Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1997. / Includes bibliographical references (p. 193-195). / by Robert Hugh Headrick. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1991. / Includes bibliographical references (leaves 108-111) / by Daniel Rudge and Chen-Yang Fei. / M.S.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 2004. / Includes bibliographical references (p. 101-102). / Studying and understanding acoustic scattering pattern from underwater targets has been of interest to various communities such as the archeologists and the navy for several reasons and applications. The present state-of-the-art technique in this area involves such methods as analytical approach and FEM/BEM numerical technique. This thesis aims to study and demonstrate the power of using the hybrid virtual source/FE approach where the physical presence of a target is replaced by virtual sources placed in the vicinity of the target and in a manner where the pressure/displacement relationship on the target surface is satisfied by the virtual sources when the target is being insonified. Accurate results for the far-field radiation of the target can be obtained by superposition of the point source Green's function of each virtual source. The hybrid virtual source/FE approach shows potential to be a computationally efficient method for computing acoustic scattering. The derivation of the dynamic flexibility matrix for an elastic conical shell with endcaps will be illustrated in this thesis. It will be shown that the dynamic flexibility matrix corresponds to the acoustic admittance matrix in the virtual source approach where the scattering functions are computed in the MIT's program OASES/SCATT. / (cont.) Moreover, the benchmarking and validation of the approach will be conducted with the hybrid analytical/ virtual source approach. Firstly, the approach predicts natural frequencies close to the theoretical analysis for higher order modes with more than 2 circumferential transverse vibration lobes. Secondly, it produces displacement profile that conforms to analytical results. The scattering functions are also in agreement those computed by the hybrid analytical/ virtual source approach, with discrepancies observed at lower frequencies. In exact terms, discrepancies start to appear for frequency in the range of 1000 to 2000 Hz for a 0.01m thick, 2 m long, 0.3m radius steel cylinder without endcaps. The scattering functions will be compared with the SCATT/OASES virtual source approach for pressure release and rigid cylinders and cones. For the hybrid FE/virtual source approach, the structural sound speed and density approach zero and infinity for pressure-release and rigid target respectively. On the other hand, in the SCATT/OASES virtual source approach, the pressure and displacement are required to vanish on the target surface respectively. It will be shown that the two approaches agree with each other. / (cont.) Moreover, scattering functions over steel cones and cylinders for various frequencies have also been derived in this research. The results will be interpreted physically and theoretically in this thesis. The importance of including structural damping in the finite element formulation of the target so as to reflect the effect of resonance on scattering will be illustrated. Other issues, such as effect of target orientations on scattering, will also be investigated in this thesis. The code has shown good potential for adaptation to compute scattering over other axisymmteric shapes using conical shells and circular plates as building blocks and the hybrid FE/ virtual source approach. / by Hwee Min Charles Low. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 2004. / Includes bibliographical references (leaves 74-75). / A high bit rate acoustic link for underwater video transmission is examined. Currently, encoding standards support video transmission at bit rates as low as 64 kbps. While this rate is still above the limit of commercially available acoustic modems, prototype acoustic modems based on phase coherent modulation/detection have demonstrated successful transmission at 30 kbps over a deep water channel. The key to bridging the remaining gap between the bit-rate needed for video transmission and that supported by the acoustic channel lies in two approaches: use of efficient image/video compression algorithms and use of high-level bandwidth-efficient modulation methods. An experimental system, based on discrete cosine transform (DCT) and Huffman entropy coding for image compression, and variable rate M-ary quadrature amplitude modulation (QAM) was implemented. Phase-coherent equalization is accomplished by joint operation of a decision feedback equalizer (DFE) and a second order phase locked loop (PLL). System performance is demonstrated experimentally, using transmission rate of 25000 symbols/sec at a carrier frequency of 75 kHz over a 10 m vertical path. / (cont.) Excellent results were obtained, thus demonstrating bit rates as high as 150 kbps, which are sufficient for real-time transmission of compressed video. As an alternative to conventional QAM signaling, whose high-level constellations are sensitive to phase distortions induced by the channel, M-ary differential amplitude and phase shift keying (DAPSK) was used. DAPSK does not require explicit carrier phase synchronization at the receiver, but instead relies on simple differentially coherent detection. Receiver processing includes a linear equalizer whose coefficients are adjusted using a modified linear least square (LMS) algorithm. Simulation results confirm good performance of the differentially coherent equalization scheme employed. / by Konstantinos Pelekanakis. / S.M.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1987. / Includes bibliographical references. / by Josko A. Catipovic. / Sc.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1994. / Includes bibliographical references (leaves 112-113). / by Viglundur Thor viglundsson. / M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1994. / Includes bibliographical references (leaves 66-68). / by Douglas A. Hamrin. / M.S.

Thesis (Nav. E. and S.M.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 2004. / Leaf 185 blank. / Includes bibliographical references (leaves 172-174). / Diesel and gas turbine electric ship propulsion are of current interest for several types of vessels that are important for commercial shipping and for the next generation of war ships. During the design process of a platform, a choice has to be made between two different fundamental concepts regarding propulsion; a conventional arrangement, and a diesel or gas turbine electric propulsion. For both concepts, the electrical installation is present and the demand for additional electric energy becomes a dominant parameter. In both cases, the selection of the prime mover significantly influences the effectiveness of the design. In this thesis, the simulation modeling of a complete propulsion system will be attempted, with overall emphasis on the prime movers. In the first part a diesel engine is considered. The time delay between changing a set point for the revolutions of the engine and the change of the real revolutions is often modeled as a first order system. However, this modeling is too simple to describe the real behavior of the diesel engine. More complex models exist, but in general they are too complex, describing the full thermodynamic behavior of diesels. / (cont.) So there is a need for a model that is more advanced than a first order system and less complex than complete thermodynamic models. Such a model has been derived, based on the Seiliger (thermodynamic) process. The results of the model show that the diesel engine behaves like a second order system when operating in the governor area and more like a first order system in the constant torque (overload) area. The simulation model of a diesel engine can be regarded as an explanation of the real engine operation, which combines the mathematical relationship between the relative components and can be used to simulate dynamic loading of the diesel engine. In the second part, a development of a nonlinear gas turbine model for loop- shaping control purposes is presented. The nonlinear dynamic equations of the gas turbine are based on first engineering principles. In order to complete the model, constitutive algebraic equations are also needed. These equations describe the static behavior of the gas turbine at various operating points. / (cont.) The complete, substituted nonlinear model is presented along with its model verification results based on a simulator and measured data. A mathematical description for the electric part of the propulsion and energy generation system with respect to numbers of components such as generators and thruster drives is attempted. Other electrical loads may be represented with an aggregate load. Based on the control functions focus on power production, advanced dynamic models shall be used for the generators and simplified static models shall be used for thruster drives and other loads. The final model shall be in a state-space vector form, suitable for control design. As a conclusion, a reliability analysis on the decision for the electric propulsion system is utilized based on market data, speed and electric energy requirements studies. The purpose of this study is to justify the employment of innovative and efficient electric propulsion systems for the future needs of the commercial and naval ship industries. / y Charalambos Soultatis. / Nav.E.and S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1989. / Includes bibliographical references (leaves 233-238). / by Moo-Hyun Kim. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 2001. / "February 2001." / Includes bibliographical references (leaves 99-100). / by Ami Salomon. / S.M.