http://hdl.handle.net/1721.1/7605 2013-06-19T20:15:24Z http://hdl.handle.net/1721.1/79330 Study of directional ocean wavefield evolution and rogue wave occurrence using large-scale phase-resolved nonlinear simulations Xiao, Wenting, 1982- It is challenging to obtain accurate predictions of ocean surface wavefield evolutions due to several complex dynamic processes involved, including nonlinear wave interaction, wave breaking and wind forcing, and also wave interactions with currents and bottom bathymetry. With fast computational algorithms for nonlinear phase-resolved wave simulations and modern computational capabilities, we now develop and apply a direct large-scale nonlinear phase-resolved wavefield simulation tool, which we call SNOW (Simulation of Nonlinear Ocean Wavefields), to study the evolution of directional ocean waves and occurrence of rogue waves (extremely large waves). Using SNOW, we obtain an ensemble of nonlinear deep-water wavefield simulations, initialized by JONSWAP spectrum with a broad range of spectral parameters, over large space-time scales. Spectral evolutions, nonlinear wave statistics and rogue wave occurrence are investigated based on the simulated wavefields. The SNOW results are compared to available wave basin experiments and predictions from linear theory and approximate nonlinear-Schr6dinger-equation (NLS) type models. SNOW predictions give an overall better comparison with wave-basin experiments than NLStype model predictions. For initially narrow-banded and narrow-directional-spreading wavefields, we find modulational instability develops over short time, resulting in considerable spectral broadening, strongly non-Gaussian statistics and probability of rogue wave occurrence an order of magnitude higher than linear theory prediction. For longer time, the wave spectrum in SNOW simulations reaches a non-Gaussian quasi-stationary state, and this is not predicted by NLS-type models, where a continuous spectral broadening is observed. When waves spread broadly in frequency and direction, the modulational-instability effect is reduced and the wave statistics and rogue wave probability are close to linear theory prediction. Number and area-based probabilities are introduced to measure the likelihood of rogue wave occurrence. To effectively predict rogue wave occurrence in directional seas, we propose a new modified Benjamin-Feir index (MBFI), which accounts for the effects of wave directionality. It is shown that the occurrence probabilities of rogue waves are well correlated with MBFI over a broad range of spectral parameters. Based on a large catalogue of rogue waves found from SNOW simulations, the geometric shapes of rogue waves are analyzed using proper orthogonal decomposition (POD). It is found that rogue wave profiles can generally be described by a small number of POD modes. SNOW simulations are also used to investigate the influence of finite depth on the evolution of nonlinear wavefields. As water depth decreases, the modulational instability decreases and finally diminishes. It is found that the occurrence probability of rogue waves and wave kurtosis decrease as water depth decreases. The wave statistics and rogue wave occurrence in bimodal wavefields are also studied. The influence of swell on the wave statistics of single-modal wind sea is not monotonic. The occurrence probability of rogue waves can either increase or decrease depending on the bimodal spectrum shape. We find the rogue wave probability and wave kurtosis are minimized when the propagation directions of swell and wind sea are orthogonal. By assimilating wave measurements from in-situ buoy and/or remote sensing into SNOW, we develop and demonstrate the capability of phase-resolved reconstruction and forecasting of wavefield evolution and rogue wave occurrence. Such capability could significantly enhance marine design and operation. This research paves the way for a new-generation wave forecasting model that is capable of providing heretofore unavailable large-scale phase-resolved information on the ocean wave evolution. Such capability is critically useful such as in the understanding of rogue wave dynamics and in the practical marine operations and safety. Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 257-267). 2013-01-01T00:00:00Z http://hdl.handle.net/1721.1/79316 Manufacturing of lab-on-a-chip devices : characterizing seals for on-board reagent delivery Inamda, Tejas Satish The reagent delivery mechanism in a point-of-care, HIV diagnostic, microfluidic device is studied. Reagents held in an aluminum pack are released on the opening of a fluidic seal. Fluidic seals, controlling the flow of reagents, are characterized to reduce anomalies in the desired flow pattern. The nature of the current seal was investigated. Four seal patterns - line hemisphere, line flat, chevron hemisphere and chevron flat were created and tested for reagent delivery using a flow sensor and a force gauge. Preliminary experiments suggested that one of the patterns - "line-flat" - resulted in fewer flow anomalies. A parameter scoping exercise involving sealing process parameters (temperature, time, gap and distance) was performed for the line flat seal. The findings of this research can be divided into three categories - 1) bonding phenomenon in current seals, 2) influence of seal pattern