Glegg, Stewart A. L.

Knowledge of the effects of sensor mislocation is necessary for interpreting the outputs from vertical acoustic receiver arrays deployed in the ocean. The first phase of this study consists of deriving a numerical method to determine the geometric shape of a flexible array cable, anchored at its upper end and displaced by horizontal ocean currents. Analytic methods to verify the numerical method are then derived. The second phase of this study considers measurements of a simple sound source by perturbed array cables. The results are compared with those from straight cables to ascertain the effect of the sensor mislocation. These effects are evaluated in terms of array cable curvature and tilt.

When acoustic measurements of moving vehicles are made by a stationary observer, the Doppler shift has two detrimental effects on the interpretation of the data. The spectra are smeared by the change in Doppler factor during the vehicle pass by, and the motion induced phase shift in the signals causes errors. The measured signals can be corrected back to source time if a moving time delay correction is applied. However, when the signals are sampled digitally this time delay correction requires an estimate to be made of the signal level between samples. This can be achieved by using a digital filter with time varying coefficients which estimates the signal from at least two adjacent samples. Results of both numerical simulations and real applications of this technique will be given.

Knowledge of vehicle source heights is necessary for noise barrier design, but currently, no reliable method exists to measure this parameter. This study involves the development of system to evaluate source heights using a method known as the source breakdown technique. The first phase of this study consists of the demonstration of the source breakdown technique on loudspeakers in an anechoic chamber. The second phase involves tests outdoors using loudspeakers in the presence of ground reflections. The third phase consists of an assessment of sources on a single vehicle. In the first two phases, source breakdown results are compared with actual measurements for verification. Noise and source mislocation errors are considered, as well as methods to reduce their detrimental effects.

The theoretical solution for acoustic propagation in a wedge-shaped ocean with ideal pressure-release boundaries predicts a well-defined beam which diverges as the energy propagates out towards deep water. Outside of the beam, shadow zones are formed, and the beam's spatial extent is determined by the lowest mode of propagation. The purpose of the experimental results presented here is to check the theoretical analyses, as part of an ongoing investigation into three dimensional propagation in the ocean environment, and to investigate further the propagation characteristics of this type. It has been found that for downslope propagation, the spatial characteristics of the field in a direction parallel to the shore line are in good agreement with the theoretical solution and the azimuthal extent of the beam depends on the wedge angle and frequency.

With the increase of air traffic and the introduction of larger aircraft and therefore larger engines, the noise generated by aircraft engines have become of greater importance. In order to address these problems, noise prediction codes must be developed in order to better understand the noise generating process. This thesis addresses important issues related to broadband self-noise from ducted fans based on the prediction model developed by Glegg and Jochault [1]. By addressing issues regarding the prediction of broadband self-noise from an isolated airfoil with the observer in the far field directly overhead (at 90° above), improvements can be made to Glegg and Jochault's approach for ducted fans. The prediction of broadband self-noise at 90° above a single airfoil is done by considering boundary layer parameters, the results obtained are compared with theoretical approaches, as well as experimental results obtained by Brooks [2] in order to verify its accuracy.

The aim of this thesis is to develop a theory for non stationary propulsor flow noise. The model which is proposed is based on Amiet's paper "Acoustic Radiation from an Airfoil in a Turbulent Stream" [1], which describes broad band noise when a simple model of airfoil interacts with a turbulent flow, under the assumption of stationarity. The Karhunen-Loeve method provides a set of modes which describe the turbulent flow without the assumption of stationarity. A method is described to obtain broad band noise calculations when the mean turbulent flow varies with time and produces non stationary turbulence. A comparison of the numerical results obtained with the results from the paper of reference [1] shows the characteristics of time varying sound radiation. The various mathematical formulae will give a starting point to the analysis of real time varying flows, which are not considered in this thesis.

The purpose of this research is to study the modification of a turbulent flow as it passes through a cascade of flat plates. The results will then be compared with experimental results obtained in a companion experimental study being conducted at Virginia Tech. In a typical marine propulsor turbulent flow passes through a set of inlet guide vanes (IGVs) and then interacts with the propeller blades: this process creates unwanted vibration and sound. The purpose of this research is to determine if the arrangement of the IGVs can be used to reduce the propulsor noise generation. In this study the incoming flow to the propeller is modeled as homogeneous turbulence and the IGVs are represented by a cascade of flat plates. We will consider the equations, which describe the blade response to an incoming harmonic gust, and we will represent the turbulent flow using a modal description.

An integrated coastal ocean and acoustic propagation model has been implemented to determine the effects of the ocean variations on the acoustic propagation field applied specifically to SFOMC. The ocean dynamics were modeled using the sigma coordinate, orthogonal curvilinear grid, Princeton Ocean Model. By using forcing conditions of tide, river runoff, wind and realistic bottom topography, the resulting time variant regional sound velocity outputs from the model were used as inputs to the range dependent, parabolic equation, acoustic propagation model, RAM. The results show that the fluctuations in the ocean result in scintillation in the acoustic propagation field, and for higher frequencies this variability is uniformly distributed and at times as much as +/-3 dB. High resolution in the POM grid and the range and depth sizes for RAM were important for obtaining reliable simulation results.

High-resolution sound propagation measurements were made on a 1/10000 th-scale model of the Santa Lucia Escarpment, located off the Southern California coast. The tank was modified from previous experiments using a rubber coating on the tank bottom. High frequency, high resolution, Transmission Loss measurements were made on the SFTF range, Dania Florida. The Parabolic Equation Model RAM was used to validate these measurement sets. A new approach to account for shear wave effects on the Transmission Loss for the RAM model was developed. Using this new approach, the scaled low frequency Santa Lucia measurements showed excellent agreement with the RAM calculated TL, but there were discrepancies in the predictions of the high frequency at sea measurements at ranges greater than 1 km.

Experimental measurements have been conducted to investigate the effects of a three dimensional bathymetry on ocean acoustic propagation and our abilities to use array processing for localizing sources. This work is unique because it uses laboratory scale measurements to isolate the effects of the bottom bathymetry. Previous investigations using laboratory scale measurements have only used simplistic bottom profiles. In addition, experiments which have investigated the effects of the bottom bathymetry at sea have encountered difficulties isolating these effects due to range dependent sound speed profiles and the uncertainties of ocean acoustic experiments. The first part of this dissertation investigates the tracking of an acoustic source in a three dimensional shallow water environment. This work is comprised of two studies. The first study uses matched field processing for identifying the trajectory of a source. The second investigation uses experimental measurements and theoretical predictions to evaluate the beating angle bias caused by the use of plane-wave beamforming in the presence of bathymetric refraction. The second part of this dissertation uses laboratory scale measurements to analyze two and three dimensional propagation over a realistic bottom bathymetry. This series of investigations uses an inverse approach based on normal mode theory. The inversion algorithm is used to extract the normal mode amplitudes for the purpose of analyzing the measurements for two dimensional mode coupling and bathymetric refraction. The results of this investigation show that the bathymetry has a strong influence on the three dimensional acoustic field. Analysis of the experimental measurements identify that mode coupling and bathymetric refraction are important for propagation over the laboratory scale model and these effects adversely influence our abilities to localize sources in three dimensional shallow water environments. It is also shown that by incorporating three dimensional propagation models into the signal replica used by the array processor a significant improvement in performance can be achieved.