Recent developments in estimation of the survivability of a U.S. Navy transport barge in random seas are extended to improve accuracy. The single Degree-of-Freedom (DOF) model of a extreme roll response of a barge used in previous research is replaced by a 3-DOF roll-heave-sway model to include linear and nonlinear static and kinematic coupling between roll, sway and heave. The predominant nonlinearity in the model arises in an improved approximation of the roll righting moment and heave buoyant restoring force by coupling roll with heave. Kinematic coupling is introduced by allowing extreme displacements and rotations in the barge response. System coefficients in the 3-DOF roll-heave-sway model and a simpler 2-DOF roll-heave model are identified by comparing time domain simulations with measured physical model tests of barge motions. Predictions of the 3-DOF and 2-DOF models are compared to measured test data for the case of random waves. Monte Carlo simulations of the equations of motions are performed to predict the reliability of the barge in an operational sea state for a specified mission duration. Use of parallel computer processing is found to make this a viable option for stability estimations as we move into the next century. The stochastic nature of the ocean waves are modeled via filtered white noise. Estimations of the joint probability of the barge responses are presented after application of density estimation kernels. Both the 3-DOF roll-heave-sway model and 2-DOF roll-heave model are tested and compared. Last, examples are provided of some observed nonlinear behavior of the barge motions for variation in damping or ocean wave amplitude. Transient and intermittent chaotic responses are observed for deterministic input waves and quasiperiodic cases are illustrated. / Graduation date: 1996

The response of lightweight equipment in structures supported on resilient-friction-base isolators (R-FBI) subjected to harmonic ground motion and various earthquake ground motions is examined. The equipment-structure base system is modeled as a three degree-of-freedom discrete system (SDOF subsystems). An efficient semi-analytical numerical solution procedure for the determination of equipment response is presented. Parametric studies to examine the effects of subsystem frequency (isolator, structure, equipment), subsystem damping, mass ratio, friction coefficient and frequency content of the ground motion on the response of the equipment are performed. The equipment response on a fixed-base structure subjected to ground motion is also calculated. Friction type isolation devices can induce high frequency effects in the isolated structure due to the stick-slip action. These effects on equipment response are examined. The results show that the high frequency effect in the structure generated from a friction-type base isolator doesn't, in general, cause amplifications in the response. The R-FBI system appears to be an effective aseismic base isolator for protecting both the structure and sensitive internal equipment. / Graduation date: 1992

Human stance in quiet mode, relies on feedback from eyes, skin, muscles and the inner ear and the control produced is a combination of strategies which enable a person to stay standing. This thesis presents the simulation and control of a hip actuated robotic model of human standing posture. / The first part of the thesis is devoted to recalling basic elements of the human balance system and to describe the balance strategies it uses to maintain an upright stance. Of the strategies presented, we consider the hip strategy which motivated the formulation of a hip actuated robot. An investigation into the control of nonlinear underactuated robots by linear controllers is done to verify the range and efficiency of the controlled system. / The second part of the thesis includes the investigation of two simplified models of the robot. Results using linear state feedback control are presented. The two models used are compared to clarify the use of one over the other. / We found that for linear controls, the size of the region of convergence decreased underactuated systems of increasing complexity. For our four degrees of freedom robot, the region of convergence is of 2.3 degrees for the actuated joints and of 1 degree for the unactuated joints. Our system is Lyapunov stable when the fully simplified model is assumed.

