Browsing by Subject "Propagation"
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Item Bispectral analysis of nonlinear acoustic propagation(2011-05) Gagnon, David Edward; Hamilton, Mark F.; Wochner, Mark S.Higher-order spectral analysis of acoustical waveforms can provide phase information that is not retained in calculations of power spectral density. In the propagation of high intensity sound, nonlinearity can cause substantial changes in the waveform as frequency components interact with one another. The bispectrum, which is one order higher than power spectral density, may provide a useful measure of nonlinearity in propagation by highlighting spectral regions of interaction. This thesis provides a review of the bispectrum, places it in the context of nonlinear acoustic propagation, and presents spectra calculated as a function of distance for numerically propagated acoustic waveforms. The calculated spectra include power spectral density, quad-spectral density, bispectrum, spatial derivative of the bispectrum, bicoherence, and skewness function.Item Experimental and computational characterization of thermal runaway propagation in lithium-ion pouch cell arrays(2021-12-01) Kennedy, Robert Wade; Ezekoye, Ofodike A.; Hall, Matthew; Ellzey, Janet; Varghese, Philip; Marr, Kevin COver the past two decades, Lithium-ion (Li-ion) batteries have become ubiquitous in society. Their relatively high energy density, and recharge efficiency have resulted in their mass adoption. With falling prices, batteries have moved from single cell applications to increasingly larger systems comprised of many cells, such as electric vehicles (EVs), and fixed systems used in grid-scale load-leveling and uninterruptible power supplies (UPS). These systems typically have thousands, or tens of thousands, of cells in order to meet the desired capacity and power draw capabilities. Li-ion energy storage systems (ESS) are generally safe. However, Li-ion cells can fail under abnormal conditions potentially resulting in a catastrophic event known as thermal runaway (TR). During this failure process, exothermic reactions within the cell can result in excessive temperatures (∼ 800-1000°C), and produce highly flammable, toxic vented gases. As has been done for legacy battery technologies, such as Lead-acid, it is important to characterize these hazards, and to develop prevention, detection, mitigation strategies for TR and TR propagation. Experiments were conducted with arrays of Li-ion, pouch-format cells of two cathode chemistries: LCO and NMC. These experiments represent pseudo-modules to characterize TR failure propagation. This work makes the assumption that a single cell can and will fail for a specific system. This failure is represented using an external heater applied to one cell on one side of the array. From the initial failure, this work analyzes the failure propagation process, discusses the patterns seen, and evaluates different experimental techniques. One of the key hazards resulting from TR and TR propagation is the previously mentioned vent gas production. This vent gas is primarily composed of hydrogen, carbon dioxide, carbon monoxide, and various hydrocarbons. Altogether the produced vent gas can result in fire if ignited, or an explosion if allowed to accumulate. Furthermore, Li-ion cells have been found to produce sparks and eject relatively hot soot particulates during TR that can result in vent gas ignition. This work characterizes the vent gas release process for the two cell chemistries tested, including the total volume production, rate of release, and gas composition. While more toxic gases have also been detected in trace amounts in Li-ion vent gas, the primary goal of this work is to characterize the TR failure propagation process as it relates to fire and explosion related hazards. Thus, toxic gas characterization is beyond the scope of this work and is left to other studies in the literature. The second hazard from Li-ion TR propagation is the resulting elevated cell temperatures from the internal exothermic reactions. This work discusses the internal reactions responsible for TR proposed in the literature. Furthermore, this work evaluates the effect of a cell’s state of charge (SOC) on these reactions. Experimentally, thermocouples (TCs) were used to measure the temperatures between cells. These experimental data were then used to create and calibrate a computational model for predicting TR propagation in cell arrays of varying SOC. The effect of SOC is significant from the modeled internal kinetics. However, the heat transfer process between cells was found to be the primary challenge in modeling cell-to-cell TR propagation. Finally, the computational model was used to evaluate the performance of thermal separation materials between cells to inhibit cell-to-cell TR propagation. This work found that insulation, and thermal sink layers, i.e. aluminum plates between cells, could slow TR propagation, but may still lead to TR propagation via other modes of heat transfer to the cell array, including heat convection from the produced hot vent gases. Additionally, this work does not fully study the effect of external flaming combustion which was found to accelerate TR propagation in preliminary tests. Thus, external flaming combustion could reduce the efficacy of thermal separation materials between cells. Regardless, the computational model is capable of accurately predicting TR