The increasing demand to integrate unmanned aircraft systems (UAS) into the national airspace is motivated by the rapid growth of the UAS industry, especially small UAS weighing less than 55 pounds. Their use however has been limited by the Federal Aviation Administration regulations due to collision risk they pose, safety and regulatory concerns. Therefore, before civil aviation authorities can approve routine UAS flight operations, UAS must be equipped with sense-and-avoid technology comparable to the see-and-avoid requirements for manned aircraft. The sense-and-avoid problem includes several important aspects including regulatory and system-level requirements, design specifications and performance standards, intruder detecting and tracking, collision risk assessment, and finally path planning and collision avoidance. In this dissertation, our primary focus is on developing an collision detection, risk assessment and avoidance framework that is computationally affordable and suitable to run on-board small UAS. To begin with, we address the minimum sensing range for the sense-and-avoid (SAA) system. We present an approximate close form analytical solution to compute the minimum sensing range to safely avoid an imminent collision. The approach is then demonstrated using a radar sensor prototype that achieves the required minimum sensing range. In the area of collision risk assessment and collision prediction, we present two approaches to estimate the collision risk of an encounter scenario. The first is a deterministic approach similar to those been developed for Traffic Alert and Collision Avoidance (TCAS) in manned aviation. We extend the approach to account for uncertainties of state estimates by deriving an analytic expression to propagate the error variance using Taylor series approximation. To address unanticipated intruders maneuvers, we propose an innovative probabilistic approach to quantify likely intruder trajectories and estimate the probability of collision risk using the uncorrelated encounter model (UEM) developed by MIT Lincoln Laboratory. We evaluate the proposed approach using Monte Carlo simulations and compare the performance with linearly extrapolated collision detection logic. For the path planning and collision avoidance part, we present multiple reactive path planning algorithms. We first propose a collision avoidance algorithm based on a simulated chain that responds to a virtual force field produced by encountering intruders. The key feature of the proposed approach is to model the future motion of both the intruder and the ownship using a chain of waypoints that are equally spaced in time. This timing information is used to continuously re-plan paths that minimize the probability of collision. Second, we present an innovative collision avoidance logic using an ownship centered coordinate system. The technique builds a graph in the local-level frame and uses the Dijkstra's algorithm to find the least cost path. An advantage of this approach is that collision avoidance is inherently a local phenomenon and can be more naturally represented in the local coordinates than the global coordinates. Finally, we propose a two step path planner for ground-based SAA systems. In the first step, an initial suboptimal path is generated using A* search. In the second step, using the A* solution as an initial condition, a chain of unit masses connected by springs and dampers evolves in a simulated force field. The chain is described by a set of ordinary differential equations that is driven by virtual forces to find the steady-state equilibrium. The simulation results show that the proposed approach produces collision-free plans while minimizing the path length. To move towards a deployable system, we apply collision detection and avoidance techniques to a variety of simulation and sensor modalities including camera, radar and ADS-B along with suitable tracking schemes.

Collision warning systems represent a promising means to reduce rear-end crash involvement. However, these systems experience failures in the real-world that may promote driver distrust and diminish drivers' willingness to comply with warnings. Recent research suggests that not all false alarms (FAs) are detrimental to drivers. However, very few studies have examined how different alarms influence different driving populations. The purpose of this research was to examine how younger, middle-aged, and older drivers (with and without UFOV impairments) evaluated and responded to four different alarm contexts - false alarm (FA), nuisance alarm (NA), unnecessary alarm (UA) and true alarm (TA) - when they did and did not receive warnings. FA contexts represent out-of-path conflict scenarios where it is difficult for the driver to identify the source of the alarm. NA contexts represent out-of-path conflict scenarios that occur in a predictable manner that allows drivers to identify the source of the alarm. UA contexts are transitioning host conflict scenarios where the system issues an alert but the situation resolves itself before the driver needs to intervene. TA contexts represent in-host conflict scenarios where the situation requires the driver to intervene to avoid a collision. The results suggest that alarm context does matter. Compared to response data that differentiates FA and NA from UA and TA, subjective data shows greater sensitivity and differentiates between all four alarm contexts (FA Younger drivers indicated a high degree of confidence in their own ability across the different conditions. While they adopted a similar response pattern as middle-aged drivers during the TA contexts, these drivers responded less frequently than middle-aged and older drivers during the UA context. Diminished hazard perception ability and the tendency to consider these situations less hazardous likely account for the fewer responses made during these situations by younger drivers. Older drivers with and without UFOV impairments indicated similar hazard ratings for UA and TA contexts, yet drivers with UFOV impairments responded less frequently in both alarm contexts. Diminished hazard perception ability, slower simple response times, and degraded contrast sensitivity likely account for the fewer and slower responses. Interestingly older drivers with impairments did respond more frequently when warned during the TA context. They also rated FAs and NAs more positively than the other driver groups. The results of this study suggest applying signal detection theory without concern for the alarm context and driver characteristics is insufficient for understanding how different alarms influence operators and that subjective data can inform design. Researchers are encouraged to combine multiple perspectives that incorporate of both an engineering and human perspective.