on flow and rupture mechanics and 3) process parameters which result in the least flow anomalies. The current seal is found to be a bond between the exposed aluminum on the lid film and the heat seal coating on the dome film. The two chevron patterns result in large amounts of flow anomalies, the line hemisphere pattern also resulted in some instances of flow anomalies. The line-flat pattern creates a seal with the least flow anomalies. A specific set of temperature, time, gap and distance which minimizes flow anomalies was found. The flow performance of the reservoir improved and delamination decreased as the distance of the seal from the reservoir was reduced. The thesis also develops a model to better understand the deformation of the reagent carrying dome and flow dynamics prior to and following the opening of the seal. The dome deformation model provides a framework for relating volume, delamination and flow rate to the geometry and material properties of the reservoir pack. Thesis (M. Eng. in Manufacturing)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 93-94). 2013-01-01T00:00:00Z http://hdl.handle.net/1721.1/79315 Manufacturability of lab on chip devices : reagent-filled reservoirs bonding process and its effect on reagents flow pattern Saber, Aabed (Aabed Saud) In its lab-on-a-chip product, Daktari Diagnostics utilizes "reagent-filled reservoirs" as a means of storing and delivering the liquid reagent. During the clinical trials of the product, undesired reagent flow patterns (namely, flow anomaly 1 & flow anomaly 2) were noticed. This work focused on optimizing the bonding process of the reagent-filled reservoirs to the backbone. Also, the relationship between the bonding process parameters and the reagent flow pattern was studied in depth. To achieve the objective of this thesis, an experiment was designed in which independent variables were the heat sealing parameters (x, y, z) and accelerated aging and the dependent variables were bond strength and the reagent flow pattern. Experiments showed that optimal Heat Sealing parameters are: parameter x = 4.5, parameter y = 110 and parameter z = 1.5. At the optimal settings of bonding process, the highest bond strength was attained and the reagent flow improved considerably but flow anomalies were not completely resolved. Also, results showed that accelerated aging affected the bond strength negatively. Accelerated aging also affected the flow pattern negatively, but this effect was not statistically significant. Thesis (M. Eng. in Manufacturing)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, February 2013.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 71-73). 2013-01-01T00:00:00Z http://hdl.handle.net/1721.1/79314 Constraint-based navigation for safe, shared control of ground vehicles Anderson, Sterling J., Ph. D. Massachusetts Institute of Technology Human error in machine operation is common and costly. This thesis introduces, develops, and experimentally demonstrates a new paradigm for shared-adaptive control of human-machine systems that mitigates the effects of human error without removing humans from the control loop. Motivated by observed human proclivity toward navigation in fields of safe travel rather than along specific trajectories, the planning and control framework developed in this thesis is rooted in the design and enforcement of constraints rather than the more traditional use of reference paths. Two constraint-planning methods are introduced. The first uses a constrained Delaunay triangulation of the environment to identify, cumulatively evaluate, and succinctly circumscribe the paths belonging to a particular homotopy with a set of semi autonomously enforceable constraints on the vehicle's position. The second identifies a desired homotopy by planning - and then laterally expanding - the optimal path that traverses it. Simulated results show both of these constraint-planning methods capable of improving the performance of one or multiple agents traversing an environment with obstacles. A method for predicting the threat posed to the vehicle given the current driver action, present state of the environment, and modeled vehicle dynamics is also presented. This threat assessment method, and the shared control approach it facilitates, are shown in simulation to prevent constraint violation or vehicular loss of control with minimal control intervention. Visual and haptic driver feedback mechanisms facilitated by this constraint-based control and threat-based intervention are also introduced. Finally, a large-scale, repeated measures study is presented to evaluate this control framework's effect on the performance, confidence, and cognitive workload of 20 drivers teleoperating an unmanned ground vehicle through an outdoor obstacle course. In 1,200 trials, the constraint-based framework developed in this thesis is shown to increase vehicle velocity by 26% while reducing the occurrence of collisions by 78%, improving driver reaction time to a secondary task by 8.7%, and increasing overall user confidence and sense of control by 44% and 12%, respectively. These performance improvements were realized with the autonomous controller usurping less than 43% of available vehicle control authority, on average. Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 138-147). 2013-01-01T00:00:00Z