propagation in these arrays without external flaming combustion, and can be used to quickly evaluate several potential mitigation strategies. These modeled mitigation strategies can then be further validated with additional experimental testing.Item Experimental investigation of geomechanical aspects of hydraulic fracturing unconventional formations(2014-08) Alabbad, Emad Abbad; Olson, Jon E.Understanding the mechanisms that govern hydraulic fracturing applications in unconventional formations, such as gas-bearing shales, is of increasing interest to the petroleum upstream industry. Among such mechanisms, the geomechanical interactions between hydraulic fractures and pre-existing fractures on one hand, and simultaneous multiple hydraulic fractures on the other hand are seen of high importance. Although the petroleum engineering and related literature contains a number of studies that discusses such topics of hydraulic fracture interactions, there still remain some aspects that require answers, validations, or further supporting data. Particularly, experimental evidence is fairly scarce and keenly needed to solidify the understanding of such complex applications. In this work, the investigation methodology uses a series of hydraulic fracturing laboratory tests performed on synthetic rocks made of gypsum-based cements such as hydrostone and plaster in various experimental set ups. Those laboratory tests aim to closely investigate hydraulic fracture intersection with pre-existing fractures by assessing some factors that govern its outcomes. Specifically, the roles of the pre-existing fracture cementation, aperture, and relative height on the intersection mode are examined. The results show dominant effect of the cement-fill type relative to the host-rock matrix in determining whether hydraulic fracture crossing the pre-existing interface may occur. Similarly, hydraulic fracture height relative to the height of the pre-existing fracture may dictate the intersection results. However, the intersection mode seems to be insensitive of the pre-existing fracture aperture. Moreover, simultaneous multi-fracture propagation is examined and found to be impacted by the interference of the stresses induced from each fracturing source on neighboring fracturing sources. Such stress interference increases as the number of the propagating hydraulic fractures increase. While hydraulic fractures initiating from fracturing sources located in the middle of the fracturing stage seem to have inhibited propagation, outer hydraulic fractures may continue propagating with outward curvatures. Overall, the experimental results and analyses offer more insights for understanding hydraulic fracture complexity in unconventional formations.Item A general poro-elastic model for pad-scale fracturing of horizontal wells(2015-12) Manchanda, Ripudaman; Sharma, Mukul M.; Espinoza, David N; McClure, Mark W; Olson, Jon E; Roussel, Nicolas PEconomic production of oil and gas from tight rocks requires horizontal well drilling with multiple hydraulic fractures along the length of the horizontal wells. Multiple horizontal wells are drilled and fractured close to each other to increase the recovery of oil and gas from a single location or pad. Interference between fractures in a horizontal well pad is commonly observed in the field. There is no clear understanding of the impact of various operational and reservoir parameters on the observed interference. This inter-well interference can occur through the creation of complex fracture networks and/or poro-elastic stress changes. In this research, the development of a poro-elastic numerical simulator was undertaken to evaluate hydraulic fracturing practices in pad-scale scenarios. The primary motivation was to assess the impact of various operational parameters such as fracture spacing, well spacing and fracture sequencing on the geometry of the created fractures. Two approaches were used to understand the problem at hand. In the first approach, static fractures were simulated in 3-D and the impact of their stress shadow on subsequent fractures was studied. It was observed that fracture spacing, injection volume, and time between successive fractures were the most important parameters that could be used to optimize the creation of fractures in a well. Formation properties such as Young’s modulus and horizontal stress contrast modified the magnitude and spatial extent of the stress shadow and the extent of stress reorientation. It was shown that stage spacing, well spacing and fracture sequencing together with fracture designs (volume of sand pumped and fluids used) can be adjusted to obtain non-intersecting, transverse fractures that efficiently drain the reservoir. A hypothesis, time dependent closure of induced unpropped fractures, was presented to explain why zipper fracturing often outperforms conventional sequential fracturing. The hypothesis was tested and confirmed with a field data set made available to us by Shell from the Eagle Ford shale. In the second approach, a novel finite volume based 3-D, geomechanical, field-scale numerical simulator was developed to simulate propagation of multiple fractures simultaneously in a poro-elastic reservoir. This provided a more realistic model of the pad-scale fracturing process. The ability of the model to perform realistic pad-scale simulations was demonstrated for a variety of field situations such as multi-cluster multi-stage