[pt] Nesta dissertação são apresentados cálculos detalhados de seções de choque de ionização múltipla de gases nobres por prótons de velocidades intermediárias. Foi realizado um estudo detalhado sobre a importância das probabilidades tanto de ionização direta de elétrons de cada subcamada como das probabilidades de emissão pós-colisional. Uma comparação com resultados teóricos e experimentais disponíveis na literatura também foi realizada. Com o intuito de investigar a influência dos valores adotados para as probabilidades de ionização em cada subcamada, foram realizadas versões que deixavam de levar em conta, gradativamente, a participação das diversas subcamadas do alvo. Foi analisada também a utilização de diferentes fontes de probabilidades de emissão pós-colisional. Observou-se que, para os estados de carga finais mais elevados, as seções de choque de ionização múltipla se tornam cada vez mais sensíveis tanto às probabilidades de ionização direta quanto às probabilidades de emissão pós-colisional. / [en] In this dissertation, detailed calculations of multiple ionization cross sections of noble gases by intermediate-velocity protons are presented. A detailed study on the relative importance of the probabilities for both the direct ionization of electrons from a given sub-shell and the postcollisional emission has been performed. The cross sections obtained in this work have also been compared to the experimental data and theoretical results available in the literature. In order to investigate the influence of the adopted values for the probabilities of direct ionization of each target sub-shell, different versions of the general equations have been used, in which the participation of each sub-shell has been gradually neglected. The use of probabilities for postcollisional emission from different authors has also been analyzed. It was observed that, for higher final charge states, the multiple ionization cross sections become more and more sensitive to both the direct ionization and the post-collisions emission probabilities.

Particle motion, clustering and agglomeration play an important role in natural phenomena and industrial processes. In classical computational fluid dynamics (CFD), there are three major methods which can be used to predict the flow field and consequently the behavior of particles in flow-fields: 1) direct numerical simulation (DNS) which is very expensive and time consuming, 2) large eddy simulation (LES) which resolves the large scale but not the small scale fluctuations, and 3) Reynolds-Averaged Navier-Stokes (RANS) which can only predict the mean flow. In order to make LES and RANS usable for studying the behavior of small suspended particles, we need to introduce small scale fluctuations to these models, since these small scales have a huge impact on the particle behavior. The first part of this dissertation both extends and critically examines a new method for the generation of small scale fluctuations for use with RANS simulations. This method, called the stochastic vortex structure (SVS) method, uses a series of randomly positioned and oriented vortex tubes to induce the small-scale fluctuating flow. We first use SVS in isotropic homogenous turbulence and validate the predicted flow characteristics and collision and agglomeration of particles from the SVS model with full DNS computations. The calculation speed for the induced velocity from the vortex structures is improved by about two orders of magnitude using a combination of the fast multiple method and a local Taylor series expansion. Next we turn to the problem of extension of the SVS method to more general turbulent flows. We propose an inverse method by which the initial vortex orientation can be specified to generate a specific anisotropic Reynolds stress field. The proposed method is validated for turbulence measures and colliding particle transport in comparison to DNS for turbulent jet flow. The second part of the dissertation uses DNS to examine in more detail two issues raised during developing the SVS model. The first issue concerns the effect of two-way coupling on the agglomeration of adhesive particles. The SVS model as developed to date does not account for the effect of particles on the flow-field (one-way coupling). We focused on examination of the local flow around agglomerates and the effect of agglomeration on modulation of the turbulence. The second issue examines the microphysics of turbulent agglomeration by examining breakup and collision of agglomerates in a shear flow. DNS results are reported both for one agglomerate in shear and for collision of two agglomerates, with a focus on the physics and role of the particle-induced flow field on the particle dynamics.

