VictorJuiz/thesis huggingface dataset:
Three-dimensional hydraulic fracturing simulation in heterogeneous materials using Lattice Element Method

Hydraulic fracturing is a technique for extracting unconventional resources by injecting high pressure fluid to crack the reservoir layer. This is a multi-scale and multi-physics problem that involves the behavior of rock matrix and injected fluid flow, as well as their interaction and resulting fracture propagation. The heterogeneities in the reservoir rocks make the fracture propagation a complex phenomenon and hydraulic fracturing an even more challenging problem./nIn this study, a C++ based three dimensional Lattice Element Method (LEM) simulator is developed, which is capable of simulating both mechanical and fluid induced fracturing behavior in heterogeneous media. In this model, the mechanical response of solid is represented with a 3D lattice structure. The Timoshenko beam with embedded discontinuity is used as a solid lattice model, and exponential softening law is incorporated. The fracture surface is represented by the discontinuity generated in the lattice once it yields. While the fluid lattice network is a pipe flow system generated at fracture surfaces. This fluid lattice model follows Darcy's law and the principle of mass conservation, and interacts with solid lattice network through hydrostatic pressure. The 3D multi-physics problem is simplified into a network composed of 1D beam and pipe lattice, which can be simulated with a relatively low computational cost. The validations and comparisons between numerical and experimental results indicate that LEM is a promising tool for investigating the process of fracture development. Its simplicity makes it capable of simulating branching and complex interactions between multiple fractures, which is extremely difficult in continuum-based methods. A nonlinear softening law is implemented in the solid lattice model, which allows the simulation of the failure of materials with different degrees of ductility. In addition, the potential application of LEM to early crack detection is explored./nThe model is used to investigate the influence of rock heterogeneities on the hydraulic fracturing process, especially the fracture pattern. The results show that, all the factors, including in situ stress field, scale of weakness and statistical distribution of properties, affect the results to a different degree. The nodal arrangement is one of the main factors that affect potential fracture path. The interactions between hydraulic fractures and pre-existing natural joints are also investigated. The hydraulic fracture penetrates into a joint when the normal stress on the joint surface is too small to stop the joint from opening under fluid pressure. Whereas a fracture crossing is more likely to occur when there is little or no slippage between two sides of the joints. This implies a possible mechanism of fracture crossing in which the fracture reinitiates on the other side of the joint due to tensile stress generated by the friction applied on the joint surface.

Advisor: Soga, Kenichi
Analysis of Uganda’s Plastic Waste Challenge from an Exergy, Thermal Phase Change, and Development Perspective to Improve Resource Efficiency and Social Impact

In low-income countries, 93% of waste is burned or dumped while only 2% of waste in high incomecountries is improperly disposed of in this manner. This research focuses on the plastic waste problem in Uganda—a country where 600 tonnes of plastic waste is generated daily and only 6% is collected. Polyethylene terephthalate (PET) is the most prevalent type of plastic waste in Uganda, but it cannot be recycled anywhere in East Africa. Plastic waste in developing countries is a complex, multifaceted problem, and little comprehensive research exists on the subject. Through a technical, social, and business approach, I attempt to present a holistic analysis./nAn extended exergy analysis was conducted to quantify the resource use and environmentalimpact of seven different disposal and recycling options for plastic waste that are feasible with Uganda’s limited infrastructure. The results revealed that recycling into a new product is the most resource efficient end-of-life option for PET plastic in Uganda and that transportation can be a major exergy cost. To reduce transportation and keep economic benefits within a community rather than exporting the majority of the profits, a local, circular economy recycling solution was proposed. Additionally, a fundamental phase change heat transfer analysis was used to model the solar drying process many Ugandan recyclers use to dry their plastic after washing./nTo demonstrate the feasibility of this proposed distributed recycling model, I co-foundeda social enterprise in Gulu, Uganda. We developed a process and built local machines to transform PET waste into durable, beautiful wall tiles making us the only recyclers of PET in Uganda. Using a community-based approach, emphasizing product-market fit, and pushing the bounds of recommended polymer processing practices, we developed and proved the viability of a solution that did not before exist. This dissertation presents a framework and an example of how to create sustainable solutions for complex technical and social problems that are technically viable, financially sustainable, and embraced by community members.

