise decrease and no tones were observed.