By the year 2020 the number of aircraft will have increased substantially and will be in �Free Flight�(that is, ATC will be devolved to the aircraft rather than being ground based). As an aid to navigation a more advanced form of collision avoidance will be required. This thesis proposes a method of collision avoidance planning using Automatic Dependent Surveillance-Broadcast (ADS-B) and Dynamic Programming (DP). It in essence enables Air Traffic Control (ATC) from within the cockpit for remote or uncontrolled airspace and is a step toward Free Flight. Free Flight requires quite different strategies than those used in the present collision avoidance schemes. This thesis reviews the approaches to collision avoidance used in the Air traffic navigation and to similar problems in other industries. In particular it considers the extended problem of collision avoidance within the framework of path planning. This is a key departure from the approach to aircraft collision avoidance used in the industry to date. Path planning reflects the real goal of an aircraft, which is to reach a particular destination efficiently and safely. Dynamic Programming is one solution method used in other industries for the problem of path planning to avoid collisions with fixed obstacles. The solution proposed herein for the Aircraft case uses Dynamic Programming applied to the moving obstacle case. The problem is first simplified by assuming fixed (static) obstacles for the cost minimisation algorithms. These fixed obstacles are then moved with time and the minimisation process is repeated at each time increment. Although this method works well in most cases, situations can be constructed where this method fails, allowing a collision. A modified approach is then used, whereby the movement of obstacles is included more explicitly (by modifying the shapes of the obstacles to represent motion) in the cost minimisation algorithm and a safe manoeuvre distance for each aircraft is used (by expanding the object size), to allow space for aircraft to execute safe evasive manoeuvres in difficult cases. This modification allows solutions which are complete (with no known cases of failure � collision situations) and should be considered as an important extension to the current Aircraft and Collision Avoidance System (ACAS). The testing of these solutions is focussed on the most difficult cases, and includes aircraft movement in �real space� (that is simulations using real aircraft dynamics together with dynamic programming algorithms running in discrete time steps).

L'avènement de l'imagerie médicale et de nouvelles techniques opératoires a bouleversé les méthodes de travail des médecins. Mais ce changement nécessitera une formation renforcée des praticiens et chirurgiens. C'est pourquoi le dévelopment d'outils appropriés comme les simulateurs médico-chirurgicaux se fait de plus en plus ressentir. Dans ce cadre, nous nous sommes intéressés au problème de la modélisation des phénomènes de déformation de tissu biologique et à la détection des collisions dans un environment virtuel. Dans un premier temps, nous présentons les différents modèles physiques existants et les différentes méthodes de résolution numérique associées aux objets déformable. Nous proposons ensuite un modèle développé pour la simulation de tissu biologique, en présentant successivement les aspects liés à la formulation du modèle, à la résolution du modèle, et au traitement des interactions physiques. Ce modèle, basé sur l'utilisation du principe de Pascal, permet de modéliser de manière relativement satisfaisante des corps biologiques, tout en permettant une simulation interactive. Dans un deuxième temps, nous présentons les différents algorithmes existants pour la détection de collision, ainsi que la difficulté d'adapter ces algorithmes aux simulateurs médicaux où les objets déformables complexes forment la base du modèle. Nous proposons ensuite les algorithmes développés pour traiter ce problème dans le cadre des simulateurs médicaux. Ces algorithmes présentent des caractéristiques de robustesse numérique et d'efficacité supérieures á l'existant, et permettent de traiter des corps déformables. Nous appliquons ces résultats dans le cadre d'un simulateur échographique de la cuisse humaine et d'une simulateur de chirurgie arthroscopique du LCA (ligament croisé antérieur du genou).

