An experimental investigation of axial and near - axial flow over long slender cylinders, which involved both flow visualisation and hot - wire anemometry, is detailed. The investigation of this type of flow was instigated by the current interest in towed underwater sonar arrays. The need to discriminate between background noise of mechanical origin and the flow - induced noise generated on a moving underwater soundrecording device has produced a requirement for a greater understanding of the larger scale, lower frequency, turbulent flow processes in the wake and the boundary layer of a cylinder in both axial and near - axial flow. Of particular interest are any regular periodic fluid - dynamic processes. Thick axisymmetric boundary layers with the ratio of outer - layer length scale ( the boundary - layer thickness δ ) to cylinder radius a in the range 31 [approximately equal to or less than] δ / a [approximately equal to or less than] 38 and the corresponding ratio of cylinder radius to the inner - layer length scale ( the viscous length v / U [subscript τ] ) in the range 22 [approximately equal to or less than] aU [subscript τ] / v = a [superscript +] [approximately equal to or less than] 41 have been investigated. In accord with previous experimental results their mean - flow and turbulence properties are found to be strongly influenced by transverse curvature and to diverge significantly from those of flat - plate boundary layers. A characteristic feature of such thick axisymmetric layers is the occurrence of " spots " of low - speed fluid which are attributed to displacement of inner - layer fluid by large - scale turbulent cross - flows. A front of low - speed fluid which propagates radially across the boundary layer is identified as the primary large - scale, low - frequency, coherent structure within the boundary layer turbulence. A flow mechanism that describes the process by which these fronts are formulated on the basis of the experimental evidence formed from low - speed spots is obtained. The stripping of low - speed fluid from the cylinder surface by large - scale crossflows within the turbulent boundary layer is seen as an additional vorticity - and turbulence - generating mechanism, which cannot occur in a flat - plate layer. When the cylinder is yawed to the free - stream, an attached boundary layer persists over a small range of yaw angle, before flow separation occurs. In this range the boundary layer becomes extremely asymmetric, even at yaw angles less than 1 °. The asymmetry and mean - flow properties of such layers have been investigated for yaw angles of 0.25 ° and 0.5 ° at several Reynolds numbers in the range 300 [approximately equal to or less than] Re [subscript a] [approximately equal to or less than] 600. At somewhat larger yaw angles, a new regime of regular vortex - shedding in near - axial flow has been identified. From the experimental results, an empirical relation for the vortex - shedding frequency ( in terms of yaw angle, vortex - shedding angle, and a Reynolds number based on the component of free - stream velocity normal to the vortex axes ) has been derived as an extension of the Roshko formula for the frequency of vortex shedding from cylinders with their axes normal to the flow. The results presented advance the current understanding of the fundamental fluid mechanics of cylinders in axial and near - axial flow, and thereby have the potential to contribute to the advancement of the signal - processing techniques applied to towed underwater sonar arrays. / Thesis (Ph.D.)--School of Mechanical Engineering, 2005.

The objective of this project is to develop a device that can monitor the heart rate and respiration of cetaceans. This would provide a way to quantitatively measure stress and determine the impact of human activity on cetaceans, especially for certain species that have been difficult to monitor in the past. There are many challenges to developing such a device, including determining the appropriate type of sensor, reducing the effect of flow noise, and designing an effective attachment method; this paper primarily focuses on determining the most suitable acoustic transducer. Experiments were conducted to compare various acoustic sensors in detecting heart rate. The electronic stethoscope performed the best in the experiments, but the results showed that other transducers, such as accelerometers and pressure sensors, also performed well and could be successful options with further development. Data processing methods to identify heartbeats and characterize signals are also discussed in this paper. Future work on the project involves subsequent tests to address other design variables as well as replicate experiments on animals.