While the locomotion performance of legged robots over flat terrain or known obstacles has improved over the past few decades, they have yet to equal the performance of their animal counterparts over variable terrain. This work analyzes a multi-legged reduced order model of cockroach locomotion on variable slopes which will be used as an inspiration for a future sprawled posture legged robot. The cockroach is modeled as a point mass, and each leg of the cockroach is modeled as a massless, tangentially rigid, linearly elastic spring attached at the center of mass. All of the springs are actuated to allow changes in energy to the system. This is accomplished by varying the force free length of each leg in a feed-forward manner without reliance on feedback to change the actuation scheme. Fixed points of the model are found using a numerical solver that varies the velocity and phase shift parameters while leaving all other parameters at fixed values selected to match true cockroach motion. Each fixed point is checked for stability and robustness representing how effective the model is at staying on the predetermined gait, and transport cost as a measure of how efficient this gait is. Stable and robust fixed points were successfully found for the range of heading angles encompassing those of representative cockroach motion at each slope. Cockroaches may select the gait used based on stability or efficiency. Thus, additional fixed points were found in combination with a search routine that varies the leg actuation parameters in order to optimize either stability or metabolic efficiency, gaining insights into why cockroaches use the gaits that they do. Optimized fixed points were found based on four different leg functional combination families depending on whether each leg pushes or pulls. Optimized fixed point gaits exist for every incline slope studied between level ground and vertical slopes, at a range of initial heading angles that encompass those typically used by cockroaches. The selected gaits using both a stability based and an efficiency based optimization on the modeled cockroach are very similar. Both are also similar to gaits used by real cockroaches. The forces generated by the model are qualitatively similar to the experimental forces. / Graduation date: 2012

Thesis (Ph. D.)--University of Hawaii at Manoa, 1990. / Includes bibliographical references (leaves 125-128) / Microfiche. / xii, 128 leaves, bound ill. 29 cm

No description available.

Rotating machines such as motors, generators, turbines, etc. are crucial mechanical components of modern industrial and power generation facilities. For proper functioning of these facilities during and after an earthquake, it is essential that the rotating machines in these facilities also function as desired. The dynamics of a rotating machine is quite complex. It is further complicated by the presence of earthquake induced base motions. The response spectrum methods, which are now commonly used for calculating seismic design response of civil structures, cannot be used as such for calculating the design response of rotating machines. In this thesis, a response spectrum method which can be applied to the rotating machines is developed. To develop the response spectrum approach, a generalized modal superposition method is utilized. The random vibration analysis is applied to incorporate the stochastic characteristics of the seismic inputs. The applicability of the proposed response spectrum approach is verified by a simulation study where fifty sets of acceleration time histories are used. The proposed method considers the fact that earthquake induced base motions have several components, including rotational inputs. To define the correlation between the rotational and translational input components of the excitation, the correlation matrix and a travelling seismic wave approaches are used. The numerical results are obtained to evaluate the effect of rotational input components on the response of a rotating machine. It is observed that the rotational components are important only when they are very strong. In actual practice, such strong rotational inputs are not expected to excite rotors which are either directly placed on ground or are placed in common buildings. In the proposed spectrum approach, nevertheless, the effect of rotational input components can be easily incorporated if the correlation between various excitation components is specified. / Master of Science

Halo currents (HCs) in post-disruption plasmas can be large enough to exert significant electromagnetic loads on structures surrounding the plasma. These currents have axisymmetric and non-axisymmetric components, both of which pose threats to the vacuum vessel and other components. However, the non-axisymmetric forces can rotate, amplifying the displacements they cause when the rotation is close to the structures’ resonant frequencies. A new physically motivated scaling law has been developed that describes the rotation frequencies of these HCs and has been validated against measurements on HBT-EP, Alcator C-Mod, and other tokamaks. This scaling law can describe the time-evolution of the asymmetric HC rotation throughout disruptions on HBT-EP as well as the time-averaged rotation on C-Mod. The scaling law can also be modified to include the edge safety factor at the onset of rotation (𝒒_𝑜𝑛𝑠𝑒𝑡), which significantly improves its validity when applied to machines like C-Mod, where 𝒒_𝑜𝑛𝑠𝑒𝑡 changes frequently. The 𝒒_𝑜𝑛𝑠𝑒𝑡 dependence is explained by the relationship between the poloidal structure of the HC asymmetries and the MHD instabilities that drive them, which has been observed experimentally for the first time using a novel set of current sensing limiter tiles installed on HBT-EP. The 1/𝑎² and 𝒒_𝑜𝑛𝑠𝑒𝑡-dependence of the rotation suggest that the HCs predominantly rotate poloidally. This remains consistent with the toroidal rotation observed on HBT-EP and other tokamaks through the “Barber Pole Illusion” and the direction of rotation’s dependence on the direction of 𝐼_𝑝. This scaling law is used to make projections for next generation tokamaks like ITER and SPARC, which predicts that rotation will be resonant on ITER. However, resonant effects can still be avoided if the duration of the disruption is kept short enough to prevent two rotations from being completed.