Free space is represented as a union of (possibly overlapping) generalized cones. An algorithm is presented which efficiently finds good collision free paths for convex polygonal bodies through space littered with obstacle polygons. The paths are good in the sense that the distance of closest approach to an obstacle over the path is usually far from minimal over the class of topologically equivalent collision free paths. The algorithm is based on characterizing the volume swept by a body as it is translated and rotated as a generalized cone and determining under what conditions generalized cone is a subset of another.

Bridges with low clearance are vulnerable to collision with overheight vehicles. Collisions of overheight vehicles can cause fatalities and injuries to the drivers and passengers of the overheight vehicles, and damage to bridge girders. The repair of the damaged bridges can be costly and time consuming. This research investigates the feasibility of developing a bridge bumper that minimizes the physical injuries and the likelihood of fatalities and protects the structural elements of bridges by absorbing the impact energy. The thesis describes a small-scale impact experiment using the proposed bridge bumper with several options of energy absorbing materials to protect a reinforced concrete beam. A finite element analysis is done to simulate the small-scale impact experiments. Optimization of the finite element model is carried out for the response quantities of interest with respect to the geometrical parameters and the material properties of the proposed bridge bumper. Such analysis can guide the design of an optimal bridge bumper that maximizes the energy dissipation and minimizes the damage to the bridge girder and the likelihood of fatalities and injuries.

This study was conducted with the purpose of furthering the knowledge of collisionsinvolving Swedish icebreakers in the Baltic Sea. It strived to identify the causes and directeffects of this type of collision. It also sought to establish if regulations and operationalguidelines, governing Swedish icebreakers and icebreaker assistance, were sufficient and ifthey were being adhered to.The method used to accomplish this was a qualitative literature study of all data regardingSwedish icebreaker collisions from the archives of the Swedish Maritime Administration andthe Swedish Transport Agency. Several accident investigation authorities from around theworld assisted in developing a method for processing the raw data.Having processed and examined the data the prevalence of different causes contributing to thecollisions could be established. All collisions occurred during icebreaker assistance. It wasfound that the all encompassing cause of the collisions was the difficulties of evaluating theice conditions and the behavior that difficult ice conditions necessitated the vessels to adopt.It was further made clear that the direct effects for the vessels, their crew and the environmentwere of limited severity. It was also concluded that the regulations and operational guidelinesgoverning Swedish icebreaker assistance, were generally being adhered to, and weresatisfactory in enabling a sufficiently safe and effective icebreaker assistance service. / Denna studie genomfördes i syfte att utöka den akademiska kunskapen om kollisioner sominvolverar svenska isbrytare i Östersjön. Den ämnade identifiera orsakerna och de omedelbaraeffekterna av dessa kollisioner. Vidare sökte den fastslå huruvida regelverk och riktlinjer förden svenska isbrytartjänsten var tillräckliga och om de efterlevdes.Studien genomfördes som en kvalitativ litteraturstudie av all data, med anknytning tillisbrytarkollisioner, som fanns tillgänglig i Sjöfartsverkets och Transportstyrelsens arkiv. Ettflertal utländska myndigheter som utreder olyckor assisterade vid framtagandet av modellenför bearbetning av datan.Efter att ha bearbetat och undersökt datan kunde förekomsten av olika bidragande orsaker tillisbrytarkollisionerna slås fast. Alla kollisioner inträffade under isbrytarassistans. Det stodklart att den övergripande orsaken var svårigheten i att utvärdera isförhållandena och deåtgärder fartygen tvingas vidta för att kunna ta sig fram genom isen. Vidare fastslogs att deomedelbara effekterna, för såväl fartyg som besättning och miljö, inte var förödande utan avbegränsad karaktär. Det fastslogs även att de regelverk och riktlinjer som styr den svenskaisbrytarverksamheten överlag efterlevdes och var tillräckliga för att säkerställa entillfredställande säker och effektiv isbrytartjänst.

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