Tonpilz-type transducers are the most commonly used projectors in underwater acoustic applications. However, no complete design approach is available in the literature for such transducers. The present study aims to fill this gap in the literature by providing a systematic design approach for the Tonpilz-type transducers. The proposed methodology involves the use of different analytical models and a finite element model of such transducers. Each model provides a different level of accuracy that is tightly correlated with the models complexity and computational cost. By using these models sequentially starting with the simplest and fastest model to yield an initial design and concluding with the most detailed and accurate model to yield an optimized final design the overall design time is reduced and greater flexibility is given to the designer. An overview of each of these four models is given. The constructed models are benchmarked against published experimental data. The overall design methodology is demonstrated by systematically applying the four models to design a Tonpilz-type transducer. Possible improvements to the proposed methodology are discussed.

Shallow water acoustics is one of the most challenging areas of underwater acoustics; it deals with strong sea bottom and surface interactions, multipath propagation, and it often involves complex variability in the water column. The sea bottom is the dominant environmental influence in shallow water. An accurate solution to the Helmholtz equation in a shallow water waveguide requires accurate seabed acoustic parameters (including seabed sound speed and attenuation) to define the bottom boundary condition. Direct measurement of these bottom acoustic parameters is excessively time consuming, expensive, and spatially limited. Thus, inverted geo-acoustic parameters from acoustic field measurements are desirable. Because of the lack of convincing experimental data, the frequency dependence of attenuation in sandy bottoms at low frequencies is still an open question in the ocean acoustics community. In this thesis, geo-acoustic parameters are inverted by matching different characteristics of a measured sound field with those of a simulated sound field. The inverted seabed acoustic parameters are obtained from long range broadband acoustic measurements in the Yellow Sea '96 experiment and the Shallow Water '06 experiment using the data-derived mode shape, measured modal attenuation coefficients, measured modal arrival times, measured modal amplitude ratios, measured spatial coherence, and transmission loss data. These inverted results can be used to test the validity of many seabed geo-acoustic models (including Hamilton model and Biot-Stoll model) in sandy bottoms at low frequencies. Based on the experimental results in this thesis, the non-linear frequency dependence of seabed effective attenuation is justified.

Recent theoretical and experimental studies have demonstrated that an estimate of the Green's function between two hydrophones can be extracted passively from the cross‐correlation of ambient noise recorded at these two points. Hence monitoring the temporal evolution of these estimated Green's functions can provide a means for noise‐based acoustic tomography using a distributed sensor network. However, obtaining unbiased Green's function estimate requires a sufficiently spatially and temporally diffuse ambient noise field. Broadband ambient noise ([200 Hz-20 kHz]) was recorded continuously for 2 days during the SWAMSI09 experiment (next to Panama City, FL) using two moored vertical line arrays (VLAs) spanning 7.5m of the 20‐m water column and separated by 150 m. The feasibility of noise‐based acoustic tomography ([300-1000 Hz]) was assessed in this dynamic coastal environment over the whole recording period. Furthermore, coherent array processing of the computed ocean noise cross‐correlations between all pairwise combinations of hydrophones was used to separate acoustic variations between the VLAs caused by genuine environmental fluctuations-such as internal waves-from the apparent variations in the same coherent arrivals caused when the ambient noise field becomes strongly directional, e.g., due to an isolated ship passing in the vicinity of the VLAs.

The development of low-frequency sonar systems, using for instance a network of autonomous systems in unmanned vehicles, provides a practical means for bistatic measurements (i.e. when the source and receiver are widely separated) allowing for multiple viewpoints of the target of interest. Time-frequency analysis, in particular Wigner-Ville analysis, takes advantage of the evolution time dependent aspect of the echo spectrum to differentiate a man-made target (e.g. elastic spherical shell) from a natural one of the similar shape (e.g. solid). A key energetic feature of fluid loaded and thin spherical shell is the coincidence pattern, or mid-frequency enhancement echoes (MFE), that result from antisymmetric Lamb-waves propagating around the circumference of the shell. This thesis investigates numerically the bistatic variations of the MFE (with respect to the monostatic configuration) using the Wigner-Ville analysis. The observed time-frequency shifts of the MFE are modeled using a previously derived quantitative ray theory for spherical shell's scattering. Additionally, the advantage of an optimal array beamformer, based on joint time delays and frequency shifts (over a conventional time-delay beamformer) is illustrated for enhancing the detection of the MFE recorded across a bistatic receiver array.