Advisor: Carey, Van P
Never the same river twice: On the causes and consequences of stream drying across space and time

Rivers act as dynamic forces that shape Earth’s changing surface, and whose scars we can still see punctuating our landscapes. Characteristically everchanging, they have inspired many poets, thinkers, and wanderers with their mutable nature. The quote “no man steps in the same river twice, for it is not the same river and he is not the same man” – attributed to the Ancient Greek philosopher Heraclitus – is known to be true for anyone who has seen a river swell beyond its banks, only to watch it shrink back to a narrow passage. But what about the streams that dry? The temporary, the seasonal, the irregular, the ephemeral, the discontinuous? These waterways, termed in this dissertation as ‘intermittent’, periodically, and naturally, cease to flow. They are similarly unpredictable, where the patterns and extent of drying are highly variable, with some reaches going dry year after year, regardless of previous conditions, while others respond more proportionally to the amount of antecedent rainfall. While riverine theory and study has been traditionally based on perennial model systems, recognition of the importance of intermittent streams has grown in recent years. However, critical gaps remain around the causes and consequences of intermittent stream drying across space and time. These systems are especially sensitive to climate fluctuations, and there is considerable potential for such ecosystems, and the species and services they support, to shift in response to climate change. Yet, relatively little attention has been given to identifying and evaluating the factors that influence variability in intermittent stream drying. Without documenting how intermittent streams vary within and across years, an essential component of their character is lost. With increasing precipitation volatility and rising temperatures, there is an urgent need to characterize where and when intermittent streams will be buffered from, or vulnerable to, altered climate conditions. In this dissertation, I explore the causes and consequences of intermittent stream variability, with a focus on the interannual variation in the hydrological and ecological patterns within these systems. These themes are explored at multiple scales, from the watershed to the stream reach, in three core chapters. I first evaluate the challenges and opportunities of monitoring intermittent streams in Chapter 1, before moving to understanding the controls on wetted channel extent and variability in Chapter 2, to finally characterizing the shifting habitat mosaic in Chapter 3. These three chapters together illustrate the importance of intermittent streams in shaping the environments they flow (and cease to flow) through, and underscore the need to capture the patterns of drying across space and time.

Advisor: Grantham, Theodore E
Effects of Wavelength-Shifting Plates on the Light Collection and Vertex Reconstruction in a Large-Volume Water-Cherenkov Detector

Antineutrino detectors may be able to fill an important role in the global nuclear nonproliferation regime by discovering an undeclared nuclear reactor or monitoring a known reactor tens of kilometers away. However, such detectors are large capital investments, and a substantial portion of their cost is the photomultiplier tubes that collect the light from antineutrino interactions in the detector volume and convert it into a usable electronic signal. As such, maximizing the light collection efficiency, and thus performance, of each photomultiplier tube is paramount. Wavelength-shifting plates may be able to aid in this goal. Wavelength-shifting plates increase the amount of light collected by each photomultiplier tube, which translates into a greater energy resolution of the detector. At the same time, however, the wavelength-shifting plates smear out the collection of this light in time, as light captured by the plates is delayed for different amounts of time before reaching a photomultiplier tube. This reduces the ability of post-processing algorithms to successfully reconstruct the location at which an event occurred in the detector. This is expected to have a countervailing effect to the improved energy resolution provided by the wavelength-shifting plates. This work explores the trade-off between these two effects, utilizing Monte Carlo simulations validated against experiments in air and in a water-Cherenkov detector to predict the effects that wavelength-shifting plates will have on large-volume water-Cherenkov detector performance. It was found that wavelength shifting plates improve the overall signal-to-background ratio of a 16 meter diameter, 16 meter height cylindrical detector by 13%, allowing a 3000 MWth undeclared nuclear reactor to be discovered 22% faster, in 16 days rather than 20.4 days when a detector without wavelength-shifting plates is utilized. This improvement is significant as it demonstrates that the improvement wavelength-shifting plates provide to light collection outweighs their effects on timing, so that wavelength-shifting plates may be a cost-effective way to improve the overall performance of large-volume water-Cherenkov antineutrino detectors intended for nuclear nonproliferation missions.