fracturing, infill-well fracturing, re-fracturing, mini-frac analysis and fracture network simulations. The inclusion of poro-elastic effects and reservoir heterogeneity in the model allowed us to examine the effects of reservoir depletion on fracture geometry in refraced and infill wells.Item Mittag-Leffler moments and weighted L∞ estimates for solutions to the Boltzmann equation for hard potentials without cutoff(2016-05) Tasković, Maja; Martínez Gamba, Irene, 1957-; Pavlović, Nataša; Caffarelli, Luis A.; Chen, Thomas; Figalli, Alessio; Morrison, Philip J.; Vasseur, Alexis F.In this thesis we study analytic properties of solutions to the spatially homogeneous Boltzmann equation for collision kernels corresponding to hard potentials without the angular cutoff assumption, i.e. the angular part of the kernel is non-integrable with prescribed singularity rate. We study behavior in time of such solutions for large velocities i.e. their tails. We do this in two settings - L¹ and L∞. In the L¹ setting, we study Mittag-Leffler moments of solutions of the Cauchy problem under consideration. These moments, obtained by integrating the solution against a Mittag-Leffler function, are a generalization of exponential moments since Mittag-Leffler functions asymptotically behave like exponential functions. Mittag-Leffler moments can be also represented as infinite sums of renormalized polynomial moments. However, instead of considering renormaliztion by integer factorials that would lead to classical exponential moments, we renormalize by Gamma functions with non-integer arguments. By analyzing the convergence of partial sums sequences of these infinite sums, we prove the propagation and generation in time of Mittag-Leffler moments. In the case of propagation, orders of these moments depend on the singularity rate of the angular collision kernel. In the case of generation, the orders depend on the potential rate of the kernel. The proof uses a subtle combination of angular averaging and angular singularity cancellation, to show that partial sums satisfy an ordinary differential inequality with a negative term of the highest order while controlling all positive terms, whose solutions are uniformly bounded in time and number of terms. These techniques apply to both generation and propagation of Mittag-Leffler moments, with some variations depending on the case. In the L∞ setting, we prove that solutions to the Boltzmann equation that satisfy propagation in time of weightedL¹ bounds also satisfy propagation in time of weighted L∞ bounds. To emphasize that the propagation in time of weighted L∞ bounds relies on the propagation in time of weighted L¹ bounds, we express our main result using certain general weights. Consequently we apply the main result to cases of exponential and Mittag-Leffler weights, for which propagation in time of weighted L¹ bounds holds. Hence we obtain propagation in time of exponentially or Mittag-Leffler weighted L∞ bounds on the solution.Item Sediment characterization using in situ measurements of acoustic properties(2018-08-17) Dubin, Justin Thomas; Wilson, Preston S.; Ballard, Megan S.; Lee, Kevin Michael, 1977-Three related studies associated with the acoustics of marine sediments were performed and described here. The first study was a direct exploration of the microscopic properties of marine sediment collected in the field. High-resolution images, acquired through scanning electron microscopy, are presented alongside sediment analysis and in situ sound speed data in order to better understand the affect microscopic geometries have on acoustic propagation in naturally occurring marine sediment. This microscopy work is followed by a description of the apparatus, methodology, and results from a field experiment conducted in a shallow water seagrass-bearing environment in which the acoustic properties of both the seagrass canopy and underlying sediment were measured in situ. The results are compared to predictions from several effective medium models to help explain the observed propagation behavior. The final study in this thesis describes acoustic directivity characterization for the compressional wave transducer probes used to collect in situ data presented in the previous studies as well as for a re-designed pair of prototype probes. These laboratory measurements are compared to both analytical and finite element models.Item Simulating refracturing treatments that employ diverting agents on horizontal wells(2013-08) Bryant, Stephen Andrew; Sharma, Mukul M.The use of hydraulic fracturing has increased rapidly and is now a necessary technique for the development of shale oil and gas resources. However, production rates from these plays typically exhibit high levels of decline. After one year, rates often decrease by over fifty percent. Refracturing – the process of hydraulically fracturing a well that has previously been fractured – is a proposed technique designed to offset these high decline rates and provide a sustainable increase in production. Benefits from refracturing can occur due to a variety of reasons, including the extension of fracture length, the increase in fracture conductivity or the reorientation of the fracture into new areas of the reservoir. In this thesis, the simulation of refracturing treatments on horizontal wells with the use of a diverting agent is described. Diverting agents are used to distribute flow more evenly along the wellbore and to