Thesis (Ph. D.)--Joint Program in Applied Ocean Sciences (Massachusetts Institute of Technology, Dept. of Ocean Engineering; and the Woods Hole Oceanographic Institution), 2002. / Includes bibliographical references (leaves 161-170).

Thesis (M.Eng.)--Memorial University of Newfoundland, 2002. / Bibliography: leaves 115-116.

The propagation of acoustic waves through water-saturated granular sediments has been widely studied, yet existing propagation models can not adequately predict the speed and attenuation of sound across the range of frequencies of interest in underwater acoustics, especially in loosely packed sediments that have been recently disturbed by storms or wave action. Advances in modeling are currently dependent on experimental validation of various components of existing models. To begin to address these deficiencies, three well-controlled laboratory experiments were performed in gravity-settled glass beads and reconstituted sand sediments. Sound speed and attenuation measurements in the 0.5 kHz to 10 kHz range are scarce in the literature, so a resonator method was used to investigate a reconstituted sand sediment in this range. The literature contains laboratory and in situ measurements of sound speed and attenuation at higher frequencies, but existing models can not predict both the speed of sound and attenuation simultaneously in some sediments. A time-of-flight technique was used to determine the speed of sound and attenuation in monodisperse water-saturated glass beads, binary glass bead mixtures, and reconstituted sediment samples in the frequency range 200 kHz to 900 kHz to investigate the effect of sediment inhomogeneity. The effect of porosity, independent of changes in other sediment physical properties, has not been demonstrated in the experimental literature. Therefore, a fluidized bed technique was used to independently vary the porosity of monodisperse glass bead samples from 0.37 to 0.43 and a Fourier phase technique was used to determine the speed and attenuation of sound. Collecting these results together, measured sound speeds showed positive dispersion below 50 kHz while negative dispersion was observed above 200 kHz for some samples. Attenuation measurements showed an approximately f⁰̇⁵ dependence in the low frequency regime and an approximately f³̇⁵ dependence for large-grained samples in the high frequency regime. The laboratory experiments presented in this work demonstrate that both sound speed and attenuation in idealized loosely packed water-saturated sediments can not be simultaneously predicted by existing models within the uncertainties of the model input parameters, but the independent effect of porosity on sound speed can be predicted. / text

To positively identify marine mammals using passive acoustics, large volumes of data are often collected that need to be processed by a trained analyst. To reduce acoustic analyst workload, an automatic detector can be implemented that produces many detections, which feed into an automatic classifier to significantly reduce the number of false detections. This requires the development of a robust classifier capable of performing inter-species classification as well as discriminating cetacean vocalizations from anthropogenic noise sources. A prototype aural classifier was developed at Defence Research and Development Canada that uses perceptual signal features which model the features employed by the human auditory system. The dataset included anthropogenic passive transients and vocalizations from five cetacean species: bowhead, humpback, North Atlantic right, minke and sperm whales. Discriminant analysis was implemented to replace principal component analysis; the projection obtained using discriminant analysis improved between-species discrimination during multiclass cetacean classification, compared to principal component analysis. The aural classifier was able to successfully identify the vocalizing cetacean species. The area under the receiver operating characteristic curve (AUC) is used to quantify the two-class classifier performance and the M-measure is used when there are three or more classes; the maximum possible value of both AUC and M is 1.00 – which is indicative of an ideal classifier model. Accurate classification results were obtained for multiclass classification of all species in the dataset (M = 0.99), and the challenging bowhead/ humpback (AUC = 0.97) and sperm whale click/anthropogenic transient (AUC = 1.00) two-class classifications.