Advisor: Vujic, Jasmina
Dynamics & Control for Collaborative Aerial Manipulation

Aerial vehicles for physical interactions as a form of a freely-floating manipulator are ofgrowing interest in recent times. Aerial vehicles like quadrotors enable us to address issues such as last-mile delivery and search & rescue. Using multiple such vehicles collaboratively increases the scope of manipulation beyond what a single vehicle can achieve. This dissertation presents theoretical and experimental contributions toward using multiple quadrotors for collaborative tasks, with an emphasis on cable-suspended payloads./nThe dissertation studies the problem of collaborative aerial manipulation from two overarchingviews. The first half of the dissertation has an “individualistic view” and presents methods and algorithms for a single quadrotor/quadrotor with a cable-suspended payload. An L1 adaptation scheme is implemented on a geometric attitude control for the quadrotor on SO(3) in the presence of model uncertainties and disturbances. Next, the extended Kalman filter is modified to estimate states on S2. The concept of variation on manifolds is employed to linearize the system states and compute the variations in the state. Optimal and obstacle-free trajectories for a quadrotor with a suspended payload are generated using direct collocation. The planning method exploits the differential flatness for generating the trajectories in the flat space and converts non-differentiable obstacle avoidance constraints into smooth constraints using dual variables./nThe second part of the dissertation has a “collaborative view” and presents results for twotypes of aerial manipulation, multiple quadrotors carrying a payload using suspended cables (parallel-aerial-manipulator) and a series of quadrotors connected using a flexible cable (serial-aerial-manipulator). The dissertation presents experimental results for grasping and controlling payload using a cable-suspended gripper using more than one quadrotor. Finally, the dissertation models and computes coordinate-free geometric dynamics for multiple quadrotors connected in series using a single flexible cable and shows that the system is differentially-flat.

Advisor: Sreenath, Koushil
Learning Objective Functions from Many Diverse Signals

Specifying the correct objective function for a machine learning system is often difficult and error-prone. This thesis focuses on learning objective functions from many kinds of human inputs. It is comprised of three parts. First, we present our formalism, reward-rational choice (RRC), that unifies reward learning from many diverse signals. The key insight is that human behavior can often be modeled as a reward-rational implicit choice – a choice from an implicit set of options, that is approximately rational for the intended reward. We show how much of the prior work, despite using many diverse modalities of feedback, can be seen as an instantiation of RRC./nIn the second part, we discuss implications of the RRC formalism. In particular, it allows us to learn from multiple feedback types at once. Through case studies and experiments, we show how RRC can be used to combine and actively select from feedback types. Furthermore, once a person has access to multiple types of feedback, even their choice of feedback type itself provides information to learn the reward function from. We use RRC to formalize and learn from a person's meta-choice, the choice of feedback type itself./nFinally, in the third part, we study settings in which the human may violate the reward-rational assumption. First, we consider the case where the human may be pedagogic, i.e., optimizing for teaching the reward function. We show that the reward-rational assumption provides robust reward inference even when the human is pedagogic. Second, we consider the case where the human may face temptation and act in ways that systematically deviate from their target preferences. We theoretically analyze such a setting and show that, with the right feedback type, one can still efficiently recover the individual's preferences. Lastly, we consider the recommender system setting. There, it is difficult to model all user behaviors as rational, but by leveraging one strong, explicit signal (e.g. ``don't show me this''), we are still able to operationalize and optimize for a notion of ``value'' on these systems.

Advisor: Dragan, Anca D
Hardt, Moritz
Ax-Kochen/Ershov style results in model theory of henselian valued fields

This thesis is concerned with developing a model theoretic understanding of henselian valued fields. The model theory of henselian valued fields has been a major topic of study during the last century, it was initiated by Robinson’s model completeness results for algebraically closed valued fields in [Rob56]. Remarkable work has been achieved by Haskell, Hrushovski and Macpherson to understand the model theory of algebraically closed valued fields, more precisely in [HHM05] and [HHM06] they clarify completely the picture for elimination of imaginaries showing that it is sufficient to add the geometric sorts. They also develop a no- tion of stable domination and independence in algebraically closed valued fields, that rather than being understood as a new form of stability should be grasp as a technique to lift ideas from stability to the setting of valued fields. This approach is for example illustrated in [HRK19], where notions of stability (e.g. germs, genericity, domintion, etc) are being used to give a complete description of definable abelian groups in algebraically closed valued fields.The starting point of this thesis relies on the Ax-Kochen principle, which states that the first order theory of a henselian valued field of equicharacteristic zero or of mixed char- acteristic, unramified and with perfect residue field is completely determined by the first order theory of its residue field and its value group. A natural principle follows from this theorem: model theoretic questions about the value group itself can be understood by reduc- ing them to questions into the residue field, the value group and their interaction in the field. A fruitful application of this principle has been achieved to describe the class of definable sets, see for example: [Pas90], [Bas91], [Kuh94]. The next natural step for understanding the model theory of henselian valued fields was obtaining an elimination of imaginaries statement. The first part of this thesis studies elimination of imaginaries in the setting of henselian valued fields of equicharacteristic zero with residue field algebraically closed. The obtained results are sensitive to the complexity of the value group, which is an ordered abelian group. In the first chapter we study elimination of imaginaries in ordered abelian groups, while in the second chapter we analyze imaginaries in henselian valued fields of equicharacteristic zero. The second part of this thesis studies domination results in an Ax-Kochen style in the setting of henselian valued fields of equicharacteristic zero. We use a more abstract notion of domination present in [EHM19] that generalizes the definition of stable domination present in [HHM05].

