Variable Fidelity Studies in Wake Vortex Evolution, Safety, and Control

The purpose of this research is to develop a variable-fidelity approach for addressing the safety of unmanned aerial system (UAS) operations in the national aerospace system (NAS). This task is implemented on the basis of safety investigation toolkit for analysis and reporting wake vortex safety system (SITAR WVSS) code, which is a dynamic low-fidelity model addressing generation, evolution, and interaction of the leader-aircraft wake vortex with the follower-aircraft lifting surfaces. The first part of the dissertation deals with the generation, evolution, and interaction of the wake vortices produced by an aircraft. In particular, it presents the results of the vortex safety analysis conducted for selected UAS operating alongside commercial aircraft in the terminal zone. The work further investigates and compares decay and transport of the wake vortex in the vicinity of various grounds including a solid surface, a forest canopy, and a water surface, representative of various terminal zone environments. The obtained high-fidelity results form the basis for reduced-order models to be integrated into the fast-analysis code under development for in-situ wake vortex safety predictions. The second part of the dissertation introduces a robust nonlinear control method that is proven to achieve altitude regulation in the presence of unmodeled external disturbances (e.g. wind gust, wake vortex disturbance) and actuator parametric uncertainty. This method is designed as a part of “Interaction” sub-module of the SITAR WVSS model. The results demonstrate the capability of the proposed nonlinear controller to asymptotically reject wind gust/wake-vortex disturbances and the parametric uncertainty. The proposed controller is a great choice for small UAV applications with limited computational resources

Global Topology of 3D Symmetric Tensor Fields

There have been recent advances in the analysis and visualization of 3D symmetric tensor fields, with a focus on the robust extraction of tensor field topology. However, topological features such as degenerate curves and neutral surfaces do not live in isolation. Instead, they intriguingly interact with each other. In this paper, we introduce the notion of {\em topological graph} for 3D symmetric tensor fields to facilitate global topological analysis of such fields. The nodes of the graph include degenerate curves and regions bounded by neutral surfaces in the domain. The edges in the graph denote the adjacency information between the regions and degenerate curves. In addition, we observe that a degenerate curve can be a loop and even a knot and that two degenerate curves (whether in the same region or not) can form a link. We provide a definition and theoretical analysis of individual degenerate curves in order to help understand why knots and links may occur. Moreover, we differentiate between wedges and trisectors, thus making the analysis more detailed about degenerate curves. We incorporate this information into the topological graph. Such a graph can not only reveal the global structure in a 3D symmetric tensor field but also allow two symmetric tensor fields to be compared. We demonstrate our approach by applying it to solid mechanics and material science data sets.Comment: IEEE VIS 202

Physical and numerical modeling of cross-flow turbines

Cross-flow (often vertical-axis) turbines (CFTs), despite being thoroughly investigated and subsequently abandoned for large scale wind energy, are seeing renewed interest for smaller scale wind turbine arrays, offshore wind, and marine hydrokinetic (MHK) energy applications. Though they are similar to the large scale Darrieus wind turbines, today\u27s CFT rotors are often designed with higher solidity, or blade chord-to-radius ratios, which makes their behavior more difficult to predict with numerical models. Furthermore, most experimental datasets used for numerical model validation were acquired with low solidity rotors. An experimental campaign was undertaken to produce high quality open datasets for the performance and near-wake flow dynamics of CFTs. An automated experimental setup was developed using the University of New Hampshire\u27s towing tank. The tank\u27s linear motion, control, and data acquisition systems were redesigned and rebuilt to facilitate automated cross-flow turbine testing at large laboratory (on the order of 1 meter) scale. Two turbines were designed and built---one high solidity (dubbed the UNH Reference Vertical-Axis Turbine or UNH-RVAT) and one medium-to-low solidity, which was a scaled model of the US Department of Energy and Sandia National Labs\u27 Reference Model 2 (RM2) cross-flow MHK turbine. A baseline performance and near-wake dataset was acquired for the UNH-RVAT, which revealed that the relatively fast wake recovery observed in vertical-axis wind turbine arrays could be attributed to the mean vertical advection of momentum and energy, caused by the unique interaction of vorticity shed from the blade tips. The Reynolds number dependence of the UNH-RVAT was investigated by varying turbine tow speeds, indicating that the baseline data had essentially achieved a Reynolds number independent state at a turbine diameter Reynolds number $Re_D \sim 10^6$ or chord based Reynolds number $Re_c \sim 10^5$. A similar study was undertaken for the RM2, with similar results. An additional dataset was acquired for the RM2 to investigate the effects of blade support strut drag on overall performance, which showed that these effects can be quite significant---on the order of percentage points of the power coefficient---especially for lower solidity rotors, which operate at higher tip speed ratio. The wake of the RM2 also showed the significance of mean vertical advection on wake recovery, though the lower solidity made these effects weaker than for the UNH-RVAT. Blade-resolved Reynolds-averaged Navier--Stokes (RANS) computational fluid dynamics (CFD) simulations were performed to assess their ability to model performance and near-wake of the UNH-RVAT baseline case at optimal tip speed ratio. In agreement with previous studies, the 2-D simulations were a poor predictor of both the performance and near-wake. 3-D simulations faired much better, but the choice of an appropriate turbulence model remains uncertain. Furthermore, 3-D blade-resolved RANS modeling is computationally expensive, requiring high performance computing (HPC), which may preclude its use for array analysis. Finally, an actuator line model (ALM) was developed to attempt to drive down the cost of 3-D CFD simulations of cross-flow turbines, since previously, the ALM had only been investigated for a very low Reynolds number 2-D CFT. Despite retaining some of the disadvantages of the lower fidelity blade element momentum and vortex methods, the ALM, when coupled with dynamic stall, flow curvature, added mass, and end effects models, was able to predict the performance of cross-flow turbines reasonably well. Near-wake predictions were able to match some of the important qualitative flow features, which warrants further validation farther downstream and with multiple turbines. Ultimately, the ALM provides an attractive alternative to blade-resolved CFD, with computational savings of two to four orders of magnitude for large eddy simulation and RANS, respectively