Donaldson's single-point second-order model [13] is used to close the Reynolds stress transport equations in cylindrical coordinates. A reduced set of equations are then solved for the decay of axisymmetric vortices and jets. A self-similar solution to the axisymmetric vortices is obtained numerically. The characteristics of the mean flow variables as well as the Reynolds stresses in this solution are discussed. Comparisons of the current results with Donaldson[13J and Donaldson and Sullivan[16] are also presented. The results show that the vortex core is free from turbulent shear stresses. The turbulent kinetic energy is also found to be relatively weak within the core region. The overshoot of the circulation is found to be 5% of the circulation at infinity over a wide range of Reynolds numbers. The effects of Reynolds number on the decay of the vortices are computed and discussed. Some of the quantities, such as mean flow circulation and turbulent kinetic energy, are found to be sensitive to the Reynolds number. However, the overshoot is found to be insensitive to the Reynolds number but its location does. A set of suitable model constants for the axisymmetric jets is also found and a self similar solution for the jet case is obtained. Comparisons of the computed results with some recent experimental data are presented. / Master of Science

A novel method of reducing the spin of a rotating symmetric body, similar to many earth orbit satellites, is by allowing small, despin weights to unwind from about the satellite so that they absorb some, or all, of the satellite angular momentum. This technique which has been used successfully on several U.S. satellites is commonly referred to as yo-yo despin. Several studies of the motion of a system such as this have been published where it was assumed that the motion was two-dimensional (i.e., without coning). This dissertation presents a comprehensive study of the yo-yo despin problem which includes a derivation of two-dimensional results as well as a three-dimensional or exact solution. The results presented are sufficient for rudimentary design computations and provide examples of the corrections necessary to apply to two-dimensional computations for their applications as estimates for the general motion. An approximate solution of the three-dimensional equations of motion is also presented along with an example of the accuracy obtained by the approximation. The equations of motion are derived in a straightforward manner using the vectorial methods of Newtonian mechanics. The Euler equations for a rigid body are used to describe the motion of the rigid body itself. The moment acting on the body through the tension in the yo-yo cables is unknown and it is necessary to apply the second law of Newton to a despin weight so that sufficient independent differential equations are available for the solution of the problem variables. These relations give three first-order differential equations and two second-order ones. An expression for the cable tension is also obtained. This system of equations is integrated numerically by a standard Runge-Kutta process. Two singularities require special attention: first, at the initial instant the fundamental inversion matrix for the Newton equations is singular; and second, special care must be taken at a point in the integration where a discontinuity is found to occur. Outside of these special points, the integration process is quite routine although some cases require precautions near the end of the despinning process in order that the integration is stopped before violent tumbling occurs. In order to discuss the motion relative to a fixed reference axis the Euler angles, and Euler angle rate equations are also integrated. A point of interest concerning the derivation of the equations of motion is that the Lagrange technique cannot be used without modification due to internal constraints which do work. After a numerical study of several typical examples, one concludes that for initial coning angles of less than 10° a two-dimensional analysis is sufficient for determining many important design variables such as maximum cable tension and despin time, although the cable length is somewhat overestimated and problems may occur in the release of the weights if only a two-dimensional analysis is considered. If one desires information on the angular trajectory of the body in inertial coordinates, a study of the problem must be made using the exact three-dimensional relations or the approximate three-dimensional relations. The approximate expressions save the investigator a great deal of effort and apparently provides excellent results. / Ph. D.