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.

In this dissertation large-eddy simulation(LES) is used to study the transitional and turbulent structures of vortices and free shear layers. The recently developed dynamic model and the basic Smagorinsky model are utilized to model the subgrid-scale(SGS) stress tensor. The dynamic model has many advantages over the existing SGS models. This model has the ability to vary in time and space depending on the local turbulence conditions. This eliminates the need to tune the model constants a priori to suit the flow field being simulated. Three different flow fields are considered. First, the evolution of large-scale turbulent structures in centrifugally unstable vortices is studied. It is found that these structures appear as counter rotating vortex rings encircling the vortex core. The interaction of these structures with the core results in the transfer of angular momentum between the core and the surroundings. The mean tangential velocity decays due to this exchange of angular momentum. Second, the generation and decay of turbulent structures in a vortex with an axial velocity deficit are studied. The presence of a destabilizing wake-like axial velocity field in an otherwise centrifugally stable vortex results in a very complex flow field. The inflectional instability mechanism of the axial velocity deficit amplifies the initial disturbances and results in the generation of large-scale turbulent structures. These structures appear as branches sprouting out of the vortex core. The breakdown of these structures leads to small-scale motions. But the stabilizing effects of the rotational flow field tend to quench the small-scale motions and the vortex returns to its initial laminar state. The mean axial velocity deficit is weakened, but the mean tangential velocity shows no significant decay. Third, a transitional mixing layer calculation is performed.The growth and breakdown to small scales of vortical structures are studied. Emphasis is given to the identification of late transition structures and their subsequent break down. Formation of streamwise vortices in place of the original Kelvin-Helmholtz vortices and the subsequent appearance of hair-pin vortices at the edges of the mixing layer mark the completion of transition. The basic Smagorinsky model is also used in the mixing layer simulations. The performance of the dynamic model is compared with the previous results obtained using the basic Smagorinsky model. As expected, the basic Smagorinsky model is found to be more dissipative. / Ph. D.

The interactions of trailing vortex pairs shed from the tips of two rectangular wings have been studied through helium bubble flow visualizations and extensive hot wire velocity measurements made between 10 and 30 chord lengths downstream. The wings were placed tip to tip at equal and opposite angles of attack, generating pairs of co-rotating and counter rotating vortices. Meaningful hot wire measurements could be made because the vortices were found to be insensitive to probe interference and experienced very small wandering motions. The co-rotating pairs were observed to rotate around each other and merge. Upstream of the merging location, the vortices have approximately elliptical cores. These are surrounded by the two wing wakes which join together around the two cores. Flow in the vicinity of the cores appears fully developed. During the merging process, the cores rotate rapidly about each other, winding the wing wakes into a fine spiral structure. Merger roughly doubles the core size and appears to produce turbulence over abroad range of frequencies. The counter rotating pairs move sideways under their mutual induction and slightly apart; their flow structure changing little with downstream location. These cores remain fairly circular and do not become fully developed within 30 chord lengths of the measurements. / Master of Science