On Simulating Tip-Leakage Vortex Flow to Study the Nature of Cavitation Inception

Cavitation is detrimental to the performance of ships and submarines, causing noise, erosion, and vibration. This study seeks to understand cavitation inception and delay on a typical ducted propulsor by utilizing the SimCenter\u27s unstructured simulation and design system: U2NCLE. Specifically, three fundamental questions are addressed: 1. What are the macroscale flow physics causing cavitation inception? 2. How does cavitation inception scale with Reynolds number? 3. How can tip-leakage vortex cavitation inception be suppressed? To study the physics of cavitation inception, a ducted propulso simulation is developed and extensively validated with experimental results. The numerical method is shown to agree very well with experimental measurements made in the vortex core. It was discovered that the interaction of the leakage and trailing edge vortices cause the pressure to drop to a local minimum, providing ideal conditions for inception to occur. However, experimental observation shows that inception does not occur at the minimum pressure location, but rather at the point where the two vortices completely coalesce. At the point of coalescence, the simulation reveals that the streamwise core velocity decelerates, causing the air nuclei to stretch and burst. A Reynolds number scaling analysis is performed for the minimum pressure and maximum velocity in the vortex core. First, the numerical method is validated on a flate plate at various Reynolds numbers to assess the ability of typical turbulence models to predict Reynolds numbers ranging from one million to one billion. This scaling analysis methodology is then applied to the propulsor simulation, revealing that the minimum pressure in the vortex core is much less dependent on Reynolds number than was previously hypothesized. Lastly, to investigate means of delaying cavitation inception, the propulsor is parameterized and studied using design optimization theory. Concepts of vortex alleviation evident in nature are used to suggest suitable parameterizations. Also, dimension reduction is used to reduced the number of design variables. Finally, the concepts are implemented, evaluated, and shown to completely decouple the two vortices causing cavitation inception. Moreover, the minimum pressure in the vortex core is significantly increased

Understanding the Nanotube Growth Mechanism: A Strategy to Control Nanotube Chirality during Chemical Vapor Deposition Synthesis

For two decades, single-wall carbon nanotubes (SWCNTs) have captured the attention of the research community, and become one of the flagships of nanotechnology. Due to their remarkable electronic and optical properties, SWCNTs are prime candidates for the creation of novel and revolutionary electronic, medical, and energy technologies. However, a major stumbling block in the exploitation of nanotube-based technologies is the lack of control of nanotube structure (chirality) during synthesis, which is intimately related to the metallic or semiconductor character of the nanotube. Incomplete understanding of the nanotube growth mechanism hinders a rationale and cost-efficient search of experimental conditions that give way to structural (chiral) control. Thus, computational techniques such as density functional theory (DFT), and reactive molecular dynamics (RMD) are valuable tools that provide the necessary theoretical framework to guide the design of experiments. The nanotube chirality is determined by the helicity of the nanotube and its diameter. DFT calculations show that once a small nanotube 'seed' is nucleated, growth proceeds faster if the seed corresponds to a high chiral angle nanotube. Thus, a strategy to gain control of the nanotube structure during chemical vapor deposition synthesis must focus on controlling the structure of the nucleated nanotube seeds. DFT and RMD simulations demonstrate the viability of using the structures of catalyst particles over which nanotube growth proceeds as templates guiding nanotube growth toward desired chiralities. This effect occurs through epitaxial effects between the nanocatalyst and the nanotube growing on it. The effectiveness of such effects has a non-monotonic relationship with the size of the nanocatalyst, and its interaction with the support, and requires fine-tuning reaction conditions for its exploitation. RMD simulations also demonstrate that carbon bulk-diffusion and nanoparticle supersaturation are not needed to promote nanotube growth, hence reaction conditions that increase nanoparticle stability, but reduce carbon solubility, may be explored to achieve nanotube templated growth of desired chiralities. The effect of carbon dissolution was further demonstrated through analyses of calculated diffusion coefficients. The metallic nanocatalyst was determined to be in viscous solid state throughout growth, but with a less solid character during the induction/nucleation stage