Advisor: Scanlon, Thomas W
Simon, Pierre A
Safety-Guaranteed Autonomy under Uncertainty

Reachability analyzes a dynamic system’s abilities to reach goals or maintain safety. This analysis plays an essential role in various safety-critical applications. Previous reachability theory characterizes the success or failure of reachability tasks. However, this does not tell us the degree to which the goal will be achieved, or the safety will be maintained. Our new reachability formulation aims to provide measures for each goal-reaching and safety./nThis dissertation introduces three bodies of work. The first presents state-constrained reachability problems that provide the goal-reaching or safety metrics, and the corresponding Hamilton-Jacobi (HJ) frameworks. The HJ frameworks guarantee performance and safety metrics for general nonlinear systems with non-convex constraints. Despite this advantage, its computational complexity is exponential in the state dimension. Thus, it is not scalable for high-dimensional systems. In order to alleviate this computational complexity, the second presents efficient Hopf-Lax theory, which provides analytic solutions to HJ partial differential equations (PDEs) for state-constrained reachability problems, and the third presents reinforcement-learning approaches.

Advisor: Tomlin, Claire
Tomizuka, Masayoshi
Developing Laboratory Seismic Inversions

Earthquakes are simulated in the laboratory using a homogeneous rock analog material, taking advantage of highly controlled conditions to investigate the source properties of asperity rupture. The frictional interface consists of two transparent Poly(methyl-methacrylate) (PMMA) blocks with self-similar roughness profiles imposed by sand blasting and laser engraving to match our understanding of rock-rock interfaces. The resulting interface is imagedby a pressure sensitive film revealing the real contact area spread over sparsely distributed 1 mm patches called asperities. Stick slip events on the direct shear fault are recorded by an array of absolutely calibrated, single component displacement acoustic emissions (AE) sensors. Rupture events which are fully contained within the fault surface have radiation patterns consistent with double couple source mechanisms aligned with the fault plane and strike, making them earthquake-like events at the millimeter scale. A catalog is developed containing 50 Mw-7.5 to Mw-6.5 events which exhibit self-similar scaling properties in estimated magnitude and rupture area, which connects these events to previously observed earthquake scaling behavior at laboratory and natural scales./nThe acoustic emissions sensors are absolutely calibrated by a method combining well characterized, broadband calibration sources with a Green’s function model for wave propagationthrough the PMMA base plate. The Green’s functions are carefully tuned for the samplespecific material properties of the plate, including anelastic attenuation. Each sensor is individually calibrated and found to have a sensitivity on the order of 1 V/nm. Displacement records for the experimental stick-slip events have sudden onsets, with first arriving pulses around 0.01 - 1 nm in amplitude and 1 - 4 μs in duration. The arrival pulses are often complex in shape, such as having two peaks rather than one smooth pulse. Displacement arrival pulses are directly proportional to the moment-rate function which describes the distribution of slip during the event source. Furthermore, the magnitude analysis of the events indicates a rupture area at the appropriate scale but generally larger than any one asperity observed on the fault surface. Together, these observations indicate that rupture events recorded likely have coseismic slip spreading over multiple nearby asperities./nIn order to study the complex, multiple-asperity sources, a finite source inversion methodfollowing Hartzell and Heaton [1] is developed. The method is tested on forward modeled synthetic asperity ruptures, beginning with simple ruptures on single, circular asperity models. Although a brief attempt is made at applying the method to recorded experimental data, more testing and sensitivity analysis is required before the method may be reliably used. Continued work in laboratory scale seismic inversions shows great promise in providing a highly controlled environment for detailed source study of earthquake-like events.

Advisor: Soga, Kenichi