Thesis (Nav.E. and S.M.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1999. / Includes bibliographical references (leaves 57-59). / by Konstantinos P. Mihanetzis. / Nav.E.and S.M.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1999. / Includes bibliographical references (p. 372-390). / The performance and agility of fish swimming motions have intrigued both biologists and fluid mechanicians for many years. Both have endeavored to understand the mechanics of fish swimming and to resolve the paradoxical observations surrounding the performance of fish, yet the unsteady hydrodynamics are not well-understood. In this thesis, the hydrodynamics of the fish-like swimming motions of a flexible body are examined through numerical simulation. Two- and three-dimensional boundary integral panel methods are developed which can model the steady straight-line swimming and unsteady maneuvering motions of a flexible-body of arbitrary thickness. Multiple, desingularized, infinitesimal wake sheet representation models the nonlinear dynamics of thin shear layer vorticity shed from an arbitrary number of predefined wake separation edges. The integrated performance quantities and the near-body unsteady flow features are corroborated through experimental comparisons. Employing this numeric scheme for a variety of fish forms, the unsteady flow dynamics are resolved in great detail and are found to be much more complex than that predicted by linear theory. In addition, fundamental mechanisms of near-body flow actuation, body-generated vorticity release, and wake vorticity control are found which allow the fish to generate thrust efficiently, to achieve outstanding performance, and to generate large, short-duration maneuvering forces. Specifically, the straight-line swimming motions of a flexible-body are studied through simulation of several fish geometries. Comparison to classical linear theory highlights the importance of the vortical dynamics in achieving performance and the complexity of the near-body flow patterns. The flow around the body is found to be highly longitudinal through systematic visualization of the body cross-sectional and waterline planes. A body-generated vortex ring structure, created through localized body undulations, actuates the smooth near-body longitudinal flow patterns around much of the fish body, resulting in strong vertical vorticity components bounding the wake thrust jet. Regions of high propulsive efficiency are identified for certain prescribed kinematics, and the performance is found to be strongly dependent on kinematic variation, recoil motions, and geometric modeling choices. Maneuvering hydrodynamics of fish swimming are studied through the simulation of a 60° "C" -turn of a Giant Dania. The formation and controlled release of body-generated vorticity through local contortions of the backbone is shown to affect the formation of a turning thrust jet for rapid maneuvering. The interaction body-generated free vorticity and regions of high fluid momentum with the sweeping motion of the tail fin is similarly shown to affect both the strength and direction of the turning jet. Through simulation of these straight-line swimming and unsteady maneuvering motions, fundamental mechanisms of vorticity control utilized by the fish are identified. Body-generated vorticity released by the body upstream is actuated by the motion of the oscillating tail fin, resulting in complex wake-wake-body interactions for varying kinematics. These interactions may enhance the performance by increasing thrust or increasing efficiency; similarly, large drag forces may be enhanced through constructive interaction of the wakes. Several novel vorticity control modes are elucidated for both straight-line steady swimming and unsteady maneuvering motions. Mechanisms of near-body flow actuation and vorticity control by the motions of swimming flexible-bodies and oscillating lifting surfaces may have tremendous potential for application to vehicle design and to unsteady maneuvering systems. / by Meldon John Wolfgang, IV. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 2004. / Includes bibliographical references (leaf 25). / This paper describes a scatter diagram approach for the classification of large numbers of current profiles for use in the prediction of riser fatigue damage due to vortex-induced vibration. Scatter diagrams have long been used to characterize the probability of various combinations of wave height and period, which are then used to assess wave forces. To predict VIV fatigue damage the designer needs to know which current profiles have the combined property of long regions of relatively constant velocity and relatively high speed. A sorting algorithm is proposed which searches every current profile for long regions of relatively constant flow speed. The probability of each length and speed combination is assessed and the data is used to populate the bins of the scatter diagram. The designer need only select relatively few representative profiles for detailed VIV analysis from those bins that would account for the most damage. The method is tested by making comparison to a brute force approach in which each of many thousands of profiles is evaluated for fatigue damage by running it in the SHEAR7 VIV response prediction program. / by Jessica Mary Donnelly. / S.M.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1994. / Includes bibliographical references (leaves 106-108). / by David Scott Barrett. / M.S.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 2003. / Includes bibliographical references (p. 151-153). / Experiments were performed on a three-dimensional NACA 0015 foil with a 5.5 cm average chord and 24.3 cm span performing a combined roll and pitch motion. The trailing edge of the foil was tapered. The motion was produced by harmonically rolling and pitching the foil near the root using two-axis control. Tests to determine the mean thrust coefficient of the foil were conducted over a wide parametric space. This parameter space included two roll amplitudes, with induced heave (at the 0.7 span) to chord ratio of hO.7 / c = 1.00 and 1.50; respectively, with Strouhal numbers ranging from 0.20 to 0.80; and maximum angle of attack varying between 15 and 50 degrees. The angle of attack and Strouhal number were also defined using the motion at the 0.7 span. A planform area mean thrust coefficient of 2.07 was recorded for 40 degrees maximum angle of attack and h0.7 / c = 1.50. Experiments to measure the mean lift and thrust coefficient were performed after adding a static bias to the foil pitch. Mean lift coefficients of near 4 were achieved in this manner. Further maneuvering tests were accomplished by measuring the forces produced by an impulsively starting foil in still water and at U = 0.4 m/s. Peak forces and impulse were measured for pitch angles between 30 and 90 degrees and for three different roll velocities. The highest propulsive impulse measured was 2.25 Newton-seconds at a maximum roll amplitude of 1.28 m/s and pitch angle of 40 degrees. Wake velocities were measured using Laser Doppler Velocimetry (LDV) and Digital Particle Imaging Velocimetry (DPIV) in three ancillary experiments. First, mean velocities across the wake at five spanwise locations were recorded using LDV. The wake width was found to be the excursion of the foil at the 70% span, validating our previous assumption. In the second experiment, using LDV, we measured the phase-average velocity and produced a three-dimensional plot of the wake behind the foil at the 0.7 span. Finally, DPIV for the same flapping parameters map the velocity field in the wake for various foil positions in a half-cycle. / by Melissa Dawn Flores. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1999. / Includes bibliographical references (leaves 161-166). / by Pubudu C. Wariyapola. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 2001. / Includes bibliographical references (p. 90-92). / United States Special Forces use high-speed planing boats in the performance of their missions. Operation of these boats, particularly in rough seas, exposes the occupants to severe mechanical shock exposure that has been linked to significant increase in the rates of acute and chronic injury. While many government and civilian organizations have researched various aspects of this problem over the past decade or more, no effective solution has yet been implemented in the fleet. In response to this problem, the Commander Naval Special Warfare Command in San Diego, CA forwarded a request to MIT's Ocean Engineering Department calling for a study of the problem. The object of this thesis is to conduct a comprehensive analysis of the problem, to research methods by which the problem can be mitigated, and to develop and validate a method for laboratory design, test, and evaluation, of shock mitigation systems. First, a theoretical and empirical study is conducted of the hydrodynamic interaction between a boat's hull and the seaway, and how this interaction results in the generation of mechanical shock. Actual acceleration data is obtained from the boats while underway in typical operating conditions, and other similar data is obtained from previous studies. Second, the mechanisms by which exposure to mechanical shock and vibration causes acute and chronic injury are investigated. Past human and animal testing is reviewed, along with information on the transmissibility and mechanical impedance of the human body. Information of this type, along with other injury data compilation studies, have contributed to existing injury prediction. Third, a study and is made of the methods by which mechanical shock exposure on high-speed boats can be mitigated. Interfaces (e.g.- hull-seaway) are identified where shock mitigation can be achieved, and existing or conceptual shock mitigation systems are discussed. Additionally, operational methods (such as training) of reducing shock exposure effects are discussed. Finally, a laboratory drop table apparatus is fabricated for use in the design, test and evaluation of shock mitigation systems. This test apparatus is validated by successful reproduction of shock events such as those experienced on high-speed boats, as well as by excellent repeatability and controllability. / by Sean D. Kearns. / S.M.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1995. / Includes bibliographical references. / by Bhavani Rajaram. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 2001. / Includes bibliographical references (p. 502-508). / When waves propagate through a medium with small irregularities of the size of the order of many wavelengths, a number of interesting phenomena may happen such as wave localization and large sensitivity of the wave field behavior with respect to the medium irregularity variation. These phenomena occur due to the interaction of the incident wave field with the medium small irregularities, and open the possibility of designing the system irregularity to achieve a desired vibratory response. This has a variety of engineering applications, such as the the design of the sea bottom of coastal areas to provide protection against the incoming swells, or the design of the material and geometrical properties of the cross section of pipelines, risers and mooring lines such that vibration transmission is minimized. The objective of this thesis is to understand how to tune the medium small irregularity such that the interaction of the incident wave field with the medium irregularity generates a desired reflected wave field. A particular design problem of interest is the prediction of the minimum amount of changes in the medium irregularity needed to minimize wave transmission to a desired level for a given range of frequencies of interest. As a model problem, we considered disordered chains of repetitive systems with the size of the order of many wavelengths of the incident wave. We applied an asymptotic theory for wave propagation along the non-uniform chain. For weak coupling between subsystems, the asymptotic theory predicted new results, such as exponential small transmission due to wave tunneling and explained localization phenomena as a turning point problem. For strong coupling, / (cont.) the asymptotic theory provided fundamental understanding of the effects of the irregularity on wave propagation. Pipelines and risers can be modeled as slender beams under tensile force. To describe well the effects of small irregularity in beams vibration, we derived asymptotically a simpler governing equation for the vibration problem. This new equation is asymptotic with respect to the beam irregularity steepness, but under the restriction of constant product of the flexural rigidity by the mass per unit length and constant tensile force, this new equation is an exact equation for the beam vibration and has a Helmholtz-like form. Inverse scattering methods for the Helmholtz-like equations can be applied to design the beam non-uniformity such that desired wave scattering properties are achieved. We also constructed a high order asymptotic solution for the scattering of mono-chromatic waves by the irregularity in slender beams. The asymptotic method used is the WKB method, which is basically a wave refraction theory, but we improved it such that wave reflection and wave mode conversion were captured. The asymptotic approach developed in the previous problems is extended and applied to the interaction of linear surface gravity waves with a bottom topography which varies slowly with respect to the length scale of the incident wave field. The asymptotic theory captured wave reflection and transmission and wave mode conversion, which leads to a more complete asymptotic representation of the wave field ... / by Karl Peter Burr. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1991. / Includes bibliographical references (leaves 119-125). / by George Anagnostou. / Ph.D.