replace the use of costly downhole equipment employed to isolate sections of the wellbore. When diverting agent is deposited, a cake forms with an associated permeability. Flow is diverted from the fractures with high amounts of diverting agent because the larger cake results in a greater resistance to flow. The diverting agent cake breaks down with time at reservoir temperature so that production is uninhibited. Two different models are used to account for the application of diverting agent. One assumes the diverting agent cake forms in the perforation tunnel and the other assumes it forms in the fracture. The propagation of competing fractures is calculated using a computer code developed at the University of Texas called UTWID. In both models, the simulations showed successful diversion of flow. Previously understimulated fractures – that is, shorter fractures or fractures that would grow less preferentially under normal fracturing treatments – grew at a faster pace after pumping of the diverting agent. A sensitivity analysis was conducted on several of the key refracturing design parameters, and the interdependence of the parameters was demonstrated. The simulations support the concept that diverting agents can be used to more evenly stimulate the entire length of the lateral.Item Single station Doppler tracking for satellite orbit prediction and propagation(2015-05) Dykstra, Matthew C.; Fowler, Wallace T.; Lightsey, E. GlennPresently, there are two main methods of launching a cube satellite into Earth orbit. The first method is to purchase a secondary payload slot on a major launch vehicle. For the second method, the satellite must first be transported via a major launch vehicle to the International Space Station. From there, the satellite is loaded into one of two deployment mechanisms, and deployed at a specified time. In each case, the satellite's initial orbit is not accurately known. For ground operators this poses a problem of position uncertainty. In order to solve this problem, a satellite tracking algorithm was developed to use an initial two-line element set for coarse orbit prediction, followed by Doppler measurements for continuous processing and updating. The system was tested using simulated data. The analysis showed that this low-cost, scalable system will satisfy the tracking requirements of many cube satellite missions, including current missions at the University of Texas.Item Space object translational and rotational state prediction and sensitivity calculation(2016-12) Hatten, Noble Ariel; Russell, Ryan Paul, 1976-; Akella, Maruthi R; Bettadpur, Srinivas V; Jones, Brandon A; Weisman, Ryan MWhile computing power has grown monumentally during the space age, the demands of astrodynamics applications have more than kept pace. Resources are taxed by the ever-growing number of Earth-orbiting space objects (SOs) that must be tracked to maintain space situational awareness (SSA) and by increasingly popular but computationally expensive tools like Monte Carlo techniques and stochastic optimization algorithms. In this dissertation, methods are presented to improve the accuracy, efficiency, and utility of SO state prediction and sensitivity calculation algorithms. The dynamical model of the low Earth orbit regime is addressed through the introduction of an upgraded Harris-Priester atmospheric density model, which introduces a smooth polynomial dependency on solar flux. Additional modifications eliminate singularities and provide smooth partial derivatives of the density with respect to SO state, time, and solar conditions. The numerical solution of the equations of motion derived from dynamics models is also addressed, with particular emphasis placed on six-degree-of-freedom (6DOF) state prediction. Implicit Runge-Kutta (IRK) methods are applied to the 6DOF problem, and customizations, including variable-fidelity dynamics models and parallelization, are introduced to maximize efficiency and take advantage of modern computing architectures. Sensitivity calculation -- a necessity for SSA and other applications -- via RK methods is also examined. Linear algebraic systems for first- and second-order state transition matrix calculation are derived by directly differentiating either the first- or second-order form of the RK update equations. This approach significantly reduces the required number of Jacobian and Hessian evaluations compared to the ubiquitous augmented state vector approach for IRK methods, which can result in more efficient calculations. Parallelization is once again leveraged to reduce the runtime of IRK methods. Finally, a hybrid special perturbation/general perturbation (SP/GP) technique is introduced to address the notoriously slow speed of fully coupled 6DOF state prediction. The hybrid method uses a GP rotational state prediction to provide low-fidelity attitude information for a high-fidelity 3DOF SP routine. This strategy allows for the calculation of body forces using arbitrary shape models without adding attitude to the propagated state or taking the small step sizes often required by full 6DOF propagation. The attitude approximation is obtained from a Lie-Deprit perturbation result previously applied to SOs in circular orbits subject to gravity-gradient torque and extended here to SOs in elliptical orbits. The hybrid method is shown to produce a meaningful middle ground between 3DOF SP and 6DOF SP methods in the accuracy vs. efficiency space.