Oceanographic submersibles

A robotic ribbon fin with twelve independent fin rays, elastic fin membrane, and a body
of adjustable height was developed for this thesis specifically to test the 1990 theory put forth
by Lighthill and Blake that a multiplicative propulsive enhancement exists for Gymnotiform and
Balisiform swimmers based on the ratio of body and fin heights. Until now, the theory has not
been experimentally tested. Proof of such a momentum enhancement could have a profound effect
on unmanned underwater vehicle design and shed light on the evolutionary advantage to body-fin
ratios found in nature, shown as optimal for momentum enhancement in Lighthill and Blake’s theory.
Thrust tests for various body heights were conducted in a recirculating flow tank at different flow
speeds and fin flapping frequencies. When comparing different body heights at different frequencies
to a ’no-body’ thrust test case at each frequency no momentum enhancement factor was found. Data
in this thesis indicate there is no momentum enhancement factor due to the presence of a body on
top of an undulating fin.

This thesis presents an experimental analysis of the acoustic signature of an Ocean Explorer class AUV. The experimental analysis consists of three parts. The first part reports the measurements performed in an open water environment at NSWC in Lake Pend Oreille, Idaho. The second part reports on measurements performed at the FAU test tank on a mock model of the AUV and the third part reports the measurements also in the FAU test tank of the AUV under typical operating conditions. The model measurement results were also used to verify the prediction capabilities of a numerical FE model of the AUV using the reciprocity method. The measurements in the FAU tank considered different operating conditions and different mounting of the podule inside the AUV. The podule contains the main mechanical components of the AUV, which are the propulsion motor and the control surface motors. Also considered in these measurements is the influence of the propeller and the influence of covering the aft section of the AUV with a compliant layer. The results of this analysis show that the type of mounting of the podule is not very significant and that significant energy is transferred through the water trapped in between the podule and the hull. Furthermore, the propeller has a significant influence on the acoustic signature since it generates distinct tones. These tones were also observed in the results of the open water measurements.

Autonomous Underwater Vehicles (AUV) rely on acoustics for a number of mission functions such as communications (Acoustic Modem) and vision (Forward and Side Looking Sonars). The AUV acoustic signature (self-noise and vibration) can thus interfere with AUV operations. Additionally, underwater measurements such as turbulence measurements can be contaminated by interference between the AUV generated acoustics pressures and the low pressures of the turbulence. In this thesis a Finite Element and Boundary Element approach is developed to characterize the self-noise (vibration and radiated sound pressure) of a simplified FAU Ocean Explorer AUV. Mechanical excitation from the "podule", which contains the motors for the propulsion and motion control, is assumed in the analysis. The low frequency (less than 1Khz) results are dominated by two types of modes. One type associated with the motion of the "podule" as a rigid body on the vibration isolation supports that connects it to the rest of the AUV structure. The second type is associated with local structural deformations of the "podule", support frame, and AUV hull. Modifying the stiffness of the supports reduces the frequency of the rigid body modes of the "podule", but does not influence the frequencies of the local structural deformations of the "podule" and the rest of the AUV. Decreasing the stiffness of the supports should result in a reduced AUV acoustic signature.

The radiated noise from Autonomous Underwater Vehicles (AUV's) can interfere with on-board sensors and with certain type of missions. It is thus important to understand the parameters controlling the AUV self noise. In this thesis, measurement techniques and analyses are developed to investigate the mechanisms contributing to the acoustic noise of an Ocean Explorer class AUV. Measurements of the AUV acoustic signature are performed in a reverberant tank, after the tank is qualified to establish a reliable procedure to measure the AUV source levels. The measurement results are compared that obtained in an anechoic tank and in open-water. Acoustic measurements are correlated with vibration measurements performed on various components of the AUV, in order to identify the dominant components. From the results, some preliminary mitigation procedures to reduce the AUV acoustic signature are developed.

Recent successes in Autonomous Underwater Vehicle (AUV) technology have generated demand for broader, more general use of these vehicles, along with demand for longer, more complicated missions. However, AUVs are becoming more complex, and hence more difficult to program, test and maintain. A discrete event system would provide conditional execution for mission management, failure detection, and resource allocation techniques. The goal of this thesis is to provide a convenient formalism for discrete event systems that reduces the apparent complexity of the system while maintaining its robust capabilities. The chosen formalism allows the convenient representation of hierarchies of concurrent hierarchical state machines. The formal semantics of the model are discussed, along with complete data structures and algorithms. As an example, a complete design of the navigator state machine is provided.

Having the ability to dock an Autonomous Underwater Vehicle (AUV) can significantly enhance the operation of such vehicles. In order to dock an AUV, the vehicle's position must be known precisely and a guidance algorithm must be used to drive the AUV to its dock. This thesis will examine and implement a low cost acoustic positioning system to meet the positioning requirements. At-sea tests will be used as a method of verifying the systems specifications and proper incorporation into the AUV. Analyses will be run on the results using several methods of interpreting the data. The second portion of this thesis will develop and test a fuzzy logic docking algorithm which will guide the AUV from a location within the range of the sonar system to the docking station. A six degree of freedom simulation incorporating the Ocean Explorer's hydrodynamic coefficients will be used for the simulation.

The research envisaged and reported in this thesis refers to finding comprehensive algorithms to determine the handoff probabilities of new and handoff calls encountered in mobile communications. The traditional expressions for these probabilities that are reported in the literature, are deduced only on the basis of call arrival statistics applied to RF links between base station (BS) and the mobile unit (MU). However, such radio links inevitably suffer from fading. These channels are normally modeled by appropriate probability density functions (pdfs) of the faded signal envelope. Rayleigh, Rician and Nakagami-m distributions are popularly considered in depicting such fading channel characteristics. The traditional (queueing-theoretic) based estimation of handoff probabilities does not account for the hysteresis-specific handoff statistics in the relevant fading channels. This is in contrary to the reality, inasmuch as fading is an inherent part of RF channels in mobile communications. The present study offers a tractable method of combining queuing-theoretic (call arrival) statistics and the hysteresis-crossing statistics of a RSS metric so as to obtain proper expressions for new and handoff call handoff probabilities. The (upper and lower) bound specified spread of the handoff probabilities indicates that care should be exercised in resource allocation efforts with a margin. To the best of the knowledge of the author, this research exercise is new and has not been reported elsewhere in open literature.