Courbure discrète : théorie et applications

International audienceThe present volume contains the proceedings of the 2013 Meeting on discrete curvature, held at CIRM, Luminy, France. The aim of this meeting was to bring together researchers from various backgrounds, ranging from mathematics to computer science, with a focus on both theory and applications. With 27 invited talks and 8 posters, the conference attracted 70 researchers from all over the world. The challenge of finding a common ground on the topic of discrete curvature was met with success, and these proceedings are a testimony of this wor

Control of objects with a high degree of freedom

In this thesis, I present novel strategies for controlling objects with high degrees of freedom for the purpose of robotic control and computer animation, including articulated objects such as human bodies or robots and deformable objects such as ropes and cloth. Such control is required for common daily movements such as folding arms, tying ropes, wrapping objects and putting on clothes. Although there is demand in computer graphics and animation for generating such scenes, little work has targeted these problems. The difficulty of solving such problems are due to the following two factors: (1) The complexity of the planning algorithms: The computational costs of the methods that are currently available increase exponentially with respect to the degrees of freedom of the objects and therefore they cannot be applied for full human body structures, ropes and clothes . (2) Lack of abstract descriptors for complex tasks. Models for quantitatively describing the progress of tasks such as wrapping and knotting are absent for animation generation. In this work, we employ the concept of a task-centric manifold to quantitatively describe complex tasks, and incorporate a bi-mapping scheme to bridge this manifold and the configuration space of the controlled objects, called an object-centric manifold. The control problem is solved by first projecting the controlled object onto the task-centric manifold, then getting the next ideal state of the scenario by local planning, and finally projecting the state back to the object-centric manifold to get the desirable state of the controlled object. Using this scheme, complex movements that previously required global path planning can be synthesised by local path planning. Under this framework, we show the applications in various fields. An interpolation algorithm for arbitrary postures of human character is first proposed. Second, a control scheme is suggested in generating Furoshiki wraps with different styles. Finally, new models and planning methods are given for quantitatively control for wrapping/ unwrapping and dressing/undressing problems

Computational Studies of Structure and Surface Reactivity in Metal Nanoclusters

Metal nanoparticles exhibit physical and chemical properties unique to their length scale that have the potential to shape next generation technologies. The large structural and chemical space that determines nanoparticle properties requires feedback between computational and experimental studies to drive the discovery of both new architectures and target properties of these systems. This dissertation describes the application and development of theoretical methods to study metal nanoparticle electronic structure, developing new structure-property relationships and new concepts that govern nanoparticle behavior while connecting theoretical insight with laboratory observations. In Chapter 1, the dissertation is introduced by detailing how the connection between geometry and electronic structure has shaped the way we teach and understand chemistry across length scale, and projects these concepts onto the 1-3 nm length scale where traditional descriptors of electronic structure break down. In Chapter 2, the optoelectronic impact of alloying Cu with a Au nanocluster is studied, revealing how atomic descriptors and position of the heteroatom determine optical absorption in [Au25(SR)18] , providing an easily accessible experimental readout of electronic structure. Building on hypotheses tested in Chapter 2, Chapter 3 explores the size dependence of Cu/Au alloying. Here, the atom concentration and composition architecture are key parameters predicted to drive the emergence of plasmonic behavior in Au144-xCux(SR)60 nanocluster. In addition to the composition dependence of nanoparticle properties, the surface chemistry is known to dominate overall nanocluster electronic structure. In Chapter 4, both the type and specific molecular descriptors of the ligand were shown to impact the total magnetic moment of Co13 and Co55 model nanoclusters. Chapter 5 extends these concepts of surface chemistry to address the size dependent evolution of the ligand mediated magnetic properties in CoN nanoclusters, demonstrating how energy level alignment and orbital symmetry contribute to size dependent trends. Finally, Chapter 6 describes a reduced scaling computational method that improves the approximation of mean field excited state energy predictions, increasing the size and complexity of systems that can be treated with high accuracy

Compression of 4D medical image and spatial segmentation using deformable models

Ph.DDOCTOR OF PHILOSOPH